Depression of heart sarcolemmal Ca2+-pump activity by oxygen free radicals.

Although oxygen free radicals have been implicated as mediators of cellular injury in myocardial ischemia-reperfusion, the exact nature of defects produced by these radicals is not clear. Because sarcolemmal Ca2+-pump is involved in the efflux of Ca2+ from the cell, this study was undertaken to examine the effects of oxygen free radicals on sarcolemmal ATP-dependent Ca2+ accumulation and Ca2+-stimulated Mg2+-dependent adenosinetriphosphatase (ATPase) activities as well as lipid peroxidation of membrane phospholipids. Isolated rat heart sarcolemmal membranes were incubated with xanthine + xanthine oxidase [a superoxide anion radical (O2-)-generating system], H2O2, or H2O2 + Fe2+ [a hydroxyl radical (HO.)-generating system] and assayed for Ca2+-pump activities. O2- inhibited the Ca2+-pump activities in a time-dependent manner; a significant inhibition of Ca2+-stimulated ATPase activity was seen after 1 min of incubation. Superoxide dismutase showed a protective effect on depression in Ca2+-pump activities caused by O2-.H2O2 inhibited Ca2+-pump activities in a dose-dependent manner; this inhibition was protected by the addition of catalase. HO. depressed the Ca2+-pump activities to a greater extent in comparison with H2O2. Mannitol showed a protective effect on HO.-induced inhibition of Ca2+-pump activities. The promotion of lipid peroxidation by free radicals was evident from increased formation of malondialdehyde. These results indicate that the sarcolemmal membrane is altered on exposure to oxygen free radicals, and this may result in depressing the Ca2+-pump mechanism for Ca2+ efflux from the myocardial cell.