TIME-COURSE OF THE REGENERATIVE RESPONSE IN BUPIVACAINE INJURED ORBICULARIS OCULI MUSCLE

Local anesthetics, particularly bupivacaine, are known to be myotoxic to skeletal muscle. Injury is followed by satellite cell mediated rege neration. The eyelid is a common site for the injection of local anest hetics. Due to the complex anatomy of this region and the unique prope rties of facial musculature compared to limb skeletal muscle, the resp onse of the orbicularis oculi to local injection of bupivacaine was ex amined to determine the time course of maximum satellite cell activati on and division. The lower eyelids of rabbits were injected with two d oses of a combination of bupivacaine and hyaluronidase, spaced 18 h ap art. To assess the time course of satellite cell division, bromodeoxyu ridine (BrdU) was injected immediately or, 1, 2, 3, 6 or 13 days after the second bupivacaine injury. The rabbits were sacrificed 24 h later . The eyelids were prepared for immunohistological examination and mor phometric analysis of the presence of CD Ii-positive monocytes, neutro phils and macrophages, MyoD expression in satellite cells and/or myobl asts, and co-expression of BrdU and the developmental myosin heavy cha in isoform. One day after bupivacaine injury of the orbicularis oculi, there was a large influx of CD11-positive cells which gradually decre ased over time. Maximum activation of satellite cells, as defined by M yoD expression, occurred 2 and 3 days after the injury. Using double l abeling techniques, the peak of BrdU incorporation occurred on day 3 a nd was identified in developmental myosin co-labeled cells 4 days afte r injury. The peak of satellite cell activation and division occurred 3 days after bupivacaine induced injury, as demonstrated by both MyoD expression and after pulse labeling with BrdU as identified in double labeled cells positive for BrdU and the developmental myosin heavy cha in isoform. The process of regeneration in this muscle extended beyond the duration of this study. Muscle fibers remained small in cross-sec tional area and positive for developmental myosin 2 weeks after injury , at a time when the fiber number had reached control, uninjured level s.