Depletion of brown fat norepinephrine content by acute cold exposure and adrenoceptor blockade

Experiments were conducted to characterize the effects of acute cold exposu re, with and without adrenoceptor blockade, on intrascapular brown adipose tissue (IBAT) and adrenal catecholamine content in male Sprague-Dawley rats . Groups of animals with indwelling temperature transmitters were tested fo llowing treatment with saline, the alpha-adrenoceptor blocker phentolamine, the beta-adrenoceptor blocker propranolol, combined blockade with phentola mine plus propranolol, and the ganglionic blocker chlorisondamine. IBAT nor epinephrine (NE) content was not affected in animals tested at 22 degreesC, but was reduced in 4 degreesC-exposed animals treated with phentolamine (- 57%), phentolamine plus propranolol (-97%), and chlorisondamine (-42%). Adr enal NE and epinephrine (EPI) content were not altered by the treatments at 4 degreesC or 22 degreesC. None of the treatments affected the temperature of animals at 22 degreesC, but significant hypothermia occurred at 4 degre esC after chlorisondamine (-2.3 +/- 0.3 degreesC) and the combination of ph entolamine and propranolol (-1.5 +/- 0.4 degreesC). These results suggest t hat cold exposure alone did not affect IBAT NE content, but when cold expos ure was combined with adrenoceptor blockade, the sympathetic activation was sufficient to cause a reduction in IBAT NE content. In addition, alpha- an d beta-adrenoceptor-mediated mechanisms contribute to the maintenance of co re temperature. However, both alpha- and beta-receptor mechanisms had to be interrupted before a deficit in body temperature was detected. (C) 2001 El sevier Science Inc. All rights reserved.