Slow gating charge immobilization in the human potassium channel Kv1.5 and its prevention by 4-aminopyridine.
1. The relationship between ionic current inactivation and immobilization of 'off'-gating charge in human Kv1.5 channels expressed in human embryonic kidney (HEK293) cells was studied using 4-aminopyridine (4-AP) and tetraethylammonium chloride (TEA-Cl). 2. The charge transferred during short (< 10 ms) depolarizations (Q(on)) was conserved on repolarization (Q(off)) although peak off-gating current (off-Ig) was reduced and the time course prolonged (tau decay increased from 0.4 to > 1.2 ms). For +80 mV pulses longer than 50 ms, Q(off) at 20 ms was less than Q(on) (Q(off)/Q(on) ratio was 0.26 +/- 0.06 at 450 ms). We attribute this to a relative 'immobilization' of gating charge during long depolarizations. 3. 4-AP (0.1-1 mM) prevented slowing of off-Ig, allowing saturation of peak off-Ig. 4-AP also completely prevented immobilization of off-Ig after long depolarizations. In 1 mM 4-AP, off-Ig waveforms decayed rapidly and the charge ratio Q(off)/Q(on) remained at 1.0. 4. In addition to its effects on Ig, 1 mM 4-AP prevented the slow inactivation of ionic current seen during strong depolarizations. An initial block was caused by 4-AP or 1 mM intracellular TEA internally applied. However, only 4-AP prevented the slower, later development of C-type inactivation. 5. We suggest that slow current inactivation is accompanied by a gating charge immobilization in Kv1.5. 4-AP potently inhibits the changes in Q(off)/Q(on0, off-Ig, and ionic currents that underlie slow inactivation. Some actions of 4-AP appear independent of its properties as a blocker of open K+ channels, and are not mimicked by internal TEA.
Fedida
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Department of Physiology, Queen's University, Kingston, Ontario, Canada. fedidad@post.queensu.ca
Bouchard
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Chen
F S
FS
D015761:4-Aminopyridine / Q000494:pharmacology*; D002460:Cell Line; D004553:Electric Conductivity; D006801:Humans; D015640:Ion Channel Gating; D007668:Kidney; D007700:Kinetics; D051666:Kv1.5 Potassium Channel; D008564:Membrane Potentials / Q000187:drug effects; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D024642:Potassium Channels, Voltage-Gated; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013997:Time Factors; D014162:Transfection
C496981:KCNA5 protein, human; D051666:Kv1.5 Potassium Channel; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D015761:4-Aminopyridine