Voltage activation of BK_B channels. (A) Representative single-channel currents recorded from a BK_B channel (DM4) in saturating 95 µM Ca²⁺_i. Arrows indicate the closed channel levels. (B) Plots of Po versus voltage for four separate BK_B channels, as indicated. The lines are fits with the Boltzmann equation Po = Po_max/(1 + exp[(V_0.5 − V)/b]), where b is the voltage sensitivity expressed as the voltage change (in millivolts) per e-fold change in Po. The parameters for the lines are given in Table I. (C and D) Voltage dependence of mean open and mean closed times for the same four channels as in B. The lines plot the predicted mean open and closed times for channels DM1 (continuous lines) and DM4 (dashed lines) calculated with Scheme V. A similar superposition of observed and predicted mean open and mean closed times was observed for channels DM2 and DM3. Parameters used in the predictions for channel DM4 in C and D of this figure and in Figs. 4–7,⁵⁶⁷ are given in Table S3.