Role of potassium ion channels in detrusor smooth muscle function and dysfunction

Contraction and relaxation of the detrusor smooth muscle (DSM), which makes up the wall of the urinary bladder, facilitates the storage and voiding of urine. Several families of K(+) channels, including voltage-gated K(+) (K(V)) channels, Ca(2+)-activated K(+) (K(Ca)) channels, inward-rectifying ATP-sensitive K(+) (K(ir), K(ATP)) channels, and two-pore-domain K(+) (K(2P)) channels, are expressed and functional in DSM. They control DSM excitability and contractility by maintaining the resting membrane potential and shaping the action potentials that determine the phasic nature of contractility in this tissue. Defects in DSM K(+) channel proteins or in the molecules involved in their regulatory pathways may underlie certain forms of bladder dysfunction, such as overactive bladder. K(+) channels represent an opportunity for novel pharmacological manipulation and therapeutic intervention in human DSM. Modulation of DSM K(+) channels directly or indirectly by targeting their regulatory mechanisms has the potential to control urinary bladder function. This Review summarizes our current state of knowledge of the functional role of K(+) channels in DSM in health and disease, with special emphasis on current advancements in the field.     

Caveolae localize protein kinase A signaling to arterial ATP-sensitive potassium channels

Arterial ATP-sensitive K+ (K(ATP)) channels are critical regulators of vascular tone, forming a focal point for signaling by many vasoactive transmitters that alter smooth muscle contractility and so blood flow. Clinically, these channels form the target of antianginal and antihypertensive drugs, and their genetic disruption leads to hypertension and sudden cardiac death through coronary vasospasm. However, whereas the biochemical basis of K(ATP) channel modulation is well-studied, little is known about the structural or spatial organization of the signaling pathways that converge on these channels. In this study, we use discontinuous sucrose density gradients and Western blot analysis to show that K(ATP) channels localize with an upstream signaling partner, adenylyl cyclase, to smooth muscle membrane fractions containing caveolin, a protein found exclusively in cholesterol and sphingolipid-enriched membrane invaginations known as caveolae. Furthermore, we show that an antibody against the K(ATP) pore-forming subunit, Kir6.1 co-immunoprecipitates caveolin from arterial homogenates, suggesting that Kir6.1 and caveolin exist together in a complex. To assess whether the colocalization of K(ATP) channels and adenylyl cyclase to smooth muscle caveolae has functional significance, we disrupt caveolae with the cholesterol-depleting agent, methyl-beta-cyclodextrin. This reduces the cAMP-dependent protein kinase A-sensitive component of whole-cell K(ATP) current, indicating that the integrity of caveolae is important for adenylyl cyclase-mediated channel modulation. These results suggest that to be susceptible to protein kinase A-dependent activation, arterial K(ATP) channels need to be localized in the same lipid compartment as adenylyl cyclase; the results also provide the first indication of the spatial organization of signaling pathways that regulate K(ATP) channel activity.     

New expression profiles of voltage-gated ion channels in arteries exposed to high blood pressure

The diameters of small arteries and arterioles are tightly regulated by the dynamic interaction between Ca(2+) and K(+) channels in the vascular smooth muscle cells. Calcium influx through voltage-gated Ca(2+) channels induces vasoconstriction, whereas the opening of K(+) channels mediates hyperpolarization, inactivation of voltage-gated Ca(2+) channels, and vasodilation. Three types of voltage-sensitive ion channels have been highly implicated in the regulation of resting vascular tone. These include the L-type Ca(2+) (Ca(L)) channels, voltage-gated K(+) (K(V)) channels, and high-conductance voltage- and Ca(2+)-sensitive K(+) (BK(Ca)) channels. Recently, abnormal expression profiles of these ion channels have been identified as part of the pathogenesis of arterial hypertension and other vasospastic diseases. An increasing number of studies suggest that high blood pressure may trigger cellular signaling cascades that dynamically alter the expression profile of arterial ion channels to further modify vascular tone. This article will briefly review the properties of Ca(L), K(V), and BK(Ca) channels, present evidence that their expression profile is altered during systemic hypertension, and suggest potential mechanisms by which the signal of elevated blood pressure may result in altered ion channel expression. A final section will discuss emerging concepts and opportunities for the development of new vasoactive drugs, which may rely on targeting disease-specific changes in ion channel expression as a mechanism to lower vascular tone during hypertensive diseases.     

Modulation of endothelial cell KCa3.1 channels during endothelium-derived hyperpolarizing factor signaling in mesenteric resistance arteries

Arterial hyperpolarization to acetylcholine (ACh) reflects coactivation of K(Ca)3.1 (IK(Ca)) channels and K(Ca)2.3 (SK(Ca)) channels in the endothelium that transfers through myoendothelial gap junctions and diffusible factor(s) to affect smooth muscle relaxation (endothelium-derived hyperpolarizing factor [EDHF] response). However, ACh can differentially activate K(Ca)3.1 and K(Ca)2.3 channels, and we investigated the mechanisms responsible in rat mesenteric arteries. K(Ca)3.1 channel input to EDHF hyperpolarization was enhanced by reducing external [Ca(2+)](o) but blocked either with forskolin to activate protein kinase A or by limiting smooth muscle [Ca(2+)](i) increases stimulated by phenylephrine depolarization. Imaging [Ca(2+)](i) within the endothelial cell projections forming myoendothelial gap junctions revealed increases in cytoplasmic [Ca(2+)](i) during endothelial stimulation with ACh that were unaffected by simultaneous increases in muscle [Ca(2+)](i) evoked by phenylephrine. If gap junctions were uncoupled, K(Ca)3.1 channels became the predominant input to EDHF hyperpolarization, and relaxation was inhibited with ouabain, implicating a crucial link through Na(+)/K(+)-ATPase. There was no evidence for an equivalent link through K(Ca)2.3 channels nor between these channels and the putative EDHF pathway involving natriuretic peptide receptor-C. Reconstruction of confocal z-stack images from pressurized arteries revealed K(Ca)2.3 immunostain at endothelial cell borders, including endothelial cell projections, whereas K(Ca)3.1 channels and Na(+)/K(+)-ATPase alpha(2)/alpha(3) subunits were highly concentrated in endothelial cell projections and adjacent to myoendothelial gap junctions. Thus, extracellular [Ca(2+)](o) appears to modify K(Ca)3.1 channel activity through a protein kinase A-dependent mechanism independent of changes in endothelial [Ca(2+)](i). The resulting hyperpolarization links to arterial relaxation largely through Na(+)/K(+)-ATPase, possibly reflecting K(+) acting as an EDHF. In contrast, K(Ca)2.3 hyperpolarization appears mainly to affect relaxation through myoendothelial gap junctions. Overall, these data suggest that K(+) and myoendothelial coupling evoke EDHF-mediated relaxation through distinct, definable pathways.     

Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

Ouabain, a specific inhibitor of the Na(+)/K(+)-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na(+)/K(+)-pump and the Na(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](i) in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance. Smooth muscle cells were uncoupled when the Na(+)/K(+)-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca(2+)](i) near the membrane, or by ATP depletion. Reduction of Na(+)/K(+)-pump activity by removal of extracellular potassium ([K(+)](o)) also uncoupled cells, but only after inhibition of K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na(+) and clamping of [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.     

Calcium modulation of vascular smooth muscle ATP-sensitive K(+) channels: role of protein phosphatase-2B

ATP-sensitive K(+) (K(ATP)) channels are broadly distributed in the vasculature and regulate arterial tone. These channels are inhibited by intracellular ATP ([ATP](i)) and vasoconstrictor agents and can be activated by vasodilators. It is widely assumed that K(ATP) channels are insensitive to Ca(2+), although regulation has not been examined in the intact cell where cytosolic regulatory processes may be important. Thus we investigated the effects of Ca(2+) on whole-cell K(ATP) current in rat aortic smooth muscle cells recorded in a physiological [ATP](i) and K(+) gradient. Under control recording conditions, cells had a resting potential of approximately -40 mV when bathed in 1.8 mmol/L Ca(2+). The K(ATP) channel inhibitor glibenclamide caused membrane depolarization (9 mV) and inhibited a small, time-independent background current. Reducing [ATP](i) from 3 to 0.1 mmol/L hyperpolarized cells to approximately -60 mV and increased glibenclamide-sensitive current by 2- to 4-fold. Similar effects were observed when Ca(2+) levels were decreased either externally or internally by increasing EGTA from 1 to 10 mmol/L. Dialysis with solutions containing different free [Ca(2+)](i) showed that K(ATP) current was maximally activated at 10 nmol/L [Ca(2+)](i) and almost totally inhibited at 300 nmol/L. Moreover, under control conditions, when rat aortic smooth muscle cells were dialyzed with either cyclosporin A, FK-506, or calcineurin autoinhibitory peptide (structurally unrelated inhibitors of Ca(2+)-dependent protein phosphatase, type 2B), glibenclamide-sensitive currents were large and the resting potential was hyperpolarized by approximately 20 to 25 mV. We report for the first time that K(ATP) channels can be modulated by Ca(2+) at physiological [ATP](i) and conclude that modulation occurs via protein phosphatase type 2B.     

Effect of adenosine triphosphate-sensitive potassium channel inhibitors on coronary metabolic vasodilation

The ATP-sensitive potassium (K(ATP)) channel is a distinct type of potassium ion channel that is found in the vascular smooth muscle cells of a variety of mammalian species, including humans. The activity of K(ATP) channels is determined by many factors including cellular ATP and ADP levels, thus providing a link between cellular metabolism and vascular tone through its effects on membrane potential. Experimental studies using inhibitors of K(ATP) channels, such as the sulfonuylurea class of drugs, indicate that these channels modulate coronary vascular tone including the hyperaemia induced by increased myocardial metabolism. This review examines the evidence linking K(ATP) channels to the regulation of coronary vascular tone and the potential clinical implications of pharmacologic therapies that act on K(ATP) channels.     

Calcium exerts a larger regulatory effect on potassium+ channels in small mesenteric artery mycoytes from spontaneously hypertensive rats compared to Wistar-Kyoto rats

Hypertension is associated with a remodeling of arterial smooth muscle K(+) channels with Ca(2+)-gated K(+) channel (BK(Ca)) activity being enhanced and voltage-gated K(+) channel (K(v)) activity depressed. Because both of these channel types are modulated by intracellular Ca(2+), we tested the hypothesis that Ca(2+) had a larger effect on both BK(Ca) and K(v) channels in arterial myocytes from hypertensive animals. Myocytes were enzymatically dispersed from small mesenteric arteries (SMA) of 12-week-old Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Using whole cell patch clamp methods, BK(Ca) and K(v) current components were determined as iberiotoxin-sensitive and -insensitive currents, respectively. The effects of Ca(2+) on these K(+) current components were determined from measurements made with 0.2 and 2 mmol/L external Ca(2+). Increasing external Ca(2+) from 0.2 to 2 mmol/L Ca(2+) increased BK(Ca) currents recorded using myocytes from both WKY rats and SHR with a larger effect in SHR. Increasing external Ca(2+) decreased K(v) currents recorded using myocytes from both WKY and SHR also with a larger effect in SHR. In other experiments, currents through voltage-gated Ca(2+) channels (Ca(v)) measured at 0.2 mmol/L external Ca(2+) were 12 +/- 2% (n = 12) of those recorded at 2 mmol/L Ca(2+) with no differences in percent effect between WKY and SHR. In isolated SMA segments, isometric force development in response to 140 mmol/L KCl at 0.2 mmol/L external Ca(2+) was about 23 +/- 6% (n = 8) of that measured at 2 mmol/L external Ca(2+). These results suggest that an increase in Ca(2+) influx through Ca(v) or in intracellular Ca(2+) secondary to an increase in external Ca(2+) augments BK(Ca) currents and inhibits K(v) currents in SMA myocytes with a larger effect in SHR compared to WKY. This mechanism may contribute to the functional remodeling of K(+) currents of arterial myocytes in hypertensive animals.     

Functional roles of cardiac and vascular ATP-sensitive potassium channels clarified by Kir6.2-knockout mice

-ATP-sensitive potassium (K(ATP)) channels were discovered in ventricular cells, but their roles in the heart remain mysterious. K(ATP) channels have also been found in numerous other tissues, including vascular smooth muscle. Two pore-forming subunits, Kir6.1 and Kir6.2, contribute to the diversity of K(ATP) channels. To determine which subunits are operative in the cardiovascular system and their functional roles, we characterized the effects of pharmacological K(+) channel openers (KCOs, ie, pinacidil, P-1075, and diazoxide) in Kir6.2-deficient mice. Sarcolemmal K(ATP) channels could be recorded electrophysiologically in ventricular cells from Kir6.2(+/+) (wild-type [WT]) but not from Kir6.2(-/-) (knockout [KO]) mice. In WT ventricular cells, pinacidil induced an outward current and action potential shortening, effects that were blocked by glibenclamide, a K(ATP) channel blocker. KO ventricular cells exhibited no response to KCOs, but gene transfer of Kir6.2 into neonatal ventricular cells rescued the electrophysiological response to P-1075. In terms of contractile function, pinacidil decreased force generation in WT but not KO hearts. Pinacidil and diazoxide produced concentration-dependent relaxation in both WT and KO aortas precontracted with norepinephrine. In addition, pinacidil induced a glibenclamide-sensitive current of similar magnitude in WT and KO aortic smooth muscle cells and comparable levels of hypotension in anesthetized WT and KO mice. In both WT and KO aortas, only Kir6.1 mRNA was expressed. These findings indicate that the Kir6.2 subunit mediates the depression of cardiac excitability and contractility induced by KCOs; in contrast, Kir6.2 plays no discernible role in the arterial tree.     

Angiotensin II-activated protein kinase C targets caveolae to inhibit aortic ATP-sensitive potassium channels

OBJECTIVE: The vasoconstrictor angiotensin II (Ang II) acts at G(q/11)-coupled receptors to suppress ATP-sensitive potassium (K(ATP)) channel activity via activation of protein kinase C (PKC). The aim of this study was to determine the PKC isoforms involved in the Ang II-induced inhibition of aortic K(ATP) channel activity and to investigate potential mechanisms by which these isoforms specifically target these ion channels. METHODS AND RESULTS: We show that the inhibitory effect of Ang II on pinacidil-evoked whole-cell rat aortic K(ATP) currents persists in the presence of G?6976, an inhibitor of the conventional PKC isoforms, but is abolished by intracellular dialysis of a selective PKCepsilon translocation inhibitor peptide. This suggests that PKC-dependent inhibition of aortic K(ATP) channels by Ang II arises exclusively from the activation and translocation of PKCepsilon. Using discontinuous sucrose density gradients and Western blot analysis, we show that Ang II induces the translocation of PKCepsilon to cholesterol-enriched rat aortic smooth muscle membrane fractions containing both caveolin, a protein found exclusively in caveolae, and Kir6.1, the pore-forming subunit of the vascular K(ATP) channel. Immunogold electron microscopy of rat aortic smooth muscle plasma membrane sheets confirms both the presence of Kir6.1 in morphologically identifiable regions of the membrane rich in caveolin and Ang II-evoked migration of PKCepsilon to these membrane compartments. CONCLUSIONS: Ang II induces the recruitment of the novel PKC isoform, PKCepsilon, to arterial smooth muscle caveolae. This translocation allows PKCepsilon access to K(ATP) channels compartmentalized within these specialized membrane microdomains and highlights a potential role for caveolae in targeting PKC isozymes to an ion channel effector.     

G-protein control of cardiac potassium channels

Two cardiac potassium (K(+)) channels are activated by pertussis toxin (PTX)-sensitive G proteins either directly or in a "membrane-delimited" manner. They are muscarinic K(+)(K(ACH)) and ATP-sensitive K(+)(K(ATP)) channels. K(ACH) channels are responsible for acetylcholine (ACh)- or adenosine-induced deceleration of the heartbeat and atrioventricular conduction, while K(ATP) channels are responsible for the ischemia-induced shortening of the cardiac action potential and possibly for the adenosine-mediated protection from ischemic damage. Distinct molecular mechanisms underlie G-protein activation of these cardiac K(+) channels; the α subunit of PTX-sensitive G proteins activates the K(ATP) channels, while βγ subunits activate the K(ACh) channel. The physiologic significance of this heterogeneous mechanism remains to be determined.     

ATP-sensitive K+ channels of vascular smooth muscle cells

ATP-sensitive potassium channels (K(ATP)) of vascular smooth muscle cells represent potential therapeutic targets for control of abnormal vascular contractility. The biophysical properties, regulation and pharmacology of these channels have received intense scrutiny during the past twenty years, however, the molecular basis of vascular K(ATP) channels remains ill-defined. This review summarizes the recent advancements made in our understanding of the molecular composition of vascular K(ATP) channels with a focus on the evidence that hetero-octameric complexes of Kir6.1 and SUR2B subunits constitute the vascular K(ATP) subtype responsible for control of arterial diameter by vasoactive agonists.     

Reduction of Ca(2+) channel activity by hypoxia in human and porcine coronary myocytes.

OBJECTIVE: Oxygen (O(2)) tension is a major regulator of blood flow in the coronary circulation. Hypoxia can produce vasodilation through activation of ATP regulated K(+) (K(ATP)) channels in the myocyte membrane, which leads to hyperpolarization and closure of voltage-gated Ca(2+) channels. However, there are other O(2)-sensitive mechanisms intrinsic to the vascular smooth muscle since hypoxia can relax vessels precontracted with high extracellular K(+), a condition that prevents hyperpolarization following opening of K(+) channels. The objective of the present study was to determine whether inhibition of Ca(2+) influx through voltage-dependent channels participates in the response of coronary myocytes to hypoxia. METHODS: Experiments were performed on porcine anterior descendent coronary arterial rings and on enzymatically dispersed human and porcine myocytes of the same artery. Cytosolic [Ca(2+)] was measured by microfluorimetry and whole-cell currents were recorded with the patch clamp technique. RESULTS: Hypoxia (O(2) tension approximately 20 mmHg) dilated endothelium-denuded porcine coronary arterial rings precontracted with high K(+) in the presence of glibenclamide (5 microM), a blocker of K(ATP) channels. In dispersed human and porcine myocytes, low O(2) tension decreased basal cytosolic [Ca(2+)] and transmembrane Ca(2+) influx independently of K(+) channel activation. In patch clamped cells, hypoxia reversibly inhibited L-type Ca(2+) channels. RT-PCR indicated that rHT is the predominant mRNA variant of the alpha(1C) Ca(2+) channel subunit in human coronary myocytes. CONCLUSION: Our study demonstrates, for the first time in a human preparation, that voltage-gated Ca(2+)channels in coronary myocytes are under control of O(2) tension.     

Ion channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a hemodynamic abnormality that ultimately results in mortality due to right heart failure. Although the clinical manifestations of primary and secondary PAH are diverse, medial hypertrophy and arterial vasoconstriction are key components in the vascular remodeling leading to PAH. Abnormalities in the homeostasis of intracellular Ca(2+), transmembrane flux of ions, and membrane potential may play significant roles in the processes leading to pulmonary vascular remodeling. Decreased activity of K(+) channels causes membrane depolarization, leading to Ca(2+) influx. The elevated cytoplasmic Ca(2+) is a major trigger for pulmonary vasoconstriction and an important stimulus for vascular smooth muscle proliferation. Dysfunctional K(+) channels have also been linked to inhibition of apoptosis and contribute further to the medial hypertrophy. This review focuses on the relative role of K(+) and Ca(2+) ions and channels in human pulmonary artery smooth muscle cells in the development of PAH.     

脑血管平滑肌钾通道与脑血管痉挛

在脑血管平滑肌细胞膜上存在多种钾通道,一般分为4类:电压门控钾通道、钙离子激活钾通道、内向整流钾通道和ATP敏感钾通道.它们可调节脑血管张力,从而影响脑血流量以适应不同的生理病理学情况.蛛网膜下腔出血后,钾通道结构和功能发生变化,这些变化可能与脑血管痉挛的发生和发展有关.应用钾通道开放剂可松弛脑血管平滑肌,缓解脑血管痉挛.     

脑血管平滑肌钾通道与脑血管痉挛

在脑血管平滑肌细胞膜上存在多种钾通道,一般分为4类:电压门控钾通道、钙离子激活钾通道、内向整流钾通道和ATP敏感钾通道.它们可调节脑血管张力,从而影响脑血流量以适应不同的生理病理学情况.蛛网膜下腔出血后,钾通道结构和功能发生变化,这些变化可能与脑血管痉挛的发生和发展有关.应用钾通道开放剂可松弛脑血管平滑肌,缓解脑血管痉挛.     

Inhibition by protein kinase C of the K(NDP) subtype of vascular smooth muscle ATP-sensitive potassium channel

ATP-sensitive K(+) channels (K(ATP)) contribute to the regulation of tone in vascular smooth muscle cells. We determined the effects of protein kinase C (PKC) activation on the nucleoside diphosphate-activated (K(NDP)) subtype of vascular smooth muscle K(ATP) channel. Phorbol 12,13-dibutyrate (PdBu) and angiotensin II inhibited K(NDP) activity of C-A patches of rabbit portal vein (PV) myocytes, but an inactive phorbol ester was without effect, and pretreatment with PKC inhibitor prevented the actions of PdBu. Constitutively active PKC inhibited K(NDP) in I-O patches but was without effect in the presence of a specific peptide inhibitor of PKC. PdBu increased the duration of a long-lived interburst closed state but was without effect on burst duration or intraburst kinetics. PdBu treatment inhibited K(NDP), but not a 70-pS K(ATP) channel of rat PV. The results indicate that the K(NDP) subtype of vascular smooth muscle K(ATP) channel is inhibited by activation of PKC. Control of K(NDP) activity by intracellular signaling cascades involving PKC may, therefore, contribute to control of tone and arterial diameter by vasoconstrictors.     

Role of ATP dependent potassium channels in myocardial ischaemia.

Recently, a class of potassium (K) channels has been discovered which are regulated by the intracellular level of ATP. These channels have been termed ATP dependent K channels (KATP) and have been found to exist in the heart, skeletal muscle, pancreatic beta cells, brain, and smooth muscle. In this article, we discuss the function of the KATP channel in the ischaemic myocardium and present evidence to suggest that activation of these channels may, on the one hand, result in a marked cardioprotective effect from reversible or irreversible electrical, functional or biochemical damage or, on the other hand, have the potential to produce electrical instability and a proarrhythmic effect. The therapeutic potential of potassium channel modulators is also discussed.     

Role of potassium channels in bronchodilator responses in human airways.

The plasma membrane of airway smooth muscle contains a high density of K+ channels of various types that mainly regulate membrane potential. To examine whether these K+ channels are involved in bronchodilating mechanisms in human airways, relaxation concentration-response studies to isoproterenol, theophylline, and a K(+)-channel opener, lemakalim (BRL 38227), were obtained in the presence or absence of charybdotoxin (ChTX) (10 or 100 nM), an inhibitor of large conductance Ca(2+)-activated K+ channels (KCa) in smooth muscle. The effects of other potassium channel blockers, apamin (0.1 microM, a small-conductance KCa blocker) and BRL 31660 (10 microM, an ATP-sensitive K(+)-channel blocker) on isoproterenol-induced bronchodilation were also examined. All relaxation studies were performed on spontaneous tone and in the presence of 1 microM indomethacin. ChTX produced a dose-dependent significant rightward shift in the isoproterenol relaxation response curves without changing maximum relaxation; geometric mean values of EC50 were 4.6 nM without and 19 nM with 10 nM ChTX (n = 7, p less than 0.005), and 3.4 nM without and 41 nM with 100 nM ChTX (n = 4, p less than 0.05), respectively. The theophylline relaxation responses were inhibited to a lesser extent by ChTX (10 nM) (ED50 of 32 microM without and 71 microM with ChTX, n = 7, p less than 0.05), whereas lemakalim-induced relaxation response was not affected. Other K(+)-channel blockers, apamin and BRL31660, failed to affect isoproterenol-induced bronchodilation. These results suggest that ChTX-sensitive K+ channels are involved in bronchodilation induced by beta-agonists and theophylline in human airways.     

Focus on Kir6.2: a key component of the ATP-sensitive potassium channel

ATP-sensitive potassium (K(ATP)) channels are found in a wide variety of cell types where they couple cell metabolism to electrical activity. In glucose-sensing tissues, these channels respond to fluctuating changes in blood glucose concentration, but in other tissues they are activated only under ischemic conditions or in response to hormonal stimulation. Although K(ATP) channels in different tissues have different regulatory subunits, in almost all cases (except vascular smooth muscle) the pore-forming subunit is the inwardly rectifying K(+) channel Kir6.2. This article reviews recent studies of Kir6.2, focussing on the relation between channel structure and function, and on naturally occurring mutations in Kir6.2 that lead to human disease. New insights into the location of the ATP-binding site, the permeation pathway for K(+), and the gating of the pore provided by homology modelling are discussed in relation to functional studies. Gain-of-function mutations in Kir6.2 cause permanent neonatal diabetes mellitus (PNDM) by reducing the ATP sensitivity of the K(ATP) channel and increasing the K(ATP) current, which is predicted to inhibit beta-cell electrical activity and insulin secretion. Mutations at specific residues, that cause a greater decrease in ATP sensitivity, are associated with additional neurological symptoms. The molecular mechanism underlying the differences in ATP sensitivity produced by these two classes of mutations is discussed. We speculate on how some mutations lead to neurological disease and why no obvious cardiac symptoms are observed. We also consider the implications of these studies for type-2 diabetes.