Comparison of accumulative inactivation between the Aplysia K+ channel (AKv1.1a) and its amino-terminal deletion mutant.

Accumulative inactivation of a cloned Aplysia K+ channel (AKv1.1a) was examined in Xenopus oocyte expression system by the patch clamp technique. AKv1.1a inactivates by both N-type and C-type mechanisms. The amino-terminal domain of the channel is indispensable for N-type inactivation, whereas other parts of the channel is involved in C-type inactivation. The accumulative inactivation induced by repetitive pulses (0.2-0.5 Hz) was relatively insensitive to the pulse duration (10-900 msec). The accumulative inactivation was inhibited when the external K+ concentration ([K+]out) was increased, or when tetraethylammonium (TEA) was added in the external solution. The accumulative inactivation of the amino-terminal deletion mutant (delta N) which lacks N-type inactivation was dependent on the pulse duration such that it was less pronounced for short repetitive pulses (< 100 msec). The accumulative inactivation of delta N was also inhibited by high [K+]out and external TEA. By contrast, the accumulative inactivation induced by pair-pulsed protocol was not perturbed by external TEA, and was not observed in delta N. The accumulative inactivation of AKv1.1a was enhanced when the membrane patch was excised out of the cell. Paradoxically, the macroscopic inactivation of AKv1.1a became slower in the excised patch. The accumulative inactivation of the delta N was less sensitive to the patch excision. Some synthetic peptides which were designed based on the amino-terminal sequences of K+ channels induced a use-dependent block of delta N which was apparently similar to the inactivation of AKv1.1a. Our results suggest that either N-type or C-type inactivation can induce the accumulative inactivation of K+ channels, and the C-type inactivation coupled to N-type inactivation plays substantial roles in the frequency dependent accumulative inactivaton of AKv1.1a.