Electrophysiological characteristics of Y780C, Y780T, Y780S, and Y780H. (A) Ouabain-sensitive currents in 125 mM Na⁺, elicited by 50 ms pulses to the indicated voltages. (B) Mean Q–V curve, obtained from the integral of the transient currents when the voltage returns to −50 mV, for WT, Y780C, Y780T, Y780S, and Y780H. (C) Normalized fitted curves from B, to illustrate changes in the Boltzmann distributions fitted to the mean data, with centers V_1/2 = −54 mV for WT, V_1/2 = −75 mV for Y780C, V_1/2 = −98 mV for Y780S, V_1/2 = −101 mV for Y780T, and V_1/2 = −104 mV for Y780H. WT and Y780F are shown as black and red dotted lines, respectively. (D and E) K_0.5 for K⁺ activation of pump current when applied in NMDG⁺ external solution (D) or Na⁺ solution (E). Grey circles show individual data points from each oocyte. (F) Current–voltage relationship for the ouabain-sensitive currents in 125 mM Na⁺ at four K⁺ concentrations, for WT (left, n = 12) and the disease-causing Y780C (center, n = 2) and Y780H (right, n = 7). The dotted line marks the 0-current level.