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 (IC(50)) 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 IC(50) values (±SEM) for tonic Na⁺ channel block were 29 ± 3 μM and 44 ± 3 μM, respectively. IC(50) values for flicker K⁺ channel block were 0.15 ± 0.02 μM and 11 + 1 μM, respectively, resulting in a high stereopotency ratio (±) of 73. Simultaneously applied enantiomers competed for a single binding site. Rate constants k₂ and k_-1 were 0.83 ± 0.13 x 10⁶ M^-1 · s^-1 and 0.13 ± 0.03 s^-1, respectively, for R(+)- bupivacaine and 1.90 ± 0.20 X 10⁶ 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(+)-bupivacaine.