Block of human heart hH1 sodium channels by the enantiomers of bupivacaine

Background: S(-)-bupivacaine reportedly exhibits lower cardiotoxicity but s imilar local anesthetic potency compared with R(+)-bupivacaine, The bupivac aine binding site in human heart (hH1) Na+ channels has nor been studied to date. The authors investigated the interaction of bupivacaine enantiomers with hH1 Na+ channels, assessed the contribution of putatively relevant res idues to binding, and compared the intrinsic affinities to another isoform, the rat skeletal muscle (mu 1) Na+ channel. Methods: Human heart and mu 1 Na+ channel alpha subunits were transiently e xpressed in HEK293t cells and investigated during whole cell voltage-clamp conditions. Using site-directed mutagenesis, the authors created point muta tions at positions hH1-F1760, hH1-N1765, hH1-Y1767, and hH1-N406 by introdu cing the positively charged lysine (K) or the negatively charged aspartic a cid (D) and studied their influence on state-dependent block by bupivacaine enantiomers. Results: Inactivated hH1 Na+ channels displayed a weak stereoselectivity wi th a stereopotency ratio (+/-) of 1.5. In mutations hH1-F1760K and hH1-N176 5K, bupivacaine affinity of inactivated channels was reduced by similar to 20- to 40-fold, in mutation hH1-N406K by similar to sevenfold, and In mutat ions hH1-Y176TK and hH1-Y1767D by similar to twofold to threefold. Changes In recovery of inactivated mutant channels from block paralleled those of i nactivated channel affinity. Inactivated hH1 Na+ channels exhibited a sligh tly higher intrinsic affinity than mu 1 Na+ channels. Conclusions: Differences in bupivacaine stereoselectivity and intrinsic aff inity between hill and mu 1 Na+ channels are small and most likely of minor clinical relevance. Amino acid residues in positions hH1-F1760, hH1-N1765, and hH1-N406 may contribute to binding of bupivacaine enantiomers in hH1 N a+ channels, whereas the role of hH1-Y1767 remains unclear.