Relative roles of the thyroid hormones and noradrenaline on the thermogenic activity of brown adipose tissue in the rat.

We have assessed the relative contribution of the thyroid hormones and noradrenaline (NA) on the calorigenic function of brown adipose tissue (BAT) as indicated by GDP binding and O 2 consumption of BAT mitochondria. Male Wistar rats of 200g body weight were made hypothyroid with 131 I. Groups of animals were injected s.c., in divided doses, daily for 10 days, with thyroxine (2 μg/100g body weight) or tri-iodothyronine (T 3 ; 0.3 μg/100g body weight). Animals were used 7 days after bilateral or unilateral sympathetic nerve excision of BAT (Sx). Sham-operated rats were used as controls. In normal rats kept at 22°C, GDP binding reached 94 ± 24 pmol/mg protein ; untreated hypothyroid rats had normal binding values whereas the T 3 -treated group showed an increased binding. Sx induced a sharp fall in the three groups (P<0.01). After 24-h exposure to 4°C GDP binding increased in normal rats to about 410% (P<0.01) whereas binding failed to increase in response to cold in the untreated hypothyroid and the T 3 -treated groups. Sx reduced GDP binding in the three groups significantly (P<0.01). The consumption of O 2 by BAT mitochondria showed similar variations in response to Sx and to cold exposure as did GDP binding. The data indicated that, at room temperature, BAT calorigenesis can function without the thyroid hormones, though not without the catecholamines. The findings in rats exposed to cold showed that the lack of NA was significantly more effective than the lack of thyroid hormones in preventing the BAT hyperactive response. This does not negate an active role for T 3 in BAT calorigenesis.