Mechanism of ATP-induced Ca2+ efflux from freshly isolated adult rat cardiomyocytes: possible involvement of Na+/Ca2+ exchange.

Physiological stimulation causes a rise in intracellular Ca2+ concentration in cardiomyocytes. However, it has not yet been elucidated what mechanisms are involved in the reduction of cytosolic free Ca2+ levels, including those which underlie Ca2+ efflux from the cells. In the present study, we examined the effect of extracellular adenosine 5'-triphosphate (ATP) on Ca2+, efflux from freshly isolated adult rat cardiomyocytes. The isolated cardiomyocytes were preloaded with 45CaCl2 for one hour. Then, the fractional release of 45Ca2+ from the cells was measured consecutively. ATP stimulated the efflux of 45Ca2+ from isolated adult rat cardiomyocytes in a concentration-dependent manner (0.01-1 mM). The 45Ca2+ efflux from the cells was also stimulated by adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma S), alpha, beta-methylene-ATP and adenosine 5'-diphosphate (ADP), but not by adenosine 5'-monophosphate (AMP) or adenosine. The effect of ATP was inhibited by a specific purinergic P2-receptor antagonist, but not by a P1-receptor antagonist. From these results, it is conceivable that the effect of ATP on Ca2+ efflux from cardiomyocytes is mediated through P2-purinoceptors. The ATP-stimulated 45Ca2+ efflux was not affected by removal of extracellular Ca2+, but was dependent on the presence of extracellular Na+. Moreover, ATP caused a 22Na+ influx into the cells. These results suggest that ATP stimulates extracellular Na(+)-dependent 45Ca2+ efflux from freshly isolated adult rat cardiomyocytes, probably through its stimulatory effect on plasma membrane P2-purinoceptors which may couple to Na+/Ca2+ exchange.