Inhibitors of ATP-sensitive potassium channels in guinea pig isolated ischemic hearts

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

KATP channels and cardiovascular disease: suddenly a syndrome

ATP-sensitive potassium (KATP) channels were first discovered in the heart 30 years ago. Reconstitution of KATP channel activity by coexpression of members of the pore-forming inward rectifier gene family (Kir6.1, KCNJ8, and Kir6.2 KCNJ11) with sulfonylurea receptors (SUR1, ABCC8, and SUR2, ABCC9) of the ABCC protein subfamily has led to the elucidation of many details of channel gating and pore properties. In addition, the essential roles of Kir6.x and SURx subunits in generating cardiac and vascular KATP(2) and the detrimental consequences of genetic deletions or mutations in mice have been recognized. However, despite this extensive body of knowledge, there has been a paucity of defined roles of KATP subunits in human cardiovascular diseases, although there are reports of association of a single Kir6.1 variant with the J-wave syndrome in the ECG, and 2 isolated studies have reported association of loss of function mutations in SUR2 with atrial fibrillation and heart failure. Two new studies convincingly demonstrate that mutations in the SUR2 gene are associated with Cantu syndrome, a complex multi-organ disorder characterized by hypertrichosis, craniofacial dysmorphology, osteochondrodysplasia, patent ductus arteriosus, cardiomegaly, pericardial effusion, and lymphoedema. This realization of previously unconsidered consequences provides significant insight into the roles of the KATP channel in the cardiovascular system and suggests novel therapeutic possibilities.

Mitochondrial uncoupling agents trigger ventricular fibrillation in isolated rat hearts

Sudden cardiac death resulting from ventricular fibrillation (VF) remains a major cause of mortality. The purpose of this study was to investigate the roles of loss of oxidative phosphorylation and activation of the mitochondrial ATP-sensitive K+ channel and permeability transition pore in VF development during myocardial ischemia by using mitochondrial uncoupling agents (carbonyl cyanide m-chlorophenylhydrazone and 2,4-dinitrophenol) and channel blockers (5-hydroxydecanoate and cyclosporine A) at concentrations that have been demonstrated to block the intended targets selectively. Isolated rat hearts (n = 8 per group) were perfused with 0.3 μM carbonyl cyanide m-chlorophenylhydrazone, 100 μM 2,4-dinitrophenol, 0.2 μM cyclosporine A, 100 μM 5-hydroxydecanoate, or vehicle solution and regional ischemia induced after 10 minutes. Carbonyl cyanide m-chlorophenylhydrazone and 2,4 dinitrophenol caused profound QT shortening and triggered VF in 100% of hearts before ischemia. During ischemia, neither cyclosporine A (88%) nor 5-hydroxydecanoate (100%) reduced VF incidence compared with control (100% VF). In separate hearts, carbonyl cyanide m-chlorophenylhydrazone decreased tissue ATP content, and glibenclamide or glimepiride delayed the QT shortening and onset of VF triggered by carbonyl cyanide m-chlorophenylhydrazone. In conclusion, mitochondrial uncoupling agents trigger VF, likely as a result of ATP depletion with subsequent activation of sarcolemmal ATP-sensitive K+ currents. The mechanism of VF in ischemia does not involve activation of the mitochondrial ATP-sensitive K+ channel or permeability transition pore.

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 (86)Rb(+) 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 (86)Rb(+) 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.

Sex-specific and exercise-acquired cardioprotection is abolished by sarcolemmal KATP channel blockade in the rat heart

The present study was conducted to determine whether the infarct sparing effect of short-term exercise is dependent on the operation of the myocardial sarcolemmal ATP-sensitive K+ (KATP) channel. Adult male and female Sprague-Dawley rats were exercised on a motorized treadmill for 5 days. Twenty-four hours following the training or sedentary period, hearts were isolated and exposed to 1 h of regional ischemia followed by 2 h of reperfusion on a modified Langendorf apparatus in the presence or absence of the sarcolemmal KATP channel antagonist HMR-1098 (30 μM). Following the ischemia-reperfusion protocol, infarct size was determined as a percentage of the total ischemic zone at risk (ZAR). Short-term exercise reduced infarct size by 24% in males (32 ± 2% of ZAR; P < 0.01) and by 18% in females (26 ± 2% of ZAR; P < 0.05). Sarcolemmal KATP channel blockade abolished the training-induced cardioprotection in both males and females, increasing infarct size to 43 ± 3% and 52 ± 4% of ZAR, respectively. In the absence of HMR-1098, infarct size was significantly lower in sedentary females than in males (33 ± 4% vs. 42 ± 2% of ZAR, respectively; P < 0.01). However, the presence of HMR-1098 abolished this sex difference, increasing infarct size by 58% in the sedentary females ( P < 0.01) but having no effect on infarct size in sedentary males. This study demonstrates that the sex-specific and exercise-acquired resistance to myocardial ischemia-reperfusion injury is dependent on sarcolemmal KATP activity during ischemia.

Selectivity of repaglinide and glibenclamide for the pancreatic over the cardiovascular K(ATP) channels

AIMS/HYPOTHESIS

Sulfonylureas and glinides close beta cell ATP-sensitive K(+) (K(ATP)) channels to increase insulin release; the concomitant closure of cardiovascular K(ATP) channels, however, leads to complications in patients with cardiac ischaemia. The insulinotrope repaglinide is successful in therapy, but has been reported to inhibit the recombinant K(ATP) channels of beta cells, cardiocytes and non-vascular smooth muscle cells with similar potencies, suggesting that the (patho-)physiological role of the cardiovascular K(ATP) channels may be overstated. We therefore re-examined repaglinide's potency at and affinity for the recombinant pancreatic, myocardial and vascular K(ATP) channels in comparison with glibenclamide.

METHODS

K(ATP) channel subunits (i.e. inwardly rectifying K(+) channels [Kir6.x] and sulfonylurea receptors [SURx]) were expressed in intact human embryonic kidney cells and assayed in whole-cell patch-clamp and [(3)H]glibenclamide binding experiments at 37 degrees C.

RESULTS

Repaglinide and glibenclamide, respectively, were >or=30 and >or=1,000 times more potent in closing the pancreatic than the cardiovascular channels and they did not lead to complete inhibition of the myocardial channel. Binding assays showed that the selectivity of glibenclamide was essentially based on high affinity for the pancreatic SUR, whereas binding of repaglinide to the SUR subtypes was rather non-selective. After coexpression with Kir6.x to form the assembled channels, however, the affinity of the pancreatic channel for repaglinide was increased 130-fold, an effect much larger than with the cardiovascular channels. This selective effect of coexpression depended on the piperidino substituent in repaglinide.

CONCLUSIONS/INTERPRETATION

Repaglinide and glibenclamide show higher potency and efficacy in inhibiting the pancreatic than the cardiovascular K(ATP) channels, thus supporting their clinical use.

Effects of the blockade of cardiac sarcolemmal ATP-sensitive potassium channels on arrhythmias and coronary flow in ischemia–reperfusion model in isolated rat hearts

Activation of ATP-sensitive K+ channels (K ATP) during ischemia leads to arrhythmias and blockade of these channels exert antiarrhythmic action. In this study, we investigated the effects of HMR1098, a sarcolemmal K ATP channel blocker and 5-hydroxydeconoate (5-HD), a mitochondrial K ATP channel blocker on cardiac function and arrhythmias in isolated rat hearts. The hearts were subjected to 30 min coronary occlusion, followed by 30 min reperfusion. In the preischemic period, both HMR 1098 and 5-HD slightly increased coronary perfusion pressure. Coronary occlusion increased the perfusion pressure and decreased the left ventricular developed pressure (LVDP) in both control and drug-treated hearts. However, inhibition of LVDP was greater and recovery of the perfusion pressure was lower in 30 micromol/l HMR1098 and 100 micromol/l 5-HD-treated hearts compared to control (P < 0.05). HMR1098, at 3 micromol/l, but not at 30 micromol/l, significantly reduced the ratio of bigeminis, couplets and salvos (P < 0.05). Ventricular tachycardia and ventricular fibrillation were not prevented by HMR1098, at both concentrations, and with 5-HD (100 micromol/l). These results suggest that blockade of sarcK ATP and mitoK ATP channels exert weak antiarrhythmic action, but reduce the recovery of coronary perfusion and contractile force, implying that both types of K(ATP) channels have beneficial role in the recovery of ischemic rat myocardium.