The removal of Ca2+ from the cardioplegic solutions could cause the danger of inducing a "calcium paradox" during reperfusion. Since intracellular Ca2+ activities are coupled to Na+ activities via Na(+)-Ca2+ exchange, an increase in intracellular Na+ activities during the cardioplegia could cause an abrupt Ca2+ influx when reperfused. To study the effects of Na+ and Ca2+ concentrations in cardioplegic solutions on intracellular Ca2+ activities during the cardioplegia and subsequent recovery period, the membrane potential and intracellular Na+ and Ca2+ activities of guinea pig ventricular papillary were measured. 1) A cardioplegia with low Ca2+ cardioplegic solution significantly decreased the overshoot and duration of the first action potential after cardioplegia, but the changes in action potential configuration were minimized after a cardioplegia with Ca2+ concentration adjusted according to the Na(+)-Ca2+ exchange mechanism. 2) Intracellular Na+ activity was continuously decreased during the cardioplegia, and the intracellular Na+ activity 20 minutes after cardioplegia was the highest with low Ca2+ cardioplegic solution. 3) Intracellular Na+ and Ca2+ activities were continuously decreased during the cardioplegia with Ca2+ concentration adjusted according to the Na(+)-Ca2+ exchange mechanism. 4) During a reperfusion of Tyrode solution after cardioplegia intracellular Na+ and Ca2+ activities were increased. Intracellular Ca2+ activity was increased more rapidly than intracellular Na+ activity. 5) The rate of increase in intracellular Ca2+ activity with reperfusion of Tyrode solution was dependent upon intracellular Na+ activity during cardioplegia, in such a way that the higher the intracellular Na+ activity was, the faster the intracellular Ca2+ activity increased. These data suggest that Na(+)-Ca2+ exchange mechanism may play an important role in the regulation of intracellular Ca2+ activity during recovery after cardioplegia.
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