Inhibition of shivering as a cause of metabolic suppression with norepinephrine in warm- and cold-acclimated rats.

The metabolic response of warm-acclimated (25 degrees C for 4 weeks) and cold-acclimated (5 degrees C for 4 weeks) rats to infused norepinephrine (NE) (0.5-4 micrograms/(kg.min)) was measured at an ambient temperature of 13 degrees C, either before or after sinoaortic denervation. In warm-acclimated rats, vigorous shivering consistently occurring at 13 degrees C was greatly inhibited by NE in a dose of 2-4 microgram/(kg.min). After sinoaortic denervation, no such inhibition of shivering was observed. In cold-acclimated rats, NE did not suppress but increased heat production at this temperature, no visible shivering being noticed. Phenylephrine, a alpha-adrenergic stimulant, increased blood pressure and decreased heart rate to the same extent in both groups of rats at this temperature. NE suppressed heat production even in cold-acclimated rats at -5 degrees C, where the animals exhibited shivering. These results indicated that NE infusion inhibits shivering via the sinoaortic baroreceptor reflex, in both warm-acclimated and cold-acclimated rats. The non-occurrence of metabolic suppression in cold-acclimated rats after norepinephrine infusion in the 13 degrees C environment may be due to an absence of shivering which is suppressed at this temperature.