The dependence of the action potential of the frog's heart on the external and intracellular sodium concentration

Abstract

The over shoot of the action potential of the frog''s heart was reduced when external NaCl was replaced by sucrose. However, the potential decre ment was only 17.3 mV for a 10-fold reduction of Na as compared with 58 mV expected on the basis of the Na hypothesis of excitation. Replacement of up to 75% of the external sodium by choline did not reduce the overshoot, provided atropine was present in sufficient con centrations to suppress any parasympathomimetic action. The maxi mum rate of rise of the action potential markedly declined in low Na fluids whether sucrose or choline chloride was used to replace NaCl. The maximum rate of rise was deduced to only a small extent when external sodium was replaced by Li. Increasing the intracellular Na concentration in exchange for lost K caused overshoots to decline. The effects resembled those obtained in similar experiments with skeletal muscle fibers (Desmedt, 1953). Action potentials occurring under certain conditions even in the presence of very low external sodium concentrations (< 5% normal) also declined in height when the intracellular sodium concentration was increased. The behavior of the action potential in low external Na concentrations may be explained by an action of Ca on the excitable membrane.