Physiological and pathophysiological roles of ATP-sensitive K+ channels

Immunolocalization of KATP channel subunits in mouse and rat cardiac myocytes and the coronary vasculature

BACKGROUND

Electrophysiological data suggest that cardiac KATP channels consist of Kir6.2 and SUR2A subunits, but the distribution of these (and other KATP channel subunits) is poorly defined. We examined the localization of each of the KATP channel subunits in the mouse and rat heart.

RESULTS

Immunohistochemistry of cardiac cryosections demonstrate Kir6.1 protein to be expressed in ventricular myocytes, as well as in the smooth muscle and endothelial cells of coronary resistance vessels. Endothelial capillaries also stained positive for Kir6.1 protein. Kir6.2 protein expression was found predominantly in ventricular myocytes and also in endothelial cells, but not in smooth muscle cells. SUR1 subunits are strongly expressed at the sarcolemmal surface of ventricular myocytes (but not in the coronary vasculature), whereas SUR2 protein was found to be localized predominantly in cardiac myocytes and coronary vessels (mostly in smaller vessels). Immunocytochemistry of isolated ventricular myocytes shows co-localization of Kir6.2 and SUR2 proteins in a striated sarcomeric pattern, suggesting t-tubular expression of these proteins. Both Kir6.1 and SUR1 subunits were found to express strongly at the sarcolemma. The role(s) of these subunits in cardiomyocytes remain to be defined and may require a reassessment of the molecular nature of ventricular KATP channels.

CONCLUSIONS

Collectively, our data demonstrate unique cellular and subcellular KATP channel subunit expression patterns in the heart. These results suggest distinct roles for KATP channel subunits in diverse cardiac structures.

Mutations in the Kir6.2 subunit of the KATP channel and permanent neonatal diabetes: new insights and new treatment

Permanent neonatal diabetes (PNDM) is diagnosed in the first three months of life and is a major management problem as patients require lifelong insulin injections. Recently, activating mutations in the KCNJ11 gene which encodes the Kir6.2 subunit of the KATP channels in the pancreatic beta-cells were found to be an important cause of PNDM. The mutated KATP channels do not close in the presence of adenosine triphosphate (ATP) so the beta-cell membrane is hyperpolarized and insulin secretion does not occur. Some patients have DEND syndrome (developmental delay, epilepsy and neonatal diabetes) with the neurological features arising from mutated KATP channels in muscle, nerve and brain. Defining a genetic aetiology has not only given insights into clinical classification and disease mechanism, but has also influenced treatment. Sulphonylureas, by binding the sulphonylurea receptor, can close the KATP channel. This has led to patients who were insulin-dependent being able to discontinue insulin injections and achieve excellent control with sulphonylurea tablets. In this article we discuss the work that established Kir6.2 mutations as a common cause of neonatal diabetes, the clinical features, the underlying mechanism and the impact on patient treatment.

Molecular basis of Kir6.2 mutations associated with neonatal diabetes or neonatal diabetes plus neurological features

Inwardly rectifying potassium channels (Kir channels) control cell membrane K(+) fluxes and electrical signaling in diverse cell types. Heterozygous mutations in the human Kir6.2 gene (KCNJ11), the pore-forming subunit of the ATP-sensitive (K(ATP)) channel, cause permanent neonatal diabetes mellitus (PNDM). For some mutations, PNDM is accompanied by marked developmental delay, muscle weakness, and epilepsy (severe disease). To determine the molecular basis of these different phenotypes, we expressed wild-type or mutant (R201C, Q52R, or V59G) Kir6.2/sulfonylurea receptor 1 channels in Xenopus oocytes. All mutations increased resting whole-cell K(ATP) currents by reducing channel inhibition by ATP, but, in the simulated heterozygous state, mutations causing PNDM alone (R201C) produced smaller K(ATP) currents and less change in ATP sensitivity than mutations associated with severe disease (Q52R and V59G). This finding suggests that increased K(ATP) currents hyperpolarize pancreatic beta cells and impair insulin secretion, whereas larger K(ATP) currents are required to influence extrapancreatic cell function. We found that mutations causing PNDM alone impair ATP sensitivity directly (at the binding site), whereas those associated with severe disease act indirectly by biasing the channel conformation toward the open state. The effect of the mutation on ATP sensitivity in the heterozygous state reflects the different contributions of a single subunit in the Kir6.2 tetramer to ATP inhibition and to the energy of the open state. Our results also show that mutations in the slide helix of Kir6.2 (V59G) influence the channel kinetics, providing evidence that this domain is involved in Kir channel gating, and suggest that the efficacy of sulfonylurea therapy in PNDM may vary with genotype.

Y-26763: ATP-sensitive K+ channel activation and the inhibition of insulin release from human pancreatic beta-cells

The effect of Y-26763 [(-)-(3S,4R)-4-(N-acetyl-N-hydroxyamino)-6-cyano-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-3-ol], a novel ATP-sensitive K(+) (K(ATP)) channel activator, was tested on insulin secretion from human pancreatic islets in vitro. Y-26763 was able to inhibit both glucose- and tolbutamide-induced insulin secretion from islets as assessed by radioimmunoassay. The mechanism for inhibition of insulin secretion was characterised using patch clamp electrophysiology on dispersed human pancreatic beta-cells which express K(ATP) channels comprised of Kir6.2 and SUR1, and the NES2Y human beta-cell line, transfected with Kir6.2DeltaC26. Y-26763 activated K(ATP) channels in a reversible manner with a similar activity to diazoxide. This required the presence of hydrolysable nucleotides and appeared to be mediated by interaction of Y-26763 with SUR1 since: (a) tolbutamide was able to reverse the actions of Y-26763 and (b) Y-26763 failed to activate Kir6.2DeltaC26 in the absence of SUR1. We conclude that Y-26763-induced inhibition of insulin release is dependent upon the activation of K(ATP) channels in human beta-cells.

Identification of a functionally important negatively charged residue within the second catalytic site of the SUR1 nucleotide-binding domains

The ATP-sensitive K+ channel (KATP channel) couples glucose metabolism to insulin secretion in pancreatic beta-cells. It is comprised of sulfonylurea receptor (SUR)-1 and Kir6.2 proteins. Binding of Mg nucleotides to the nucleotide-binding domains (NBDs) of SUR1 stimulates channel opening and leads to membrane hyperpolarization and inhibition of insulin secretion. To elucidate the structural basis of this regulation, we constructed a molecular model of the NBDs of SUR1, based on the crystal structures of mammalian proteins that belong to the same family of ATP-binding cassette transporter proteins. This model is a dimer in which there are two nucleotide-binding sites, each of which contains residues from NBD1 as well as from NBD2. It makes the novel prediction that residue D860 in NBD1 helps coordinate Mg nucleotides at site 2. We tested this prediction experimentally and found that, unlike wild-type channels, channels containing the SUR1-D860A mutation were not activated by MgADP in either the presence or absence of MgATP. Our model should be useful for designing experiments aimed at elucidating the relationship between the structure and function of the KATP channel.

Metabolic regulation of the pancreatic beta-cell ATP-sensitive K+ channel: a pas de deux

Closure of ATP-sensitive K+ channels (KATP channels) is a key step in glucose-stimulated insulin secretion. The precise mechanism(s) by which glucose metabolism regulates KATP channel activity, however, remains controversial. It is widely believed that the principal determinants are the intracellular concentrations of the metabolic ligands, ATP and ADP, which have opposing actions on KATP channels, with ATP closing and MgADP opening the channel. However, the sensitivity of the channel to these nucleotides in the intact cell, and their relative contribution to the regulation of channel activity, remains unclear. The precise role of phosphoinositides and long-chain acyl-CoA esters, which are capable of modulating the channel ATP sensitivity, is also uncertain. Furthermore, it is still a matter of debate whether it is changes in the concentration of ATP, of MgADP, or of other agents, which couples glucose metabolism to KATP channel activity. In this article, we review current knowledge of the metabolic regulation of the KATP channel and provide evidence that MgADP (or MgATP hydrolysis), acting at the regulatory subunit of the channel, shifts the ATP concentration-response curve into a range in which the channel pore can respond to dynamic changes in cytosolic ATP. This metabolic pas de deux orchestrates the pivotal role of ATP in metabolic regulation of the KATP channel.

The impact of ATP-sensitive K+ channel subtype selectivity of insulin secretagogues for the coronary vasculature and the myocardium

Insulin secretagogues (sulfonylureas and glinides) increase insulin secretion by closing the ATP-sensitive K+ channel (KATP channel) in the pancreatic beta-cell membrane. KATP channels subserve important functions also in the heart. First, KATP channels in coronary myocytes contribute to the control of coronary blood flow at rest and in hypoxia. Second, KATP channels in the sarcolemma of cardiomyocytes (sarcKATP channels) are required for adaptation of the heart to stress. In addition, the opening of sarcKATP channels and of KATP channels in the inner membrane of mitochondria (mitoKATP channels) plays a central role in ischemic preconditioning. Opening of sarcKATP channels also underlies the ST-segment elevation of the electrocardiogram, the primary diagnostic tool for initiation of lysis therapy in acute myocardial infarction. Therefore, inhibition of cardiovascular KATP channels by insulin secretagogues is considered to increase cardiovascular risk. Electrophysiological experiments have shown that the secretagogues differ in their selectivity for the pancreatic over the cardiovascular KATP channels, being either highly selective (approximately 1,000x; short sulfonylureas such as nateglinide and mitiglinide), moderately selective (10-20x; long sulfonylureas such as glibenclamide [glyburide]), or essentially nonselective (<2x; repaglinide). New binding studies presented here give broadly similar results. In clinical studies, these differences are not yet taken into account. The hypothesis that the in vitro selectivity of the insulin secretagogues is of importance for the cardiovascular outcome of diabetic patients with coronary artery disease needs to be tested.

Effect of two amino acids in TM17 of Sulfonylurea receptor SUR1 on the binding of ATP-sensitive K+ channel modulators

The sulfonylurea receptor (SUR) is the important regulatory subunit of ATP-sensitive K+ channels. It is an ATP-binding cassette protein comprising 17 transmembrane helices. SUR is endowed with binding sites for channel blockers like the antidiabetic sulfonylurea glibenclamide and for the chemically very heterogeneous channel openers. SUR1, the typical pancreatic SUR isoform, shows much higher affinity for glibenclamide but considerably lower affinity for most openers than SUR2. In radioligand binding assays, we investigated the role of two amino acids, T1285 and M1289, located in transmembrane helix (TM)-17, in opener binding to SUR1. These amino acids were exchanged for the corresponding amino acids of SUR2. In competition experiments using [3H]glibenclamide as radioligand, SUR1(T1285L, M1289T) showed much higher affinity toward the cyanoguanidine openers pinacidil and P1075 than SUR1 wild type. The affinity for the thioformamide aprikalim was also markedly increased. In contrast, the affinity for the benzopyrans rilmakalim and levcromakalim was unaffected; however, the amount of displaced [3H]glibenclamide binding was nearly doubled. The binding properties of the opener diazoxide and the blocker glibenclamide were unchanged. In conclusion, mutation of two amino acids in TM17 of SUR1, especially of M1289, leads to class-specific effects on opener binding by increasing opener affinity or by changing allosteric coupling between opener and glibenclamide binding.

ATP-sensitive K+ channel knockout compromises the metabolic benefit of exercise training, resulting in cardiac deficits

Exercise training elicits a metabolic and cardiovascular response that underlies fitness. The molecular mechanisms that orchestrate this adaptive response and secure the wide-ranging gains of a regimented exercise program are poorly understood. Formed through association of the Kir6.2 pore and the sulfonylurea receptor, the stress-responsive ATP-sensitive K(+) channels (K(ATP) channels), with their metabolic-sensing capability and broad tissue expression, are potential candidates for integrating the systemic adaptive response to repetitive exercise. Here, the responses of mice lacking functional Kir6.2-containing K(ATP) channels (Kir6.2-KO) were compared with wild-type controls following a 28-day endurance swimming protocol. While chronic aquatic training resulted in lighter, leaner, and fitter wild-type animals, the Kir6.2-KO manifested less augmentation in exercise capacity and lacked metabolic improvement in body fat composition and glycemic handling with myocellular defects. Moreover, the repetitive stress of swimming unmasked a survival disadvantage in the Kir6.2-KO, associated with pathologic calcium-dependent structural damage in the heart and impaired cardiac performance. Thus, Kir6.2-containing K(ATP) channel activity is required for attainment of the physiologic benefits of exercise training without injury.

Targeting ischemic stroke with a novel opener of ATP-sensitive potassium channels in the brain

During cerebral ischemia, the opening of neuronal ATP-sensitive potassium channels (K(ATP) channels) affords intrinsic protection by regulating membrane potential. To augment this endogenous mechanism, we have synthesized iptakalim, a K(ATP) opener. Through K(ATP) channel activation, iptakalim affected multiple pathways of the glutamatergic system, limiting glutamate release and receptor actions, which are involved in excitotoxicity during ischemic damage. The molecule readily penetrated the brain and showed low toxicity in animal experiments. In different animal models of stroke as well as in cell cultures, iptakalim provided significant neuroprotection, not only in promoting behavioral recovery but also in protecting neurons against necrosis and apoptosis. This compound thus has promise as a neuroprotective drug for the treatment of stroke and other forms of neuronal damage.

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.

Conserved mechanisms of glucose sensing and regulation by Drosophila corpora cardiaca cells

Antagonistic activities of glucagon and insulin control metabolism in mammals, and disruption of this balance underlies diabetes pathogenesis. Insulin-producing cells (IPCs) in the brain of insects such as Drosophila also regulate serum glucose, but it remains unclear whether insulin is the sole hormonal regulator of glucose homeostasis and whether mechanisms of glucose-sensing and response in IPCs resemble those in pancreatic islets. Here we show, by targeted cell ablation, that Drosophila corpora cardiaca (CC) cells of the ring gland are also essential for larval glucose homeostasis. Unlike IPCs, CC cells express Drosophila cognates of sulphonylurea receptor (Sur) and potassium channel (Ir), proteins that comprise ATP-sensitive potassium channels regulating hormone secretion by islets and other mammalian glucose-sensing cells. They also produce adipokinetic hormone, a polypeptide with glucagon-like functions. Glucose regulation by CC cells is impaired by exposure to sulphonylureas, drugs that target the Sur subunit. Furthermore, ubiquitous expression of an akh transgene reverses the effect of CC ablation on serum glucose. Thus, Drosophila CC cells are crucial regulators of glucose homeostasis and they use glucose-sensing and response mechanisms similar to islet cells.

Activation of rat mesenteric arterial KATP channels by 11,12-epoxyeicosatrienoic acid

Epoxyeicosatrienoic acids (EETs), the cytochrome P-450 epoxygenase metabolites of arachidonic acid, are candidates of endothelium-derived hyperpolarizing factors. We have previously reported that EETs are potent activators of cardiac ATP-sensitive K(+) (K(ATP)) channels, but their effects on the vascular K(ATP) channels are unknown. With the use of whole cell patch-clamp techniques with 0.1 mM ATP in the pipette and holding at -60 mV, freshly isolated smooth muscle cells from rat mesenteric arteries had small glibenclamide-sensitive currents at baseline (13.1 +/- 3.9 pA, n = 5) that showed a 7.2-fold activation by 10 microM pinacidil (94.1 +/- 21.9 pA, n = 7, P < 0.05). 11,12-EET dose dependently activated the K(ATP) current with an apparent EC(50) of 87 nM. Activation of the K(ATP) channels by 500 nM 11,12-EET was inhibited by inclusion of the PKA inhibitor peptide (5 microM) but not by the inclusion of the PKC inhibitor peptide (100 microM) in the pipette solution. These results were corroborated by vasoreactivity studies. 11,12-EET produced dose-dependent vasorelaxation in isolated small mesenteric arteries, and this effect was reduced by 50% with glibenclamide (1 microM) preincubation. The 11,12-EET effects on vasorelaxation were also significantly attenuated by preincubation with cell-permeant PKA inhibitor myristoylated PKI(14-22), and, in the presence of PKA inhibitor, glibenclamide had no additional effects. These results suggest that 11,12-EET is a potent activator of the vascular K(ATP) channels, and its effects are dependent on PKA activities.

Potassium channel regulation

The sulphonylurea receptor (SUR) is a member of the ATP-binding cassette (ABC) family of membrane proteins. It functions as the regulatory subunit of the ATP-sensitive potassium (KATP) channel, which comprises SUR and Kir6.x proteins. Here, we review data demonstrating functional differences between the two nucleotide binding domains (NBDs) of SUR1. In addition, to explain the structural basis of these functional differences, we have constructed a molecular model of the NBD dimer of human SUR1. We discuss the experimental data in the context of this model, and show how the model can be used to design experiments aimed at elucidating the relationship between the structure and function of the KATP channel.

Potassium channel regulation

The sulphonylurea receptor (SUR) is a member of the ATP-binding cassette (ABC) family of membrane proteins. It functions as the regulatory subunit of the ATP-sensitive potassium (KATP) channel, which comprises SUR and Kir6.x proteins. Here, we review data demonstrating functional differences between the two nucleotide binding domains (NBDs) of SUR1. In addition, to explain the structural basis of these functional differences, we have constructed a molecular model of the NBD dimer of human SUR1. We discuss the experimental data in the context of this model, and show how the model can be used to design experiments aimed at elucidating the relationship between the structure and function of the KATP channel.

Arylcyanoguanidines as activators of Kir6.2/SUR1K ATP channels and inhibitors of insulin release

Phenylcyanoguanidines substituted with lipophilic electron-withdrawing functional groups, e.g. N-cyano-N'-[3,5-bis-(trifluoromethyl)phenyl]-N' '-(cyclopentyl)guanidine (10) and N-cyano-N'-(3,5-dichlorophenyl)-N' '-(3-methylbutyl)guanidine (12) were synthesized and investigated for their ability to inhibit insulin release from beta cells, to repolarize beta cell membrane potential, and to relax precontracted rat aorta rings. Structural modifications gave compounds, which selectively inhibit insulin release from betaTC6 cells (e.g. compound 10: IC(50) = 5.45 +/- 1.9 microM) and which repolarize betaTC3 beta cells (10: IC(50) = 4.7 +/- 0.5 microM) without relaxation of precontracted aorta rings (10: IC(50) > 300 microM). Inhibition of insulin release from rat islets was observed in the same concentration level as for betaTC6 cells (10: IC(50) = 1.24 +/- 0.1 microM, 12: IC(50) = 3.8 +/- 0.4 microM). Compound 10 (10 microM) inhibits calcium outflow and insulin release from perifused rat pancreatic islets. The mechanisms of action of 10 and 12 were further investigated. The compounds depolarize mitochondrial membrane from smooth muscle cells and beta cell and stimulate glucose utilization and mitochondrial respiration in isolated liver cells. Furthermore, 10 was studied in a patch clamp experiment and was found to activate Kir6.2/SUR1 and inhibit Kir6.2/SUR2B type of K(ATP) channels. These studies indicate that the observed effects of the compounds on beta cells result from activation of K(ATP) channels of the cell membrane in combination with a depolarization of mitochondrial membranes. It also highlights that small structural changes can dramatically shift the efficacy of the cyanoguanidine type of selective activators of Kir6.2/SUR2 potassium channels.

Altered gene expression and increased bursting activity of colonic smooth muscle ATP-sensitive K+ channels in experimental colitis

The ATP-sensitive K(+) channel (K(ATP)) is a complex composed of an inwardly rectifying, pore-forming subunit (Kir 6.1 and Kir 6.2) and the sulfonylurea receptor (SUR1 and SUR2). In gastrointestinal smooth muscle, these channels are important in regulating cell excitability. We examined the molecular composition of the K(ATP) channel in mouse colonic smooth muscle and determined its activity in the pathophysiological setting of experimental colitis. Following 7 days of dextran sulfate sodium (DSS) treatment in drinking water, colonic inflammation was scored by histology and physical signs. In whole cell recordings, levcromakalim-induced currents were significantly larger in inflamed cells. In cell-attached patch recordings of single-channel events, levcromakalim enhanced the bursting duration in inflamed cells. The single-channel conductance of approximately 42 pS was not altered with inflammation. mRNA for both Kir 6.1 and 6.2 were detected by RT-PCR. Kir 6.1 was localized to the plasma membrane, whereas Kir 6.2 was mainly detected in the cytosol by immunohistochemistry. Quantitative PCR showed that Kir 6.1 gene expression was upregulated by almost 22-fold, whereas SUR2B was downregulated by threefold after inflammation. Thus decreased motility of the colon during inflammation may be associated with changes in the transcriptional regulation of Kir 6.1 and SUR2B gene expression.

Zinc is both an intracellular and extracellular regulator of KATP channel function

Extracellular Zn(2+) has been identified as an activator of pancreatic K(ATP) channels. We further examined the action of Zn(2+) on recombinant K(ATP) channels formed with the inward rectifier K(+) channel subunit Kir6.2 associated with either the pancreatic/neuronal sulphonylurea receptor 1 (SUR1) subunit or the cardiac SUR2A subunit. Zn(2+), applied at either the extracellular or intracellular side of the membrane appeared as a potent, reversible activator of K(ATP) channels. External Zn(2+), at micromolar concentrations, activated SUR1/Kir6.2 but induced a small inhibition of SUR2A/Kir6.2 channels. Cytosolic Zn(2+) dose-dependently stimulated both SUR1/Kir6.2 and SUR2A/Kir6.2 channels, with half-maximal effects at 1.8 and 60 microm, respectively, but it did not affect the Kir6.2 subunit expressed alone. These observations point to an action of both external and internal Zn(2+) on the SUR subunit. Effects of internal Zn(2+) were not due to Zn(2+) leaking out, since they were unaffected by the presence of a Zn(2+) chelator on the external side. Similarly, internal chelators did not affect activation by external Zn(2+). Therefore, Zn(2+) is an endogenous K(ATP) channel opener being active on both sides of the membrane, with potentially distinct sites of action located on the SUR subunit. These findings uncover a novel regulatory pathway targeting K(ATP) channels, and suggest a new role for Zn(2+) as an intracellular signalling molecule.

Muscle membrane polarisation after provocative tests, and after cooling: the normal CMAP changes to be expected

OBJECTIVE

To know the range of changes of compound muscle action potentials (CMAPs) in the muscles innervated by the ulnar nerve after diverse provocative tests, 14 healthy patients were studied with the same protocol.

METHODS

CMAPs were measured at rest, just after a short exercise test (SET), during short 5 and 10c/s repetitive nerve stimulation (RNS) trains, at approximately 32 and approximately 20 degrees C.

RESULTS

At 32 degrees C, the SET induced a significant but transient enlargement of the CMAPs (amplitude increased by 8.3%, duration decreased by 9%) that was only partially reproduced by RNS trains, except for a significant shortening of the CMAPs at 10c/s. At 20 degrees C without exercise, CMAPs increased significantly by approximately 30% in amplitude, duration and area, and after the SET the inverse of what has been seen at 32 degrees C was observed (amplitude decreased by 1.7% and duration increased by 9%). RNS at 20 degrees C produced a marked interpatient heterogeneity except for a significant shortening of the CMAPs at 10c/s. In one pure autonomic failure patient, the infusion of norepinephrine induced potentiation of the responses at rest and a decrease in the expected changes after provocative tests.

CONCLUSIONS

CMAP amplitude and duration are significantly modified just after the SET at 32 degrees C, at rest at 20 degrees C and after RNS at 10c/s but not at 5c/s. Although providing indirect evidence, these findings indicate that provocative tests make the muscle membrane hyperexcitable by the way of a direct influence on the electrical events and by an indirect local catecholamine spillover.

Evidence for mitochondrial K+ channels and their role in cardioprotection

Twenty years after the discovery of sarcolemmal ATP-sensitive K+ channels and 12 years after the discovery of mitochondrial K(ATP) (mitoK(ATP)) channels, progress has been remarkable, but many questions remain. In the case of the former, detailed structural information is available, and it is well accepted that the channel couples bioenergetics to cellular electrical excitability; however, in the heart, a clear physiological or pathophysiological role has yet to be defined. For mitoK(ATP), structural information is lacking, but there is abundant evidence linking the opening of the channel to protection against ischemia-reperfusion injury or apoptosis. This review updates recent progress in understanding the physiological role of mitoK(ATP) and highlights outstanding questions and controversies, with the intent of stimulating additional investigation on this topic.

NO stimulation of ATP-sensitive potassium channels: Involvement of Ras/mitogen-activated protein kinase pathway and contribution to neuroprotection

ATP-sensitive potassium (K(ATP)) channels regulate insulin release, vascular tone, and neuronal excitability. Whether these channels are modulated by NO, a membrane-permeant messenger in various physiological and pathological processes, is not known. The possibility of NO signaling via K(ATP) channel modulation is of interest because both NO and K(ATP) have been implicated in physiological functions such as vasodilation and neuroprotection. In this report, we demonstrate a mechanism that leads to K(ATP) activation via NO/Ras/mitogen-activated protein kinase pathway. By monitoring K(ATP) single-channel activities from human embryonic kidney 293 cell-attached patches expressing sulfonylurea receptor 2B and Kir6.2, we found K(ATP) stimulation by NO donor Noc-18, a specific NO effect abolished by NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) but not guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). NO stimulation of K(ATP) is indirect and requires Ras and mitogen-activated protein kinase kinase activities. Blockade of Ras activation by pharmacological means or by coexpressing either a dominant-negative or an S-nitrosylation-site mutant Ras protein significantly abrogates the effects of NO. Inhibition of mitogen-activated protein kinase kinase abolishes the NO activation of K(ATP) but suppression of phosphatidylinositol 3-kinase does not. The NO precursor l-Arg also stimulates K(ATP) via endogenous NO synthase and the Ras signaling pathway. In addition, in rat hippocampal neurons, the protective effect of ischemic preconditioning induced by oxygen-glucose deprivation requires K(ATP) and NO synthase activity during preconditioning. Thus, neuroprotection caused by NO released during the short episode of sublethal ischemia may be mediated partly by K(ATP) stimulation.

Interaction of a novel dihydropyridine K+ channel opener, A-312110, with recombinant sulphonylurea receptors and KATP channels: comparison with the cyanoguanidine P1075

1. ATP-sensitive K(+) channels (K(ATP) channels) are composed of pore-forming subunits (Kir6.x) and of regulatory subunits, the sulphonylurea receptors (SURx). Synthetic openers of K(ATP) channels form a chemically heterogeneous class of compounds that are of interest in several therapeutic areas. We have investigated the interaction of a novel dihydropyridine opener, A-312110 ((9R)-9-(4-fluoro-3-iodophenyl)-2,3,5,9-tetrahydro-4H-pyrano[3,4-b]thieno [2,3-e]pyridin-8(7H)-one-1,1-dioxide), with SURs and Kir6/SUR channels in comparison to the cyanoguanidine opener P1075. 2. In the presence of 1 mM MgATP, A-312110 bound to SUR2A (the SUR in cardiac and skeletal muscle) and to SUR2B (smooth muscle) with K(i) values of 14 and 18 nM; the corresponding values for P1075 were 16 and 9 nM, respectively. Decreasing the MgATP concentration reduced the affinity of A312110 binding to SUR2A significantly more than that to SUR2B; for P1075, the converse was true. At SUR1 (pancreatic beta-cell), both openers showed little binding up to 100 microM. 3. In the presence of MgATP, both openers inhibited [(3)H]glibenclamide binding to the SUR2 subtypes in a biphasic manner. In the absence of MgATP, the high-affinity component of the inhibition curves was absent. 4. In inside-out patches, the two openers activated the Kir6.2/SUR2A and Kir6.2/SUR2B channels with similar potency (approximately 50 nm). Both were almost 2 x more efficacious in opening the Kir6.2/SUR2B than the Kir6.2/SUR2A channel. 5. The results show that the novel dihydropyridine A-312110 is a potent K(ATP) channel opener with binding and channel-opening properties similar to those of P1075.

Mapping the architecture of the ATP-binding site of the KATP channel subunit Kir6.2

ATP-sensitive potassium (K(ATP)) channels comprise Kir6.2 and SUR subunits. The site at which ATP binds to mediate K(ATP) channel inhibition lies on Kir6.2, but the potency of block is enhanced by coexpression with SUR1. To assess the structure of the ATP-binding site on Kir6.2, we used a range of adenine nucleotides as molecular measuring sticks to map the internal dimensions of the binding site. We compared their efficacy on Kir6.2-SUR1, and on a truncated Kir6.2 (Kir6.2DeltaC) that expresses in the absence of SUR. We show here that SUR1 modifies the ATP-binding pocket of Kir6.2, by increasing the width of the groove that binds the phosphate tail of ATP, without changing the length of the groove, and by enhancing interaction with the adenine ring.

Molecular analysis of the subtype-selective inhibition of cloned KATP channels by PNU-37883A

1. In this study, we have used Kir6.1/Kir6.2 chimeric proteins and current recordings to investigate the molecular basis of PNU-37883A inhibition of cloned K(ATP) channels. 2. Rat Kir6.1, Kir6.2 and Kir6.1/Kir6.2 chimeras were co-expressed with either SUR2B or SUR1, following RNA injection into Xenopus oocytes, and fractional inhibition of K(ATP) currents by 10 microm PNU-37883A reported. 3. Channels containing Kir6.1/SUR2B were more sensitive to inhibition by PNU-37883A than those containing Kir6.2/SUR2B (mean fractional inhibition: 0.70, cf. 0.07). 4. On expression with SUR2B, a chimeric channel with the Kir6.1 pore and the Kir6.2 amino- and carboxy-terminal domains was PNU-37883A insensitive (0.06). A chimera with the Kir6.1 carboxy-terminus and Kir6.2 amino-terminus and pore was inhibited (0.48). These results, and those obtained with other chimeras, suggest that the C-terminus is an important determinant of PNU-37883A inhibition of Kir6.1. Similar results were seen when constructs were co-expressed with SUR1. Further chimeric constructs localised PNU-37883A sensitivity to an 81 amino-acid residue section in the Kir6.1 carboxy-terminus. 5. Our data show that structural differences between Kir6.1 and Kir6.2 are important in determining sensitivity to PNU-37883A. This compound may prove useful in probing the structural and functional differences between the two channel subtypes.

Binding of sulfonylurea by AtMRP5, an Arabidopsis multidrug resistance-related protein that functions in salt tolerance

Recently, a new member of the ABC transporter superfamily of Arabidopsis, AtMRP5, was identified and characterized. In the present work, we found that AtMRP5 can bind specifically to sulfonurea when it is expressed in HEK293 cells. We also present evidence for a new role of AtMRP5 in the salt stress response of Arabidopsis. We used reverse genetics to identify an Arabidopsis mutant (atmrp5-2) in which the AtMRP5 gene was disrupted by transferred DNA insertion. In root-bending assays using Murashige and Skoog medium supplemented with 100 mm NaCl, root growth of atmrp5-2 was substantially inhibited in contrast to the almost normal growth of wild-type seedlings. This hypersensitive response of the atmrp5-2 mutant was not observed during mannitol treatment. The root growth of the wild-type plant grown in Murashige and Skoog medium supplemented with the MRP inhibitor glibenclamide and NaCl was inhibited to a very similar extent as the root growth of atmrp5-2 grown in NaCl alone. The Na(+)-dependent reduction of root growth of the wild-type plant in the presence of glibenclamide was partially restored by diazoxide, a known K+ channel opener that reverses the inhibitory effects of sulfonylureas in animal cells. Moreover, the atmrp5-2 mutant was defective in 86Rb+ uptake. When seedlings were treated with 100 mm NaCl, atmrp5-2 seedlings accumulated less K+ and more Na+ than those of the wild type. These observations suggest that AtMRP5 is a putative sulfonylurea receptor that is involved in K+ homeostasis and, thus, also participates in the NaCl stress response.

Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Dunne, Mark J., Karen E. Cosgrove, Ruth M. Shepherd, Albert Aynsley-Green, and Keith J. Lindley. Hyperinsulinism in Infancy: From Basic Science to Clinical Disease. Physiol Rev 84: 239–275, 2004; 10.1152/physrev.00022.2003.—Ion channelopathies have now been described in many well-characterized cell types including neurons, myocytes, epithelial cells, and endocrine cells. However, in only a few cases has the relationship between altered ion channel function, cell biology, and clinical disease been defined. Hyperinsulinism in infancy (HI) is a rare, potentially lethal condition of the newborn and early childhood. The causes of HI are varied and numerous, but in almost all cases they share a common target protein, the ATP-sensitive K+channel. From gene defects in ion channel subunits to defects in β-cell metabolism and anaplerosis, this review describes the relationship between pathogenesis and clinical medicine. Until recently, HI was generally considered an orphan disease, but as parallel defects in ion channels, enzymes, and metabolic pathways also give rise to diabetes and impaired insulin release, the HI paradigm has wider implications for more common disorders of the endocrine pancreas and the molecular physiology of ion transport.

Regulation of action potential duration under acute heat stress by I(K,ATP) and I(K1) in fish cardiac myocytes

The mechanism underlying temperature-dependent shortening of action potential (AP) duration was examined in the fish (Carassius carassius L.) heart ventricle. Acute temperature change from +5 to +18 degrees C (heat stress) shortened AP duration from 2.8 +/- 0.3 to 1.3 +/- 0.1 s in intact ventricles. In 56% (18 of 32) of enzymatically isolated myocytes, heat stress also induced reversible opening of ATP-sensitive K+ channels and increased their single-channel conductance from 37 +/- 12 pS at +8 degrees C to 51 +/- 13 pS at +18 degrees C (Q10 = 1.38) (P < 0.01; n = 12). The ATP-sensitive K+ channels of the crucian carp ventricle were characterized by very low affinity to ATP both at +8 degrees C [concentration of Tris-ATP that produces half-maximal inhibition of the channel (K1/2)= 1.35 mM] and +18 degrees C (K1/2 = 1.85 mM). Although acute heat stress induced ATP-sensitive K+ current (IK,ATP) in patch-clamped myocytes, similar heat stress did not cause any glibenclamide (10 microM)-sensitive changes in AP duration in multicellular ventricular preparations. Examination of APs and K+ currents from the same myocytes by alternate recording under current-clamp and voltage-clamp modes revealed that changes in AP duration were closely correlated with temperature-specific changes in the voltage-dependent rectification of the background inward rectifier K+ current IK1. In approximately 15% of myocytes (4 out of 27), IK,ATP-dependent shortening of AP followed the IK1-induced AP shortening. Thus heat stress-induced shortening of AP duration in crucian carp ventricle is primarily dependent on IK1. IK,ATP is induced only in response to prolonged temperature elevation or perhaps in the presence of additional stressors.

Potent and selective activation of the pancreatic beta-cell type K(ATP) channel by two novel diazoxide analogues

AIMS/HYPOTHESIS

We investigated the pharmacological properties of two novel ATP sensitive potassium (K(ATP)) channel openers, 6-Chloro-3-isopropylamino-4 H-thieno[3,2- e]-1,2,4-thiadiazine 1,1-dioxide (NNC 55-0118) and 6-chloro-3-(1-methylcyclopropyl)amino-4 H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (NN414), on the cloned cardiac (Kir6.2/SUR2A), smooth muscle (Kir6.2/SUR2B) and pancreatic beta cell (Kir6.2/SUR1) types of K(ATP) channel.

METHODS

We studied the effects of these compounds on whole-cell currents through cloned K(ATP) channels expressed in Xenopus oocytes or mammalian cells (HEK293). We also used inside-out macropatches excised from Xenopus oocytes.

RESULTS

In HEK 293 cells, NNC 55-0118 and NN414 activated Kir6.2/SUR1 currents with EC(50) values of 0.33 micromol/l and 0.45 micromol/l, respectively, compared with that of 31 micro mol/l for diazoxide. Neither compound activated Kir6.2/SUR2A or Kir6.2/SUR2B channels expressed in oocytes, nor did they activate Kir6.2 expressed in the absence of SUR. Current activation was dependent on the presence of intracellular MgATP, but was not supported by MgADP.

CONCLUSION/INTERPRETATION

Both NNC 55-0118 and NN414 selectively stimulate the pancreatic beta-cell type of K(ATP) channel with a higher potency than diazoxide, by interaction with the SUR1 subunit. The high selectivity and efficacy of the compounds could prove useful for treatment of disease states where inhibition of insulin secretion is beneficial.

Sulphonylurea action revisited: the post-cloning era

Hypoglycaemic agents such as sulphonylureas and the newer group of "glinides" stimulate insulin secretion by closing ATP-sensitive potassium (K(ATP)) channels in pancreatic beta cells, but have varying cross-reactivity with related channels in extrapancreatic tissues such as heart, vascular smooth and skeletal muscle. Experiments on the structure-function relationships of recombinant K(ATP) channels and the phenotypes of mice deficient in different K(ATP) channel subunits have provided important insights into the mechanisms underlying sulphonylurea selectivity, and the potential consequences of K(ATP) channel blockade outside the pancreatic beta cell. The different pharmacological properties of K(ATP) channels from beta cells compared with those from cardiac, smooth and skeletal muscle, are accounted for by the expression of alternative types of sulphonylurea receptor, with non-identical drug binding sites. The sulphonylureas and glinides are found to fall into two groups: one exhibiting selectivity for beta cell sulphonylurea receptors (SUR1), and the other blocking cardiovascular and skeletal muscle sulphonylurea receptors (SUR2) with potencies similar to their action on SUR1. In seeking potential side effects of K(ATP) channel inhibitors in humans, it is essential to take these drug differences into account, along with the probability (suggested by the studies on K(ATP) channel knockout mice) that the effects of extrapancreatic K(ATP) channel inhibition might be either subtle or rare. Further studies are still required before a final decision can be made on whether non-selective agents are appropriate for the therapy of Type 2 diabetes.

Identification of residues contributing to the ATP binding site of Kir6.2

The ATP-sensitive potassium (K(ATP)) channel links cell metabolism to membrane excitability. Intracellular ATP inhibits channel activity by binding to the Kir6.2 subunit of the channel, but the ATP binding site is unknown. Using cysteine-scanning mutagenesis and charged thiol-modifying reagents, we identified two amino acids in Kir6.2 that appear to interact directly with ATP: R50 in the N-terminus, and K185 in the C-terminus. The ATP sensitivity of the R50C and K185C mutant channels was increased by a positively charged thiol reagent (MTSEA), and was reduced by the negatively charged reagent MTSES. Comparison of the inhibitory effects of ATP, ADP and AMP after thiol modification suggests that K185 interacts primarily with the beta-phosphate, and R50 with the gamma-phosphate, of ATP. A molecular model of the C-terminus of Kir6.2 (based on the crystal structure of Kir3.1) was constructed and automated docking was used to identify residues interacting with ATP. These results support the idea that K185 interacts with the beta-phosphate of ATP. Thus both N- and C-termini may contribute to the ATP binding site.

Cellular remodeling in heart failure disrupts K(ATP) channel-dependent stress tolerance

ATP-sensitive potassium (K(ATP)) channels are required for maintenance of homeostasis during the metabolically demanding adaptive response to stress. However, in disease, the effect of cellular remodeling on K(ATP) channel behavior and associated tolerance to metabolic insult is unknown. Here, transgenic expression of tumor necrosis factor alpha induced heart failure with typical cardiac structural and energetic alterations. In this paradigm of disease remodeling, K(ATP) channels responded aberrantly to metabolic signals despite intact intrinsic channel properties, implicating defects proximal to the channel. Indeed, cardiomyocytes from failing hearts exhibited mitochondrial and creatine kinase deficits, and thus a reduced potential for metabolic signal generation and transmission. Consequently, K(ATP) channels failed to properly translate cellular distress under metabolic challenge into a protective membrane response. Failing hearts were excessively vulnerable to metabolic insult, demonstrating cardiomyocyte calcium loading and myofibrillar contraction banding, with tolerance improved by K(ATP) channel openers. Thus, disease-induced K(ATP) channel metabolic dysregulation is a contributor to the pathobiology of heart failure, illustrating a mechanism for acquired channelopathy.