The brown adipose tissue of hyperthyroid rats. A biochemical and ultrastructural study.

Male Sprague-Dawley rats were treated with triiodothyronine (100 micrograms/100 g/day) for 2, 4, 7, 14 and 21 days and the biochemical and ultrastructural changes of the brown adipose tissue were investigated. Results showed that the tissue weight, DNA and phospholipid content increased very early (by day 2 or 4) and that triglycerides increased later. These hormonal effects are not inhibited by the beta 1-antagonist propranolol. From the morphological point of view, triiodothyronine administration induced the early proliferation and maturation of adipocyte precursors (interstitial cells and preadipocytes). It is concluded that triiodothyronine administration causes a very early hyperplasia in the brown adipose tissue similar to that observed during exposure to cold by mechanisms that may not be secondary to the involvement of norepinephrine.