51
|
Abstract
Potassium channels display considerable functional diversity. Alternative pre-mRNA splicing represents one of the most powerful post-transcriptional mechanisms to create physiological diversity by generating multiple protein products from a single gene. Due to the modular nature of proteins, alternative splicing can profoundly modify potassium channel structure, function and regulation. Alternative pre-mRNA splicing is exploited by most genes but is particularly prevalent in single gene families as exemplified by the gene (KCNMA1), which encodes large conductance calcium- and voltage-gated potassium (BK) channel alpha-subunits. Importantly, alternative pre-mRNA splicing is kept under spatiotemporal control by circulating hormones and cellular activity, as well as being differentially modified during development and in different tissues. While the sequencing of numerous genomes has further demonstrated the importance of splicing in generating diversity from a limited genome size, a major challenge is to define splice variants that are expressed in tissues and their functional role. Here we describe strategies and protocols to experimentally define and isolate splice variant mRNA transcripts in multiple tissues and provide a platform to characterise the effect of splice variants on channel function and physiology.
Collapse
Affiliation(s)
- Lie Chen
- Centre for Integrative Physiology, College of Medicine and Veterinary Medicine, University of Edinburgh, UK
| | | |
Collapse
|
52
|
Differential expression of BK channel isoforms and β-subunits in rat neuro-vascular tissues. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2009; 1788:380-9. [DOI: 10.1016/j.bbamem.2008.10.001] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/29/2008] [Revised: 09/18/2008] [Accepted: 10/06/2008] [Indexed: 12/30/2022]
|
53
|
Palmitoylation gates phosphorylation-dependent regulation of BK potassium channels. Proc Natl Acad Sci U S A 2008; 105:21006-11. [PMID: 19098106 DOI: 10.1073/pnas.0806700106] [Citation(s) in RCA: 94] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Large conductance calcium- and voltage-gated potassium (BK) channels are important regulators of physiological homeostasis and their function is potently modulated by protein kinase A (PKA) phosphorylation. PKA regulates the channel through phosphorylation of residues within the intracellular C terminus of the pore-forming alpha-subunits. However, the molecular mechanism(s) by which phosphorylation of the alpha-subunit effects changes in channel activity are unknown. Inhibition of BK channels by PKA depends on phosphorylation of only a single alpha-subunit in the channel tetramer containing an alternatively spliced insert (STREX) suggesting that phosphorylation results in major conformational rearrangements of the C terminus. Here, we define the mechanism of PKA inhibition of BK channels and demonstrate that this regulation is conditional on the palmitoylation status of the channel. We show that the cytosolic C terminus of the STREX BK channel uniquely interacts with the plasma membrane via palmitoylation of evolutionarily conserved cysteine residues in the STREX insert. PKA phosphorylation of the serine residue immediately upstream of the conserved palmitoylated cysteine residues within STREX dissociates the C terminus from the plasma membrane, inhibiting STREX channel activity. Abolition of STREX palmitoylation by site-directed mutagenesis or pharmacological inhibition of palmitoyl transferases prevents PKA-mediated inhibition of BK channels. Thus, palmitoylation gates BK channel regulation by PKA phosphorylation. Palmitoylation and phosphorylation are both dynamically regulated; thus, cross-talk between these 2 major posttranslational signaling cascades provides a mechanism for conditional regulation of BK channels. Interplay of these distinct signaling cascades has important implications for the dynamic regulation of BK channels and physiological homeostasis.
Collapse
|
54
|
Saleem F, Rowe ICM, Shipston MJ. Characterization of BK channel splice variants using membrane potential dyes. Br J Pharmacol 2008; 156:143-52. [PMID: 19068078 DOI: 10.1111/j.1476-5381.2008.00011.x] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
BACKGROUND AND PURPOSE Large conductance calcium- and voltage-activated potassium (BK) channels are encoded by a single gene that displays extensive pre-mRNA splicing. Here we exploited a membrane potential assay to investigate the sensitivity of different BK splice variants to elevations in intracellular free calcium and their inhibition by the BK channel blocker paxilline. EXPERIMENTAL APPROACH Murine BK channel splice variants were expressed in human embryonic kidney 293 cells and their properties analysed in response to ionomycin-induced calcium influx in both fluorescent membrane potential (fluorescent-imaging plate reader) and patch clamp electrophysiological assays. The dose-dependent inhibition of distinct splice variants by the BK channel-specific blocker paxilline was also investigated. KEY RESULTS Ionomycin-induced calcium influx induced a robust hyperpolarization of human embryonic kidney 293 cells expressing distinct BK channel splice variants: stress regulated exon (STREX), e22 and ZERO. Splice variant expression resulted in membrane hyperpolarization that displayed a rank order of potency in response to calcium influx of STREX > e22 > ZERO. The BK channel inhibitor paxilline exhibited very similar potency on all three splice variants with IC(50)s in membrane potential assays of 0.35 +/- 0.04, 0.37 +/- 0.03 and 0.70 +/- 0.02 micromol x L(-1) for STREX, ZERO and e22 respectively. CONCLUSIONS AND IMPLICATIONS BK channel splice variants can be rapidly discriminated using membrane potential based assays, based on their sensitivity to calcium. BK channel splice variants are inhibited by the specific blocker paxilline with similar IC(50)s. Thus, paxilline may be used in functional assays to inhibit BK channel function, irrespective of the variant expressed.
Collapse
Affiliation(s)
- F Saleem
- Centre for Integrative Physiology, College of Medicine and Veterinary Medicine, University of Edinburgh, Edinburgh, UK
| | | | | |
Collapse
|
55
|
Rollo CD. Dopamine and Aging: Intersecting Facets. Neurochem Res 2008; 34:601-29. [DOI: 10.1007/s11064-008-9858-7] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2008] [Accepted: 07/03/2008] [Indexed: 10/21/2022]
|
56
|
Heitzmann D, Warth R. Physiology and pathophysiology of potassium channels in gastrointestinal epithelia. Physiol Rev 2008; 88:1119-82. [PMID: 18626068 DOI: 10.1152/physrev.00020.2007] [Citation(s) in RCA: 104] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.
Collapse
Affiliation(s)
- Dirk Heitzmann
- Institute of Physiology and Clinic and Policlinic for Internal Medicine II, Regensburg, Germany
| | | |
Collapse
|
57
|
Zhao H, Sokabe M. Tuning the mechanosensitivity of a BK channel by changing the linker length. Cell Res 2008; 18:871-8. [DOI: 10.1038/cr.2008.88] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
|
58
|
Yan J, Olsen JV, Park KS, Li W, Bildl W, Schulte U, Aldrich RW, Fakler B, Trimmer JS. Profiling the phospho-status of the BKCa channel alpha subunit in rat brain reveals unexpected patterns and complexity. Mol Cell Proteomics 2008; 7:2188-98. [PMID: 18573811 DOI: 10.1074/mcp.m800063-mcp200] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Molecular diversity of ion channel structure and function underlies variability in electrical signaling in nerve, muscle, and non-excitable cells. Protein phosphorylation and alternative splicing of pre-mRNA are two important mechanisms to generate structural and functional diversity of ion channels. However, systematic mass spectrometric analyses of in vivo phosphorylation and splice variants of ion channels in native tissues are largely lacking. Mammalian large-conductance calcium-activated potassium (BK(Ca)) channels are tetramers of alpha subunits (BKalpha) either alone or together with beta subunits, exhibit exceptionally large single channel conductance, and are dually activated by membrane depolarization and intracellular Ca(2+). The cytoplasmic C terminus of BKalpha is subjected to extensive pre-mRNA splicing and, as predicted by several algorithms, offers numerous phospho-acceptor amino acids. Here we use nanoflow liquid chromatography tandem mass spectrometry on BK(Ca) channels affinity-purified from rat brain to analyze in vivo BKalpha phosphorylation and splicing. We found 7 splice variations and identified as many as 30 Ser/Thr in vivo phosphorylation sites; most of which were not predicted by commonly used algorithms. Of the identified phosphosites 23 are located in the C terminus, four were found on splice insertions. Electrophysiological analyses of phospho- and dephosphomimetic mutants transiently expressed in HEK-293 cells suggest that phosphorylation of BKalpha differentially modulates the voltage- and Ca(2+)-dependence of channel activation. These results demonstrate that the pore-forming subunit of BK(Ca) channels is extensively phosphorylated in the mammalian brain providing a molecular basis for the regulation of firing pattern and excitability through dynamic modification of BKalpha structure and function.
Collapse
Affiliation(s)
- Jiusheng Yan
- Department of Neurobiology, Physiology and Behavior, College of Biological Sciences, University of California, Davis, California 95616, USA
| | | | | | | | | | | | | | | | | |
Collapse
|
59
|
Skalska J, Piwońska M, Wyroba E, Surmacz L, Wieczorek R, Koszela-Piotrowska I, Zielińska J, Bednarczyk P, Dołowy K, Wilczynski GM, Szewczyk A, Kunz WS. A novel potassium channel in skeletal muscle mitochondria. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2008; 1777:651-9. [PMID: 18515063 DOI: 10.1016/j.bbabio.2008.05.007] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/01/2007] [Revised: 05/07/2008] [Accepted: 05/12/2008] [Indexed: 11/30/2022]
Abstract
In this work we provide evidence for the potential presence of a potassium channel in skeletal muscle mitochondria. In isolated rat skeletal muscle mitochondria, Ca(2+) was able to depolarize the mitochondrial inner membrane and stimulate respiration in a strictly potassium-dependent manner. These potassium-specific effects of Ca(2+) were completely abolished by 200 nM charybdotoxin or 50 nM iberiotoxin, which are well-known inhibitors of large conductance, calcium-activated potassium channels (BK(Ca) channel). Furthermore, NS1619, a BK(Ca)-channel opener, mimicked the potassium-specific effects of calcium on respiration and mitochondrial membrane potential. In agreement with these functional data, light and electron microscopy, planar lipid bilayer reconstruction and immunological studies identified the BK(Ca) channel to be preferentially located in the inner mitochondrial membrane of rat skeletal muscle fibers. We propose that activation of mitochondrial K(+) transport by opening of the BK(Ca) channel may be important for myoprotection since the channel opener NS1619 protected the myoblast cell line C2C12 against oxidative injury.
Collapse
Affiliation(s)
- Jolanta Skalska
- Laboratory of Intracellular Ion Channels, Nencki Institute of Experimental Biology, 3 Pasteur Street, 02-093 Warsaw, Poland
| | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
60
|
Tjong YW, Li MF, Hung MW, Fung ML. Melatonin ameliorates hippocampal nitric oxide production and large conductance calcium-activated potassium channel activity in chronic intermittent hypoxia. J Pineal Res 2008; 44:234-43. [PMID: 18339118 DOI: 10.1111/j.1600-079x.2007.00515.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
Melatonin protects against hippocampal injury induced by intermittent hypoxia (IH). IH-induced oxidative stress is associated with decreases in constitutive production of nitric oxide (NO) and in the activity of large conductance calcium-activated potassium (BK) channels in hippocampal neurons. We tested the hypothesis that administration of melatonin alleviates the NO deficit and impaired BK channel activity in the hippocampus of IH rats. Sprague-Dawley rats were injected with melatonin (10 mg/kg, i.p.) or vehicle before daily IH exposure for 8 hr for 7 days. The NO and intracellular calcium ([Ca2+]i) levels in the CA1 region of hippocampal slices were measured by electrochemical microsenor and spectrofluorometry, respectively. The activity of BK channels was recorded by patch-clamping electrophysiology in dissociated CA1 neurons. Malondialdehyde levels were increased in the hippocampus of hypoxic rats and were lowered by the melatonin treatment. Levels of NO under resting and hypoxic conditions, and the protein expression of neuronal NO synthase (nNOS) were significantly reduced in the CA1 neurons of hypoxic animals compared with the normoxic controls. These deficits were mitigated in the melatonin-treated hypoxic rats with an improved [Ca2+]i response to acute hypoxia. The open probability of BK channels was decreased in the hypoxic rats and was partially restored in the melatonin-treated animals, without alterations in the expression of channel subunits and unitary conductance. Acute treatment of melatonin had no significant effects on the BK channel activity or on the [Ca2+]i response to hypoxia. Collectively, these results suggest that melatonin ameliorates the constitutive NO production and BK channel activity via an antioxidant mechanism against an IH-induced down-regulation of nNOS expression in hippocampal neurons.
Collapse
Affiliation(s)
- Y W Tjong
- Department of Physiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, China
| | | | | | | |
Collapse
|
61
|
The RCK1 high-affinity Ca2+ sensor confers carbon monoxide sensitivity to Slo1 BK channels. Proc Natl Acad Sci U S A 2008; 105:4039-43. [PMID: 18316727 DOI: 10.1073/pnas.0800304105] [Citation(s) in RCA: 92] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Carbon monoxide (CO) is a lethal gas, but it is also increasingly recognized as a physiological signaling molecule capable of regulating a variety of proteins. Among them, large-conductance Ca(2+)- and voltage-gated K(+) (Slo1 BK) channels, important in vasodilation and neuronal firing, have been suggested to be directly stimulated by CO. However, the molecular mechanism of the stimulatory action of CO on the Slo1 BK channel has not been clearly elucidated. We report here that CO reliably and repeatedly activates Slo1 BK channels in excised membrane patches in the absence of Ca(2+) in a voltage-sensor-independent manner. The stimulatory action of CO on the Slo1 BK channel requires an aspartic acid and two histidine residues located in the cytoplasmic RCK1 domain, and the effect persists under the conditions known to inhibit the conventional interaction between CO and heme in other proteins. We propose that CO acts as a partial agonist for the high-affinity divalent cation sensor in the RCK1 domain of the Slo1 BK channel.
Collapse
|
62
|
Tjong YW, Li M, Hung MW, Wang K, Fung ML. Nitric oxide deficit in chronic intermittent hypoxia impairs large conductance calcium-activated potassium channel activity in rat hippocampal neurons. Free Radic Biol Med 2008; 44:547-57. [PMID: 17996205 DOI: 10.1016/j.freeradbiomed.2007.10.033] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/06/2007] [Revised: 09/14/2007] [Accepted: 10/12/2007] [Indexed: 12/22/2022]
Abstract
Sleep apnea associated with chronic intermittent hypoxia (IH) impairs hippocampal functions but the pathogenic mechanisms involving dysfunction of nitric oxide (NO) and ionic channels remain unclear. We examined the hypothesis that hippocampal NO deficit impairs the activity of large conductance calcium-activated potassium (BK) channels in rats with chronic IH, mimicking conditions in patients with sleep apnea. A patch-clamp study was performed on hippocampal CA1 neurons acutely dissociated from IH and control rats. The levels of endogenous NO and intracellular calcium in the CA1 region of the hippocampal slices were measured respectively by electrochemical microsensors and spectrofluorometry. We found that the open probability of BK channels remarkably decreased in the CA1 pyramidal neurons in a time-dependent manner with the IH treatment, without changes in the unitary conductance and reversal potential. NO donors, SNP or DETA/NO, significantly restored the activity of BK channels in the IH neurons, which was prevented by blockade of S-nitrosylation with NEM or MTSES but not by inhibition of the cGMP pathway with ODQ or 8-bromo-cGMP. Endogenous NO levels were substantially lowered in the IH hippocampus during resting and hypoxia. Also, the level of protein expression of neuronal NO synthase was markedly lessened in the IH neurons with decreased intracellular calcium response to hypoxia. Collectively, the results suggest that the IH-induced NO deficit mediated by a down-regulation of the expression of neuronal NO synthase plays a causative role in the impaired activity of BK channels, which could account for the hippocampal injury in patients with sleep apnea.
Collapse
Affiliation(s)
- Yung-Wui Tjong
- Department of Physiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, China
| | | | | | | | | |
Collapse
|
63
|
Abstract
Alternative pre-mRNA splicing has an important role in the control of neuronal gene expression. Many neuronal proteins are structurally diversified through the differential inclusion and exclusion of sequences in the final spliced mRNA. Here, we discuss common mechanisms of splicing regulation and provide examples of how alternative splicing has important roles in neuronal development and mature neuron function. Finally, we describe regulatory proteins that control the splicing of some neuronally expressed transcripts.
Collapse
Affiliation(s)
- Qin Li
- Howard Hughes Medical Institute, University of California, Los Angeles, 6-762 MacDonald Research Laboratories, 675 Charles E. Young Drive South, Los Angeles, California 90095, USA
| | | | | |
Collapse
|
64
|
Petrik D, Brenner R. Regulation of STREX exon large conductance, calcium-activated potassium channels by the beta4 accessory subunit. Neuroscience 2007; 149:789-803. [PMID: 17945424 DOI: 10.1016/j.neuroscience.2007.07.066] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2007] [Revised: 06/22/2007] [Accepted: 08/07/2007] [Indexed: 12/31/2022]
Abstract
Large conductance (BK-type) calcium-activated potassium channels utilize alternative splicing and association with accessory beta subunits to tailor BK channel properties to diverse cell types. Two important modulators of BK channel gating are the neuronal-specific beta4 accessory subunit (beta4) and alternative splicing at the stress axis hormone-regulated exon (STREX). Individually, these modulators affect the gating properties of the BK channel as well as its response to phosphorylation. In this study, the combined functional consequences of STREX and the mouse beta4 subunit on mouse BK channel biophysical properties were investigated in transfected HEK 293 cells. Surprisingly, we found that the combined effects of STREX and beta4 are non-additive and even opposite for some properties. At high calcium, beta4 and the STREX individually share properties that promote BK channel opening via slowing of deactivation. However, the combined effects are a speeding of deactivation and a decreased open probability. beta4 also inhibits BK channel opening by a slowing of activation. This effect occurs across calcium concentrations in the absence of STREX, but predominates only at low calcium for STREX containing channels. BK channel responses to phosphorylation status are also altered by the combination of the beta4 subunit and STREX. beta4/STREX channels show a slowing of activation kinetics following dephosphorylation whereas beta4 channels lacking STREX do not. In contrast, beta4 confers a speeding of activation in response to cyclic AMP-dependent phosphorylation in channels lacking STREX, but not in channels containing STREX. These results indicate that the combination of the beta4 subunit and STREX confers non-additive and unique properties to BK channels. Analysis of expression in brain slices suggests that STREX and beta4 mRNA overlap expression in the dentate gyrus of the hippocampus and the cerebellar Purkinje cells, suggesting that these unique properties of BK channels may underlie BK channel gating in these cells.
Collapse
Affiliation(s)
- D Petrik
- Department of Physiology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA
| | | |
Collapse
|
65
|
Sun XH, Ding JP, Li H, Pan N, Gan L, Yang XL, Xu HB. Activation of large-conductance calcium-activated potassium channels by puerarin: the underlying mechanism of puerarin-mediated vasodilation. J Pharmacol Exp Ther 2007; 323:391-7. [PMID: 17652634 DOI: 10.1124/jpet.107.125567] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Puerarin is the main isoflavone found in Pueraria lobata (Willd) Ohwi, which has been used in therapy for various cardiovascular diseases. The present study examined the effects of puerarin on the large-conductance voltage- and Ca2+-activated potassium (BK(Ca)) channel and on rat thoracic aortas. BK(Ca) channels encoded with either alpha (BK-alpha) or alpha/beta subunits (BK-alpha+beta1) were heterologously expressed in Xenopus oocytes or human embryonic kidney 293 cells. The activities of BK(Ca) channels were measured using excised patch-clamp recordings. Puerarin activated BK-alpha+beta1 currents with a half-maximal concentration (EC50) of 0.8 nM and a Hill coefficient of 1.11 at 10 microM Ca2+ and with an EC50 of 12.6 nM and a Hill coefficient of 1.08 at 0 microM Ca2+. Puerarin (1 nM) induced a 16-mV leftward shift in the conductance-voltage curve for BK-alpha+beta1 currents at 10 microM Ca2+ and at 100 nM induced a 26-mV leftward shift at 0 microM Ca2+. Puerarin mainly increased the BK-alpha+beta1 channel open probability without changing the unitary conductance. Activation was also detected in the absence of the beta1 subunit. A deglycosylated analog of puerarin, daidzein, also activated BK(Ca) channels with weaker potency. In addition, puerarin (0.1 to 1000 microM) caused concentration-dependent relaxations of rat thoracic aortic rings contracted with 1 microM noradrenaline bitartrate (EC50 = 1.1 microM). These were significantly inhibited by 50 nM iberiotoxin, a specific blocker of BK(Ca) channels. This is the first study demonstrating that puerarin activates BK(Ca) channels, especially BK-alpha+beta1 channels. The activation of the BK(Ca) channel probably contributes to the puerarin-mediated vasodilation action.
Collapse
Affiliation(s)
- Xiao-Hui Sun
- Key Laboratory of Molecular Biophysics of the Ministry of Education, College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China
| | | | | | | | | | | | | |
Collapse
|
66
|
Yi L, Ragsdale SW. Evidence that the heme regulatory motifs in heme oxygenase-2 serve as a thiol/disulfide redox switch regulating heme binding. J Biol Chem 2007; 282:21056-67. [PMID: 17540772 PMCID: PMC3957417 DOI: 10.1074/jbc.m700664200] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Heme oxygenase (HO) catalyzes the O(2)- and NADPH-dependent conversion of heme to biliverdin, CO, and iron. The two forms of HO (HO-1 and HO-2) share similar physical properties but are differentially regulated and exhibit dissimilar physiological roles and tissue distributions. Unlike HO-1, HO-2 contains heme regulatory motifs (HRMs) (McCoubrey, W. K., Jr., Huang, T. J., and Maines, M. D. (1997) J. Biol. Chem. 272, 12568-12574). Here we describe UV-visible, EPR, and differential scanning calorimetry experiments on human HO-2 variants containing single, double, and triple mutations in the HRMs. Oxidized HO-2, which contains an intramolecular disulfide bond linking Cys(265) of HRM1 and Cys(282) of HRM2, binds heme tightly. Reduction of the disulfide bond increases the K(d) for ferric heme from 0.03 to 0.3 microm, which is much higher than the concentration of the free heme pool in cells. Although the HRMs markedly affect the K(d) for heme, they do not alter the k(cat) for heme degradation and do not bind additional hemes. Because HO-2 plays a key role in CO generation and heme homeostasis, reduction of the disulfide bond would be expected to increase intracellular free heme and decrease CO concentrations. Thus, we propose that the HRMs in HO-2 constitute a thiol/disulfide redox switch that regulates the myriad physiological functions of HO-2, including its involvement in the hypoxic response in the carotid body, which involves interactions with a Ca(2+)-activated potassium channel.
Collapse
Affiliation(s)
- Li Yi
- Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664
| | - Stephen W. Ragsdale
- Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664
| |
Collapse
|
67
|
Gonzalez C, Agapito MT, Rocher A, Gonzalez-Martin MC, Vega-Agapito V, Gomez-Niño A, Rigual R, Castañeda J, Obeso A. Chemoreception in the context of the general biology of ROS. Respir Physiol Neurobiol 2007; 157:30-44. [PMID: 17331812 DOI: 10.1016/j.resp.2007.01.016] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2006] [Revised: 01/23/2007] [Accepted: 01/23/2007] [Indexed: 11/28/2022]
Abstract
Superoxide anion is the most important reactive oxygen species (ROS) primarily generated in cells. The main cellular constituents with capabilities to generate superoxide anion are NADPH oxidases and mitochondrial respiratory chain. The emphasis of our article is centered in critically examining hypotheses proposing that ROS generated by NADPH oxidase and mitochondria are key elements in O(2)-sensing and hypoxic responses generation in carotid body chemoreceptor cells. Available data indicate that chemoreceptor cells express a specific isoform of NADPH oxidase that is activated by hypoxia; generated ROS acting as negative modulators of the carotid body (CB) hypoxic responses. Literature is also consistent in supporting that poisoned respiratory chain can produce high amounts of ROS, making mitochondrial ROS potential triggers-modulators of the CB activation elicited by mitochondrial venoms. However, most data favour the notion that levels of hypoxia, capable of strongly activating chemoreceptor cells, would not increase the rate of ROS production in mitochondria, making mitochondrial ROS unlikely triggers of hypoxic responses in the CB. Finally, we review recent literature on heme oxygenases from two perspectives, as potential O(2)-sensors in chemoreceptor cells and as generators of bilirubin which is considered to be a ROS scavenger of major quantitative importance in mammalian cells.
Collapse
Affiliation(s)
- C Gonzalez
- Departamento de Bioquímica y Biología Molecular y Fisiología e, Instituto de Biología y Genética Molecular, Facultad de Medicina, Universidad de Valladolid y Consejo Superior de Investigaciones Científicas, Valladolid, Spain.
| | | | | | | | | | | | | | | | | |
Collapse
|
68
|
Gu XQ, Siemen D, Parvez S, Cheng Y, Xue J, Zhou D, Sun X, Jonas EA, Haddad GG. Hypoxia increases BK channel activity in the inner mitochondrial membrane. Biochem Biophys Res Commun 2007; 358:311-6. [PMID: 17481584 DOI: 10.1016/j.bbrc.2007.04.110] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2007] [Accepted: 04/19/2007] [Indexed: 10/23/2022]
Abstract
To explore the potential function of the BK channel in the inner mitochondrial membrane under physiological and hypoxic conditions, we used on-mitoplast and whole-mitoplast patches. Single BK channels had a conductance of 276+/-9 pS under symmetrical K(+) solutions, were Ca(2+)- and voltage-dependent and were inhibited by 0.1 microM charybdotoxin. In response to hypoxia, BK increased open probability, shifted its reversal potential (9.3+/-2.4 mV) in the positive direction and did not change its conductance. We conclude that (1) the properties at rest of this mitoplast K(+) channel are similar to those of BK channels in the plasma membrane; (2) hypoxia induces an increase, rather than a decrease (as in the plasmalemma), in the open probability of this K(+) channel, leading to K(+) efflux from the mitochondrial matrix to the outside. We speculate that this increase in K(+) efflux from mitochondria into the cytosol is important during hypoxia in maintaining cytosolic K(+).
Collapse
Affiliation(s)
- Xiang Q Gu
- Department of Pediatrics (Section of Respiratory Medicine), University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0735, USA
| | | | | | | | | | | | | | | | | |
Collapse
|
69
|
Zhao G, Adebiyi A, Xi Q, Jaggar JH. Hypoxia reduces KCa channel activity by inducing Ca2+ spark uncoupling in cerebral artery smooth muscle cells. Am J Physiol Cell Physiol 2007; 292:C2122-8. [PMID: 17314264 PMCID: PMC2241735 DOI: 10.1152/ajpcell.00629.2006] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Arterial smooth muscle cell large-conductance Ca(2+)-activated potassium (K(Ca)) channels have been implicated in modulating hypoxic dilation of systemic arteries, although this is controversial. K(Ca) channel activity in arterial smooth muscle cells is controlled by localized intracellular Ca(2+) transients, termed Ca(2+) sparks, but hypoxic regulation of Ca(2+) sparks and K(Ca) channel activation by Ca(2+) sparks has not been investigated. We report here that in voltage-clamped (-40 mV) cerebral artery smooth muscle cells, a reduction in dissolved O(2) partial pressure from 150 to 15 mmHg reversibly decreased Ca(2+) spark-induced transient K(Ca) current frequency and amplitude to 61% and 76% of control, respectively. In contrast, hypoxia did not alter Ca(2+) spark frequency, amplitude, global intracellular Ca(2+) concentration, or sarcoplasmic reticulum Ca(2+) load. Hypoxia reduced transient K(Ca) current frequency by decreasing the percentage of Ca(2+) sparks that activated a transient K(Ca) current from 89% to 63%. Hypoxia reduced transient K(Ca) current amplitude by attenuating the amplitude relationship between Ca(2+) sparks that remained coupled and the evoked transient K(Ca) currents. Consistent with these data, in inside-out patches at -40 mV hypoxia reduced K(Ca) channel apparent Ca(2+) sensitivity and increased the K(d) for Ca(2+) from approximately 17 to 32 microM, but did not alter single-channel amplitude. In summary, data indicate that hypoxia reduces K(Ca) channel apparent Ca(2+) sensitivity via a mechanism that is independent of cytosolic signaling messengers, and this leads to uncoupling of K(Ca) channels from Ca(2+) sparks. Transient K(Ca) current inhibition due to uncoupling would oppose hypoxic cerebrovascular dilation.
Collapse
Affiliation(s)
- Guiling Zhao
- Dept. of Physiology, University of Tennessee Health Science Center, Memphis, TN 38163, USA
| | | | | | | |
Collapse
|
70
|
Thompson RJ, Buttigieg J, Zhang M, Nurse CA. A rotenone-sensitive site and H2O2 are key components of hypoxia-sensing in neonatal rat adrenomedullary chromaffin cells. Neuroscience 2007; 145:130-41. [PMID: 17207576 DOI: 10.1016/j.neuroscience.2006.11.040] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2006] [Revised: 11/17/2006] [Accepted: 11/21/2006] [Indexed: 10/23/2022]
Abstract
In the perinatal period, adrenomedullary chromaffin cells (AMC) directly sense PO2 and secrete catecholamines during hypoxic stress, and this response is lost in juvenile ( approximately 2 week-old) chromaffin cells following postnatal innervation. Here we tested the hypothesis that a rotenone-sensitive O2-sensor and ROS are involved in the hypoxic response of AMC cultured from neonatal and juvenile rats. In whole-cell recordings, hypoxia (PO2=5-15 mm Hg) inhibited outward current in neonatal AMC; this response was reversed by exogenous H2O2 and mimicked and occluded by intracellular catalase (1000 units/ml), as well as the antioxidants, N-acetyl-L-cysteine (NAC; 50 microM) and Trolox (200 microM). Acute hypoxia decreased ROS levels and stimulated ATP secretion in these cells, as measured by luminol and luciferin-luciferase chemiluminescence, respectively. Of several mitochondrial electron transport chain (ETC) inhibitors tested, only rotenone, a complex I blocker, mimicked and occluded the effects of hypoxia on outward current, cellular ROS, and ATP secretion. Succinate donors, which act as complex II substrates, reversed the effects of hypoxia and rotenone in neonatal AMC. In contrast, in hypoxia-insensitive juvenile AMC, neither NAC nor rotenone stimulated ATP secretion though they both caused a decrease in ROS levels. We propose that O2-sensing by neonatal AMC is mediated by decreased ROS generation via a rotenone-sensitive site that is coupled to outward current inhibition and secretion. Interestingly, juvenile AMC display at least two modifications, i.e. an uncoupling of the O2-sensor from ROS regulation, and an apparent insensitivity of outward current to decreased ROS.
Collapse
Affiliation(s)
- R J Thompson
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1.
| | | | | | | |
Collapse
|
71
|
Moudgil R, Michelakis ED, Archer SL. The role of k+ channels in determining pulmonary vascular tone, oxygen sensing, cell proliferation, and apoptosis: implications in hypoxic pulmonary vasoconstriction and pulmonary arterial hypertension. Microcirculation 2006; 13:615-32. [PMID: 17085423 DOI: 10.1080/10739680600930222] [Citation(s) in RCA: 126] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Abstract
Potassium channels are tetrameric, membrane-spanning proteins that selectively conduct K+ at near diffusion-limited rates. Their remarkable ionic selectivity results from a highly-conserved K+ recognition sequence in the pore. The classical function of K+ channels is regulation of membrane potential (EM) and thence vascular tone. In pulmonary artery smooth muscle cells (PASMC), tonic K+ egress, driven by a 145/5 mM intracellular/extracellular concentration gradient, contributes to a EM of about -60 mV. It has been recently discovered that K+ channels also participate in vascular remodeling by regulating cell proliferation and apoptosis. PASMC express voltage-gated (Kv), inward rectifier (Kir), calcium-sensitive (KCa), and two-pore (K2P) channels. Certain K+ channels are subject to rapid redox regulation by reactive oxygen species (ROS) derived from the PASMC's oxygen-sensor (mitochondria and/or NADPH oxidase). Acute hypoxic inhibition of ROS production inhibits Kv1.5, which depolarizes EM, opens voltage-sensitive, L-type calcium channels, elevates cytosolic calcium, and initiates hypoxic pulmonary vasoconstriction (HPV). Hypoxia-inhibited K+ currents are not seen in systemic arterial SMCs. Kv expression is also transcriptionally regulated by HIF-1alpha and NFAT. Loss of PASMC Kv1.5 and Kv2.1 contributes to the pathogenesis of pulmonary arterial hypertension (PAH) by causing a sustained depolarization, which increases intracellular calcium and K+, thereby stimulating cell proliferation and inhibiting apoptosis, respectively. Restoring Kv expression (via Kv1.5 gene therapy, dichloroacetate, or anti-survivin therapy) reduces experimental PAH. Electrophysiological diversity exists within the pulmonary circulation. Resistance PASMC have a homogeneous Kv current (including an oxygen-sensitive component), whereas conduit PASMC current is a Kv/KCa mosaic. This reflects regional differences in expression of channel isoforms, heterotetramers, splice variants, and regulatory subunits as well as mitochondrial diversity. In conclusion, K+ channels regulate pulmonary vascular tone and remodeling and constitute potential therapeutic targets in the regression of PAH.
Collapse
Affiliation(s)
- Rohit Moudgil
- Vascular Biology Group, Division of Cardiology, University of Alberta, Edmonton, Canada
| | | | | |
Collapse
|
72
|
Wyatt CN, Mustard KJ, Pearson SA, Dallas ML, Atkinson L, Kumar P, Peers C, Hardie DG, Evans AM. AMP-activated protein kinase mediates carotid body excitation by hypoxia. J Biol Chem 2006; 282:8092-8. [PMID: 17179156 PMCID: PMC1832262 DOI: 10.1074/jbc.m608742200] [Citation(s) in RCA: 120] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
Early detection of an O2 deficit in the bloodstream is essential to initiate corrective changes in the breathing pattern of mammals. Carotid bodies serve an essential role in this respect; their type I cells depolarize when O2 levels fall, causing voltage-gated Ca2+ entry. Subsequent neurosecretion elicits increased afferent chemosensory fiber discharge to induce appropriate changes in respiratory function (1). Although depolarization of type I cells by hypoxia is known to arise from K+ channel inhibition, the identity of the signaling pathway has been contested, and the coupling mechanism is unknown (2). We tested the hypothesis that AMP-activated protein kinase (AMPK) is the effector of hypoxic chemotransduction. AMPK is co-localized at the plasma membrane of type I cells with O2-sensitive K+ channels. In isolated type I cells, activation of AMPK using 5-aminoimidazole-4-carboxamide riboside (AICAR) inhibited O2-sensitive K+ currents (carried by large conductance Ca2+-activated (BKCa) channels and TASK (tandem pore, acid-sensing potassium channel)-like channels, leading to plasma membrane depolarization, Ca2+ influx, and increased chemosensory fiber discharge. Conversely, the AMPK antagonist compound C reversed the effects of hypoxia and AICAR on type I cell and carotid body activation. These results suggest that AMPK activation is both sufficient and necessary for the effects of hypoxia. Furthermore, AMPK activation inhibited currents carried by recombinant BKCa channels, whereas purified AMPK phosphorylated thealpha subunit of the channel in immunoprecipitates, an effect that was stimulated by AMP and inhibited by compound C. Our findings demonstrate a central role for AMPK in stimulus-response coupling by hypoxia and identify for the first time a link between metabolic stress and ion channel regulation in an O2-sensing system.
Collapse
Affiliation(s)
- Christopher N. Wyatt
- From the Department of Biomedical Sciences, School of Biology, Bute Building, University of St Andrews, St. Andrews, Fife. KY16 9TS, UK
| | - Kirsty J.W. Mustard
- Division of Molecular Physiology, College of Life Sciences, Sir James Black Centre, University of Dundee, Dow Street,DD1 5EH, UK
| | - Selina A. Pearson
- Department of Physiology, The Medical School, University of Birmingham, Birmingham B15 2TT, UK
| | - Mark L Dallas
- School of Medicine, University of Leeds, Leeds LS2 9JT, UK
| | - Lucy Atkinson
- School of Medicine, University of Leeds, Leeds LS2 9JT, UK
| | - Prem Kumar
- Department of Physiology, The Medical School, University of Birmingham, Birmingham B15 2TT, UK
| | - Chris Peers
- School of Medicine, University of Leeds, Leeds LS2 9JT, UK
| | - D. Grahame Hardie
- Division of Molecular Physiology, College of Life Sciences, Sir James Black Centre, University of Dundee, Dow Street,DD1 5EH, UK
| | - A. Mark Evans
- From the Department of Biomedical Sciences, School of Biology, Bute Building, University of St Andrews, St. Andrews, Fife. KY16 9TS, UK
| |
Collapse
|
73
|
Peers C, Wyatt CN. The role of maxiK channels in carotid body chemotransduction. Respir Physiol Neurobiol 2006; 157:75-82. [PMID: 17157084 DOI: 10.1016/j.resp.2006.10.010] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2006] [Revised: 10/27/2006] [Accepted: 10/28/2006] [Indexed: 01/10/2023]
Abstract
MaxiK channels are a unique class of K(+) channels activated by both voltage and intracellular Ca(2+). Derived from a single gene, their diversity arises from extensive splicing, and their wide distribution has led to their implication in a large variety of cellular functions. In the carotid body, they have been proposed to contribute to the resting membrane potential of type I cells, and also to be O(2) sensitive. Thus, they have been suggested to have an important role in hypoxic chemotransduction. Their O(2) sensitivity is preserved when the channels are expressed in HEK 293 cells, permitting detailed studies of candidate mechanisms underlying hypoxic inhibition of maxiK channels. In this article, we review evidence for and against an important role for maxiK channels in chemotransduction. We also consider different mechanisms proposed to account for hypoxic channel inhibition and suggest that, although our understanding of this important physiological process has advanced significantly in recent years, there remain important, unanswered questions as to the importance of maxiK in carotid body chemoreception.
Collapse
Affiliation(s)
- Chris Peers
- School of Medicine, University of Leeds, Leeds, UK.
| | | |
Collapse
|
74
|
Tian L, Chen L, McClafferty H, Sailer CA, Ruth P, Knaus HG, Shipston MJ. A noncanonical SH3 domain binding motif links BK channels to the actin cytoskeleton via the SH3 adapter cortactin. FASEB J 2006; 20:2588-90. [PMID: 17065230 DOI: 10.1096/fj.06-6152fje] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Calcium-activated potassium (BK) channels play a central role in regulating multiple physiological processes, from the control of blood flow to neuronal excitability. Coordinated regulation of BK channel activity by changes in actin cytoskeleton dynamics has been implicated in several of these processes and related disease states such as epilepsy and stroke. However, how BK channels interact with the actin cytoskeleton is essentially unknown. Here we demonstrate noncanonical Src homology domain 3 (SH3) binding site motifs in the intracellular C terminus of the BK channel pore-forming alpha-subunit that are conserved from fish to humans. These noncanonical motifs target multiple SH3 domain cellular signaling proteins to BK channels, including the SH3 adapter protein cortactin (EMS1). We demonstrate that cortactin provides a molecular bridge between BK channels and the cortical actin cytoskeleton in cells. Disruption of the SH3-mediated interaction prevents the regulation of BK channel activity controlled by changes in actin cytoskeletal dynamics. Targeting of cortactin to BK channels via a novel, noncanonical SH3 domain binding motif has important implications for the coordination of BK channel function in normal physiology and disease.
Collapse
Affiliation(s)
- Lijun Tian
- Centre for Integrative Physiology, School of Biomedical Science, University of Edinburgh, Edinburgh EH8 9XD, Scotland, UK
| | | | | | | | | | | | | |
Collapse
|
75
|
MacDonald SHF, Ruth P, Knaus HG, Shipston MJ. Increased large conductance calcium-activated potassium (BK) channel expression accompanied by STREX variant downregulation in the developing mouse CNS. BMC DEVELOPMENTAL BIOLOGY 2006; 6:37. [PMID: 16872513 PMCID: PMC1562363 DOI: 10.1186/1471-213x-6-37] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/25/2006] [Accepted: 07/27/2006] [Indexed: 11/30/2022]
Abstract
Background Large conductance calcium- and voltage activated potassium (BK) channels are important determinants of neuronal excitability through effects on action potential duration, frequency and synaptic efficacy. The pore- forming subunits are encoded by a single gene, KCNMA1, which undergoes extensive alternative pre mRNA splicing. Different splice variants can confer distinct properties on BK channels. For example, insertion of the 58 amino acid stress-regulated exon (STREX) insert, that is conserved throughout vertebrate evolution, encodes channels with distinct calcium sensitivity and regulation by diverse signalling pathways compared to the insertless (ZERO) variant. Thus, expression of distinct splice variants may allow cells to differentially shape their electrical properties during development. However, whether differential splicing of BK channel variants occurs during development of the mammalian CNS has not been examined. Results Using quantitative real-time polymerase chain reaction (RT-PCR) Taqman™ assays, we demonstrate that total BK channel transcripts are up regulated throughout the murine CNS during embryonic and postnatal development with regional variation in transcript levels. This upregulation is associated with a decrease in STREX variant mRNA expression and an upregulation in ZERO variant expression. Conclusion As BK channel splice variants encode channels with distinct functional properties the switch in splicing from the STREX phenotype to ZERO phenotype during embryonic and postnatal CNS development may provide a mechanism to allow BK channels to control distinct functions at different times of mammalian brain development.
Collapse
Affiliation(s)
- Stephen H-F MacDonald
- Centre for Integrative Physiology, School of Biomedical Science, Hugh Robson Building, University of Edinburgh, Edinburgh, Scotland, EH8 9XD, UK
- Current address: Trinity Institute of Molecular Medicine, St. James's Hospital, Dublin 8, Republic of Ireland
| | - Peter Ruth
- Pharmacology and Toxicology, University Tuebingen, Institute of Pharmacy, 72076 Tuebingen, Germany
| | - Hans-Guenther Knaus
- Division for Molecular and Cellular Pharmacology, Department of Medical Genetics, Molecular and Clinical Pharmacology, Medical University Innsbruck, Peter-Mayr Strasse 1, 6020 Innsbruck, Austria
| | - Michael J Shipston
- Centre for Integrative Physiology, School of Biomedical Science, Hugh Robson Building, University of Edinburgh, Edinburgh, Scotland, EH8 9XD, UK
| |
Collapse
|
76
|
Abstract
The majority of physiological processes proceed most favourably when O(2) is in plentiful supply. However, there are a number of physiological and pathological circumstances in which this supply is reduced either acutely or chronically. A crucial homeostatic response to such arterial hypoxaemia is carotid body excitation and a resultant increase in ventilation. Central to this response in carotid body, and many other chemosensory tissues, is the rapid inhibition of ion channels by hypoxia. Since the first direct demonstration of hypoxia-evoked depression in K(+) channel activity, the numbers of mechanisms which have been proposed to serve as the primary O(2) sensor have been almost as numerous as the experimental strategies with which to probe their nature. Three of the current favourite candidate mechanisms are mitochondria, AMP-activated kinase and haemoxygenase-2; a fourth proposal has been NADPH oxidase, but recent evidence suggests that this enzyme plays a secondary role in the O(2)-sensing process. All of these proposals have attractive points, but none can fully reconcile all of the data which have accumulated over the last two decades or so, suggesting that there may, in fact, not be a unique sensing system even within a single cell type. This latter point is key, because it implies that the ability of a cell to respond appropriately to decreased O(2) availability is biologically so important that several mechanisms have evolved to ensure that cellular function is never compromised during moderate to severe hypoxic insult.
Collapse
Affiliation(s)
- Paul J Kemp
- School of Biosciences, Museum Avenue, Cardiff University, Cardiff CF11 9BX, UK.
| |
Collapse
|