Changes in thyroid hormone and insulin status of the brown adipose tissue of cold acclimated rats on short term exposure to heat

Acclimation of rats to cold caused a 45% increase in the concn. of $T_3$ and a 35% increase in the concn. of $T_4$ in serum. Exposure of cold-acclimated rats to heat (12 h, 37°) failed to decrease the concns. of thyroid hormones in circulation. The concn. of $T_3$ in brown adipose tissue (BAT) increased almost 10-fold on cold acclimation. Iodothyronine deiodinase activity also registered a 3-fold increase. Exposure of cold-acclimated animals to heat caused a decrease in the concn. of $T_3$ in BAT without appreciably affecting $T_4$ concn. In liver tissue, the changes in hormone concns. were quite small compared to those in BAT. On thyroidectomy or when fed with PTU, rats did not survive exposure to the cold. The concn. of insulin in circulation showed a small increase, while that in the tissues decreased on acclimation of rats to the cold. The concn. of the hormone in BAT increased on exposure of cold-acclimated animals to heat (12 h, 37°). The increase in liver was marginal. The temp.-dependent response of $T_3$ indicates an important role for this hormone in rapid physiol. response in BAT