Effect of outside pH (pH_o) on H⁺ currents in JME cells. (A–D) Whole cell currents elicited by voltage pulses (as shown in top inset in A) at pH_o 6, 7, and 8 (A–C), and in the presence of 10 µM ZnCl₂ at pH_o 8 (D). All recordings are from one cell; pH_i 6. Y axis and 10-s time bar in A are the same for B–D. Selected voltage steps are labeled. The bottom insets in A–C are respective expanded tail currents on a scale as shown in A. (E) Average steady-state current–voltage relations at pH_o 6, 7, and 8 as indicated. Currents were normalized to membrane capacitance. The number of experiments is given in F in parentheses and is the same for E–H. (F) Reversal potential (E_rev) was determined from tail currents resulting in a slope of −53 ± 5.1 mV/pH. (G) Threshold potential (V_th) as determined from 8-s voltage steps resulted in a slope of −31.5 ± 1.9 mV/pH. (H) Relationship between V_th and V_rev (using data from F and G) resulted in a slope of 0.62 ± 0.12 mV/mV and an offset of 35.5 ± 7.4 mV.