mSlo1 channels are extremely Cu²⁺ sensitive. (A) Mean G_K-V relations with [Ca²⁺]_i = 3 μM at different [Cu²⁺]: 0 (○), 0.1 (•), 0.2 (□), 0.5(▴), 1.0 (▵), 2.5 (▪), 5 (▿), 10 (▾), 20 (⋄), 50 (♦), 100 () and 200 μM (⋄), are normalized by G_Kmax in 0 Cu²⁺ and fit by Boltzmann functions. (B) Mean G_K-V relations from panel A are normalized by G_Kmax at each [Cu²⁺] based on Boltzmann fits. (C) Cu²⁺ dose–response relations for ΔV_0.5 = (V_0.5 [Cu²⁺] − V_0.5[0]) (•) and G_K[Cu²⁺] at 110 mV (○), 150 mV (▵) and 190 mV (▿) were determined from fits in A. Solid lines are fits to a Hill equation. Thick dotted curve is a fit to Eq. 1 with most parameters set to previously determined values (z_J = 0.58 e, L₀ = 10⁻⁶, z_L = 0.3 e, K_D(Ca²⁺) = 11 μM, C = 8, D = 25, E = 2.4) (Horrigan and Aldrich, 2002). V_hC = 162 mV was adjusted to fit V_0.5 of the 0 Cu²⁺ control, and Cu²⁺-dependent parameters (K_Dcu = 0.75 μM, E_cu = 0.124) were then varied to fit the V_0.5-[Cu²⁺] relation. Thin dotted curve is a fit to Eq. 2 with K_Dcu = 2.1 μM, E_cu = 0.129, and all other parameters the same as for Eq. 1. (D) IC₅₀ and (E) n_H obtained by fitting G_K-[Cu²⁺] relations in 3 μM Ca²⁺ (•) and 0 Ca²⁺ (□) are plotted at different voltages. Dashed lines are IC₅₀ = 1.97 μM, n_H = 1.01 from the ΔV_0.5-[Cu²⁺] relation fit in panel C. (F) Apparent charge (Z_app) from Boltzmann G_K-V fits in A are plotted versus [Cu²⁺] and fit by a Hill equation (IC₅₀ = 0.72 ± 0.12 μM, n_H = 1.1 ± 0.2). Curves A and B are the predictions of Eqs. 1 and 2, respectively, using parameters from C.