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.     

Ischaemic preconditioning may abolish the protection afforded by ATP-sensitive potassium channel openers in isolated human atrial muscle

The ATP-sensitive potassium channel (K_ATP channel) has been implicated in the mechanism underlying ischaemic preconditioning protection. This study based on human atrium compared the protective effects of ischaemic preconditioning with pre-operative nicorandil (a K_ATP channel opener with nitrate actions). We also examined the added effect of ischaemic preconditioning to that of nicorandil on ischaemic protection. The protective effects of other K_ATP channel openers devoid of nitrate actions were also examined.Atrial trabeculae harvested from patients undergoing routine myocardial revascularisation were divided on the basis of whether patients had been ingesting nicorandil orally preoperatively. Trabeculae were superfused with oxygenated Tyrode's solution and following stabilisation underwent 90 minutes simulated ischaemia followed by 120 minutes reoxygenation (n=6 per group). Atrial trabeculae exposed to nicorandil underwent either no treatment (N), or ischaemic preconditioning (N+PC) using 3 minutes simulated ischaemia and 7 minutes reoxygenation prior to the 90 minutes simulated ischaemia. Similarly trabeculae not exposed to nicorandil underwent either no treatment, controls (C), or ischaemic preconditioning (PC). The experimental endpoint was recovery of contractile function presented as percentage baseline function. Further groups were examined using other K_ATP channels openers with and without ischaemic preconditioning.In the control group, following 120 minutes reoxygentation the recovery of function reached 28.8±3.5%. In contrast, exposure to nicorandil alone improved recovery of function (55.5%±5.3) to a similar extent as PC (55.3%±2.5) when compared to controls (p<0.05, ANOVA). The addition of ischaemic preconditioning to nicorandil exposure abolished protection (29.7%±3.1). Findings were confirmed using the other K_ATP channels openers.Clinically available nicorandil appears to afford ischaemic protection to isolated human atrial muscle. The addition of a short ischaemic episode to nicorandil exposure seems to completely abolish this protection. Although the mechanism underlying this effect remains unknown, we believe that this observation may have clinical implications.     

Nicorandil protects ATP-sensitive potassium channels against oxidation-induced dysfunction in cardiomyocytes of aging rats

ATP-sensitive potassium channels (K_ATP channels) regulate vascular tone and cardiac contraction through their action on the membrane potential of smooth muscle cells and cardiomyocytes. Because aging and diseases alter K_ATP channel activity, many pharmacological treatments aimed at improving their function, therefore cardiovascular function, have been evaluated. Nicorandil, a K_ATP channel opener, nitric oxide donor and antioxidant, is used as a treatment of angina pectoris and induces vasodilation, blood pressure decrease and cardioprotection in aging as well as after ischemia-reperfusion. Here, using the patch-clamp technique, we have studied the effect a chronic low dose of nicorandil (0.1 mg/kg per day for 2 months), on the activity of cardiomyocyte K_ATP channels as a function of age, in newborn, 4-, 12- and 24-month old rats. Nicorandil exerted an anti-oxidant and protective action on cardiomyocyte K_ATP channels, especially in aged animals, leading to restoration of a normal channel activity. These findings could justify further therapeutical applications.     

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     

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.     

Facilitation of β-cell KATP channel sulfonylurea sensitivity by a cAMP analog selective for the cAMP-regulated guanine nucleotide exchange factor Epac

Clinical studies demonstrate that combined administration of sulfonylureas with exenatide can induce hypoglycemia in type 2 diabetic subjects. Whereas sulfonylureas inhibit β-cell K_ATP channels by binding to the sulfonylurea receptor-1 (SUR1), exenatide binds to the GLP-1 receptor, stimulates β-cell cAMP production, and activates both PKA and Epac. In this study, we hypothesized that the adverse in vivo interaction of sulfonylureas and exenatide to produce hypoglycemia might be explained by Epac-mediated facilitation of K_ATP channel sulfonylurea sensitivity. We now report that the inhibitory action of a sulfonylurea (tolbutamide) at K_ATP channels was facilitated by 2’-O-Me-cAMP, a selective activator of Epac. Thus, under conditions of excised patch recording, the dose-response relationship describing the inhibitory action of tolbutamide at human beta cell or rat INS-1 cell K_ATP channels was left-shifted in the presence of 2’-O-Me-cAMP, and this effect was abolished in INS-1 cells expressing a dominant-negative Epac2. Using an acetoxymethyl ester prodrug of an Epac-selective cAMP analog (8-pCPT-2'-O-Me-cAMP-AM), the synergistic interaction of an Epac activator and tolbutamide to depolarize INS-1 cells and to raise [Ca²⁺]_i was also measured. This effect of 8-pCPT-2'-O-Me-cAMP-AM correlated with its ability to stimulate phosphatidylinositol 4,5-bisphosphate hydrolysis that might contribute to the changes in K^_ATP channel sulfonylurea-sensitivity reported here. On the basis of such findings, we propose that the adverse interaction of sulfonylureas and exenatide to induce hypoglycemia involves at least in part, a functional interaction of these two compounds to close K_ATP channels, to depolarize β-cells, and to promote insulin secretion.     

Differential Role of Epicardial and Endocardial KATP Channels in Potassium Accumulation During Regional Ischemia Induced by Embolization of a Coronary Artery with Latex

Epicardial and and Endocardial [K⁺]₀ Rise and K_ATP Channels. Introduction: K_ATP channels are activated predominantly in the epicardium during regional ischemia. Therefore, the role of K_ATP channels in ischemia-induced rise of extracellular potassium concentration ([K⁺]_o) might he greater in the epicardium. Methods and Results: In 18 anesthetized dogs, the left anterior descending coronary artery (LAD) was ligated, followed by injection of 23-μm latex heads into the occluded artery to interrupt collateral flow, by which accumulated [K⁺]_o might wash out. Epicardial and endocardial [K⁺]_o were measured during a 20-minute period of ischemia using a valinomycin membrane. The dogs were divided into three groups: 6 control dogs (CTRL); 7 dogs pretreated with intravenous glibenclamide (0.3 mg/kg [GLIB]), a blocker of K_ATP channels: and 5 dogs pretreated with intravenous nicorandil (0.2 to 0.25 mg/kg [NCR]), a K_ATP channel opener. Before LAD occlusion, there was no difference in [K⁺]_o among the three groups. In the control group, epicardial and endocardial [K⁺]_o were increased to a similar level as a function of time after occlusion (CTRL) at both layers. Ischemia-induced epicardial [K⁺]_o rise was suppressed by GLIB (8.4 ± 0.4 vs 6.7 ± 0.5 mM, P < 0.05) but augmented by NCR (12.9 ± 2.0 mM, P < 0.05). In contrast, endocardial [K⁺]_o, rise remained unaffected (7.6 ± 0.2 mM CTRL, 7.6 ± 1.3 mM GLIB, and 9.4 ± 2.2 mM NCR, P = NS). Conclusion: Activation of K_ATP channels plays an important role in epicardial [K⁺]_o rise, but not in endocardial [K⁺]_o rise, during regional ischemia. Another mechanism(s) may he important for endocardial [K⁺]_o accumulation.     

Calcitonin gene-related peptide activates the K+ channels of vascular smooth muscle cells via adenylate cyclase

The aim of the present study was to examine the effects of calcitonin gene-related peptide (CGRP) on the K⁺ channels of vascular smooth muscle cells. Cultured smooth muscle cells from a porcine coronary artery were studied using the patch-clamp technique. Extracellular application of 100 nM CGRP activated two types of K⁺ channels the Ca²⁺-activated K⁺ channel (K_Ca channel) and the ATP-sensitive K⁺ channel (K_ATP channel) in cell-attached patch configurations. In cells pretreated with Rp-cAMPS, a membrane-permeable inhibitor of cAMP-dependent protein kinase (PKA), extracellular application of 100 nM CGRP could not activate the K_Ca or K_ATP channel, indicating that the activation of the K⁺ channels by CGRP occurs in connection with PKA. In the cell-attached patch configurations, extracellular application of 1 mM dibutyryl cAMP, a membrane permeable cAMP, activated K_Ca and K_ATP channels. In inside-out patch configurations, application of PKA to the cytosolic side activated both the K_Ca and K_ATP channels. These results indicate that CGRP modulates the K⁺ channels of vascular smooth muscle cells via adenylate cyclase, i.e., cAMP-PKA pathway, and contributes to control of vascular tone.     

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⁺.     

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.     

Role of K+ATP Channels in Ischemic Preconditioning and Cardioprotection

Summary. Since the phenomenon of ischemic preconditioning was first described some 15 years ago, interest in strategies aimed at reducing infarct size has increased. During the past 10 years, investigations into the mechanism of ischemic preconditioning have clearly demonstrated the cardioprotective effect of K⁺ _ATP channel opening. Thus, K⁺ _ATP channel activation has been shown to be involved in this protection by a variety of stimuli, including a brief period of complete ischemia (classic ischemic preconditioning) or a partial coronary artery occlusion. In addition, ischemia in remote organs and nonischemic stimuli in the heart such as ventricular pacing, stretch, and heat stress also confer protection via K⁺ _ATP channel activation. Pharmacological agents that open K⁺ _ATP channels reduce infarct size, but K⁺ _ATP channel opening must occur prior to or early during the sustained infarct-producing coronary artery occlusion, while the degree and memory of cardioprotection are less than those produced by classic ischemic preconditioning. Although the exact mechanism by which K⁺ _ATP channel activation protects is still incompletely understood, recent studies indicate a role for the mitochondrial K⁺ _ATP channels. Before K⁺ _ATP channel opening can be employed in patients at increased risk of developing myocardial infarction (e.g., unstable angina), it is mandatory to determine whether tolerance (tachyphylaxia) occurs with repeated administration of K⁺ _ATP channel openers in a fashion similar to what occurs with ischemic preconditioning.     

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.     

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     

Pancreatic islets from hypothalamic obese rats maintain K+ATP channel-dependent but not -independent pathways on glucose-induced insulin release process

One of the main features of obesity is hyperinsulinemia, which is related to insulin oversecretion. Glucose is by far the major physiological stimulator of insulin secretion. Glucose promotes an increase in the ATP/ADP ratio, which inactivates ATP-sensitive K⁺ channels (K⁺ _ATP) and induces beta cell depolarization with consequent calcium influx. Increased intracellular calcium concentration triggers insulin exocytosis. K⁺ _ATP channel function is important for K⁺ _ATP channel-dependent pathways involved in glucose-stimulated insulin secretion (GSIS). However, K⁺ _ATP channel-independent pathway has been identified and it has been found that this pathway sustains GSIS. Both pathways are critical to better GSIS control. GSIS was studied in pancreatic islets from hyperinsulinemic adult obese rats obtained by monosodium l-glutamate (MSG) neonatal treatment. Islets from MSG-obese rats were more glucose responsive than control ones. Diazoxide, a drug which maintains the K⁺ _ATP channels open without interfering with cell metabolism, blocked GSIS in islets from both groups. High extracellular potassium concentration plus diaz-oxide was used to study an alternative to the K⁺ _ATP channel pathway; in these conditions islets from MSG-obese rats did not respond, while islets from control animals showed enhanced GSIS. Results indicate that MSG-obese rats oversecreted insulin, even though the K⁺ _ATP channel-independent pathway is impaired in their beta cells.     

Fructose induces glucose‐dependent insulinotropic polypeptide,glucagon‐like peptide‐1 and insulin secretion: Role of adenosine triphosphate‐sensitive K+ channels

Adenosine triphosphate-sensitive K⁺ (K_ATP) channels play an essential role in glucose-induced insulin secretion from pancreatic β-cells. It was recently reported that the K_ATP channel is also found in the enteroendocrine K-cells and L-cells that secrete glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), respectively. In the present study, we investigated the involvement of the K_ATP channel in fructose-induced GIP, GLP-1 and insulin secretion in mice. Fructose stimulated GIP secretion, but pretreatment with diazoxide, a K_ATP channel activator, did not affect fructose-induced GIP secretion under streptozotocin-induced hyperglycemic conditions. Fructose significantly stimulated insulin secretion in Kir6.2^+/+ mice, but not in mice lacking K_ATP channels (Kir6.2^−/−), and fructose stimulated GLP-1 secretion in both Kir6.2^+/+ mice and Kir6.2^−/− mice under the normoglycemic condition. In addition, diazoxide completely blocked fructose-induced insulin secretion in Kir6.2^+/+ mice and in MIN6-K8 β-cells. These results show that fructose-induced GIP and GLP-1 secretion is K_ATP channel-independent and that fructose-induced insulin secretion is K_ATP channel-dependent.     

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.     

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.     

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.     

HMR 1098 is not an SUR isotype specific inhibitor of heterologous or sarcolemmal K ATP channels

Murine ventricular and atrial ATP-sensitive potassium (K_ATP) channels contain different sulfonylurea receptors (ventricular K_ATP channels are Kir6.2/SUR2A complexes, while atrial K_ATP channels are Kir6.2/SUR1 complexes). HMR 1098, the sodium salt of HMR 1883 {1-[[5-[2-(5-chloro-o-anisamido)ethyl]-2-methoxyphenyl]sulfonyl]-3-methylthiourea}, has been considered as a selective sarcolemmal (i.e. SUR2A-dependent) K_ATP channel inhibitor. However, it is not clear whether HMR 1098 would preferentially inhibit ventricular K_ATP channels over atrial K_ATP channels. To test this, we used whole-cell patch clamp techniques on mouse atrial and ventricular myocytes as well as ⁸⁶Rb⁺ efflux assays and excised inside-out patch clamp techniques on Kir6.2/SUR1 and Kir6.2/SUR2A channels heterologously expressed in COSm6 cells. In mouse atrial myocytes, both spontaneously activated and diazoxide-activated K_ATP currents were effectively inhibited by 10 μM HMR 1098. By contrast, in ventricular myocytes, pinacidil-activated K_ATP currents were inhibited by HMR 1098 at a high concentration (100 μM) but not at a low concentration (10 μM). Consistent with this finding, HMR 1098 inhibits ⁸⁶Rb⁺ effluxes through Kir6.2/SUR1 more effectively than Kir6.2/SUR2A channels in COSm6 cells. In excised inside-out patches, HMR 1098 inhibited Kir6.2/SUR1 channels more effectively, particularly in the presence of MgADP and MgATP (mimicking physiological stimulation). Finally, dose-dependent enhancement of insulin secretion from pancreatic islets and decrease of blood glucose level confirm that HMR 1098 is an inhibitor of Kir6.2/SUR1-composed K_ATP channels.     

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