The desensitization time course of GLIC I9′A. Macroscopic-current responses of the indicated constructs upon stepping the pH of the extracellular solution from 9.0 to 4.5. The displayed traces are representative recordings obtained in the whole-cell configuration at −80 mV. (A) WT GLIC. In this recording, the current decayed to a zero-current level with a time-constant of ∼0.66 s. This means that it took only ∼3.0 s for the WT current to decay to ∼1% of its peak value. (B) GLIC I9′A. The brief downward current deflections reflect the instability of the membrane during prolonged exposure to pH 4.5; eventually, the seals broke, and the recordings ended. The duration of the pH 4.5 application before the seals broke is indicated for each I9′A trace. Evidently, desensitization is greatly slowed down by the mutation. The time scale is the same for all traces in A and B (with the exception of those in the inset of A).