title: Fast-onset lidocaine block of rat NaV1.4 channels suggests involvement of a second high-affinity open state.
abstract: Local anesthetics (LAs) block resting, open, and inactivated states of voltage-gated Na(+) channels where inactivated states are thought to bind with highest affinity. However, reports of fast-onset block occurring over milliseconds hint at high-affinity block of open channels. Movement of voltage-sensor domain IV-segment 4 (DIVS4) has been associated with high affinity LA block termed voltage-sensor block (VSB) that also leads to a second open state. These observations point to a second high-affinity open state that may underlie fast-onset block. To test for this state, we analyzed the modulation of Na(+) currents by lidocaine and its quaternary derivative (QX222) from heterologously expressed (Xenopus laevis oocytes) rat skeletal muscle μ1 NaV1.4 (rSkM1) with β1 (WT-β1), and a mutant form (IFM-QQQ mutation in the III-IV interdomain, QQQ) lacking fast inactivation, in combination with Markov kinetic gating models. 100 μM lidocaine induced fast-onset (τonset≈2 ms), long-lived (τrecovery≈120 ms) block of WT-β1 macroscopic currents. Lidocaine blocked single-channel and macroscopic QQQ currents in agreement with our previously described mechanism of dual, open-channel block (DOB mechanism). A DOB kinetic model reproduced lidocaine effects on QQQ currents. The DOB model was extended to include trapping fast-inactivation and activation gates, and a second open state (OS2); the latter arising from DIVS4 translocation that precedes inactivation and exhibits high-affinity, lidocaine binding (apparent Kd=25 μM) that accords with VSB (DOB-S2VSB mechanism). The DOB-S2VSB kinetic model predicted fast-onset block of WT-β1. The findings support the involvement of a second, high-affinity, open state in lidocaine modulation of Na(+) channels.
authors.affiliation: Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, Dallas, TX, United States; Department of Anesthesiology, University of Rochester School of Medicine, Rochester, NY, United States; Department of Pharmacology and Physiology, University of Rochester School of Medicine, Rochester, NY, United States. Electronic address: kevin.gingrich@utsouthwestern.edu.
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Gingrich
authors.affiliation: Department of Anesthesiology, University of Rochester School of Medicine, Rochester, NY, United States; Department of Pharmacology and Physiology, University of Rochester School of Medicine, Rochester, NY, United States.
authors.forename: Larry E
authors.initials: LE
authors.lastname: Wagner
mesh_terms: D000818:Animals; D008012:Lidocaine / Q000494:pharmacology*; D009124:Muscle Proteins / Q000037:antagonists & inhibitors* / Q000737:chemistry; D051381:Rats; D015222:Sodium Channels / Q000737:chemistry; D061567:Voltage-Gated Sodium Channel Blockers / Q000494:pharmacology*; D014982:Xenopus laevis
chemical_list: D009124:Muscle Proteins; C466180:Scn4a protein, rat; D015222:Sodium Channels; D061567:Voltage-Gated Sodium Channel Blockers; D008012:Lidocaine
keywords: Local anesthetic; Markov models; Open-channel block; Sodium channels; Voltage-clamp; Xenopus oocytes