Fast-onset lidocaine block of rat Na-v1.4 channels suggests involvement of a second high-affinity open state

摘要

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 mu 1 Na-v1.4 (rSkM1) with beta(1) (WT-beta(1)), and a mutant form (IFM-QQQ mutation in the III-IV interdomain, QQQ) lacking fast inactivation, in combination with Markov kinetic gating models. 100 mu M lidocaine induced fast-onset (T-onset approximate to 2 ms), long-lived (T-recovery approximate to 120 ms) block of WT-beta(1) 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 QQQcurrents. The DOB model was extended to include trapping fast-inactivation and activation gates, and a second open state (OS2); the latter arising from DIVS4 trans location that precedes inactivation and exhibits high-affinity, lidocaine binding (apparent K-d = 25 mu M) that accords with VSB (DOB-S2(VSB) mechanism). The DOB-S2(VSB) kinetic model predicted fast-onset block of WT-beta(1). The findings support the involvement of a second, high-affinity, open state in lidocaine modulation of Na+ channels. (C) 2016 Elsevier B.V. All rights reserved.