Similar Nao dependence and voltage dependence of transient charge movements in parent C113Y pumps and in C113Y pumps with C-terminal truncations. (A–C) Superimposed ouabain-sensitive current records (obtained by subtraction as in Fig. 8 [C and G]) determined in the same oocyte for C113Y pumps exposed to 125 mM (A), 62.5 mM (B), and 15.6 mM (C) [Nao]; Na was replaced by equimolar TMA. (E–G) Similar ouabain-sensitive transient currents in an oocyte expressing C113Y-ΔYY pumps exposed to 125 mM (E), 62.5 mM (F), and 15.6 mM (G) [Nao]; note that the steady inward currents at negative test voltages became larger as [Nao] was lowered (E–G). (D and H) Pre–steady-state charge determined as integrals of OFF transients was plotted against test voltage and fitted with Boltzmann relations and then normalized and averaged for four C113Y oocytes (D) and three C113Y-ΔYY oocytes (H). ΔV_0.5 for the twofold and eightfold reductions of [Nao], respectively, averaged as follows: C113Y (red), −22 ± 1 mV and −80 ± 3 mV, n = 4; C113Y-ΔYY (olive), −25 ± 2 mV and −90 ± 10 mV, n = 3; C113Y-ΔKESYY, −33 mV and −102 mV, n = 1. Fractional electrical distance, λ, estimated from these data via ΔV_0.5 = (RT/λF)ln([Nao]₁/[Nao]₂) is ∼0.7 (0.73 for C113Y and 0.65 for C113Y-ΔKESYY), as for WT.