Unexpected mexiletine responses of a mutant cardiac Na+ channel implicate the selectivity filter as a structural determinant of antiarrhythmic drug access.
Gating properties of Na(+) channels are the critical determinants for the state-dependent block by class I antiarrhythmic drugs; however, recent site-directed mutagenesis studies have shown that the Na(+) channel selectivity filter region controls drug access to and dissociation from the binding site. To validate these observations, we have exploited a naturally occurring cardiac Na(+) channel mutation, S1710L, located next to the putative selectivity filter residue of domain 4, and evaluated the pharmacological properties to mexiletine using whole-cell, patch-clamp recordings. Consistent with the large negative shift of steady-state inactivation and the enhanced slow inactivation, the S1710L channel showed greater mexiletine tonic block than wild-type (WT) channel. In contradiction, S1710L showed attenuated use-dependent block by mexiletine and accelerated recovery from block, suggesting that the drug escape though the external access path is facilitated. Extracellularly applied QX-314, a membrane-impermeant derivative of lidocaine, elicited significantly enhanced tonic block in S1710L similar to mexiletine. However, recovery from internally applied QX-314 was accelerated by 4.4-fold in S1710L compared with WT. These results suggest that the drug access to and dissociation from the binding site through the hydrophilic path are substantially altered. Moreover, K(+) permeability was 1.9-fold increased in S1710L, verifying that the mutated residue is located in the ion-conducting pore. We propose that the Na(+) channel selectivity filter region is a structural determinant for the antiarrhythmic drug sensitivity in addition to gating properties of the indigenous Na(+) channels that govern the state-dependent drug block.
Sasaki
Koji
K
Department of Cardiovascular Medicine, Hokkaido University Graduate School of Medicine, Sapporo, Japan.
Makita
Naomasa
N
Sunami
Akihiko
A
Sakurada
Harumizu
H
Shirai
Nobumasa
N
Yokoi
Hisataka
H
Kimura
Akinori
A
Tohse
Noritsugu
N
Hiraoka
Masayasu
M
Kitabatake
Akira
A
D019943:Amino Acid Substitution; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology*; D006321:Heart / Q000187:drug effects*; D006801:Humans; D007930:Leucine / Q000235:genetics; D008012:Lidocaine / Q000031:analogs & derivatives* / Q000494:pharmacology; D008801:Mexiletine / Q000494:pharmacology*; D009154:Mutation; D009206:Myocardium / Q000378:metabolism*; D012694:Serine / Q000235:genetics; D026941:Sodium Channel Blockers / Q000494:pharmacology; D015222:Sodium Channels / Q000235:genetics / Q000378:metabolism*
D000889:Anti-Arrhythmia Agents; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D008801:Mexiletine; C012647:QX-314; D012694:Serine; D008012:Lidocaine; D007930:Leucine