Nicorandil, a potent cardioprotective agent, reduces QT dispersion during coronary angioplasty

BACKGROUND: Previous studies have found that ST-segment elevation and QT dispersion are smaller in second coronary occlusions than in first occlusions, a trend that suggests ischemic preconditioning. It has not been established whether nicorandil reduces ST-segment elevation and QT dispersion during coronary angioplasty. METHODS AND RESULTS: Thirty patients with stable angina undergoing coronary angioplasty in the proximal left anterior descending artery were randomly assigned to one of two groups, receiving either 5 mg oral nicorandil 3 times daily (n = 15) or placebo (n = 15). In the control patients, the total ST-segment elevation decreased from 14 +/- 3 mm during the first inflation to 7 +/- 2 mm during the second inflation (P < .01). In contrast, in the nicorandil-treated patients, the total ST-segment elevation during the second inflation was roughly equivalent to that during the first inflation (8 +/- 3 mm vs 8 +/- 3 mm, P = not significant). After the first reperfusion, a significantly smaller increase in QT dispersion was observed in the nicorandil-treated patients than in the control patients (43 +/- 15 ms vs 54 +/- 15 ms, P < .001). However, after the second reperfusion, QT dispersion was similar for the two groups (32 +/- 15 ms vs 34 +/- 13 ms, P = not significant). CONCLUSIONS: Nicorandil may precondition the myocardium and may prevent the occurrence of ventricular arrhythmias after coronary reperfusion by suppressing the increase in QT dispersion.     

Prostacyclin reduces microvascular fluid conductivity in cat skeletal muscle through opening of ATP-dependent potassium channels

Prostacyclin is suggested to reduce microvascular permeability, but the cellular mechanisms mediating this response in the microvascular endothelial cells are still unknown. Considering that prostacyclin relaxes vascular smooth muscle cells via opening of ATP-dependent potassium channels, and opening of ATP-dependent potassium channels in the endothelial cells is suggested to influence microvascular permeability, this study was designed to test (1) if ATP-dependent potassium channels are involved in the regulation of microvascular hydraulic permeability, (2) if the permeability-reducing effect of prostacyclin is mediated through opening of ATP-dependent potassium channels, and (3) if cAMP is involved in this process. An autoperfused cat calf hindlimb was used as experimental model, and microvascular hydraulic permeability (conductivity) was estimated by a capillary filtration coefficient (CFC) technique. The potassium channel opener PCO-400 (0.5 microg x min(-1) per 100 g muscle, intra-arterially), prostacyclin (1 ng x min(-1) per kg body weight, intravenously) and the cAMP analogue dibutyryl-cAMP (24 microg x min(-1) per 100 g muscle, intra-arterially), decreased CFC to 77, 72 and 69% compared to control, respectively (p < 0.01). The decrease in CFC obtained by these substances was completely restituted after the start of a simultaneous infusion of the ATP-dependent potassium channel blocker glibenclamide (6 microg x min(-1) per 100 g muscle, intra-arterially; p < 0.01). Infusion of glibenclamide alone increased CFC to 107% of control (p < 0.05). In conclusion, the ATP-dependent potassium channels contribute to the regulation of microvascular hydraulic conductivity, and the prostacyclin permeability-reducing effect may act through this mechanism via increase in intracellular cAMP.     

ATP-Sensitive potassium channels: a review of their cardioprotective pharmacology

ATP-sensitive potassium channels (K(ATP)) have been thought to be a mediator of cardioprotection for the last ten years. Significant progress has been made in learning the pharmacology of this channel as well as its molecular regulation with regard to cardioprotection. K(ATP)openers as a class protect ischemic/reperfused myocardium and appear to do so by conservation of energy. The reduced rate of ATP hydrolysis during ischemia exerted by these openers is not due to a cardioplegic effect and is independent of action potential shortening. Compounds have been synthesized which retain the cardioprotective effects of first generation K(ATP)openers, but are devoid of vasodilator and cardiac sarcolemmal potassium outward currents. These results suggest receptor or channel subtypes. Recent pharmacologic and molecular biology studies suggest the activation of mitochondrial K(ATP)as the relevant cardioprotective site. Implications of these results for future drug discovery and preconditioning are discussed.     

The cardioprotective effect of uridine and uridine-5'-monophosphate: the role of the mitochondrial ATP-dependent potassium channel

The activity of mitochondrial ATP-dependent potassium channel (mitoKATP) of rat heart and liver mitochondria was shown to decrease during aging. This partially explains the increase of risk of ischemia at a mature age since mitoKATP activation provides cardioprotection. We demonstrated that uridine-5'-diphosphate (UDP) possesses the property to activate mitoKATP. At a concentration of 30 microM, it reactivated mitoKATP in mitochondria, and 5-hydroxydecanoate (5-HD) eliminated this effect. In experimental animals, UDP precursors uridine and uridine-5'-monophosphate (UMP) (both 30 mg/kg, administered intravenously 5 min before coronary occlusion) decreased the myocardium ischemic alteration index (1.9 and 3.5 times, respectively) and the T-wave amplitude within 60 min after occlusion. Both effects were inhibited by Glibenclamide (Glib) and 5-HD. UMP and uridine decreased the number of premature ventricular beats 5.6 and 1.9 times and the duration of ventricular tachycardia 9.4 and 4.1 times, respectively. Glib and 5-HD inhibited the anti-arrhythmic parameters, 5-HD being less effective. Uridine and UMP decreased the duration of fibrillation 10.8 and 3.6 times, respectively, and this effect was not abolished by Glib and 5-HD. Thus, uridine and UMP, which are the precursors of UDP in the cell, possess cardioprotective properties. MitoKATP prevents mainly ischemic injuries and partially rhythm disorders.     

Nicorandil inhibits oxidative stress-induced apoptosis in cardiac myocytes through activation of mitochondrial ATP-sensitive potassium channels and a nitrate-like effect

The anti-anginal drug nicorandil has been shown to inhibit apoptosis by activating mitochondrial ATP-sensitive potassium (K(ATP)) channels. The possible contribution of the nitrate moiety of this drug to its anti-apoptotic effect has now been investigated in neonatal rat ventricular myocytes subjected to oxidative stress. Exposure of cultured myocytes to 100 micromol/l hydrogen peroxide (H(2)O(2)) increased the number of nuclei stained by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling technique as well as induced internucleosomal DNA fragmentation, loss of mitochondrial membrane potential, cytochrome c release into the cytosol, and activation of caspases-3 and -9, all of which are characteristics of apoptosis. Pretreatment of cells with nicorandil (100 micromol/l) inhibited these effects of H(2)O(2). Both the mitochondrial K(ATP) channel antagonist 5-hydroxydecanoate (5-HD) and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase, attenuated the anti-apoptotic effect of nicorandil in concentration-dependent manners. Coapplication of ODQ (10 micromol/l) and 5-HD (500 micromol/l) completely abolished nicorandil-induced cytoprotection. The effect of nicorandil was also reduced by an inhibitor of cGMP-dependent protein kinase (KT5823, 1 micromol/l). The nitric oxide donor (+/-)-S-nitroso-N-acetylpenicillamine (SNAP, 50 micromol/l) mimicked the protective effect of nicorandil in a manner sensitive to ODQ but not to 5-HD. A cell-permeable cGMP analog, 8-bromo-cGMP, also reduced H(2)O(2)-induced apoptosis. The inhibition of the H(2)O(2)-induced activation of caspase-3, but not that of caspase-9, by nicorandil in the presence of 5-HD or by SNAP was reversed by the addition of dithiothreitol to the enzyme assay. Nicorandil inhibits oxidative stress-induced apoptosis in cardiac myocytes through a nitric oxide/cGMP-dependent mechanism as well as by activating mitochondrial K(ATP) channels.     

Pharmacological evidence for a role of ATP-dependent potassium channels in myocardial stunning.

BACKGROUND. Several recent studies suggest that activation of ATP-dependent potassium (K(ATP)) channels in the myocardium plays an important cardioprotective role during ischemia. The present study was undertaken to examine further the role of this ion channel in vivo in a model of "stunned" myocardium. METHODS AND RESULTS. Barbital-anesthetized dogs were subjected to 15 minutes of left anterior descending (LAD) coronary artery occlusion followed by 3 hours of reperfusion. Regional myocardial blood flow was measured by radioactive microspheres and segment function by sonomicrometry. Intravenous administration of the potassium channel opener aprikalim (RP 52891) at a dose that produced no significant systemic hemodynamic effects (10 micrograms/kg plus 0.1 microgram/kg/min) resulted in a marked improvement in segment shortening in the ischemic/reperfused myocardium compared with control animals (p less than 0.05) when given before the ischemic insult. However, administration of aprikalim immediately before reperfusion had no beneficial effect. Furthermore, pretreatment with the K(ATP) channel antagonist glibenclamide antagonized the recovery of contractile function afforded by aprikalim when administered at a low dose (0.3 mg/kg) that alone had no effect on postischemic recovery. In contrast, pretreatment with either a higher dose of glibenclamide (1.0 mg/kg) or the related sulfonylurea K(ATP) channel antagonist tolbutamide (100 mg/kg) resulted in a worsening of segment function after reperfusion. The ability of aprikalim and the K(ATP) channel antagonists to alter postischemic wall function occurred independently of differences in systemic hemodynamics, area at risk, and collateral blood flow during occlusion, the major determinants of the extent of myocardial stunning. CONCLUSIONS. These results suggest that opening myocardial K(ATP) channels in the ischemic heart results in a marked cardioprotective effect in stunned myocardium and that these channels may serve an endogenous function, which is to provide protection from ischemic insults.     

Ischemic myocardial cell protection conferred by the opening of ATP-sensitive potassium channels

The responses of the cardiac myocyte to a potentially injurious ischemic stress are multiple. The opening of the ATP-sensitive K⁺ channels may constitute one such response. These channels are present in the plasmalemma at very elevated density and have a large unitary conductance. Consequently, the opening of a small fraction (0.01–0.1%) of these channels during ischemia can help to drive the myocyte into an emergency state, in which its syncytial functions become rapidly downregulated and strategies appropriate to preserving cell viability are implemented. Thus, ATP-sensitive K⁺ channels in cardiac myocytes would appear to be an efficient and apparently redundant natural means of defense against metabolic stress. These channels can undergo physiologic modulation, as occurs during cardiac ischemic preconditioning in several species, including humans. The termcardioprotection refers to an endogenous cardioprotective strategy, whereby the myocardium slows its energy demands, produces fewer toxic glycolytic products, and exhibits reduced injury following a potentially lethal ischemic stress. Openers of cardiac ATP-sensitive K⁺ channels, a class of drugs that includes, in particular, aprikalim and nicorandil, also afford cardioprotection by reducing the functional and biochemical damage produced by ischemia. Hence, these compounds can improve the recovery of cardiac contractility, reduce the extracellular leakage of intracellular enzymes, delay the loss of ATP, and preserve the cell ultrastructure in isolated heart preparations subjected to transient ischemic conditions. Furthermore, when segmental contractility has been strongly depressed by a stunning insult, nicorandil and aprikalim can accelerate recovery at the reperfusion. Finally, nicorandil and aprikalim decrease substantially the size of the necrotic region that results from a prolonged ischemic insult followed by reperfusion. All of these desirable effects of K⁺-channel openers can be abolished by blockers of ATP-sensitive K⁺ channels, such as glibenclamide. The fundamental mechanism of the myocyte viability protection conferred by K⁺-channel openers is not yet clear. It may exploit some of the same pathways that mediate cardiac ischemic preconditioning. If this suggestion holds true, drugs opening cardiac ATP-sensitive K⁺ channels would mimic, exploit, or intensify those cardioprotective means that are naturally available to the cardiac myocyte for overcoming metabolic stress.     

Expression and function of ATP-dependent potassium channels in late post-infarction remodeling

Myocardial remodeling late after infarction is associated with increased incidence of fatal arrhythmias. Heterogeneous prolongation of the action potential in the surviving myocardium is one of the predominant causes. Sarcolemmal ATP-dependent potassium (K(ATP)) channels are important metabolic sensors regulating electrical activity of cardiomyocytes and are capable of considerably shortening the action potential. We determined whether ATP-dependent potassium channels generate or, on the contrary prevent the heterogeneity in action potential prolongation. Cardiomyocytes were obtained from the infarct border zone, the septum and the right ventricle of rat hearts 20 weeks after coronary occlusion when rats developed signs of heart failure. Expression of the conductance subunit Kir6.1, but not Kir6.2, and of all SUR regulatory subunits was increased up to 3-fold in cardiomyocytes from the infarct border zone. Concomitantly, there was a prominent response of the K(ATP) current to diazoxide that was not detectable in control cardiomyocytes. The action potential was prolonged in cardiomyocytes from the infarct border zone (74 ms) relative to sham (41 ms). However, activation of the K(ATP) channels by diazoxide reduced action potential duration to 42 ms. In myocytes of the septum and right ventricle, expression of channel subunits, duration of action potential, and sensitivity to diazoxide were only slightly increased relative to shams. In conclusion, the myocardium remodeled after infarction displays alterations of K(ATP) expression and function with spatial heterogeneity matching that of the action potential prolongation. Drugs selectively activating diazoxide-sensitive sarcolemmal K(ATP) channels should be considered in the prevention of arrhythmias in post-infarction heart failure.     

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.     

Nicorandil increases coronary blood flow predominantly by K-channel opening mechanism

Summary Nicorandil has a hybrid property between nitrates and potassium (K)-channel openers. In order to clarify which mechanism of action is responsible for its effect in increasing coronary blood flow, we investigated how this effect was antagonized by glibenclamide, which was recently found to behave as a pharmacologic antagonist of K-channel openers. Cromakalim, one of the most specific K-channel openers currently available, and nitroglycerin were used as reference drugs. In isolated, blood-perfused papillary muscle preparations of dogs, intraarterial injections of nicorandil and cromakalim increased (coronary) blood flow, and at high doses a negative inotropic effect and ventricular fibrillation occurred. Dose-response curves for the increase in coronary blood flow produced by nicorandil or cromakalim were shifted to the right in a parallel manner and to similar extents by glibenclamide given intravenously to support dogs. Schild analysis yielded pA₂ values of 6.08 and 6.34 for glibenclamide versus nicorandil and cromakalim, respectively. Nitroglycerin injected intraarterially produced only an increase in coronary blood flow. This effect was not affected by glibenclamide. These results indicate that the effect of nicorandil in increasing coronary blood flow, like that of cromakalim, is predominantly due to its mechanism of action as a K-channel opener. The negative inotropy and ventricular fibrillation seen with high doses of nicroandil and cromakalim were also antagonized by glibenclamide, indicating that these effects are also due to K-channel opening.     

Effect of thimerosal on arrhythmia induced by coronary ligation: the involvement of ATP-dependent potassium channels

Thiol-modifying agents induce the release of nitric oxide (NO) from endothelial epithelium and the release of reactive oxygen free radicals in the vascular system. Moreover, thiol groups are essential for the functioning of the ATP dependent potassium channel (K-ATP). The effects of thiol-modifying agents and their molecular mechanisms on arrhythmia have not been widely studied. In this study, we investigated the effect of the hydrophilic SH-group-oxidizing substance thimerosal on the arrhythmia induced by reperfusion/ischemia after coronary artery ligation in rats. We studied the possible involvement of the K-ATP and NOS on the effect of thimerosal. Thimerosal pretreatment (3, 30 mg/kg dose iv. 10 minutes before coronary occlusion) significantly decreased the length of total arrhythmia, ventricular tachycardia, and the arrhythmia score. This effect of thimerosal was reversed by the K-ATP opener pinacidil but not by the K-ATP blocker glibenclamide. The inhibition of iNOS by L-NAME did not alter the antiarrhythmic effect of thimerosal. These data clearly suggest that the antiarrhythmic effect of thimerosal is dependent upon the blockage of K-ATP.     

Nicorandil improves post-ischemic myocardial dysfunction in association with opening the mitochondrial K(ATP) channels and decreasing hydroxyl radicals in isolated rat hearts.

BACKGROUND: Nicorandil has been reported to induce cardioprotection by opening the mitochondrial K(ATP) channels. However, whether nicorandil affects reactive oxygen species is unclear. METHODS AND RESULTS: The hearts of male Sprague-Dawley rats were excised and perfused on a Langendorff apparatus with Krebs-Henseleit solution with a gas mixture of 95% O(2) and 5% CO(2). 1 mmol/L of nicorandil was given 10 min before ischemia. Left ventricular developed pressure (LVDP, mmHg), +/-dP/dt (mmHg/s) and coronary flow (ml/min) were continuously monitored. All hearts were perfused for a total of 120 min consisting of a 30 min pre-ischemic period, followed by a 30 min global ischemia and 60 min reperfusion with and without 5-hydroxydecanoic acid sodium salt (5-HD), a mitochondrial K(ATP) channel blocker. The concentrations of 2,3-dihydroxybenzoic acid (2,3-DHBA), an indicator of hydroxyl radicals, in the perfusate during reperfusion period were also measured. Nicorandil significantly improved LVDP and +/-dP/dt, and increased coronary flow during reperfusion. Pretreatment with 5-HD abolished the improvement of LVDP and +/-dP/dt, and the increase in coronary flow induced by nicorandil. Nicorandil significantly attenuated the concentrations of 2,3-DHBA during reperfusion, which were restored by 5-HD. CONCLUSION: Nicorandil is protective against post-ischemic left ventricular dysfunction in association with opening the mitochondrial K(ATP) channels, decreasing hydroxyl radicals and increasing coronary flow in the isolated rat heart.     

Purification, sequence, and model structure of charybdotoxin, a potent selective inhibitor of calcium-activated potassium channels.

Charybdotoxin (ChTX), a protein present in the venom of the scorpion Leiurus quinquestriatus var. hebraeus, has been purified to homogeneity by a combination of ion-exchange and reversed-phase chromatography. Polyacrylamide gel electrophoresis, amino acid analysis, and complete amino acid sequence determination of the pure protein reveal that it consists of a single polypeptide chain of 4.3 kDa. Purified ChTX is a potent and selective inhibitor of the approximately 220-pS Ca2+-activated K+ channel present in GH3 anterior pituitary cells and primary bovine aortic smooth muscle cells. The toxin reversibly blocks channel activity by interacting at the external pore of the channel protein with an apparent Kd of 2.1 nM. The primary structure of ChTX is similar to a number of neurotoxins of diverse origin, which suggests that ChTX is a member of a superfamily of proteins that modify ion-channel activities. On the basis of this similarity, the three-dimensional structure of ChTX has been modeled from the known crystal structure of alpha-bungarotoxin. These studies indicate that ChTX is useful as a probe of Ca2+-activated K+-channel function and suggest that the proposed tertiary structure of ChTX may provide insight into the mechanism of channel block.