The addition of dilute epinephrine produces equieffectiveness of bupivacaine enantiomers for cutaneous analgesia in the rat.

UNLABELLED: We investigated the effectiveness for cutaneous analgesia of bupivacaine (Bup) stereoisomers in male rats. As a model of infiltration anesthesia, inhibition of a nocifensive reflex by subcutaneous injection of 0.6 mL of different concentrations of R-, S-, and racemic-Bup was evaluated quantitatively by the fraction of times a pinprick failed to evoke a nocifensive motor response. R-Bup was more potent in the extent of block; however, S-Bup had a longer-lasting action at smaller doses. This significant difference was apparent when R-Bup and S-Bup were administered in equipotent doses of 0.06% and 0.075%, respectively. Co-injection of epinephrine (Epi) with these equipotent doses enhanced and prolonged the blocking effects of both Bup stereoisomers, although at dilutions of 1:100,000 to 1:1,000,000 Epi itself induced partial, transient analgesia. At 1:2,000,000 dilution, Epi alone had no analgesic effect; however, when co-injected with the shorter-acting R-Bup (0. 06%), Epi prolonged its blocking effect to equal the duration of block evoked by equipotent S-Bup (0.075%). We conclude R-Bup is more potent for cutaneous analgesia and that the longer duration of block by S-Bup probably originates from vasoconstrictor activity. IMPLICATIONS: Here we show that the more potent optical R-isomer of bupivacaine (Bup) can be used at a smaller dose (80%) than the S-isomer of Bup to give equal pain relief of a skin prick. Although the analgesia from R-Bup is briefer than that from equipotent S-Bup solutions, the durations become equal when a very dilute solution of the vasoconstrictor epinephrine is mixed with the R-isomer. The resulting vasoconstriction thus reduces vascular drug uptake and peak blood levels of systemic drug, reducing potential toxicity.