Voltage dependence of Na/K pump currents and K-induced capacitance changes with 120 and 7 mM extracellular Na. Cytoplasmic K was omitted to ensure minimal K conductance. (A) Na/K pump currents and capacitance changes recorded at five voltages from −60 to +60 mV. K-induced capacitance changes decrease and Na/K pump current decay attenuates progressively with depolarization. (B) Mean peak (blue) and 20 s (red) pump currents plotted against membrane voltage, together with the fractional decay of pump currents (black) and the magnitude of K-induced capacitance changes (red). The voltage range is too narrow to reveal the bell shape of capacitance changes with 120 mM Na. Solid curves are simulation results scaled to the data points. According to the model, capacitance changes reflect primarily the voltage dependence of Na-dependent charge movements. The attenuation of current decay with depolarization reflects a shift of pumps from E1 to E2 configurations. Although the dissociation of Na in E2 conformations is favored by depolarization, thereby promoting a shift to E1, extracellular K binding is progressively hindered and Na occlusion from the cytoplasmic side is progressively favored. With depolarization, these latter influences become dominant and pumps shift on average to E2 from E1. (C and D) Equivalent results for experiments in which 7 mM extracellular Na was replaced with 7 mM K to activate pump currents. Voltage dependence of pump current is reduced in characteristic fashion, whereas other patterns are principally similar.