The ΔpH dependence of WT hH_V1 saturates at high pH_o or pH_i. (A–D) Families of currents at several pH_o are illustrated in a COS-7 cell expressing WT hH_V1 with pH_i 7 at holding potentials of −80 (A), −60 (B), or −40 mV (C and D). In some families, shorter pulses were applied for large depolarizations to minimize proton depletion. (E) The g_H–V relationships are plotted, showing that the shift between pH_o 8 and 9 is distinctly <40 mV. Measured V_rev in A–D was −95, −53, 5, and 57 mV, respectively. (G–J) Current families in an inside-out patch are shown, with pipette pH_o 7 and holding potentials of −60 (G), −40 mV (H and I), and −20 mV (J), with g_h–V relationships plotted in F. Measured V_rev in G–J was −54, 0, 60, and 115 mV, respectively. For g_H calculation, the current amplitude I_H was obtained by fitting rising currents to a single-exponential function, with the exception of J, where because of the difficulty of fitting the slowly activating inward and outward currents, the tail current amplitude was used.