Effects of Ca ²⁺ on BK channel modulation by γ subunits. (A) Representative current traces of BK channels formed by BKα alone or together with the γ subunit in the virtual absence of [Ca²⁺]_i (no label) or in the presence of 10 µM Ca²⁺ in response to depolarization of the membrane potential from −80 mV, −120 mV, or −200 mV in 20-mV steps. (B–D) Voltage dependence of BK channel activation for channels formed by the BKα and γ1 (E), γ2 (F), or γ3 (G) subunits in the virtual absence of [Ca²⁺]_i and in the presence of 10 µM Ca²⁺ (n = 4–10). 10 µM Ca²⁺ was included directly in the bath solution. (E) Summary of 10 µM Ca²⁺-induced reduction (ΔΔV_1/2) in the V_1/2-shifting capabilities (ΔV_1/2) of the γ1–3 subunits (n = 4–10). (F) Predicted voltage-dependence of BK channel open probability (P_o) of unmodulated (D = 21) and modulated (D = 412) BK channel in the absence and presence of 10 µM Ca²⁺ according to the HA allosteric model. The values of the volage-gating parameters were taken from previous reports (Horrigan and Aldrich, 2002; Yan and Aldrich, 2012) as such: L₀ = 3.6 × 10⁻⁶, Z_L = 0.29, J₀ = 0.048, Z_J = 0.59, D = 21 or 412, K_D = 11, C = 8, E = 2.4. Error bars in B–E represent ± SEM.