Local anesthetics disrupt energetic coupling between the voltage-sensing segments of a sodium channel

Abstract

Local anesthetics block sodium channels in a state-dependent fashion, binding with higher affi nity to open and/or inactivated states. Gating current measurements show that local anesthetics immobilize a fraction of the gating charge, suggesting that the movement of voltage sensors is modifi ed when a local anesthetic binds to the pore of the sodium channel. Here, using voltage clamp fl uorescence measurements, we provide a quantitative description of the effect of local anesthetics on the steady-state behavior of the voltage-sensing segments of a sodium channel. Lidocaine and QX-314 shifted the midpoints of the fl uorescence-voltage (F-V) curves of S4 domain III in the hyperpolarizing direction by 57 and 65 mV, respectively. A single mutation in the S6 of domain IV (F1579A), a site critical for local anesthetic block, abolished the effect of QX-314 on the voltage sensor of domain III. Both local anesthetics modestly shifted the F-V relationships of S4 domain IV toward hyperpolarized potentials. In contrast, the F-V curve of the S4 domain I was shifted by 11 mV in the depolarizing direction upon QX-314 binding. These antagonistic effects of the local anesthetic indicate that the drug modifi es the coupling between the voltage-sensing domains of the sodium channel. Our fi ndings suggest a novel role of local anesthetics in modulating the gating apparatus of the sodium channel. © 2009 Muroi and Chanda.