β1 increases the Ca²⁺ sensitivity of BK_Ca-channel activation. (A and B) Macroscopic currents recorded from ΔE channels in the absence and presence of β1. Currents are from inside-out macropatches from TSA 201 cells exposed to 9.7 µM Ca²⁺. (C and D) G-V relations determined at the following Ca²⁺ concentrations: 0.003, 0.360, 1.4, 5.3, 113, and 2,500 µM for the mutant channel ΔE (C) and ΔE + β1 (D). Each curve represents the average of between 4 and 22 individual curves. Error bars indicate SEM. The solid curves are Boltzmann fits with the following parameters: ΔE, 3 nM Ca²⁺: Q = 1.21 e, V_1/2 = 183 mV; 360 nM Ca²⁺: Q = 1.18 e, V_1/2 = 151 mV; 1.4 µM Ca²⁺: Q = 1.47 e, V_1/2 = 101 mV; 5.4 µM Ca²⁺: Q = 1.38 e, V_1/2 = 59 mV; 113 µM Ca²⁺: Q = 1.00 e, V_1/2 = 13 mV; 2.5 mM Ca²⁺: Q = 1.04 e, V_1/2 = −5.7 mV. ΔE + β1, 3 nM Ca²⁺: Q = 0.46 e, V_1/2 = 230 mV; 360 nM Ca²⁺: Q = 0.51 e, V_1/2 = 197 mV; 1.4 µM Ca²⁺: Q = 0.70 e, V_1/2 = 131 mV; 5.4 µM Ca²⁺: Q = 0.94 e, V_1/2 = 46 mV; 113 µM Ca²⁺: Q = 0.83 e, V_1/2 = −57 mV; 2.5 mM Ca²⁺: Q = 0.87 e, V_1/2 = −90 mV. (E) Plots of half-maximal activation voltage (V_1/2) versus [Ca²⁺] for traces shown in C and D. (F) Ca²⁺ dose–response curves determined for ΔE in the presence and absence of β1 at 0 mV. The curves are fitted with the Hill equation:Fit parameters are as follows: ΔE (Kd = 68 µM, n = 1.4); ΔE + β1 (Kd = 32 µM, n = 1.1).