Effect of ambient temperature on the paradoxical metabolic responses to norepinephrine.

Four experiments were conducted to examine the effect of ambient temperature (Ta) 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 Ta between -5 degrees and 25 degrees C at a fixed dose of NE (250 micrograms/kg, IP) resulted in a temperature-dependent decrease in metabolism in nonacclimated rats as a function of Ta. 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 micrograms/kg at a Ta of 25 degrees C resulted in a maximal thermogenic response at a dose of 250 micrograms/kg with diminished responsiveness at higher and lower doses. At 5 degrees C, NE inhibited metabolism maximally at a dose of 250 micrograms/kg. Propranolol, a nonspecific beta-antagonist, attenuated the hypometabolic effect of NE in the cold, while the alpha 2-antagonist yohimbine completely blocked this effect. These results suggest that the metabolic suppressive effect of NE may be mediated by the presynaptic alpha 2-receptor and that beta-adrenoceptors may also contribute to this effect.