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

Background: S(-)-bupivacaine reportedly exhibits lower cardiotoxicity but similar local anesthetic potency compared with R(+)-bupivacaine. The bupivacaine binding site in human heart (hH1) Na⁺ channels has not been studied to date. The authors investigated the interaction of bupivacaine enantiomers with hH1 Na⁺ channels, assessed the contribution of putatively relevant residues to binding, and compared the intrinsic affinities to another isoform, the rat skeletal muscle (μ1) Na⁺ channel. Methods: Human heart and μ1 Na⁺ channel α subunits were transiently expressed in HEK293t cells and investigated during whole cell voltage-clamp conditions. Using site-directed mutagenesis, the authors created point mutations at positions hH1-F1760, hH1-N1765, hH1-Y1767, and hH1-N406 by introducing the positively charged lysine (K) or the negatively charged aspartic acid (D) and studied their influence on state-dependent block by bupivacaine enantiomers. Results: Inactivated hH1 Na⁺ channels displayed a weak stereoselectivity with a stereopotency ratio (+/-) of 1.5. In mutations hH1-F1760K and hH1-N1765K, bupivacaine affinity of inactivated channels was reduced by ~ 20- to 40-fold, in mutation hH1-N406K by ~ sevenfold, and in mutations hH1-Y1767K and hH1-Y1767D by ~ twofold to threefold. Changes in recovery of inactivated mutant channels from block paralleled those of inactivated channel affinity. Inactivated hH1 Na⁺ channels exhibited a slightly higher intrinsic affinity than μ1 Na⁺ channels. Conclusions: Differences in bupivacaine stereoselectivity and intrinsic affinity between hH1 and μ1 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 Na⁺ channels, whereas the role of hH1-Y1767 remains unclear.