INFLUENCE OF OXYGEN FREE-RADICALS ON HEART SARCOLEMMAL NA+-CA-2+ EXCHANGE

Because oxygen free radicals are considered to play an important role in myocardial injury due to ischemia-reperfusion and because sarcolemmal Na+-Ca2+ exchange is markedly altered in ischemic or hypoxic-reperfused hearts, this study was undertaken to investigate the effects of oxygen free radicals on rat heart sarcolemmal Na+-Ca2+ exchange activity. Incubation of sarcolemma for 10 minutes with 2 mM xanthine plus 0.03 U/mL xanthine oxidase inhibited the Na+-dependent Ca2+ uptake activity. This change was associated with significant formation of malondialdehyde. These alterations were significantly prevented by 80-mu-g/mL superoxide dismutase, a scavenger of superoxide anions, but were completely prevented by a combination of superoxide dismutase (80-mu-g/mL) and catalase (10-mu-g/mL). Inhibitory effects on Na+-Ca2+ exchange and increased formation of malondialdehyde were observed with 0.5 mM H2O2 or 0.01 mM H2O2 plus 0.01 mM Fe2+ and were prevented by 10-mu-g/mL catalase, a scavenger of H2O2, and 2 mM mannitol, a scavenger of hydroxyl radicals, respectively. The depressant effects of all these activated types of oxygen were not associated with any changes in nonspecific Ca2+ uptake, passive Ca2+ permeability or passive Na+ permeability of the sarcolemmal vesicles. The decrease of Na+-dependent Ca2+ uptake by these free radical systems was also evident when the sarcolemmal vesicles (prepared by different methods of isolation from hearts of different animal species) were employed. These results indicate that the sarcolemmal membrane is altered on exposure to oxygen free radicals and this may depress the Na+-Ca2+ exchange system in the myocardial cell under different pathophysiologic conditions.