Binding affinity and stereoselectivity of local anesthetics in single batrachotoxin-activated Na+ channels

Several local anesthetics (LA) have been previously shown to block muscle batrachotoxin (BTX)-activated Na+ channels in planar bilayers. The mean dwell time of different LA drugs, however, varies widely, from less than 10 ms to longer than several seconds. In this study, we have examined the structural determinants that govern the dwell time, the binding affinity, and the stereoselectivity of LA drugs using cocaine and bupivacaine homologues, RAC compounds, and their available stereoisomers. Our results from the structure-activity experiments reveal that (a) there are two apparent hydrophobic binding domains present in the LA binding site; one interacts with the aromatic moiety of the LA drugs, and the other interacts with the alkyl group attached to the tertiary amine of the LA drugs; (b) the LA mean dwell time and the binding affinity are largely determined by the hydrophobic interactions; (c) the LA binding site is highly stereoselective, with a difference in KD values over 50- and 6-fold for (+/-) cocaine and (+/-) bupivacaine, respectively; (d) the cocaine stereoselectivity is comparable among muscle, brain, and heart BTX-activated Na+ channels; and finally and most unexpectedly, (e) the stereoselectivity of LA drugs in BTX-activated Na+ channels appears greatly different from that reported in normal Na+ channels. Possible explanations for this difference are discussed.