Sodium Current in Ventricular Myocardial Fibers

Membrane currents were measured in thin bundles of dog ventricular myocardium under voltage-clamp conditions. A rather large initial inward current which had an equilibrium potential at about + 55 millivolts could be recorded. When the external sodium concentration was reduced, the equilibrium potential for this current was shifted by the amount predicted theoretically for a current carried solely by sodium ions. The size of the sodium inward current (I$_{Na}$) was largely dependent on the preceding membrane potential. The I$_{Na}$ was completely inactivated if the membrane potential was as low as - 45 millivolts. Sodium ions are the main carrier of charge during the rapid depolarization phase of the action potential.