THERMOGENESIS IN BROWN ADIPOCYTES IN INHIBITED BY VOLATILE ANESTHETICAGENTS - A FACTOR CONTRIBUTING TO HYPOTHERMIA IN INFANTS

Background. In infants, nonshivering thermogenesis from brown adipose tissue provides an important source of heat for thermoregulation. Infa nts are known to have a high susceptibility to hypothermia during anes thesia. To investigate whether this could be due to an inhibition of n onshivering thermogenesis by anesthetics, the effect of preincubation with volatile anesthetics on the norepinephrine-induced heat productio n of brown adipocytes was investigated. Methods. Brown adipocytes from hamsters were isolated with a collagenase digestion method and preinc ubated with volatile anesthetics. The cells were stimulated with norep inephrine, and heat production, measured as oxygen consumption, was mo nitored polarographically. Results. Norepinephrine addition led to a 2 0-fold increase in the rate of oxygen consumption (thermogenesis). How ever, preincubation of cells with 3% halothane reduced the response to norepinephrine by more than 70%. The potency of norepinephrine (the m edian effective concentration) was not affected by halothane. Full eff ect of halothane was reached quickly, and after halothane withdrawal, the thermogenic response recovered, although rather slowly. Halothane, isoflurane, and enflurane were approximately equipotent inhibitors of thermogenesis, with concentrations of approximately 0.7% resulting In 50% inhibition. The inhibitory effect of 1% halothane was unaffected by the presence of 74% nitrous oxide, but nitrous oxide alone also red uced thermogenesis. Conclusions. Volatile anesthetics severely attenua ted the thermogenic response to norepinephrine of isolated brown-fat c ells. It is inferred that brown-adipose-tissue heat production is redu ced during (and probably also some time after) anesthesia. Because inf ants are dependent on brown-fat-derived nonshivering thermogenesis for thermal balance, the inhibition by volatile anesthetic agents of brow n-adipocyte heat production may at least partly explain the susceptibi lity of infants to hypothermia during and after anesthesia.