Analysis of Ni²⁺ effects on current ratios during large depolarizations. Current ratios were calculated as in Fig. 6, for the same cells as Fig. 2. Ratios were measured at the end of 5-ms depolarizations in Ca²⁺ (A) and Ba²⁺ (D). Time constants, from single exponential fits to current ratios in Ca²⁺ (B) or Ba²⁺ (E). Rate constants for relief of block by Ni²⁺, calculated from the fractional block (A and D) and time constants (B and E), from Eqs. 2 and 3, in Ca²⁺ (C) and Ba²⁺ (F).