Block by 4-aminopyridine of a Kv1.2 delayed rectifier K+ current expressed in Xenopus oocytes.
1. The blocking action of 4-aminopyridine (4-AP) on a delayed rectifier Kv1.2 K+ channel expressed in oocytes was investigated at room temperature (22 degrees C) and physiological temperature (34 degrees C) using the double-electrode voltage clamp and patch clamp techniques. 2. At room temperature, 4-AP (100 microM) inhibition occurred only after activation of current. The rate of onset of block was dependent upon the length of time current was activated by a depolarizing step. Similarly, removal of block required current activation. The degree of steady-state block by 4-AP was not reduced by increasingly more depolarized step potentials. The degree of steady-state block also did not change over the duration of a 1 s step. 3. When channels were nearly fully inactivated, 4-AP produced no additional block of a subsequent depolarizing step, suggesting that 4-AP did not bind when channels were in the inactivated state. In single channel experiments, 4-AP decreased the mean open time in a dose-dependent manner but did not alter the single-channel current amplitude. 4. At 34 degrees C the I-V relationship and inactivation curve shifted to more negative potentials. Increasing the temperature to 34 degrees C did not alter the degree of block by 4-AP, although the rate of onset of block was greatly enhanced. 5. Results suggest that 4-AP binds to the open state of the Kv1.2 channel and is trapped when the channel closes. 4-AP cannot bind when the channel is closed or inactivated prior to the addition of the drug. C-type inactivation and 4-AP binding to the channel are mutually exclusive. A model for the proposed mechanism of action of 4-AP on the Kv1.2 channel is proposed based on experimental data.
Russell
S N
SN
Department of Physiology, University of Nevada, School of Medicine, Reno 89557.
Publicover
N G
NG
Hart
P J
PJ
Carl
A
A
Hume
J R
JR
Sanders
K M
KM
Horowitz
B
B
D015761:4-Aminopyridine / Q000494:pharmacology*; D000818:Animals; D018076:DNA, Complementary / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D004594:Electrophysiology; D005260:Female; D006358:Hot Temperature; D007700:Kinetics; D008954:Models, Biological; D009865:Oocytes; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000187:drug effects* / Q000502:physiology; D014981:Xenopus
D018076:DNA, Complementary; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D015761:4-Aminopyridine