Fast-onset lidocaine block of rat Na_V1.4 channels suggests involvement of a second high-affinity open state

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 Na_V1.4 (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 (τ_onset 2 ms), long-lived (τ_recovery 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 K_d = 25 μM) that accords with VSB (DOB-S2_VSB mechanism). The DOB-S2_VSB 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.