BI-STABLE BLOCK BY 4-AMINOPYRIDINE OF A TRANSIENT K-CENTER-DOT-4) CLONED FROM FERRET VENTRICLE AND EXPRESSED IN XENOPUS OOCYTES( CHANNEL (KV1)

1. Using the two-microelectrode, 'cut open' oocyte, and 'torn off' mac ropatch voltage clamp techniques, we studied the blocking effects of 4 -aminopyridine (4-AP) on two cloned K+ channels expressed in Xenopus o ocytes, 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, imperme ant 4-AP derivative, 4-aminopyridine-methyliodide, indicated that the cationic form of 4-AP blocks at an intracellular site. 3. Block accumu lated from pulse to pulse and was sensitive to the applied potential d uring 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 duratio n. Block of FK1 was much more sensitive to pulse duration than was blo ck of Delta Nco, consistent with competition between N-type inactivati on and 4-AP binding. 4. To elucidate these mechanisms further, in the absence of fast N-type inactivation the following results were obtaine d 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 t hat 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 blockin g scheme.