EFFECTS OF VOLATILE ANESTHETICS ON DIRECTLY AND INDIRECTLY STIMULATED SKELETAL-MUSCLE

Isolated guinea pig nerve-lumbrical muscle preparations were exposed to halothane, methoxyflurane, isoflurane, enflurane, fluroxene, and diethyl ether. The temporal courses of the effects on indirectly and directly elicited twitch responses were determined over a range of concentrations for each agent. When the anesthetics were compared at concentrations equivalent in terms of minimum alveolar concentration (MAC), a spectrum was observed in which halothane, methoxyflurane and isoflurane depressed the indirect twitch response at 3.5-5 MAC and the direct twitch response at 8-10 MAC. Diethyl ether and fluroxene depressed the indirect twitch response at 2-3.5 MAC and the direct twitch response at 3-6 MAC. Enflurane depressed the indirect response at 1.5-2.5 MAC and the direct response at 6-8 MAC. When the anesthetics were compared at concentrations equivalent in terms of their abilities to depress end-plate depolarization, however, all anesthetics were equipotent. Depression of the indirect twitch response occurred only when anesthetic concentrations were great enough to depress depolarization by 50 per cent.