Extracellular Cd²⁺ shifts the Q-V relationship for hERG1 to more positive potentials. (A) Normalized Q_OFF is plotted as a function of V_t and [Cd²⁺]_e. Averaged data for control and each [Cd²⁺]_e were fitted to a Boltzmann function (smooth curves). (B) Plot of change in V_1/2 for Q-V and G-V relationships as a function of [Cd²⁺]_e. Data were fitted to a Hill equation to determine EC₅₀ and Hill coefficient (n_H). ΔV_1/2max was set at +90 mV (Fernandez et al., 2005). For Q-V relationship, EC₅₀ = 171 ± 13 µM; n_H = 0.53 ± 0.03. For G-V relationship, EC₅₀ = 416 ± 20 µM; n_H = 0.57 ± 0.02. The number of oocytes used to calculate the average data is indicated next to each data point. (C) [Cd²⁺]_e-dependent effects on V_1/2 for Q-V relationship. (D) [Cd²⁺]_e-dependent effects on effective valence, z, of Boltzmann function for Q-V relationship. For A, C, and D, the number of oocytes was as follows: control, 20; 3 µM Cd²⁺ , 8; 10 µM Cd²⁺, 7; 30 µM Cd²⁺, 12; 100 µM Cd²⁺, 10; 300 µM Cd²⁺, 14.