Dependence of BK fractional availability during bbTBA block on [Ca²⁺] and [bbTBA]. (A) Currents were activated with 4 µM of internal Ca²⁺. Control GV curves in the absence of bbTBA were generated from tail currents (open circles), and then fractional availability was determined either with 20 µM (filled circles) or 75 µM (diamonds) bbTBA using the protocol shown in Fig. 9. For 20 µM bbTBA, V_h = 67.1 ± 3.6 mV with z = 0.76 ± 0.02 e, whereas for 75 µM bbTBA, V_h = 5.1 ± 3.9 mV with z = 0.59 ± 0.01 e. The activation GV yielded V_h = 97.7 ± 0.7 mV (q = 1.32 ± 0.07 e). (B) Fractional availability and activation were defined in patches bathed with 100 µM of internal Ca²⁺. With 20 µM bbTBA (filled squares), V_h = −32.6 ± 1.33 mV (z = 0.88 ± 0.03 e) and, for 75 µM bbTBA (filled triangles), V_h = −58.4 ± 1.4 mV (z = 0.80 ± 0.02 e). The activation GV (open squares) yielded V_h = 22.7 ± 1.15 mV (q = 0.98 ± 0.04 e). For each condition, five to six patches were used. (C) Curves from A and B are replotted to show the shift in fractional availability with 20 µM bbTBA as [Ca²⁺] is increased from 4 to 100 µM. (D) Curves from A and B are replotted to show the shift in fractional availability with 75 µM bbTBA with increases in [Ca²⁺] from 4 to 100 µM.