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Corradi J, Thompson B, Fletcher PA, Bertram R, Sherman AS, Satin LS. K ATP channel activity and slow oscillations in pancreatic beta cells are regulated by mitochondrial ATP production. J Physiol 2023; 601:5655-5667. [PMID: 37983196 PMCID: PMC10842208 DOI: 10.1113/jp284982] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2023] [Accepted: 10/16/2023] [Indexed: 11/22/2023] Open
Abstract
Pancreatic beta cells secrete insulin in response to plasma glucose. The ATP-sensitive potassium channel (KATP ) links glucose metabolism to islet electrical activity in these cells by responding to increased cytosolic [ATP]/[ADP]. It was recently proposed that pyruvate kinase (PK) in close proximity to beta cell KATP locally produces the ATP that inhibits KATP activity. This proposal was largely based on the observation that applying phosphoenolpyruvate (PEP) and ADP to the cytoplasmic side of excised inside-out patches inhibited KATP . To test the relative contributions of local vs. mitochondrial ATP production, we recorded KATP activity using mouse beta cells and INS-1 832/13 cells. In contrast to prior reports, we could not replicate inhibition of KATP activity by PEP + ADP. However, when the pH of the PEP solutions was not corrected for the addition of PEP, strong channel inhibition was observed as a result of the well-known action of protons to inhibit KATP . In cell-attached recordings, perifusing either a PK activator or an inhibitor had little or no effect on KATP channel closure by glucose, further suggesting that PK is not an important regulator of KATP . In contrast, addition of mitochondrial inhibitors robustly increased KATP activity. Finally, by measuring the [ATP]/[ADP] responses to imposed calcium oscillations in mouse beta cells, we found that oxidative phosphorylation could raise [ATP]/[ADP] even when ADP was at its nadir during the burst silent phase, in agreement with our mathematical model. These results indicate that ATP produced by mitochondrial oxidative phosphorylation is the primary controller of KATP in pancreatic beta cells. KEY POINTS: Phosphoenolpyruvate (PEP) plus adenosine diphosphate does not inhibit KATP activity in excised patches. PEP solutions only inhibit KATP activity if the pH is unbalanced. Modulating pyruvate kinase has minimal effects on KATP activity. Mitochondrial inhibition, in contrast, robustly potentiates KATP activity in cell-attached patches. Although the ADP level falls during the silent phase of calcium oscillations, mitochondria can still produce enough ATP via oxidative phosphorylation to close KATP . Mitochondrial oxidative phosphorylation is therefore the main source of the ATP that inhibits the KATP activity of pancreatic beta cells.
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Affiliation(s)
- Jeremías Corradi
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan, USA
- Brehm Diabetes Research Center, University of Michigan Medical School, Ann Arbor, Michigan, USA
| | - Benjamin Thompson
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan, USA
- Brehm Diabetes Research Center, University of Michigan Medical School, Ann Arbor, Michigan, USA
| | - Patrick A. Fletcher
- Laboratory of Biological Modeling, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland
| | - Richard Bertram
- Department of Mathematics and Programs in Neuroscience and Molecular Biophysics, Florida State University, Tallahassee, Florida, USA
| | - Arthur S. Sherman
- Laboratory of Biological Modeling, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland
| | - Leslie S. Satin
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan, USA
- Brehm Diabetes Research Center, University of Michigan Medical School, Ann Arbor, Michigan, USA
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Chareyron I, Christen S, Moco S, Valsesia A, Lassueur S, Dayon L, Wollheim CB, Santo Domingo J, Wiederkehr A. Augmented mitochondrial energy metabolism is an early response to chronic glucose stress in human pancreatic beta cells. Diabetologia 2020; 63:2628-2640. [PMID: 32960311 PMCID: PMC7641954 DOI: 10.1007/s00125-020-05275-5] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/05/2020] [Accepted: 08/04/2020] [Indexed: 01/15/2023]
Abstract
AIMS/HYPOTHESIS In islets from individuals with type 2 diabetes and in islets exposed to chronic elevated glucose, mitochondrial energy metabolism is impaired. Here, we studied early metabolic changes and mitochondrial adaptations in human beta cells during chronic glucose stress. METHODS Respiration and cytosolic ATP changes were measured in human islet cell clusters after culture for 4 days in 11.1 mmol/l glucose. Metabolomics was applied to analyse intracellular metabolite changes as a result of glucose stress conditions. Alterations in beta cell function were followed using insulin secretion assays or cytosolic calcium signalling after expression of the calcium probe YC3.6 specifically in beta cells of islet clusters. RESULTS At early stages of glucose stress, mitochondrial energy metabolism was augmented in contrast to the previously described mitochondrial dysfunction in beta cells from islets of diabetic donors. Following chronic glucose stress, mitochondrial respiration increased (by 52.4%, p < 0.001) and, as a consequence, the cytosolic ATP/ADP ratio in resting human pancreatic islet cells was elevated (by 27.8%, p < 0.05). Because of mitochondrial overactivation in the resting state, nutrient-induced beta cell activation was reduced. In addition, chronic glucose stress caused metabolic adaptations that resulted in the accumulation of intermediates of the glycolytic pathway, the pentose phosphate pathway and the TCA cycle; the most strongly augmented metabolite was glycerol 3-phosphate. The changes in metabolites observed are likely to be due to the inability of mitochondria to cope with continuous nutrient oversupply. To protect beta cells from chronic glucose stress, we inhibited mitochondrial pyruvate transport. Metabolite concentrations were partially normalised and the mitochondrial respiratory response to nutrients was markedly improved. Furthermore, stimulus-secretion coupling as assessed by cytosolic calcium signalling, was restored. CONCLUSION/INTERPRETATION We propose that metabolic changes and associated mitochondrial overactivation are early adaptations to glucose stress, and may reflect what happens as a result of poor blood glucose control. Inhibition of mitochondrial pyruvate transport reduces mitochondrial nutrient overload and allows beta cells to recover from chronic glucose stress. Graphical abstract.
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Affiliation(s)
- Isabelle Chareyron
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland
- Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
| | - Stefan Christen
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland
| | - Sofia Moco
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland
| | - Armand Valsesia
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland
| | - Steve Lassueur
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland
| | - Loïc Dayon
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland
- Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
| | - Claes B Wollheim
- Department of Cell Physiology and Metabolism, University Medical Center, Geneva, Switzerland
| | - Jaime Santo Domingo
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland
| | - Andreas Wiederkehr
- Nestlé Institute of Health Sciences, Nestlé Research, EPFL Innovation Park, Lausanne, Switzerland.
- Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
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Santo-Domingo J, Dayon L, Wiederkehr A. Protein Lysine Acetylation: Grease or Sand in the Gears of β-Cell Mitochondria? J Mol Biol 2019; 432:1446-1460. [PMID: 31628953 DOI: 10.1016/j.jmb.2019.09.011] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2019] [Revised: 09/16/2019] [Accepted: 09/18/2019] [Indexed: 02/06/2023]
Abstract
Mitochondria carry out many essential functions in metabolism. A central task is the oxidation of nutrients and the generation of ATP by oxidative phosphorylation. Mitochondrial metabolism needs to be tightly regulated for the cell to respond to changes in ATP demand and nutrient supply. Here, we review how protein lysine acetylation contributes to the regulation of mitochondrial metabolism in insulin target tissues and the insulin-secreting pancreatic β-cell. We summarize recent evidence showing that in pancreatic β-cells, lysine acetylation occurs on a large number of proteins involved in metabolism. Furthermore, we give a brief overview of the molecular mechanism that controls lysine acetylation dynamics. We propose that protein lysine acetylation is an important mechanism for the fine-tuning of mitochondrial activity in β-cells during normal physiology. In contrast, nutrient oversupply, oxidative stress, or inhibition of the mitochondrial deacetylase SIRT3 leads to protein lysine hyperacetylation, which impairs mitochondrial function. By perturbing mitochondrial activity in β-cells and insulin target tissues, protein lysine hyperacetylation may contribute to the development of type 2 diabetes.
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Affiliation(s)
- Jaime Santo-Domingo
- Mitochondrial Function, Nestlé Institute of Health Sciences, 1015 Lausanne, Switzerland
| | - Loïc Dayon
- Proteomics, Nestlé Institute of Health Sciences, 1015 Lausanne, Switzerland
| | - Andreas Wiederkehr
- Mitochondrial Function, Nestlé Institute of Health Sciences, 1015 Lausanne, Switzerland.
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4
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Gerzanich V, Stokum JA, Ivanova S, Woo SK, Tsymbalyuk O, Sharma A, Akkentli F, Imran Z, Aarabi B, Sahuquillo J, Simard JM. Sulfonylurea Receptor 1, Transient Receptor Potential Cation Channel Subfamily M Member 4, and KIR6.2:Role in Hemorrhagic Progression of Contusion. J Neurotrauma 2018; 36:1060-1079. [PMID: 30160201 PMCID: PMC6446209 DOI: 10.1089/neu.2018.5986] [Citation(s) in RCA: 27] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
In severe traumatic brain injury (TBI), contusions often are worsened by contusion expansion or hemorrhagic progression of contusion (HPC), which may double the original contusion volume and worsen outcome. In humans and rodents with contusion-TBI, sulfonylurea receptor 1 (SUR1) is upregulated in microvessels and astrocytes, and in rodent models, blockade of SUR1 with glibenclamide reduces HPC. SUR1 does not function by itself, but must co-assemble with either KIR6.2 or transient receptor potential cation channel subfamily M member 4 (TRPM4) to form KATP (SUR1-KIR6.2) or SUR1-TRPM4 channels, with the two having opposite effects on membrane potential. Both KIR6.2 and TRPM4 are reportedly upregulated in TBI, especially in astrocytes, but the identity and function of SUR1-regulated channels post-TBI is unknown. Here, we analyzed human and rat brain tissues after contusion-TBI to characterize SUR1, TRPM4, and KIR6.2 expression, and in the rat model, to examine the effects on HPC of inhibiting expression of the three subunits using intravenous antisense oligodeoxynucleotides (AS-ODN). Glial fibrillary acidic protein (GFAP) immunoreactivity was used to operationally define core versus penumbral tissues. In humans and rats, GFAP-negative core tissues contained microvessels that expressed SUR1 and TRPM4, whereas GFAP-positive penumbral tissues contained astrocytes that expressed all three subunits. Förster resonance energy transfer imaging demonstrated SUR1-TRPM4 heteromers in endothelium, and SUR1-TRPM4 and SUR1-KIR6.2 heteromers in astrocytes. In rats, glibenclamide as well as AS-ODN targeting SUR1 and TRPM4, but not KIR6.2, reduced HPC at 24 h post-TBI. Our findings demonstrate upregulation of SUR1-TRPM4 and KATP after contusion-TBI, identify SUR1-TRPM4 as the primary molecular mechanism that accounts for HPC, and indicate that SUR1-TRPM4 is a crucial target of glibenclamide.
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Affiliation(s)
- Volodymyr Gerzanich
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Jesse A Stokum
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Svetlana Ivanova
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Seung Kyoon Woo
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Orest Tsymbalyuk
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Amit Sharma
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Fatih Akkentli
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Ziyan Imran
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Bizhan Aarabi
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland
| | - Juan Sahuquillo
- 2 Neurotraumatology and Neurosurgery Research Unit, Vall d'Hebron University Hospital, Universitat Autònoma de Barcelona, Barcelona, Spain.,3 Department of Neurosurgery, Vall d'Hebron University Hospital, Universitat Autònoma de Barcelona, Barcelona, Spain
| | - J Marc Simard
- 1 Department of Neurosurgery, University of Maryland School of Medicine, Baltimore, Maryland.,4 Department of Pathology, University of Maryland School of Medicine, Baltimore, Maryland.,5 Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland
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5
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Esguerra JLS, Nagao M, Ofori JK, Wendt A, Eliasson L. MicroRNAs in islet hormone secretion. Diabetes Obes Metab 2018; 20 Suppl 2:11-19. [PMID: 30230181 DOI: 10.1111/dom.13382] [Citation(s) in RCA: 38] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/17/2018] [Revised: 05/10/2018] [Accepted: 05/23/2018] [Indexed: 12/12/2022]
Abstract
Pancreatic islet hormone secretion is central in the maintenance of blood glucose homeostasis. During development of hyperglycaemia, the β-cell is under pressure to release more insulin to compensate for increased insulin resistance. Failure of the β-cells to secrete enough insulin results in type 2 diabetes (T2D). MicroRNAs (miRNAs) are short non-coding RNA molecules suitable for rapid regulation of the changes in target gene expression needed in β-cell adaptations. Moreover, miRNAs are involved in the maintenance of α-cell and β-cell phenotypic identities via cell-specific, or cell-enriched expression. Although many of the abundant miRNAs are highly expressed in both cell types, recent research has focused on the role of miRNAs in β-cells. It has been shown that highly abundant miRNAs, such as miR-375, are involved in several cellular functions indispensable in maintaining β-cell phenotypic identity, almost acting as "housekeeping genes" in the context of hormone secretion. Despite the abundance and importance of miR-375, it has not been shown to be differentially expressed in T2D islets. On the contrary, the less abundant miRNAs such as miR-212/miR-132, miR-335, miR-130a/b and miR-152 are deregulated in T2D islets, wherein the latter three miRNAs were shown to play key roles in regulating β-cell metabolism. In this review, we focus on β-cell function and describe miRNAs involved in insulin biosynthesis and processing, glucose uptake and metabolism, electrical activity and Ca2+ -influx and exocytosis of the insulin granules. We present current status on miRNA regulation in α-cells, and finally we discuss the involvement of miRNAs in β-cell dysfunction underlying T2D pathogenesis.
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Affiliation(s)
- Jonathan L S Esguerra
- Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Clinical Research Centre, SUS, Malmö, Sweden
| | - Mototsugu Nagao
- Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Clinical Research Centre, SUS, Malmö, Sweden
| | - Jones K Ofori
- Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Clinical Research Centre, SUS, Malmö, Sweden
| | - Anna Wendt
- Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Clinical Research Centre, SUS, Malmö, Sweden
| | - Lena Eliasson
- Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Clinical Research Centre, SUS, Malmö, Sweden
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Rorsman P, Ashcroft FM. Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men. Physiol Rev 2018; 98:117-214. [PMID: 29212789 PMCID: PMC5866358 DOI: 10.1152/physrev.00008.2017] [Citation(s) in RCA: 424] [Impact Index Per Article: 70.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2017] [Revised: 05/30/2017] [Accepted: 06/18/2017] [Indexed: 12/14/2022] Open
Abstract
The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.
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Affiliation(s)
- Patrik Rorsman
- Oxford Centre for Diabetes, Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, United Kingdom; Department of Neuroscience and Physiology, Metabolic Research Unit, Göteborg, Sweden; and Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
| | - Frances M Ashcroft
- Oxford Centre for Diabetes, Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, United Kingdom; Department of Neuroscience and Physiology, Metabolic Research Unit, Göteborg, Sweden; and Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
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7
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Proks P, Puljung MC, Vedovato N, Sachse G, Mulvaney R, Ashcroft FM. Running out of time: the decline of channel activity and nucleotide activation in adenosine triphosphate-sensitive K-channels. Philos Trans R Soc Lond B Biol Sci 2017; 371:rstb.2015.0426. [PMID: 27377720 PMCID: PMC4938026 DOI: 10.1098/rstb.2015.0426] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/17/2016] [Indexed: 11/29/2022] Open
Abstract
KATP channels act as key regulators of electrical excitability by coupling metabolic cues—mainly intracellular adenine nucleotide concentrations—to cellular potassium ion efflux. However, their study has been hindered by their rapid loss of activity in excised membrane patches (rundown), and by a second phenomenon, the decline of activation by Mg-nucleotides (DAMN). Degradation of PI(4,5)P2 and other phosphoinositides is the strongest candidate for the molecular cause of rundown. Broad evidence indicates that most other determinants of rundown (e.g. phosphorylation, intracellular calcium, channel mutations that affect rundown) also act by influencing KATP channel regulation by phosphoinositides. Unfortunately, experimental conditions that reproducibly prevent rundown have remained elusive, necessitating post hoc data compensation. Rundown is clearly distinct from DAMN. While the former is associated with pore-forming Kir6.2 subunits, DAMN is generally a slower process involving the regulatory sulfonylurea receptor (SUR) subunits. We speculate that it arises when SUR subunits enter non-physiological conformational states associated with the loss of SUR nucleotide-binding domain dimerization following prolonged exposure to nucleotide-free conditions. This review presents new information on both rundown and DAMN, summarizes our current understanding of these processes and considers their physiological roles. This article is part of the themed issue ‘Evolution brings Ca2+ and ATP together to control life and death’.
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Affiliation(s)
- Peter Proks
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Michael C Puljung
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Natascia Vedovato
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Gregor Sachse
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Rachel Mulvaney
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Frances M Ashcroft
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
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8
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Proks P, de Wet H, Ashcroft FM. Sulfonylureas suppress the stimulatory action of Mg-nucleotides on Kir6.2/SUR1 but not Kir6.2/SUR2A KATP channels: a mechanistic study. ACTA ACUST UNITED AC 2015; 144:469-86. [PMID: 25348414 PMCID: PMC4210431 DOI: 10.1085/jgp.201411222] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
Sulfonylureas suppress the stimulatory effect of Mg-nucleotides on recombinant β-cell (Kir6.2/SUR1) but not cardiac (Kir6.2/SUR2A) KATP channels. Sulfonylureas, which stimulate insulin secretion from pancreatic β-cells, are widely used to treat both type 2 diabetes and neonatal diabetes. These drugs mediate their effects by binding to the sulfonylurea receptor subunit (SUR) of the ATP-sensitive K+ (KATP) channel and inducing channel closure. The mechanism of channel inhibition is unusually complex. First, sulfonylureas act as partial antagonists of channel activity, and second, their effect is modulated by MgADP. We analyzed the molecular basis of the interactions between the sulfonylurea gliclazide and Mg-nucleotides on β-cell and cardiac types of KATP channel (Kir6.2/SUR1 and Kir6.2/SUR2A, respectively) heterologously expressed in Xenopus laevis oocytes. The SUR2A-Y1206S mutation was used to confer gliclazide sensitivity on SUR2A. We found that both MgATP and MgADP increased gliclazide inhibition of Kir6.2/SUR1 channels and reduced inhibition of Kir6.2/SUR2A-Y1206S. The latter effect can be attributed to stabilization of the cardiac channel open state by Mg-nucleotides. Using a Kir6.2 mutation that renders the KATP channel insensitive to nucleotide inhibition (Kir6.2-G334D), we showed that gliclazide abolishes the stimulatory effects of MgADP and MgATP on β-cell KATP channels. Detailed analysis suggests that the drug both reduces nucleotide binding to SUR1 and impairs the efficacy with which nucleotide binding is translated into pore opening. Mutation of one (or both) of the Walker A lysines in the catalytic site of the nucleotide-binding domains of SUR1 may have a similar effect to gliclazide on MgADP binding and transduction, but it does not appear to impair MgATP binding. Our results have implications for the therapeutic use of sulfonylureas.
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Affiliation(s)
- Peter Proks
- Oxford Centre for Gene Function and Department of Physiology, Anatomy, and Genetics, University of Oxford, Oxford OX1 3PT, England, UK Oxford Centre for Gene Function and Department of Physiology, Anatomy, and Genetics, University of Oxford, Oxford OX1 3PT, England, UK
| | - Heidi de Wet
- Oxford Centre for Gene Function and Department of Physiology, Anatomy, and Genetics, University of Oxford, Oxford OX1 3PT, England, UK Oxford Centre for Gene Function and Department of Physiology, Anatomy, and Genetics, University of Oxford, Oxford OX1 3PT, England, UK
| | - Frances M Ashcroft
- Oxford Centre for Gene Function and Department of Physiology, Anatomy, and Genetics, University of Oxford, Oxford OX1 3PT, England, UK Oxford Centre for Gene Function and Department of Physiology, Anatomy, and Genetics, University of Oxford, Oxford OX1 3PT, England, UK
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Molecular determinants of ATP-sensitive potassium channel MgATPase activity: diabetes risk variants and diazoxide sensitivity. Biosci Rep 2015; 35:BSR20150143. [PMID: 26181369 PMCID: PMC4613687 DOI: 10.1042/bsr20150143] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2015] [Accepted: 07/03/2015] [Indexed: 11/17/2022] Open
Abstract
Molecular interactions between two residues in the sulfonylurea receptor (SUR) subunit of the ATP-sensitive potassium channel influence MgATPase activity. This interaction may provide a mechanism for the increased diabetes risk associated with a common channel variant and determines sensitivity to diazoxide. ATP-sensitive K+ (KATP) channels play an important role in insulin secretion. KATP channels possess intrinsic MgATPase activity that is important in regulating channel activity in response to metabolic changes, although the precise structural determinants are not clearly understood. Furthermore, the sulfonylurea receptor 1 (SUR1) S1369A diabetes risk variant increases MgATPase activity, but the molecular mechanisms remain to be determined. Therefore, we hypothesized that residue–residue interactions between 1369 and 1372, predicted from in silico modelling, influence MgATPase activity, as well as sensitivity to the clinically used drug diazoxide that is known to increase MgATPase activity. We employed a point mutagenic approach with patch-clamp and direct biochemical assays to determine interaction between residues 1369 and 1372. Mutations in residues 1369 and 1372 predicted to decrease the residue interaction elicited a significant increase in MgATPase activity, whereas mutations predicted to possess similar residue interactions to wild-type (WT) channels elicited no alterations in MgATPase activity. In contrast, mutations that were predicted to increase residue interactions resulted in significant decreases in MgATPase activity. We also determined that a single S1369K substitution in SUR1 caused MgATPase activity and diazoxide pharmacological profiles to resemble those of channels containing the SUR2A subunit isoform. Our results provide evidence, at the single residue level, for a molecular mechanism that may underlie the association of the S1369A variant with type 2 diabetes. We also show a single amino acid difference can account for the markedly different diazoxide sensitivities between channels containing either the SUR1 or SUR2A subunit isoforms.
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10
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Fatehi M, Raja M, Carter C, Soliman D, Holt A, Light PE. The ATP-sensitive K(+) channel ABCC8 S1369A type 2 diabetes risk variant increases MgATPase activity. Diabetes 2012; 61:241-9. [PMID: 22187380 PMCID: PMC3237651 DOI: 10.2337/db11-0371] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Pancreatic β-cell ATP-sensitive K(+) (K(ATP)) channels are composed of Kir6.2 and SUR1 subunits encoded by the KCNJ11 and ABCC8 genes, respectively. Although rare monogenic activating mutations in these genes cause overt neonatal diabetes, the common variants E23K (KCNJ11) and S1369A (ABCC8) form a tightly heritable haplotype that is associated with an increased susceptibility to type 2 diabetes (T2D) risk. However, the molecular mechanism(s) underlying this risk remain to be elucidated. A homology model of the SUR1 nucleotide-binding domains (NBDs) indicates that residue 1369 is in close proximity to the major MgATPase site. Therefore, we investigated the intrinsic MgATPase activity of K(ATP) channels containing these variants. Electrophysiological and biochemical techniques were used to study the MgATPase activity of recombinant human K(ATP) channels or glutathione S-transferase and NBD2 fusion proteins containing the E23/S1369 (nonrisk) or K23/A1369 (risk) variant haplotypes. K(ATP) channels containing the K23/A1369 haplotype displayed a significantly increased stimulation by guanosine triphosphate compared with the E23/S1369 haplotype (3.2- vs. 1.8-fold). This effect was dependent on the presence of the A1369 variant and was lost in the absence of Mg(2+) ions or in the presence of the MgATPase inhibitor beryllium fluoride. Direct biochemical assays also confirmed an increase in MgATPase activity in NBD2 fusion proteins containing the A1369 variant. Our findings demonstrate that the A1369 variant increases K(ATP) channel MgATPase activity, providing a plausible molecular mechanism by which the K23/A1369 haplotype increases susceptibility to T2D in humans homozygous for these variants.
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MESH Headings
- ATP-Binding Cassette Transporters/chemistry
- ATP-Binding Cassette Transporters/genetics
- ATP-Binding Cassette Transporters/metabolism
- ATP-Binding Cassette Transporters/physiology
- Adenosine Triphosphatases/metabolism
- Adenosine Triphosphate/metabolism
- Alanine/genetics
- Amino Acid Substitution/physiology
- Cells, Cultured
- Diabetes Mellitus, Type 2/genetics
- Enzyme Activation
- Genetic Predisposition to Disease
- Humans
- KATP Channels/chemistry
- KATP Channels/genetics
- KATP Channels/physiology
- Models, Molecular
- Polymorphism, Single Nucleotide/physiology
- Potassium Channels, Inwardly Rectifying/chemistry
- Potassium Channels, Inwardly Rectifying/genetics
- Potassium Channels, Inwardly Rectifying/metabolism
- Potassium Channels, Inwardly Rectifying/physiology
- Protein Structure, Tertiary/genetics
- Receptors, Drug/chemistry
- Receptors, Drug/genetics
- Receptors, Drug/metabolism
- Receptors, Drug/physiology
- Risk Factors
- Serine/genetics
- Sulfonylurea Receptors
- Transfection
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Proks P, de Wet H, Ashcroft FM. Activation of the K(ATP) channel by Mg-nucleotide interaction with SUR1. ACTA ACUST UNITED AC 2011; 136:389-405. [PMID: 20876358 PMCID: PMC2947056 DOI: 10.1085/jgp.201010475] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
The mechanism of adenosine triphosphate (ATP)-sensitive potassium (KATP) channel activation by Mg-nucleotides was studied using a mutation (G334D) in the Kir6.2 subunit of the channel that renders KATP channels insensitive to nucleotide inhibition and has no apparent effect on their gating. KATP channels carrying this mutation (Kir6.2-G334D/SUR1 channels) were activated by MgATP and MgADP with an EC50 of 112 and 8 µM, respectively. This activation was largely suppressed by mutation of the Walker A lysines in the nucleotide-binding domains of SUR1: the remaining small (∼10%), slowly developing component of MgATP activation was fully inhibited by the lipid kinase inhibitor LY294002. The EC50 for activation of Kir6.2-G334D/SUR1 currents by MgADP was lower than that for MgATP, and the time course of activation was faster. The poorly hydrolyzable analogue MgATPγS also activated Kir6.2-G334D/SUR1. AMPPCP both failed to activate Kir6.2-G334D/SUR1 and to prevent its activation by MgATP. Maximal stimulatory concentrations of MgATP (10 mM) and MgADP (1 mM) exerted identical effects on the single-channel kinetics: they dramatically elevated the open probability (PO > 0.8), increased the mean open time and the mean burst duration, reduced the frequency and number of interburst closed states, and eliminated the short burst states. By comparing our results with those obtained for wild-type KATP channels, we conclude that the MgADP sensitivity of the wild-type KATP channel can be described quantitatively by a combination of inhibition at Kir6.2 (measured for wild-type channels in the absence of Mg2+) and activation via SUR1 (determined for Kir6.2-G334D/SUR1 channels). However, this is not the case for the effects of MgATP.
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Affiliation(s)
- Peter Proks
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, England, UK
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12
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Hibino H, Inanobe A, Furutani K, Murakami S, Findlay I, Kurachi Y. Inwardly rectifying potassium channels: their structure, function, and physiological roles. Physiol Rev 2010; 90:291-366. [PMID: 20086079 DOI: 10.1152/physrev.00021.2009] [Citation(s) in RCA: 1070] [Impact Index Per Article: 76.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.
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Affiliation(s)
- Hiroshi Hibino
- Department of Pharmacology, Graduate School of Medicine and The Center for Advanced Medical Engineering and Informatics, Osaka University, Osaka 565-0871, Japan
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13
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Modeling K(ATP) channel gating and its regulation. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2008; 99:7-19. [PMID: 18983870 DOI: 10.1016/j.pbiomolbio.2008.10.002] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
ATP-sensitive potassium (K(ATP)) channels couple cell metabolism to plasmalemmal potassium fluxes in a variety of cell types. The activity of these channels is primarily determined by intracellular adenosine nucleotides, which have both inhibitory and stimulatory effects. The role of K(ATP) channels has been studied most extensively in pancreatic beta-cells, where they link glucose metabolism to insulin secretion. Many mutations in K(ATP) channel subunits (Kir6.2, SUR1) have been identified that cause either neonatal diabetes or congenital hyperinsulinism. Thus, a mechanistic understanding of K(ATP) channel behavior is necessary for modeling beta-cell electrical activity and insulin release in both health and disease. Here, we review recent advances in the K(ATP) channel structure and function. We focus on the molecular mechanisms of K(ATP) channel gating by adenosine nucleotides, phospholipids and sulphonylureas and consider the advantages and limitations of various mathematical models of macroscopic and single-channel K(ATP) currents. Finally, we outline future directions for the development of more realistic models of K(ATP) channel gating.
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14
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Kibbey RG, Pongratz RL, Romanelli AJ, Wollheim CB, Cline GW, Shulman GI. Mitochondrial GTP regulates glucose-stimulated insulin secretion. Cell Metab 2007; 5:253-64. [PMID: 17403370 PMCID: PMC1876711 DOI: 10.1016/j.cmet.2007.02.008] [Citation(s) in RCA: 130] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/11/2006] [Revised: 12/01/2006] [Accepted: 02/15/2007] [Indexed: 11/15/2022]
Abstract
Nucleotide-specific isoforms of the tricarboxylic acid (TCA) cycle enzyme succinyl-CoA synthetase (SCS) catalyze substrate-level synthesis of mitochondrial GTP (mtGTP) and ATP (mtATP). While mtATP yield from glucose metabolism is coupled with oxidative phosphorylation and can vary, each molecule of glucose metabolized within pancreatic beta cells produces approximately one mtGTP, making mtGTP a potentially important fuel signal. In INS-1 832/13 cells and cultured rat islets, siRNA suppression of the GTP-producing pathway (DeltaSCS-GTP) reduced glucose-stimulated insulin secretion (GSIS) by 50%, while suppression of the ATP-producing isoform (DeltaSCS-ATP) increased GSIS 2-fold. Insulin secretion correlated with increases in cytosolic calcium, but not with changes in NAD(P)H or the ATP/ADP ratio. These data suggest a role for mtGTP in controlling pancreatic GSIS through modulation of mitochondrial metabolism, possibly involving mitochondrial calcium. Furthermore, in light of its tight coupling to TCA oxidation rates, mtGTP production may serve as an important molecular signal of TCA-cycle activity.
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Affiliation(s)
- Richard G. Kibbey
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Rebecca L. Pongratz
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Anthony J. Romanelli
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Claes B. Wollheim
- Department of Cell Physiology and Metabolism University Medical Center CH-1211 Geneva 4, Switzerland
| | - Gary W. Cline
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
| | - Gerald I. Shulman
- Department of Internal Medicine, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
- Department of Cellular, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
- Department of Molecular Physiology, Howard Hughes Medical Institute Yale University School of Medicine New Haven, CT 06520, USA
- *To whom correspondence should be addressed. E-mail:
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15
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Ashcroft FM. ATP-sensitive potassium channelopathies: focus on insulin secretion. J Clin Invest 2005; 115:2047-58. [PMID: 16075046 PMCID: PMC1180549 DOI: 10.1172/jci25495] [Citation(s) in RCA: 457] [Impact Index Per Article: 24.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
ATP-sensitive potassium (K(ATP)) channels, so named because they are inhibited by intracellular (ATP), play key physiological roles in many tissues. In pancreatic beta cells, these channels regulate glucose-dependent insulin secretion and serve as the target for sulfonylurea drugs used to treat type 2 diabetes. This review focuses on insulin secretory disorders, such as congenital hyperinsulinemia and neonatal diabetes, that result from mutations in K(ATP) channel genes. It also considers the extent to which defective regulation of K(ATP) channel activity contributes to the etiology of type 2 diabetes.
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Affiliation(s)
- Frances M Ashcroft
- University Laboratory of Physiology, Oxford University, Oxford, United Kingdom.
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16
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Hattersley AT, Ashcroft FM. Activating mutations in Kir6.2 and neonatal diabetes: new clinical syndromes, new scientific insights, and new therapy. Diabetes 2005; 54:2503-13. [PMID: 16123337 DOI: 10.2337/diabetes.54.9.2503] [Citation(s) in RCA: 337] [Impact Index Per Article: 17.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Closure of ATP-sensitive K(+) channels (K(ATP) channels) in response to metabolically generated ATP or binding of sulfonylurea drugs stimulates insulin release from pancreatic beta-cells. Heterozygous gain-of-function mutations in the KCJN11 gene encoding the Kir6.2 subunit of this channel are found in approximately 47% of patients diagnosed with permanent diabetes at <6 months of age. There is a striking genotype-phenotype relationship with specific Kir6.2 mutations being associated with transient neonatal diabetes, permanent neonatal diabetes alone, and a novel syndrome characterized by developmental delay, epilepsy, and neonatal diabetes (DEND) syndrome. All mutations appear to cause neonatal diabetes by reducing K(ATP) channel ATP sensitivity and increasing the K(ATP) current, which inhibits beta-cell electrical activity and insulin secretion. The severity of the clinical symptoms is reflected in the ATP sensitivity of heterozygous channels in vitro with wild type > transient neonatal diabetes > permanent neonatal diabetes > DEND syndrome channels. Sulfonylureas still close mutated K(ATP) channels, and many patients can discontinue insulin injections and show improved glycemic control when treated with high-dose sulfonylurea tablets. In conclusion, the finding that Kir6.2 mutations can cause neonatal diabetes has enabled a new therapeutic approach and shed new light on the structure and function of the Kir6.2 subunit of the K(ATP) channel.
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17
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Tarasov A, Dusonchet J, Ashcroft F. Metabolic regulation of the pancreatic beta-cell ATP-sensitive K+ channel: a pas de deux. Diabetes 2004; 53 Suppl 3:S113-22. [PMID: 15561898 DOI: 10.2337/diabetes.53.suppl_3.s113] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
Closure of ATP-sensitive K+ channels (KATP channels) is a key step in glucose-stimulated insulin secretion. The precise mechanism(s) by which glucose metabolism regulates KATP channel activity, however, remains controversial. It is widely believed that the principal determinants are the intracellular concentrations of the metabolic ligands, ATP and ADP, which have opposing actions on KATP channels, with ATP closing and MgADP opening the channel. However, the sensitivity of the channel to these nucleotides in the intact cell, and their relative contribution to the regulation of channel activity, remains unclear. The precise role of phosphoinositides and long-chain acyl-CoA esters, which are capable of modulating the channel ATP sensitivity, is also uncertain. Furthermore, it is still a matter of debate whether it is changes in the concentration of ATP, of MgADP, or of other agents, which couples glucose metabolism to KATP channel activity. In this article, we review current knowledge of the metabolic regulation of the KATP channel and provide evidence that MgADP (or MgATP hydrolysis), acting at the regulatory subunit of the channel, shifts the ATP concentration-response curve into a range in which the channel pore can respond to dynamic changes in cytosolic ATP. This metabolic pas de deux orchestrates the pivotal role of ATP in metabolic regulation of the KATP channel.
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Affiliation(s)
- Andrei Tarasov
- University Laboratory of Physiology, Parks Rd., Oxford OX1 3PT, UK
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18
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Ribalet B, John SA, Weiss JN. Regulation of cloned ATP-sensitive K channels by phosphorylation, MgADP, and phosphatidylinositol bisphosphate (PIP(2)): a study of channel rundown and reactivation. J Gen Physiol 2000; 116:391-410. [PMID: 10962016 PMCID: PMC2233681 DOI: 10.1085/jgp.116.3.391] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2000] [Accepted: 07/24/2000] [Indexed: 11/20/2022] Open
Abstract
Kir6.2 channels linked to the green fluorescent protein (GFP) (Kir6. 2-GFP) have been expressed alone or with the sulfonylurea receptor SUR1 in HEK293 cells to study the regulation of K(ATP) channels by adenine nucleotides, phosphatidylinositol bisphosphate (PIP(2)), and phosphorylation. Upon excision of inside-out patches into a Ca(2+)- and MgATP-free solution, the activity of Kir6.2-GFP+SUR1 channels spontaneously ran down, first quickly within a minute, and then more slowly over tens of minutes. In contrast, under the same conditions, the activity of Kir6.2-GFP alone exhibited only slow rundown. Thus, fast rundown is specific to Kir6.2-GFP+SUR1 and involves SUR1, while slow rundown is a property of both Kir6.2-GFP and Kir6.2-GFP+SUR1 channels and is due, at least in part, to Kir6.2 alone. Kir6. 2-GFP+SUR1 fast phase of rundown was of variable amplitude and led to increased ATP sensitivity. Excising patches into a solution containing MgADP prevented this phenomenon, suggesting that fast rundown involves loss of MgADP-dependent stimulation conferred by SUR1. With both Kir6.2-GFP and Kir6.2-GFP+SUR1, the slow phase of rundown led to further increase in ATP sensitivity. Ca(2+) accelerated this process, suggesting a role for PIP(2) hydrolysis mediated by a Ca(2+)-dependent phospholipase C. PIP(2) could reactivate channel activity after a brief exposure to Ca(2+), but not after prolonged exposure. However, in both cases, PIP(2) reversed the increase in ATP sensitivity, indicating that PIP(2) lowers the ATP sensitivity by increasing P(o) as well as by decreasing the channel affinity for ATP. With Kir6.2-GFP+SUR1, slow rundown also caused loss of MgADP stimulation and sulfonylurea inhibition, suggesting functional uncoupling of SUR1 from Kir6.2-GFP. Ca(2+) facilitated the loss of sensitivity to MgADP, and thus uncoupling of the two subunits. The nonselective protein kinase inhibitor H-7 and the selective PKC inhibitor peptide 19-36 evoked, within 5-15 min, increased ATP sensitivity and loss of reactivation by PIP(2) and MgADP. Phosphorylation of Kir6.2 may thus be required for the channel to remain PIP(2) responsive, while phosphorylation of Kir6.2 and/or SUR1 is required for functional coupling. In summary, short-term regulation of Kir6.2+SUR1 channels involves MgADP, while long-term regulation requires PIP(2) and phosphorylation.
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Affiliation(s)
- B Ribalet
- Department of Physiology, Cardiovascular Research Laboratory, University of California, Los Angeles, School of Medicine, Los Angeles, California 90095, USA.
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19
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Shi H, Moustaid-Moussa N, Wilkison WO, Zemel MB. Role of the sulfonylurea receptor in regulating human adipocyte metabolism. FASEB J 1999; 13:1833-8. [PMID: 10506587 DOI: 10.1096/fasebj.13.13.1833] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
A regulatory role for intracellular Ca2+ ([Ca2+]i) in adipocyte lipogenesis, lipolysis and triglyceride accumulation has been demonstrated. Compounds acting on the pancreatic sulfonylurea receptor (SUR) to increase (e.g., glibenclamide) or decrease (e.g., diazoxide) [Ca2+]i cause corresponding increases and decreases in weight gain. However, these weight gain and loss effects have been attributed to insulin release rather than to the primary effects of these compounds on the adipocyte SUR and its associated K(ATP) channel. Accordingly, we have evaluated the direct role of the human adipocyte SUR in regulating adipocyte metabolism. We used RT-PCR with primers for a highly conserved region of SUR1 to demonstrate that human adipocytes express SUR1. The PCR product was confirmed by sequence analysis and used as a probe to demonstrate adipocyte SUR1 expression by Northern blot analysis. Adipocytes exhibited glibenclamide dose-responsive (0-20 microM) increases in [Ca2+]i (P<0.05). Similarly, glibenclamide (10 microM) caused a 67% increase in adipocyte fatty acid synthase activity (P<0.001), a 48% increase in glycerol-3-phosphate dehydrogenase activity (P<0.01) and a 68% inhibition in lipolysis (P<0.01), whereas diazoxide (10 microM) completely prevented each of these effects. These data demonstrate that human adipocytes express a SUR that regulates [Ca2+]i and, consequently, exerts coordinate control over lipogenesis and lipolysis. Accordingly, the adipocyte SUR1 may represent an important target for the development of therapeutic interventions in obesity.
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Affiliation(s)
- H Shi
- Department of Nutrition, The University of Tennessee, Knoxville, Tennessee 37996, USA
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20
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Takano M, Xie LH, Otani H, Horie M. Cytoplasmic terminus domains of Kir6.x confer different nucleotide-dependent gating on the ATP-sensitive K+ channel. J Physiol 1998; 512 ( Pt 2):395-406. [PMID: 9763630 PMCID: PMC2231215 DOI: 10.1111/j.1469-7793.1998.395be.x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
1. In order to investigate the structural basis for the nucleotide-dependent gating of ATP-sensitive K+ channels (KATP), Kir6.1 (uKATP-1), Kir6.2 (BIR1) and chimeric channels were co-expressed with a common subtype of sulphonylurea receptor, SUR1, in COS7 cells. Representing the amino terminal domain-transmembrane domain-carboxyl-terminal domain of Kir6.1 as 1-1-1 and of Kir6.2 as 2-2-2, chimeric Kir6.x channels were constructed by swapping the amino and/or carboxyl terminal domains between Kir6.1 and Kir6.2 to give the chimeric x-1-x channels 1-1-2, 2-1-1 and 2-1-2, and the chimeric x-2-x channels 2-2-1, 1-2-2 and 1-2-1. 2. Inside-out patch clamp experiments revealed that both wild-type Kir6.1 and Kir6.2 formed inwardly rectifying K+ channels. Single-channel conductances were 36.3 and 66.1 pS, respectively. Chimeric x-1-x channels, whose transmembrane domain was that of Kir6.1, showed similar ion-pore properties to wild-type Kir6.1. Likewise, chimeric x-2-x channels had similar ion-pore properties to wild-type Kir6.2. 3. Wild-type Kir6.1 and Kir6.2 possessed distinct gating properties towards intracellular nucleotides. The activity of Kir6.1 was entirely dependent on Mg2+ and nucleotide diphosphates (NDPs) such as UDP. In contrast, Kir6.2 was activated upon excision of patch membrane. When Kir6.2 underwent rundown, UDP reactivated the channel. 4. In order to eliminate UDP dependence from Kir6.1, it was necessary to replace both N- and C-termini; chimera 2-1-2 opened in UDP-free conditions. With Kir6.2, substitution of the N-terminus with that of Kir6.1 conferred UDP dependence on chimeras 1-2-2 and 1-2-1. Chimera 2-2-1 opened in UDP-free conditions, but UDP potentiated the channel activity by > 20-fold. 5. The kinetics of UDP-dependent activation were significantly different between Kir6.1 and Kir6.2. Kir6.1 maximally activated by UDP was sensitive to intracellular ATP, although its ATP sensitivity was significantly lower than that of Kir6.2 measured in identical conditions. The kinetics of UDP-dependent activation and ATP sensitivity could be transferred between Kir6.1 and Kir6.2 only when both N- and C-termini were replaced. We therefore concluded that nucleotide-dependent gating was regulated by the N- and C-terminal domains irrespective of the transmembrane domains.
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Affiliation(s)
- M Takano
- Department of Physiology and Biophysics, Graduate School of Medicine, Kyoto University, 606-8501 Kyoto, Japan.
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21
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Gribble FM, Tucker SJ, Haug T, Ashcroft FM. MgATP activates the beta cell KATP channel by interaction with its SUR1 subunit. Proc Natl Acad Sci U S A 1998; 95:7185-90. [PMID: 9618560 PMCID: PMC22779 DOI: 10.1073/pnas.95.12.7185] [Citation(s) in RCA: 141] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/1998] [Indexed: 02/07/2023] Open
Abstract
ATP-sensitive potassium (KATP) channels in the pancreatic beta cell membrane mediate insulin release in response to elevation of plasma glucose levels. They are open at rest but close in response to glucose metabolism, producing a depolarization that stimulates Ca2+ influx and exocytosis. Metabolic regulation of KATP channel activity currently is believed to be mediated by changes in the intracellular concentrations of ATP and MgADP, which inhibit and activate the channel, respectively. The beta cell KATP channel is a complex of four Kir6.2 pore-forming subunits and four SUR1 regulatory subunits: Kir6.2 mediates channel inhibition by ATP, whereas the potentiatory action of MgADP involves the nucleotide-binding domains (NBDs) of SUR1. We show here that MgATP (like MgADP) is able to stimulate KATP channel activity, but that this effect normally is masked by the potent inhibitory effect of the nucleotide. Mg2+ caused an apparent reduction in the inhibitory action of ATP on wild-type KATP channels, and MgATP actually activated KATP channels containing a mutation in the Kir6.2 subunit that impairs nucleotide inhibition (R50G). Both of these effects were abolished when mutations were made in the NBDs of SUR1 that are predicted to abolish MgATP binding and/or hydrolysis (D853N, D1505N, K719A, or K1384M). These results suggest that, like MgADP, MgATP stimulates KATP channel activity by interaction with the NBDs of SUR1. Further support for this idea is that the ATP sensitivity of a truncated form of Kir6.2, which shows functional expression in the absence of SUR1, is unaffected by Mg2+.
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Affiliation(s)
- F M Gribble
- University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, United Kingdom
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22
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Alekseev AE, Brady PA, Terzic A. Ligand-insensitive state of cardiac ATP-sensitive K+ channels. Basis for channel opening. J Gen Physiol 1998; 111:381-94. [PMID: 9450949 PMCID: PMC2222775 DOI: 10.1085/jgp.111.2.381] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
The mechanism by which ATP-sensitive K+ (KATP) channels open in the presence of inhibitory concentrations of ATP remains unknown. Herein, using a four-state kinetic model, we found that the nucleotide diphosphate UDP directed cardiac KATP channels to operate within intraburst transitions. These transitions are not targeted by ATP, nor the structurally unrelated sulfonylurea glyburide, which inhibit channel opening by acting on interburst transitions. Therefore, the channel remained insensitive to ATP and glyburide in the presence of UDP. "Rundown" of channel activity decreased the efficacy with which UDP could direct and maintain the channel to operate within intraburst transitions. Under this condition, the channel was sensitive to inhibition by ATP and glyburide despite the presence of UDP. This behavior of the KATP channel could be accounted for by an allosteric model of ligand-channel interaction. Thus, the response of cardiac KATP channels towards inhibitory ligands is determined by the relative lifetime the channel spends in a ligand-sensitive versus -insensitive state. Interconversion between these two conformational states represents a novel basis for KATP channel opening in the presence of inhibitory concentrations of ATP in a cardiac cell.
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Affiliation(s)
- A E Alekseev
- Division of Cardiovascular Diseases (G-7), Department of Medicine, Mayo Clinic, Mayo Foundation, Rochester, Minnesota 55905, USA
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Shyng S, Ferrigni T, Nichols CG. Regulation of KATP channel activity by diazoxide and MgADP. Distinct functions of the two nucleotide binding folds of the sulfonylurea receptor. J Gen Physiol 1997; 110:643-54. [PMID: 9382893 PMCID: PMC2229399 DOI: 10.1085/jgp.110.6.643] [Citation(s) in RCA: 227] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/1997] [Accepted: 10/03/1997] [Indexed: 02/05/2023] Open
Abstract
KATP channels were reconstituted in COSm6 cells by coexpression of the sulfonylurea receptor SUR1 and the inward rectifier potassium channel Kir6.2. The role of the two nucleotide binding folds of SUR1 in regulation of KATP channel activity by nucleotides and diazoxide was investigated. Mutations in the linker region and the Walker B motif (Walker, J.E., M.J. Saraste, M.J. Runswick, and N.J. Gay. 1982. EMBO [Eur. Mol. Biol. Organ.] J. 1:945-951) of the second nucleotide binding fold, including G1479D, G1479R, G1485D, G1485R, Q1486H, and D1506A, all abolished stimulation by MgADP and diazoxide, with the exception of G1479R, which showed a small stimulatory response to diazoxide. Analogous mutations in the first nucleotide binding fold, including G827D, G827R, and Q834H, were still stimulated by diazoxide and MgADP, but with altered kinetics compared with the wild-type channel. None of the mutations altered the sensitivity of the channel to inhibition by ATP4-. We propose a model in which SUR1 sensitizes the KATP channel to ATP inhibition, and nucleotide hydrolysis at the nucleotide binding folds blocks this effect. MgADP and diazoxide are proposed to stabilize this desensitized state of the channel, and mutations at the nucleotide binding folds alter the response of channels to MgADP and diazoxide by altering nucleotide hydrolysis rates or the coupling of hydrolysis to channel activation.
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Affiliation(s)
- S Shyng
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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24
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Gribble FM, Tucker SJ, Ashcroft FM. The interaction of nucleotides with the tolbutamide block of cloned ATP-sensitive K+ channel currents expressed in Xenopus oocytes: a reinterpretation. J Physiol 1997; 504 ( Pt 1):35-45. [PMID: 9350615 PMCID: PMC1159933 DOI: 10.1111/j.1469-7793.1997.00035.x] [Citation(s) in RCA: 136] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
1. We have examined the mechanism by which nucleotides modulate the tolbutamide block of the beta-cell ATP-sensitive K+ channel (KATP channel), using wild-type and mutant KATP channels heterologously expressed in Xenopus oocytes. This channel is composed of sulphonylurea receptor (SUR1) and pore-forming (Kir6.2) subunits. 2. The dose-response relation for tolbutamide block of wild-type KATP currents in the absence of nucleotide showed both a high-affinity (Ki = 2.0 microM) and a low-affinity (Ki = 1.8 mM) site. 3. The dose-response relation for tolbutamide block of Kir6.2 delta C36 (a truncated form of Kir6.2 which is expressed independently of SUR1) was best fitted with a single, low-affinity site (Ki = 1.7 mM). This indicates that the high-affinity site resides on SUR1, whereas the low-affinity site is located on Kir6.2. 4. ADP (100 microM) had a dual effect on wild-type KATP currents: the nucleotide enhanced the current in the presence of Mg2+, but was inhibitory in the absence of Mg2+. Kir6.2 delta C36 currents were blocked by 100 microM ADP in the presence of Mg2+. 5. For wild-type KATP currents, the blocking effect of 0.5 mM tolbutamide appeared greater in the presence of 100 microM MgADP (84 +/- 2%) than in its absence (59 +/- 4%). When SUR1 was mutated to abolish MgADP activation of KATP currents (K719A or K1384M), there was no difference in the extent of tolbutamide inhibition in the presence or absence of MgADP. 6. The Ki for tolbutamide interaction with either the high- or low-affinity site was unaffected by 100 microM MgADP, for both wild-type and K719A-K1384M currents. 7. MgGDP (100 microM) enhanced wild-type KATP currents and was without effect on K719A-K1384M currents. It did not affect the Ki for tolbutamide block at either the high- or low-affinity site. 8. Our results indicate that interaction of tolbutamide with the high-affinity site (on SUR1) abolishes the stimulatory action of MgADP. This unmasks the inhibitory effect of ADP and leads to an apparent increase in channel inhibition. Under physiological conditions, abolition of MgADP activation is likely to constitute the principal mechanism by which tolbutamide inhibits the KATP channel.
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Affiliation(s)
- F M Gribble
- University Laboratory of Physiology, Oxford, UK
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25
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Gribble FM, Tucker SJ, Ashcroft FM. The essential role of the Walker A motifs of SUR1 in K-ATP channel activation by Mg-ADP and diazoxide. EMBO J 1997; 16:1145-52. [PMID: 9135131 PMCID: PMC1169713 DOI: 10.1093/emboj/16.6.1145] [Citation(s) in RCA: 262] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
The ATP-sensitive K-channel (K-ATP channel) plays a key role in insulin secretion from pancreatic beta-cells. It is closed by glucose metabolism, which stimulates insulin secretion, and opened by the drug diazoxide, which inhibits insulin release. Metabolic regulation is mediated by changes in ATP and Mg-ADP, which inhibit and potentiate channel activity, respectively. The beta-cell K-ATP channel consists of a pore-forming subunit, Kir6.2, and a regulatory subunit, SUR1. We have mutated (independently or together) two lysine residues in the Walker A (W(A)) motifs of the first (K719A) and second (K1384M) nucleotide-binding domains (NBDs) of SUR1. These mutations are expected to inhibit nucleotide hydrolysis. Our results indicate that the W(A) lysine of NBD1 (but not NBD2) is essential for activation of K-ATP currents by diazoxide. The potentiatory effects of Mg-ADP required the presence of the W(A) lysines in both NBDs. Mutant currents were slightly more sensitive to ATP than wild-type currents. Metabolic inhibition led to activation of wild-type and K1384M currents, but not K719A or K719A/K1384M currents, suggesting that there may be a factor in addition to ATP and ADP which regulates K-ATP channel activity.
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Affiliation(s)
- F M Gribble
- University Laboratory of Physiology, Oxford, UK
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26
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Dunn-Meynell AA, Routh VH, McArdle JJ, Levin BE. Low-affinity sulfonylurea binding sites reside on neuronal cell bodies in the brain. Brain Res 1997; 745:1-9. [PMID: 9037388 DOI: 10.1016/s0006-8993(96)01006-2] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
The antidiabetic sulfonylurea drugs bind to sites associated with an ATP-sensitive potassium (Katp) channel on cell bodies and terminals of neurons which increase their firing rates or transmitter release when glucose concentrations rise or sulfonylureas are present. High-affinity sulfonylurea binding sites are concentrated in areas such as the substantia nigra (SN) where glucose and sulfonylureas increase transmitter release from GABA neurons. But there is a paucity of high-affinity sites in areas such as the hypothalamic ventromedial nucleus (VMN) where many neurons increase their activity when glucose rises. Here we assessed both high- and low--affinity sulfonylurea binding autoradiographically with 20 nM [3H]glyburide in the presence of absence of Gpp(NH)p. Neurotoxin lesions with 6-hydroxydopamine (6-OHDA), 5,7-dihydroxytryptamine (5,7-DHT) and ibotenic acid were used to elucidate the cellular location of the two sites in the VMN, SN and locus coeruleus (LC). In the VMN, 25% of the sites were of low affinity. Neither 6-OHDA nor 5,7-DHT affected [3H]glyburide binding, while ibotenic acid reduced the number of VMN neurons and abolished low-affinity without changing high-affinity binding. In cell-attached patches of isolated VMN neurons, both 10 mM glucose and 100 microM glyburide decreased the open probability of the Katp channel suggesting that the low-affinity binding site resides on these neurons. In the SN pars reticulata, ibotenic acid reduced the number of neurons and high-affinity [3H]glyburide binding was decreased by 20%, while 6-OHDA had no effect. In the SN pars compacta, both 6-OHDA and ibotenic acid destroyed endogenous dopamine neurons and selectivity ablated low-affinity binding. In the LC, 6-OHDA destroyed norepinephrine neurons and abolished low-affinity binding. These data suggest that low-affinity sulfonylurea binding sites reside on cell bodies on VMN, SN dopamine and LC norepinephrine neuron cell bodies and that high-affinity sites may be on axon terminals of GABA neurons in the SN.
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Affiliation(s)
- A A Dunn-Meynell
- Neurology Service (127C), DVA Medical Center, E. Orange, Newark, NJ 07103, USA
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27
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Robson L, Hunter M. Regulation of an outwardly rectifying Cl- conductance in single proximal tubule cells isolated from frog kidney. J Physiol 1997; 498 ( Pt 2):409-17. [PMID: 9032688 PMCID: PMC1159210 DOI: 10.1113/jphysiol.1997.sp021867] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
1. A previous study has identified a Cl- conductance (GCl) in single proximal tubule cells isolated from frog kidney, which was activated by a protein kinase C (PKC)-dependent mechanism. 2. The whole-cell patch clamp technique was employed to examine further the properties and regulation of GCl. 3. GCl showed outward rectification, outward conductance was significantly greater than the inward conductance (56.1 +/- 15.6 vs. 16.8 +/- 6.4 microS cm-2, respectively, n = 8). DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) blocked GCl in a dose- and voltage-dependent manner. 4. Other anions permeated the conductance. The anion selectivity sequence, I- > Br- > Cl- > gluconate, followed Eisenman's sequence I. 5. GCl could be activated by ATP. This process was dependent on ATP hydrolysis and channel phosphorylation. 6. G-protein activation inhibited the ATP-dependent activation of GCl. 7. These data support the hypothesis that activation of GCl by an increase in cell volume is dependent on ATP hydrolysis and channel phosphorylation via PKC.
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Affiliation(s)
- L Robson
- Department of Physiology, Medical School, Leeds University, UK.
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Detimary P, Van den Berghe G, Henquin JC. Concentration dependence and time course of the effects of glucose on adenine and guanine nucleotides in mouse pancreatic islets. J Biol Chem 1996; 271:20559-65. [PMID: 8702800 DOI: 10.1074/jbc.271.34.20559] [Citation(s) in RCA: 114] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Changes in the ATP:ADP ratio in pancreatic B cells may participate in the regulation of insulin secretion by glucose. Here, we have investigated the possible role of guanine nucleotides. Mouse islets were incubated in a control medium (when K+-ATP channels are the major site of regulation) or in a high K+ medium (when glucose modulates the effectiveness of cytosolic Ca2+ on exocytosis). Glucose induced a concentration-dependent (0-20 m) increase in GTP and a decrease in GDP in both types of medium, thus causing a progressive rise of the GTP:GDP ratio. ATP and ADP levels were 4-5-fold higher but varied in a similar way as those of guanine nucleotides. Insulin secretion was inversely correlated with ADP and GDP levels and positively correlated with the ATP:ADP and GTP:GDP ratios between 6 and 20 m glucose in control medium and between 0 and 20 m glucose in high K+ medium. The increases in the GTP:GDP and ATP:ADP ratios induced by a rise of glucose were faster than the decreases induced by a fall in glucose, but the changes of both ratios were again parallel. In conclusion, glucose causes large, concentration-dependent changes in guanine as well as in adenine nucleotides in islet cells. This raises the possibility that both participate in the regulation of nutrient-induced insulin secretion.
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Affiliation(s)
- P Detimary
- Unité d'Endocrinologie et Métabolisme, University of Louvain, Faculty of Medicine, UCL 55.30, Avenue Hippocrate 55, B-1200 Brussels, Belgium
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29
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Larsson O, Kindmark H, Brandstrom R, Fredholm B, Berggren PO. Oscillations in KATP channel activity promote oscillations in cytoplasmic free Ca2+ concentration in the pancreatic beta cell. Proc Natl Acad Sci U S A 1996; 93:5161-5. [PMID: 8643546 PMCID: PMC39425 DOI: 10.1073/pnas.93.10.5161] [Citation(s) in RCA: 78] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Pancreatic beta cells exhibit oscillations in electrical activity, cytoplasmic free Ca2+ concentration ([Ca2+](i)), and insulin release upon glucose stimulation. The mechanism by which these oscillations are generated is not known. Here we demonstrate fluctuations in the activity of the ATP-dependent K+ channels (K(ATP) channels) in single beta cells subject to glucose stimulation or to stimulation with low concentrations of tolbutamide. During stimulation with glucose or low concentrations of tolbutamide, K(ATP) channel activity decreased and action potentials ensued. After 2-3 min, despite continuous stimulation, action potentials subsided and openings of K(ATP) channels could again be observed. Transient suppression of metabolism by azide in glucose-stimulated beta cells caused reversible termination of electrical activity, mimicking the spontaneous changes observed with continuous glucose stimulation. Thus, oscillations in K(ATP) channel activity during continuous glucose stimulation result in oscillations in electrical activity and [Ca2+](i).
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Affiliation(s)
- O Larsson
- The Rolf Luft Center for Diabetes Research, Department of Molecular Medicine, Karolinska Institute, Karolinska Hospital, Stockholm, Sweden
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30
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Larsson O, Deeney JT, Bränström R, Berggren PO, Corkey BE. Activation of the ATP-sensitive K+ channel by long chain acyl-CoA. A role in modulation of pancreatic beta-cell glucose sensitivity. J Biol Chem 1996; 271:10623-6. [PMID: 8631866 DOI: 10.1074/jbc.271.18.10623] [Citation(s) in RCA: 128] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Long-term exposure to elevated levels of long chain free fatty acids decreases glucose-induced insulin secretion from pancreatic islets and clonal pancreatic beta-cells. The mechanism for this loss of glucose sensitivity is at present not known. In this study, we evaluated the possibility that increases in long chain acyl-CoA esters (LC-CoA), the metabolically active form of free fatty acids, might mediate the loss of glucose sensitivity. We observed that cellular levels of LC-CoA increased more than 100% in response to overnight incubation with 0.5 mM palmitic acid complexed to albumin. In the same studies, the total CoA pool increased by about 40%. Patch-clamp studies demonstrated that saturated and unsaturated LC-CoA, but not malonyl-CoA or free CoASH, induced a rapid and slowly reversible opening of ATP-sensitive K+ channels. The effect was concentration-dependent between 10 nM and 1 microM. These findings indicate that the ATP-regulated K/ channels is a sensitive target for LC-CoA and suggest that high levels of LC-CoA, which accumulate in response to hyperglycemia or prolonged exposure to free fatty acids, may prevent channel closure and contribute to the development of beta-cell glucose insensitivity.
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Affiliation(s)
- O Larsson
- Department of Molecular Medicine, Rolf Luft Center for Diabetes Research, Karolinska Institute, Stockholm, Sweden
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31
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Aguilar-Bryan L, Nichols CG, Wechsler SW, Clement JP, Boyd AE, González G, Herrera-Sosa H, Nguy K, Bryan J, Nelson DA. Cloning of the beta cell high-affinity sulfonylurea receptor: a regulator of insulin secretion. Science 1995; 268:423-6. [PMID: 7716547 DOI: 10.1126/science.7716547] [Citation(s) in RCA: 954] [Impact Index Per Article: 32.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Sulfonylureas are a class of drugs widely used to promote insulin secretion in the treatment of non-insulin-dependent diabetes mellitus. These drugs interact with the sulfonylurea receptor of pancreatic beta cells and inhibit the conductance of adenosine triphosphate (ATP)-dependent potassium (KATP) channels. Cloning of complementary DNAs for the high-affinity sulfonylurea receptor indicates that it is a member of the ATP-binding cassette or traffic ATPase superfamily with multiple membrane-spanning domains and two nucleotide binding folds. The results suggest that the sulfonylurea receptor may sense changes in ATP and ADP concentration, affect KATP channel activity, and thereby modulate insulin release.
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Affiliation(s)
- L Aguilar-Bryan
- Department of Cell Biology, Baylor College of Medicine, Houston, TX 77030, USA
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32
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McKillen HC, Davies NW, Stanfield PR, Standen NB. The effect of intracellular anions on ATP-dependent potassium channels of rat skeletal muscle. J Physiol 1994; 479 ( Pt 3):341-51. [PMID: 7837093 PMCID: PMC1155754 DOI: 10.1113/jphysiol.1994.sp020300] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
1. We have used excised inside-out patches to study the effects of anions bathing the cytoplasmic surface of the membrane on ATP-dependent K+ channels of rat flexor digitorum brevis muscle. Channels were closed by ATP applied to the cytoplasmic face of the patch with a concentration for half-closure (Ki) of 14 microM, were highly selective for K+ and had unitary conductances of 62 pS in symmetrical 155 mM K+ and 27 pS in 5 mM [K+]o. 2. In 139 mM Cl- internal solution channel activity declined rapidly after excision of the patch. Inclusion of 40 mM potassium gluconate (substituted for KCl) in the solution both restored channel activity and greatly slowed its subsequent run-down. 3. The action of gluconate was concentration dependent. The effect did not involve a change in ATP binding, since the Ki for ATP was not significantly changed by gluconate, and was specific for the cytoplasmic face of the patch. 4. The anions pyruvate, lactate and acetate were all able to restore channel activity after run-down, though less well than gluconate, while sulphate and methylsulphate were without effect. 5. Analysis of single channel kinetics showed that gluconate did not affect mean open lifetime, but led to a decrease in the number and duration of long closings. 6. Anions are most likely to act by stabilizing the structure of the channel protein. Changes in the intracellular concentration of certain anions may play a role in regulating channel activity.
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Affiliation(s)
- H C McKillen
- Department of Cell Physiology and Pharmacology, University of Leicester
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33
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Takano M, Ashcroft FM. Effects of internal chloride on ATP-sensitive K-channels in mouse pancreatic beta-cells. Pflugers Arch 1994; 428:194-6. [PMID: 7971177 DOI: 10.1007/bf00374859] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The effect of internal Cl- on the K-ATP channel of pancreatic beta-cells was examined. Reducing Cl- from 140mM to 14mM potentiated channel activity (NPO) approximately 4.5 fold in 60% of patches but was without effect in 40% of patches. The Ki for channel inhibition by ATP was not changed. The inhibitory effect of Cl- interacted with the stimulatory action of MgGDP. In 140mM Cl-, 1mM MgGDP increased NPO in Cl(-)-sensitive patches only: when NPO was potentiated by 14mM Cl-, no further increase was produced by MgGDP. These observations suggest that MgGDP and low Cl- solution may increase channel activity via a similar mechanism.
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Affiliation(s)
- M Takano
- University Laboratory of Physiology, Oxford, UK
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34
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Schwanstecher C, Dickel C, Panten U. Interaction of tolbutamide and cytosolic nucleotides in controlling the ATP-sensitive K+ channel in mouse beta-cells. Br J Pharmacol 1994; 111:302-10. [PMID: 8012711 PMCID: PMC1910016 DOI: 10.1111/j.1476-5381.1994.tb14060.x] [Citation(s) in RCA: 28] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
1. In mouse pancreatic beta-cells the role of cytosolic nucleotides in the regulation of the sulphonylurea sensitivity of the adenosine 5'-triphosphate-sensitive K+ channel (KATP-channel) was examined. Patch-clamp experiments with excised inside-out membrane patches were carried out using an experimental protocol favouring phosphorylation of membrane proteins. 2. In the absence of Mg2+, the KATP-channel-inhibiting potency of cytosolic nucleotides decreased in the order ATP = adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) > adenosine 5'-diphosphate (ADP) > adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) = adenylyl-imidodiphosphate (AMP-PNP) > 2'-deoxyadenosine 5'-triphosphate (dATP) > uridine 5'-triphosphate (UTP) > 2'-deoxyadenosine 5'-diphosphate (dADP) > guanosine 5'-triphosphate (GTP) > guanosine 5'-diphosphate (GDP) > uridine 5'-diphosphate (UDP). 3. In the presence of Mg2+, the inhibitory potency of cytosolic nucleotides decreased in the order ATP gamma S > ATP > AMP-PNP > ADP beta S > dATP > UTP. In the presence of Mg2+, the KATP-channels were activated by dADP, GTP, GDP and UDP. 4. Tolbutamide inhibited the KATP-channels not only in the presence but also in the prolonged absence of Mg2+. In nucleotide-free solutions, the potency of tolbutamide was very low. When about half of the KATP-channel activity was inhibited by ATP, AMP-PNP, ADP beta S or ADP (absence of Mg2+), the potency of tolbutamide was increased. 5. Tolbutamide (100 microM) slightly enhanced the channel-inhibiting potency of AMP-PNP and inhibited the channel-activating effect of MgGDP in a non-competitive manner. 6. Channel activation by MgGDP (0.5 mM) competitively antagonized the inhibitory responses to AMP-PNP (1 MicroM- 1 mM). This effect of GDP was neutralized by tolbutamide (100 MicroM).7. The stimulatory effect of 0.5 mM MgGDP was neutralized by 200 MicroM AMP-PNP. Under these conditions the potency of tolbutamide was much higher than in the presence of 0.5 mM MgGDP alone or in the absence of any nucleotides.8. dADP (0.3-1 mM) increased the potency of tolbutamide. Additional application of 200 MicroM AMPPNP caused a further increase in the potency of tolbutamide.9. In conclusion, in the simultaneous presence of inhibitory and stimulatory nucleotides, binding of sulphonylureas to their receptor causes direct inhibition of channel activity, non-competitive inhibition of the action of stimulatory nucleotides and interruption of the competitive interaction between stimulatory and inhibitory nucleotides. The latter effect increases the proportion of KATP- channels staying in the nucleotide-blocked state. In addition, this state potentiates the direct effect of sulphonylureas.
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Affiliation(s)
- C Schwanstecher
- Institute of Pharmacology & Toxicology, University of Göttingen, Germany
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35
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Abstract
During acute myocardial ischemia, passage of potassium ions across the sarcolemma to the extracellular space is a well-established phenomenon. A recent hypothesis is that the ATP-dependent potassium channel plays a role in contributing to the potassium loss. As the potassium loss starts while the overall level of ATP is still relatively high, and as the channel is inhibited by rather low concentrations of ATP, the question arises as to how the channel is opened. Among the proposals are that, in addition to the total concentration of ATP, there is modulation of the regulation by its breakdown products, such as ADP and adenosine. Alternatively, or in addition, breakdown products of anaerobic glycolysis, such as lactate and protons, may also play a role. Extracellular acidosis may help to activate the channel, and internal lactate accumulation may have a similar effect. In certain circumstances there is evidence that ATP produced by glycolysis plays a significant role in the control of potassium channel activity. The concept of subsarcolemmal ATP is another explanation for the activation of the channel at relatively high ATP concentrations. Potassium channel closing drugs, such as glibenclamide, may prolong the action potential duration (shortened by ischemia) and thereby decrease the incidence of early ventricular arrhythmias. This same category of drugs may reduce early potassium loss from the ischemic tissue, thereby lessening the potentially protective effect of the external accumulation of potassium on the ischemic zone, the so-called local cardioplegic effect. Conversely, drugs of the potassium channel activating group are likely to have opposite effects on these arrhythmias and on myocardial protection.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- L H Opie
- Ischaemic Heart Disease Research Unit, Medical Research Council, Cape Town, South Africa
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Abstract
The inside-out configuration of the patch-clamp method was used to study the effects of trypsin on the activity of ATP-sensitive potassium (K-ATP) channels from isolated mouse pancreatic beta-cells. Trypsin (20 micrograms/ml) irreversibly enhanced channel activity around twofold by reducing the interburst intervals without altering the burst kinetics. No effect on the single channel conductance or the inward rectification produced by internal Mg2+ was observed: however, the protease did reduce the inhibitory effect of Mg2+ on channel activity. Trypsin both prevented rundown of K-ATP channel activity and reactivated the channels after complete rundown. These effects of trypsin were absent in the presence of trypsin inhibitor. The protease also reduced the inhibitory effect of ATP on channel activity, increasing the dissociation constant from 7 to 49 microM. Trypsin removed the activating effect of ADP (0.1 mmol/l) on channel activity and reduced the inhibitory effect of tolbutamide (0.5 mmol/l). Carboxypeptidase A did not activate K-ATP channels in excised patches, although it was able to slightly reactivate channels after complete rundown, whereas chymotrypsin increased K-ATP channel activity but it did not produce reactivation. The effects of papain were similar to those of trypsin.
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Affiliation(s)
- P Proks
- University Laboratory of Physiology, Oxford, UK
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Larsson O, Ammälä C, Bokvist K, Fredholm B, Rorsman P. Stimulation of the KATP channel by ADP and diazoxide requires nucleotide hydrolysis in mouse pancreatic beta-cells. J Physiol 1993; 463:349-65. [PMID: 8246187 PMCID: PMC1175347 DOI: 10.1113/jphysiol.1993.sp019598] [Citation(s) in RCA: 74] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
1. The mechanisms by which ADP and the hyperglycaemic compound diazoxide stimulate the activity of the ATP-regulated K+ channel (KATP channel) were studied using inside-out patches isolated from mouse pancreatic beta-cells maintained in tissue culture. 2. The ability of diazoxide and ADP to increase KATP channel activity declined with time following patch excision and no stimulation was observed after 15-40 min. 3. Activation of KATP channels by ADP required the presence of intracellular Mg2+. The stimulatory effect of ADP was mimicked by AMP but only in the presence of ATP. Replacement of ATP with the non-hydrolysable analogue beta, gamma-methylene ATP did not interfere with the ability of ADP to stimulate KATP channel activity. By contrast, enhancement of KATP channel activity was critically dependent on hydrolysable ADP and no stimulation was observed after substitution of alpha,beta-methylene ADP for standard ADP. 4. The ability of diazoxide to enhance KATP channel activity was dependent on the presence of both internal Mg2+ and ATP. Diazoxide stimulation of KATP channel activity was not observed after substitution of beta,gamma-methylene ATP for ATP. However, in the presence of ADP, at a concentration which in itself had no stimulatory action (10 microM), diazoxide was stimulatory also in the presence of the stable ATP analogue. 5. The stimulatory action of diazoxide on KATP channel activity in the presence of ATP was markedly enhanced by intracellular ADP. This potentiating effect of ADP was not reproduced by the stable analogue alpha,beta-methylene ADP and was conditional on the presence of intracellular Mg2+. A similar enhancement of channel activity was also observed with AMP (0.1 mM). In the absence of ATP, diazoxide was still capable of stimulating channel activity provided ADP was present. This effect was not reproduced by AMP. 6. In both nucleotide-free solution and in the presence of 0.1 mM ATP, the distribution of the KATP channel open times were described by a single exponential with a time constant of approximately 20 ms. Addition of ADP or diazoxide resulted in the appearance of a second component with a time constant of > 100 ms which comprised 40-70% of the total number of events. Under the latter experimental conditions, the open probability of the channel increased more than fivefold relative to that observed in the presence of ATP alone.(ABSTRACT TRUNCATED AT 400 WORDS)
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Affiliation(s)
- O Larsson
- Department of Medical Physics, Gothenburg University, Sweden
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Ashcroft SJ, Niki I, Kenna S, Weng L, Skeer J, Coles B, Ashcroft FM. The beta-cell sulfonylurea receptor. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 1993; 334:47-61. [PMID: 8249695 DOI: 10.1007/978-1-4615-2910-1_4] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- S J Ashcroft
- Nuffield Department of Clinical Biochemistry, John Radcliffe Hospital, Headington, Oxford, U.K
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39
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Hopkins WF, Fatherazi S, Peter-Riesch B, Corkey BE, Cook DL. Two sites for adenine-nucleotide regulation of ATP-sensitive potassium channels in mouse pancreatic beta-cells and HIT cells. J Membr Biol 1992; 129:287-95. [PMID: 1433280 DOI: 10.1007/bf00232910] [Citation(s) in RCA: 74] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
ATP-inhibited potassium channels (K(ATP)) were studied in excised, inside-out patches from cultured adult mouse pancreatic beta-cells and HIT cells. In the absence of ATP, ADP opened K(ATP) channels at concentrations as low as 10 microM and as high as 500 microM, with maximal activation between 10 and 100 microM ADP in mouse beta-cell membrane patches. At concentrations greater than 500 microM, ADP inhibited K(ATP) channels while 10 mM virtually abolished channel activity. HIT cell channels had a similar biphasic response to ADP except that more than 1 mM ADP was required for inhibition. The channel opening effect of ADP required magnesium while channel inhibition did not. Using creatine/creatine phosphate solutions with creatine phosphokinase to fix ATP and ADP concentrations, we found substantially different K(ATP)-channel activity with solutions having the same ATP/ADP ratio but different absolute total nucleotide levels. To account for ATP-ADP competition, we propose a new model of channel-nucleotide interactions with two kinds of ADP binding sites regulating the channel. One site specifically binds MgADP and increases channel opening. The other, the previously described ATP site, binds either ATP or ADP and decreases channel opening. This model very closely fits the ADP concentration-response curve and, when incorporated into a model of beta-cell membrane potential, increasing ADP in the 10 and 100 microM range is predicted to compete very effectively with millimolar levels of ATP to hyperpolarize beta-cells. The results suggest that (i) K(ATP)-channel activity is not well predicted by the "ATP/ADP ratio," and (ii) ADP is a plausible regulator of K(ATP) channels even if its free cytoplasmic concentration is in the 10-100 microM range as suggested by biochemical studies.
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Affiliation(s)
- W F Hopkins
- Division of Metabolism, Seattle Veterans Affairs Medical Center, Seattle 98108
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Fournier L, Bégin-Heick N, Whitfield JF, Schwartz JL. Comparison of the properties of the ATP-sensitive K+ channels of pancreatic beta-cells of lean and obese (ob/ob) C57BL/6J mice. J Membr Biol 1992; 129:267-76. [PMID: 1433278 DOI: 10.1007/bf00232908] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Cultures of pancreatic islet cells from obese and lean mice of the C57BL/6J strain were established and their secretory response to glucose stimulation was measured. Insulin secretion (as % of total cellular insulin content) from the cells of the obese mouse cultures was significantly higher than from lean mouse cells. The properties of the glucose- and ATP-sensitive potassium channels present in these cultured beta-cells were compared using the cell-attached and the inside-out configurations of the patch-clamp technique. The channels of both types of mouse were indistinguishable in terms of conductance, ionic selectivity, kinetic behavior, voltage dependence or sensitivity to glucose, ATP and ADP. It is concluded that the depolarized state and the hypersecretory response of obese mouse beta-cells are not related to an altered behavior of their ATP-sensitive potassium channels.
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Affiliation(s)
- L Fournier
- Department of Biochemistry, University of Ottawa, Ontario, Canada
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