Myotoxicity of single and repeated injections of mepivacaine (Carbocaine) in the rat.

Young rats received single or repeated injections of 2% mepivacaine (Carbocaine) into the tibialis anterior or extensor digitorum longus muscles. Repeated injections consisted of six injections of the anesthetic (100 microliter per injection into the tibialis anterior) on three different schedules, at intervals of 2 1/2 hours, 24 hours, or 4 days. The muscles that had been given injections were examined histologically for evidence of myotoxicity at 0 to 7 and 20 days after the last injection. Single injection studies showed that mepivacaine is a myotoxic drug, producing a lesion which ultimately results in the degeneration and subsequent regeneration of large amounts of muscle. A similar picture was seen with repeated injections except that greater tissue destruction was noted. Long-term studies following a single injection of mepivacaine showed restoration of the original muscle structure whereas after repeated injections some muscles showed persisting foci of increased intersitial connective tissue. This study shows that in rats 2% mepivacaine is a myotoxic drug, but that the damage it produces is to a large extent restored by the regeneration of new muscle fibers.