Effects of alcohols and anesthetics on recombinant voltage-gated Na+ channels

Voltage-gated Na+ channels (Na+ channels) mediate the rising phase of action potentials in neurons and excitable cells. Nine subtypes of the alpha subunit (Na(v)1.1-Na(v)1.9) have been shown to form functional Na+ channels to date. Recently, anesthetic concentrations of volatile anesthetics and ethanol were reported to inhibit Na+ channel functions, but it is not known whether all subtypes are inhibited by anesthetics. To investigate possible subtype-specific effects of anesthetics on Na+ channels, mRNA of Na(v)1.2, Na(v)1.4, Na(v)1.6, and Na(v)1.8 alpha subunit-encoded genes were injected individually or together with a beta subunit mRNA into Xenopus oocytes. Na+ currents were recorded using the two-electrode voltage-clamp technique. Isoflurane, at clinically relevant concentrations, inhibited the currents produced by Na(v)1.2, Na(v)1.4, and Na(v)1.6 by similar to10% at the holding potential of -90 mV and by similar to30% at -60 mV, but it did not affect the Na(v)1.8-mediated current. An anesthetic fluorocyclobutane (1-chloro-1,2,2-trifluorocyclobutane) also inhibited the Na(v)1.2 channel, whereas the nonanesthetic fluorocyclobutane (1,2-dichlorohexafluorocyclobutane) had no effect. The perfluorinated heptanol [CF3(CF2),CH2OH], which produces anesthesia, inhibited the Na(v)1.2 channel like other alcohols tested (ethanol, heptanol, and CF3CH2OH), even though this compound does not affect GABA, glycine, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, or kainate receptors. In contrast, most intravenous anesthetics did not have significant effects on the Na(v)1.2 channel at clinically relevant concentrations although urethane inhibited. These results show that isoflurane inhibits the Na+ channel functions except Na(v)1.8 in a voltage-dependent manner. These findings indicate that the Na+ channel is a neuronal target for anesthetic action.