Bi‐stable block by 4‐aminopyridine of a transient K+ channel (Kv1.4) cloned from ferret ventricle and expressed in Xenopus oocytes.

1. Using the two‐microelectrode, ‘cut open’ oocyte, and ‘torn off’ macropatch voltage clamp techniques, we studied the blocking effects of 4‐aminopyridine (4‐AP) on two cloned K+ channels expressed in Xenopus oocytes, an inactivating K+ channel isolated from ferret ventricle (FK1), and its NH2‐terminal deletion mutant (delta NCO) which lacks fast N‐type inactivation. 2. Experiments with a permanently charged, impermeant 4‐AP derivative, 4‐aminopyridine‐methyliodide, indicated that the cationic form of 4‐AP blocks at an intracellular site. 3. Block accumulated from pulse to pulse and was sensitive to the applied potential during hyperpolarizing deactivating pulses, indicating trapping of 4‐AP in deactivated channels. For long trains of depolarizing pulses (‐90 to +50 mV, 0.1 Hz), 4‐AP block increased with decreasing pulse duration. Block of FK1 was much more sensitive to pulse duration than was block of delta NCO, consistent with competition between N‐type inactivation and 4‐AP binding. 4. To elucidate these mechanisms further, in the absence of fast N‐type inactivation the following results were obtained on delta NCO channels: (1) application of 4‐AP caused the appearance of apparent inactivation; (2) 4‐AP, however, did not cause cross‐over of deactivating tail currents; (3) 4‐AP block developed with time for potentials positive to ‐40 mV; and (4) trapping of 4‐AP by delta NCO was insensitive to the degree of C‐type inactivation. 5. We conclude that the kinetics of 4‐AP block of FK1 and delta NCO channels cannot be accounted for by either a pure open channel or closed channel blocking scheme.