Intracellular calcium and high-energy phosphates in isolated cardiac myocytes

The relationship between intracellular calcium (Cai) and high-energy phosphates was studied in adult cardiac myocytes. Cai and high-energy phosphates were measured in the same population of cells. Cai, reported by the fluorescence of fura-2, was maintained at normal levels in the presence of increased transsarcolemmal calcium gradients, up to 5 mM extracellular calcium concentration. Cai was experimentally elevated by increasing calcium influx from the extracellular medium and/or by diminishing calcium efflux by Na-Ca exchange. Under these conditions, cells contracted and relengthened repetitively. The regulation of high-energy phosphates was challenged by increasing ATP utilization and by inhibiting ATP synthesis. Cai regulation was not affected by inhibition of glycolysis or NADH oxidation, so long as ATP concentration remained unchanged. High-energy phosphates were not depleted in beating cells with intact NADH oxidation, but inhibition of NADH oxidation caused a significant drop in phosphocreatine, demonstrating the increased rate of ATP consumption during beating. In beating cells, as in the working heart, ATP supply is increased to meet ATP demand, and steady-state ATP and phosphocreatine concentrations remain unchanged.