Characterization of BK F380 mutant ionic currents. (A) BK-WT currents (I_K) evoked by voltage steps (−100 to 300 mV; in 10-mV steps of 10 ms) at two different [Ca²⁺]_i (0 and 100 µM). (B) BK-F380A macroscopic currents recorded at 0 and 100 µM (−100 to 350 mV; in 10-mV steps of 2 ms). Note the difference in time scale because of the fast activation in the case of BK-F380A. (C) BK-F380W currents (I_K) evoked by voltage steps (−220 to 300 mV; in 10-mV steps of 10 ms) in two different [Ca²⁺]_i (0 and 100 µM). (D–F) Relative conductance versus voltage ((G/G_MAX) − V) curves were obtained from tail current measurements from experiments as those shown in A–C. Data were fitted using a Boltzmann function (solid lines). For the BK-WT channel, parameters were (mean ± SEM) V_h = 176 ± 8 mV, z_G = 0.96 ± 0.08 e₀ at [Ca⁺²]_i = 0 µM, n = 5; V_h = 120 ± 10 mV, z_G = 1.30 ± 0.10 e₀ at [Ca⁺²]_i = 0.1 µM, n = 5; V_h = 110 ± 8 mV, z_G = 1.40 ± 0.10 e₀ at [Ca⁺²]_i = 0.2 µM, n = 6; V_h = 81 ± 5 mV, z_G = 1.33 ± 0.12 e₀ at [Ca⁺²]_i = 0.5 µM, n = 5; V_h = 52 ± 8 mV, z_G = 1.40 ± 0.06 e₀ at [Ca⁺²]_i = 1.5 µM, n = 5; V_h = 20 ± 6 mV, z_G = 1.34 ± 0.11 e₀ at [Ca⁺²]_i = 3 µM, n = 6; V_h = −43.7 ± 11 mV, z_G = 1.12 ± 0.04 e₀ at [Ca⁺²]_i = 100 µM, n = 5. BK-F380A: V_h = 620 ± 34 mV, z_G = 0.32 ± 0.03 e₀ at [Ca⁺²]_i = 0.2 µM, n = 4; V_h = 492 ± 19 mV, z_G = 0.43 ± 0.02 e₀ at [Ca⁺²]_i = 0.5 µM, n = 4; V_h = 349.8 ± 11.7 mV, z_G = 0.42 ± 0.01 e₀ at [Ca⁺²]_i = 1.5 µM, n = 5; V_h = 321 ± 9 mV, z_G = 0.45 ± 0.01 e₀ at [Ca⁺²]_i = 3 µM, n = 4; V_h = 232 ± 25 mV, z_G = 0.48 ± 0.05 e₀ at [Ca⁺²]_i = 100 µM, n = 5; BK-F380W: V_h = 133 ± 3 mV, z_G = 0.73 ± 0.03 e₀ at [Ca⁺²]_i = 0 µM, n = 17; V_h = 102 ± 6 mV, z_G = 0.52 ± 0.02 e₀ at [Ca⁺²]_i = 0.1 µM, n = 6; V_h = 92 ± 7 mV, z_G = 0.52 ± 0.01 e₀ at [Ca⁺²]_i = 0.2 µM, n = 7; V_h = 80 ± 5 mV, z_G = 0.55 ± 0.03 e₀ at [Ca⁺²]_i = 0.5 µM, n = 7; V_h = 26 ± 5 mV, z_G = 0.70 ± 0.01 e₀ at [Ca⁺²]_i = 3 µM, n = 7; V_h = −58 ± 3 mV, z_G = 0.52 ± 0.03 e₀ at [Ca⁺²]_i = 100 µM, n = 14. (G and H) BK-F380I and BK-F380L currents (I_K) evoked by voltage steps (−100 to 350 mV; in 10-mV steps of 2 ms) at [Ca²⁺]_i = 100 µM, respectively. (I) Relative conductance versus voltage ((G/G_MAX) − V) curves were obtained from tail current measurements from the experiments shown in G and H. Data were fitted using a Boltzmann function (solid lines). For the BK-F380I channel, parameters were (mean ± SEM) V_h = 261 ± 6 mV, z_G = 0.72 ± 0.08 e₀ at [Ca⁺²]_i = 100 µM, n = 4. BK-F380L: V_h = 267 ± 6 mV, z_G = 0.58 ± 0.04 e₀ at [Ca⁺²]_i = 100 µM, n = 7. Black and red lines are the G/G_MAX − V curves and Boltzmann fitting of the BK channel WT and F380A mutant at [Ca⁺²]_i = 100 µM (as is seen in D and E), respectively.