A molecular basis for the different local anesthetic affinities of resting versus open and inactivated states of the sodium channel.
Voltage-gated sodium channels are inhibited by local anesthetic drugs. This inhibition has complex voltage- and frequency-dependent properties, consistent with a model in which the sodium channel has low affinity for local anesthetics when it is in resting states and higher affinity when it is in open or inactivated states. Two residues, a phenylalanine (F1710) and a tyrosine (Y1717), in transmembrane segment IVS6 of the channel alpha subunit are critical for state-dependent block. We examined how these residues determine channel sensitivity to local anesthetics by introducing mutations that varied their size, hydrophobicity, and aromaticity. Block of resting channels by tetracaine was correlated with hydrophobicity at position 1710, as if hydrophobic drug-receptor interactions stabilize binding to resting states. In contrast, drug action on open or inactivated channels required an aromatic residue at this position. We propose that the native phenylalanine at position 1710 stabilizes drug binding to open or inactivated states by either cation-pi or aromatic-aromatic interactions between the aromatic side chain of the amino acid and charged or aromatic moieties on the drug molecule. We also consider the alternative possibility that mutations at this position affect drug action by either altering access to the receptor or by allosteric changes in receptor conformation. Mutations at position 1717 also altered drug action; however, these effects were not well-correlated with the size, hydrophobicity, or aromaticity of the substituted amino acid. These results suggest that the residue at this position does not contribute directly to the drug receptor.