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PubMed2005

Role of ubiquitin-proteasome degradation pathway in biogenesis efficiency of {beta}-cell ATP-sensitive potassium channels.

ATP-sensitive potassium (K(ATP)) channels of pancreatic beta-cells mediate glucose-induced insulin secretion by linking glucose metabolism to membrane excitability. The number of plasma membrane K(ATP) channels determines the sensitivity of beta-cells to glucose stimulation. The K(ATP) channel is formed in the endoplasmic reticulum (ER) on coassembly of four inwardly rectifying potassium channel Kir6.2 subunits and four sulfonylurea receptor 1 (SUR1) subunits. Little is known about the cellular events that govern the channel's biogenesis efficiency and expression. Recent studies have implicated the ubiquitin-proteasome pathway in modulating surface expression of several ion channels. In this work, we investigated whether the ubiquitin-proteasome pathway plays a role in the biogenesis efficiency and surface expression of K(ATP) channels. We provide evidence that, when expressed in COS cells, both Kir6.2 and SUR1 undergo ER-associated degradation via the ubiquitin-proteasome system. Moreover, treatment of cells with proteasome inhibitors MG132 or lactacystin leads to increased surface expression of K(ATP) channels by increasing the efficiency of channel biogenesis. Importantly, inhibition of proteasome function in a pancreatic beta-cell line, INS-1, that express endogenous K(ATP) channels also results in increased channel number at the cell surface, as assessed by surface biotinylation and whole cell patch-clamp recordings. Our results support a role of the ubiquitin-proteasome pathway in the biogenesis efficiency and surface expression of beta-cell K(ATP) channels.

ATP-Binding Cassette Transporters↗
PubMed2000

Dominantly inherited hyperinsulinism caused by a mutation in the sulfonylurea receptor type 1.

ATP-sensitive potassium channels play a major role in linking metabolic signals to the exocytosis of insulin in the pancreatic beta cell. These channels consist of two types of protein subunit: the sulfonylurea receptor SUR1 and the inward rectifying potassium channel Kir6.2. Mutations in the genes encoding these proteins are the most common cause of congenital hyperinsulinism (CHI). Since 1973, we have followed up 38 pediatric CHI patients in Finland. We reported previously that a loss-of-function mutation in SUR1 (V187D) is responsible for CHI of the most severe cases. We have now identified a missense mutation, E1506K, within the second nucleotide binding fold of SUR1, found heterozygous in seven related patients with CHI and in their mothers. All patients have a mild form of CHI that usually can be managed by long-term diazoxide treatment. This clinical finding is in agreement with the results of heterologous coexpression studies of recombinant Kir6.2 and SUR1 carrying the E1506K mutation. Mutant K(ATP) channels were insensitive to metabolic inhibition, but a partial response to diazoxide was retained. Five of the six mothers, two of whom suffered from hypoglycemia in infancy, have developed gestational or permanent diabetes. Linkage and haplotype analysis supported a dominant pattern of inheritance in a large pedigree. In conclusion, we describe the first dominantly inherited SUR1 mutation that causes CHI in early life and predisposes to later insulin deficiency.

ATP-Binding Cassette Transporters↗
PubMed1998

Microglia generate external proton and potassium ion gradients utilizing a member of the H/K ATPase family.

An ion selective electrode in self-referencing mode was employed to detect ionic concentration gradients at the vicinity of microglia isolated from newborn rat brains. At 5 mM extracellular potassium concentration, a gradient of -9.43+/-4.2 microM (n=48) was recorded dissipating over a distance of 10 microm from the outer surface of the cell membrane. Pharmacological studies indicated that neither the Na+/K+-ATPase nor the inward rectifier potassium channel makes significant contributions to generation of this gradient. The recorded potassium gradient was found to be augmented by increase in extracellular potassium or proton concentrations and could be inhibited by Omeprazole (10 microM) and by the specific H+/K+-ATPase blocker SCH28080 (1 microM). These, along with the coexistence of a gradient of excess of protons, strongly suggest that a K/H ATPase is the major generator of both the potassium and the proton gradients. The Kd of the glial transporter for K ions is an order of magnitude higher (3.7 mM) than that of the epithelial H+/K+-ATPase. This is a first report of an H+/K+ transporter in microglia cells with a Kd in the physiological range of [K+]out. Implications of the H+/K+-ATPase on potassium homeostasis in microglia under high extracellular potassium and low pH, as found at the site of brain injury, are discussed.

Animals↗
PubMed1998

Inwardly rectifying whole cell potassium current in human blood eosinophils.

1. Membrane currents were studied in single human blood eosinophils using the whole cell voltage clamp technique. The whole cell current-voltage relationship exhibited rectification about the membrane potential which followed the potassium equilibrium potential when [K+]o was raised. Elevation of [K+]o considerably potentiated inward current amplitude, and in some cells channel activity was discernible in the whole cell membrane current recordings. The single channel conductance was 24 +/- 1 pS ([K+]o, 100 mM; [K+]i, 140 mM), and eosinophils were found to have as few as three, and on average twenty, inward rectifier channels each. 2. The inward current was inhibited in a voltage-dependent manner by extracellular cations in order of potency Ba2+ > Cs+ > Na+. Intracellular acidification inhibited while alkalization augmented the inward current. Mg2+ contributed to rectification as dialysis with nominally Mg(2+)-free pipette solution was associated with an increase in the outward current during membrane polarization. 3. By reverse transcription-polymerase chain reaction (RT-PCR) using suitable primers on human eosinophils mRNA, an inward rectifier channel, Kir2.1, was identified, which is known from expression studies to have very similar properties to those found in this study. 4. Superoxide anion production or its stimulation by phorbol 12-myristate 13-acetate (PMA) was not significantly affected by depolarization with 140 mM [K+]o, or by 1 mM BaCl2. 5. It is concluded that the single channel currents and the whole cell current rectification observed in human blood eosinophils resulted from the presence of an inwardly rectifying potassium channel, probably Kir2.1.

Action Potentials↗
PubMed1999

Characterization of the mouse sulfonylurea receptor 1 promoter and its regulation.

The ATP-sensitive potassium channels (K+ATP channels) are heteromultimeric structures formed by a member of the sulfonylurea receptor (SUR) family and a member of the inwardly rectifying potassium channel family (Kir6.x). The K+ATP channels play an essential role in nutrient-induced insulin secretion from the pancreatic beta-cell. We have cloned and characterized the promoter region of the mouse SUR1 gene, and have shown that it lacks CAAT and TATA boxes or an initiator element. Studies of transcription initiation in several tissues showed that there is a common SUR1 promoter in brain, heart, and pancreas and in the pancreatic beta-cell line, betaTC3. The SUR1 gene uses multiple transcription start sites with the major site located 54 base pairs 5'-upstream of the translation initiation site. Transient transfection experiments in pancreatic beta-cell lines showed that the proximal promoter fragment -84/+54 is sufficient for significant transcriptional activity. The proximal promoter region contains multiple SP1-binding sites, and cotransfection experiments of the SUR1 promoter-luciferase vector with SP1 expression vector in Drosophila SL2 cells demonstrated a stimulatory effect of SP1 on SUR1 transcriptional activity. The mobility shift assays confirmed the interaction of the SP1 transcription factor with the proximal promoter region of the SUR1 gene. Together, these results indicate that SP1 may mediate transcription initiation of the SUR1 gene. In addition, we have described the coordinate regulation of the gene expression of both K+ATP channel subunits by glucocorticoids. SUR1 and Kir6.2 mRNA levels are down-regulated by approximately 40-50% in response to glucocorticoid treatment. Interestingly, the extent of the inhibitory effect as well as the kinetics and sensitivity are very similar for both mRNAs. Studies of mRNA turnover demonstrate that glucocorticoids most likely decrease the transcriptional activity of both SUR1 and Kir6.2 genes since glucocorticoids failed to affect the stability of each mRNA. Likewise, the reduction in mRNA levels was correlated with a decrease in SUR1 and Kir6.2 protein levels.

ATP-Binding Cassette Transporters↗
PubMed2006

Involvement of Golgin-160 in cell surface transport of renal ROMK channel: co-expression of Golgin-160 increases ROMK currents.

The weak inward rectifier potassium channel ROMK is important for water and salt reabsorption in the kidney. Here we identified Golgin-160 as a novel interacting partner of the ROMK channel. By using yeast two-hybrid assays and co-immunoprecipitations from transfected cells, we demonstrate that Golgin-160 associates with the ROMK C-terminus. Immunofluorescence microscopy confirmed that both proteins are co-localized in the Golgi region. The interaction was further confirmed by the enhancement of ROMK currents by the co-expressed Golgin-160 in Xenopus oocytes. The increase in ROMK current amplitude was due to an increase in cell surface density of ROMK protein. Golgin-160 also stimulated current amplitudes of the related Kir2.1, and of voltage-gated Kv1.5 and Kv4.3 channels, but not the current amplitude of co-expressed HERG channel. These results demonstrate that the Golgi-associated Golgin-160 recognizes the cytoplasmic C-terminus of ROMK, thereby facilitating the transport of ROMK to the cell surface. However, the stimulatory effect on the activity of more distantly-related potassium channels suggests a more general role of Golgin-160 in the trafficking of plasma membrane proteins.

Amino Acid Sequence↗
PubMed1998

Cloning of the promoters for the beta-cell ATP-sensitive K-channel subunits Kir6.2 and SUR1.

The beta-cell ATP-sensitive potassium channel (K-ATP channel), which regulates insulin secretion, is composed of two types of subunits: 1) a sulfonylurea receptor (SUR1) and 2) an inwardly rectifying potassium channel (Kir6.2). We have isolated clones containing 5'-flanking DNA for both genes by hybridization screening of a human genomic library. Sequencing of over one kilobase of each upstream region has revealed that the putative promoters are G + C rich, with no TATA box. Several E-boxes and potential Sp1 sites are present in both promoters, and the Kir6.2 upstream region contains an Alu repeat. Using a luciferase reporter gene in transient transfection assays, we demonstrate that the upstream DNA contains promoters that are active in the beta-cell lines HIT T15 and MIN6. The promoters are completely inactive in the fibroblast cell line COS7 but show some activity in HepG2 (liver) and HEK293 (epithelial) cell lines. Deletion analysis suggests that a short (173-base pair [bp]) fragment of SUR1 5'-flanking sequence is sufficient for maximal promoter activity. In contrast, over 900 bp of Kir6.2 5' sequence are required for similar high level expression, and deletion of the Alu repeat results in an increase in promoter activity.

ATP-Binding Cassette Transporters↗
PubMed2005

Inhibition of mechanosensitivity in visceral primary afferents by GABAB receptors involves calcium and potassium channels.

GABA(B) receptors inhibit mechanosensitivity of visceral afferents. This is associated with reduced triggering of events that lead to gastro-esophageal reflux, with important therapeutic consequences. In other neuronal systems, GABA(B) receptor activation may be linked via G-proteins to reduced N-type Ca(2+) channel opening, increased inward rectifier K(+) channel opening, plus effects on a number of intracellular messengers. Here we aimed to determine the role of Ca(2+) and K(+) channels in the inhibition of vagal afferent mechanoreceptor function by the GABA(B) receptor agonist baclofen. The responses of three types of ferret gastro-esophageal vagal afferents (mucosal, tension and tension mucosal receptors) to graded mechanical stimuli were investigated in vitro. The effects of baclofen (200 microM) alone on these responses were quantified, and the effects of baclofen in the presence of the G-protein-coupled inward rectifier potassium channel blocker Rb(+) (4.7 mM) and/or the N-type calcium channel blocker omega-conotoxin GVIA (0.1 microM). Baclofen inhibition of mucosal receptor mechanosensitivity was abolished by both blockers. Its inhibitory effect on tension mucosal receptors was partly reduced by both. The inhibitory effect of baclofen on tension receptors was unaffected. The data indicate that the inhibitory action of GABA(B) receptors is mediated via different pathways in mucosal, tension and tension mucosal receptors via mechanisms involving both N-type Ca(2+) channels and inwardly rectifying K(+) channels and others.

Action Potentials↗
PubMed2005

ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart.

Cardiac ATP-sensitive K(+) (K(ATP)) channels, gated by cellular metabolism, are formed by association of the inwardly rectifying potassium channel Kir6.2, the potassium conducting subunit, and SUR2A, the ATP-binding cassette protein that serves as the regulatory subunit. Kir6.2 is the principal site of ATP-induced channel inhibition, while SUR2A regulates K(+) flux through adenine nucleotide binding and catalysis. The ATPase-driven conformations within the regulatory SUR2A subunit of the K(ATP) channel complex have determinate linkage with the states of the channel's pore. The probability and life-time of ATPase-induced SUR2A intermediates, rather than competitive nucleotide binding alone, defines nucleotide-dependent K(ATP) channel gating. Cooperative interaction, instead of independent contribution of individual nucleotide binding domains within the SUR2A subunit, serves a decisive role in defining K(ATP) channel behavior. Integration of K(ATP) channels with the cellular energetic network renders these channel/enzyme heteromultimers high-fidelity metabolic sensors. This vital function is facilitated through phosphotransfer enzyme-mediated transmission of controllable energetic signals. By virtue of coupling with cellular energetic networks and the ability to decode metabolic signals, K(ATP) channels set membrane excitability to match demand for homeostatic maintenance. This new paradigm in the operation of an ion channel multimer is essential in providing the basis for K(ATP) channel function in the cardiac cell, and for understanding genetic defects associated with life-threatening diseases that result from the inability of the channel complex to optimally fulfill its physiological role.

ATP-Binding Cassette Transporters↗
PubMed2005

Redistribution of the water channel protein aquaporin-4 and the K+ channel protein Kir4.1 differs in low- and high-grade human brain tumors.

The blood-brain barrier (BBB) regulation is characterized by an interplay between endothelial cells, subendothelial basal laminae and astrocytic cells. Astroglial cells are highly polarized by the differentiation of perivascular membrane domains. These domains are characterized by the aggregation of, among other molecules, the water channel protein aquaporin-4 (AQP4), the dystrophin-dystroglycan complex, and the inwardly rectifying potassium channel protein Kir4.1. Normally, this ion channel plays an important role in spatial buffering of extracellular K(+) in the central nervous system, which only can be performed due to the non-uniform distribution of Kir4.1 across the surface of the glial cell. In this study, we observed a mislocalization of Kir4.1 in various human brain tumors (low- and high-grade astrocytomas and oligodendrogliomas), suggesting that buffering capacity of glial cells may be compromised, leading to water influx (cytotoxic edema). Interestingly, whereas dystrophin remained regularly restricted at the endfeet membranes in all cases investigated, AQP4 was found to be redistributed only in high-grade astrocytomas, not in low-grade astrocytomas. If the mechanisms of redistribution of AQP4 and Kir4.1 are different in low- and high-grade gliomas, this may suggest that the mechanisms of clustering of AQP4 and Kir4.1 at the glial endfeet membrane domains are also different. The redistribution of AQP4 in glioblastoma cells is discussed as a reaction to the vasogenic edema, as induced by the breakdown of the BBB, to facilitate reabsorption of excess fluid.

Aquaporin 4↗
PubMed1999

Localization and age-dependent expression of the inward rectifier K+ channel subunit Kir 5.1 in a mammalian reproductive system.

Kir 5.1 is a member of the inward rectifier potassium channel superfamily which does not form functional channels when expressed by itself in Xenopus laevis oocytes. rt-PCR reveals high levels of Kir 5.1 mRNA expression in testis but the function of this channel remains unknown. To determine the cell-specific expression of this channel in the testis we raised a polyclonal antibody against an external epitope of Kir 5.1 and tested its specificity in Xenopus oocytes expressing several cloned Kir subunits. Strong immunoreactivity for Kir 5.1 was found in seminiferous tubules of rat testis and, particularly, in spermatogonia, primary and secondary spermatocytes, spermatids and in the head and body of spermatozoa. The intensity of Kir 5.1 immunofluorescence, quantified using laser scanning microscopy, increased with age at every stage in the development of sperm from spermatogonia and reached a peak in 60-day-old rats. In contrast, the immunofluorescence decreased in 90-day-old animals and was detected mostly in spermatozoa. The results demonstrate that Kir 5.1 expression in the testis is localised to cells involved in spermatogenesis, showing a temporal pattern of expression during sexual maturity.

Aging↗
PubMed1995

Susceptibility of cloned K+ channels to reactive oxygen species.

Free radical-induced oxidant stress has been implicated in a number of physiological and pathophysiological states including ischemia and reperfusion-induced dysrhythmia in the heart, apoptosis of T lymphocytes, phagocytosis, and neurodegeneration. We have studied the effects of oxidant stress on the native K+ channel from T lymphocytes and on K+ channels cloned from cardiac, brain, and T-lymphocyte cells and expressed in Xenopus oocytes. The activity of three Shaker K+ channels (Kv1.3, Kv1.4, and Kv1.5), one Shaw channel (Kv3.4), and one inward rectifier K+ channel (IRK3) was drastically inhibited by photoactivation of rose bengal, a classical generator of reactive oxygen species. Other channel types (such as Shaker K+ channel Kv1.2, Shab channels Kv2.1 and Kv2.2, Shal channel Kv4.1, inward rectifiers IRK1 and ROMK1, and hIsK) were completely resistant to this treatment. On the other hand tert-butyl hydroperoxide, another generator of reactive oxygen species, removed the fast inactivation processes of Kv1.4 and Kv3.4 but did not alter other channels. Xanthine/xanthine oxidase system had no effect on all channels studied. Thus, we show that different types of K+ channels are differently modified by reactive oxygen species, an observation that might be of importance in disease states.

Animals↗
PubMed1997

Regulation of KATP channel activity by diazoxide and MgADP. Distinct functions of the two nucleotide binding folds of the sulfonylurea receptor.

KATP channels were reconstituted in COSm6 cells by coexpression of the sulfonylurea receptor SUR1 and the inward rectifier potassium channel Kir6.2. The role of the two nucleotide binding folds of SUR1 in regulation of KATP channel activity by nucleotides and diazoxide was investigated. Mutations in the linker region and the Walker B motif (Walker, J.E., M.J. Saraste, M.J. Runswick, and N.J. Gay. 1982. EMBO [Eur. Mol. Biol. Organ.] J. 1:945-951) of the second nucleotide binding fold, including G1479D, G1479R, G1485D, G1485R, Q1486H, and D1506A, all abolished stimulation by MgADP and diazoxide, with the exception of G1479R, which showed a small stimulatory response to diazoxide. Analogous mutations in the first nucleotide binding fold, including G827D, G827R, and Q834H, were still stimulated by diazoxide and MgADP, but with altered kinetics compared with the wild-type channel. None of the mutations altered the sensitivity of the channel to inhibition by ATP4-. We propose a model in which SUR1 sensitizes the KATP channel to ATP inhibition, and nucleotide hydrolysis at the nucleotide binding folds blocks this effect. MgADP and diazoxide are proposed to stabilize this desensitized state of the channel, and mutations at the nucleotide binding folds alter the response of channels to MgADP and diazoxide by altering nucleotide hydrolysis rates or the coupling of hydrolysis to channel activation.

ATP-Binding Cassette Transporters↗
PubMed2000

Sodium pump activity, not glial spatial buffering, clears potassium after epileptiform activity induced in the dentate gyrus.

A number of mechanisms have been proposed to play a role in the regulation of activity-dependent variations in extracellular potassium concentration ([K(+)](o)). We tested possible regulatory mechanisms for [K(+)](o) during spontaneous recurrent epileptiform activity induced in the dentate gyrus of hippocampal slices from adult rats by perfusion with 8 mM potassium and 0-added calcium medium in an interface chamber. Local application of tetrodotoxin blocked local [K(+)](o) changes, suggesting that potassium is released and taken up locally. Perfusion with barium or cesium, blockers of the inward rectifying potassium channel, did not alter the baseline [K(+)](o), the ceiling level of [K(+)](o) reached during the burst, or the rate of [K(+)](o) recovery after termination of the bursts. Decreasing gap junctional conductance did not change the baseline [K(+)](o) or the half-time of recovery of the [K(+)](o) after the bursts but did cause a decrease in the ceiling level of [K(+)](o). Perfusion with furosemide, which will block cation/chloride cotransporters, or perfusion with low chloride did not change the baseline [K(+)](o) or the half-time of recovery of the [K(+)](o) after the bursts but did increase the ceiling level of [K(+)](o). Bath or local application of ouabain, a Na(+)/K(+)-ATPase inhibitor, increased the baseline [K(+)](o), slowed the rate of [K(+)](o) recovery, and induced spreading depression. These findings suggest that potassium redistribution by glia only plays a minor role in the regulation of [K(+)](o) in this model. The major regulator of [K(+)](o) in this model appears to be uptake via a Na(+)/K(+)-ATPase, most likely neuronal.

Animals↗
PubMed2004

Kir4.1 expression by astrocytes and oligodendrocytes in CNS white matter: a developmental study in the rat optic nerve.

Deletion studies in transgenic mice indicate that the potassium inward rectifying channel Kir4.1 is crucial for oligodendrocyte differentiation and has a special role in regulation of extracellular potassium (K(+)), a major function of astrocytes. However, there are conflicting reports on whether Kir4.1 is expressed by white matter astrocytes and oligodendrocytes, raising doubts over its functions. Here, we have examined Kir4.1 expression in astrocytes and oligodendrocytes of the rat optic nerve, a typical central nervous system white matter tract. Single and double immunofluorescence labelling was performed on frozen sections from optic nerves aged postnatal day (P)5, 10, 15, 20 and adult, using anti-Kir4.1 antibodies and the glia-specific antibodies glial fibrillary acidic protein (GFAP, astrocytes), carbonic anhydrase II (CAII, oligodendrocyte somata and processes) and myelin basic protein (MBP, oligodendrocyte myelin sheaths). The results demonstrate Kir4.1 expression in rows of glial cells as early as P5, and this pattern persisted throughout development and into adulthood, consistent with early expression of Kir4.1 on developing oligodendrocytes. Clear co-expression of Kir4.1 and GFAP is first evident at P10 and increases to adult levels by P15 and P20, which correlates with the development of K(+) regulation between P15 and P20. Astrocyte expression of Kir4.1 is localized to perivascular end-feet and fine processes within the fascicles of myelinated axons, consistent with a role in K(+) spatial buffering between nodes of Ranvier and blood vessels. By contrast, Kir4.1 is concentrated in the cell bodies of oligodendrocytes, and there is no apparent co-expression with MBP(+) myelin sheaths, suggesting oligodendroglial Kir4.1 channels are not involved in K(+) regulation. The results support roles for Kir4.1 in both oligodendrocyte differentiation and K(+) regulation by astrocytes.

Animals↗
PubMed2001

Reciprocal regulation of expression of pore-forming KATP channel genes by hypoxia.

The ATP-sensitive potassium (KATP) channel is thought to play an important role in the protection of heart and brain against tissue hypoxia. The genetic regulation of the components of the channel by hypoxia has not been previously described. Here, we investigated the regulation of the two pore-forming channel proteins, Kir6.1 and Kir6.2, in response to hypoxia in vivo and in vitro. We find that these two structurally-related inwardly-rectifying potassium channel proteins are reciprocally regulated by hypoxia in vivo, with upregulation of Kir6.1 and down-regulation of Kir6.2, thereby resulting in a significant change in the composition of the channel complex in response to hypoxia. In vitro we describe neuronal and cardiac cell lines in which Kir6.1 is up-regulated by hypoxia, demonstrating that Kir6.1 is a hypoxia-inducible gene. We conclude that the heart and brain display genetic plasticity in response to hypoxic stress through specific genetic reprograming of cytoprotective channel genes.

Adenosine Triphosphate↗
PubMed1999

Potassium channel-mediated hyperpolarization of mesenteric vascular smooth muscle by isoflurane.

BACKGROUND: A primary source of calcium (Ca2+) necessary for excitation contraction in vascular smooth muscle (VSM) is influx via voltage-dependent Ca2+ channels. Thus, force generation in VSM is coupled closely to resting transmembrane potential, which itself is primarily a function of potassium conductance. Previously, the authors reported that volatile anesthetics hyperpolarize VSM of small mesenteric resistance arteries and capacitance veins. The current study was designed to determine whether isoflurane-mediated hyperpolarization is the result of specific effects on one or more of four types of potassium channels known to exist in VSM. METHODS: Transmembrane potentials (Em) were recorded from in situ mesenteric capacitance and resistance vessels in Sprague-Dawley rats weighing 250-300 g. In separate experiments, selective inhibitors of each of four types of potassium channels known to exist in VSM were administered in the superfusate of the vessel preparations to assess their effects on isoflurane-mediated hyperpolarization. RESULTS: Resting VSM Em ranged from -38 to -43 mV after local sympathetic denervation. Isoflurane produced a significant hyperpolarization (2.7-4.3 mV), whereas each potassium channel inhibitor significantly depolarized (2.8-8.5 mV) the VSM. Both 100 nM iberiotoxin (inhibitor of high conductance calcium-activated potassium channels) and 1 microM glybenclamide (inhibitor of adenosine triphosphatase-sensitive potassium channels) significantly inhibited VSM hyperpolarization induced by 1 MAC (minimum alveolar concentration) levels of inhaled isoflurane (0.1-0.9 mV Em change, which was not significant). In contrast, isoflurane hyperpolarized the VSM significantly despite the presence of 3 mM 4 aminopyridine (inhibitor of voltage-dependent potassium channels) or 10 microM barium chloride (an inhibitor of inward rectifier potassium channels) (3.7-8.2 mV change in VSM Em). CONCLUSIONS: These results suggest that isoflurane-mediated hyperpolarization (and associated relaxation) of VSM can be attributed in part to an enhanced (or maintained) opening of calcium-activated and adenosine triphosphate-sensitive potassium channels but not voltage-dependent or inward rectifier potassium channels.

Anesthetics, Inhalation↗