EFFECT OF AMBIENT-TEMPERATURE ON THE PARADOXICAL METABOLIC RESPONSES TO NOREPINEPHRINE

Four experiments were conducted to examine the effect of ambient temperature (T(a)) on norepinephrine (NE)-induced metabolism. NE increased oxygen consumption at ambient temperatures above and at acclimation temperature and decreased oxygen consumption at ambient temperatures below acclimation temperature in both nonacclimated (25-degrees-C) and cold-acclimated (5-degrees-C) rats. Varying the T(a) between -5-degrees and 25-degrees-C at a fixed dose of NE (250 mug/kg, IP) resulted in a temperature-dependent decrease in metabolism in nonacclimated rats as a function of T(a). A similar effect was demonstrated in cold-acclimated rats tested at -15, 5, and 25-degrees-C. Varying the dose of NE between 100 and 1,000 mug/kg at a T(a) of 25-degrees-C resulted in a maximal thermogenic response at a dose of 250 mug/kg with diminished responsiveness at higher and lower doses. At 5-degrees-C, NE inhibited metabolism maximally at a dose of 250 mug/kg. Propranolol, a nonspecific beta-antagonist, attenuated the hypometabolic effect of NE in the cold, while the alpha2-antagonist yohimbine completely blocked this effect. These results suggest that the metabolic suppressive effect of NE may be mediated by the presynaptic alpha2-receptor and that beta-adrenoceptors may also contribute to this effect.