Effect of prostacyclin on cardiac transmembrane currents

The relatively stable sodium salt of prostacyclin, PGI2-Na, kindly supplied by Chinoin Works (Budapest), was found to induce a dose-dependent increase in the inward-going membrane current of the sinoauricular fiber of the frog. Threshold concentration was 0.05 microgram/ml, and maximal effect was reached at a concentration of about 0.1 microgram/ml. Supramaximal concentrations of PGI2-Na evoked a maximal response, in contrast to primary prostaglandins, the dose-response curves of which decline at supramaximal values. The stimulatory effect of PGI2-Na was confined to the fast component of the inward current and was also readily demonstrable in the presence of Mn ions, i.e., with the slow channel blocked. The PGI2-Na was, however, ineffective in sodium-free medium (choline Ringer) or in the presence of 1 microgram/ml tetrodotoxin. Endogenous prostacyclin is likely to contribute to the regulation of cardiac cell membrane processes.