Voltage dependence of Mg²⁺ and Ca²⁺ block of SK_Ca. (A) Voltage dependence of the apparent K_d of Mg²⁺ calculated with Eq. 2. Dashed line was fitted to data with Eq. 3, with a K_d at 0 mV of 3 mM and θ (slope factor indicating voltage sensitivity of K_d) = −0.20. (B) Voltage dependence of the apparent K_d of Ca²⁺ calculated with Eq. 2. Dashed line was fitted to data with Eq. 3, with a K_d at 0 mV of 3 μM and θ = −0.20.