Abstract
Background
Although oxygen free radicals have been shown to induce myocardial cell damage and cardiac dysfunction, the exact mechanism by which these radicals affect the heart function is not clear. Since the occurrence of intracellular Ca2+ overload is critical in the genesis of cellular damage and cardiac dysfunction, and since the sarcolemmal Na+–Ca2+ exchange is intimately involved in Ca2+ movements in myocardium, this study was undertaken to examine the effects of oxygen free radicals on the relationship between changes in cardiac contractile force development and sarcolemmal Na+–Ca2+ exchange activity.
Methods and Results
Isolated rat hearts were perfused with a medium containing xanthine plus xanthine oxidase for different times, and changes in contractile force as well as sarcolemmal Na+–Ca2+ exchange activity were monitored. Perfusion of the heart with xanthine plus xanthine oxidase resulted in a transient increase followed by a marked decrease in contractile activity; the resting tension was markedly increased. The xanthine plus xanthine oxidase-induced depression in developed tension, rate of contraction, and rate of relaxation, except the transient increase in contractile activity, was prevented by the addition of catalase, but not by superoxide dismutase, in the perfusion medium. A time-dependent depression in sarcolemmal Na+–Ca2+ was also evident upon perfusing the heart with xanthine plus xanthine oxidase. This depression in Na+-dependent Ca2+ uptake was associated with a decrease in the maximal velocity of reaction without any changes in the affinity of Na+–Ca2+ exchanger for Ca2+. The presence of catalase, unlike superoxide dismutase, prevented the decrease in sarcolemmal Na+–Ca2+ exchange activity in hearts perfused with xanthine plus xanthine oxidase.
Conclusion
The results support the view that a depression in the sarcolemmal Na+–Ca2+ exchange activity may contribute to the occurrence of intracellular Ca2+ overload and subsequent decrease in contractile activity. Furthermore, these actions of xanthine plus xanthine oxidase in the whole heart appear to be a consequence of H2O2 production rather than the ‘ generation of superoxide radicals.
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