MOLECULAR DETERMINANTS OF STATE-DEPENDENT BLOCK OF NA-ANESTHETICS( CHANNELS BY LOCAL)

Sodium ion (Na+) channels, which initiate the action potential in elec trically excitable cells, are the molecular targets of local anestheti c drugs. Site-directed mutations in transmembrane segment S6 of domain IV of the Na+ channel alpha subunit from rat brain selectively modifi ed drug binding to resting or to open and inactivated channels when ex pressed in Xenopus oocytes. Mutation F1764A, near the middle of this s egment, decreased the affinity of open and inactivated channels to 1 p ercent of the wild-type value, resulting in almost complete abolition of both the use-dependence and voltage-dependence of drug block, where as mutation N1769A increased the affinity of the resting channel 15-fo ld. Mutation I1760A created an access pathway for drug molecules to re ach the receptor site from the extracellular side. The results define the location of the local anesthetic receptor site in the pore of the Na+ channel and identify molecular determinants of the state-dependent binding of local anesthetics.