SODIUM-CHANNEL SELECTIVITY FILTER REGULATES ANTIARRHYTHMIC DRUG-BINDING

Local anesthetic antiarrhythmic drugs block Na+ channels and have impo rtant clinical uses. However, the molecular mechanism by which these d rugs block the channel has not been established. The family of drugs i s characterized by having an ionizable amino group and a hydrophobic t ail. We hypothesized that the charged amino group of the drug may inte ract with charged residues in the channel's selectivity filter. Mutati on of the putative domain III selectivity filter residue of the adult rat skeletal muscle Na+ channel (mu 1) K1237E increased resting lidoca ine block, but no change was observed in block by neutral analogs of l idocaine. An intermediate effect on the lidocaine block resulted from K1237S and there was no effect from K1237R, implying an electrostatic effect of Lys, Mutation of the other selectivity residues, D400A (doma in I), E755A (domain II), and A1529D (domain IV) allowed block by exte rnally applied quaternary membrane-impermeant derivatives of lidocaine (QX314 and QX222) and accelerated recovery from block by internal QX3 14. Neo-saxitoxin and tetrodotoxin, which occlude the channel pore, re duced the amount of QX314 bound in D400A and A1529D, respectively. Blo ck by outside QX314 in E755A was inhibited by mutation of residues in transmembrane segment S6 of domain IV that are thought to be part of a n internal binding site. The results demonstrate that the Naf channel selectivity filter is involved in interactions with the hydrophilic pa rt of the drugs, and it normally limits extracellular access to and es cape from their binding site just within the selectivity filter. Parti cipation of the selectivity ring in antiarrhythmic drug binding and ac cess locates this structure adjacent to the S6 segment.