A modified HA model can explain the effect of Ca²⁺ on the steady-state gating properties of the BK_Ca channel. Shown are series of mSlo1α G-V relations determined at the following [Ca²⁺]: 0.003, 0.070, 0.130, 0.360, 0.8, 10, and 100 μM and fit simultaneously with the modified model that includes two types of Ca²⁺ binding sites, one type interacts with the voltage sensor, and the other type is independent. The data are the same as shown in Fig. 8. The parameters were held as follows: K_O1 = 0.88 μM, K_C1 = 3.18 μM, K_O2 = 4.88 μM, K_C2 = 23.2 μM, L_O = 2.2 × 10⁻⁶, z_L = 0.41 e, V_hc = 151 mV, V_ho = 27 mV, and z_J = 0.58 e. The value of E coupling voltage sensor activation and Ca²⁺ binding at one type of site was allowed to vary. Shown is the best fit obtained. The value of E was calculated to be 2.84 ± 0.13.