ATP-regulated potassium channels and voltage-gated calcium channels in pancreatic alpha and beta cells: similar functions but reciprocal effects on secretion

Closure of ATP-regulated K⁺ channels (K_ATP channels) plays a central role in glucose-stimulated insulin secretion in beta cells. K_ATP channels are also highly expressed in glucagon-producing alpha cells, where their function remains unresolved. Under hypoglycaemic conditions, K_ATP channels are open in alpha cells but their activity is low and only ~1% of that in beta cells. Like beta cells, alpha cells respond to hyperglycaemia with K_ATP channel closure, membrane depolarisation and stimulation of action potential firing. Yet, hyperglycaemia reciprocally regulates glucagon (inhibition) and insulin secretion (stimulation). Here we discuss how this conundrum can be resolved and how reduced K_ATP channel activity, via membrane depolarisation, paradoxically reduces alpha cell Ca²⁺ entry and glucagon exocytosis. Finally, we consider whether the glucagon secretory defects associated with diabetes can be attributed to impaired K_ATP channel regulation and discuss the potential for remedial pharmacological intervention using sulfonylureas.     

Nicorandil ameliorates impulse conduction disturbances during ischemia in isolated arterially perfused canine atria

BackgroundNicorandil has protective effects on the ischemic atrial myocardium. However, effects of nicorandil on ischemia-induced impulse conduction disturbances are still uncertain.MethodsOptical action potentials were recorded from 256 sites of the left atrium in isolated arterially perfused canine atria during the left atrial ischemia. Constant pacing (BCL = 350 ms) from the left superior pulmonary vein (LSPV) and the posterior left atrium (PLA) was performed, and local conduction velocity (CV) was calculated at the LSPV–left atrial (LA) junction and the right inferior PV (RIPV)–LA junction. Impulse conduction failure was elucidated within the optical mapping field during sinus rhythm.ResultsIn the control, ischemia slowed the local CV at both regions regardless of the pacing site, and impulse conduction failure occurred within the mapping field during sinus rhythm. Nicorandil suppressed the ischemic conduction slowing at both regions and prevented the conduction failure. Nicorandil also reduced the dispersion of local CV during ischemia. HMR1098, a blocker of cardiac sarcolemmal K_ATP channels abolished suppression of the ischemic conduction slowing by nicorandil at the RIPV–LA junction but not at the LSPV–LA junction and induced the conduction failure. 5-HD, a blocker of mitochondrial K_ATP channels also abolished it at both regions and induced the conduction failure. 5-HD abolished the decreased dispersion of local CV by nicorandil, and HMR1098 further increased the dispersion of local CV compared with the control.ConclusionsThese results indicate that nicorandil suppresses ischemia-induced impulse conduction disturbances by its action on both the mitochondrial and sarcolemmal K_ATP channels.     

Nicorandil opens mitochondrial K<Subscript>ATP</Subscript> channels not only directly but also through a NO-PKG-dependent pathway

Nicorandil, a hybrid of nitrate generator and potassium channel opener, protects ischemic myocardium by opening mitochondrial ATP sensitive potassium (mitoK_ATP) channels. We recently found that nitric oxide (NO) opened K_ATP channels in rabbit hearts by a protein kinase G (PKG) mechanism. This study examined whether the NO-donor property of nicorandil also contributes to opening of mitoK_ATP channels through PKG. MitoK_ATP channel opening was monitored in adult rabbit cardiomyocytes by measuring reactive oxygen species (ROS) production, an established marker of channel opening. Nicorandil increased ROS production in a dose-dependent manner. The selective mitoK_ATP channel inhibitor 5-hydroxydecanoate (200 μM) completely blocked ROS production by nicorandil at all doses. The PKG inhibitor 8-bromoguanosine-3’,5’-cyclic monophosphorothioate, Rpisomer (Rp-8-Br-cGMPs, 50 μM) shifted the dose-ROS production curve to the right with an increase of the EC₅₀ from 2.4 x 10^–5 M to 6.9 x 10^–5 M. Rp- 8-Br-cGMPs did not affect the increase in ROS production by the selective mitoK_ATP channel opener diazoxide while it completely blocked increased ROS production from the NO donor S-nitroso-N-acetylpenicillamine (1 μM). Furthermore ODQ, an antagonist of soluble guanylyl cyclase, blocked nicorandil’s ability to increase ROS generation. These results indicate that nicorandil, in addition to its direct effect on the channels, opens mitoK_ATP channels indirectly via a NO-PKG signaling pathway.     

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.Abbreviations K_ATP channel ATP sensitive potassium channel - Kir inwardly-rectifying potassium channel - SUR sulphonylurea receptor - TMD transmembrane domain - NBD nucleotide binding domain - CL cytoplasmic linker - CHI congenital hyperinsulinism     

Leptin-stimulated KATP channel trafficking

Insulin secretion from pancreatic β-cells is initiated by the closure of ATP-sensitive K⁺ channels (K_ATP) in response to high concentrations of glucose, and this action of glucose is counteracted by the hormone leptin, an adipokine that signals through the Ob-R_b receptor to increase K_ATP channel activity. Despite intensive investigations, the molecular basis for K_ATP channel regulation remains uncertain, particularly from the standpoint of whether fluctuations in plasma membrane K_ATP channel content underlie alterations of K_ATP channel activity in response to glucose or leptin. Surprisingly, newly published findings reveal that leptin stimulates AMP-activated protein kinase (AMPK) in order to promote trafficking of K_ATP channels from cytosolic vesicles to the plasma membrane of β-cells. This action of leptin is mimicked by low concentrations of glucose that also activate AMPK and that inhibit insulin secretion. Thus, a new paradigm for β-cell stimulus-secretion coupling is suggested in which leptin exerts a tonic inhibitory effect on β-cell excitability by virtue of its ability to increase plasma membrane K_ATP channel density and whole-cell K_ATP channel current. One important issue that remains unresolved is whether high concentrations of glucose suppress AMPK activity in order to shift the balance of membrane cycling so that K_ATP channel endocytosis predominates over vesicular K_ATP channel insertion into the plasma membrane. If so, high concentrations of glucose might transiently reduce K_ATP channel density/current, thereby favoring β-cell depolarization and insulin secretion. Such an AMPK-dependent action of glucose would complement its established ability to generate an increase of ATP/ADP concentration ratio that directly closes K_ATP channels in the plasma membrane.     

Intractable hyperkalemia due to nicorandil induced potassium channel syndrome

Nicorandil is a commonly used antianginal agent, which has both nitrate-like and ATP-sensitive potassium (K_ATP) channel activator properties. Activation of potassium channels by nicorandil causes expulsion of potassium ions into the extracellular space leading to membrane hyperpolarization, closure of voltage-gated calcium channels and finally vasodilatation. However, on the other hand, being an activator of K_ATP channel, it can expel K⁺ ions out of the cells and can cause hyperkalemia. Here, we report a case of nicorandil induced hyperkalemia unresponsive to medical treatment in a patient with diabetic nephropathy.     

MgATP activates the β cell KATP channel by interaction with its SUR1 subunit

ATP-sensitive potassium (K_ATP) channels in the pancreatic β cell membrane mediate insulin release in response to elevation of plasma glucose levels. They are open at rest but close in response to glucose metabolism, producing a depolarization that stimulates Ca²⁺ influx and exocytosis. Metabolic regulation of K_ATP channel activity currently is believed to be mediated by changes in the intracellular concentrations of ATP and MgADP, which inhibit and activate the channel, respectively. The β cell K_ATP channel is a complex of four Kir6.2 pore-forming subunits and four SUR1 regulatory subunits: Kir6.2 mediates channel inhibition by ATP, whereas the potentiatory action of MgADP involves the nucleotide-binding domains (NBDs) of SUR1. We show here that MgATP (like MgADP) is able to stimulate K_ATP channel activity, but that this effect normally is masked by the potent inhibitory effect of the nucleotide. Mg²⁺ caused an apparent reduction in the inhibitory action of ATP on wild-type K_ATP channels, and MgATP actually activated K_ATP channels containing a mutation in the Kir6.2 subunit that impairs nucleotide inhibition (R50G). Both of these effects were abolished when mutations were made in the NBDs of SUR1 that are predicted to abolish MgATP binding and/or hydrolysis (D853N, D1505N, K719A, or K1384M). These results suggest that, like MgADP, MgATP stimulates K_ATP channel activity by interaction with the NBDs of SUR1. Further support for this idea is that the ATP sensitivity of a truncated form of Kir6.2, which shows functional expression in the absence of SUR1, is unaffected by Mg²⁺.     

Early opening of sarcolemmal ATP-sensitive potassium channels is not a key step in PKC-mediated cardioprotection

ATP-sensitive potassium (K_ATP) channels are abundantly expressed in the myocardium. Although a definitive role for the channel remains elusive they have been implicated in the phenomenon of cardioprotection, but the precise mechanism is unclear. We set out to test the hypothesis that the channel protects by opening early during ischemia to shorten action potential duration and reduce electrical excitability thus sparing intracellular ATP. This could reduce reperfusion injury by improving calcium homeostasis. Using a combination of contractile function analysis, calcium fluorescence imaging and patch clamp electrophysiology in cardiomyocytes isolated from adult male Wistar rats, we demonstrated that the opening of sarcolemmal K_ATP channels was markedly delayed after cardioprotective treatments: ischemic preconditioning, adenosine and PMA. This was due to the preservation of intracellular ATP for longer during simulated ischemia therefore maintaining sarcolemmal K_ATP channels in the closed state for longer. As the simulated ischemia progressed, K_ATP channels opened to cause contractile, calcium transient and action potential failure; however there was no indication of any channel activity early during simulated ischemia to impart an energy sparing hyperpolarization or action potential shortening. We present compelling evidence to demonstrate that an early opening of sarcolemmal K_ATP channels during simulated ischemia is not part of the protective mechanism imparted by ischemic preconditioning or other PKC-dependent cardioprotective stimuli. On the contrary, channel opening was actually delayed. We conclude that sarcolemmal K_ATP channel opening is a consequence of ATP depletion, not a primary mechanism of ATP preservation in these cells.     

Sarcolemmal KATP Channels in the Heart: Molecular Mechanisms Brought to Light, but Physiologic Consequences Still in the Dark

Sarcolemmal K_ATP Channels in the Heart. ATP‐sensitive potassium (K_ATP) channels are inhibited by intracellular ATP and thus couple the metabolic state of the cell to its electrical activity. Tremendous progress has been made in the identification of the molecular basis of K_ATP channel function and regulation. The answer to one key question, however, has proven elusive: What are the precise conditions for, and functional consequences of, sarcolemmal K_ATP activation in physiologic and pathophysiologic states? Here we consider recent studies of the molecular basis of cardiac K_ATP channel activity and the role of these channels in cardiac function during ischemia.     

Glucose‐stimulated insulin secretion: A newer perspective

Existing concepts and models for glucose‐stimulated insulin secretion (GSIS) are overviewed and a newer perspective has been formulated toward the physiological understanding of GSIS. A conventional model has been created on the basis of in vitro data on application of a square wave high glucose in the absence of any other stimulatory inputs. Glucose elicits rapid insulin release through an adenosine triphosphate‐sensitive K⁺ channel (K_ATP channel)‐dependent mechanism, which is gradually augmented in a K_ATP channel‐independent manner. Biphasic GSIS thus occurs. In the body, the β‐cells are constantly exposed to stimulatory signals, such as glucagon‐like peptide 1 (GLP‐1), parasympathetic inputs, free fatty acid (FFA), amino acids and slightly suprathreshold levels of glucose, even at fasting. GLP‐1 increases cellular cyclic adenosine monophosphate, parasympathetic stimulation activates protein kinase C, and FFA, amino acids and glucose generate metabolic amplification factors. Plasma glucose concentration gradually rises postprandially under such tonic stimulation. We hypothesize that these stimulatory inputs together make the β‐cells responsive to glucose independently from its action on K_ATP channels. Robust GSIS in patients with a loss of function mutation of the sulfonylurea receptor, a subunit of K_ATP channels, is compatible with this hypothesis. Furthermore, pre‐exposure of the islets to an activator of protein kinase A and/or C makes β‐cells responsive to glucose in a K_ATP channel‐ and Ca²⁺‐independent manner. We hypothesize that GSIS occurs in islet β‐cells without glucose regulation of K_ATP channels in vivo, for which priming with cyclic adenosine monophosphate, protein kinase C and non‐glucose nutrients are required. To understand the physiology of GSIS, comprehensive integration of accumulated knowledge is required.     

Phentolamine block of KATP channels is mediated by Kir6.2

The ATP-sensitive K⁺-channel (K_ATP channel) plays a key role in insulin secretion from pancreatic β cells. It is closed both by glucose metabolism and the sulfonylurea drugs that are used in the treatment of noninsulin-dependent diabetes mellitus, thereby initiating a membrane depolarization that activates voltage-dependent Ca²⁺ entry and insulin release. The β cell K_ATP channel is a complex of two proteins: Kir6.2 and SUR1. The former is an ATP-sensitive K⁺-selective pore, whereas SUR1 is a channel regulator that endows Kir6.2 with sensitivity to sulfonylureas. A number of drugs containing an imidazoline moiety, such as phentolamine, also act as potent stimulators of insulin secretion, but their mechanism of action is unknown. We have used a truncated form of Kir6.2, which expresses independently of SUR1, to show that phentolamine does not inhibit K_ATP channels by interacting with SUR1. Instead, our results argue that phentolamine may interact directly with Kir6.2 to produce a voltage-independent reduction in channel activity. The single-channel conductance is unaffected. Although the ATP molecule also contains an imidazoline group, the site at which phentolamine blocks is not identical to the ATP-inhibitory site, because phentolamine block of an ATP-insensitive mutant (K185Q) is normal. K_ATP channels also are found in the heart where they are involved in the response to cardiac ischemia: they also are blocked by phentolamine. Our results suggest that this may be because Kir6.2, which is expressed in the heart, forms the pore of the cardiac K_ATP channel.     

An adenylate kinase is involved in K<Subscript>ATP</Subscript> channel regulation of mouse pancreatic beta cells

Aims/hypothesis In a previous study, we demonstrated that a creatine kinase (CK) modulates K_ATP channel activity in pancreatic beta cells. To explore phosphotransfer signalling pathways in more detail, we examined whether K_ATP channel regulation in beta cells is determined by a metabolic interaction between adenylate kinase (AK) and CK. Methods Single channel activity was measured with the patch–clamp technique in the inside-out (i/o) and open-cell attached (oca) configuration. Results The ATP sensitivity of K_ATP channels was higher in i/o patches than in permeabilised beta cells (oca). One reason for this observation could be that the local ATP:ADP ratio in the proximity of the channels is determined by factors not active in i/o patches. AMP (0.1 mmol/l) clearly increased open channel probability in the presence of ATP (0.125 mmol/l) in permeabilised cells but not in excised patches. This suggests that AK-catalysed ADP production in the vicinity of the channels is involved in K_ATP channel regulation. The observation that the stimulatory effect of AMP on K_ATP channels was prevented by the AK inhibitor P ¹,P ⁵-di(adenosine-5′)pentaphosphate (Ap₅A; 20 μmol/l) and abolished in the presence of the non-metabolisable ATP analogue adenosine 5′-(β,γ-imido)triphosphate tetralithium salt (AMP-PNP; 0.12 mmol/l) strengthens this idea. In beta cells from AK1 knockout mice, the effect of AMP was less pronounced, though not completely suppressed. The increase in K_ATP channel activity induced by AMP in the presence of ATP was outweighed by phosphocreatine (1 mmol/l). We suggest that this is due to an elevation of the ATP concentration by CK. Conclusions/interpretation We propose that phosphotransfer events mediated by AK and CK play an important role in determining the effective concentrations of ATP and ADP in the microenvironment of pancreatic beta cell K_ATP channels. Thus, these enzymes determine the open probability of K_ATP channels and eventually the actual rate of insulin secretion.     

Pharmacological Profile of U‐37883A,a Channel Blocker of Smooth Muscle‐Type ATP‐Sensitive K+ Channels

U‐37883A (PNU‐37883A, guanidine; 4‐morpholinecarboximidine‐N‐1‐adamantyl‐N′‐cyclohexyl hydrochloride) was originally developed as a potential diuretic with specific binding in kidney and vascular smooth muscle rather than in brain or pancreatic β cells. U‐37883A inhibits ATP‐sensitive K⁺ channels (K_ATP channels) in vascular smooth muscle at submicromolar concentrations whilst even at high concentrations (≥10 μM) it has no inhibitory effect at pancreatic, cardiac or skeletal K_ATP channels. Thus, it is generally thought that U‐37883A is a selective inhibitor of vascular smooth muscle K_ATP channels. Approximately one decade ago, K_ATP channels were cloned and found to consist of at least two subunits: an inwardly‐rectifying K⁺ channel six family (K_ir6.x; K_ir6.1 and K_ir6.2) which forms the ion conducting pore and a modulatory sulphonylurea receptor (SUR.x; SUR1, SUR2A, and SUR2B) that accounts for several pharmacological properties. It is generally believed that different combinations of K_ir6.x and SUR.x determine the molecular properties of K_ATP channels. Thus, K_ir6.2/SUR1 channel represents the pancreatic β‐cell K_ATP channel, K_ir6.2/SUR2A channel is thought to represent the cardiac K_ATP channel, whereas K_ir6.1/SUR2B channel is likely to represent the vascular smooth muscle K_ATP channel. Recent molecular studies have shown that U‐37883A selectively suppresses the activity of recombinant K_ATP channels which contain K_ir6.1 subunits in the channel pore unit. It was thus thought that U‐37883A was a selective pharmacological tool which could be used to investigate the activity of vascular smooth muscle K_ATP channels. However, due to its multiple pharmacological actions on several ion channels and poor tissue selectivity, U‐37883A should not be viewed as a selective blocker of smooth muscle K_ATP channels.     

Long-Chain CoA esters activate human pancreatic beta-cell K<Subscript>ATP</Subscript> channels: potential role in Type 2 diabetes

Aims/hypothesis The ATP-regulated potassium (K_ATP) channel in the pancreatic beta cell couples the metabolic state to electrical activity. The primary regulator of the K_ATP channel is generally accepted to be changes in ATP/ADP ratio, where ATP inhibits and ADP activates channel activity. Recently, we showed that long-chain CoA (LC-CoA) esters form a new class of potent K_ATP channel activators in rodents, as studied in inside-out patches.Methods In this study we have investigated the effects of LC-CoA esters in human pancreatic beta cells using the inside-out and whole-cell configurations of the patch clamp technique.Results Human K_ATP channels were potently activated by acyl-CoA esters with a chain length exceeding 12 carbons. Activation by LC-CoA esters did not require the presence of Mg²⁺ or adenine nucleotides. A detailed characterization of the concentration-dependent relationship showed an EC ₅₀ of 0.7±0.1 µmol/l. Furthermore, in the presence of an ATP/ADP ratio of 10 (1.1 mmol/l total adenine nucleotides), whole-cell K_ATP channel currents increased approximately six-fold following addition of 1 µmol/l LC-CoA ester. The presence of 1 µmol/l LC-CoA in the recording pipette solution increased beta-cell input conductance, from 0.5±0.2 nS to 2.5±1.3 nS.Conclusion/interpretation Taken together, these results show that LC-CoA esters are potent activators of the K_ATP channel in human pancreatic beta cells. The fact that LC-CoA esters also stimulate K_ATP channel activity recorded in the whole-cell configuration, points to the ability of these compounds to have an important modulatory role of human beta-cell electrical activity under both physiological and pathophysiological conditions.Abbreviations K_ATP ATP sensitive potassium channel - LC-CoA Long-chain Co-enzyme A ester     

Gliclazide produces high-affinity block of KATP channels in mouse isolated pancreatic beta cells but not rat heart or arterial smooth muscle cells

Aims/hypothesis: Sulphonylureas stimulate insulin secretion by closing ATP-sensitive potassium (K_ATP) channels in the pancreatic beta-cell membrane. K_ATP channels are also found in other tissues, including heart and smooth muscle, where they link cellular metabolism to electrical activity. The sulphonylurea gliclazide blocks recombinant beta-cell K_ATP channels (Kir6.2/SUR1) but not heart (Kir6.2/SUR2A) or smooth muscle (Kir6.2/SUR2B) K_ATP channels with high potency. In this study, we examined the specificity of gliclazide for the native (as opposed to recombinant) K_ATP channels in beta cells, heart and smooth muscle. Methods: The action of the drug was studied by whole-cell current recordings of native K_ATP channels in isolated pancreatic beta-cells and myocytes from heart and smooth muscle. Results: Gliclazide blocked whole-cell beta-cell K_ATP currents with an IC ₅₀ of 184 ± 30 nmol/l (n = 6–10) but was much less effective in cardiac and smooth muscle (IC ₅₀s of 19.5 ± 5.4 μmol/l (n = 6–12) and 37.9 ± 1.0 μmol/l (n = 5–10), respectively). In all three tissues, the action of the drug on whole-cell K_ATP currents was rapidly reversible. In inside-out patches on beta-cells, gliclazide (1 μmol/l) produced a maximum of 66 ± 13 % inhibition (n = 5), compared with more than 98 % block in the whole-cell configuration. Conclusion/interpretation: Gliclazide is a high-potency sulphonylurea which shows specificity for the pancreatic beta-cell K_ATP channel over heart and smooth muscle. In this respect, it differs from glibenclamide. The difference in the maximal block observed in the excised patch and whole-cell recordings from beta-cells, may be due to the absence of intracellular Mg-nucleotides in the excised patch experiments. [Diabetologia (2001) 44: 1019–1025] Received: 21 March 2001 and in revised form: 30 April 2001     

Nicorandil increases adenosine 5′-monophosphate-primed interstitial adenosine via activation of ecto-5′-nucleotidase in rat hearts

With the use of microdialysis techniques, we examined the effects of nicorandil, a hybrid of an ATP-sensitive K⁺ (K_ATP) channel opener and a nitrate compound, on the production of interstitial adenosine in rat hearts in situ. The level of dialysate adenosine measured under a constant supply of adenosine 5′-monophosphate (AMP) reflected the activity of endogenous ecto-5′-nucleotidase. Nicorandil (0.3–3 mM) increased the level of AMP (100 μM)-primed dialysate adenosine in a concentration-dependent manner, and this effect was completely abolished by the guanylate cyclase inhibitor, methylene blue (100 μM), but not by the K_ATP channel blocker, glibenclamide (10 μM). Another K_ATP channel opener, cromakalim (0.1–1 mM), did not increase the production of AMP-primed dialysate adenosine. These results suggest that nicorandil increases the level of interstitial adenosine via cyclic guanosine monophosphate-mediated activation of ecto-5′-nucleotidase. Received: March 16, 2000 / Accepted: June 30, 2000     

Pharmacologic Profile of the Selective Mitochondrial‐KATP Opener BMS‐191095 for Treatment of Acute Myocardial Ischemia

ATP‐sensitive potassium channel (K_ATP) openers as a class protect ischemic myocardium. The protective effects are independent of vasodilator activity and effects on action potential shortening, actions typically associated with sarcolemmal K_ATP activation. BMS‐191095 is a novel mitochondrial K_ATP opener which protects ischemic myocardium while having no electrophysiologic or vasodilator effects (determined in vitro and in vivo). The cardioprotective effects were determined in isolated rat hearts subjected to ischemia and reperfusion. Protective effects were deduced from increased time to contracture formation during ischemia, improved reperfusion recovery of contractile function, and reduced reperfusion LDH release. The cardioprotective effects of BMS‐191095 were observed at concentrations at which this compound selectively opened cardiac mitochondrial K_ATP channels. This effect was consistent with the pharmacologic profile of this agent. The protective effects were abolished by mitochondrial K_ATP inhibition. Unlike first‐generation K_ATP openers, BMS‐191095 is expected to protect ischemic myocardium with little hemodynamic sequelae and without any proarrhythmic potential. BMS‐191095 is potentially useful clinically as a cardioprotective agent. It is also a useful tool for basic research.     

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K_ATP) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K_ATP channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K_ATP channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K_ATP channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K_ATP channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.     

The Molecular Genetics of Sulfonylurea Receptors in the Pathogenesis and Treatment of Insulin Secretory Disorders and Type 2 Diabetes

Sulfonylurea receptors (SURs) form an integral part of the ATP-sensitive potassium (K_ATP) channel complex that is present in most excitable cell types. K_ATP channels couple cellular metabolism to electrical activity and provide a wide range of cellular functions including stimulus secretion coupling in pancreatic β cells. K_ATP channels are composed of SURs and inward rectifier potassium channel (Kir6.x) subunits encoded by the ABCC8/9 and KCNJ8/11 genes, respectively. Recent advances in the genetics, molecular biology, and pharmacology of SURs have led to an increased understanding of these channels in the etiology and treatment of rare genetic insulin secretory disorders. Furthermore, common genetic variants in these genes are associated with an increased risk for type 2 diabetes. In this review we summarize the molecular biology, pharmacology, and physiology of SURs and K_ATP channels, highlighting recent advances in their genetics and understanding of rare insulin secretory disorders and susceptibility to type 2 diabetes.     

Leptin promotes KATP channel trafficking by AMPK signaling in pancreatic β-cells

Leptin is a pivotal regulator of energy and glucose homeostasis, and defects in leptin signaling result in obesity and diabetes. The ATP-sensitive potassium (K_ATP) channels couple glucose metabolism to insulin secretion in pancreatic β-cells. In this study, we provide evidence that leptin modulates pancreatic β-cell functions by promoting K_ATP channel translocation to the plasma membrane via AMP-activated protein kinase (AMPK) signaling. K_ATP channels were localized mostly to intracellular compartments of pancreatic β-cells in the fed state and translocated to the plasma membrane in the fasted state. This process was defective in leptin-deficient ob/ob mice, but restored by leptin treatment. We discovered that the molecular mechanism of leptin-induced AMPK activation involves canonical transient receptor potential 4 and calcium/calmodulin-dependent protein kinase kinase β. AMPK activation was dependent on both leptin and glucose concentrations, so at optimal concentrations of leptin, AMPK was activated sufficiently to induce K_ATP channel trafficking and hyperpolarization of pancreatic β-cells in a physiological range of fasting glucose levels. There was a close correlation between phospho-AMPK levels and β-cell membrane potentials, suggesting that AMPK-dependent K_ATP channel trafficking is a key mechanism for regulating β-cell membrane potentials. Our results present a signaling pathway whereby leptin regulates glucose homeostasis by modulating β-cell excitability.The K_ATP channel, an inwardly rectifying K⁺ channel that consists of pore-forming Kir6.2 and regulatory sulfonylurea receptor 1 (SUR1) subunits (1), functions as an energy sensor: its gating is regulated mainly by the intracellular concentrations of ATP and ADP. In pancreatic β-cells, K_ATP channels are inhibited or activated in response to the rise or fall in blood glucose levels, leading to changes in membrane excitability and insulin secretion (2, 3). Thus, K_ATP channel gating has been considered an important mechanism in coupling blood glucose levels to insulin secretion. Recently, trafficking of K_ATP channels to the plasma membrane was highlighted as another important mechanism for regulating K_ATP channel activity (4–6).AMP-activated protein kinase (AMPK) is a key enzyme regulating energy homeostasis (7). We recently demonstrated that K_ATP channels are recruited to the plasma membrane in glucose-deprived conditions via AMPK signaling in pancreatic β-cells (6). Inhibition of AMPK signaling significantly reduces K_ATP currents, even after complete wash-out of intracellular ATP (6). Given these results, we proposed a model that recruitment of K_ATP channels to the plasma membrane via AMPK signaling is crucial for K_ATP channel activation in low-glucose conditions. However, the physiological relevance of this model remains unclear because pancreatic β-cells had to be incubated in media containing less than 3 mM glucose to recruit a sufficient number of K_ATP channels to the plasma membrane (6). We thus hypothesized that there should be an endogenous ligand in vivo that promotes AMPK-dependent K_ATP channel trafficking sufficiently to stabilize pancreatic β-cells at physiological fasting glucose levels.Leptin is an adipocyte-derived hormone that regulates food intake, body weight, and glucose homeostasis (8, 9). In addition to its central action, leptin regulates the release of insulin and glucagon, the key hormones regulating glucose homeostasis, by direct actions on β- and α-cells of pancreatic islets, respectively (10–12). It thus was proposed that the adipoinsular axis is crucial for maintaining nutrient balance and that dysregulation of this axis contributes to obesity and diabetes (12). However, intracellular signaling mechanisms underlying leptin effects are largely unknown. Leptin was shown to increase K_ATP currents in pancreatic β-cells (13, 14), but the possibility that K_ATP channel trafficking mediates leptin-induced K_ATP channel activation has not been explored.In the present study, we demonstrate that the surface levels of K_ATP channels increase in pancreatic β-cells under fasting conditions in vivo. Translocation of K_ATP channels to the plasma membrane in fasting was absent in pancreatic β-cells from ob/ob mice, but restored by treatment with leptin, suggesting a role for leptin in K_ATP channel trafficking in vivo. We further show that leptin-induced AMPK activation, which is essential for K_ATP channel trafficking to the plasma membrane, is mediated by activation of canonical transient receptor potential 4 (TRPC4) and calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ). Our results highlight the importance of trafficking regulation in K_ATP channel activation and provide insights into the action of leptin on glucose homeostasis.