The role of oxygen free radicals and extracellular calcium ions in reperfusion-induced arrhythmias

Effects of a Novel Cyclohexane Dicarboximide Derivative, ST-6, on Reperfusion-Induced Arrhythmia in Rats

The present study was designed to determine whether a novel cyclohexane dicarboximide derivative, ST-6, 2-[4-[4-(chlorophenyl)-4-hydroxy-1-piperidinyl]butyl]hexahydro-1H-isoindol-1,3(2H)-dione, prevents reperfusion-induced ventricular arrhythmias. Pentobarbital-anesthetized rats were subjected to left coronary artery occlusion for 4 min followed by 4-min reperfusion, and the incidence of their ventricular arrhythmias was examined. The coronary occlusion of control rats induced ventricular tachycardia and fibrillation, eventually leading to sudden death. The intravenous injection of 0.1 to 2 mg/kg ST-6 prior to the occlusion resulted in a dose-dependent suppression of the ventricular arrhythmias. The suppression of ventricular fibrillation was also observed on the intraperitoneal and intradoudenal administration of 2 to 10 mg/kg ST-6 15 min prior to coronary occlusion. Antiarrhythmic effects of this agent (0.5 mg/kg per min) were compared with those of other antiarrhythmic agents including lidocaine (0.1 mg/kg per min), sematilide (0.3 mg/kg per min), and diltiazem (0.5 mg/kg per min) by administrating the agents from 1 min after the coronary occlusion to the end of 4-min reperfusion. Antiarrhythmic effects of ST-6 were similar in degree to those of lidocaine and diltiazem, whereas no significant prevention by sematilide was seen. The results suggest that ST-6 may be capable of suppressing reperfusion-induced arrhythmias following oral or intravenous administration.

Hypothyroidism provides resistance to reperfusion injury following myocardium ischemia

A growing body of evidence has demonstrated that reperfusion injury may be mediated, in part, by mitochondrial Ca2+ overload that promotes non-selective permeability of the inner membrane. In this regard it is known that mitochondria from hypothyroid rats are resistant to membrane damage as induced by Ca2+. The purpose of this study was to evaluate the sensitivity of hearts from hypothyroid rats, to the damage by reperfusion, after an ischemic period of 5 min. The results were compared with those from control and hyperthyroid rats. Hypothyroidism was established by surgical removal of the thyroid gland; in turn hyperthyroidism was induced after a daily injection of 2 mg/kg of 3,5,3'-triiodothyronine for 4 days. ECG tracings from hypothyroid rats showed a total absence of post-reperfusion arrhythmias conversely to what was observed in control and hyperthyroid rats. The release of creatine kinase and aspartate amino transferase to the plasma in hypothyroid rats was found to be lower than that found in hyperthyroid and euthyroid rats. The histological studies showed that myocardial fibers from hypothyroid rats were in good condition and retained their striae and a remarkable near absence of edema was clearly observed.

Oxidative injury of isolated cardiomyocytes: dependence on free radical species

The contribution of lipid peroxidation to myocardial injury by free radicals (FR) is still unclear. Consequently, we examined the functional damages inflicted on cultured rat cardiomyocytes (CM) during FR stress provoked by the xanthine/xanthine oxidase system (X/XO) or by a hydroperoxidized fatty acid ((9 Z, 11 E, 13 (S), 15 Z)-13-hydroperoxyocta-decatrienoic acid; 13-HpOTrE), in order to simulate in vitro the initial phase and the propagation phase of the FR attack, respectively. Transmembrane potentials were recorded with glass microelectrodes and contractions were monitored photometrically. The EPR spectroscopy showed that X/XO produced superoxide and hydroxyl radicals during 10 min. The X/XO system altered sharply and irreversibly the spontaneous electrical and mechanical activities of the CM. However, the gas chromatographic analysis showed that these drastic functional damages were associated with comparatively moderate membrane PUFA degradation. Moreover, the EPR analysis did not reveal the production of lipid-derived FR. 13-HpOTrE induced a moderate and reversible decrease in electrical parameters, with no change in CM contractions. These results indicate that the functional consequences of FR attack are dependent on the radical species present and do not support the idea that the membrane lipid breakdown is a major factor of myocardial oxidant dysfunction.

On the protection by ketorolac of reperfusion-induced heart damage

This study shows that the nonsteroidal antiinflammatory drug, ketorolac, protects against myocardial damage induced by reperfusion. This effect was analyzed after 5 min of coronary occlusion in rat hearts. The results indicate that ketorolac, at a dose of 1 mg/kg, effectively protects the heart against reperfusion arrhythmias. Furthermore, it protects from the release of lactate dehydrogenase and creatine kinase to the plasma. We propose that the protective effect of the drug might be due to its chelating action on calcium ions, thus preventing the overload of such cation in myocardial cells.

Ischemic heart disease and antioxidants: mechanistic aspects of oxidative injury and its prevention

The disease state of myocardial ischemia results from a hypoperfusion-induced insufficiency of heart-muscle oxidative metabolism due to inadequate coronary circulation. Myocardial ischemia is an important, lifespan-limiting medical problem and a major economic health-care concern. Reperfusion, although avidly pursued in the clinic as essential to the ultimate survival of acutely ischemic heart muscle, may itself carry an injury component. Cardiac reperfusion injury appears to reflect, at least in part, an oxidant burden established upon reoxygenation of ischemic myocardium. Laboratory evidence demonstrates that oxidative stress to the heart-muscle cell (cardiomyocyte) can elicit the three known types of ischemia-reperfusion injury that directly affect the myocardium: arrhythmia, stunning, and infarction. The limited clinical occurrence of serious reperfusion arrhythmias has restricted the importance of antioxidants as antiarrhythmic agents against this form of myocardial ischemia-reperfusion damage. Despite the utmost clinical significance of lethal cardiomyocyte injury as a negative prognostic indicator for the ischemic heart-disease patient, inconsistent results of antioxidant interventions in reducing infarct size have somewhat tempered interest in antioxidant infarct trials. By contrast, the negative clinical consequences of stunning may indeed be preventable by utilizing antioxidants to help restore postischemic cardiac pump function. Several as yet unanswered questions remain regarding oxidative stress in the reperfused heart, its significance to cardiomyocyte damage, and its ability to elicit specific postischemic myocardial derangements. Targeted mechanistic studies are required to address these questions and to define the pathogenic role of oxidative stress (and, hence, the therapeutic potential of antioxidant intervention) in myocardial ischemia-reperfusion injury. The overall aim of current research in this area is to enable the cardiac surgeon/cardiologist to advance beyond the largely palliative drugs now available for management of the coronary heart-disease patient and attack directly the pathogenic determinants of heart-muscle ischemia-reperfusion injury. Optimal use of antioxidants may help address this important medical need.