On the mechanism by which 4-Aminopyridine occludes quinidine block of the cardiac K+ channel, hKv1.5.
4-Aminopyridine (4-AP) binds to potassium channels at a site or sites in the inner mouth of the pore and is thought to prevent channel opening. The return of hKv1.5 off-gating charge upon repolarization is accelerated by 4-AP and it has been suggested that 4-AP blocks slow conformational rearrangements during late closed states that are necessary for channel opening. On the other hand, quinidine, an open channel blocker, slows the return or immobilizes off-gating charge only at opening potentials (>-25 mV). The aim of this study was to use quinidine as a probe of open channels to test the kinetic state of 4-AP-blocked channels. In the presence of 0.2-1 mM 4-AP, quinidine slowed charge return and caused partial charge immobilization, corresponding to an increase in the Kd of approximately 20-fold. Peak off-gating currents were reduced and decay was slowed approximately 2- to 2.5-fold at potentials negative to the threshold of channel activation and during depolarizations shorter than normally required for channel activation. This demonstrated access of quinidine to 4-AP-blocked channels, a lack of competition between the two drugs, and implied allosteric modulation of the quinidine binding site by 4-AP resident within the channel. Single channel recordings also showed that quinidine could modulate the 4-AP-induced closure of the channels, with the result that frequent channel reopenings were observed when both drugs were present. We propose that 4-AP-blocked channels exist in a partially open, nonconducting state that allows access to quinidine, even at more negative potentials and during shorter depolarizations than those required for channel activation.
Chen
F S
FS
Department of Physiology, Botterell Hall, Queen's University, Kingston, Ontario, Canada K7L 3N6.
Fedida
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D015761:4-Aminopyridine / Q000494:pharmacology*; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology*; D002460:Cell Line; D004553:Electric Conductivity; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D051666:Kv1.5 Potassium Channel; D009206:Myocardium / Q000378:metabolism*; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000378:metabolism / Q000502:physiology; D024642:Potassium Channels, Voltage-Gated; D011802:Quinidine / Q000494:pharmacology*; D013997:Time Factors
D000889:Anti-Arrhythmia Agents; C496981:KCNA5 protein, human; D051666:Kv1.5 Potassium Channel; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D015761:4-Aminopyridine; D011802:Quinidine