Effects of 3 mM Ni²⁺, with 110 mM Ca²⁺ or Ba²⁺. (A and B) Inhibition of IIV currents by Ni²⁺ in Ca²⁺ (A) and Ba²⁺ (B). (C) Inhibition of chord conductances by Ni²⁺, calculated from the data of A and B. (D and E) Effects of Ni²⁺ on tail current time constants in Ca²⁺ (D) and Ba²⁺ (E). (F) Pseudo first-order rate constants for Ni²⁺ block, calculated from biexponential fits. (G and H) Effects of 3 mM Ni²⁺ on I-V relationships in 110 mM Ca²⁺ (G) and 110 mM Ba²⁺ (H). (I) Effect of Ni²⁺ on chord conductances, from the data of G and I. Smooth curves are Woodhull (1973) fits to data in Ca²⁺ (solid curve) and Ba²⁺ (dotted curve), constrained to have the same voltage dependence (see text for parameters). n = 5 (Ca²⁺), n = 4 (Ba²⁺).