Stereoselectivity of bupivacaine in local anesthetic-sensitive ion channels of peripheral nerve

Background: The local anesthetic bupivacaine exists in two stereoisomeric forms, R(+)- and S(-)-bupivacaine. Because of its lower cardiac and central nervous system toxicity, attempts were made recently to introduce S(-)-bupivacaine into clinical anesthesia We investigated stereoselective actions of R(+)and S(-)-bupivacaine toward two local anesthetic-sensitive ion channels in peripheral nerve, the Na+ and the flicker K+ channel. Methods: In patch-clamp experiments on enzymatically demyelinated peripheral amphibian nerve fibers, Na+ and flicker K+ channels were investigated in outside-out patches. Half-maximum inhibiting concentrations (IC50) were determined. For the flicker K+ channel, simultaneous block by R(+)-bupivacaine and S(-)-bupivacaine was analyzed for competition and association (k(1)) and dissociation rate constants (k(-1)) were determined. Results: Both channels were reversibly blocked by R(+)- and S(-)-bupivacaine. The IC50 values (+/-SEM) for tonic Na+ channel block were 29 +/- 3 mu M and 44 +/- 3 mu M, respectively. IC50 values for flicker K+ channel block were 0.15 +/- 0.02 mu M and 11 +/- 1 mu M, respectively, resulting in a high stereopotency ratio (+/-) of 73. Simultaneously applied enantiomers competed for a single binding site. Rate constants k(1) and k(-1) were 0.83 +/- 0.13 x 10(6) M-1.s(-1) and 0.13 +/- 0.03 s(-1), respectively, for R(+)-bupivacaine and 1.90 +/- 0.20 x 10(6) M-1.s(-1) and 8.3 +/- 1.0 s(-1), respectively, for S(-)-bupivacaine. Conclusions: Bupivacaine block of Na+ channels shows no salient stereoselectivity. Block of flicker K+ channels has the highest stereoselectivity ratio of bupivacaine action known so far. This stereoselectivity derives predominantly from a difference in k(-1) suggesting a tight fit between R(+)-bupivacaine and the binding site. The flicker K+ channel may play an important role in yet unknown toxic mechanisms of R(S)-bupivacaine.