Involvement of the central nervous system in the protective effect of melanocortins in myocardial ischaemia/reperfusion injury.

Melanocortin peptides exert, in rats, a protective effect in myocardial ischaemia followed by reperfusion, or permanent occlusion of a coronary artery. Moreover, melanocortins have an anti-shock effect. Since the mechanism of the life-saving effect of these peptides in haemorrhagic shock includes an essential brain loop, we aimed to determine whether the central nervous system (CNS) is also involved in the protective effect against the outcome of short-term myocardial ischaemia followed by reperfusion. Ischaemia was produced in anaesthetized rats by ligature of the left anterior descending coronary artery for 5 min. Reperfusion-induced ventricular tachycardia (VT), ventricular fibrillation (VF) and lethality, and the time-course of arterial blood pressure over 5 min following reperfusion were evaluated. Groups of 8-14 rats were used. Intravenous (i.v.) injection of ACTH-(1-24) (0.16-0.48 mg/kg) during the ischaemic period dose dependently reduced the incidence of VT, VF and of lethality. In saline-treated rats, coronary reperfusion caused VT in 100% animals, VF in 86%, and death in 86%. The highest dose of ACTH-(1-24) (0.48 mg/kg) completely prevented the occurrence of VT, VF and death in all rats (P<0.005). Moreover, the melanocortin peptide prevented the fall in mean arterial pressure (MAP) occurring during reperfusion. Treatment with ACTH-(1-24) by the intracerebroventricular (i.c.v.) route also reduced the incidence of VT, VF and lethality, and prevented the fall in MAP in a dose dependent manner. Complete (100%) protection occurred with an i.c.v. dose (0.048 mg/kg) 10 times less than that needed by the i.v. route. The present data show that in the protective effect of melanocortin peptides against the injury after myocardial ischaemia/reperfusion, the i.c.v. route of administration is more effective than the i.v. route. They suggest that a CNS mechanism, not yet identified, may be involved.     

EFFECT OF SOTALOL AGAINST REPERFUSION-INDUCED ARRHYTHMIAS IN SPRAGUE-DAWLEY AND WISTAR RATS

Summary— The effect of (±)-sotalol (aβ-blocker with class III antiarrhythmic activity) against reperfusion-induced arrhythmias was studied in artificially ventilated, open-chest, Sprague-Dawley and Wistar rats. Coronary artery occlusion was produced for 5 min and reperfusion allowed for 10 min. A somewhat different arrhythmic profile was observed between saline-treated rats of the 2 strains studied, with more Sprague-Dawley rats experiencing an irreversible ventricular fibrillation (VF) upon reperfusion, compared to Wistar rats, in whom a combination of reversible ventricular tachycardia (VT) and VF was more frequently observed. No difference in action potential characteristics and ventricular effective refractory period determined in vitro , nor in myocardial noradrenaline content, was found between the 2 strains of rats. (±)-Sotalol (5 and 10mg/kg, IV) showed marked β-blocking activity and reduced the mean duration of VT-VF in both strains studied. It also produced similar increases in action potential duration and refractory period in vitro in these 2 strains. In a different series of experiments, the antiarrhythmic action of the racemic form was compared to that of (+)-sotalol using Wistar rats. (+)-Sotalol had much less β-blocking activity, and was found to be similarly effective against reperfusion-induced VT-VF. It is concluded that the antiarrhythmic effect of sotalol against reperfusion-induced arrhythmias may not be related to β-adrenergic blockade but probably to class III type activity. Despite differences in the profile of reperfusion-induced arrhythmias between Sprague-Dawley and Wistar rats, both strains were sensitive to the antiarrhythmic action of (±)-sotalol.     

Antiarrhythmic effect of caffeic acid phenethyl ester (CAPE) on myocardial ischemia/reperfusion injury in rats

OBJECTIVES: The present study was designed to determine the antiarrhythmic effect of caffeic acid phenethyl ester (CAPE), an active component of propolis, which exhibits antioxidant properties, in rats subjected to myocardial ischemia and ischemia-reperfusion (I/R) injury. DESIGN AND METHODS: Rats were subjected to 30 min coronary artery occlusion for evaluating the effect of CAPE on the myocardial ischemia injury. While in the myocardial I/R injury study, the coronary artery was ligated for a 5-min period of ischemia followed by a 30-min period of reperfusion. Animals were pretreated with or without CAPE before coronary artery ligation and the severity of myocardial ischemia- and I/R-induced arrhythmias and mortality were compared. RESULTS: Pretreatment of CAPE (0.1 and 1 microg/kg) not only reduced both the incidence and duration of ventricular tachycardia (VT) and ventricular fibrillation (VF) but also decreased the mortality during the myocardial ischemia and I/R injury period. CONCLUSIONS: Our results suggest that CAPE is a potent antiarrhythmic agent with cardioprotective effects in myocardial ischemia and I/R injury rats.     

Loss of anti-arrhythmic effect of vagal nerve stimulation on ischemia-induced ventricular tachyarrhythmia in aged rats

Reduced vagal activity is associated with increased risk for life-threatening arrhythmia during myocardial ischemia (MI); conversely, the increase in vagal tone may provide protective effect against ventricular arrhythmias. In fact, vagal nerve stimulation (VNS) exerted an anti-arrhythmic effect by preserving connexin 43 (Cx43), a gap junction protein in ventricles, in a rat model of MI. We investigated the effects of VNS on ventricular tachyarrhythmia during acute MI and the expression of Cx43 in aged rats. Both adult (3-4 months) and aged (≥ 24 months) male rats were subjected to ischemia of 30 min. VNS was applied before ischemia either alone or in combination with atropine (0.5 mg/kg) or carbenoxolone, a gap junction inhibitor (10 mg/kg). During the 30-min ischemia, the incidence of ventricular tachycardia (VT) or ventricular fibrillation (VF) was higher in aged rats compared with adult rats. VNS significantly suppressed VT and VF in adult rats and these effects were eliminated by atropine or carbenoxolone. In contrast, VNS did not suppress VT and VF in the aged rats. Moreover, ischemia did not change the expression levels of total Cx43 protein in adult and aged rat ventricles. However, the expression level of total Cx43 protein was two times lower in sham-operated aged rats than that in sham-operated adult rats. Thus, in aged rats, loss of anti-arrhythmic effect of VNS is associated with reduced expression of Cx43 protein. These findings suggest that Cx43 may be an important target for inhibiting ischemia-induced VT in adult patients but not in aged patients.     

Mechanical conversion of post-ischaemic ventricular fibrillation: effects on function and myocyte injury in isolated rat hearts

Ventricular fibrillation (VF) and ventricular tachycardia (VT) are common phenomena during reperfusion. In experimental research many hearts have to be excluded from haemodynamic evaluation because of severe arrhythmias. Theoretically, electroconversion or mechanical conversion (MC) might be used to convert VF or VT. MC induces a physical shock analogous to a chest thump. The aim of this study was to investigate the efficacy of MC in isolated, perfused rat hearts, and to see whether MC itself induced myocardial cell injury and functional impairment. Langendorff-perfused rat hearts (n = 89) from several experimental series subjected to 30 min of global ischaemia and 60 min of reperfusion were retrospectively analysed. Left ventricular systolic (LVSP), end-diastolic (LVEDP), and developed (LVDP) pressures, coronary flow (CF), and heart rate (HR) were measured. If VF or VT continued for 1 min during reperfusion, MC was attempted by a flick of the forefinger to the right ventricle. If VF or VT still occurred, MC was repeated. Hearts that did not have regular beating after 20 min of reperfusion were excluded. Release of cardiac troponin T (cTnT) was measured before ischaemia and after 20 min of reperfusion. Forty-four out of 89 hearts had VF or VT during reperfusion. Thirty-five hearts were converted, 18 of which were converted by one or two MCs only. The higher the total number of MCs employed, the more cTnT was released. After 20 min of reperfusion, LVEDP, LVDP and CF were better in hearts with a higher number of MCs and with increased release of cTnT. After 60 min of reperfusion, LVEDP was still improved in hearts with more cTnT release, whereas LVSP was lower, and LVDP and CF were independent of the number of MCs. There was no consistent correlation between release of cTnT and heart dysfunction. In conclusion, MC effectively converted VF or VT. MCs increased post-ischaemic myocardial cell damage, as judged from increased cTnT release. Post-ischaemic dysfunction was partly attenuated in hearts with multiple MCs, and did not correlate with release of cTnT. We feel that MCs should not be used in isolated, perfused hearts.     

Activation of an efferent cholinergic pathway produces strong protection against myocardial ischemia/reperfusion injury in rats

OBJECTIVE: A vagus nerve-mediated, brain cholinergic protective mechanism activated by melanocortin peptides is operative in conditions of circulatory shock; moreover, there is anatomical evidence of dual vagal-cardiac efferent pathways in rats, which could play different roles in controlling heart function. Therefore, we investigated the role and functional mechanism of such vagal efferent pathway(s) in an experimental model of ischemic heart disease. DESIGN: Randomized experimental study. SETTING: Research laboratory. SUBJECTS: Adult Wistar rats of either sex. INTERVENTIONS: After bilateral cervical vagotomy (with or without pretreatment with atropine), efferent vagal fibers were electrically stimulated in rats subjected to coronary artery occlusion (5 mins) followed by reperfusion (5 mins). Other rats (intact, vagotomized, or pretreated with atropine) were treated with nanomolar doses of melanocortin peptides. MEASUREMENTS AND MAIN RESULTS: Electrical stimulation of efferent vagal fibers (5 V, 2 m secs, 1-9 Hz, for the whole period of ischemia/reperfusion) strongly reduced the high incidence of severe arrhythmias and lethality, reduced the increase in free radical blood levels and left-ventricle histologic alterations, and augmented the extracellular signal-regulated kinase activation. Treatment with the melanocortin peptides adrenocorticotropin and gamma2-melanocyte-stimulating hormone (162 nmol/kg intravenously or 16.2 nmol/kg intracerebroventricularly, during coronary occlusion) produced the same protective effects of electrical stimulation and with the same muscarinic acetylcholine receptor-dependent mechanism, seemingly through brain activation (mediated by melanocortin MC3 receptors, as previously described) of such efferent vagal pathway. CONCLUSIONS: The present results give evidence for the identification of a protective, melanocortin-activated, efferent vagal cholinergic pathway, operative in conditions of myocardial ischemia/reperfusion. These data suggest that melanocortins and pertinent compounds able to activate such a pathway could provide the potential for development of a new class of drugs for a novel approach to management of ischemic heart disease.     

Ketanserin inhibits digoxin-induced arrhythmias in the anaesthetized guinea-pig

Digoxin inhibits the membrane-bound ATPase enzyme, resulting in a rise in intracellular sodium and activated outward potassium current, predisposing to arrhythmias. In this study, the effect of ketanserin, thought to block outward potassium currents, was investigated on digoxin-induced arrhythmias. Twenty-four guinea-pigs were studied in four groups (control, ketanserin 0.5 mg/kg, ketanserin 1 mg kg, ketanserin 2 mg/kg). Under pentobarbital anaesthesia (40 mg/kg), 15 min after injection of saline or ketanserin, digoxin (0.6 mg/kg) was administered through the jugular vein. Carotid artery blood pressure and electrocardiogram (ECG) were recorded. The time for the onset of the first arrhythmia and incidence of ventricular tachycardia (VT), ventricular fibrillation (VF), and premature ventricular contraction (PVC) were determined. Arrhythmias were scored according to the MacLeod scale. Ketanserin produced minor haemodynamic effects and lacked, by itself, arrhythmogenic effects at the doses studied. However, it increased the time for the onset of the first digoxin-induced arrhythmia and decreased the incidence of VT, VF and PVC. We conclude that ketanserin inhibits digoxin-induced arrhythmias in guinea-pigs.     

Effects of an Angiotensin II Antagonist on Reperfusion Arrhythmias in Dogs

Losartan, an angiotensin II receptor antagonist with no bradykinin potentiating property, provides the opportunity to study the consequences of blocking angiotensin II. The objective of this study was to evaluate the antiarrhythmic responses of reperfusion arrhythmia to hsartan in dogs. The effects of losartan on ventricular tachyarrhythmias induced during occlusion and reperfusion of the left anterior descending coronary artery were investigated in 30 dogs. The animals were randomized to receive either losartan (n = 15) or saline (n = 15). The VF inducing threshold was measured before occlusion and after reperfusion. Losartan (50 μg/kg per min) or saline was intravenously administered 5 minutes before occlusion and continued throughout the entire study period. The incidence of ventricular tachyarrhythmias during reperfusion was lower in the losartan group than in the control group (4/15 vs 6/15). There was no significant change in VF inducing threshold between the period before occlusion and during reperfusion in the losartan group (10.9 ± 5.7 vs 11.1 ± 5.7mA, P = NS), whereas there was a significant decrease in the control group (15.5 ± 4.4 vs 7.7 ± 3.9 mA, P < 0.01). Blockade of the angiotensin II receptor has beneficial effects on reperfusion arrhythmias.     

Thromboxane A2 antagonist and diltiazem-induced enhancement of contractile function: the effect of timing of treatment

We determined the ability of the thromboxane A2 antagonist SQ 29,548 or diltiazem to enhance postischemic myocardial function and if the effects of these compounds were occurring during occlusion or reperfusion periods. Anesthetized open-chest dogs were pretreated with i.v. saline, SQ 29,548 (0.20 mg/kg + 0.20 mg/kg/hr) or diltiazem (0.18 mg/kg) 15 min before the left anterior descending coronary artery was occluded. In another group, animals were given saline, SQ 29,548 or diltiazem 1 min before reperfusion. The occlusion was maintained for 15 min and reperfusion instituted for 5 hr. Subendocardial segmental shortening was monitored throughout the experiment using sonomicrometry. Left anterior descending coronary artery occlusion resulted in marked systolic bulging to similar levels in all groups. Upon reperfusion, function returned immediately but, after several min, hypokinesia existed in saline-treated animals. At 5-hr postreperfusion, percentage of shortening returned to only 20% of base-line values in saline-treated animals. Both diltiazem and SQ 29,548 pretreatment resulted in significant (P less than .05) improvements in function such that at 5-hr postreperfusion, shortening returned to 60% of base-line values. When given immediately before reperfusion, SQ 29,548 still resulted in significant protection of function, although this occurred much later compared to pretreated animals. Diltiazem did not improve function when given immediately before reperfusion. SQ 29,548 improves reperfusion function and, thus by inference, thromboxane A2 may play a role in postischemic hypokinesia and some of its protective effects may occur during reperfusion. Diltiazem seems to protect reperfusion function only when present during ischemia per se.     

Effects of chronic trimetazidine treatment on myocardial preconditioning in anesthetized rats

Trimetazidine is a widely used anti-ischemic agent, but effects of its chronic treatment on myocardial preconditioning in anesthetized animals have not been investigated. The aim of this study was to examine the effects of 15-day treatment of trimetazidine on ischemic preconditioning and carbachol-induced preconditioning in anesthetized rats. Ischemic preconditioning, induced by 5 min of coronary artery occlusion and 5 min of reperfusion, significantly decreased the total number of ventricular ectopic beats, the incidence of ventricular tachycardia and abolished the occurrence of ventricular fibrillation (VF) during 30 min of ischemia. Trimetazidine (10 mg/kg/day, i.p. for 15 days and 10 mg/kg, i.v.) itself attenuated these arrhythmia parameters with no marked effect on hemodynamic effects. In the presence of trimetazidine, anti-arrhythmic effects of ischemic preconditioning were present. Carbachol infusion induced preconditioning with a marked depression of mean arterial blood pressure, heart rate and the total number of ventricular ectopic beats. No VF was observed in carbachol-induced preconditioning. The marked reductions in arrhythmia parameters that induced carbachol-induced preconditioning were also preserved in the presence of trimetazidine. Arrhythmia scores and myocardial infarct size were reduced significantly with ischemic preconditioning or carbachol-induced preconditioning and were not modified by trimetazidine. Lactate and malondialdehyde levels were suppressed significantly with preconditioning or trimetazidine + preconditioning groups. These results show that chronic treatment of trimetazidine protects the heart against ischemia-induced arrhythmias, reduces myocardial infarct size, plasma lactate and malondialdehyde levels, and preserves the effects of ischemic and pharmacological preconditioning in anesthetized rats.     

Protective effect of fenofibrate against ischemia‐/reperfusion‐induced cardiac arrhythmias in isolated rat hearts

Fenofibrate is a peroxisome proliferator‐activated receptor (PPAR)‐α activator that lowers triglycerides and influences cytochrome P‐450 (CYP‐450) epoxygenase‐dependent arachidonic acid (AA) metabolism. CYP‐450 epoxygenase metabolizes AA to epoxyeicosatrienoic acids (EETs). EETs have coronary dilating and cardiac and renal protective properties. Fibrates possess similar properties due to their CYP‐450 epoxygenase‐inducing properties that lead to increase in endogenous EET production. In the current investigations, fenofibrate (100 mg/kg, orally) for 2 weeks decreased ischemia‐/reperfusion (I/R)‐induced premature ventricular contractions (PVCs), ventricular tachycardia (VT), and ventricular fibrillation (VF) in the isolated rat hearts. Fenofibrate caused marked inhibition of the reperfusion‐induced cardiac arrhythmias. The incidence of reperfusion‐induced VF decreased from 80% in the control vehicle‐treated animals to 33% in the fenofibrate‐treated animals (P < 0.001). PVCs were also significantly (P < 0.01) decreased from 223.2 ± 51 in control vehicle‐treated animals to 136.8 ± 22 in fenofibrate‐treated animals. Total duration of reperfusion‐induced VT decreased from 29.2 ± 6.3 s in control, vehicle‐treated animals to 4.8 ± 1.3 s in fenofibrate‐treated animals, P < 0.001. Heart rate and perfusion pressure were not significantly affected by fenofibrate pretreatment. Diltiazem, a clinically used anti‐arrhythmic agent, produced complete protection against I/R‐induced cardiac arrhythmias in this model reducing the incidence of VF from 80% in control, vehicle‐treated animals to 10% in diltiazem‐treated hearts. These findings indicate that fenofibrate suppresses arrhythmias in isolated rat hearts subjected to I/R‐induced injury.     

Delayed cardioprotective effects of exercise in dogs are aminoguanidine sensitive: possible involvement of nitric oxide

Dogs were subjected to exercise on a treadmill, using a protocol in which the speed and slope were increased every 3 min, and which elevated both heart rate (to a mean of 198+/-14 beats.min(-1)) and mean arterial blood pressure (to 150+/-4 mmHg). Then, 24 or 48 h later, the dogs were anaesthetized with a mixture of alpha-chloralose and urethane and subjected to a 25 min occlusion of the left anterior descending coronary artery. The control dogs (instrumented but not exercised) were subjected to the same procedure. In some dogs the nitric oxide synthase inhibitor aminoguanidine (50 mg.kg(-1); intravenous) was administered 30 min before occlusion. Baroreflex sensitivity (BRS) was determined by the rapid bolus injection of phenylephrine 60 min before, and again 3 min after, the onset of occlusion. Exercise markedly reduced the consequences of coronary artery occlusion 24 h (but not 48 h) later, without modifying myocardial tissue blood flow. In the exercised dogs there were reductions in arrhythmia severity [ventricular fibrillation (VF) during occlusion, 0%; survival from the combined ischaemia/reperfusion insult, 70%] compared with controls (VF during occlusion, 36%; survival, 9%). BRS was preserved during occlusion in the exercised dogs (before occlusion, 1.60+/-0.54 ms.mmHg(-1); 3 min after occlusion, 1.37+/-0.4 ms.mmHg(-1)), but not in controls (before occlusion, 1.28+/-0.29 ms.mmHg(-1); 3 min after occlusion, 0.45+/-0.12 ms.mmHg(-1); P<0.05), and other ischaemic changes (inhomogeneity of electrical activation and changes in the ST-segment, recorded over the ischaemic region) were also less marked in the exercised dogs. Exercise-induced cardioprotection was abolished by aminoguanidine (VF during occlusion, 25%; survival, 0%). The results show that even a single period of exercise affords delayed protection against ischaemia/reperfusion-induced VF and other ischaemic changes. Since this protection is abolished by aminoguanidine, and since (inducible) NO synthase activity was elevated 3-fold in left ventricular samples 24 h after exercise, we suggest that this protection is mediated by nitric oxide.     

Reduction in QT dispersion and ventricular arrhythmias by ischaemic preconditioning in anaesthetized, normotensive and spontaneously hypertensive rats

QT dispersion is a marker for dispersion of ventricular repolarization and electrical instability of the heart. However, QT dispersion remains undocumented in both normotensive rats (NTRs) and spontaneously hypertensive rats (SHRs), in particular in conditions of myocardial ischaemia/reperfusion (isch./rep.) and ischaemic preconditioning (IP). Therefore, we assessed the effects of IP on the dynamic change of QT and QTc dispersion during isch./rep., and on isch.- and rep.-induced ventricular arrhythmias in both NTRs and SHRs. Isch. and rep. were produced by occlusion and release of a snare around the left coronary artery in all rats. The effect of IP (three cycles of 3 min coronary artery occlusion and 5 min rep.) on myocardial repolarization and on development of isch.- and rep.-induced ventricular arrhythmias was studied in 12 NTRs and 12 SHRs. Another 12 NTRs or 12 SHRs were subjected to 10 min of isch. followed by 10 min rep. without IP. SHRs have significantly longer QT- and QTc-intervals as well as QT and QTc dispersion before isch. compared to NTRs. Myocardial isch. and early rep. largely increased QT and QTc dispersion in both NTRs and SHRs and resulted in a high incidence of isch.- and rep.-induced ventricular tachycardia (VT) and fibrillation (VF). IP significantly reduced QT and QTc dispersion in SHRs before isch., and remarkably reduced the elevation of QT and QTc dispersion during a prolonged period of isch. and rep. in all rats. This protective effect on electrophysiology of IP was associated with an antiarrhythmic effect against both isch.- and rep.-induced ventricular arrhythmias in NTRs and SHRs. Our data indicate that: 1) SHRs have a significantly higher baseline dispersion of ventricular repolarization than NTRs; 2) IP provides protection against ventricular arrhythmias in SHRs; 3) the increasing QT dispersion provoked by myocardial isch. and rep. is associated with a high incidence of isch.- and rep.-induced ventricular arrhythmias and; 4) the reduction of QT dispersion by IP may be involved in its protective effect against isch.- and rep.-induced arrhythmias in both NTRs and SHRs.     

Effect of two new PBN-derived phosphorylated nitrones against postischaemic ventricular dysrhythmias

Spin traps might exert antioxidant cardioprotective effects during myocardial ischaemia-reperfusion where free radicals are thought to be responsible for the occurrence of reperfusion injury. The aim of our study was to investigate the effects of two new alpha-phenyl N-tert-butylnitrone (PBN)-derived beta-phosphorylated nitrones: 2-N-oxy-N-[benzylidène amino] diéthyl propyl-2-phosphate (PPN) and 1-diethoxyphosphoryl-1-methyl-N-[(1-oxido-pyridin-1-ium-4-yl) methylidene] ethylamine N-oxide (4-PyOPN) compared with PBN on (1) the evolution of cardiovascular parameters and (2) the postischaemic recovery. Anaesthetized rats were injected with 120 micro mol/kg of the nitrones or 14 micro mol/kg of amiodarone, used as a reference antidysrhythmic drug. Ischaemia was induced in vivo through ligation of the left anterior descending coronary artery for 5 min followed by 15 min of reperfusion after release. Cardiovascular parameters and occurrence of ventricular premature beats (VPB), ventricular tachycardia (VT) and fibrillation (VF) were recorded throughout the experiment. Under nonischaemic conditions, none of the three spin traps was shown to modify cardiovascular parameters during the 25-min measurement period. Solvent-treated (NaCl 0.9%) animals challenged with ischaemia-reperfusion exhibited 39 +/- 10 VPB, 156 +/- 39 s of VT and 60% mortality caused by sustained VF. Nitrones improved slightly postischaemic recovery, reducing the occurrence of VF and mortality to 33% whereas amiodarone injection totally suppressed rhythm disturbances and mortality. Our study has shown only limited antidysrhythmic cardioprotective effects of PBN-derived beta-phosphorylated nitrones during reperfusion after a regional myocardial ischaemia but also minor antioxidant properties of these spin trapping agents.     

Effect of the thromboxane A2 receptor antagonist SQ 30,741 on ultimate myocardial infarct size, reperfusion injury and coronary flow reserve

We determined if the thromboxane A2 antagonist SQ 30,741 can reduce ultimate myocardial infarct size and reperfusion injury. Anesthetized dogs were subjected to left circumflex coronary artery occlusion for 90 min at which time reperfusion was instituted. In one study, SQ 30,741 (1 mg/kg + 1 mg/kg/hr) was given either 10 min postocclusion (n = 7) or 2 min (n = 9) before reperfusion along with their appropriate vehicle controls in a model of 90 min of occlusion and 5 hr of reperfusion. Infarct size was reduced 50% (P less than .05) when SQ 30,741 was given 10 min postocclusion and 30% (P less than .05) when given only during reperfusion. Flow reserve using maximally dilating doses of adenosine was determined 3 hr postreperfusion in vehicle (10 min postocclusion, n = 10), SQ 30,741 (10 min postocclusion, n = 6) and nonischemic (n = 5) animals. Maximal subendocardial flow was reduced during reperfusion in ischemic animals, but SQ 30,741 improved this compared to vehicle animals (400 +/- 95, 88 +/- 25 and 208 +/- 48 ml/min/100 g; nonischemic, vehicle, SQ 30,741 groups, respectively). To determine if myocardial salvage can be observed 24 hr postocclusion with SQ 30,741 or the cyclooxygenase inhibitor aspirin, dogs were given vehicle (n = 9), SQ 30,741 (10 min postocclusion up to 4 hr postreperfusion) or aspirin (n = 9, 40 mg/kg 30 min preocclusion) and infarct size was determined 24 hr postocclusion (90 min left circumflex coronary artery occlusion + reperfusion).(ABSTRACT TRUNCATED AT 250 WORDS)     

Nicotinic acid, free fatty acids and myocardial function during coronary occlusion and reperfusion in the dog

The effect of nicotinic acid on regional myocardial blood flow, percentage of segment shortening and myocardial uptake of free-fatty acids during a 15-min occlusion of the left anterior descending coronary artery and 3-hr reperfusion period was compared to a saline-treated control group. Nicotinic acid (2.4 mumol/kg/min i.v.) was infused 30 min before and throughout the occlusion period. Heart rate, arterial blood pressure and left ventricular systolic and end diastolic pressures were not different during occlusion and reperfusion in the nicotinic acid or saline-treated groups. However, left ventricular dP/dt, an index of global myocardial function and percentage of segment shortening in the ischemic region were greater during occlusion and reperfusion after nicotinic acid. Even though myocardial blood flow was unaltered in the normal or ischemic region during nicotinic acid infusion, subendocardial blood flow during reperfusion was enhanced significantly when compared to the control group. Nicotinic acid also decreased free-fatty acid uptake by the heart during occlusion which returned gradually to the pretreatment control during 3 hr of reperfusion. Thus, the improvement in percentage of segment shortening, dP/dt and subendocardial blood flow during reperfusion may be related to the ability of nicotinic acid to reduce free-fatty acid uptake by the heart during coronary occlusion.     

Effect of diltiazem on infarct size, reperfusion flow and flow reserve: the effect of timing of treatment

This study was performed to determine if improved subendocardial reflow seen with diltiazem pretreatment after left circumflex coronary (LCX) occlusion is due to a direct vasodilatory effect of diltiazem on the reperfused bed or due to an increased flow maximum or reserve. In the first part of this study anesthetized dogs were subjected to saline or diltiazem (infused starting before, 10 min after LCX occlusion or 2 min before reperfusion; 0.18 mg/kg + 0.45 mg/kg/hr i.v. for all groups) treatment with a 90-min LCX occlusion and 5-hr reperfusion and myocardial blood flow and infarct size were determined at the end of the experiment. In the second part, maximal flow using intracoronary adenosine was determined at 1 and 3 hr postreperfusion in the ischemic bed when pretreated with saline or diltiazem. Myocardial infarct size was reduced significantly only in animals pretreated with diltiazem compared to saline-treated animals. At 1-hr postreperfusion, subendocardial flow (microspheres) was significantly higher only with diltiazem pretreatment compared to the saline group (100 +/- 17 vs. 54 +/- 8 ml/min/100 g, respectively) and subendocardial reperfusion flows were negatively correlated to infarct size (r = 0.97, P less than .05). Thus, diltiazem only improves reflow and infarct size when infused before occlusion and this improved reflow does not occur via a direct vasodilator action of diltiazem. When maximal vasodilating doses of adenosine were given, flow in the ischemic region was nearly identical for saline and diltiazem pretreated groups despite higher preadenosine flows in the diltiazem group (higher resting flow occurred at the expense of the existing flow reserve).(ABSTRACT TRUNCATED AT 250 WORDS)     

Coronary vascular injury due to ischemia-reperfusion is reduced by pentoxifylline.

Myocardial ischemia and reperfusion cause coronary vascular injury involving both the large epicardial arteries and the microcirculation. Although the mechanisms are unclear, leukocytes appear to play an important role. Since the methylxanthine derivative pentoxifylline (PTX) decreases neutrophil activity in vitro, we hypothesized that it might diminish coronary vascular injury due to ischemia and reperfusion. We investigated the effects of PTX on coronary microvascular and epicardial artery injury in open chest, anesthetized dogs undergoing moderate (60 min) or more prolonged (90 min) ischemia due to left anterior descending coronary artery occlusion followed by 60 min of reperfusion. As an index of microvascular injury, we assessed regional permeability with a dual radioisotope protein leak index (PLI) method. Both ischemic periods with reperfusion increased the PLI of severely ischemic (flow less than or equal to 20/ml/min/100 g) myocardium by 2.5- and 3-fold, respectively, compared to nonischemic (flow greater than or equal to 100 ml/min/100 g) myocardium. Treated dogs received PTX (20 mg/kg bolus plus 0.1 mg/kg/min infusion) before ischemia. PTX reduced the increase in the PLI by 40% after 60 min of ischemia (PLI = 5.87 +/- 0.48 vs. 4.10 +/- 0.52 untreated vs. PTX-treated; P less than .05), and by 25% after 90 min of ischemia (6.84 +/- 0.49 vs. 4.84 +/- 0.42; P less than .05). The amount of protein leak was inversely related to ischemic blood flow, and the magnitude of this relationship was significantly reduced in PTX-treated animals. In arterial rings from untreated dogs exposed to 90 min of ischemia followed by reperfusion, there was impaired relaxation to ADP and acetylcholine, but not to sodium nitroprusside.(ABSTRACT TRUNCATED AT 250 WORDS)     

Close association between the reduction in myocardial energy metabolism and infarct size: dose-response assessment of cyclosporine

Cyclosporine protects the heart against ischemia/reperfusion injury, but its effect on cardiac metabolism is largely unknown. We assessed cyclosporine-induced metabolic changes in the rat heart prior to occlusion using magnetic resonance spectroscopy (MRS) and correlated effects with infarct size in a coronary occlusion/reperfusion model. The two study groups were cyclosporine and cyclosporine + coronary occlusion (n = 20/group). Rats were pretreated with cyclosporine (5, 10, 15, and 25 mg/kg/day) or the vehicle by oral gavage for 3 days (n = 4/dose). On day 4, hearts of rats in the cyclosporine group were excised, and extracted cell metabolites were measured using (1)H and (31)P MRS. The second group was subjected to 30 min of coronary artery occlusion followed by 24 h of reperfusion. Infarct size and area at risk were measured using a double staining method. In the cyclosporine group, cyclosporine reduced cardiac energy metabolism (ATP: r = -0.89, P < 0.001) via depression of oxidative phosphorylation and the Krebs' cycle in a dose-dependent manner. The decrease of ATP levels was positively correlated with changes of NAD(+) (r = 0.89), glutamate (r = 0.95), glutamine (r = 0.84), and glucose concentrations (r = 0.92, all P < 0.002). It was inversely correlated with lactate (r = -0.93, P < 0.001). In the coronary occlusion group, cyclosporine dose dependently reduced the ratio [area of infarct/area of the left ventricle] (r = -0.86, P < 0.01), with 15 mg/kg/day being the most effective cyclosporine dose. The reduction in infarct size correlated with the reduction in oxidative phosphorylation (ATP: r = 0.97; NAD(+): r = 0.82, P < 0.01). The reduction in cardiac energy metabolism before occlusion may be the cause of myocardial preservation during ischemia/reperfusion.     

Evaluation of LY203647 on cardiovascular leukotriene D4 receptors and myocardial reperfusion injury.

In vitro studies have shown LY203647 to be a selective antagonist of contractile responses to leukotriene (LT) D4 and LTE4 in guinea pig ileum, trachea and lung parenchyma. In pithed rat, i.v. injection of LTD4 produced pressor responses that were selectively antagonized by LY203647 in a dose-dependent manner [ED50 7.5 (6.0-9.5) mg/kg, i.v.]. In normal anesthetized rats and dogs, LTD4 reduced aortic blood flow and stroke volume in association with systemic vasoconstriction, variable blood pressure responses and no change in cardiac rate. LTD4 did not alter myocardial contractility corrected for alterations in afterload. Pretreatment of rats and dogs with LY203647 (1-10 mg/kg, i.v.) produced dose-related inhibition of the myocardial and systemic hemodynamic effects of LTD4, whereas coadministration of LY203647 reversed established myocardial depression and systemic and pulmonary vasoconstriction during continuous infusion of LTD4 in dogs. LY203647 (10 mg/kg over 90 min, i.v.) infusion in normal dogs abolished or greatly antagonized hemodynamic responses to LTD4 for 6 hr. In subsequent experiments, myocardial infarct size was measured after 1 hr of occlusion of the circumflex coronary artery and 5 hr of reperfusion. LY203647 (10 mg/kg over 90 min, i.v.) treatment did not alter cardiovascular parameters when compared to time-related alterations observed in control dogs. ST segment deviation and the intensity and duration of cardiac arrhythmias associated with coronary artery occlusion and reperfusion also were similar between groups. Resultant infarct sizes were 46 +/- 1 and 45 +/- 1% of the left ventricular mass placed at risk in control and LY203647-treated dogs, respectively. Present data illustrate the prominent cardiac and systemic hemodynamic effects of LTD4 and indicate that LY203647 produces selective and sustained antagonism of cardiovascular LTD4 receptors. Lack of containment of infarction by LY203647 suggests that endogenous cysteinyl-LT do not contribute to reperfusion injury of ischemic myocardium.