TRIIODOTHYRONINE REDUCES GROWTH-HORMONE SECRETION AND PITUITARY GROWTH-HORMONE MESSENGER-RNA IN THE CHICKEN, IN-VIVO AND IN-VITRO

The influence of triiodothyronine (T-3) on growth hormone (GH) mRNA an d GH secretion has been examined in the chicken. Initially T-3 treatme nt in the diet for three days did not alter plasma concentrations of G H. Plasma concentrations of GH were depressed with seven and 14 days o f T-3 treatment (1 or 5 ppm In the diet). There was a concomitant decl ine in pituitary GH mRNA with T-3 treatment. Pituitary GH content was reduced with 14 but not seven days of T-3 treatment. No effect of T-3 was observed on the percentage by somatotroph cells in the anterior pi tuitary gland by fluorescence flow cytometry analysis. While exposure of pituitary cells in vitro from young chickens to GHRF for 2 hr incre ased GH mRNA, no effect was observed with T-3. The presence of T-3, fo r 48 hr in vitro, tended to reduce GH mRNA in adenohypophyseal cells f rom young chickens and decreased GH mRNA with anterior pituitary cells from adult chicks. It is concluded that T-3 chronic administration of T-3 depresses circulating concentrations of GH, at least in part, by decreasing GH mRNA and hence GH synthesis.