A, the fast rate of recovery from inactivation. B, the slow rate of recovery. C, the ratio of the amplitude of the fast and slow rate constants, Afast/Aslow. The slow time constant dominates recovery in fKv1.4ΔN channels (no quinidine present, Afast/Aslow= 0.33 ± 0.05), and the fast time constant dominates fKv1.4[V561A] recovery from inactivation (no quinidine present, Afast/Aslow= 2.40 ± 0.37). In the presence of 500 μm quinidine the amplitude ratio of fKv1.4[V561A]ΔN recovery is close to 1 (Afast/Aslow= 0.98 ± 0.27), and in the presence of 1 mm quinidine the ratio of the time constants in fKv1.4[V561A]ΔN (Afast/Aslow= 0.22 ± 0.10) reverts to a ratio similar to that found in the unmutated fKv1.4ΔN construct. Significant differences from fKv1.4ΔN data are denoted by asterisks (P < 0.01).