Concentration dependence for Zn²⁺ effects on activation kinetics. (A) Concentration dependence for the slowing of activation time constants at 10 mV (τ_act(10mV)) determined with 4 mM free Ca²⁺ as the charge carrier at pH 7.4 without (open squares, n = 5–12 cells per Zn²⁺ concentration) or with DEPC pretreatment (red squares, n = 5–11 cells per Zn²⁺ concentration), 2 mM free Ca²⁺ as the charge carrier at pH 7.4 (green diamonds, n = 5–8 cells per Zn²⁺ concentration) or 4 mM free Ca²⁺ as the charge carrier at pH 7.0 (purple triangles, n = 5–11 cells per Zn²⁺ concentration). Solid lines are the combination of two binding isotherms for simple bimolecular reactions. (B and C) Extrapolated maximum slowing (E_max; B) and logarithm of apparent dissociation constants (logK_Zn; C) for the high-affinity (left) and low-affinity (right) components obtained from fits to the data in A. Shown are best-fit values and 84% CIs.