Interaction of Y780L/Q/A with extracellular ions. (A) Na⁺/K⁺ pump current induced by 10 mM K⁺ in NMDG⁺. (B) K_0.5.K obtained from Hill fits (Eq. 2) to the K⁺-induced current at −50 mV in NMDG⁺ or with 125 mM Na⁺. Grey circles show each independent measurement in individual oocytes. (C) Representative ouabain-sensitive currents elicited by 50-ms-long pulses from −50 mV to the indicated voltages, in oocytes bathed by 125 mM Na⁺ solution, expressing Y780L, Y780Q, or Y780A. (D) Mean Q_OFF–V curve from 6 to 12 experiments. Continuous lines are Boltzmann distribution fits to the average data, with kT/z_qe = 36 mV and Q_tot = 6.65 nC for WT and kT/z_qe = 50 mV; Q_tot = 7.32 nC for Y780F, kT/z_qe = 45 mV, Q_tot = 4.24 nC for Y780Q and kT/z_qe = 60 mV, Q_tot = 2.24 nC (Y780A). The mean V_1/2 from individual experiments is listed in Table 1. The inset shows normalized Boltzmann equations to illustrate shifts in Q–V, except for Y780L which could not be fit, see Results.