Reduction of calcium channel antagonist binding sites by oxygen free radicals in rat heart

Abstract In view of the importance of Ca 2+ -channels in controlling the entry of Ca 2+ into the myocardium, this study was undertaken to examine the effects of oxygen free radicals on the binding of Ca 2+ -channel antagonists in rat heart by employing [ 3 H]-nitrendipine as a ligand. Isolated heart membranes were incubated with xanthine + xanthine oxidase (a superoxide anion radicals generating system), hydrogen peroxide (an activated species of oxygen), or hydrogen peroxide + Fe 2+ (a hydroxyl radicals generating system). The assay of the [ 3 H]-nitrendipine binding activity revealed that the maximal number of binding sites ( B max ) were reduced in a time-dependent manner by superoxide radicals without any changes in the binding constant ( K d ); a significant reduction of B max was seen after incubating membranes with xanthine + xanthine oxidase for a 10-min-period. Superoxide dismutase showed a protective effect on the superoxide radicals induced reduction in B max . Both hydrogen peroxide and hydroxyl radicals also depressed the B max for [ 3 H]-nitrendipine binding without any significant change in K d ; catalase and mannitol showed protective effects on hydrogen peroxide or hydroxyl radicals induced depression in B max , respectively. These results indicate that oxygen free radicals may reduce the number of Ca 2+ -channels in the cell membrane and this change may contribute towards decreasing the voltage-dependent Ca 2+ influx in the cardiac cell.