Brown Adipose Tissue Thermogenesis in T₃-Treated Rats

Summary

T₄ treatment results in an inactivation of brown adipose tissue (BAT) which has been attributed to a reduced need of thermoregulatory heat production. Since T₃ formation in brown adipocytes is governed by a type II T₄ 5'-deiodinase which is inhibited by T₄, we analyzed the possibility that results obtained by T₄ treatment were due to a lack of T₃ in the tissue. Hyperthyroidism was induced in adult rats by administration of T₃ (50 μg/kg body weight daily s. c.). Euthyroid and hyperthyroid rats were maintained at 23 °C or exposed at 6 °C for 3 weeks. Hyperthyroid rats at 23 °C showed an increase in BAT mass and in DNA and total lipids contents; however, BAT thermogenic activity was depressed. BAT from cold-exposed hyperthyroid rats showed the same mass and DNA content than at 23 °C, but it showed an increase in thermogenic activity, this increase being lower than in cold-exposed euthyroid rats. We conclude that high levels of T₃ in BAT do not stimulate the thermogenic activity of the tissue. On the contrary, they inhibit it in response to lower requirements of facultative thermogenesis, both at 23 °C and at 6 °C.

Key words

Brown Adipose Tissue - T₃ Treatment - Cold-Exposure