Studies on the specificity of the effects of oxygen metabolites on cardiac sodium pump.

Isolated myocytes of rat heart, and sealed sarcolemmal vesicles of bovine heart, were used to examine the selectivity of the effects of partially reduced oxygen species (generated by a mixture of xanthine and xanthine oxidase) on cardiac sodium pump and several other ion transporters of the plasma membrane. When myocytes were exposed to xanthine plus xanthine oxidase, there were time-dependent inhibitions of ouabain-sensitive 86Rb+ uptake and (Na+ + K+)-ATPase activity that could be prevented by allopurinol, or by catalase and superoxide dismutase; suggesting the involvements of H2O2 or oxygen free radicals in the inhibition of the pump. This inhibition preceded any significant decrease in cellular ATP or in the number of viable cells. While ouabain increased 45Ca2+ uptake by myocytes as expected, exposure to xanthine plus xanthine oxidase decreased 45Ca2+ uptake; suggesting that the Na+, Ca2(+)-exchanger of the intact myocytes is also inhibited by oxygen metabolites. Simultaneous inhibitions of the pump, the Na+, Ca2(+)-exchange, the Na+, H(+)-exchange, and the Na+, Pi-cotransport activities also occurred in sarcolemmal vesicles that were treated with xanthine plus xanthine oxidase. These findings indicate that inactivations of the sodium pump and other sarcolemmal ion carriers are early events in the oxidant-induced damage to the cardiomyocyte. In the rat heart myocytes, a fraction of (Na+ + K+)-ATPase that seems to be more sensitive to ouabain, was inactivated more rapidly upon exposure of myocytes to xanthine plus xanthine oxidase; raising the possibility of the existence of different pump populations with different sensitivities to extracellularly generated oxygen metabolites.