Comparative effects of nicorandil, a nicotinamide nitrate derivative, and nifedipine on myocardial reperfusion injury in dogs

The effects of the nicotinamide nitrate compound nicorandil (SG-75) and the slow channel calcium entry blocker nifedipine on the recovery of subendocardial segment shortening (% SS) were compared with a vehicle-treated group following 30 min of left anterior descending coronary artery (LAD) occlusion and 3 h of reperfusion. Sonomicrometry was used to determine % SS in ischemic and nonischemic myocardium, and radioactive microspheres were used to determine regional myocardial blood flow. Nicorandil (100-micrograms/kg bolus followed by 25 micrograms/kg/min i.v.), nifedipine (10-micrograms/kg bolus followed by 3 micrograms/kg/min i.v.), or vehicle (saline) was administered 15 min prior to and throughout the occlusion period. Both drugs produced equivalent decreases in the heart rate X systolic pressure product before and during LAD occlusion. In addition, total left ventricular weights, the area at risk, the percent of the left ventricle at risk, and collateral blood flow were similar in all three groups. During coronary occlusion, % SS in the ischemic region was equally depressed in each series and passive systolic lengthening resulted. However, following reperfusion, only the nicorandil-treated animals showed an improvement in myocardial segment function through 3 h of reperfusion as compared with the control group. Transmural myocardial blood flow within the ischemic region during reperfusion returned to control values in all three groups; however, the endocardial/epicardial blood flow ratio (endo/epi) was significantly decreased in the control and nicorandil-treated dogs. In contrast, the endo/epi was greater than the preocclusion control in the nifedipine series during reperfusion. Thus, although the mechanism of action of nicorandil in this model is unknown, the improvement in % SS in the nicorandil-treated group was not related to changes in peripheral hemodynamics or improved regional blood flow, since nifedipine produced similar changes in hemodynamics and resulted in a better recovery of perfusion.     

Effects of hydroxyethyl starch conjugated deferoxamine on myocardial functional recovery following coronary occlusion and reperfusion in dogs.

The effect of hydroxyethyl starch-conjugated deferoxamine (HES-DFO) on the recovery of regional myocardial function after 15 min of coronary artery occlusion followed by 3 h of reperfusion of the left anterior descending coronary artery (stunned myocardium) was investigated in anesthetized dogs. Regional myocardial blood flow was measured by radioactive microspheres and regional myocardial segment shortening (%SS) by sonomicrometry. HES-DFO (equivalent of 50 mg/kg DFO), iron saturated HES-DFO (HES-FO), deferoxamine (DFO, 50 mg/kg), or saline were administered by intravenous infusion starting 30 min before occlusion and throughout occlusion. Ischemic bed size and collateral blood flow were similar in all four groups. HES-DFO significantly improved %SS in the ischemic-reperfused region during reperfusion; however, HES-FO and DFO had no effect on %SS as compared to the saline-treated group. HES-DFO and HES-FO had no effect on hemodynamics; however, DFO produced a marked reduction in systemic blood pressure. Since HES-FO had no effect on the recovery of %SS, the beneficial effect of HES-DFO is thought to be due to its iron chelating characteristics. Plasma concentrations of HES-DFO not only reached a higher peak level but also had a longer half life (3 h) than that of regular DFO (20 min). Thus, high-molecular-weight HES-DFO is effective in enhancing the recovery of regional wall motion in stunned myocardium. The reason for the lack of efficacy of DFO compared to HES-DFO at this high dose may be related to the formation of a toxic deferoxamine free radical species.     

Effects of intracoronary nifedipine on blood flow and segment shortening in normal and ischemic myocardium: potentiation by ischemia of the negative inotropic effect

The effects of intracoronary nifedipine on myocardial blood flow (flow probe or microspheres) and regional function (ultrasonic crystals in subendocardium) were examined both in the normal myocardium and in myocardium made ischemic by a partial coronary occlusion in the open-chest anesthetized dog. In a first group of experiments (n = 7), without ischemia, nifedipine infused into the left anterior descending coronary artery (LAD) during a 1-min period (doses 0.75-8 nmol/kg body weight) decreased coronary vascular resistance with a maximal effect at 4 nmol/kg. Systolic segment shortening was decreased from 10.7 to 7.4% (p less than 0.05) by 6 nmol/kg, whereas lower doses had no effect. In a second experimental group (n = 7), a partial LAD occlusion was applied to decrease subendocardial segment shortening by about 50%. Nifedipine (2 nmol/kg) injected into the partially occluded LAD induced a marked segmental bulging during early systole and systolic segment shortening was eliminated (from 4.2 to -3.1%, p less than 0.02) in the LAD-dependent myocardium. Concomitant with the decreased regional function, nifedipine caused a transmural redistribution of myocardial blood flow in the ischemic area, the endocardial/epicardial blood flow ratio increasing from 0.49 to 0.61 (p less than 0.02). It is concluded that ischemia potentiates the direct depressant effect of nifedipine on myocardial regional function.     

Antianginal effects of hydroxyfasudil, a Rho-kinase inhibitor, in a canine model of effort angina.

1. The effects of Rho-kinase inhibitor, fasudil, and of a more specific Rho-kinase inhibitor, hydroxyfasudil, on pacing-induced myocardial ischaemia were determined in anaesthetized open-chest dogs. 2. The dogs were subjected to left anterior descending coronary artery (LAD) stenosis producing a sufficient ischaemia as measured by ST-segment depression on electrocardiograms only when the hearts were paced 60 beats min(-1) above the baseline. After a recovery (nonpacing) period, drugs or saline were infused intravenously over 30 min. The animals were again subjected to 5 min of pacing 25 min after the initiation of the treatment. 3. Hydroxyfasudil (0.1 and 0.3 mg kg(-1)) and fasudil (0.3 mg kg(-1)) suppressed the ST-segment depression. Hydroxyfasudil and fasudil also increased the regional blood flow of the LAD perfused endomyocardium region in the canine model of effort angina. 4. To determine the flow profile for hydroxyfasudil in dogs, blood flow in three vascular beds was measured. Hydroxyfasudil (0.3 mg kg(-1) for 30 min) significantly increased coronary blood flow and vertebral blood flow, without significantly changing the femoral blood flow. 5. Hydroxyfasudil had no inotropic or chronotropic effect on the isolated hearts of guinea-pigs. Hydroxyfasudil (2 mg kg(-1) for 20 min) did not affect the PR or QTc interval in anaesthetized dogs. 6. Inhibition of Rho-kinase appears to protect myocardium subjected to pacing-induced ischaemia through the increase in the regional myocardial blood flow. Hydroxyfasudil may be categorized as a novel type of anti-anginal drug, without any inotropic or chronotropic effects.     

Examination of diltiazem for preservation of myocardial function after brief regional ischemia

Diltiazem (750 micrograms/kg plus 600 micrograms/kg/h X 1 h, i.v.) and vehicle were examined in open-chest anesthetized dogs subjected to 15 min of occlusion of the left circumflex coronary artery (LCCA). Regional segment lengths in myocardium supplied by the LCCA and by the left anterior descending coronary (LAD) were measured with piezoelectric crystals implanted in the subendocardium. Diltiazem decreased heart rate and mean arterial pressure, and increased coronary blood flow, determined with an electromagnetic flowmeter. Vehicle had no significant effects. Occlusion of the LCCA increased end diastolic segment length (EDL), and produced akinesis or paradoxical systolic lengthening: diltiazem -2.5 +/- 2.7% and vehicle 0.0 +/- 1.2% segmental shortening (SS). EDL and SS in the LAD zone showed small increases. After 15 min, the LCCA was reperfused and recovery of SS was followed for 3 h. Significantly greater recovery of SS was observed with diltiazem compared to vehicle throughout reperfusion: at 5 min, diltiazem 105 +/- 22% and vehicle 43 +/- 7% and at 180 min, diltiazem 73 +/- 0% and vehicle 33 +/- 8% of baseline SS. The LCCA and LAD zones both responded to isoproterenol 0.3 microgram/kg given 2.5 h after reperfusion. During the isoproterenol challenge SS for LCCA in the diltiazem group (122 +/- 21%) was not different than that of vehicle (99 +/- 15% of baseline). Calcium entry blockade with diltiazem resulted in improved myocardial function during reperfusion. The stunned myocardium showed significant stimulation of shortening by isoproterenol in both groups.     

Effects of the prostanoid EP3-receptor agonists M&B 28767 and GR 63799X on infarct size caused by regional myocardial ischaemia in the anaesthetized rat

1. This study investigates the effects of two agonists of the prostanoid EP3-receptor (M&B 28767 and GR 63799X) on the infarct size caused by regional myocardial ischaemia and reperfusion in the anaesthetized rat. 2. One hundred and sixty-seven, male Wistar rats were anaesthetized (thiopentone, 120 mg kg(-1) i.p.), ventilated (8-10 ml kg(-1), 70 strokes min(-1), inspiratory oxygen concentration: 30%; PEEP: 1-2 mmHg) and subjected to occlusion of the left anterior descending coronary artery (LAD, for 7.5, 15, 25, 35, 45 or 60 min) followed by reperfusion (2 h). Infarct size was determined by staining of viable myocardium with a tetrazolium stain (NBT), histological evaluation by light and electron microscopy and determination of the plasma levels of cardiac troponin T. 3. M&B 28767 (0.5 microg kg(-1) min(-1), i.v., n=7) or GR 63799X (3 microg kg(-1) min(-1), i.v., n=7) caused significant reductions in infarct size from 60+/-3% (25 min ischaemia and 2 h reperfusion; saline-control, n=8) to 39+/-6 and 38+/-4% of the area at risk, without causing a significant fall in blood pressure. Pretreatment of rats with 5-hydroxydecanoate (5-HD), an inhibitor of ATP-sensitive potassium channels, attenuated the cardioprotective effects of both EP3-receptor agonists. The reduction in infarct size afforded by M&B 28767 was also abolished by glibenclamide and the protein kinase C (PKC) inhibitors staurosporine and chelerythrine. 4. Thus, M&B 28767 and GR 63799X reduce myocardial infarct size in the rat by a mechanism(s) which involves the activation of PKC and the opening of ATP-sensitive potassium channels.     

Nicorandil attenuates myocardial dysfunction associated with transient ischemia by opening ATP-dependent potassium channels.

The objective of this study was to determine whether ATP-dependent potassium channel activation is involved in the mechanism by which nicorandil reduces postischemic contractile dysfunction produced by a brief period of ischemia (myocardial stunning). Barbital-anesthetized dogs were subjected to 15-min left anterior descending (LAD) coronary artery occlusion followed by 3-h reperfusion. Saline or nicorandil (100 micrograms/kg + 25 micrograms/kg/min) were infused 15 min before and throughout occlusion with or without addition of the KATP channel antagonist, glibenclamide 0.3 mg/kg as an intravenous (i.v.) bolus. Regional myocardial blood flow was measured by radioactive microspheres, and left ventricular (LV) segment function was measured by sonomicrometry. There were no significant differences between the groups in area-at-risk size or collateral blood flow. In contrast, nicorandil significantly reduced mean aortic blood pressure (BP) and the rate-pressure product (RPP) which persisted throughout the occlusion period. In addition, nicorandil markedly accelerated recovery of segment shortening in the ischemic/reperfused region as compared with control dogs. Pretreatment of dogs with glibenclamide blocked none of the hemodynamic effects of nicorandil, but it did prevent improvement in reperfusion segment function. The small dose of glibenclamide used had no effect on hemodynamics or the degree of stunning. Thus, these results suggest that nicorandil attenuates stunning in anesthetized dogs by a direct cardioprotective effect as a result of KATP channel activation in ischemic myocardium.     

Effects of an ATP‐sensitive potassium channel opener,YM934, on hypoxia/reoxygenation injury of isolated canine cardiac cells

The cardioprotective effects of YM934, an ATP‐sensitive potassium channel opener (KCO), on isolated canine cardiomyocytes were examined. Exposure of isolated canine cardiomyocytes to YM934 (10 μM) resulted in an increase of potassium channel opening activity in whole cell patch clamp technique. Glibenclamide (1 μM) reversed the channel opening activity induced by YM934. Pretreatment of the cardiomyocytes with YM934 caused a potent inhibition KCL (20 mM)‐induced increase in [Ca²⁺]_i. Cromakalim also inhibited the KCL‐induced increase in [Ca²⁺]_i under the same experimental conditions. The ratio of alive to dead cells and the release of creatine kinase from isolated cardiomyocytes during a 120‐min hypoxia / 15‐min reoxygenation period were dose‐dependently (YM934; 0.1–10 μM) attenuated; 71–93% of the preischemic value for the ratio of alive/dead cells and 76–58% of the nontreated hearts for the release of creatine kinase. These results suggest that YM934 has a direct cardioprotective effect against ischemia/reperfusion injury of canine cardiac myocytes, possibly through opening ATP‐sensitive potassium channels. Drug Dev. Res. 48:113–120, 1999. © 1999 Wiley‐Liss, Inc.     

Comparative effects of azelnidipine and amlodipine on myocardial function and mortality after ischemia/reperfusion in dogs

Effects of azelnidipine were examined and compared with those of amlodipine on stunned myocardium in dogs. The left anterior descending (LAD) coronary artery was ligated for 20 min and subsequently released for 60 min. A vehicle, azelnidipine (0.3 mg/kg), or amlodipine (0.3 or 1 mg/kg) was injected intravenously 20 min before LAD ligation. The heart rate increased after a depressor response in the presence of amlodipine, while it decreased despite a decrease in arterial pressures in the presence of azelnidipine. After reperfusion, the coronary flow (CF) significantly increased in the presence of azelnidipine, but did not change with amlodipine after reperfusion. A positive inotropic effect was observed after treatment with both calcium antagonists. Ischemia significantly decreased the percentage of segment shortening (%SS) in all groups. Treatment with both calcium antagonists significantly increased %SS after reperfusion, although high-energy phosphate levels did not improve in the presence of calcium antagonists 60 min after reperfusion. Mortality with azelnidipine was significantly lower than that with 0.3 mg/kg amlodipine immediately after reperfusion. In conclusion, improvement in myocardial stunning after pretreatment with azelnidipine is associated with an increase in CF after reperfusion. The negative chronotropic action may have contributed to decreased mortality due to reperfusion arrhythmias. Azelnidipine is more beneficial than amlodipine and may provide an additional advantage to patients with angina and hypertension.     

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.     

Coronary collateral blood flow in acute myocardial ischemia is not increased by dihydropyridine-induced coronary vasodilatation

The effect of two dihydropyridine derivatives, nifedipine and felodipine, on myocardial blood flow distribution 1 h after ligation of the left anterior descending coronary artery (LAD) was studied in open-chest dogs by means of radioactive microspheres. The myocardium normally perfused from the LAD was first labeled with 125I-labeled microspheres injected directly into the LAD before ligation. Microspheres used for blood flow measurements were given in the left atrium. An intravenous infusion rate of 0.3 nmol/kg/min felodipine slightly depressed mean aortic blood pressure (approximately 5 mm Hg) and decreased coronary vascular resistance in normal myocardium. Nifedipine, at three times the dose of felodipine, had a comparable hypotensive effect, but the decrease in coronary vascular resistance was not statistically significant. The two dihydropyridines were also compared in a dose range that was four times higher. The mean arterial blood pressure reduction (approximately 30% for both drugs) was counterbalanced by inflation of an intraaortic balloon to avoid a drastic decrease in afterload and coronary perfusion pressure. Under these circumstances, felodipine and nifedipine decreased coronary vascular resistance and increased blood flow to nonischemic myocardium comparably. However, in severely ischemic, truly collateral-dependent myocardium without admixture of interdigitating healthy myocardium, the blood flow was unaffected after administration of both felodipine and nifedipine. Although felodipine was three times more potent than nifedipine with regard to vasodilatation in the normal myocardium, the difference in vascular selectivity between the two agents did not influence the effect on the "true" collateral blood flow in acute myocardial ischemia.     

Nicorandil improves postischemic contractile function independently of direct myocardial effects.

We determined whether any of the antiischemic effects of nicorandil were due to direct cardioprotective effects such as potassium channel activation or to its peripheral hemodynamic effects. Nicorandil was administered either intravenously (i.v.) or directly into the ischemic coronary artery (i.c.) and compared with i.c. cromakalim (a potassium channel activator previously shown to improve reperfusion function directly in rat hearts) or vehicle for their ability to improve postischemic contractile function as measured by ultrasonic crystals in anesthetized dogs or in isolated perfused rat hearts. In a model of 25-min global ischemia and reperfusion in isolated perfused rat hearts, nicorandil (10-100 microM) did not improve reperfusion function or decrease LDH release, although 300 microM nicorandil did protect the hearts. Cromakalim (7 microM) significantly improved reperfusion function and reduced lactate dehydrogenase (LDH) release. In the dog studies, the left anterior descending coronary artery (LAD) was occluded for 15 min and was reperfused for 3 h. Nicorandil improved reperfusion function only when administered i.v., although i.c. cromakalim was efficacious in improving function. Neither nicorandil nor cromakalim improved collateral flow, although cromakalim significantly improved preischemic and reperfusion blood flows, particularly in the subepicardial region. Although i.c. treatment with cromakalim and nicorandil did not result in significant changes in peripheral hemodynamic status, i.v. nicorandil reduced both preload and afterload. Thus, at the dose used, nicorandil does not appear to have direct myocardial protective effects and the beneficial effects of nicorandil do not appear to be related to potassium channel activation in the myocardium. Potassium channel activation by cromakalim does result in direct cardioprotective effects whereas nicorandil appears to be dependent on peripheral actions for its efficacy.     

Effect of calcium channel blocking agents on infarct size after ischaemia-reperfusion in anaesthetised pigs: relationship between cardioprotection and cardiodepression.

We compared the abilities of verapamil and nicardipine to protect the porcine myocardium from the consequences of ischaemia-reperfusion in vivo. Infusion of verapamil (50 micrograms/kg) into the left anterior descending coronary artery LAD (i.c.a.), in 15 min immediately before ligation depressed regional contractile function, reduced infarct size by 80%, and enabled contractile function to recover partially during reperfusion. Verapamil (10 micrograms/kg i.c.a.) did not depress contractile function before ligation or permit its recovery during reperfusion, despite reducing infarct size by 80%. Lower doses of verapamil were not cardioprotective. Nicardipine (10 and 30 micrograms/kg i.c.a.) depressed contractile function before ligation but did not permit its recovery during reperfusion. Nicardipine did not reduce infarct size development. Thus, drug-induced negative inotropic activity (which presumably reflects myocardial calcium channel blockade) and cardioprotection are not linked. Verapamil can markedly reduce infarct size development at a dose that exerts no detectable negative inotropic activity. This cardioprotective effect of verapamil was greatly reduced by intravenous (i.v) pretreatment with aspirin (30 mg/kg), which alone did not alter infarct size development. Thus, the cardioprotective effect of verapamil (10 micrograms/kg i.c.a.) appears to be mediated by a cyclooxygenase product, possibly prostacyclin.     

Postjunctional alpha-adrenergic stimulation of inotropy in hypoperfused myocardium outside an acute infarct.

The functional significance of myocardial postjunctional alpha-adrenergic support of inotropy in the vicinity of an acute regional ischemic zone was addressed in pentobarbital-anesthetized, beta-adrenergic blocked cats with circumflex coronary artery occlusion. Regional myocardial performance was measured by ultrasonic crystals in the anterior wall perfused by the left anterior descending coronary artery (LAD) before and during postjunctional alpha-adrenergic antagonism (SK&F 104078 2 mg/kg). A group with unrestricted flow in the LAD (control group) was compared with a group perfused below the autoregulatory pressure range (stenosis group). End-systolic pressure-length relations during dynamic after-load elevation were calculated for assessment of regional contractility. Regional myocardial blood flow (RMBF) was measured by radioactive microspheres. SK&F 104078 did not alter regional myocardial shortening or the slope of end-systolic pressure-length relations in the control group. In the stenosis group, however, alpha-adrenergic antagonism produced significant deterioration of shortening as well as consistent reduction of the slope of the end-systolic pressure-length relations (p < 0.05). As a reflection of reduced demands for perfusion, impairment of midmyocardial and endocardial blood flow occurred in the stenosis group (p < 0.05). These findings imply a negative inotropic effect of SK&F 104078 in metabolically vasodilated myocardium in the vicinity of an acute ischemic region.     

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.     

Effects of minoxidil on ischemia-induced mechanical and metabolic dysfunction in dog myocardium

Effects of minoxidil on ischemia-induced myocardial mechanical and metabolic dysfunction were examined in anesthetized open-chest dogs. A regional portion of the left ventricle was made ischemic for 20 min by ligating the left anterior descending coronary artery, and then reperfused for 120 min. Dimethylsulfoxide or minoxidil (0.3, or 1.0 mg/kg) was injected intravenously 10 min before ligation. Ischemia decreased regional myocardial contraction, and reperfusion recovered it but incompletely. Myocardial metabolic derangement was observed during ischemia, such as decreases in the myocardial levels of ATP and creatine phosphate. These metabolic changes caused by ischemia were restored by reperfusion. Minoxidil injection at 0.3 and 1.0 mg/kg significantly decreased blood pressures but increased coronary flow. Pretreatment with minoxidil significantly enhanced the recovery of myocardial contraction during reperfusion after ischemia. The levels of ATP and creatine phosphate in the ischemic myocardium were significantly preserved by minoxidil at 0.3 mg/kg. No significant effect of minoxidil on the metabolism was observed in the 120 min reperfused myocardium. In conclusion, minoxidil improved the mechanical dysfunction in the reperfused heart and the drug at low dose preserved high-energy phosphates during ischemia.     

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.     

Daltroban, a thromboxane receptor antagonist, protects the myocardium against reperfusion injury following myocardial ischemia without protecting the coronary endothelium

The effects of daltroban, a specific thromboxane receptor antagonist, were investigated in a model of myocardial ischemia consisting of 1.5 h of coronary artery occlusion followed by reperfusion for 4.5 h in anesthetized cats. Daltroban (1 mg/kg) was infused as a bolus 10 min prior to reperfusion of the left anterior descending (LAD) coronary artery. Daltroban infusion resulted in a significantly lower necrotic area expressed as a percentage of the myocardial area-at-risk compared to the MI + vehicle group. However, daltroban failed to retard increases in myeloperoxidase activity in the ischemic myocardium, indicating no reduction in neutrophil accumulation. Moreover, left anterior descending coronary artery ring preparations isolated from daltroban treated MI cats exhibited endothelial dysfunction following ischemia reperfusion. Thus, daltroban significantly protected the myocardium from reperfusion injury without protecting the coronary endothelium or retarding neutrophil accumulation.     

Effect of dilazep on decrease in myocardial pH during ischemia in dogs

The present study was undertaken in order to examine whether dilazep (1,4-bis-[3-(3,4,5-trimethoxybenzoyloxy)propyl]perhydro-1, 4-diazepine dihydrochloride monohydrate) attenuates myocardial acidosis induced by coronary artery occlusion in dogs. In dogs with nonischemic normal heart, dilazep (300 or 500 micrograms/kg i.v.) increased blood flow in the left anterior descending coronary artery (LAD) with a decrease in heart rate and diastolic blood pressure. In other dogs, LAD flow was reduced by an occluder by 57 to 68% (partial occlusion) for 90 min. Partial occlusion for 30 min decreased myocardial pH by 0.67 to 0.87 pH units, increased ST segment of the surface electrocardiogram, and decreased regional myocardial contractile force. Dilazep was injected i.v. 30 min after partial occlusion. The decrease in myocardial pH induced by partial occlusion was attenuated by the injection of 300 micrograms/kg of dilazep insignificantly and by that of 500 micrograms/kg of dilazep significantly. Restoration of myocardial [H+] induced by dilazep was calculated from the myocardial pH data. Dilazep (500 micrograms/kg) restored myocardial [H+] induced by partial occlusion by 56.7%, and saline solution restored it by 27.4% 60 min after the drug injection, the actual restoration induced by dilazep being 29.3%. Dilazep, however, did not restore the ST segment elevation and contractile force decrease. It is concluded that dilazep attenuates myocardial acidosis during ischemia.     

Ventricular function and cardiac hypertrophy after coronary thrombolysis with tissue-type plasminogen activator (t-PA) in dogs with coronary artery thrombi

Subacute prognosis of cardiac function after thrombolysis with a modified tissue-type plasminogen activator (t-PA) YM866 was determined in dogs with coronary artery thromboses induced by injection of a thrombin, fibrinogen and autogenous blood mixture. The left ventricular ejection fraction (LVEF) decreased 30 min after occlusion and had not improved 1 week later. Examination after sacrifice revealed myocardial infarction as well as increases in both the left ventricular myocardial area and heart mass. Occluded coronary arteries reperfused by YM866 (0.1 mg kg(-1) i.v.) treatment 30 min after occlusion, by contrast, had improved LVEF and inhibited myocardial infarction development. In addition, the left ventricular myocardial area and heart mass were significantly reduced compared with the vehicle control group 1 week after administration. Although occluded coronary arteries reperfused by YM866 (0.1 mg kg(-1) i.v.) treatment 3 h after occlusion did not show an improvement in the LVEF or inhibition of myocardial infarction development, the left ventricular myocardial area and heart mass decreased significantly compared with the vehicle control group 1 week after administration. In conclusion, early reperfusion by t-PA treatment 30 min after occlusion improved the ventricular function and cardiac hypertrophy, whereas late reperfusion by t-PA treatment 3 h after occlusion did not improve the ventricular function but did inhibit hypertrophy in dogs with coronary artery thrombi.