title: State-dependent block of wild-type and inactivation-deficient Na+ channels by flecainide.
abstract: The antiarrhythmic agent flecainide appears beneficial for painful congenital myotonia and LQT-3/DeltaKPQ syndrome. Both diseases manifest small but persistent late Na+ currents in skeletal or cardiac myocytes. Flecainide may therefore block late Na+ currents for its efficacy. To investigate this possibility, we characterized state-dependent block of flecainide in wild-type and inactivation-deficient rNav1.4 muscle Na+ channels (L435W/L437C/A438W) expressed with beta1 subunits in Hek293t cells. The flecainide-resting block at -140 mV was weak for wild-type Na+ channels, with an estimated 50% inhibitory concentration (IC50) of 365 micro M when the cell was not stimulated for 1,000 s. At 100 micro M flecainide, brief monitoring pulses of +30 mV applied at frequencies as low as 1 per 60 s, however, produced an approximately 70% use-dependent block of peak Na+ currents. Recovery from this use-dependent block followed an exponential function, with a time constant over 225 s at -140 mV. Inactivated wild-type Na+ channels interacted with flecainide also slowly at -50 mV, with a time constant of 7.9 s. In contrast, flecainide blocked the open state of inactivation-deficient Na+ channels potently as revealed by its rapid time-dependent block of late Na+ currents. The IC50 for flecainide open-channel block at +30 mV was 0.61 micro M, right within the therapeutic plasma concentration range; on-rate and off-rate constants were 14.9 micro M-1s-1 and 12.2 s-1, respectively. Upon repolarization to -140 mV, flecainide block of inactivation-deficient Na+ channels recovered, with a time constant of 11.2 s, which was approximately 20-fold faster than that of wild-type counterparts. We conclude that flecainide directly blocks persistent late Na+ currents with a high affinity. The fast-inactivation gate, probably via its S6 docking site, may further stabilize the flecainide-receptor complex in wild-type Na+ channels.
authors.affiliation: Department of Anesthesia, Brigham and Women's Hospital, 75 Francis St., Boston, MA 02115, USA. wang@zeus.bwh.harvard.edu
authors.forename: Ging Kuo
authors.initials: GK
authors.lastname: Wang
authors.forename: Corinna
authors.initials: C
authors.lastname: Russell
authors.forename: Sho-Ya
authors.initials: SY
authors.lastname: Wang
mesh_terms: D000889:Anti-Arrhythmia Agents / Q000008:administration & dosage / Q000494:pharmacology*; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D004594:Electrophysiology; D005424:Flecainide / Q000008:administration & dosage / Q000494:pharmacology*; D006801:Humans; D015640:Ion Channel Gating; D009124:Muscle Proteins / Q000187:drug effects / Q000502:physiology; D009154:Mutation; D026941:Sodium Channel Blockers / Q000008:administration & dosage / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism / Q000502:physiology
chemical_list: D000889:Anti-Arrhythmia Agents; D009124:Muscle Proteins; C466180:Scn4a protein, rat; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D005424:Flecainide