Figure 7. Comparison of block of BK currents by 4-AP at pH 7.0 and 8.5. In (A), currents activated with 300 μM Ca2+ at +100 mV by the indicated voltage protocol are compared for different 4-AP concentrations at pH 7.0. The trace for 50 mM 4-AP exhibits greater than 50% block at +100 mV with almost full and instantaneous recovery during the tail current at -100 mV. In (B), currents from the same patch as in (A) were activated as in (A) except at pH 8.5. The trace at 50 mM 4-AP shows strong block at +100 mV, with less complete recovery from block during the tail current in comparison to currents at pH 7.0. In (C and D), G-V curves at plotted for different concentrations of 4-AP either for pH 7.0 (C) or 8.5 (D). In (C), lines correspond to the best fit of to all points for pH 7.0 yielding Kb = 58.3 ± 3.3 mM with zδ = 0.18 ± 0.02 e. In (D), similarly for pH 8.5, yielded Kbo = 24.9 ± 2.6 mM with zδ = 0.05 ± 0.06 e. However, at pH 8.5, the state-independent yielded a better fit than , with Kbo= Kbc = 31.0 ± 2.7 mM with zδ = 0.11 ± 0.03 e, while for pH 7.0 yielded a poorer fit than . In (E), the G/V curves obtained at both pH 7.0 (left) and pH 8.5 (right) were fit simultaneously with , which assumes two independent 4-AP blocking sites, one for the uncharged non-protonated species defined by binding constant Kb(u) and a voltage-dependent site sensitive to protonated 4-AP defined by binding constant Kb(p). Best fit values for the three free parameters were Kb(u) = 9.07 ± 0.46 mM, Kb(p) = 63.5 ± 3.8 mM, and z(p) = 0.19 ± 0.02 e.