Representative Na/K pump currents and capacitance transients in the presence of 25 mM Na and 90 mM K on the cytoplasmic side. (A) Pump currents are activated by substituting 7 mM Na for 7 mM K on the extracellular side. Outward pump current decays by 70% with a τ of ∼2 s and recovers with a τ of ∼8 s. Membrane capacitance falls immediately upon pump activation by 1 to 2% and recovers with a time course that matches the recovery of pump currents. Dots above the current and capacitance records are scaled to one another to illustrate the close temporal correlation between recovery of pump current and membrane capacitance. (B) Plot of Na/K pump current changes versus capacitance changes during pump current recovery. (C) Voltage dependence of K-induced capacitance changes in the presence of 120 mM extracellular Na (blue, n = 7) and for the substitution of 7 mM Na for 7 mM K (red, n = 13, except for −120 mV, where n = 7). The bell shaped capacitance curve with 120 mM Na reflects the major charge movement of the pump with a Boltzmann slope of 29 mV and a midpoint of −60 mV. (D) Voltage dependence of capacitance in the presence of 120 mM Na in the absence (blue) and the presence (green) of 7 mM K. Note that the midpoint of the bell shifts at most 10 mV to more negative potentials. Assuming that Na and K binding are mutually exclusive, this result requires that time-dependent steps separate Na and K binding.