Effects of Ni²⁺ on I-V relations. Currents were measured at the time of maximal current during 5-ms steps. The full voltage range is shown for Ca²⁺ (A) and Ba²⁺ (D); B and E are expanded views to better show inward currents. The ratios of chord conductances (Ni²⁺/control) are shown for Ca²⁺ (C) and Ba²⁺ (F). The smooth curves in C and F are fits assuming Ni²⁺ blocks the pore by binding at a site in the electrical field of the membrane, and cannot permeate (Woodhull, 1973), with the location of the binding site constrained to be the same for Ca²⁺ and Ba²⁺. Same cells as Fig. 2.