Quantitative analysis of the electrogenic ion exchange current using the cell dialysis method in the isolated cardiac myocytes.

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The intracellular Ca^<2+> concentration is lower by 10^4 than the extracellular Ca^<2+> concentration. This concentration gradient is essential to achieve various kinds of cell functions, such as the muscle contraction, and electrical activity of the cell. The Na/Ca exchange mechanism is mainly responsible for pumping out Ca^<2+> which enters the cardiac muscle during the systolic period. The aim of our project is to determine the stoichiometry of the Na/Ca exchange by recording the Na/Ca exchange current. We clarified that the intracellular ion concentrations may change, if we stimulate the Na/Ca exchange continuously. Therefore, we used a holding potential which is equal to the theoretical equilibrium potential for the 3Na:1Ca exchange. The reversal potential of the Na/Ca exchange current were successfully measured at various external Na^+ and Ca^<2+> concentrations. The reversal potential was in agreement with the theoretical values of the 3Na:1Ca exchange.