HALOTHANE AND TEMPERATURE INTERACT TO INCREASE SUCCINYLCHOLINE-INDUCED JAW CONTRACTURE IN THE RAT

Background: The agonist actions of succinylcholine (SCh) have recently come under study because of their involvement in the clinical problem of masseter muscle rigidity, and their possible involvement in malign ant hyperthermia. The authors investigated factors affecting SCh-induc ed contractures in an animal preparation. Methods. Rats were anestheti zed with either halothane (1-2%) or pentobarbital. Resting and twitch isometric tension were measured from the jaw muscles. Succinylcholine (500 or 750 mug/kg) was administered intravenously, producing increase s in resting tension (i.e., contractures). Jaw muscle temperature was controlled by radiant heat. Results. Succinylcholine increased jaw mus cle tension for several seconds. These contractures exhibited tachyphy laxis, and were antagonized by vecuronium (0.8-1.5 mg/kg), indicating mediation by acetylcholine receptors (AChR). In the presence of 2% hal othane, contractures were tenfold greater at a rectal temperature of 4 1-degrees-C than at 37-degrees-C. In contrast, under 50 mg/kg intraper itoneal pentobarbital anesthesia, contractures were not affected by re ctal temperature. Neither the half-decay time of contracture nor twitc h tension (0.2 Hz, preceding SCh) were increased in the presence of ha lothane at 41-degrees-C. In a set of experiments in which rectal tempe rature was maintained at 37-degrees-C but jaw temperature was varied b etween 36-41-degrees-C, there was a significant regression of SCh-indu ced jaw contracture on temperature in the presence of halothane. In co ntrast, there was no significant relationship between jaw temperature and contracture in the presence of pentobarbital. Conclusions. These r esults in the rat demonstrate a temperature-dependent interaction betw een halothane and SCh that has not previously been described.