Evidence against a Mediatory Role of Brown Adipose Tissue in the Calorigenic Response of Cold-Acclimated Rats to Noradrenaline

Abstract

The calorigenic response to infused noradrenaline (NA) of barbital-anesthetized, cold-acclimated, adult, gnotobiotic, albino rats tested 1 h after removal of interscapular brown adipose tissue (I.B.A.T.) was 14% lower than before surgery and 15% lower than the response of either sham-operated or non-operated rats; but 4 days later, the response of rats without I.B.A.T. was not significantly different from that of the controls. In unanesthetized rats, response to NA was also unaffected by interruption of blood flow to I.B.A.T. 1–3 days before the measurements. Since these results do not conform with previous findings of 30–60% reductions in response to NA following removal of I.B.A.T. or interference with the tissue's vascular connections, they contradict the current hypothesis that the B.A.T. of cold-acclimated rodents has a unique role as a mediator of calorigenesis in other tissues.A supplementary feature of the study was the discovery of apparent thermolability in the process by which cold-acclimated rats respond calorigenically to NA. If, during infusion of NA, colonic temperature exceeded about 41.5 °C, an apparently critical degree of hyperthermia response to the hormone during a subsequent infusion was reduced. This reduction was linearly related to the previous maximum colonic temperature over the range 41.6–42.4 °C and amounted to approximately 70% at 42.4 °C. These results indicate the necessity for monitoring the body temperatures of animals during infusion of NA, particularly in experiments in which two or more tests of response to NA are done on the same animal. Since such temperature measurements were not reported in those studies that have suggested a mediatory role of B.A.T. in calorigenesis in rodents, it is not possible to resolve the discrepancies on this basis.