Mitochondrial K ATP channel activation is important in the antiarrhythmic and cardioprotective effects of non-hypotensive doses of nicorandil and cromakalim during ischemia/reperfusion: a study in an intact anesthetized rabbit model.

The roles of cardiomyocyte sarcolemmal ATP-sensitive K(+) (K(ATP)) and mitochondrial K(ATP) channels in the cardioprotection and antiarrhythmic activity induced by K(ATP) channel openers remain obscure, though the mitochondrial K(ATP) channels have been proposed to be involved as a subcellular mediator in cardioprotection afforded by ischemic preconditioning. In the present study, we investigated the effects of administration of non-hypotensive doses of ATP-sensitive K(+) channel (K(ATP)) openers (nicorandil and cromakalim), a specific mitochondrial K(ATP) channel blocker (5-hydroxydecanoate (5-HD)) and a specific sarcolemmal K(ATP) channel blocker (HMR 1883; (1-[5-[2-(5-chloro-o-anisamido)ethyl]-2-methoxyphenyl]sulfonyl-3-methylthiourea) prior to and during coronary occlusion as well as prior to and during post-ischemic reperfusion on survival rate, ischemia-induced and reperfusion-induced arrhythmias and myocardial infarct size in anesthetized albino rabbits.The thorax was opened in the left 4th intercostal space and after pericardiotomy the heart was exposed. In Group I (n=80), occlusion of the left main coronary artery and hence, myocardial ischemia-induced arrhythmias were achieved by tightening a previously placed loose silk ligature for 30min. In Group II (n=184), arrhythmias were induced by reperfusion following a 20 min ligation of the left main coronary artery.Both in Groups I and II, early intravenous infusion of nicorandil (100 micro g/kg bolus+10 micro g/kg/min), cromakalim (0.2 micro g/kg/min), HMR 1883 (3mg/kg)/nicorandil and HMR 1883 (3mg/kg)/cromakalim just prior to and during ischemia increased survival rate (75%, 67%, 86% and 75% versus 60% in the control subgroup in Group I; 75%, 75%, 75% and 67% versus 50% in the control subgroup in Group II), significantly decreased the incidence and severity of life-threatening arrhythmias and significantly decreased myocardial infarct size. However, late intravenous administration of nicorandil or cromakalim at the onset and during reperfusion did neither increase survival rate nor confer any antiarrhythmic or cardioprotective effects. The antiarrhythmic and cardioprotective effects of both nicorandil and cromakalim were abolished by pretreating the rabbits with 5-HD (5mg/kg, i.v. bolus), a selective mitochondrial K(ATP) channel blocker but not by HMR 1883 (3mg/kg). In the present study, higher levels of malondialdehyde (MDA) and lower levels of reduced glutathione (GSH) and superoxide dismutase (SOD) in necrotic zone of myocardium in all the 16 subgroups in Group II suggest little anti-free radical property of nicorandil and cromakalim.We, therefore, conclude that intervention by intravenous administration of nicorandil and cromakalim (through the selective activation of mitochondrial K(ATP) channels), increased survival rate and exhibited antiarrhythmic and cardioprotective effects during coronary occlusion and reperfusion in anesthetized rabbits when administered prior to and during coronary occlusion. The mitochondrial K(ATP) channel may be a potential site of cardioprotection and antiarrhythmic activity.     

Selective mitochondrial KATP channel activation by nicorandil and 3-pyridyl pinacidil results in antiarrhythmic effect in an anesthetized rabbit model of myocardial ischemia/reperfusion

The roles of cardiomyocyte sarcolemmal ATP-sensitive K+ (KATP) and mitochondrial KATP channels in cardioprotection and antiarrhythmic activity induced by KATP channel openers remain obscure. However, it has been suggested that the mitochondrial KATP channels are involved as a subcellular mediator in cardioprotection afforded by ischemic preconditioning. In the present study, we investigated the effects of the administration of non-hypotensive doses of ATP-sensitive K+ channel (KATP) openers (nicorandil and 3-pyridyl pinacidil), a specific mitochondrial KATP channel blocker (5-hydroxydecanoate) and a specific sarcolemmal KATP channel blocker (HMR 1883; 1-[5-[2-(5-chloro-o-anisamido)ethyl]-2-methoxyphenyl]sulfonyl-3- methylthiourea) prior to and during coronary occlusion, as well as prior to and during post-ischemic reperfusion, on survival rate, ischemia-induced and reperfusion-induced arrhythmias and myocardial infarct size in anesthetized albino rabbits. The thorax was opened in the left 4th intercostal space and after pericardiotomy the heart was exposed. In Group I (n = 80), occlusion of the left main coronary artery and hence, myocardial ischemia-induced arrhythmias were achieved by tightening a previously placed loose silk ligature for 30 min. In Group II (n = 186), arrhythmias were induced by reperfusion following a 20 min ligation of the left main coronary artery. In both Group I and Group II, early intravenous infusion of nicorandil (100 micrograms/kg bolus + 10 micrograms/kg/min), 3-pyridyl pinacidil (3.0 micrograms/kg bolus + 1.0 microgram/kg/min), HMR 1883 (3 mg/kg)/nicorandil and HMR 1883 (3 mg/kg)/3-pyridyl pinacidil, just prior to and during ischemia, increased survival rate (75%, 67%, 86% and 75% vs. 60% in the control subgroup in Group I; 67%, 75%, 75% and 67% vs. 43% in the control subgroup in Group II), significantly decreased the incidence and severity of life-threatening arrhythmias and significantly decreased myocardial infarct size. However, late intravenous administration of nicorandil or 3-pyridyl pinacidil at the onset of and during reperfusion did not increase survival rate nor confer any antiarrhythmic or cardioprotective effects. The antiarrhythmic and cardioprotective effects of both nicorandil and 3-pyridyl pinacidil were abolished by pretreating the rabbits with 5-hydroxydecanoate (5 mg/kg, i.v. bolus), a selective mitochondrial KATP channel blocker, but not by pretreatment with HMR 1883 (3 mg/kg). In the present study, higher levels of malondialdehyde (MDA) and lower levels of reduced glutathione (GSH) and superoxide dismutase (SOD) in the necrotic zone of myocardium in all sixteen subgroups in Group II suggest little anti-free radical property of nicorandil and 3-pyridyl pinacidil. Therefore, we may conclude that intervention by intravenous administration of nicorandil and 3-pyridyl pinacidil (through the selective activation of mitochondrial KATP channels), increases survival rate and exhibits antiarrhythmic and cardioprotective effects during coronary occlusion and reperfusion in anesthetized rabbits, when administered prior to and during coronary occlusion. The mitochondrial KATP channel may be considered to be a potentially important site of cardioprotection and antiarrhythmic activity.     

NO donor-activated PKC-delta plays a pivotal role in ischemic myocardial protection through accelerated opening of mitochondrial K-ATP channels

Nitric oxide (NO) can activate protein kinase C (PKC) and the activation of mitochondrial ATP-sensitive potassium (K-ATP) channels is cardioprotective. However, interactions among NO, PKC, and mitochondrial K-ATP channels remain vague. To clarify the cardioprotective mechanism induced by nicorandil, we compared its ability to activate PKC isoforms with that of the mitochondrial K-ATP channel opener, diazoxide. We induced myocardial infarction in rats by 30 minutes of ischemia followed by reperfusion, then assessed the infarct size 3 weeks later. We also examined the translocation of PKC isoforms in the isolated perfused rat heart. Nicorandil and diazoxide reduced infarct size, and the effect of nicorandil, but not of diazoxide attenuated by the PKC inhibitor, chelerythrine, or by the NO quencher, carboxy PTIO. Immunoblotting revealed that nicorandil translocated PKC-delta to the mitochondria, and that this was inhibited by carboxy PTIO. The protective effect of nicorandil against myocardial infarction partly depended on the translocation of PKC-delta to the mitochondria, which we attributed to the NO donor effect of nicorandil. The PKC-delta- dependent activation of mitochondrial K-ATP channel opening might be synergistic with its direct effect, making nicorandil an efficient opener of such channels.     

NMDA receptors are involved in dithiothreitol-induced hypothermia

The effect of nicorandil, an ATP-sensitive K(+) channel opener, on the level of intracellular Ca(2+) ([Ca(2+)](i)) and on ATP release in endothelial cells of the rat caudal artery was examined using a fluorescent confocal microscopic imaging system and high-performance liquid chromatography (HPLC) with fluorescent detection, respectively. Nicorandil significantly increased [Ca(2+)](i) and the overflow of ATP and its metabolites. The former reaction was abolished in the absence of extracellular Ca(2+), but it did not change in the presence of thapsigargin or cyclopiazonic acid. The increase in the overflow of ATP and [Ca(2+)](i) induced by nicorandil was markedly suppressed by glibenclamide, an ATP-sensitive K(+) channel blocker. The increase of [Ca(2+)](i) induced by nicorandil was significantly and inversely correlated with the level of intracellular ATP in the endothelial cells, suggesting that activation of ATP-sensitive K(+) channels by nicorandil increases Ca(2+) influx in endothelial cells. The increase of [Ca(2+)](i) might be associated with ATP release.     

Selective mitochondrial K(ATP) channel activation results in antiarrhythmic effect during experimental myocardial ischemia/reperfusion in anesthetized rabbits

We investigated the effects of administration of non-hypotensive doses of ATP-sensitive K+ channel (K(ATP)) openers (nicorandil and aprikalim), and a specific mitochondrial K(ATP) channel blocker (5-hydroxydecanoate) prior to and during coronary occlusion as well as prior to and during post-ischemic reperfusion on survival rate, ischemia/reperfusion-induced arrhythmias and myocardial infarct size in anesthetized albino rabbits. Arrhythmias were induced by reperfusion following a 20 min ligation of the left main coronary artery with a releaseable silk ligature. Early intervention by intravenous infusion of nicorandil (100 microg/kg bolus+10 microg/kg/min) or aprikalim (10 microg/kg bolus+0.1 microg/kg/min) just before and during ischemia increased survival rate (86% and 75% vs. 55% in the control group), significantly decreased the incidence and severity of life-threatening arrhythmias and myocardial infarct size. The antiarrhythmic and cardioprotective effects of both nicorandil and aprikalim were abolished by pretreating the rabbits with 5-hydroxydecanoate (5 mg/kg, i.v. bolus). In conclusion, intervention by intravenous administration of nicorandil and aprikalim (through the selective activation of mitochondrial K(ATP) channels) increased survival rate and exhibited antiarrhythmic and cardioprotective effects during coronary occlusion and reperfusion in anesthetized rabbits when administered prior to and during coronary occlusion.     

The role of endothelium-derived nitric oxide in relaxations to levcromakalim in the rat aorta

The present study was designed to examine the role of basally released nitric oxide in relaxations to an ATP-sensitive K+ channel opener. Whether relaxations to levcromakalim are modulated by endothelial removal or the inhibitors of vasodilator effects of endothelium-derived nitric oxide, were investigated in the rat aorta. During contractions to phenylephrine (3 x 10(-7) to 10(-6) M), levcromakalim (10(-8) to 10(-5) M) or a nitric oxide donor, 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC-7, 10(-9) to 10(-5) M), was added in a cumulative fashion. Relaxations to levcromakalim (10(-8) to 10(-5) M) were significantly reduced by the endothelium-removal. In aortas with endothelium, relaxations in response to levcromakalim were decreased by selective inhibitors of nitric oxide synthase (N(G)-nitro-L-arginine methyl ester, 10(-4) M) and soluble guanylate cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one; ODQ, 10(-5) M) and a scavenger of nitric oxide (carboxy-PTIO, 10(-3) M). Relaxations to levcromakalim in aortas treated with these inhibitors are comparable to those seen in aortas without endothelium. KCl (30 mM) and an ATP-sensitive K+ channel inhibitor, glibenclamide (10(-5) M), abolished relaxations to levcromakalim in aortas with or without endothelium, whereas glibenclamide did not alter relaxations to NOC-7 (10(-9) to 10(-5) M) in aortas without endothelium. These results suggest that in rat aortas, inhibition of vasodilator effects of basally released nitric oxide can reduce relaxations via ATP-sensitive K+ channels, although these channels do not mediate relaxations to exogenously applied nitric oxide.     

The relaxant action of nicorandil in bovine tracheal smooth muscle

The relaxant mechanisms of nicorandil were examined by comparing its effects with those of sodium nitroprusside and cromakalim in bovine tracheal smooth muscle. In preparations contracted with methacholine (0.3 μ mol/l) or high K(+)(40 mmol/l), nicorandil and sodium nitroprusside caused concentration-dependent relaxations. Their relaxant effects on high K(+) -contracted preparations were smaller than those on methacholine-contracted muscle. Cromakalim relaxed methacholine-contracted preparations, whereas it had no effect on high K(+) -contracted muscle. The inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 5 mol/l) completely prevented the relaxation induced by lower concentrations ( <30 μ mol/l) of nicorandil,whereas it partially attenuated relaxation caused by higher concentrations. The ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide only partially attenuated the relaxant responses to nicorandil (at 100 and 300 μ mol/l). Combination treatment with ODQ and glibenclamide almost completely prevented nicorandil-induced relaxations. The large-conductance Ca2(+) -activated K(+) channel (Maxi K(+) channel) inhibitor iberiotoxin significantly prevented the relaxations induced by lower concentrations (3 and 10 μ mol/l) of nicorandil. The preventive effect of iberiotoxin was markedly enhanced under the blockade of K(ATP) channels with glibenclamide. These results suggest that nicorandil relaxes bovine tracheal smooth muscle through 2 mechanisms: opening of K(ATP) channels and activation of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway. Nicorandil may also activate Maxi K(+) channels, possibly through the NO-cGMP pathway, and the interaction of K ATP channels and Maxi K(+) channels may affect the relaxant effect of nicorandilin bovine tracheal smooth muscle.     

Nicorandil stimulates release of adenyl purines from porcine coronary artery

1. To clarify the mechanism of the cardioprotective effect of nicorandil (2-nicotinamidoethyl-nitrate ester), the effects of nicorandil and nitric oxide (NO) donors on the release of ATP, ADP, AMP and adenosine from arterial segments and cultured endothelial cells of the porcine coronary artery were examined. 2. Nicorandil significantly increased the release of total adenyl purines from arterial segments and from cultured endothelial cells. 3. Cromakalim, an ATP-sensitive K+ channel opener, did not affect the release of total adenyl purines from coronary artery segments. 4. s-Nitroso-N-acetyl-D,L-penicillamine and isosorbide dinitrate, NO donors, significantly increased the release of total adenyl purines from coronary artery segments. 5. These results demonstrate that nicorandil stimulates ATP release from the coronary artery by acting not as an ATP-sensitive K+ channel opener, but as a nitrate, thus suggesting the cardioprotective properties of nicorandil.     

Nicorandil enhances the effect of endothelial nitric oxide under hypoxia-reoxygenation: role of the KATP channel

Nicorandil increased the anti-platelet aggregation activity of endothelial cells when endothelial cells were exposed to hypoxia-reoxygenation conditions. However, nicorandil (0.1-10 muM) did not inhibit platelet aggregation directly. The mechanism by which nicorandil increases the anti-aggregation activity of hypoxia-reoxygenation treated endothelial cells was investigated. The effect of nicorandil was observed even in indomethacin-treated endothelial cells, but the effect was eliminated by treating endothelial cells with N(G)-nitro-l-arginine methyl ester (L-NAME). This indicates that nicorandil enhances the anti-aggregation activity of endothelial nitric oxide (NO). Nicorandil did not increase the anti-aggregation activity of endothelial NO when endothelial cells were pre-treated with superoxide dismutase or 4-(2-aminophenyl)-benzenesulfonyl fluoride, an inhibitor of NADPH oxidase. Nicorandil dose-dependently inhibited the reactive oxygen species generation induced by an oxidative stress in endothelial cells. The effect of nicorandil on anti-aggregation activity was abrogated by glibenclamide, an ATP-sensitive potassium (K(ATP)) channel blocker. Pinacidil, a K(ATP) channel opener, also enhanced the anti-aggregation activity of endothelial NO. This effect was similarly abrogated by glibenclamide. These results suggest that nicorandil may inhibit the generation of superoxide (O(2)(-)) from hypoxia-reoxygenation treated endothelial cells through activation of the K(ATP) channel, and that nicorandil may prevent the disappearance of endothelial NO by O(2)(-).     

Nicorandil elevates tissue cGMP levels in a nitric-oxide-independent manner

The K(+) channel opener nicorandil is a hybrid compound that contains nitrate in its structure. It has been reported that nicorandil can relax vascular tissue in vitro via a mechanism that involves activation of K(ATP) channels and stimulation of soluble guanylyl cyclase. However, it is not known whether the increase of cGMP levels occurs through an elevation of nitric oxide (NO). The aim of the present study was to determine whether NO release was a direct effect of nicorandil. We reported here that nicorandil did not generate NO using ozone chemiluminescence detection methods in human or rat liver microsomes (P450-rich fractions) with addition of NADPH. However, nicorandil elevated cGMP levels in rat liver, aorta, and human coronary smooth muscle cells in vitro. The elevation was not inhibited by the NO trapping agent carboxy-2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (carboxy-PTIO). These results suggest that nicorandil elevates cGMP without NO generation.     

Large conductance Ca2+-activated K+ (BKCa) channels are involved in the vascular relaxations elicited by piceatannol isolated from Rheum undulatum rhizome

We previously reported that piceatannol isolated from the rhizome extract of RHEUM UNDULATUM has a potent vasorelaxant activity. In the present study, the mechanisms underlying the direct vascular relaxant effect of piceatannol were investigated in isolated rat aorta. Piceatannol induced a concentration-dependent relaxation in aortic preparations precontracted with phenylephrine (EC (50) : 2.4 +/- 0.4 microM), which was completely inhibited by endothelial removal, N(omega)-nitro- L-arginine (nitric oxide synthase inhibitor), methylene blue and 1 H- oxadiazolo [4,3- A]quinoxalin-1-one (guanylyl cyclase inhibitor). The piceatannol-induced relaxation was also blocked by raising the extracellular K (+) (45 mM), 4-aminopyridine (voltage-sensitive K (+) channel blocker) and tetraethylammonium [the non-selective Ca (2+)-activated K (+) (K (Ca)) channel blocker] but not by indomethacin (cyclooxygenase inhibitor), atropine (muscarinic receptor antagonist), propranolol (beta-adrenoceptor antagonist), verapamil and nifedipine (L-type voltage-gated Ca (2+) channel blocker), barium chloride (inward rectifier K (+) channel inhibitor) and glibenclamide (ATP-sensitive K (+) channel blocker). In further studies investigating the role of Ca (2+)-activated K (+) (K (Ca)) channels, piceatannol-induced relaxant responses were decreased by charybdotoxin [large (BK (Ca))- and intermediate (IK (Ca))-conductance K (Ca) channel blocker], iberiotoxin (selective BK (Ca) channels blocker), but not by apamin [small-conductance K (Ca) (SK (Ca)) channel blocker], TRAM-34 [intermediate-conductance K (Ca) (IK (Ca)) channel blocker]. The present results demonstrate that piceatannol-induced vascular relaxation in rat aorta may be mediated by an endothelium-dependent nitric oxide signaling pathway, at least partially, through the activation of BK (Ca).     

Effects of administration of nicorandil or bimakalim prior to and during ischemia or reperfusion on survival rate, ischemia/reperfusion-induced arrhythmias and infarct size in anesthetized rabbits

We investigated the effects of administration of non-hypotensive doses of ATP-sensitive K+ channel (KATP) openers (nicorandil and bimakalim), and a specific mitochondrial KATP channel blocker (5-hydroxydecanoate) prior to and during coronary occlusion as well as prior to and during post-ischemic reperfusion on survival rate, ischemia-induced and reperfusion-induced arrhythmias and myocardial infarct size in anesthetized albino rabbits. The thorax was opened in the left fourth intercostal space and after pericardiotomy the heart was exposed. In Part I, occlusion of the left main coronary artery and hence, myocardial ischemia-induced arrhythmias were achieved by tightening a previously placed loose silk ligature for 30 min. In Part II, arrhythmias were induced by reperfusion following a 20-min ligation of the left main coronary artery. In Part I, early intravenous infusion of nicorandil (100 microg/kg bolus + 10 microg/kg per min) or bimakalim (3 microg/kg bolus + 0.1 microg/kg per min) just prior to and during ischemia increased survival rate (75% and 67% vs. 60% in the control group), significantly decreased the incidence and severity of life-threatening arrhythmias and significantly decreased myocardial infarct size. In Part II also, early intervention by intravenous infusion of nicorandil (100 microg/kg bolus + 10 microg/kg per min) or bimakalim (3 microg/kg bolus + 0.1 microg/kg per min) just before and during ischemia increased survival rate (86% and 75% vs. 55% in the control group), significantly decreased the incidence and severity of life-threatening arrhythmias and significantly decreased myocardial infarct size. However, late intravenous administration of nicorandil or bimakalim at the onset and during reperfusion did not increase survival rate nor confer any antiarrhythmic or cardioprotective effects. The antiarrhythmic and cardioprotective effects of both nicorandil and bimakalim were abolished by pretreating the rabbits with 5-hydroxydecanoate (5 mg/kg, i.v. bolus), a selective mitochondrial KATP channel blocker. In conclusion, intervention by intravenous administration of nicorandil and bimakalim (through the activation of mitochondrial KATP channels), increased survival rate and exhibited antiarrhythmic and cardioprotective effects during coronary occlusion and reperfusion in anesthetized rabbits when administered prior to and during coronary occlusion.     

Activation of ATP-sensitive K(+) channels: mechanism of peripheral antinociceptive action of the nitric oxide donor, sodium nitroprusside

Using the rat paw pressure test, in which sensitivity is increased by intraplantar injection of prostaglandin E(2) (PGE(2)), we conducted a study using several K(+) channel blockers. The objective was to determine what types of K(+) channels could be involved in the peripheral antinociceptive action of the nitric oxide donor sodium nitroprusside (SNP). SNP elicited a dose-dependent (250 and 500 microgram/paw) peripheral antinociceptive effect, which was considered local, since only higher doses produced an effect in the contralateral paw. The effect of SNP (500 microgram/paw) was dose-dependently antagonized by intraplantar administration of the sulfonylureas tolbutamide (20, 40 and 160 microgram) and glibenclamide (40, 80 and 160 microgram), selective blockers of ATP-sensitive K(+) channels. Charybdotoxin (2 microgram/paw), a selective blocker of high conductance Ca(2+)-activated K(+) channels, and apamin (10 microgram/paw), a selective blocker of low conductance Ca(2+)-activated K(+) channels, did not modify the peripheral antinociception induced by SNP. Tetraethylammonium (2 mg/paw), 4-aminopyridine (200 microgram/paw) and cesium (800 microgram/paw) also had no effect. Based on this experimental evidence, we conclude that the activation of ATP-sensitive K(+) channels could be the mechanism by which nitric oxide, donated by SNP, induces peripheral antinociception, and that Ca(2+)-activated K(+) channels and voltage-dependent K(+) channels appear not to be involved in the process.     

Different K+ channels are involved in relaxation of arterial and venous graft induced by nicorandil

The drug nicorandil is a vasodilator approved for the treatment of angina. In addition to its well-known effect on the opening of ATP-sensitive K (KATP) channels, nicorandil-induced vasorelaxation also involves the opening of Ca-activated K channels. The aim of this study was to investigate the effects of nicorandil on the isolated human internal mammary artery (HIMA) and the human saphenous vein (HSV) and to define the contribution of different K channel subtypes in the nicorandil action on these arterial and venous grafts. Our results show that nicorandil induced a concentration-dependent relaxation of HSV and HIMA rings precontracted by phenylephrine. Glibenclamide, a selective KATP channels inhibitor, partially inhibited the response to nicorandil in both HSV and HIMA. Iberiotoxin, a most selective blocker of large-conductance Ca-activated K (BKCa) channels, partly antagonized relaxation of HIMA. A nonselective blocker of voltage-gated K channels, 4-aminopyridine caused partial inhibition of the nicorandil-induced relaxation of HSV but did not antagonize relaxation of HIMA induced by nicorandil. Margatoxin, a potent inhibitor of KV1.3 channels, did not abolish the effect of nicorandil on HSV and HIMA. Our results showed that nicorandil induced strong endothelium-independent relaxation of HSV and HIMA contracted by phenylephrine. It seems that KATP and 4-aminopyridine-sensitive K channels located in the smooth muscle of HSV mediated relaxation induced by nicorandil. In addition, KATP and BKCa channels are probably involved in the nicorandil action on HIMA.     

Glibenclamide and L-NG-nitro-arginine methyl ester modulate the ocular and hypotensive effects of calcitonin gene-related peptide.

Calcitonin gene-related peptide (CGRP) given i.v. to rabbits induced hypotension and a breakdown of the blood-aqueous barrier. Glibenclamide, a blocker of ATP-sensitive K+ (K+ATP) channels antagonized both these effects. The K+ATP channel opener diazoxide reduced blood pressure but did not damage the blood-aqueous barrier. Inhibition of nitric oxide synthase antagonized the effects of CGRP on the blood-aqueous barrier but did not attenuate the hypotensive response. The results suggest that vasodilatation induced by the opening of K+ATP channels is a prerequisite for the effect of CGRP on the blood-aqueous barrier.     

The antiarrhythmic effect of centrally administered rilmenidine involves muscarinic receptors, protein kinase C and mitochondrial signalling pathways

BACKGROUND AND PURPOSE: We have previously demonstrated that stimulation of imidazoline receptors in the CNS prevented halothane-adrenaline arrhythmias during halothane anaesthesia and that stimulation of the vagus nerve may be critical to this effect. However, details of the mechanism(s) involved are not yet available. The present study was designed to examine the role of muscarinic receptors, protein kinase C (PKC), ATP-sensitive potassium channels (K(ATP)) and the mitochondrial permeability transition pore (MPTP) in the antiarrhythmic effect of rilmenidine, an imidazoline receptor agonist.EXPERIMENTAL APPROACH: Rats were anaesthetized with halothane and monitored continuously for arterial blood pressure and premature ventricular contractions. The arrhythmogenic dose of adrenaline was defined as the lowest dose producing three or more premature ventricular contractions within a 15-s period. We confirmed that centrally administered rilmenidine prevented halothane-adrenaline arrhythmias and then examined the antiarrhythmic effect of rilmenidine in the presence of atropine methylnitrate, a muscarinic receptor antagonist, calphostin C, a PKC inhibitor, HMR-1098, a sarcolemmal K(ATP) inhibitor, 5-hydroxydecanoic acid, a mitochondrial K(ATP) inhibitor or atractyloside, an MPTP opener.KEY RESULTS: The antiarrhythmic effect of rilmenidine was significantly inhibited by atropine methylnitrate, calphostin C, 5-hydroxydecanoic acid and atractyloside, but the effects of HMR-1098 in our model were not clear.CONCLUSIONS AND IMPLICATIONS: The present results suggest that muscarinic receptors, PKC, mitochondrial K(ATP) channels and MPTP may be crucial components of the mechanism involved in the antiarrhythmic effect of rilmenidine given into the CNS.     

Participation of the nitric oxide-cyclic GMP-ATP-sensitive K(+) channel pathway in the antinociceptive action of ketorolac

The involvement of nitric oxide (NO), cyclic GMP and ATP-sensitive K(+) channels in the antinociceptive effect of ketorolac was assessed using the formalin test in the rat. Local administration of ketorolac in a formalin-injured paw produced a dose-dependent antinociceptive effect due to a local action, as drug administration in the contralateral paw was ineffective. Pretreatment of the injured paw with N(G)-L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) or glibenclamide (an ATP-sensitive K(+) channel blocker) prevented ketorolac-induced antinociception. However, pretreatment with saline or N(G)-D-nitro-arginine methyl ester (D-NAME) did not block antinociception. Local administration of S-nitroso-N-acetylpenicillamine (SNAP, an NO donor) was inactive by itself, but increased the effect of ketorolac. The present results suggest that the antinociceptive effect of ketorolac involves activation of the NO-cyclic GMP pathway, followed by an opening of ATP-sensitive K(+) channels at the peripheral level.     

Vasodilator action in the nitroxylated cyanoamidine derivative, KRN2391, in rabbit basilar artery.

KRN2391 is a cyanoamidine derivative with a pyridine ring and a nitroxyl group. This gives the molecule a dual pharmacological action as both an ATP-sensitive K channel (K(ATP)) opener and an organic nitrate. In cerebrovascular disease with endothelial dysfunction, such a compound could be advantageous to prevent the negative consequences of a reduced synthesis of endogenous nitric oxide and endothelium-derived hyperpolarizing factor. The objective of this study was to characterise the vasodilator action of KRN2391 in a cerebral artery. As shown in the rabbit basilar artery contracted by endothelin-1 KRN2391 elicited a concentration-dependent relaxation. KRN2391 was unable to relax arteries contracted by a 60 mM K solution. The KRN2391-induced relaxation of endothelin-1-contracted arteries was unaffected by N(G)-nitro-L-arginine (0.1 mM), indomethacin (10 microM) or removal of the endothelium. The guanylate cyclase inhibitors ODQ (10 microM) and LY53583 (10 microM), and the cGMP phosphodiesterase inhibitor zaprinast (10 microM) each had no effect on the KRN2391-induced relaxation. Glibenclamide (1 microM), a blocker of K(ATP), caused a rightward shift of the concentration-response curve for KRN2391. The relaxation induced by the prototype K(ATP) opener levcromakalim was inhibited to a similar extent by glibenclamide. Addition of ODQ or LY53583, or the calcium-sensitive K channel blockers apamin (0.1 microM) and charybdotoxin (0.1 microM) in the presence of glibenclamide did not produce a significant further inhibition of the KRN-induced relaxation. KRN2391 (10 microM) did not influence the content of cGMP in the basilar artery, whereas the nitric oxide donor 3-morpholino-sydnonimine (0.1 mM) increased the cGMP level three-fold. Thus, KRN2391 is an effective vasodilator of the rabbit basilar artery, acting mainly through opening of KATP . The nitro-moiety of the molecule does not seem to contribute to the relaxant effect in this artery.     

Relaxant mechanisms of 3, 5, 7, 3', 4'-pentamethoxyflavone on isolated human cavernosum

We have investigated effects and mechanisms responsible for the activity of 3, 5, 7, 3', 4'-pentamethoxyflavone (PMF) on isolated human cavernosum. PMF is the major flavone isolated from Kaempferia parviflora claimed to act as an aphrodisiac. PMF caused relaxation of phenylephrine precontracted human cavernosal strips, and this effect was slightly inhibited by N(G)-nitro-l-arginine, a nitric oxide synthase inhibitor, but not by ODQ (soluble guanylate cyclase inhibitor), TEA (tetraethylammonium, blocker of voltage-dependent K(+) channels) or glybenclamide (blocker of ATP-dependent K(+) channels). PMF did not significantly inhibit the relaxant activity of glyceryltrinitrate or acetylcholine on human cavernosal strips precontracted with phenylephrine. In contrast, sildenafil (phosphodiesterase inhibitor) potentiated the relaxant activity of glyceryl trinitrate but not of acetylcholine. In normal Krebs solution with nifedipine (blocker of l-type Ca(2+) channels), or in Ca(2+)-free Krebs solution, PMF caused a further inhibition of human cavernosum contracted with phenylephrine. In human cavernosum treated with thapsigargin (inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase) in Ca(2+)-free medium, PMF suppressed the concentration-response curve of human cavernosum to phenylephrine and a further suppression was found when SKF-96365 (a blocker of store-operated Ca(2+) channels and Y-27632 (inhibitor of Rho-kinase)), but not nifedipine, were added sequentially. Thus, PMF had only a weak effect on the release of nitric oxide, and had no effect as a K(ATP)- or K(Ca) channel opener, a phosphodiesterase inhibitor, a store-operated Ca(2+) channel blocker or a Rho-kinase inhibitor. Therefore, these studies suggest that PMF causes relaxation of human cavernosum through voltage-dependent Ca(2+) channels and other mechanisms associated with calcium mobilization.