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Cheng X, Prange-Barczynska M, Fielding JW, Zhang M, Burrell AL, Lima JD, Eckardt L, Argles IL, Pugh CW, Buckler KJ, Robbins PA, Hodson EJ, Bruick RK, Collinson LM, Rastinejad F, Bishop T, Ratcliffe PJ. Marked and rapid effects of pharmacological HIF-2α antagonism on hypoxic ventilatory control. J Clin Invest 2020; 130:2237-2251. [PMID: 31999648 PMCID: PMC7190921 DOI: 10.1172/jci133194] [Citation(s) in RCA: 25] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2019] [Accepted: 01/14/2020] [Indexed: 12/22/2022] Open
Abstract
Hypoxia-inducible factor (HIF) is strikingly upregulated in many types of cancer, and there is great interest in applying inhibitors of HIF as anticancer therapeutics. The most advanced of these are small molecules that target the HIF-2 isoform through binding the PAS-B domain of HIF-2α. These molecules are undergoing clinical trials with promising results in renal and other cancers where HIF-2 is considered to be driving growth. Nevertheless, a central question remains as to whether such inhibitors affect physiological responses to hypoxia at relevant doses. Here, we show that pharmacological HIF-2α inhibition with PT2385, at doses similar to those reported to inhibit tumor growth, rapidly impaired ventilatory responses to hypoxia, abrogating both ventilatory acclimatization and carotid body cell proliferative responses to sustained hypoxia. Mice carrying a HIF-2α PAS-B S305M mutation that disrupts PT2385 binding, but not dimerization with HIF-1β, did not respond to PT2385, indicating that these effects are on-target. Furthermore, the finding of a hypomorphic ventilatory phenotype in untreated HIF-2α S305M mutant mice suggests a function for the HIF-2α PAS-B domain beyond heterodimerization with HIF-1β. Although PT2385 was well tolerated, the findings indicate the need for caution in patients who are dependent on hypoxic ventilatory drive.
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Affiliation(s)
- Xiaotong Cheng
- Target Discovery Institute and
- Ludwig Institute for Cancer Research, University of Oxford, Oxford, United Kingdom
| | - Maria Prange-Barczynska
- Target Discovery Institute and
- Ludwig Institute for Cancer Research, University of Oxford, Oxford, United Kingdom
| | - James W. Fielding
- Target Discovery Institute and
- Ludwig Institute for Cancer Research, University of Oxford, Oxford, United Kingdom
| | | | | | | | | | | | | | - Keith J. Buckler
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
| | - Peter A. Robbins
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
| | | | - Richard K. Bruick
- Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, Texas, USA
| | | | | | | | - Peter J. Ratcliffe
- Target Discovery Institute and
- Ludwig Institute for Cancer Research, University of Oxford, Oxford, United Kingdom
- Francis Crick Institute, London, United Kingdom
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O'Donohoe PB, Turner PJ, Huskens N, Buckler KJ, Pandit JJ. Influence of propofol on isolated neonatal rat carotid body glomus cell response to hypoxia and hypercapnia. Respir Physiol Neurobiol 2018; 260:17-27. [PMID: 30389452 PMCID: PMC6336315 DOI: 10.1016/j.resp.2018.10.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2018] [Revised: 10/14/2018] [Accepted: 10/29/2018] [Indexed: 11/06/2022]
Abstract
The intravenous anaesthetic propofol acts directly on carotid body glomus cells to inhibit their response to hypoxia. Propofol acts via novel mechanisms, as we excluded action via its known target receptors (nicotinic, GABA-ergic, or K+ channel). Inhibition of the hypoxic response is clinically relevant in anaesthesia.
In humans the intravenous anaesthetic propofol depresses ventilatory responses to hypoxia and CO2. Animal studies suggest that this may in part be due to inhibition of synaptic transmission between chemoreceptor glomus cells of the carotid body and the afferent carotid sinus nerve. It is however unknown if propofol can also act directly on the glomus cell. Here we report that propofol can indeed inhibit intracellular Ca2+ responses to hypoxia and hypercapnia in isolated rat glomus cells. Neither this propofol effect, nor the glomus cell response to hypoxia in the absence of propofol, were influenced by GABA receptor activation (using GABA, muscimol and baclofen) or inhibition (using bicuculline and 5-aminovaleric acid). Suggesting that these effects of propofol are not mediated through GABA receptors. Propofol inhibited calcium responses to nicotine in glomus cells but the nicotinic antagonists vecuronium and methyllycaconitine did not inhibit calcium responses to hypoxia. TASK channel activity was not altered by propofol. The glomus cell Ca2+ response to depolarisation with 30 mM K+ was however modestly inhibited by propofol. In summary we conclude that propofol does have a direct effect upon hypoxia signalling in isolated type-1 cells and that this may be partially due to its ability to inhibit voltage gated Ca2+v channels. We also note that propofol has the capacity to supress glomus cell excitation via nicotinic receptors and may therefore also interfere with paracrine/autocrine cholinergic signalling in the intact organ. The effects of propofol on chemoreceptor function are however clearly complex and require further investigation.
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Affiliation(s)
- Peadar B O'Donohoe
- Department of Physiology, Anatomy & Genetics, Parks Road, University of Oxford, Oxford, OX1 3PT, UK; Nuffield Department of Anaesthetics, Oxford University Hospitals NHS Trust, Oxford, OX3 9DU, UK
| | - Philip J Turner
- Department of Physiology, Anatomy & Genetics, Parks Road, University of Oxford, Oxford, OX1 3PT, UK
| | - Nicky Huskens
- Department of Physiology, Anatomy & Genetics, Parks Road, University of Oxford, Oxford, OX1 3PT, UK
| | - Keith J Buckler
- Department of Physiology, Anatomy & Genetics, Parks Road, University of Oxford, Oxford, OX1 3PT, UK
| | - Jaideep J Pandit
- Nuffield Department of Anaesthetics, Oxford University Hospitals NHS Trust, Oxford, OX3 9DU, UK.
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O'Donohoe PB, Huskens N, Turner PJ, Pandit JJ, Buckler KJ. A1899, PK-THPP, ML365, and Doxapram inhibit endogenous TASK channels and excite calcium signaling in carotid body type-1 cells. Physiol Rep 2018; 6:e13876. [PMID: 30284397 PMCID: PMC6170881 DOI: 10.14814/phy2.13876] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2018] [Accepted: 08/24/2018] [Indexed: 01/10/2023] Open
Abstract
Sensing of hypoxia and acidosis in arterial chemoreceptors is thought to be mediated through the inhibition of TASK and possibly other (e.g., BKCa ) potassium channels which leads to membrane depolarization, voltage-gated Ca-entry, and neurosecretion. Here, we investigate the effects of pharmacological inhibitors on TASK channel activity and [Ca2+ ]i -signaling in isolated neonatal rat type-1 cells. PK-THPP inhibited TASK channel activity in cell attached patches by up to 90% (at 400 nmol/L). A1899 inhibited TASK channel activity by 35% at 400 nmol/L. PK-THPP, A1899 and Ml 365 all evoked a rapid increase in type-1 cell [Ca2+ ]i . These [Ca2+ ]i responses were abolished in Ca2+ -free solution and greatly attenuated by Ni2+ (2 mM) suggesting that depolarization and voltage-gated Ca2+ -entry mediated the rise in [Ca2+ ]i. Doxapram (50 μmol/L), a respiratory stimulant, also inhibited type-1 cell TASK channel activity and increased [Ca2+ ]i. . We also tested the effects of combined inhibition of BKCa and TASK channels. TEA (5 mmol/L) slightly increased [Ca2+ ]i in the presence of PK-THPP and A1899. Paxilline (300 nM) and iberiotoxin (50 nmol/L) also slightly increased [Ca2+ ]i in the presence of A1899 but not in the presence of PK-THPP. In general [Ca2+ ]i responses to TASK inhibitors, alone or in combination with BKCa inhibitors, were smaller than the [Ca2+ ]i responses evoked by hypoxia. These data confirm that TASK channel inhibition is capable of evoking membrane depolarization and robust voltage-gated Ca2+ -entry but suggest that this, even with concomitant inhibition of BKCa channels, may be insufficient to account fully for the [Ca2+ ]i -response to hypoxia.
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Affiliation(s)
- Peadar B. O'Donohoe
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
- Nuffield Department of AnaestheticsOxford University HospitalsOxfordUnited Kingdom
| | - Nicky Huskens
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
| | - Philip J. Turner
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
| | - Jaideep J. Pandit
- Nuffield Department of AnaestheticsOxford University HospitalsOxfordUnited Kingdom
| | - Keith J. Buckler
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
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Fielding JW, Hodson EJ, Cheng X, Ferguson DJP, Eckardt L, Adam J, Lip P, Maton‐Howarth M, Ratnayaka I, Pugh CW, Buckler KJ, Ratcliffe PJ, Bishop T. PHD2 inactivation in Type I cells drives HIF-2α-dependent multilineage hyperplasia and the formation of paraganglioma-like carotid bodies. J Physiol 2018; 596:4393-4412. [PMID: 29917232 PMCID: PMC6138294 DOI: 10.1113/jp275996] [Citation(s) in RCA: 36] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2018] [Accepted: 05/17/2018] [Indexed: 02/06/2023] Open
Abstract
KEY POINTS The carotid body is a peripheral arterial chemoreceptor that regulates ventilation in response to both acute and sustained hypoxia. Type I cells in this organ respond to low oxygen both acutely by depolarization and dense core vesicle secretion and, over the longer term, via cellular proliferation and enhanced ventilatory responses. Using lineage analysis, the present study shows that the Type I cell lineage itself proliferates and expands in response to sustained hypoxia. Inactivation of HIF-2α in Type I cells impairs the ventilatory, proliferative and cell intrinsic (dense core vesicle) responses to hypoxia. Inactivation of PHD2 in Type I cells induces multilineage hyperplasia and ultrastructural changes in dense core vesicles to form paraganglioma-like carotid bodies. These changes, similar to those observed in hypoxia, are dependent on HIF-2α. Taken together, these findings demonstrate a key role for the PHD2-HIF-2α couple in Type I cells with respect to the oxygen sensing functions of the carotid body. ABSTRACT The carotid body is a peripheral chemoreceptor that plays a central role in mammalian oxygen homeostasis. In response to sustained hypoxia, it manifests a rapid cellular proliferation and an associated increase in responsiveness to hypoxia. Understanding the cellular and molecular mechanisms underlying these processes is of interest both to specialized chemoreceptive functions of that organ and, potentially, to the general physiology and pathophysiology of cellular hypoxia. We have combined cell lineage tracing technology and conditionally inactivated alleles in recombinant mice to examine the role of components of the HIF hydroxylase pathway in specific cell types within the carotid body. We show that exposure to sustained hypoxia (10% oxygen) drives rapid expansion of the Type I, tyrosine hydroxylase expressing cell lineage, with little transdifferentiation to (or from) that lineage. Inactivation of a specific HIF isoform, HIF-2α, in the Type I cells was associated with a greatly reduced proliferation of Type I cells and hypoxic ventilatory responses, with ultrastructural evidence of an abnormality in the action of hypoxia on dense core secretory vesicles. We also show that inactivation of the principal HIF prolyl hydroxylase PHD2 within the Type I cell lineage is sufficient to cause multilineage expansion of the carotid body, with characteristics resembling paragangliomas. These morphological changes were dependent on the integrity of HIF-2α. These findings implicate specific components of the HIF hydroxylase pathway (PHD2 and HIF-2α) within Type I cells of the carotid body with respect to the oxygen sensing and adaptive functions of that organ.
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Affiliation(s)
- James W. Fielding
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
- Ludwig Institute for Cancer ResearchUniversity of Oxford, Oxford, UK
| | - Emma J. Hodson
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
| | - Xiaotong Cheng
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
- Ludwig Institute for Cancer ResearchUniversity of Oxford, Oxford, UK
| | | | - Luise Eckardt
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
| | - Julie Adam
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
- Ludwig Institute for Cancer ResearchUniversity of Oxford, Oxford, UK
| | - Philomena Lip
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
| | | | - Indrika Ratnayaka
- Ludwig Institute for Cancer ResearchUniversity of Oxford, Oxford, UK
| | | | - Keith J. Buckler
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUK
| | - Peter J. Ratcliffe
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
- Ludwig Institute for Cancer ResearchUniversity of Oxford, Oxford, UK
- The Francis Crick InstituteLondonUK
| | - Tammie Bishop
- Target Discovery InstituteUniversity of Oxford, Oxford, UK
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5
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Bardsley EN, Davis H, Ajijola OA, Buckler KJ, Ardell JL, Shivkumar K, Paterson DJ. RNA Sequencing Reveals Novel Transcripts from Sympathetic Stellate Ganglia During Cardiac Sympathetic Hyperactivity. Sci Rep 2018; 8:8633. [PMID: 29872217 PMCID: PMC5988725 DOI: 10.1038/s41598-018-26651-7] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2018] [Accepted: 05/15/2018] [Indexed: 12/15/2022] Open
Abstract
Cardiovascular disease is the most prevalent age-related illness worldwide, causing approximately 15 million deaths every year. Hypertension is central in determining cardiovascular risk and is a strong predictive indicator of morbidity and mortality; however, there remains an unmet clinical need for disease-modifying and prophylactic interventions. Enhanced sympathetic activity is a well-established contributor to the pathophysiology of hypertension, however the cellular and molecular changes that increase sympathetic neurotransmission are not known. The aim of this study was to identify key changes in the transcriptome in normotensive and spontaneously hypertensive rats. We validated 15 of our top-scoring genes using qRT-PCR, and network and enrichment analyses suggest that glutamatergic signalling plays a key role in modulating Ca2+ balance within these ganglia. Additionally, phosphodiesterase activity was found to be altered in stellates obtained from the hypertensive rat, suggesting that impaired cyclic nucleotide signalling may contribute to disturbed Ca2+ homeostasis and sympathetic hyperactivity in hypertension. We have also confirmed the presence of these transcripts in human donor stellate samples, suggesting that key genes coupled to neurotransmission are conserved. The data described here may provide novel targets for future interventions aimed at treating sympathetic hyperactivity associated with cardiovascular disease and other dysautonomias.
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Affiliation(s)
- Emma N Bardsley
- Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, Sherrington Building, University of Oxford, Oxford, OX1 3PT, UK.
| | - Harvey Davis
- Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, Sherrington Building, University of Oxford, Oxford, OX1 3PT, UK
| | - Olujimi A Ajijola
- UCLA Cardiac Arrhythmia Center, 100 Medical Plaza, Suite 660, Los Angeles, CA, 90095, USA
| | - Keith J Buckler
- Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, Sherrington Building, University of Oxford, Oxford, OX1 3PT, UK
| | - Jeffrey L Ardell
- UCLA Cardiac Arrhythmia Center, 100 Medical Plaza, Suite 660, Los Angeles, CA, 90095, USA
| | - Kalyanam Shivkumar
- UCLA Cardiac Arrhythmia Center, 100 Medical Plaza, Suite 660, Los Angeles, CA, 90095, USA
| | - David J Paterson
- Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, Sherrington Building, University of Oxford, Oxford, OX1 3PT, UK.
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6
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Bardsley EN, Davis H, Buckler KJ, Paterson DJ. Neurotransmitter Switching Coupled to β-Adrenergic Signaling in Sympathetic Neurons in Prehypertensive States. Hypertension 2018; 71:1226-1238. [PMID: 29686017 PMCID: PMC5959210 DOI: 10.1161/hypertensionaha.118.10844] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2018] [Revised: 01/18/2018] [Accepted: 03/26/2018] [Indexed: 01/16/2023]
Abstract
Single or combinatorial administration of β-blockers is a mainstay treatment strategy for conditions caused by sympathetic overactivity. Conventional wisdom suggests that the main beneficial effect of β-blockers includes resensitization and restoration of β1-adrenergic signaling pathways in the myocardium, improvements in cardiomyocyte contractility, and reversal of ventricular sensitization. However, emerging evidence indicates that another beneficial effect of β-blockers in disease may reside in sympathetic neurons. We investigated whether β-adrenoceptors are present on postganglionic sympathetic neurons and facilitate neurotransmission in a feed-forward manner. Using a combination of immunocytochemistry, RNA sequencing, Förster resonance energy transfer, and intracellular Ca2+ imaging, we demonstrate the presence of β-adrenoceptors on presynaptic sympathetic neurons in both human and rat stellate ganglia. In diseased neurons from the prehypertensive rat, there was enhanced β-adrenoceptor-mediated signaling predominantly via β2-adrenoceptor activation. Moreover, in human and rat neurons, we identified the presence of the epinephrine-synthesizing enzyme PNMT (phenylethanolamine-N-methyltransferase). Using high-pressure liquid chromatography with electrochemical detection, we measured greater epinephrine content and evoked release from the prehypertensive rat cardiac-stellate ganglia. We conclude that neurotransmitter switching resulting in enhanced epinephrine release, may provide presynaptic positive feedback on β-adrenoceptors to promote further release, that leads to greater postsynaptic excitability in disease, before increases in arterial blood pressure. Targeting neuronal β-adrenoceptor downstream signaling could provide therapeutic opportunity to minimize end-organ damage caused by sympathetic overactivity.
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Affiliation(s)
- Emma N Bardsley
- From the Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, University of Oxford, United Kingdom.
| | - Harvey Davis
- From the Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, University of Oxford, United Kingdom
| | - Keith J Buckler
- From the Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, University of Oxford, United Kingdom
| | - David J Paterson
- From the Wellcome Trust OXION Initiative in Ion Channels and Disease, Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, University of Oxford, United Kingdom.
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7
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Bardsley EN, Davis H, Ajijola OA, Buckler KJ, Ardell J, Shivkumar K, Paterson DJ. Identification of Novel mRNA Transcripts in the Sympathetic Stellate Ganglia using RNA Sequencing. FASEB J 2018. [DOI: 10.1096/fasebj.2018.32.1_supplement.596.4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
| | - Harvey Davis
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
| | | | - Keith J. Buckler
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
| | | | | | - David J. Paterson
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
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8
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Bardsley EN, Davis H, Buckler KJ, Paterson DJ. Neurotransmitter Switching in Sympathetic Neurons Coupled to Beta‐Adrenergic Signalling in Hypertensive States. FASEB J 2018. [DOI: 10.1096/fasebj.2018.32.1_supplement.591.1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Emma N. Bardsley
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
| | - Harvey Davis
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
| | - Keith J. Buckler
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
| | - David J. Paterson
- Department of Physiology, Anatomy and GeneticsUniversity of OxfordOxfordUnited Kingdom
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Hodson EJ, Nicholls LG, Turner PJ, Llyr R, Fielding JW, Douglas G, Ratnayaka I, Robbins PA, Pugh CW, Buckler KJ, Ratcliffe PJ, Bishop T. Regulation of ventilatory sensitivity and carotid body proliferation in hypoxia by the PHD2/HIF-2 pathway. J Physiol 2016; 594:1179-95. [PMID: 26337139 PMCID: PMC4771794 DOI: 10.1113/jp271050] [Citation(s) in RCA: 63] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2015] [Accepted: 08/26/2015] [Indexed: 12/24/2022] Open
Abstract
Ventilatory sensitivity to hypoxia increases in response to continued hypoxic exposure as part of acute acclimatisation. Although this process is incompletely understood, insights have been gained through studies of the hypoxia-inducible factor (HIF) hydroxylase system. Genetic studies implicate these pathways widely in the integrated physiology of hypoxia, through effects on developmental or adaptive processes. In keeping with this, mice that are heterozygous for the principal HIF prolyl hydroxylase, PHD2, show enhanced ventilatory sensitivity to hypoxia and carotid body hyperplasia. Here we have sought to understand this process better through comparative analysis of inducible and constitutive inactivation of PHD2 and its principal targets HIF-1α and HIF-2α. We demonstrate that general inducible inactivation of PHD2 in tamoxifen-treated Phd2(f/f);Rosa26(+/CreERT2) mice, like constitutive, heterozygous PHD2 deficiency, enhances hypoxic ventilatory responses (HVRs: 7.2 ± 0.6 vs. 4.4 ± 0.4 ml min(-1) g(-1) in controls, P < 0.01). The ventilatory phenotypes associated with both inducible and constitutive inactivation of PHD2 were strongly compensated for by concomitant inactivation of HIF-2α, but not HIF-1α. Furthermore, inducible inactivation of HIF-2α strikingly impaired ventilatory acclimatisation to chronic hypoxia (HVRs: 4.1 ± 0.5 vs. 8.6 ± 0.5 ml min(-1) g(-1) in controls, P < 0.0001), as well as carotid body cell proliferation (400 ± 81 vs. 2630 ± 390 bromodeoxyuridine-positive cells mm(-2) in controls, P < 0.0001). The findings demonstrate the importance of the PHD2/HIF-2α enzyme-substrate couple in modulating ventilatory sensitivity to hypoxia.
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Affiliation(s)
- Emma J Hodson
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
| | - Lynn G Nicholls
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
| | - Philip J Turner
- Department of Physiology, Anatomy and Genetics, Sherrington Building, South Parks Road, University of Oxford, Oxford, OX1 3PT, UK
| | - Ronan Llyr
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
| | - James W Fielding
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
- Ludwig Institute for Cancer Research, Roosevelt Drive, University of Oxford, Oxford, OX3 7DQ, UK
| | - Gillian Douglas
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
| | - Indrika Ratnayaka
- Ludwig Institute for Cancer Research, Roosevelt Drive, University of Oxford, Oxford, OX3 7DQ, UK
| | - Peter A Robbins
- Department of Physiology, Anatomy and Genetics, Sherrington Building, South Parks Road, University of Oxford, Oxford, OX1 3PT, UK
| | - Christopher W Pugh
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
| | - Keith J Buckler
- Department of Physiology, Anatomy and Genetics, Sherrington Building, South Parks Road, University of Oxford, Oxford, OX1 3PT, UK
| | - Peter J Ratcliffe
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
- Ludwig Institute for Cancer Research, Roosevelt Drive, University of Oxford, Oxford, OX3 7DQ, UK
| | - Tammie Bishop
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford, OX3 7BN, UK
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10
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Buckler KJ, Turner PJ. Functional Properties of Mitochondria in the Type-1 Cell and Their Role in Oxygen Sensing. Adv Exp Med Biol 2016; 860:69-80. [PMID: 26303469 DOI: 10.1007/978-3-319-18440-1_9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/16/2023]
Abstract
The identity of the oxygen sensor in arterial chemoreceptors has been the subject of much speculation. One of the oldest hypotheses is that oxygen is sensed through oxidative phosphorylation. There is a wealth of data demonstrating that arterial chemoreceptors are excited by inhibitors of oxidative phosphorylation. These compounds mimic the effects of hypoxia inhibiting TASK1/3 potassium channels causing membrane depolarisation calcium influx and neurosecretion. The TASK channels of Type-I cells are also sensitive to cytosolic MgATP. The existence of a metabolic signalling pathway in Type-1 cells is thus established; the contentious issue is whether this pathway is also used for acute oxygen sensing. The main criticism is that because cytochrome oxidase has a high affinity for oxygen (P50 ≈ 0.2 mmHg) mitochondrial metabolism should be insensitive to physiological hypoxia. This argument is however predicated on the assumption that chemoreceptor mitochondria are analogous to those of other tissues. We have however obtained new evidence to support the hypothesis that type-1 cell mitochondria are not like those of other cells in that they have an unusually low affinity for oxygen (Mills E, Jobsis FF, J Neurophysiol 35(4):405-428, 1972; Duchen MR, Biscoe TJ, J Physiol 450:13-31, 1992a). Our data confirm that mitochondrial membrane potential, NADH, electron transport and cytochrome oxidase activity in the Type-1 cell are all highly sensitive to hypoxia. These observations not only provide exceptionally strong support for the metabolic hypothesis but also reveal an unknown side of mitochondrial behaviour.
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Affiliation(s)
- Keith J Buckler
- Department of Physiology Anatomy and Genetics, University of Oxford, Oxford, OX1 3PT, UK,
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11
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Holmes AP, Turner PJ, Carter P, Leadbeater W, Ray CJ, Hauton D, Buckler KJ, Kumar P. Glycogen metabolism protects against metabolic insult to preserve carotid body function during glucose deprivation. J Physiol 2014; 592:4493-506. [PMID: 25063821 PMCID: PMC4287740 DOI: 10.1113/jphysiol.2014.276105] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2014] [Accepted: 07/07/2014] [Indexed: 01/08/2023] Open
Abstract
The view that the carotid body (CB) type I cells are direct physiological sensors of hypoglycaemia is challenged by the finding that the basal sensory neuronal outflow from the whole organ is unchanged in response to low glucose. The reason for this difference in viewpoint and how the whole CB maintains its metabolic integrity when exposed to low glucose is unknown. Here we show that, in the intact superfused rat CB, basal sensory neuronal activity was sustained during glucose deprivation for 29.1 ± 1.2 min, before irreversible failure following a brief period of excitation. Graded increases in the basal discharge induced by reducing the superfusate PO2 led to proportional decreases in the time to the pre-failure excitation during glucose deprivation which was dependent on a complete run-down in glycolysis and a fall in cellular energy status. A similar ability to withstand prolonged glucose deprivation was observed in isolated type I cells. Electron micrographs and immunofluorescence staining of rat CB sections revealed the presence of glycogen granules and the glycogen conversion enzymes glycogen synthase I and glycogen phosphorylase BB, dispersed throughout the type I cell cytoplasm. Furthermore, pharmacological attenuation of glycogenolysis and functional depletion of glycogen both significantly reduced the time to glycolytic run-down by ∼33 and 65%, respectively. These findings suggest that type I cell glycogen metabolism allows for the continuation of glycolysis and the maintenance of CB sensory neuronal output in periods of restricted glucose delivery and this may act as a key protective mechanism for the organ during hypoglycaemia. The ability, or otherwise, to preserve energetic status may thus account for variation in the reported capacity of the CB to sense physiological glucose concentrations and may even underlie its function during pathological states associated with augmented CB discharge.
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Affiliation(s)
- Andrew P Holmes
- School of Clinical and Experimental Medicine, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
| | - Philip J Turner
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford, OX1 3PT, UK
| | - Paul Carter
- School of Clinical and Experimental Medicine, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
| | - Wendy Leadbeater
- School of Clinical and Experimental Medicine, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
| | - Clare J Ray
- School of Clinical and Experimental Medicine, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
| | - David Hauton
- School of Clinical and Experimental Medicine, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
| | - Keith J Buckler
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford, OX1 3PT, UK
| | - Prem Kumar
- School of Clinical and Experimental Medicine, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
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13
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Turner PJ, Buckler KJ. Oxygen and mitochondrial inhibitors modulate both monomeric and heteromeric TASK-1 and TASK-3 channels in mouse carotid body type-1 cells. J Physiol 2013; 591:5977-98. [PMID: 24042502 DOI: 10.1113/jphysiol.2013.262022] [Citation(s) in RCA: 54] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
In rat arterial chemoreceptors, background potassium channels play an important role in maintaining resting membrane potential and promoting depolarization and excitation in response to hypoxia or acidosis. It has been suggested that these channels are a heterodimer of TASK-1 and TASK-3 based on their similarity to heterologously expressed TASK-1/3 fusion proteins. In this study, we sought to confirm the identity of these channels through germline ablation of Task-1 (Kcnk3) and Task-3 (Kcnk9) in mice. Background K-channels were abundant in carotid body type-1 cells from wild-type mice and comparable to those previously described in rat type-1 cells with a main conductance state of 33 pS. This channel was absent from both Task-1(-/-) and Task-3(-/-) cells. In its place we observed a larger (38 pS) K(+)-channel in Task-1(-/-) cells and a smaller (18 pS) K(+)-channel in Task-3(-/-) cells. None of these channels were observed in Task-1(-/-)/Task-3(-/-) double knock-out mice. We therefore conclude that the predominant background K-channel in wild-type mice is a TASK-1/TASK-3 heterodimer, whereas that in Task-1(-/-) mice is TASK-3 and, conversely, that in Task-3(-/-) mice is TASK-1. All three forms of TASK channel in type-1 cells were inhibited by hypoxia, cyanide and the uncoupler FCCP, but the greatest sensitivity was seen in TASK-1 and TASK-1/TASK-3 channels. In summary, the background K-channel in type-1 cells is predominantly a TASK-1/TASK-3 heterodimer. Although both TASK-1 and TASK-3 are able to couple to the oxygen and metabolism sensing pathways present in type-1 cells, channels containing TASK-1 appear to be more sensitive.
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Affiliation(s)
- Philip J Turner
- K. J. Buckler: Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK.
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Slingo ME, Turner PJ, Christian HC, Buckler KJ, Robbins PA. The von Hippel-Lindau Chuvash mutation in mice causes carotid-body hyperplasia and enhanced ventilatory sensitivity to hypoxia. J Appl Physiol (1985) 2013; 116:885-92. [PMID: 24030664 PMCID: PMC3972741 DOI: 10.1152/japplphysiol.00530.2013] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The hypoxia-inducible factor (HIF) family of transcription factors coordinates diverse cellular and systemic responses to hypoxia. Chuvash polycythemia (CP) is an autosomal recessive disorder in humans in which there is impaired oxygen-dependent degradation of HIF, resulting in long-term systemic elevation of HIF levels at normal oxygen tensions. CP patients demonstrate the characteristic features of ventilatory acclimatization to hypoxia, namely, an elevated baseline ventilation and enhanced acute hypoxic ventilatory response (AHVR). We investigated the ventilatory and carotid-body phenotype of a mouse model of CP, using whole-body plethysmography, immunohistochemistry, and electron microscopy. In keeping with studies in humans, CP mice had elevated ventilation in euoxia and a significantly exaggerated AHVR when exposed to 10% oxygen, with or without the addition of 3% carbon dioxide. Carotid-body immunohistochemistry demonstrated marked hyperplasia of the oxygen-sensing type I cells, and the cells themselves appeared enlarged with more prominent nuclei. This hypertrophy was confirmed by electron microscopy, which also revealed that the type I cells contained an increased number of mitochondria, enlarged dense-cored vesicles, and markedly expanded rough endoplasmic reticulum. The morphological and ultrastructural changes seen in the CP mouse carotid body are strikingly similar to those observed in animals exposed to chronic hypoxia. Our study demonstrates that the HIF pathway plays a major role, not only in regulating both euoxic ventilatory control and the sensitivity of the response to hypoxia, but also in determining the morphology of the carotid body.
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Affiliation(s)
- Mary E Slingo
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
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15
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Bishop T, Talbot NP, Turner PJ, Nicholls LG, Pascual A, Hodson EJ, Douglas G, Fielding JW, Smith TG, Demetriades M, Schofield CJ, Robbins PA, Pugh CW, Buckler KJ, Ratcliffe PJ. Carotid body hyperplasia and enhanced ventilatory responses to hypoxia in mice with heterozygous deficiency of PHD2. J Physiol 2013; 591:3565-77. [PMID: 23690557 PMCID: PMC3731614 DOI: 10.1113/jphysiol.2012.247254] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Oxygen-dependent prolyl hydroxylation of hypoxia-inducible factor (HIF) by a set of closely related prolyl hydroxylase domain enzymes (PHD1, 2 and 3) regulates a range of transcriptional responses to hypoxia. This raises important questions about the role of these oxygen-sensing enzymes in integrative physiology. We investigated the effect of both genetic deficiency and pharmacological inhibition on the change in ventilation in response to acute hypoxic stimulation in mice. Mice exposed to chronic hypoxia for 7 days manifest an exaggerated hypoxic ventilatory response (HVR) (10.8 ± 0.3 versus 4.1 ± 0.7 ml min−1 g−1 in controls; P < 0.01). HVR was similarly exaggerated in PHD2+/− animals compared to littermate controls (8.4 ± 0.7 versus 5.0 ± 0.8 ml min−1 g−1; P < 0.01). Carotid body volume increased (0.0025 ± 0.00017 in PHD2+/− animals versus 0.0015 ± 0.00019 mm3 in controls; P < 0.01). In contrast, HVR in PHD1−/− and PHD3−/− mice was similar to littermate controls. Acute exposure to a small molecule PHD inhibitor (PHI) (2-(1-chloro-4-hydroxyisoquinoline-3-carboxamido) acetic acid) did not mimic the ventilatory response to hypoxia. Further, 7 day administration of the PHI induced only modest increases in HVR and carotid body cell proliferation, despite marked stimulation of erythropoiesis. This was in contrast with chronic hypoxia, which elicited both exaggerated HVR and cellular proliferation. The findings demonstrate that PHD enzymes modulate ventilatory sensitivity to hypoxia and identify PHD2 as the most important enzyme in this response. They also reveal differences between genetic inactivation of PHDs, responses to hypoxia and responses to a pharmacological inhibitor, demonstrating the need for caution in predicting the effects of therapeutic modulation of the HIF hydroxylase system on different physiological responses.
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Affiliation(s)
- Tammie Bishop
- Wellcome Trust Centre for Human Genetics, Roosevelt Drive, University of Oxford, Oxford OX3 7BN, UK.
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16
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Abstract
The mechanism of oxygen sensing in arterial chemoreceptors is unknown but has often been linked to mitochondrial function. A common criticism of this hypothesis is that mitochondrial function is insensitive to physiological levels of hypoxia. Here we investigate the effects of hypoxia (down to 0.5% O2) on mitochondrial function in neonatal rat type-1 cells. The oxygen sensitivity of mitochondrial [NADH] was assessed by monitoring autofluorescence and increased in hypoxia with a P50 of 15 mm Hg (1 mm Hg = 133.3 Pa) in normal Tyrode or 46 mm Hg in Ca(2+)-free Tyrode. Hypoxia also depolarised mitochondrial membrane potential (m, measured using rhodamine 123) with a P50 of 3.1, 3.3 and 2.8 mm Hg in normal Tyrode, Ca(2+)-free Tyrode and Tyrode containing the Ca(2+) channel antagonist Ni(2+), respectively. In the presence of oligomycin and low carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP; 75 nm) m is maintained by electron transport working against an artificial proton leak. Under these conditions hypoxia depolarised m/inhibited electron transport with a P50 of 5.4 mm Hg. The effects of hypoxia upon cytochrome oxidase activity were investigated using rotenone, myxothiazol, antimycin A, oligomycin, ascorbate and the electron donor tetramethyl-p-phenylenediamine. Under these conditions m is maintained by complex IV activity alone. Hypoxia inhibited cytochrome oxidase activity (depolarised m) with a P50 of 2.6 mm Hg. In contrast hypoxia had little or no effect upon NADH (P50 = 0.3 mm Hg), electron transport or cytochrome oxidase activity in sympathetic neurons. In summary, type-1 cell mitochondria display extraordinary oxygen sensitivity commensurate with a role in oxygen sensing. The reasons for this highly unusual behaviour are as yet unexplained.
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Affiliation(s)
- Keith J Buckler
- Department of Physiology, Anatomy & Genetics, Parks Road, Oxford, UK.
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Li D, Nikiforova N, Lu CJ, Wannop K, McMenamin M, Lee CW, Buckler KJ, Paterson DJ. Targeted neuronal nitric oxide synthase transgene delivery into stellate neurons reverses impaired intracellular calcium transients in prehypertensive rats. Hypertension 2012; 61:202-7. [PMID: 23172925 DOI: 10.1161/hypertensionaha.111.00105] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
Hypertension is associated with the early onset of cardiac sympathetic hyperresponsiveness and enhanced intracellular Ca(2+) concentration [Ca(2+)](i) in sympathetic neurons from both prehypertensive and hypertensive, spontaneously hypertensive rats (SHRs). Oxidative stress is a hallmark of hypertension, therefore, we tested the hypothesis that the inhibitory action of the nitric oxide-cGMP pathway on [Ca(2+)](i) transients is impaired in cardiac sympathetic neurons from the SHR. Stellate ganglia were isolated from young prehypertensive SHRs and age-matched normotensive Wistar-Kyoto rats. [Ca(2+)](i) was measured by ratiometric fluorescence imaging. Neurons from the prehypertensive SHR ganglia had a significantly higher depolarization evoked [Ca(2+)](i) transient that was also associated with decreased expression of neuronal nitric oxide synthase (nNOS), β1 subunit of soluble guanylate cyclase and cGMP when compared with the Wistar-Kyoto rat ganglia. Soluble guanylate cyclase inhibition or nNOS inhibition increased [Ca(2+)](i) in the Wistar-Kyoto rats but had no effect in SHR neurons. A nitric oxide donor decreased [Ca(2+)](i) in both sets of neurons, although this was markedly less in the SHR. A novel noradrenergic cell specific vector (Ad.PRSx8-nNOS/Cherry) or its control vector (Ad.PRSx8-Cherry) was expressed in sympathetic neurons. In the SHR, Ad.PRSx8-nNOS/Cherry-treated neurons had a significantly reduced peak [Ca(2+)](i) transient that was associated with increased tissue levels of nNOS protein and cGMP concentration compared with gene transfer of Ad.PRSx8-Cherry alone. nNOS inhibition significantly increased [Ca(2+)](i) after Ad.PRSx8-nNOS/Cherry expression. We conclude that artificial upregulation of stellate sympathetic nNOS via targeted gene transfer can directly attenuate intracellular Ca(2+) and may provide a novel method for decreasing enhanced cardiac sympathetic neurotransmission.
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Affiliation(s)
- Dan Li
- Department of Physiology, Anatomy and Genetics, University of Oxford, Burdon Sanderson Cardiac Science Centre and BHF Centre of Research Excellence, Oxford, United Kingdom.
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Abstract
A number of tandem P-domain K(+)- channels (K(2)P) generate background K(+)-currents similar to those found in enteroreceptors that sense a diverse range of physiological stimuli including blood pH, carbon dioxide, oxygen, potassium and glucose. This review presents an overview of the properties of both cloned K(2)P tandem-P-domain K-channels and the endogenous chemosensitive background K-currents found in central chemoreceptors, peripheral chemoreceptors, the adrenal gland and the hypothalamus. Although the identity of many of these endogenous channels has yet to be confirmed they show striking similarities to a number of K(2)P channels especially those of the TASK subgroup. Moreover these channels seem often (albeit not exclusively) to be involved in pH and nutrient/metabolic sensing.
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Affiliation(s)
- Keith J Buckler
- Department of Physiology Anatomy and Genetics, University of Oxford, Parks Road, Oxford, OX1 3PT, UK.
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Abstract
Nociceptive neurons play an important role in ischemia by sensing and transmitting information to the CNS and by secreting peptides and nitric oxide, which can have local effects. While these responses are probably primarily mediated by acid sensing channels, other events occurring in ischemia may also influence neuron function. In this study, we have investigated the effects of anoxia and anoxic aglycemia on Ca2+ regulation in sensory neurons from rat dorsal root ganglia. Anoxia increased [Ca2+]i by evoking Ca2+ release from two distinct internal stores one sensitive to carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) and one sensitive to caffeine, cyclopiazonic acid (CPA), and ryanodine [assumed to be the endoplasmic reticulum (ER)]. Anoxia also promoted progressive decline in ER Ca2+ content. Despite partially depolarizing mitochondria, anoxia had relatively little effect on mitochondrial Ca2+ uptake when neurons were depolarized but substantially delayed mitochondrial Ca2+ release and subsequent Ca2+ clearance from the cytosol on repolarization. Anoxia also reduced both sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) activity and Ca2+ extrusion [probably via plasma membrane Ca2+-ATPase (PMCA)]. Thus anoxia has multiple effects on [Ca2+]i homeostasis in sensory neurons involving internal stores, mitochondrial buffering, and Ca2+ pumps. Under conditions of anoxic aglycemia, there was a biphasic and more profound elevation of [Ca2+]i, which was associated with complete ER Ca2+ store emptying and progressive, and eventually complete, inhibition of Ca2+ clearance by PMCA and SERCA. These data clearly show that loss of oxygen, and exhaustion of glycolytic substrates, can profoundly affect many aspects of cell Ca2+ regulation, and this may play an important role in modulating neuronal responses to ischemia.
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Affiliation(s)
- Michael Henrich
- Department of Physiology, Anatomy and Genetics, Oxford, United Kingdom
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20
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Abstract
Sensory neurons can detect ischemia and transmit pain from various organs. Whereas the primary stimulus in ischemia is assumed to be acidosis, little is known about how the inevitable metabolic challenge influences neuron function. In this study we have investigated the effects of anoxia, aglycemia, and acidosis upon intracellular Mg(2+) concentration [Mg(2+)](i) and intracellular pH (pH(i)) in isolated sensory neurons. Anoxia, anoxic aglycemia, and acidosis all caused a rise in [Mg(2+)](i) and a fall in pH(i). The rise in [Mg(2+)](i) in response to acidosis appears to be due to H(+) competing for intracellular Mg(2+) binding sites. The effects of anoxia and aglycemia were mimicked by metabolic inhibition and, in a dorsal root ganglia (DRG)-derived cell line, the rise in [Mg(2+)](i) during metabolic blockade was closely correlated with fall in intracellular ATP concentration ([ATP](i)). Increase in [Mg(2+)](i) during anoxia and aglycemia were therefore assumed to be due to MgATP hydrolysis. Even brief periods of anoxia (<3 min) resulted in rapid internal acidosis and a rise in [Mg(2+)](i) equivalent to a decline in MgATP levels of 15-20%. With more prolonged anoxia (20 min) MgATP depletion is estimated to be around 40%. With anoxic aglycemia, the [Mg(2+)](i) rise occurs in two phases: the first beginning almost immediately and the second after an 8- to 10-min delay. Within 20 min of anoxic aglycemia [Mg(2+)](i) was comparable to that observed following complete metabolic inhibition (dinitrophenol + 2-deoxyglucose, DNP + 2-DOG) indicating a near total loss of MgATP. The consequences of these events therefore need to be considered in the context of sensory neuron function in ischemia.
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Affiliation(s)
- Michael Henrich
- Department of Physiology, Anatomy and Genetics, Oxford OX1 3PT, United Kingdom
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Wang L, Henrich M, Buckler KJ, McMenamin M, Mee CJ, Sattelle DB, Paterson DJ. Neuronal nitric oxide synthase gene transfer decreases [Ca2+]i in cardiac sympathetic neurons. J Mol Cell Cardiol 2007; 43:717-25. [PMID: 17961591 DOI: 10.1016/j.yjmcc.2007.09.005] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/13/2007] [Revised: 08/30/2007] [Accepted: 09/06/2007] [Indexed: 01/28/2023]
Abstract
Gene transfer of neuronal nitric oxide synthase (nNOS) can decrease cardiac sympathetic outflow and facilitate parasympathetic neurotransmission. The precise pathway responsible for nitric oxide (NO) mediated inhibition of sympathetic neurotransmission is not known, but may be related to NO-cGMP activation of cGMP-stimulated phosphodiesterase (PDE2) that enhances the breakdown of cAMP to deactivate protein kinase A (PKA), resulting in a decrease in Ca(2+) influx mediated exocytosis of the neurotransmitter. We investigated depolarization evoked Ca(2+) influx in nNOS gene transduced sympathetic neurons from stellate ganglia with a noradrenergic cell specific vector (Ad.PRS-nNOS or empty vector), and examined how nNOS gene transfer affected cAMP and cGMP levels in these neurons. We found that targeting nNOS into these sympathetic neurons reduced amplitudes of voltage activated Ca(2+) transients by 44%. nNOS specific inhibition by N-[(4S)-4-Amino-5-[(2-aminoetyl](amino] pentyl]-N'-nitroguanidine (AAAN) reversed this response. nNOS gene transfer also increased intracellular cGMP (47%) and decreased cAMP (29%). A PDE2 specific inhibitor Bay60-7557 reversed the reduction in cAMP caused by Ad.PRS-nNOS. These results suggest that neuronal NO modulates cGMP and PDE2 to regulate voltage gated intracellular Ca(2+) transients in sympathetic neurons. Therefore, we propose this as a possible key step involved in NO decreasing cardiac sympathetic neurotransmission.
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Affiliation(s)
- Lijun Wang
- Burdon Sanderson Cardiac Science Centre, Department of Physiology, Anatomy and Genetics, Sherrington Building, Parks Road, Oxford, UK
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Varas R, Wyatt CN, Buckler KJ. Modulation of TASK-like background potassium channels in rat arterial chemoreceptor cells by intracellular ATP and other nucleotides. J Physiol 2007; 583:521-36. [PMID: 17615104 PMCID: PMC2156202 DOI: 10.1113/jphysiol.2007.135657] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
The carotid body's physiological role is to sense arterial oxygen, CO(2) and pH. It is however, also powerfully excited by inhibitors of oxidative phosphorylation. This latter observation is the cornerstone of the mitochondrial hypothesis which proposes that oxygen is sensed through changes in energy metabolism. All of these stimuli act in a similar manner, i.e. by inhibiting a background TASK-like potassium channel (K(B)) they induce membrane depolarization and thus neurosecretion. In this study we have evaluated the role of ATP in modulating K(B) channels. We find that K(B) channels are strongly activated by MgATP (but not ATP(4)(-)) within the physiological range (K(1/2) = 2.3 mm). This effect was mimicked by other Mg-nucleotides including GTP, UTP, AMP-PCP and ATP-gamma-S, but not by PP(i) or AMP, suggesting that channel activity is regulated by a Mg-nucleotide sensor. Channel activation by MgATP was not antagonized by either 1 mm AMP or 500 microm ADP. Thus MgATP is probably the principal nucleotide regulating channel activity in the intact cell. We therefore investigated the effects of metabolic inhibition upon both [Mg(2+)](i), as an index of MgATP depletion, and channel activity in cell-attached patches. The extent of increase in [Mg(2+)](i) (and thus MgATP depletion) in response to inhibition of oxidative phosphorylation were consistent with a decline in [MgATP](i) playing a prominent role in mediating inhibition of K(B) channel activity, and the response of arterial chemoreceptors to metabolic compromise.
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Affiliation(s)
- Rodrigo Varas
- Department of Physiology Anatomy and Genetics, Sherrington Building, Parks Road, Oxford OX1 3PT, UK
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Buckler KJ. TASK-like potassium channels and oxygen sensing in the carotid body. Respir Physiol Neurobiol 2007; 157:55-64. [PMID: 17416212 DOI: 10.1016/j.resp.2007.02.013] [Citation(s) in RCA: 86] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2006] [Revised: 02/12/2007] [Accepted: 02/13/2007] [Indexed: 11/17/2022]
Abstract
Chemosensing by type-1 cells of the carotid body involves a series of events which culminate in the calcium-dependent secretion of neurotransmitter substances which then excite afferent nerves. This response is mediated via membrane depolarisation and voltage-gated calcium entry. Studies utilising isolated cells indicates that the membrane depolarisation in response to hypoxia, and acidosis, appears to be primarily mediated via the inhibition of a background K(+)-current. The pharmacological and biophysical characteristics of these channels suggest that they are probably closely related to the TASK subfamily of tandem-P-domain K(+)-channels. Indeed they show greatest similarity to TASK-1 and -3. In addition to being sensitive to hypoxia and acidosis, the background K(+)-channels of the type-1 cell are also remarkably sensitive to inhibition of mitochondrial energy metabolism. Metabolic poisons are known potent stimulants of the carotid body and cause membrane depolarisation of type-1 cells. In the presence of metabolic inhibitors hypoxic sensitivity is lost suggesting that oxygen sensing may itself be mediated via depression of mitochondrial energy production. Thus these TASK-like background channels play a central role in mediating the chemotransduction of several different stimuli within the type-1 cell. The mechanisms by which metabolic/oxygen sensitivity might be conferred upon these channels are briefly discussed.
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Affiliation(s)
- Keith J Buckler
- Department of Physiology Anatomy and Genetics, Sherrington Building, Parks Road, Oxford, UK.
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24
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Varas R, Buckler KJ. Regulation of a TASK-like potassium channel in rat carotid body type I cells by ATP. Adv Exp Med Biol 2006; 580:167-72; discussion 351-9. [PMID: 16683714 DOI: 10.1007/0-387-31311-7_25] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Affiliation(s)
- Rodrigo Varas
- University Laboratory of Physiology, Parks Road, Oxford, UK
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25
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Buckler KJ, Williams BA, Orozco RV, Wyatt CN. The role of TASK-like K+ channels in oxygen sensing in the carotid body. Novartis Found Symp 2006; 272:73-85; discussion 85-94, 131-40. [PMID: 16686430] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
The carotid body plays an important role in initiating protective responses to hypoxemia. The primary oxygen sensing cells are the glomus or type 1 cells. Hypoxia evokes the secretion of neurotransmitters from these cells which then excite afferent nerves. This response is mediated via membrane depolarization and voltage-gated Ca2+ entry. Studies from this laboratory have revealed that membrane depolarization in response to hypoxia is primarily the result of inhibition of background K+ channels which show strong similarities to the acid sensitive tandem-P-domain K+ channels TASK-1 and TASK-3. The background K+ channels of type-1 cells are also very sensitive to inhibition of mitochondrial energy metabolism and, in excised patches, appear to be directly activated by ATP. Thus these TASK-like background channels would appear to confer the ability to sense changes in oxygen levels, pH and metabolism upon the type 1 cell. The key issue of whether the effects of hypoxia are mediated through changes in metabolism remains unanswered but the effects of inhibition of mitochondrial energy metabolism and of hypoxia upon background K+ channels is mutually exclusive suggesting that there is a close link between metabolism and oxygen sensing in the type 1 cell.
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Affiliation(s)
- Keith J Buckler
- University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK
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Affiliation(s)
- E Kenneth Weir
- Department of Medicine, Minneapolis Veterans Affairs Medical Center and University of Minnesota, Minneapolis 55417, USA.
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Abstract
TREK-1 is a member of the two-pore domain potassium (K(2P)) channel family that is mechano-, heat, pH, voltage and lipid sensitive. It is highly expressed in the central nervous system and probably encodes one of the previously described arachidonic acid-activated K(+) channels. Polyunsaturated fatty acids and lysophospholipids protect the brain against global ischaemia. Since both lipids are openers of TREK-1, it has been suggested that this K(2P) channel is directly involved in neuroprotection. Recently, however, this view has been challenged by a report claiming that TREK-1 and its activation by arachidonic acid is inhibited by hypoxia. In the present study, we demonstrate that the bubbling of saline with gases results in the loss of arachidonic acid from solution. Using experimental conditions which obviate this experimental artefact we demonstrate that TREK-1 is resistant to hypoxia and is strongly activated by arachidonic acid even at low P(O(2)) (< 4 Torr). Furthermore, hypoxia fails to affect basal as well as 2,4,6-trinitrophenol- and acid-stimulated TREK-1 currents. These data are supportive for a possible role of TREK-1 in ischaemic neuroprotection and in cell signalling via arachidonic acid.
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK.
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29
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Abstract
Inhibitors of mitochondrial energy metabolism have long been known to be potent stimulants of the carotid body, yet their mechanism of action remains obscure. We have therefore investigated the effects of rotenone, myxothiazol, antimycin A, cyanide (CN(-)) and oligomycin on isolated carotid body type I cells. All five compounds caused a rapid rise in intracellular Ca(2+), which was inhibited on removal of extracellular Ca(2+). Under current clamp conditions rotenone and CN(-) caused a rapid membrane depolarization and elevation of [Ca(2+)](i). Voltage clamping cells to -70 mV substantially attenuated this rise in [Ca(2+)](i). Rotenone, cyanide, myxothiazol and oligomycin significantly inhibited resting background K(+) currents. Thus rotenone, myxothiazol, cyanide and oligomycin mimic the effects of hypoxia in that they all inhibit background K(+) current leading to membrane depolarization and voltage-gated calcium entry. Hypoxia, however, failed to have any additional effect upon membrane currents in the presence of CN(-) or rotenone or the mitochondrial uncoupler p-trifluoromethoxyphenyl hydrazone (FCCP). Thus not only do mitochondrial inhibitors mimic the effects of hypoxia, but they also abolish oxygen sensitivity. These observations suggest that there is a close link between oxygen sensing and mitochondrial function in type I cells. Mechanisms that could account for this link and the actions of mitochondrial inhibitors are discussed.
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Affiliation(s)
- C N Wyatt
- University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK
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Williams BA, Buckler KJ. Biophysical properties and metabolic regulation of a TASK-like potassium channel in rat carotid body type 1 cells. Am J Physiol Lung Cell Mol Physiol 2003; 286:L221-30. [PMID: 14504065 DOI: 10.1152/ajplung.00010.2003] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The single channel properties of TASK-like oxygen-sensitive potassium channels were studied in rat carotid body type 1 cells. We observed channels with rapid bursting kinetics, active at resting membrane potentials. These channels were highly potassium selective with a slope conductance of 14-16 pS, values similar to those reported for TASK-1. In the absence of extracellular divalent cations, however, single channel conductance increased to 28 pS in a manner similar to that reported for TASK-3. After patch excision, channel activity ran down rapidly. Channel activity in inside-out patches was markedly increased by 2 and 5 mM ATP and by 2 mM ADP but not by 100 microM ADP or 1 mM AMP. In cell-attached patches, both cyanide and 2,4-dinitrophenol strongly inhibited channel activity. We conclude that 1) whilst the properties of this channel are consistent with it being a TASK-like potassium channel they do not precisely conform to those of either TASK-1 or TASK-3, 2) channel activity is highly dependent on cytosolic factors including ATP, and 3) changes in energy metabolism may play a role in regulating the activity of these background K+ channels.
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Tsai KL, Vaughan-Jones RD, Buckler KJ. A dual acid-influx transport system in the carotid body type I cell. Acid influx in carotid body type I cells. Adv Exp Med Biol 2000; 475:723-32. [PMID: 10849714 DOI: 10.1007/0-306-46825-5_72] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/16/2023]
Affiliation(s)
- K L Tsai
- University Laboratory of Physiology, Oxford, UK
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Williams BA, Buckler KJ. Identification of an oxygen-sensitive potassium channel in neonatal rat carotid body type I cells. Adv Exp Med Biol 2000; 475:419-24. [PMID: 10849682 DOI: 10.1007/0-306-46825-5_40] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/16/2023]
Affiliation(s)
- B A Williams
- University Laboratory of Physiology, Oxford, U.K
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Dasso LL, Buckler KJ, Vaughan-Jones RD. Interactions between hypoxia and hypercapnic acidosis on calcium signaling in carotid body type I cells. Am J Physiol Lung Cell Mol Physiol 2000; 279:L36-42. [PMID: 10893200 DOI: 10.1152/ajplung.2000.279.1.l36] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The effects of hypercapnic acidosis and hypoxia on intracellular Ca(2+) concentration ([Ca(2+)](i)) were determined with Indo 1 in enzymatically isolated single type I cells from neonatal rat carotid bodies. Type I cells responded to graded hypoxic stimuli with graded [Ca(2+)](i) rises. The percentage of cells responding was also dependent on the severity of the hypoxic stimulus. Raising CO(2) from 5 to 10 or 20% elicited a significant increase in [Ca(2+)](i) in the same cells as those that responded to hypoxia. Thus both stimuli can be sensed by each individual cell. When combinations of hypoxic and acidic stimuli were given simultaneously, the responses were invariably greater than the response to either stimulus given alone. Indeed, in most cases, the response to hypercapnia was slightly potentiated by hypoxia. These data provide the first evidence that the classic synergy between hypoxic and hypercapnic stimuli observed in the intact carotid body may, in part, be an inherent property of the type I cell.
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Affiliation(s)
- L L Dasso
- University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom
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Buckler KJ, Williams BA, Honore E. An oxygen-, acid- and anaesthetic-sensitive TASK-like background potassium channel in rat arterial chemoreceptor cells. J Physiol 2000; 525 Pt 1:135-42. [PMID: 10811732 PMCID: PMC2269923 DOI: 10.1111/j.1469-7793.2000.00135.x] [Citation(s) in RCA: 333] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
The biophysical and pharmacological properties of an oxygen-sensitive background K+ current in rat carotid body type-I cells were investigated and compared with those of recently cloned two pore domain K+ channels. Under symmetrical K+ conditions the oxygen-sensitive whole cell K+ current had a linear dependence on voltage indicating a lack of intrinsic voltage sensitivity. Single channel recordings identified a K+ channel, open at resting membrane potentials, that was inhibited by hypoxia. This channel had a single channel conductance of 14 pS, flickery kinetics and showed little voltage sensitivity except at extreme positive potentials. Oxygen-sensitive current was inhibited by 10 mM barium (57% inhibition), 200 microM zinc (53% inhibition), 200 microM bupivacaine (55% inhibition) and 1 mM quinidine (105 % inhibition). The general anaesthetic halothane (1.5%) increased the oxygen-sensitive K+ current (by 176%). Halothane (3 mM) also stimulated single channel activity in inside-out patches (by 240%). Chloroform had no effect on background K+ channel activity. Acidosis (pH 6.4) inhibited the oxygen-sensitive background K+ current (by 56%) and depolarised type-I cells. The pharmacological and biophysical properties of the background K+ channel are, therefore, analogous to those of the cloned channel TASK-1. Using in situ hybridisation TASK-1 mRNA was found to be expressed in type-I cells. We conclude that the oxygen- and acid-sensitive background K+ channel of carotid body type-I cells is likely to be an endogenous TASK-1-like channel.
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK.
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Abstract
One model of oxygen sensing by the carotid body is that hypoxia depolarises type 1 cells leading to voltage-gated calcium entry and the secretion of neurotransmitters which then excite afferent nerves. This paper revues the mechanisms responsible for the membrane depolarisation in response to hypoxia. It concludes that depolarisation is caused not through the inhibition of calcium activated or delayed rectifier K+-channels but through the inhibition of an entirely new type of background K+-channel. This channel lacks sensitivity to the classical K+-channel inhibitors TEA and 4-AP. New evidence does however reveal that background K+-channels in the type 1 cell can be inhibited by Ba2+ and that Ba2+ depolarises isolated type 1 cells. Intriguingly, Ba2+ is the only K+-channel inhibitor thus far reported to stimulate the carotid body. These studies therefore support the hypothesis that depolarisation of the type 1 cell is an integral part of the oxygen sensing pathway in the carotid body.
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, Oxford, UK.
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Buckler KJ, Vaughan-Jones RD. Effects of mitochondrial uncouplers on intracellular calcium, pH and membrane potential in rat carotid body type I cells. J Physiol 1998; 513 ( Pt 3):819-33. [PMID: 9824720 PMCID: PMC2231310 DOI: 10.1111/j.1469-7793.1998.819ba.x] [Citation(s) in RCA: 142] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
1. Mitochondrial uncouplers are potent stimulants of the carotid body. We have therefore investigated their effects upon isolated type I cells. Both 2,4-dinitrophenol (DNP) and carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) caused an increase in [Ca2+]i which was largely inhibited by removal of extracellular Ca2+ or Na+, or by the addition of 2 mM Ni2+. Methoxyverapamil (D600) also partially inhibited the [Ca2+]i response. 2. In perforated-patch recordings, the rise in [Ca2+]i coincided with membrane depolarization and was greatly reduced by voltage clamping the cell to -70 mV. Uncouplers also inhibited a background K+ current and induced a small inward current. 3. Uncouplers reduced pHi by 0.1 unit. Alkaline media diminished this acidification but had no effect on the [Ca2+]i response. 4. FCCP and DNP also depolarized type I cell mitochondria. The onset of mitochondrial depolarization preceded changes in cell membrane conductance by 3-4 s. 5. We conclude that uncouplers excite the carotid body by inhibiting a background K+ conductance and inducing a small inward current, both of which lead to membrane depolarization and voltage-gated Ca2+ entry. These effects are unlikely to be caused by cell acidification. The inhibition of background K+ current may be related to the uncoupling of oxidative phosphorylation.
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK.
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Abstract
1. Hypoxic stimuli depolarize carotid body type I cells causing voltage-gated calcium influx. This study investigates the cause of this membrane depolarization. Isolated type I cells from neonatal (11-16 day) rat carotid bodies were used in the experiments. 2. Tetraethylammonium (TEA; 10 mM), 1 and 5 mM 4-aminopyridine (4-AP) and 20 nM charybdotoxin all failed to evoke a significant rise in [Ca2+]i. Similarly, in perforated patch whole-cell recordings, a combination of 10 mM TEA and 5 mM 4-AP failed to depolarize type I cells. 3. In type I cells voltage clamped at -70 mV, anoxia evoked a small inward current under control conditions, but had no effect in the absence of pipette and extracellular K+. 4. Anoxia decreased resting membrane conductance from 322 to 131 pS. The anoxia-sensitive current (measured using voltage ramps from -100 to -40 mV) had a reversal potential of -89 mV in 4.5 mM Ko+ and -66 mV in 20 mM Ko+, indicating that this current was carried principally by potassium ions. In contrast, 10 mM TEA + 5 mM 4-AP had little effect on the current-voltage relationship of the cells over the same range. 5. This O2-sensitive K+ conductance showed only mild outward rectification over the range -90 to +30 mV, which could be approximated by the Goldman-Hodgkin-Katz current equation. In addition, there was no time-dependent activation or inactivation of membrane currents elicited by voltage steps in the range -100 to -30 mV. 6. The O2-sensitive K+ conductance was inhibited by graded reductions in PO2 to 40 Torr and below, with a K1/2 of about 12 Torr. 7. The data suggest that hypoxia depolarizes type I cells principally through the inhibition of a small voltage-insensitive resting (or background) K+ conductance, and not through the inhibition of voltage-gated TEA and 4-AP-sensitive K+ channels (e.g. maxi-K or KO2 channels), as has been previously suggested.
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, Oxford, UK.
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Abstract
1. The effects of cholinergic agonists upon intracellular free Ca2+ levels ([Ca2+]i) have been studied in enzymically isolated rat carotid body single type I cells, using indo-1. 2. Acetylcholine (ACh) dose-dependently increased [Ca2+]i in 55% of cells studied (EC50 = 13 microM). These [Ca2+]i rises were partially inhibited by atropine or mecamylamine. 3. Specific nicotinic and muscarinic agonists also elevated [Ca2+]i in a dose-dependent manner (nicotine, EC50 = 15 microM; methacholine, EC50 = 20 microM). 4. While the majority of the ACh-sensitive cells responded to both classes of cholinergic agonist, 29% responded exclusively to nicotinic stimulation and 9% responded exclusively to muscarinic stimulation. 5. In the presence of nicotinic agonists, Ca2+i responses were transient. In the presence of muscarinic agonists, Ca2+i responses consisted of an initial rise, which then declined to a lower plateau level. 6. Nicotinic responses were rapidly abolished in Ca(2+)-free medium, suggesting that they are dependent on Ca2+ influx. 7. The plateau component of the muscarinic-activated response was also abolished in Ca(2+)-free conditions. The rapid initial [Ca2+]i rise, however, could still be evoked after several minutes in Ca(2+)-free medium. Muscarine also increased Mn2+ quenching of intracellular fura-2 fluorescence. These data suggest that the full muscarinic response depends on both Ca2+ release from intracellular stores and Ca2+o influx. 8. The results indicate that, in rat carotid body type I cells, both nicotinic and muscarinic acetylcholine receptors increase [Ca2+]i, but achieve this via different mechanisms. ACh may therefore play a role in carotid body function by modulating Ca2+i in the chemosensory type I cells.
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Affiliation(s)
- L L Dasso
- University Laboratory of Physiology, University of Oxford, UK
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Abstract
It has been suggested that alterations in intracellular Ca2+ homeostasis may be responsible for the development of diabetic cardiomyopathy. We have studied the effects of streptozotocin-induced diabetes on intracellular Ca2+ concentration ([Ca2+]i) in enzymically isolated rat ventricular myocytes. [Ca2+]i was measured using indo 1 or fluo 3. Both diastolic and peak systolic [Ca2+]i were reduced in diabetic compared with normal myocytes (by 52 and 43%, respectively). The decay phase of the systolic [Ca2+]i transient was slower in the diabetic myocyte compared with normal (time constant = 89.6 +/- 3.4 ms, n = 23, normal vs. 105.2 +/- 4.05 ms, n = 20, diabetic; P < 0.01). This led to a significant prolongation of the [Ca2+]i transient duration in the diabetic myocyte. In both normal and diabetic myocytes, increasing the frequency of electrical stimulation decreased peak systolic [Ca2+]i. The relationship between stimulation frequency and normalized peak systolic [Ca2+]i was the same for both normal and diabetic myocytes. We also found that the caffeine-induced Ca2+ release [used as an index of sarcoplasmic reticulum (SR) Ca2+ content] was significantly reduced in diabetic myocytes. These data indicate that SR Ca2+ content is decreased by diabetes. In the presence of thapsigargin (2.5 microM, an inhibitor of SR Ca(2+)-adenosinetriphosphatase), the magnitude and time course of stimulus-evoked [Ca2+]i transients were identical in both groups of myocytes, suggesting that Ca2+ influx and/or efflux across the plasma membrane is not significantly affected in diabetes. We conclude that 1) diabetes is associated with significant alterations in [Ca2+]i homeostasis and 2) the decrease in systolic [Ca2+]i and lengthening of the systolic [Ca2+]i transient result primarily from dysfunction of the SR.
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Affiliation(s)
- D Lagadic-Gossmann
- Laboratoire de Physiologie Cellulaire, Equipe en Restructuration 100, Centre National de la Recherche Scientifique, Université Paris-Sud, Orsay, France
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, Oxford, UK
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Dasso LL, Buckler KJ, Vaughan-Jones RD. Acetylcholine elevates intracellular Ca2+ via muscarinic and nicotinic receptors in rat carotid body type I cells. Adv Exp Med Biol 1996; 410:239-44. [PMID: 9030305 DOI: 10.1007/978-1-4615-5891-0_35] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Affiliation(s)
- L L Dasso
- University Laboratory of Physiology, University of Oxford, United Kingdom
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Abstract
The postulated mechanisms for hypoxic and acidic chemotransduction by type I cells that we have described here are summarized in the diagrams of Fig. 4. Most if not all of these require more complete evaluation and, as we have described, there are obvious points of contention that need to be resolved. Nevertheless, it is apparent that studies of isolated type I cell preparations carried out over the last six years have provided significant advancements in our understanding of chemotransduction in the type I cell. Only when the functioning of these cells has been fully described can we hope to understand the mechanisms underlying the responses of the intact organ to chemostimuli.
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Affiliation(s)
- C Peers
- Institute for Cardiovascular Research, Leeds University, United Kingdom
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Abstract
1. An acid-induced rise in the intracellular calcium concentration ([Ca2+]i) of type I cells is thought to play a vital role in pH/PCO2 chemoreception by the carotid body. In this present study we have investigated the cause of this rise in [Ca2+]i in enzymatically isolated, neonatal rat type I cells. 2. The rise in [Ca2+]i induced by a hypercapnic acidosis was inhibited in Ca(2+)-free media, and by 2 mM Ni2+. Acidosis also increased Mn2+ permeability. The rise in [Ca2+]i is dependent, therefore, upon a Ca2+ influx from the external medium. 3. The acid-induced rise in [Ca2+]i was attenuated by both nicardipine and methoxyverapamil (D600), suggesting a role for L-type Ca2+ channels. 4. Acidosis depolarized type I cells and often (approximately 50% of cells) induced action potentials. These effects coincided with a rise in [Ca2+]i. When membrane depolarization was prevented by a voltage clamp, acidosis failed to evoke a rise in [Ca2+]i. The acid-induced rise in [Ca2+]i is a consequence, therefore, of membrane depolarization. 5. Acidosis decreased the resting membrane conductance of type I cells. The reversal potential of the acid-sensitive current was about -75 mV. 6. A depolarization (30 mM [K+]o)-induced rise in [Ca2+]i was blocked by either the removal of extracellular Ca2+ or the presence of 2 mM Ni2+, and was also substantially inhibited by nicardipine. Under voltage-clamp conditions, [Ca2+]i displayed a bell-shaped dependence on membrane potential. Depolarization raises [Ca2+]i, therefore, through voltage-operated Ca2+ channels. 7. Caffeine (10 mM) induced only a small rise in [Ca2+]i (< 10% of that induced by 30 mM extracellular K+). Ca(2+)-induced Ca2+ release is unlikely, therefore, to contribute greatly to the rise in [Ca2+]i induced by depolarization. 8. Although the replacement of extracellular Na+ with N-methyl-D-glucamine (NMG), but not Li+, inhibited the acid-induced rise in [Ca2+]i, this was due to membrane hyperpolarization and not to the inhibition of Na(+)-Ca2+ exchange or Na(+)-dependent action potentials. 9. The removal of extracellular Na+ (NMG substituted) did not have a significant effect upon the resting [Ca2+]i, and only slowed [Ca2+]i recovery slightly following repolarization from 0 to -60 mV. Therefore, if present, Na(+)-Ca2+ exchange plays only a minor role in [Ca2+]i homeostasis. 10. In summary, in the neonatal rat type I cell, hypercapnic acidosis raises [Ca2+]i through membrane depolarization and voltage-gated Ca2+ entry.
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, University of Oxford
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Abstract
1. We have studied the effects of hypoxia on membrane potential and [Ca2+]i in enzymically isolated type I cells of the neonatal rat carotid body (the principal respiratory O2 chemosensor). Isolated cells were maintained in short term culture (3-36 h) before use. [Ca2+]i was measured using the Ca(2+)-sensitive fluoroprobe indo-1. Indo-1 was loaded into cells using the esterified form indo-1 AM. Membrane potential was measured (and clamped) in single isolated type I cells using the perforated-patch (amphotericin B) whole-cell recording technique. 2. Graded reductions in PO2 from 160 Torr to 38, 19, 8, 5 and 0 Torr induced a graded rise of [Ca2+]i in both single and clumps of type I cells. 3. The rise of [Ca2+]i in response to anoxia was 98% inhibited by removal of external Ca2+ (+1 mM EGTA), indicating the probable involvement of Ca2+ influx from the external medium in mediating the anoxic [Ca2+]i response. 4. The L-type Ca2+ channel antagonist nicardipine (10 microM) inhibited the anoxic [Ca2+]i response by 67%, and the non-selective Ca2+ channel antagonist Ni2+ (2 mM) inhibited the response by 77%. 5. Under voltage recording conditions, anoxia induced a reversible membrane depolarization (or receptor potential) accompanied, in many cases, by trains of action potentials. These electrical events were coincident with a rapid rise of [Ca2+]i. When cells were voltage clamped close to their resting potential (-40 to -60 mV), the [Ca2+]i response to anoxia was greatly reduced and its onset was much slower.(ABSTRACT TRUNCATED AT 250 WORDS)
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Buckler KJ, Vaughan-Jones RD. Role of intracellular pH and [Ca2+]i in acid chemoreception in type-I cells of the carotid body. Adv Exp Med Biol 1994; 360:41-55. [PMID: 7532906 DOI: 10.1007/978-1-4615-2572-1_5] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
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Affiliation(s)
- C Peers
- Department of Pharmacology, Leeds University, UK
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Abstract
We have investigated the effects of acidic stimuli upon [Ca2+]i in isolated carotid body type I cells from the neonatal rat using indo-1 (AM-loaded). Under normocapnic, non-hypoxic conditions (23 mM HCO3-, 5% CO2 in air, pHo = 7.4), the mean [Ca2+]i for single cells was 102 +/- 5.0 nM (SEM, n = 55) with 58% of cells showing sporadic [Ca2+]i fluctuations. A hypercapnic acidosis (increase in CO2 to 10%-20% at constant HCO3-, pHo 7.15-6.85), an isohydric hypercapnia (increase in CO2 to 10% at constant pHo = 7.4) and an isocapnic acidosis (pHo = 7.0, constant CO2) all increased [Ca2+]i in single cells and cell clusters. The averaged [Ca2+]i response to both hypercapnic acidosis and isohydric hypercapnia displayed a rapid rise followed by a secondary decline. The averaged [Ca2+]i response to isocapnic acidosis displayed a slower rise and little secondary decline. The rise of [Ca2+]i in response to all the above stimuli can be attributed to no single factor other than to a fall of pHi. The hypercapnia-induced rise of [Ca2+]i was almost completely abolished in Ca(2+)-free solution, suggesting a role for Ca2+ influx in triggering and/or sustaining the [Ca2+]i response. These results are consistent with a role for type I cell [Ca2+]i in mediating pH/PCO2 chemoreception.
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Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, University of Oxford, UK
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Buckler KJ, Vaughan-Jones RD, Peers C, Lagadic-Gossmann D, Nye PC. The modulation of intracellular pH in carotid body glomus cells by extracellular pH and pCO2. Adv Exp Med Biol 1993; 337:103-9. [PMID: 8109390 DOI: 10.1007/978-1-4615-2966-8_15] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- K J Buckler
- University Laboratory of Physiology, Oxford, England
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Vaughan-Jones RD, Buckler KJ, Peers C, Nye PC. Regulation of intracellular pH in type I cells of the neonatal rat carotid body. Adv Exp Med Biol 1993; 337:251-6. [PMID: 8109408 DOI: 10.1007/978-1-4615-2966-8_35] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
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