Voltage dependence of transient and steady-state Na/K pump currents in myocytes

Experiments are reviewed here in which Na/K pump current was determined as strophanthidin-sensitive current in guinea-pig ventricular myocytes, voltage-clamped and internally-dialyzed via wide-tipped pipettes. In the presence of 150 mM extracellular [Na], both outward and inward pump current, during forward and reverse Na/K exchange respectively, were strongly voltage dependent. But reduction of external [Na] to 1.5 mM severely attenuated the voltage sensitivity of outward Na/K pump current. Voltage jumps elicited large transient pump currents during forward or reverse Na/K exchange, or when pump activity was restricted to Na translocation steps, but not when pumps were presumably engaged in K/K exchange. These findings indicate that Na translocation, but not K translocation, involves net charge movement through the membrane field, and that both forward and reverse Na/K transport cycles are rate-limited not by that voltage-sensitive step but by a subsequent voltage-insensitive step.