External calcium dependence of extensor digitorum longus muscle contractility during bupivacaine-induced regeneration.

The effect of the intramuscular injection of bupivacaine hydrochloride on selected morphological characteristics and contractile properties of adult rat extensor digitorum longus muscle was studied. Recovery of normal fiber size was already present 30 days after bupivacaine injection and at 90 days after injection, values of the normalized twitch tension (mN/mg of tissue) and of the fatigue index approached those measured in control muscle, whereas the normalized tetanic tension remained 57% of control. At 7-30 days postinjection, twitch force was decreased by reducing [Ca2+]zero (substituted by Mg2+) or adding Co2+ (5 mmol/L-1). By contrast potentiation of the twitch was recorded in the presence of Cd2+ (2 mmol/L-1). Glycerol treatment only reduced, but did not eliminate twitches developed by muscles 7 days after injection. Present results emphasize the importance of the recovery process in the loss of the susceptibility of the contractile responses to extracellular calcium in bupivacaine-injected muscles. These data may be of interest in the evaluation of functional aspects of muscles in which injections of viral vector or autologous myoblasts have been performed.