Whole-body hyperthermia provides biphasic cardioprotection against ischemia/reperfusion injury in the rat

Background - Hyperthermia increases cardiac tolerance to ischemia/reperfusion injury 24 hours after the heat stress. Free radicals and redox mechanisms have been implicated in such tolerance. However, the time course and its relation to the induction of antioxidative enzymes in the protection induced by whole-body hyperthermia against ischemia/reperfusion injury are unknown. Methods and Results-Hyperthermia was induced in anesthetized rats by placement in a temperature-controlled water bath. After the defined recovery interval(s) at room temperature, ischemia was induced by occlusion of the left coronary artery for 20 minutes, followed by reperfusion for 48 hours. The exposure to hyperthermia led to a recovery interval-dependent, biphasic reduction in the incidence of ventricular fibrillation during ischemia and in the size of the myocardial infarct as determined after 48 hours of reperfusion. The time course of the late-phase (24- to 96-hour recovery interval) but not the early-phase (0.5 hour) cardioprotection depended on the degree of hyperthermia. The time course of the increase in myocardial manganese superoxide dismutase (Mn-SOD) activity corresponded to that of the cardioprotective effects, although an increase in the content of Mn-SOD and of heat shock protein 72 corresponded only to the late-phase effects. Administration of an antioxidant before hyperthermia abolished the early- and late-phase cardioprotection and the increase in Mn-SOD activity. Conclusions - The activation of Mn-SOD mediated by free radical production during hyperthermia is important in the acquisition of early-phase and late-phase cardioprotection against ischemia/reperfusion injury in rats.