Wild-type Na⁺/K⁺ pump currents in 125 mM Na⁺_o at normal and lowered pH_o. (A) Current record at −20 mV (same oocyte as in Fig. 2, A and B) during the addition of 15 mM K⁺_o and ouabain, as indicated, at pH_o 6.0 (gray current trace) and pH_o 7.6 (black trace). (B) Ouabain-sensitive steady currents without (squares) and with (stars) 15 mM K⁺_o, at pH_o 7.6 (closed symbols) and 6.0 (open symbols), from subtraction of currents in numbered voltage trials in A (the same results obtained in n = 8 oocytes are shown averaged in Fig. S4 A).