Gating currents of BK-F380A mutant. (A) Fast ON gating charge versus voltage curves obtained in the presence of symmetrical 110 mM KMES (closed circles) and in the absence of permeant ions (open circles), the inset shows the gating and macroscopic currents in the presence of permeant ions (KMES). I_g BK-F380A was recorded in inside-out macropatches at 0 [Ca⁺²]_i (inset). (B) Open probability versus voltage curve (mean ± SEM) was obtained from F380A mutant current record shown in the inset of A and calculated as was described in Materials and methods. The best fitting of this data to a monoexponential curve was $L*(V)=L0*ezLFVRT=L0D4ezLFVRT=6.59*10−5e16.5V.$ (C and D) I_g of BK and BK-F380A was recorded from inside-out macropatches at 0 and 100 µM of [Ca⁺²]_i. To isolate I_g of all ionic currents and I_leak, our internal and external solutions contained nonpermeant ions (see Materials and methods). I_g was evoked by 2-ms voltage steps of 10 mV, going from −90 to 350 mV. (E) Q_c(V) relationships of BK and BK-F380A were obtained by integrating the fast component for each ON I_g trace. Boltzmann fitting to the experimental data (mean ± SEM) is indicated by solid lines (V₀ = 169 ± 3 mV, z_Q = 0.63 ± 0.02 e₀ in BK at [Ca⁺²]_i = 0 µM, n = 14; V₀ = 29 ± 8 mV, z_Q = 0.71 ± 0.07 e₀ in BK at [Ca⁺²]_i = 100 µM, n = 3; V₀ = 149 ± 3 mV, z_Q = 0.59 ± 0.01 e₀ in BK-F380A at [Ca⁺²]_i = 0 µM, n = 9; V₀ = 72 ± 3 mV, z_Q = 0.87 ± 0.05 e₀ in BK-F380A at [Ca⁺²]_i = 100 µM, n = 3).