Effect of external acidification on pHi in WT and V876E fibers in the presence of an external Tyrode’s solution. (A) Recording of the change in pHi in response to exposition of the cell to an external solution buffered at pH 5.0 in a BCECF-loaded V876E fiber held at −80 mV. (B) Means and SEM of pHi as a function of time in WT and in V876E fibers in response to external acidification (pHe). (C) Mean rate of change in pHi measured during the first minute of exposition of the cell to the external solution buffered at pH 5. The number of fibers tested is indicated above each histogram bar. Data are given as means ± SEM.