Outward and inward current components in wild-type Na⁺/K⁺ pumps. (A) Current in a single Na⁺-loaded oocyte overexpressing wild-type α1/β3 Xenopus Na⁺/K⁺-ATPase pumps, at −20 mV, in 125-mM Na⁺_o solutions. The addition of 15 mM K⁺_o (bar labeled “K”) activated large outward 3 Na⁺_i/2 K⁺_o exchange current that was abolished by 10 mM ouabain (bar labeled “ouab”). Vertical lines are responses to 50-ms jumps to other potentials. (B) Steady currents from the oocyte in A plotted against voltage for the trials identified by numbers 1–4 in A. (C) Average ouabain-sensitive currents (I ouab-sens norm) obtained by subtraction of currents like those in B, at every voltage and presence or absence of K⁺_o, were normalized to the mean amplitude between 0 and 60 mV of ouabain-sensitive current at 15 mM [K⁺_o] in 0 or 125 mM Na⁺_o, as appropriate; normalized currents were then averaged across oocytes; ±SEM is visible where larger than the symbols; n = 8 at 125 mM Na⁺_o; n = 5 at 0 Na⁺_o. In direct comparisons in the same oocyte, maximal outward ouabain-sensitive current at 15 mM [K⁺_o] and positive voltage is the same in the presence or absence of Na⁺_o (Vedovato and Gadsby, 2010). (D) Ouabain-sensitive currents in a representative oocyte at 0 mM Na⁺_o. At pH_o 7.6, ouabain-sensitive currents in 15 mM K⁺_o (closed triangles) or in 0 mM K⁺_o (closed circles) are like those in C, but upon lowering pH_o to 6.0 in 0 mM K⁺_o (open circles), inward current at negative potentials increased approximately fivefold.