Effect of feeding level and thyroxine on adipose tissue development and growth in postnatal lambs

We investigated the influence of exogenous thyroxine (T-4) administration i n conjunction with level of feeding on adipose tissue and liver growth in p ostnatal lambs. Pairs of lambs were fed either 100 g (i.e. low fed) or 200 g (i.e. high fed) of milk powder per litre of reconstituted milk replacer o ver the first month of life. Half of the pairs of lambs were fed a bolus do se of T-4 (15 mg (kg body weight)(-1)) daily until 8 days of age. Perirenal adipose tissue and hepatic tissue were sampled at either 8 or 35 days of a ge. High fed lambs grew faster, possessed more adipose tissue and had large r livers than low fed lambs at 8 and 35 days of age. T-4 administration res ulted in a lower thermogenic activity (i.e. GDP binding) in adipose tissue at 8 days of age in low, but not high fed lambs. There was no difference be tween groups in colonic temperature or oxygen consumption. Between 17 and 3 5 days of age high fed lambs previously treated with T-4 had lower daily mi lk consumption than untreated siblings, but still attained the same growth rate. Plasma insulin-like growth factor-I concentrations were greater in hi gh than low Fed lambs, a relationship that was not influenced by T-4 treatm ent. In adipose tissue, iodothyronine 5' deiodinase activity was not influe nced by T-4 administration and was greater in high than low fed lambs. Hepa tic iodothyronine 5' deiodinase activity was not influenced by T, administr ation in low fed lambs, but was reduced by T-4 in high fed lambs. In conclu sion, T-4 administration over the first 8 days of life can accelerate the r ate of decline in thermogenic activity of uncoupling protein-1. This effect is not observed when the level of feeding is increased. Following withdraw al of T-4 treatment, high fed T-4-treated lambs were able to maintain the s ame growth rate as untreated lambs despite having a lower food intake.