Effects of Ni²⁺ on kinetics of tail currents. Current from the protocol of Fig. 1 were fitted to single exponential functions (see Materials and methods). Time constants are shown in Ca²⁺ (A) and Ba²⁺ (B). The pseudo first-order rate constant for Ni²⁺ block was calculated from fits to two exponentials for Ca²⁺ (C) or one exponential for Ba²⁺ (D), as described in supplemental Materials and methods. Same cells as Fig. 2. The solid lines in C and D are a fit to the rate constants assuming bimolecular kinetics, with equal voltage dependence for Ca²⁺ and Ba²⁺, from −30 to −70 mV (Ba²⁺) or −10 to −50 mV (Ca²⁺), including extrapolation to voltages not included in the fit. The fit to rates for 0.3 mM Ni²⁺ in 2 mM Ca²⁺ is also shown in D, for comparison to the rates in Ba²⁺ (dashed red line).