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.
Rasmusson
R L
RL
Department of Biomedical Engineering, Duke University Durham, NC 27710, USA.
Zhang
Y
Y
Campbell
D L
DL
Comer
M B
MB
Castellino
R C
RC
Liu
S
S
Strauss
H C
HC
D015761:4-Aminopyridine / Q000494:pharmacology*; D000818:Animals; D004594:Electrophysiology; D005260:Female; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D008839:Microelectrodes; D008968:Molecular Conformation; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects*; D018075:RNA, Complementary / Q000096:biosynthesis; D014982:Xenopus laevis
D015221:Potassium Channels; D018075:RNA, Complementary; D015761:4-Aminopyridine