pH sensitivity of neurotoxic UNC-8 mutants. (A) Example of currents stimulated by voltage steps from −160 to +100 mV in an oocyte injected with UNC-8(G387E); the holding potential was −30 mV. The oocyte was perfused with the divalent cation–free plus EGTA NaCl solution at pH 7.2. (B) The same oocyte shown in A was perfused with the divalent cation–free plus EGTA NaCl solution at pH 6.5. (C and D) Same as in A and B for an oocyte expressing UNC-8(A586T). (E) pH dose–response curves for oocytes expressing UNC-8(G387E) (closed circles; n = 22) and UNC-8(A586T) (closed squares; n = 14), respectively. Before normalization, all currents were leak subtracted. Data points were fitted by sigmoidal curves that gave pH_0.5 values of 5.6 and 6.6 for UNC-8(G387E) and UNC-8(A586T), respectively.