Alterations in heart sarcolemmal Ca2+-ATPase and Ca2+-binding activities due to oxygen free radicals

Summary Effects of oxygen free radicals on Ca²⁺/Mg²⁺ ATPase and ATP-independent Ca²⁺-binding activities were examined in rat heart sarcolemma. Membranes were incubated with different oxygen radical generating media such as xanthine + xanthine oxidase, hydrogen peroxide, and hydrogen peroxide + Fe²⁺. In the presence of xanthine + xanthine oxidase, Ca²⁺ ATPase activity was stimulated and this effect was prevented by the addition of superoxide dismutase. Hydrogen peroxide also showed a significant increase in Ca²⁺-ATPase activity in a dose-dependent manner and this effect was blocked by catalase. On the other hand, a combination of hydrogen peroxide + Fe²⁺ decreased Ca²⁺-ATPase activity; this depression was prevented by the addition of D-mannitol. The observed change in Ca²⁺-ATPase activity due to oxygen free radicals was associated with changes in V_max, whereas Ka remained unaffected. Both xanthine + xanthine oxidase and hydrogen peroxide increased whereas, hydrogen peroxide + Fe²⁺ inhibited the ATP-independent Ca²⁺-binding activities. It is suggested that oxygen free radicals may influence Ca²⁺ movements in the cell by altering the Ca²⁺/Mg²⁺ ATPase and Ca²⁺-binding activities of the membrane and these effects may be oxygen-radical species specific.