Each curve represents the individual cell that approximately describes the mean of the population for the given experimental condition: S238A (A); T560A (B); S238A/T560A (C); C1 (D); C3 (E); and C4 (F) mutants. The drug was placed in the culture medium for 45 min and was present in the intracellular pipette solution as well. For the cells included in this graph the following values were obtained by curve fitting; probability values pertain to charge density measures. S238A: Qmax, 149/141 fC; α, 30.8/32.4 mV; V1/2, −51.4/−51.4 mV (P > 0.7); T560A: Qmax, 115/113 fC; α, 30.5/29.8 mV; V1/2, −63.7/−66.9 mV (P = 0.8); S238A/T560A: Qmax, 83/123 fC; α, 32.5/38.8 mV; V1/2, −38.5/−40.6 mV (P = 0.6); C1: Qmax, 263/215 fC; α, 32.9/29.8 mV; V1/2, −132.3/−126.8 mV (P = 0.7); C3: Qmax, 68/76 fC; α, 39.6/35 mV; V1/2, −63.3/−70.2 mV (P = 0.8); C4: Qmax, 90/55 fC; α, 32.1/37.5 mV; V1/2, −64/−73.6 mV (P < 0.05).