Figure 1. Voltage dependence of Na⁺-K⁺ pump current activated by 0.5 mM Na⁺_pip.

A, whole-cell recording from a guinea-pig ventricular myocyte. The patch pipette contained 0.5 mM Na⁺ and 159.5 mM NMDG⁺ as the main cation. Current traces in response to voltage steps from zero potential to potentials between −100 and +40 mV in 10 mV increments are displayed. Left panel: control in Na⁺-free solution containing 20 mM Cs⁺. Right panel: currents obtained following application of 0.1 mM ouabain. Note the inward shift of the current traces after inhibition of the Na⁺-K⁺ pump by ouabain. Cell capacitance: 111 pF. B, steady-state membrane current (I_m)-voltage relationships from the data shown in A. □, control in drug-free medium; ▪, currents measured under 0.1 mM ouabain. C, pump current (I_p)-voltage relationship. Data represent the differences between membrane currents recorded in the absence and in the presence of ouabain (shown in B). Note the steep positive slope of the I_p-V curve.