Local anesthetics disrupt energetic coupling between the voltage-sensing segments of a sodium channel.
Local anesthetics block sodium channels in a state-dependent fashion, binding with higher affinity 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 modified when a local anesthetic binds to the pore of the sodium channel. Here, using voltage clamp fluorescence 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 fluorescence-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 modifies the coupling between the voltage-sensing domains of the sodium channel. Our findings suggest a novel role of local anesthetics in modulating the gating apparatus of the sodium channel.
Muroi
Yukiko
Y
Department of Physiology, University of Wisconsin-Madison, Madison, WI 53706, USA.
Chanda
Baron
B
D000772:Anesthesia, Local; D000779:Anesthetics, Local / Q000494:pharmacology*; D000818:Animals; D007700:Kinetics; D008012:Lidocaine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D008954:Models, Biological; D009124:Muscle Proteins / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D017434:Protein Structure, Tertiary; D051381:Rats; D026941:Sodium Channel Blockers / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects* / Q000502:physiology*; D014981:Xenopus
D000779:Anesthetics, Local; D009124:Muscle Proteins; C466180:Scn4a protein, rat; D026941:Sodium Channel Blockers; D015222:Sodium Channels; C012647:QX-314; D008012:Lidocaine