DIRECT EVIDENCE THAT INITIAL OXIDATIVE STRESS TRIGGERED BY PRECONDITIONING CONTRIBUTES TO 2ND WINDOW OF PROTECTION BY ENDOGENOUS ANTIOXIDANT ENZYME IN MYOCYTES

Background We tested the hypothesis that late preconditioning is assoc iated with increased antioxidant enzyme activity induced by initial ox idative stress. Methods and Results Isolated rat myocytes were precond itioned either with two cycles of 5 minutes of anoxia and 5 minutes of reoxygenation or with exogenous superoxide anion (O-2(-)) generated b y reaction of xanthine oxidase with xanthine. Myocytes were allowed to recover for 60 minutes or 24 hours, after which they were subjected t o 60 minutes of anoxia and 60 minutes of reoxygenation. After 60 minut es or 24 hours, the protection was evidenced by decreased O-2(-) produ ction, increased Mn superoxide dismutase (Mn-SOD) activity, increased cell viability, decreased LDH release, reduced malondialdehyde formati on, high-energy phosphate preservation, and improved cell morphology i n preconditioned and O-2(-)-treated myocytes. Immediately after treatm ent with O-2(-) or repetitive, brief anoxia, O-2(-) production was inc reased in myocytes. Longer anoxia resulted in loss of Mn-SOD activity in anoxic controls 24 hours later, whereas it was significantly increa sed in preconditioned and O-2(-)-treated myocytes. O-2(-) production w as inhibited in preconditioned and O-2(-)-treated myocytes. Myocytes t reated with Mn-SOD during short, intermittent anoxia exhibited decreas ed activity of Mn-SOD and increased O-2(-) production 24 hours later. Mn-SOD activity in late preconditioning was considerably higher than t hat in classic preconditioning. Conclusions These results suggest that a burst of oxygen free radicals generated during the initial periods of brief, repetitive anoxia increases myocardial antioxidant activity 24 hours later and that it contributes to the late cardioprotective ef fect of preconditioning.