100 µM bbTBA has no inhibitory effect on NPo at voltages over which voltage sensor activation is minimal. (A) Po-V curves are plotted based on a full Horrigan-Aldrich activation scheme for 30 µM Ca²⁺ (black line) along with predictions for block in the presence of 100 µM bbTBA for either Scheme 1a (open-channel block; dotted line), Scheme 2a (state-independent block; blue line), or Scheme 3a (block dependent on voltage sensor movement; red line). The state-independent Scheme 2a predicts the same percent block at positive potentials and at potentials where voltage sensors are inactive, whereas Schemes 1a and 3a predict no change in NPo when voltage sensors are inactive. (B) Channel openings were monitored with 30 µM Ca²⁺ at −150 mV before a step to +20 mV either in the absence (black) or presence (red) of 100 µM bbTBA. 100 µM bbTBA produces >90% block of BK current at +20 mV, with minimal obvious effect at −150 mV. (C) Expanded time base records from the traces in B are shown during the periods at −150 mV. (D) Histograms of the current amplitudes of all digitized time points during the recording at −150 mV in the absence (black) and presence (red) of 100 µM bbTBA are plotted, with the bottom panel showing the same data on a different scale. bbTBA results in an increase in NPo in this patch.