“Instantaneous” inhibition of currents by Ni²⁺. Initial current amplitudes were measured (see Materials and methods) from the protocol of Fig. 1, in 2 mM Ca²⁺ (A) or 2 mM Ba²⁺ (D). (B and E) Chord conductances, calculated from the data of A and D, respectively. (C and F) The conductance in Ni²⁺ as a fraction of the control conductance in 2 mM Ca²⁺ (C) or 2 mM Ba²⁺ (F). A ratio of 1.0 (dashed lines) represents zero inhibition. Different Ni²⁺ concentrations are indicated by the symbol shapes and color coding defined in B and E in all panels, and also in Figs. 3, 5, and 8 below. In Ca²⁺, 4 cells were tested in 0.3 mM Ni²⁺, 3 cells in 1 mM, and 5 cells in 3 mM. In Ba²⁺, 3 cells were tested in 0.1 mM Ni²⁺, 4 cells in 0.3 mM, and 4 cells in 1 mM. Note that 3 mM Ni²⁺ was used only in Ca²⁺, and 0.1 mM Ni²⁺ only in Ba²⁺.