Mutant R217H. (A) Sensing currents from the mutant R217H-C363S at pH_EXT 7.4 and 6.1. The HP for the recordings was −90 mV. After a 200-ms prepulse to −120 mV, test pulses ranged from −150 to 140 mV. (B) Q_ON-V curves for the records in A. As shown, no changes were observed in the Q_ON,MAX when pH_EXT was modified. (C) Time constants versus potential (τ-V) curves display at least two components, where ON-sensing currents at pH 6.1 were slower than those observed at pH 7.4 at equivalent potentials. (D) Q_ON-V curves for C363S at pH_EXT of 7.4 and 6.1 were very similar to each other. In contrast, the mutant R217H displayed strong pH_EXT sensitivity. Q_ON-V were normalized to the Q_ON,MAX observed at pH 7.4. No changes in Q_MAX were observed as a function of pH_EXT. Q_ON-V curves had two components and the relative proportion of them changed with pH_EXT, where the component at more positive potential increases as pH_EXT decreases (see Modulation of the voltage dependence by titration of R217H). Error bars are SD.