The effect of bupivacaine hydrochloride on skeletal muscle fiber type transformation by low frequency electrical stimulation.

Bupivacaine hydrochloride (bupivacaine HCl) is known to induce the degeneration and regeneration of skeletal muscles. We examined the priming effect of chronic and continuous nerve stimulation (5 Hz) on regenerating pectoralis major muscle after a single injection of bupivacaine HCl (3 mg/kg). Pectoralis muscle stimulated for 24 days after bupivacaine HCl injection contained 55.4 +/- 8.2% type 1 muscle fibers (n = 4). Control pectoralis major muscle stimulated for 34 days without bupivacaine HCl injection, in contrast, contained 37.7 +/- 5.5% type 1 fibers (n = 5). Therefore, the stimulated pectoralis major muscle with bupivacaine HCl injection contained a significantly larger number of type 1 fibers than that without bupivacaine HCl injection. Undifferentiated myosatellite cells produced by bupivacaine HCl may be primed to differentiate into type 1 cells by chronic low frequency electrical stimulation.