Accumulation of inactivation in a cloned transient K+ channel (AKv1.1a) of Aplysia.

1. Inactivation of a cloned Aplysia K+ channel, AKv1.1a, expressed in Xenopus oocytes was examined by a cell-attached macropatch recording. A fast macroscopic inactivation (the time constant for decay was in the range of 20-40 ms) in response to a depolarizing command pulse was insensitive to the concentration of external K+ (2-100 mM KCl). 2. By contrast, recovery from inactivation was extremely slow and dependent on external K+. When the concentration of external KCl was 2-3 mM, a patched membrane had to be held at hyperpolarized potential for > 40 s for a full recovery. The recovery was greatly accelerated if external K+ concentration was increased. A tail current following a command pulse long enough to inactivate most of the channels showed a marked rising phase. 3. A consequence of the slow recovery from inactivation was that AKv1.1a showed a marked accumulation of the inactivation following repetitive pulses, even at low frequency (< 0.1 Hz). When two depolarizing pulses were applied at a short interval, the current during a second pulse was smaller than the current at the end of the preceding pulse. This is a phenomenon called "cumulative inactivation." The onset and the extent of cumulative inactivation of AKv1.1a were voltage dependent but relatively insensitive to external K+ concentration. An amino terminal deletion mutant of AKv1.1a that lacks the fast N-type inactivation did not show cumulative inactivation. 4. These results suggest that the inactivation gating by the amino terminal region of AKv1.1a has a similarity to open-channel blockade, and that the cumulative inactivation can also be dependent on the amino terminal cytoplasmic domain of K+ channels.