Halothane and temperature interact to increase succinylcholine-induced jaw contracture in the rat

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 malignant hyperthermia. The authors investigated factors affecting SCh-induced contractures in an animal preparation.

Rats were anesthetized with either halothane (1-2%) or pentobarbital. Resting and twitch isometric tension were measured from the jaw muscles. Succinylcholine (500 or 750 micrograms/kg) was administered intravenously, producing increases in resting tension (i.e., contractures). Jaw muscle temperature was controlled by radiant heat.

Succinylcholine increased jaw muscle tension for several seconds. These contractures exhibited tachyphylaxis, and were antagonized by vecuronium (0.8-1.5 mg/kg), indicating mediation by acetylcholine receptors (AChR). In the presence of 2% halothane, contractures were tenfold greater at a rectal temperature of 41 degrees C than at 37 degrees C. In contrast, under 50 mg/kg intraperitoneal pentobarbital anesthesia, contractures were not affected by rectal temperature. Neither the half-decay time of contracture nor twitch tension (0.2 Hz, preceding SCh) were increased in the presence of halothane at 41 degrees C. In a set of experiments in which rectal temperature was maintained at 37 degrees C but jaw temperature was varied between 36-41 degrees C, there was a significant regression of SCh-induced jaw contracture on temperature in the presence of halothane. In contrast, there was no significant relationship between jaw temperature and contracture in the presence of pentobarbital.

These results in the rat demonstrate a temperature-dependent interaction between halothane and SCh that has not previously been described.