FIGURE 4.

A comparison of the residual current behavior, done in 3.5 mM K_o⁺ with a concentration of H_o⁺ (A) or Ni²⁺ (B) estimated to block 80–95% of the channels, reveals divergent effects on inactivation. From a holding potential of −80 mV, the voltage protocol consisted of a 5-s step to 50 mV followed by 50-ms steps at variable intervals to 50 mV to monitor recovery from inactivation. Current during the 5-s pulse is shown expanded on the right side of the figure. (A) At pH 5.4 the inactivation of current during the 5-s pulse is well fitted by a single exponential with a time constant of 91 ms. Peak currents, marked by the filled circles, that were evoked by the 50-ms test pulses were fitted to a single exponential with a time constant of 4.3 s. (B) In the same cell after switching to solution containing 2 mM Ni²⁺ at pH 7.4, inactivation was ≈20 times slower (τ_inact = 1.69 s) and recovery from inactivation was ≈5 times slower (τ_recovery = 24.8 s) than at pH 5.4.