Cardiac-specific external paths for lidocaine, defined by isoform-specific residues, accelerate recovery from use-dependent block.

Local anesthetic antiarrhythmic drugs block voltage-gated Na(+) channels from the cytoplasmic side. In addition, cardiac Na(+) channels can be also blocked by the membrane-impermeant local anesthetic QX via external paths not present in skeletal muscle or brain channels. Introduction of cardiac isoform-specific residues into wild-type skeletal muscle or brain channels creates access paths for external QX block. These paths should affect the characteristics of use-dependent block by influencing drug on- and off-rates. We investigated the effects of these external paths on drug kinetics of lidocaine, a lipophilic drug of clinical relevance, by studying use-dependent block using a two-electrode voltage clamp in Xenopus oocytes. Recovery from use-dependent block was slowed when cardiac isoform-specific residues important for external QX access were mutated to skeletal muscle or brain isoform-specific residues. As the fraction of charged lidocaine was decreased by raising external pH, differences in recovery kinetics diminished, indicating that these mutations mostly influenced block by charged lidocaine molecules. Data were fit into a model in which bound drug distributes into charged and neutral forms based on its pK(a) and external pH with separate dissociation paths and recovery-time constants. These isoform-specific mutations altered the recovery-time constants for the charged molecules with smaller effects on those for the neutral molecules. We conclude that the external egress paths created by isoform-specific residues influence the drug kinetics of lidocaine, and these residues define cardiac-specific external paths for local anesthetic drugs.