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Deepa Maheshvare M, Raha S, König M, Pal D. A pathway model of glucose-stimulated insulin secretion in the pancreatic β-cell. Front Endocrinol (Lausanne) 2023; 14:1185656. [PMID: 37600713 PMCID: PMC10433753 DOI: 10.3389/fendo.2023.1185656] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/13/2023] [Accepted: 06/08/2023] [Indexed: 08/22/2023] Open
Abstract
The pancreas plays a critical role in maintaining glucose homeostasis through the secretion of hormones from the islets of Langerhans. Glucose-stimulated insulin secretion (GSIS) by the pancreatic β-cell is the main mechanism for reducing elevated plasma glucose. Here we present a systematic modeling workflow for the development of kinetic pathway models using the Systems Biology Markup Language (SBML). Steps include retrieval of information from databases, curation of experimental and clinical data for model calibration and validation, integration of heterogeneous data including absolute and relative measurements, unit normalization, data normalization, and model annotation. An important factor was the reproducibility and exchangeability of the model, which allowed the use of various existing tools. The workflow was applied to construct a novel data-driven kinetic model of GSIS in the pancreatic β-cell based on experimental and clinical data from 39 studies spanning 50 years of pancreatic, islet, and β-cell research in humans, rats, mice, and cell lines. The model consists of detailed glycolysis and phenomenological equations for insulin secretion coupled to cellular energy state, ATP dynamics and (ATP/ADP ratio). Key findings of our work are that in GSIS there is a glucose-dependent increase in almost all intermediates of glycolysis. This increase in glycolytic metabolites is accompanied by an increase in energy metabolites, especially ATP and NADH. One of the few decreasing metabolites is ADP, which, in combination with the increase in ATP, results in a large increase in ATP/ADP ratios in the β-cell with increasing glucose. Insulin secretion is dependent on ATP/ADP, resulting in glucose-stimulated insulin secretion. The observed glucose-dependent increase in glycolytic intermediates and the resulting change in ATP/ADP ratios and insulin secretion is a robust phenomenon observed across data sets, experimental systems and species. Model predictions of the glucose-dependent response of glycolytic intermediates and biphasic insulin secretion are in good agreement with experimental measurements. Our model predicts that factors affecting ATP consumption, ATP formation, hexokinase, phosphofructokinase, and ATP/ADP-dependent insulin secretion have a major effect on GSIS. In conclusion, we have developed and applied a systematic modeling workflow for pathway models that allowed us to gain insight into key mechanisms in GSIS in the pancreatic β-cell.
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Affiliation(s)
- M. Deepa Maheshvare
- Department of Computational and Data Sciences, Indian Institute of Science, Bangalore, India
| | - Soumyendu Raha
- Department of Computational and Data Sciences, Indian Institute of Science, Bangalore, India
| | - Matthias König
- Institute for Biology, Institute for Theoretical Biology, Humboldt-University Berlin, Berlin, Germany
| | - Debnath Pal
- Department of Computational and Data Sciences, Indian Institute of Science, Bangalore, India
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Deepa Maheshvare M, Raha S, König M, Pal D. A Consensus Model of Glucose-Stimulated Insulin Secretion in the Pancreatic β -Cell. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2023:2023.03.10.532028. [PMID: 36945414 PMCID: PMC10028967 DOI: 10.1101/2023.03.10.532028] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 03/13/2023]
Abstract
The pancreas plays a critical role in maintaining glucose homeostasis through the secretion of hormones from the islets of Langerhans. Glucose-stimulated insulin secretion (GSIS) by the pancreatic β -cell is the main mechanism for reducing elevated plasma glucose. Here we present a systematic modeling workflow for the development of kinetic pathway models using the Systems Biology Markup Language (SBML). Steps include retrieval of information from databases, curation of experimental and clinical data for model calibration and validation, integration of heterogeneous data including absolute and relative measurements, unit normalization, data normalization, and model annotation. An important factor was the reproducibility and exchangeability of the model, which allowed the use of various existing tools. The workflow was applied to construct the first consensus model of GSIS in the pancreatic β -cell based on experimental and clinical data from 39 studies spanning 50 years of pancreatic, islet, and β -cell research in humans, rats, mice, and cell lines. The model consists of detailed glycolysis and equations for insulin secretion coupled to cellular energy state (ATP/ADP ratio). Key findings of our work are that in GSIS there is a glucose-dependent increase in almost all intermediates of glycolysis. This increase in glycolytic metabolites is accompanied by an increase in energy metabolites, especially ATP and NADH. One of the few decreasing metabolites is ADP, which, in combination with the increase in ATP, results in a large increase in ATP/ADP ratios in the β -cell with increasing glucose. Insulin secretion is dependent on ATP/ADP, resulting in glucose-stimulated insulin secretion. The observed glucose-dependent increase in glycolytic intermediates and the resulting change in ATP/ADP ratios and insulin secretion is a robust phenomenon observed across data sets, experimental systems and species. Model predictions of the glucose-dependent response of glycolytic intermediates and insulin secretion are in good agreement with experimental measurements. Our model predicts that factors affecting ATP consumption, ATP formation, hexokinase, phosphofructokinase, and ATP/ADP-dependent insulin secretion have a major effect on GSIS. In conclusion, we have developed and applied a systematic modeling workflow for pathway models that allowed us to gain insight into key mechanisms in GSIS in the pancreatic β -cell.
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Matschinsky F, Wilson D. Lifting the Veil on the "Phosphate Flush," a Cryptic Phenomenon of Experimental Pancreatic Islet Physiology. Diabetes 2021; 70:27-28. [PMID: 33355310 DOI: 10.2337/dbi20-0038] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Affiliation(s)
- Franz Matschinsky
- Department of Biochemistry and Biophysics and the Institute for Diabetes, Obesity and Metabolism, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA
| | - David Wilson
- Department of Biochemistry and Biophysics and the Institute for Diabetes, Obesity and Metabolism, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA
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Matschinsky FM, Wilson DF. The Central Role of Glucokinase in Glucose Homeostasis: A Perspective 50 Years After Demonstrating the Presence of the Enzyme in Islets of Langerhans. Front Physiol 2019; 10:148. [PMID: 30949058 PMCID: PMC6435959 DOI: 10.3389/fphys.2019.00148] [Citation(s) in RCA: 140] [Impact Index Per Article: 28.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2018] [Accepted: 02/07/2019] [Indexed: 01/05/2023] Open
Abstract
It is hypothesized that glucokinase (GCK) is the glucose sensor not only for regulation of insulin release by pancreatic β-cells, but also for the rest of the cells that contribute to glucose homeostasis in mammals. This includes other cells in endocrine pancreas (α- and δ-cells), adrenal gland, glucose sensitive neurons, entero-endocrine cells, and cells in the anterior pituitary. Glucose transport is by facilitated diffusion and is not rate limiting. Once inside, glucose is phosphorylated to glucose-6-phosphate by GCK in a reaction that is dependent on glucose throughout the physiological range of concentrations, is irreversible, and not product inhibited. High glycerol phosphate shuttle, pyruvate dehydrogenase, and pyruvate carboxylase activities, combined with low pentose-P shunt, lactate dehydrogenase, plasma membrane monocarboxylate transport, and glycogen synthase activities constrain glucose-6-phosphate to being metabolized through glycolysis. Under these conditions, glycolysis produces mostly pyruvate and little lactate. Pyruvate either enters the citric acid cycle through pyruvate dehydrogenase or is carboxylated by pyruvate carboxylase. Reducing equivalents from glycolysis enter oxidative phosphorylation through both the glycerol phosphate shuttle and citric acid cycle. Raising glucose concentration increases intramitochondrial [NADH]/[NAD+] and thereby the energy state ([ATP]/[ADP][Pi]), decreasing [Mg2+ADP] and [AMP]. [Mg2+ADP] acts through control of KATP channel conductance, whereas [AMP] acts through regulation of AMP-dependent protein kinase. Specific roles of different cell types are determined by the diverse molecular mechanisms used to couple energy state to cell specific responses. Having a common glucose sensor couples complementary regulatory mechanisms into a tightly regulated and stable glucose homeostatic network.
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Affiliation(s)
- Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
| | - David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
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5
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Wilson DF, Cember ATJ, Matschinsky FM. Glutamate dehydrogenase: role in regulating metabolism and insulin release in pancreatic β-cells. J Appl Physiol (1985) 2018; 125:419-428. [PMID: 29648519 DOI: 10.1152/japplphysiol.01077.2017] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
Regulation of insulin release and glucose homeostasis by pancreatic β-cells is dependent on the metabolism of glucose by glucokinase (GK) and the influence of that activity on oxidative phosphorylation. Genetic alterations that result in hyperactivity of mitochondrial glutamate dehydrogenase (GDH-1) can cause hypoglycemia-hyperammonemia following high protein meals, but the role of GDH-1 remains poorly understood. GDH-1 activity is strongly inhibited by GTP, to near zero in the absence of ADP, and cooperatively activated ( n = 2.3) by ADP. The dissociation constant for ADP is near 200 µM in vivo, but leucine and its nonmetabolized analog 2-amino-2-norbornane-carboxylic acid (BCH) can activate GDH-1 by increasing the affinity for ADP. Under physiological conditions, as [ADP] increases GDH-1 activity remains very low until ~35 µM (threshold) and then increases rapidly. A model for GDH-1 and its regulation has been combined with a previously published model for glucose sensing that coupled GK activity and oxidative phosphorylation. The combined model (GK-GDH-core) shows that GK activity determines the energy state ([ATP]/[ADP][Pi]) in β-cells for glucose concentrations > 5 mM ([ADP] < 35 µM). As glucose falls < 5 mM the [ADP]-dependent increase in GDH-1 activity prevents [ADP] from rising above ~70 µM. Thus, GDH-1 dynamically buffers β-cell energy metabolism during hypoglycemia, maintaining the energy state and the basal rate of insulin release. GDH-1 hyperactivity suppresses the normal increase in [ADP] in hypoglycemia. This leads to hypoglycemia following a high protein meal by increasing the basal rate of insulin release (β-cells) and decreasing glucagon release (α-cells). NEW & NOTEWORTHY A model of β-cell metabolism and regulation of insulin release is presented. The model integrates regulation of oxidative phosphorylation, glucokinase (GK), and glutamate dehydrogenase (GDH-1). GDH-1 is near equilibrium under physiological conditions, but the activity is inhibited by GTP. In hypoglycemia, however, GK activity is low and [ADP], a potent activator of GDH-1, increases. Reducing equivalents from GDH dynamically buffers the intramitochondrial [NADH]/[NAD+], and thereby the energy state, preventing hypoglycemia-induced substrate deprivation.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania , Philadelphia, Pennsylvania
| | - Abigail T J Cember
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania , Philadelphia, Pennsylvania
| | - Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania , Philadelphia, Pennsylvania
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Wilson DF, Cember ATJ, Matschinsky FM. The thermodynamic basis of glucose-stimulated insulin release: a model of the core mechanism. Physiol Rep 2018; 5:5/12/e13327. [PMID: 28655753 PMCID: PMC5492210 DOI: 10.14814/phy2.13327] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2017] [Revised: 05/23/2017] [Accepted: 05/24/2017] [Indexed: 11/24/2022] Open
Abstract
A model for glucose sensing by pancreatic β-cells is developed and compared with the available experimental data. The model brings together mathematical representations for the activities of the glucose sensor, glucokinase, and oxidative phosphorylation. Glucokinase produces glucose 6-phosphate (G-6-P) in an irreversible reaction that determines glycolytic flux. The primary products of glycolysis are NADH and pyruvate. The NADH is reoxidized and the reducing equivalents transferred to oxidative phosphorylation by the glycerol phosphate shuttle, and some of the pyruvate is oxidized by pyruvate dehydrogenase and enters the citric acid cycle. These reactions are irreversible and result in a glucose concentration-dependent reduction of the intramitochondrial NAD pool. This increases the electrochemical energy coupled to ATP synthesis and thereby the cellular energy state ([ATP]/[ADP][Pi]). ATP and Pi are 10-100 times greater than ADP, so the increase in energy state is primarily through decrease in ADP The decrease in ADP is considered responsible for altering ion channel conductance and releasing insulin. Applied to the reported glucose concentration-dependent release of insulin by perifused islet preparations (Doliba et al. 2012), the model predicts that the dependence of insulin release on ADP is strongly cooperative with a threshold of about 30 μmol/L and a negative Hill coefficient near -5.5. The predicted cellular energy state, ADP, creatine phosphate/creatine ratio, and cytochrome c reduction, including their dependence on glucose concentration, are consistent with experimental data. The ability of the model to predict behavior consistent with experiment is an invaluable resource for understanding glucose sensing and planning experiments.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
| | - Abigail T J Cember
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
| | - Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
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Carrig S, Bijjiga E, Wopat MJ, Martino AT. Insulin Therapy Improves Adeno-Associated Virus Transduction of Liver and Skeletal Muscle in Mice and Cultured Cells. Hum Gene Ther 2016; 27:892-905. [PMID: 27358030 DOI: 10.1089/hum.2016.073] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
Adeno-associated virus (AAV) gene transfer is a promising treatment for genetic abnormalities. Optimal AAV vectors are showing success in clinical trials. Gene transfer to skeletal muscle and liver is being explored as a potential therapy for some conditions, that is, α1-antitrypsin (AAT) disorder and hemophilia B. Exploring approaches that enhance transduction of liver and skeletal muscle, using these vectors, is beneficial for gene therapy. Regulating hormones as an approach to improve AAV transduction is largely unexplored. In this study we tested whether insulin therapy improves liver and skeletal muscle gene transfer. In vitro studies demonstrated that the temporary coadministration (2, 8, and 24 hr) of insulin significantly improves AAV2-CMV-LacZ transduction of cultured liver cells and differentiated myofibers, but not of lung cells. In addition, there was a dose response related to this improved transduction. Interestingly, when insulin was not coadministered with the virus but given 24 hr afterward, there was no increase in the transgene product. Insulin receptor gene (INSR) expression levels were increased 5- to 13-fold in cultured liver cells and differentiated myofibers when compared with lung cells. Similar INSR gene expression profiles occurred in mouse tissues. Insulin therapy was performed in mice, using a subcutaneously implanted insulin pellet or a high-carbohydrate diet. Insulin treatment began just before intramuscular delivery of AAV1-CMV-schFIX or liver-directed delivery of AAV8-CMV-schFIX and continued for 28 days. Both insulin augmentation therapies improved skeletal muscle- and liver-directed gene transduction in mice as seen by a 3.0- to 4.5-fold increase in human factor IX (hFIX) levels. The improvement was observed even after the insulin therapy ended. Monitoring insulin showed that insulin levels increased during the brief period of rAAV delivery and during the entire insulin augmentation period (28 days). This study demonstrates that AAV transduction of liver or skeletal muscle can be improved by insulin therapy.
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Affiliation(s)
- Sean Carrig
- Department of Pharmaceutical Sciences, College of Pharmacy and Health Sciences, St. John's University , Queens, New York
| | - Enoch Bijjiga
- Department of Pharmaceutical Sciences, College of Pharmacy and Health Sciences, St. John's University , Queens, New York
| | - Mitchell J Wopat
- Department of Pharmaceutical Sciences, College of Pharmacy and Health Sciences, St. John's University , Queens, New York
| | - Ashley T Martino
- Department of Pharmaceutical Sciences, College of Pharmacy and Health Sciences, St. John's University , Queens, New York
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8
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Kuo T, Kim-Muller JY, McGraw TE, Accili D. Altered Plasma Profile of Antioxidant Proteins as an Early Correlate of Pancreatic β Cell Dysfunction. J Biol Chem 2016; 291:9648-56. [PMID: 26917725 DOI: 10.1074/jbc.m115.702183] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2015] [Indexed: 12/22/2022] Open
Abstract
Insulin resistance and β cell dysfunction contribute to the pathogenesis of type 2 diabetes. Unlike insulin resistance, β cell dysfunction remains difficult to predict and monitor, because of the inaccessibility of the endocrine pancreas, the integrated relationship with insulin sensitivity, and the paracrine effects of incretins. The goal of our study was to survey the plasma response to a metabolic challenge in order to identify factors predictive of β cell dysfunction. To this end, we combined (i) the power of unbiased iTRAQ (isobaric tag for relative and absolute quantification) mass spectrometry with (ii) direct sampling of the portal vein following an intravenous glucose/arginine challenge (IVGATT) in (iii) mice with a genetic β cell defect. By so doing, we excluded the effects of peripheral insulin sensitivity as well as those of incretins on β cells, and focused on the first phase of insulin secretion to capture the early pathophysiology of β cell dysfunction. We compared plasma protein profiles with ex vivo islet secretome and transcriptome analyses. We detected changes to 418 plasma proteins in vivo, and detected changes to 262 proteins ex vivo The impairment of insulin secretion was associated with greater overall changes in the plasma response to IVGATT, possibly reflecting metabolic instability. Reduced levels of proteins regulating redox state and neuronal stress markers, as well as increased levels of coagulation factors, antedated the loss of insulin secretion in diabetic mice. These results suggest that a reduced complement of antioxidants in response to a mixed secretagogue challenge is an early correlate of future β cell failure.
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Affiliation(s)
- Taiyi Kuo
- From the Department of Medicine and Berrie Diabetes Center, Columbia University College of Physicians and Surgeons, New York, New York 10032 and
| | - Ja Young Kim-Muller
- From the Department of Medicine and Berrie Diabetes Center, Columbia University College of Physicians and Surgeons, New York, New York 10032 and
| | - Timothy E McGraw
- the Department of Biochemistry, Weill Cornell Medical College, New York, New York 10065
| | - Domenico Accili
- From the Department of Medicine and Berrie Diabetes Center, Columbia University College of Physicians and Surgeons, New York, New York 10032 and
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9
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Nguyen TT, Quan X, Hwang KH, Xu S, Das R, Choi SK, Wiederkehr A, Wollheim CB, Cha SK, Park KS. Mitochondrial oxidative stress mediates high-phosphate-induced secretory defects and apoptosis in insulin-secreting cells. Am J Physiol Endocrinol Metab 2015; 308:E933-41. [PMID: 25852001 DOI: 10.1152/ajpendo.00009.2015] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/06/2015] [Accepted: 03/31/2015] [Indexed: 02/06/2023]
Abstract
Inorganic phosphate (Pi) plays an important role in cell signaling and energy metabolism. In insulin-releasing cells, Pi transport into mitochondria is essential for the generation of ATP, a signaling factor in metabolism-secretion coupling. Elevated Pi concentrations, however, can have toxic effects in various cell types. The underlying molecular mechanisms are poorly understood. Here, we have investigated the effect of Pi on secretory function and apoptosis in INS-1E clonal β-cells and rat pancreatic islets. Elevated extracellular Pi (1~5 mM) increased the mitochondrial membrane potential (ΔΨm), superoxide generation, caspase activation, and cell death. Depolarization of the ΔΨm abolished Pi-induced superoxide generation. Butylmalonate, a nonselective blocker of mitochondrial phosphate transporters, prevented ΔΨm hyperpolarization, superoxide generation, and cytotoxicity caused by Pi. High Pi also promoted the opening of the mitochondrial permeability transition (PT) pore, leading to apoptosis, which was also prevented by butylmalonate. The mitochondrial antioxidants mitoTEMPO or MnTBAP prevented Pi-triggered PT pore opening and cytotoxicity. Elevated extracellular Pi diminished ATP synthesis, cytosolic Ca(2+) oscillations, and insulin content and secretion in INS-1E cells as well as in dispersed islet cells. These parameters were restored following preincubation with mitochondrial antioxidants. This treatment also prevented high-Pi-induced phosphorylation of ER stress proteins. We propose that elevated extracellular Pi causes mitochondrial oxidative stress linked to mitochondrial hyperpolarization. Such stress results in reduced insulin content and defective insulin secretion and cytotoxicity. Our data explain the decreased insulin content and secretion observed under hyperphosphatemic states.
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Affiliation(s)
- Tuyet Thi Nguyen
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
| | - Xianglan Quan
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
| | - Kyu-Hee Hwang
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
| | - Shanhua Xu
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
| | - Ranjan Das
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
| | - Seong-Kyung Choi
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
| | | | - Claes B Wollheim
- Department of Cell Physiology and Metabolism, University of Geneva, Geneva, Switzerland
| | - Seung-Kuy Cha
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea
| | - Kyu-Sang Park
- Department of Physiology, Yonsei University Wonju College of Medicine, Wonju, Republic of Korea;
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Hoogwerf BJ, Kendall DM, Seaquist ER. Frederick C. Goetz, MD: a profile of the intersection among science, the fine arts, and education. Diabetes Care 2014; 37:2419-23. [PMID: 25147251 DOI: 10.2337/dc14-0283] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Affiliation(s)
| | - David M Kendall
- Eli Lilly and Co., Indianapolis, IN University of Minnesota, Minneapolis, MN
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Abstract
Glucokinase, a unique isoform of the hexokinase enzymes, which are known to phosphorylate D-glucose and other hexoses, was identified during the past three to four decades as a new, promising drug target for type 2 diabetes. Glucokinase serves as a glucose sensor of the insulin-producing pancreatic islet beta-cells, controls the conversion of glucose to glycogen in the liver and regulates hepatic glucose production. Guided by this fundamental knowledge, several glucokinase activators are now being developed, and have so far been shown to lower blood glucose in several animal models of type 2 diabetes and in initial trials in humans with the disease. Here, the scientific basis and current status of this new approach to diabetes therapy are discussed.
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Slaby O, Lebiedz D. Oscillatory NAD(P)H waves and calcium oscillations in neutrophils? A modeling study of feasibility. Biophys J 2009; 96:417-28. [PMID: 19167293 DOI: 10.1016/j.bpj.2008.09.044] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2008] [Accepted: 09/29/2008] [Indexed: 11/17/2022] Open
Abstract
The group of Howard Petty has claimed exotic metabolic wave phenomena together with mutually phase-coupled NAD(P)H- and calcium-oscillations in human neutrophils. At least parts of these phenomena are highly doubtful due to extensive failure of reproducibility by several other groups and hints that unreliable data from the Petty lab are involved in publications concerning circular calcium waves. The aim of our theoretical spatiotemporal modeling approach is to propose a possible and plausible biochemical mechanism which would, in principle, be able to explain metabolic oscillations and wave phenomena in neutrophils. Our modeling suggests the possibility of a calcium-controlled glucose influx as a driving force of metabolic oscillations and a potential role of polarized cell geometry and differential enzyme distribution for various NAD(P)H wave phenomena. The modeling results are supposed to stimulate further controversial discussions of such phenomena and potential mechanisms and experimental efforts to finally clarify the existence and biochemical basis of any kind of temporal and spatiotemporal patterns of calcium signals and metabolic dynamics in human neutrophils. Independent of Petty's observations, they present a general feasibility study of such phenomena in cells.
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Li C, Nissim I, Chen P, Buettger C, Najafi H, Daikhin Y, Nissim I, Collins HW, Yudkoff M, Stanley CA, Matschinsky FM. Elimination of KATP channels in mouse islets results in elevated [U-13C]glucose metabolism, glutaminolysis, and pyruvate cycling but a decreased gamma-aminobutyric acid shunt. J Biol Chem 2008; 283:17238-49. [PMID: 18445600 DOI: 10.1074/jbc.m709235200] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Pancreatic beta cells are hyper-responsive to amino acids but have decreased glucose sensitivity after deletion of the sulfonylurea receptor 1 (SUR1) both in man and mouse. It was hypothesized that these defects are the consequence of impaired integration of amino acid, glucose, and energy metabolism in beta cells. We used gas chromatography-mass spectrometry methodology to study intermediary metabolism of SUR1 knock-out (SUR1(-/-)) and control mouse islets with d-[U-(13)C]glucose as substrate and related the results to insulin secretion. The levels and isotope labeling of alanine, aspartate, glutamate, glutamine, and gamma-aminobutyric acid (GABA) served as indicators of intermediary metabolism. We found that the GABA shunt of SUR1(-/-) islets is blocked by about 75% and showed that this defect is due to decreased glutamate decarboxylase synthesis, probably caused by elevated free intracellular calcium. Glutaminolysis stimulated by the leucine analogue d,l-beta-2-amino-2-norbornane-carboxylic acid was, however, enhanced in SUR1(-/-) and glyburide-treated SUR1(+/+) islets. Glucose oxidation and pyruvate cycling was increased in SUR1(-/-) islets at low glucose but was the same as in controls at high glucose. Malic enzyme isoforms 1, 2, and 3, involved in pyruvate cycling, were all expressed in islets. High glucose lowered aspartate and stimulated glutamine synthesis similarly in controls and SUR1(-/-) islets. The data suggest that the interruption of the GABA shunt and the lack of glucose regulation of pyruvate cycling may cause the glucose insensitivity of the SUR1(-/-) islets but that enhanced basal pyruvate cycling, lowered GABA shunt flux, and enhanced glutaminolytic capacity may sensitize the beta cells to amino acid stimulation.
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Affiliation(s)
- Changhong Li
- Division of Endocrinology, The Children's Hospital of Philadelphia, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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14
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Panten U, Rustenbeck I. Fuel-induced amplification of insulin secretion in mouse pancreatic islets exposed to a high sulfonylurea concentration: role of the NADPH/NADP+ ratio. Diabetologia 2008; 51:101-9. [PMID: 17960358 DOI: 10.1007/s00125-007-0849-z] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/18/2007] [Accepted: 09/07/2007] [Indexed: 10/22/2022]
Abstract
AIMS/HYPOTHESIS The aim of this study was to examine whether the cytosolic NADPH/NADP+ ratio of beta cells serves as an amplifying signal in fuel-induced insulin secretion and whether such a function is mediated by cytosolic alpha-ketoglutarate. METHODS Pancreatic islets and islet cells were isolated from albino mice by collagenase digestion. Insulin secretion of incubated or perifused islets was measured by ELISA. The NADPH and NADP+ content of incubated islets was determined by enzymatic cycling. The cytosolic Ca2+ concentration ([Ca2+]c) in islets was measured by microfluorimetry and the activity of ATP-sensitive K+ channels in islet cells by patch-clamping. RESULTS Both 30 mmol/l glucose and 10 mmol/l alpha-ketoisocaproate stimulated insulin secretion and elevated the NADPH/NADP+ ratio of islets preincubated in the absence of fuel. The increase in the NADPH/NADP+ ratio was abolished in the presence of 2.7 micromol/l glipizide (closing all ATP-sensitive K+ channels). However, alpha-ketoisocaproate, but not glucose, still stimulated insulin secretion. That glipizide did not inhibit alpha-ketoisocaproate-induced insulin secretion was not the result of elevated [Ca2+]c, as glucose caused a more marked [Ca2+]c increase. Insulin release triggered by glipizide alone was moderately amplified by dimethyl alpha-ketoglutarate (which is cleaved to produce cytosolic alpha-ketoglutarate), but there was no indication of a signal function of cytosolic alpha-ketoglutarate. CONCLUSIONS/INTERPRETATION The results strongly suggest that the NADPH/NADP+ ratio in the beta cell cytosol does not serve as an amplifying signal in fuel-induced insulin release. The study supports the view that amplification results from the intramitochondrial production of citrate by citrate synthase and from the associated export of citrate into the cytosol.
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Affiliation(s)
- U Panten
- Institute of Pharmacology and Toxicology, Technical University of Braunschweig, Mendelssohnstrasse 1, 38106, Brunswick, Germany.
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15
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Schulze DU, Düfer M, Wieringa B, Krippeit-Drews P, Drews G. An adenylate kinase is involved in KATP channel regulation of mouse pancreatic beta cells. Diabetologia 2007; 50:2126-34. [PMID: 17704905 DOI: 10.1007/s00125-007-0742-9] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/24/2007] [Accepted: 05/25/2007] [Indexed: 10/22/2022]
Abstract
AIMS/HYPOTHESIS In a previous study, we demonstrated that a creatine kinase (CK) modulates K(ATP) channel activity in pancreatic beta cells. To explore phosphotransfer signalling pathways in more detail, we examined whether K(ATP) channel regulation in beta cells is determined by a metabolic interaction between adenylate kinase (AK) and CK. METHODS Single channel activity was measured with the patch-clamp technique in the inside-out (i/o) and open-cell attached (oca) configuration. RESULTS The ATP sensitivity of K(ATP) channels was higher in i/o patches than in permeabilised beta cells (oca). One reason for this observation could be that the local ATP:ADP ratio in the proximity of the channels is determined by factors not active in i/o patches. AMP (0.1 mmol/l) clearly increased open channel probability in the presence of ATP (0.125 mmol/l) in permeabilised cells but not in excised patches. This suggests that AK-catalysed ADP production in the vicinity of the channels is involved in K(ATP) channel regulation. The observation that the stimulatory effect of AMP on K(ATP) channels was prevented by the AK inhibitor P (1),P (5)-di(adenosine-5')pentaphosphate (Ap(5)A; 20 micromol/l) and abolished in the presence of the non-metabolisable ATP analogue adenosine 5'-(beta,gamma-imido)triphosphate tetralithium salt (AMP-PNP; 0.12 mmol/l) strengthens this idea. In beta cells from AK1 knockout mice, the effect of AMP was less pronounced, though not completely suppressed. The increase in K(ATP) channel activity induced by AMP in the presence of ATP was outweighed by phosphocreatine (1 mmol/l). We suggest that this is due to an elevation of the ATP concentration by CK. CONCLUSIONS/INTERPRETATION We propose that phosphotransfer events mediated by AK and CK play an important role in determining the effective concentrations of ATP and ADP in the microenvironment of pancreatic beta cell K(ATP) channels. Thus, these enzymes determine the open probability of K(ATP) channels and eventually the actual rate of insulin secretion.
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Affiliation(s)
- D U Schulze
- Institute of Pharmacy, Department of Pharmacology, University of Tübingen, Auf der Morgenstelle 8, 72076, Tübingen, Germany
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16
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Heart E, Yaney G, Corkey R, Schultz V, Luc E, Liu L, Deeney J, Shirihai O, Tornheim K, Smith P, Corkey B. Ca2+, NAD(P)H and membrane potential changes in pancreatic beta-cells by methyl succinate: comparison with glucose. Biochem J 2007; 403:197-205. [PMID: 17181533 PMCID: PMC1828901 DOI: 10.1042/bj20061209] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2006] [Revised: 12/14/2006] [Accepted: 12/20/2006] [Indexed: 11/17/2022]
Abstract
The present study was undertaken to determine the main metabolic secretory signals generated by the mitochondrial substrate MeS (methyl succinate) compared with glucose in mouse and rat islets and to understand the differences. Glycolysis and mitochondrial metabolism both have key roles in the stimulation of insulin secretion by glucose. Both fuels elicited comparable oscillatory patterns of Ca2+ and changes in plasma and mitochondrial membrane potential in rat islet cells and clonal pancreatic beta-cells (INS-1). Saturation of the Ca2+ signal occurred between 5 and 6 mM MeS, while secretion reached its maximum at 15 mM, suggesting operation of a K(ATP)-channel-independent pathway. Additional responses to MeS and glucose included elevated NAD(P)H autofluorescence in INS-1 cells and islets and increases in assayed NADH and NADPH and the ATP/ADP ratio. Increased NADPH and ATP/ADP ratios occurred more rapidly with MeS, although similar levels were reached after 5 min of exposure to each fuel, whereas NADH increased more with MeS than with glucose. Reversal of MeS-induced cell depolarization by Methylene Blue completely inhibited MeS-stimulated secretion, whereas basal secretion and KCl-induced changes in these parameters were not affected. MeS had no effect on secretion or signals in the mouse islets, in contrast with glucose, possibly due to a lack of malic enzyme. The data are consistent with the common intermediates being pyruvate, cytosolic NADPH or both, and suggest that cytosolic NADPH production could account for the more rapid onset of MeS-induced secretion compared with glucose stimulation.
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Affiliation(s)
- Emma Heart
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
- †BioCurrents Research Center, Marine Biological Laboratory, Woods Hole, MA 02543, U.S.A
| | - Gordon C. Yaney
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
| | - Richard F. Corkey
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
| | - Vera Schultz
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
- ‡Department of Biochemistry, Boston University School of Medicine, Boston, MA 02118, U.S.A
| | - Esthere Luc
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
| | - Lihan Liu
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
| | - Jude T. Deeney
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
| | - Orian Shirihai
- §Department of Pharmacology and Experimental Therapeutics, Tufts University School of Medicine, Boston, MA 02118, U.S.A
| | - Keith Tornheim
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
- ‡Department of Biochemistry, Boston University School of Medicine, Boston, MA 02118, U.S.A
| | - Peter J. S. Smith
- †BioCurrents Research Center, Marine Biological Laboratory, Woods Hole, MA 02543, U.S.A
| | - Barbara E. Corkey
- *Obesity Research Center, Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, U.S.A
- ‡Department of Biochemistry, Boston University School of Medicine, Boston, MA 02118, U.S.A
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17
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Koshkin V, Bikopoulos G, Chan CB, Wheeler MB. The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation. J Biol Chem 2004; 279:41368-76. [PMID: 15231823 DOI: 10.1074/jbc.m406914200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic beta-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 beta-cells, we revealed that both Ca(2+)-phosphate- and oxidant-induced MPT is remarkably different from other tissues. Ca(2+)-phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in beta-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in beta-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic beta-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca(2+), suggesting a novel type of nonclassical MPT in beta-cells. It results in the conversion of electrical transmembrane potential into DeltapH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca(2+)- and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in beta-cell glucose-sensing and secretory activity.
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Affiliation(s)
- Vasilij Koshkin
- Departments of Physiology and Medicine, University of Toronto, Toronto M5S 1A8, Canada
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18
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Westermark PO, Lansner A. A model of phosphofructokinase and glycolytic oscillations in the pancreatic beta-cell. Biophys J 2003; 85:126-39. [PMID: 12829470 PMCID: PMC1303071 DOI: 10.1016/s0006-3495(03)74460-9] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We have constructed a model of the upper part of the glycolysis in the pancreatic beta-cell. The model comprises the enzymatic reactions from glucokinase to glyceraldehyde-3-phosphate dehydrogenase (GAPD). Our results show, for a substantial part of the parameter space, an oscillatory behavior of the glycolysis for a large range of glucose concentrations. We show how the occurrence of oscillations depends on glucokinase, aldolase and/or GAPD activities, and how the oscillation period depends on the phosphofructokinase activity. We propose that the ratio of glucokinase and aldolase and/or GAPD activities are adequate as characteristics of the glucose responsiveness, rather than only the glucokinase activity. We also propose that the rapid equilibrium between different oligomeric forms of phosphofructokinase may reduce the oscillation period sensitivity to phosphofructokinase activity. Methodologically, we show that a satisfying description of phosphofructokinase kinetics can be achieved using the irreversible Hill equation with allosteric modifiers. We emphasize the use of parameter ranges rather than fixed values, and the use of operationally well-defined parameters in order for this methodology to be feasible. The theoretical results presented in this study apply to the study of insulin secretion mechanisms, since glycolytic oscillations have been proposed as a cause of oscillations in the ATP/ADP ratio which is linked to insulin secretion.
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Affiliation(s)
- Pål O Westermark
- PSCI/SANS, NADA, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden.
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19
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Li C, Najafi H, Daikhin Y, Nissim IB, Collins HW, Yudkoff M, Matschinsky FM, Stanley CA. Regulation of leucine-stimulated insulin secretion and glutamine metabolism in isolated rat islets. J Biol Chem 2003; 278:2853-8. [PMID: 12444083 DOI: 10.1074/jbc.m210577200] [Citation(s) in RCA: 128] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Glutamate dehydrogenase (GDH) is regulated by both positive (leucine and ADP) and negative (GTP and ATP) allosteric factors. We hypothesized that the phosphate potential of beta-cells regulates the sensitivity of leucine stimulation. These predictions were tested by measuring leucine-stimulated insulin secretion in perifused rat islets following glucose depletion and by tracing the nitrogen flux of [2-(15)N]glutamine using stable isotope techniques. The sensitivity of leucine stimulation was enhanced by long time (120-min) energy depletion and inhibited by glucose pretreatment. After limited 50-min glucose depletion, leucine, not alpha-ketoisocaproate, failed to stimulate insulin release. beta-Cells sensitivity to leucine is therefore proposed to be a function of GDH activation. Leucine increased the flux through GDH 3-fold compared with controls while causing insulin release. High glucose inhibited flux through both glutaminase and GDH, and leucine was unable to override this inhibition. These results clearly show that leucine induced the secretion of insulin by augmenting glutaminolysis through activating glutaminase and GDH. Glucose regulates beta-cell sensitivity to leucine by elevating the ratio of ATP and GTP to ADP and P(i) and thereby decreasing the flux through GDH and glutaminase. These mechanisms provide an explanation for hypoglycemia caused by mutations of GDH in children.
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Affiliation(s)
- Changhong Li
- Division of Endocrinology, Children's Hospital of Philadelphia, Pennsylvania 19104, USA
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20
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Abstract
Experiments do not support a recent claim that glutamate formed from the amination of citric acid cycle-derived alpha-ketoglutarate is a messenger in glucose-induced insulin secretion (Maechler, P., and Wollheim, C. (1999) Nature 402, 685-689). Glucose, leucine, succinic acid methyl ester, and alpha-ketoisocaproic acid all markedly stimulate insulin release but do not increase glutamate levels in pancreatic islets. Increasing the intracellular glutamate levels to 10-fold higher than basal levels by adding glutamine to islets does not stimulate insulin release. When leucine, in addition to glutamine, is applied to islets, insulin release is almost as high as with glucose alone. This is consistent with the known ability of leucine to allosterically activate glutamate deamination by glutamate dehydrogenase, which can supply alpha-ketoglutarate to the citric acid cycle. Experiments with mitochondria from pancreatic islets suggest that flux through the glutamate dehydrogenase reaction is quiescent during glucose-induced insulin secretion. These experiments support the traditional idea that when insulin release is associated with flux through glutamate dehydrogenase, the flux is in the direction of alpha-ketoglutarate.
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Affiliation(s)
- M J MacDonald
- Childrens Diabetes Center, University of Wisconsin Medical School, Madison, Wisconsin, USA.
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21
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Olson LK, Schroeder W, Robertson RP, Goldberg ND, Walseth TF. Suppression of adenylate kinase catalyzed phosphotransfer precedes and is associated with glucose-induced insulin secretion in intact HIT-T15 cells. J Biol Chem 1996; 271:16544-52. [PMID: 8663240 DOI: 10.1074/jbc.271.28.16544] [Citation(s) in RCA: 47] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Adenine nucleotide metabolism was characterized in intact insulin secreting HIT-T15 cells during the transition from non-stimulated (i. e. 0.2 mM glucose) to the glucose-stimulated secretory state. Metabolic dynamics were monitored by assessing rates of appearance of 18O-labeled phosphoryls of endogenous nucleotides in cells incubated in medium enriched in [18O]water. Most prominent of the metabolic alterations associated with stimulated insulin secretion was the suppression in the rate of adenylate kinase (AK)-catalyzed phosphorylation of AMP by ATP. This was manifest as a graded decrease of up to 50% in the rate of appearance of beta-18O-labeled species of ADP and ATP and corresponded to the magnitude of the secretory response elicited over a range of stimulatory glucose concentrations. The only nucleotide exhibiting a significant concentration change associated with suppression of AK activity was AMP, which decreased by about 50%, irrespective of the glucose concentration. Leucine-stimulated secretion also decreased the rate of AK-catalyzed phosphotransfer. This secretory stimulus-related suppression of AK-catalyzed phosphotransfer occurs within 45 s of glucose addition, precedes insulin secretion, depends on the internalization and metabolism of glucose, and is independent of membrane depolarization and the influx of extracellular calcium. The secretory stimulus-induced decrease in AK-catalyzed phosphotransfer, therefore occurs prior to or at the time of KATP+ channel closure but it is not associated with or a consequence of events occurring subsequent to KATP+ channel closure. These results indicate that AK-catalyzed phosphotransfer may be a determinant of ATP to ADP conversion rates in the KATP+ channel microenvironment; secretory stimuli-linked decreased rates of AK-catalyzed ADP generation from ATP (and AMP) would translate into an increased probability of ATP-liganded and, therefore, closed state of the channel.
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Affiliation(s)
- L K Olson
- Department of Pharmacology, Division of Diabetes, Endocrinology, and Metabolism, University of Minnesota, Minneapolis, Minnesota 55455, USA
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22
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Sener A, Malaisse WJ. Kinetics and specificity of human B-cell glucokinase: relevance to hexose-induced insulin release. BIOCHIMICA ET BIOPHYSICA ACTA 1996; 1312:73-8. [PMID: 8679719 DOI: 10.1016/0167-4889(96)00010-9] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The present study reevaluates the relevance of human B-cell glucokinase activity to the process of hexose-induced insulin release. Taking into account a phenomenon of positive cooperativity (Hill number: 1.34), the Km of the enzyme for glucose ( < or = 5.1 mM) was lower than the concentration of the hexose required to cause half-maximal stimulation of insulin release in intact islets. Likewise, there were obvious discrepancies between the kinetics of glucose, mannose and fructose phosphorylation by B-cell glucokinase, e.g. in terms of maximal velocity, and the secretory and metabolic responses to these hexoses in intact islets. Glucose 6-phosphate decreased, modestly but significantly, B-cell glucokinase activity, such an inhibitory action being of the non-competitive type. Mannoheptulose caused competitive inhibition of B-cell glucokinase. It is concluded that the intrinsic catalytic properties of B-cell glucokinase cannot fully account for the concentration dependency and sugar specificity of the secretory response to D-glucose or other hexoses in pancreatic islets.
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Affiliation(s)
- A Sener
- Laboratory of Experimental Medicine, Brussels Free University, Belgium
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23
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Detimary P, Jonas JC, Henquin JC. Possible links between glucose-induced changes in the energy state of pancreatic B cells and insulin release. Unmasking by decreasing a stable pool of adenine nucleotides in mouse islets. J Clin Invest 1995; 96:1738-45. [PMID: 7560065 PMCID: PMC185810 DOI: 10.1172/jci118219] [Citation(s) in RCA: 90] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Whether adenine nucleotides in pancreatic B cells serve as second messengers during glucose stimulation of insulin secretion remains disputed. Our hypothesis was that the actual changes in ATP and ADP are obscured by the large pool of adenine nucleotides (ATP/ADP ratio close to 1) in insulin granules. Therefore, mouse islets were degranulated acutely with a cocktail of glucose, KCl, forskolin, and phorbol ester or during overnight culture in RPMI-1640 medium containing 10 mM glucose. When these islets were then incubated in 0 glucose + azide (to minimize cytoplasmic and mitochondrial adenine nucleotides), their content in ATP + ADP + AMP was decreased in proportion to the decrease in insulin stores. After incubation in 10 mM glucose (no azide), the ATP/ADP ratio increased from 2.4 to > 8 in cultured islets, and only from 2 to < 4 in fresh islets. These differences were not explained by changes in glucose oxidation. The glucose dependency (0-30 mM) of the changes in insulin secretion and in the ATP/ADP ratio were then compared in the same islets. In nondegranulated, fresh islets, the ATP/ADP ratio increased between 0 and 10 mM glucose and then stabilized although insulin release kept increasing. In degranulated islets, the ATP/ADP ratio also increased between 0 and 10 mM glucose, but a further increase still occurred between 10 and 20 mM glucose, in parallel with the stimulation of insulin release. In conclusion, decreasing the granular pool of ATP and ADP unmasks large changes in the ATP/ADP ratio and a glucose dependency which persists within the range of stimulatory concentrations. The ATP/ADP ratio might thus serve as a coupling factor between glucose metabolism and insulin release.
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Affiliation(s)
- P Detimary
- Unité d'Endocrinologie et Métabolisme, University of Louvain Faculty of Medicine, UCL 55.30, Brussels, Belgium
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24
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Ferber S, BeltrandelRio H, Johnson J, Noel R, Cassidy L, Clark S, Becker T, Hughes S, Newgard C. GLUT-2 gene transfer into insulinoma cells confers both low and high affinity glucose-stimulated insulin release. Relationship to glucokinase activity. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(19)78155-1] [Citation(s) in RCA: 65] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
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25
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Erecińska M, Bryła J, Michalik M, Meglasson MD, Nelson D. Energy metabolism in islets of Langerhans. BIOCHIMICA ET BIOPHYSICA ACTA 1992; 1101:273-95. [PMID: 1643073 DOI: 10.1016/0005-2728(92)90084-f] [Citation(s) in RCA: 88] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Affiliation(s)
- M Erecińska
- Department of Pharmacology, University of Pennsylvania, School of Medicine, Philadelphia 19104-6084
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26
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Ohta M, Nelson D, Wilson JM, Meglasson MD, Erecińska M. Relationships between energy level and insulin secretion in isolated rat islets of Langerhans. Manipulation of [ATP]/[ADP][Pi] by 2-deoxy-D-glucose. Biochem Pharmacol 1992; 43:1859-64. [PMID: 1575778 DOI: 10.1016/0006-2952(92)90722-u] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Perifusion of islets with nominally phosphate-free buffer containing increasing concentrations of 2-deoxy-D-glucose (2.5 to 10 mM) produced increments in high alpha-ketoisocaproic acid-induced secretion of insulin beyond those observed in the absence of the sugar analogue. 3-O-methyl-D-glucose, a poorly metabolized sugar, was without effect. Insulin release evoked by 40 mM KCl was not altered by 2-deoxyglucose. The concentration of intracellular inorganic phosphate was lower in islets perifused with 2-deoxyglucose and declined to a lower level after addition of 20 mM alpha-ketoisocaproic acid. The enhancement of alpha-ketoisocaproic acid-induced hormone secretion by 2-deoxyglucose was not seen in islets perifused with medium containing 1.5 mM phosphate; instead a small inhibition was observed. It is postulated that conditions which lower intracellular [Pi] facilitate, either directly or indirectly, hormone release although the mechanism of this effect remains to be elucidated.
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Affiliation(s)
- M Ohta
- Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084
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27
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Ghosh A, Ronner P, Cheong E, Khalid P, Matschinsky F. The role of ATP and free ADP in metabolic coupling during fuel-stimulated insulin release from islet beta-cells in the isolated perfused rat pancreas. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)54437-9] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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28
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Pugliese G, Tilton RG, Williamson JR. Glucose-induced metabolic imbalances in the pathogenesis of diabetic vascular disease. DIABETES/METABOLISM REVIEWS 1991; 7:35-59. [PMID: 1935535 DOI: 10.1002/dmr.5610070106] [Citation(s) in RCA: 106] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Affiliation(s)
- G Pugliese
- Department of Experimental Medicine, La Sapienza University, Policlinico Umberto 1, Rome, Italy
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29
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Oscillatory synthesis of glucose 1,6-bisphosphate and frequency modulation of glycolytic oscillations in skeletal muscle extracts. J Biol Chem 1990. [DOI: 10.1016/s0021-9258(18)45757-2] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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30
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Allegra V, Mengozzi G, Martimbianco L, Vasile A. Glucose-induced insulin secretion in uremia: effects of aminophylline infusion and glucose loads. Kidney Int 1990; 38:1146-50. [PMID: 1963648 DOI: 10.1038/ki.1990.325] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
To explain mechanisms responsible for derangement of insulin release in uremia, we investigated glucose metabolism through three different tests in 14 patients with end-stage chronic renal failure. These tests were: intravenous glucose tolerance test with 0.33 g/kg of glucose solution (IVGTT); IVGTT with 0.5 g/kg of glucose solution (IVGTT2); IVGTT during aminophylline infusion (IVGTT + A). Twelve of the patients had IVGTT repeated after two to four months of thrice-weekly regular hemodialysis (IVGTT3). In each test we measured plasma glucose (G), immunoreactive insulin (IRI) and C-peptide. We also calculated glucose constant decay (K), insulin production (IRI area), insulinogenic index (IGI), and insulin resistance index (RI). Twenty-nine healthy volunteers formed the normal controls for IVGTT. As compared to controls, during IVGTT uremic patients showed significantly lower values in K, IRI area and IGI, and showed a significant RI value increase. During IVGTT2, IRI are values were higher than during IVGTT but IGI and K values were unchanged. During IVGTT + A both IRI area and IGI values were higher than during IVGTT. After hemodialysis treatment (IVGTT3) K, IRI areas and IGI increased significantly as compared to the predialysis period. K increase after hemodialysis correlated directly to IGI increase and inversely to RI changes. IGI increase during IVGTT3 was directly correlated to IGI rise during IVGTT + A. From these data we infer that defective insulin release in uremia is due to a decrease of beta-cell glucose sensitivity rather than to their functional exhaustion. An impaired adenyl cyclase-cAMP system may have an important role in the pathogenesis of this abnormality.
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Affiliation(s)
- V Allegra
- Servizio Emodialisi, Ospedale di Palmanova, USL No.8 Bassa Friuland, Regione Friuli-Venezia Giulia, Italy
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31
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Oxygen and temperature dependence of stimulated insulin secretion in isolated rat islets of Langerhans. J Biol Chem 1990. [DOI: 10.1016/s0021-9258(18)38196-1] [Citation(s) in RCA: 58] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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32
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Geschwind JF, Hiriart M, Glennon MC, Najafi H, Corkey BE, Matschinsky FM, Prentki M. Selective activation of Ca2+ influx by extracellular ATP in a pancreatic beta-cell line (HIT). BIOCHIMICA ET BIOPHYSICA ACTA 1989; 1012:107-15. [PMID: 2543452 DOI: 10.1016/0167-4889(89)90018-9] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
The action of exogenous ATP on cytoplasmic free Ca2+ ([Ca2+]i) was studied in insulin secreting cells using fura-2. Stimulation of clonal pancreatic beta-cells (HIT) with ATP (range 2-20 microM) evoked a sustained elevation in [Ca2+]i. ATP selectively promoted Ca2+ influx and not Ca2+ mobilization since (1) the effect required external Ca1+ and (2) was observed in cells in which internal stores were depleted with ionomycin (3) the rate of Mn2+ influx, measured as the quenching of the fura-2 signal, was accelerated by ATP. The action of ATP was unaffected by the voltage-sensitive Ca2+ channel blockers nifedipine and verapamil as well as by a depolarizing concentration of K+. The effect on [Ca2+]i was highly specific for ATP since AMP, ADP, adenosine 5'-[gamma-thio]triphosphate, adenosine 5'-[beta, gamma-methylene]triphosphate, GTP and adenosine were ineffective. In normal pancreatic islet cells, both exogenous ATP (range 0.2-2 microM) and ADP induced a transient Ca2+ elevation that did not require external Ca2+. The nucleotide specificity of the effect on [Ca2+]i suggests that ATP activates P2 gamma purinergic receptors in normal beta-cells. Thus, ATP evokes a Ca2+ signal in clonal HIT cells and normal islet cells by different transducing systems involving distinct purinoreceptors. A novel mechanism for increasing [Ca2+]i by extracellular ATP is reported in HIT cells, since the nucleotide specificity and the selective activation of Ca2+ influx without mobilization of internal Ca2+ stores cannot be explained by mechanisms already described in other cell systems.
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Affiliation(s)
- J F Geschwind
- Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia
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33
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MacDonald MJ, Mertz RJ, Rana RS. Glyceraldehyde phosphate: an insulin secretagogue with possible effects on inositol phosphate formation in pancreatic islets. Arch Biochem Biophys 1989; 269:194-200. [PMID: 2644893 DOI: 10.1016/0003-9861(89)90100-8] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
The insulinotropic action of glucose, the most potent physiologic insulin secretagogue, involves its metabolism. However, no glucose metabolite has ever been identified as a key intermediate. We tested the abilities of a number of glucose metabolites to stimulate insulin release from pancreatic islets. Of all of these metabolites, glyceraldehyde 3-phosphate was the most potent insulin secretagogue. In numerous experiments over 3 years, insulin release by 4 mM glyceraldehyde phosphate ranged from 50 to 200% of that initiated by 16.7 mM glucose--a near-maximal insulin stimulus. At concentrations of 1 and 4 mM, glyceraldehyde phosphate was even more potent than the known secretagogues glucose and glyceraldehyde. Glucose metabolites were also tested for their ability to stimulate inositol tris-, bis-, and monophosphate formation by permeabilized islets. Only glyceraldehyde phosphate stimulated inositol phosphate formation and this stimulation occurred at concentrations of glyceraldehyde phosphate which could be present in the beta cell under physiologic conditions (K0.5 = 25 microM). The current results are consistent with the idea that glyceraldehyde phosphate is a key insulinotropic glucose metabolite that might act directly (or rather directly via a receptor) on the phospholipase C that forms inositol trisphosphate in the plasma membrane.
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Affiliation(s)
- M J MacDonald
- Department of Pediatrics, University of Wisconsin Medical School, Madison 53706
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Corkey BE, Deeney JT, Glennon MC, Matschinsky FM, Prentki M. Regulation of steady-state free Ca2+ levels by the ATP/ADP ratio and orthophosphate in permeabilized RINm5F insulinoma cells. J Biol Chem 1988. [DOI: 10.1016/s0021-9258(18)68916-1] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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Rana RS, Sekar MC, Mertz RJ, Hokins LE, MacDonald MJ. Potentiation by glucose metabolites of inositol trisphosphate-induced calcium mobilization in permeabilized rat pancreatic islets. J Biol Chem 1987. [DOI: 10.1016/s0021-9258(19)76465-5] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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Abstract
The sodium dependence of insulin release was studied with the in vitro perfused principal islet of channel catfish. When extracellular sodium was replaced with choline, glucose-induced insulin release and glucose enhancement of arginine-induced insulin release were selectively abolished. Lowering the extracellular pH from 7.6 to 7.1 had a similar selective effect on glucose-induced insulin release. It is suggested that the Na:H antiport is responsible for the sodium dependence of insulin release and that a lowered cytosolic pH preferentially inhibits the glucose sensing mechanism.
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RONNER PETER. Transmembrane Ion Distribution and Insulin Secretion. Ann N Y Acad Sci 1986. [DOI: 10.1111/j.1749-6632.1986.tb54416.x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Metabolic concomitants in pure, pancreatic beta cells during glucose-stimulated insulin secretion. J Biol Chem 1986. [DOI: 10.1016/s0021-9258(18)66980-7] [Citation(s) in RCA: 33] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
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A possible role for glucose metabolites in the regulation of inositol-1,4,5-trisphosphate 5-phosphomonoesterase activity in pancreatic islets. J Biol Chem 1986. [DOI: 10.1016/s0021-9258(19)57203-9] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
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Meglasson MD, Matschinsky FM. Pancreatic islet glucose metabolism and regulation of insulin secretion. DIABETES/METABOLISM REVIEWS 1986; 2:163-214. [PMID: 2943567 DOI: 10.1002/dmr.5610020301] [Citation(s) in RCA: 371] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
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Johnson RC, Freinkel N. A new method for the simultaneous estimation of phosphate efflux and influx during glucose stimulation of isolated pancreatic islets. Biochem Biophys Res Commun 1985; 129:862-7. [PMID: 3893433 DOI: 10.1016/0006-291x(85)91971-0] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Isolated rat pancreatic islets were prelabeled with [33Pi] and then incubated with basal (2.8 mM) or stimulatory (16.7 mM) glucose in the presence of [32Pi]. Subsequent changes in islet [33P] and [32P] were utilized as respective indices of net efflux and influx. During the initial eight min, (the period usually spanning the first phase of stimulated insulin secretion) efflux was significantly greater with 16.7 than 2.8 mM glucose whereas the lesser amount of phosphate influx did not differ in the two systems. During the subsequent seven min (a time usually associated with the onset of the second phase of stimulated insulin secretion), efflux was dampened in the presence of 16.7 mM glucose and Pi influx significantly exceeded the 2.8 mM glucose values. Thus, acute stimulation with glucose effects an initial phosphate depletion in pancreatic islets as efflux exceeds influx and repletion occurs thereafter as efflux is attenuated and influx is enhanced. These oscillations in islet phosphate may contribute to the biphasic pattern of glucose-stimulated insulin release.
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Bedoya FJ, Meglasson MD, Wilson JM, Matschinsky FM. Radiometric oil well assay for glucokinase in microscopic structures. Anal Biochem 1985; 144:504-13. [PMID: 3887980 DOI: 10.1016/0003-2697(85)90147-2] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.1) plays a pivotal role in hepatic glucose metabolism and serves as the glucose sensor in pancreatic islet beta-cells. Biochemical studies of this enzyme are complicated by the cellular heterogeneity of the liver and the pancreas and because the presence of hexokinases (ATP:D-hexose 6-phosphotransferases, EC 2.7.1.1) seriously interferes with currently available analytical procedures. A radiometric assay was designed to deal with these problems. It is based on the liberation of 3H2O from D-[2-3H(N)]glucose 6-phosphate, the product of the glucokinase reaction, using exogenous phosphoglucose isomerase (D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9). Interference by hexokinases was largely eliminated by using glucose 6-phosphate as inhibitor and the sensitivity of the assay was greatly increased by using small volumes with the oil well procedure. The assay was sufficiently sensitive to detect about 1 pg of glucokinase. It thus allowed the application of quantitative histochemical procedures to the study of intralobular hepatic glucokinase profiles and the pancreatic beta-cell glucose sensor. The quantitative histochemical procedures were sufficiently sensitive and reliable for measuring important kinetic constants of glucokinase (i.e., the Km and the Hill number) in microscopic samples of tissue.
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Henquin JC, Meissner HP. Significance of ionic fluxes and changes in membrane potential for stimulus-secretion coupling in pancreatic B-cells. EXPERIENTIA 1984; 40:1043-52. [PMID: 6386515 DOI: 10.1007/bf01971450] [Citation(s) in RCA: 253] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
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Meglasson MD, Matschinsky FM. New perspectives on pancreatic islet glucokinase. THE AMERICAN JOURNAL OF PHYSIOLOGY 1984; 246:E1-13. [PMID: 6364828 DOI: 10.1152/ajpendo.1984.246.1.e1] [Citation(s) in RCA: 55] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Control of blood sugar involves the complex interaction of the pancreatic glucose-sensing beta-cells with the liver, which serves as the primary site of glucose disposal after a meal. Glucokinase occupies an important role in controlling glucose phosphorylation and metabolism both in the liver and in pancreatic islets. In the beta-cells, glucokinase functions as pacemaker of glycolysis at physiological glucose levels. It determines the unique characteristics of islet hexose usage, that is, the rate, affinity, cooperativity, and anomeric discrimination of glucose metabolism. Because glycolysis controls hexose-induced insulin release, glucokinase is considered the best-qualified candidate for the elusive glucose sensor of beta-cells. A deficiency of glucokinase would disturb glucose homeostasis. Decreased islet glucokinase would diminish islet glycolysis and would result in a higher set point of beta-cells for glucose-induced insulin release. Decreased liver glucokinase would cause less efficient hepatic glucose disposal. Human maturity-onset diabetes (type II diabetes) has these characteristics. It is thus conceivable that certain forms of type II diabetes are due to a glucokinase deficiency.
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Malaisse WJ, Carpinelli AR, Sener A. Stimulus-secretion coupling of glucose-induced insulin release. Timing of early metabolic, ionic, and secretory events. Metabolism 1981; 30:527-32. [PMID: 7015071 DOI: 10.1016/0026-0495(81)90191-8] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
The timing of the early metabolic, ionic, and secretory responses to glucose in rat pancreatic islets was monitored by measuring, at 12 sec intervals, the concentrations of glucose, lactic, and pyruvic acids, 32P, 86Rb, 45ca, and insulin in the effluent of perifused prelabeled islets. The increase in glucose concentration from zero to 16.7 mM was complete within 133 sec. The output of organic acids increased after 24 sec of exposure to glucose and, in the case of lactic acid, fell slightly after the initial elevation. The phosphate flush was initiated only after 96 sec of exposure to glucose, whereas the decreases in 86Rb and 45Ca outflow were both detectable within 72 sec of stimulation. The secondary rise in 45Ca efflux was first seen after 157 sec of stimulation and its time course was not vastly different from that of insulin release. These data indicate that, in the secretory sequence, metabolic changes precede both the remodelling of ionic fluxes and the stimulation of insulin release. The results are compatible with the view that the secondary rise in 45Ca outflow is attributable, in part at least, to the glucose-induced decrease in K conductance (but not to the increase in phosphate outflow), with resulting membrane depolarization and gating of voltage-dependent Ca channels.
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