Pituitary nuclear triiodothyronine receptors during development in the rat

Studies were undertaken to measure the pituitary nuclear triiodothyronine (T3) binding capacity (BC) during development in the rat. BC was measured in 0.4 M NaCl-solubilized receptors in 5-, 14-, 20-, 27-, 30-, 40-, 50-day-old rats and in adult animals. Results indicate that BC is lower in 5-day-old rats than in adult animals (0.397 +/- 0.02 vs. 0.797 +/- 0.06 pmol T3/mg protein) (mean +/- SE) (P less than 0.01). Hypothyroidism, induced during the neonatal period or in the adult animal, results in a significant decrease in BC (P less than 0.01) when compared with control rats. However, treatment of hypothyroid animals with T3 (0.4 micrograms/100 g body wt) for a period of 7-14 days significantly increased BC as compared with hypothyroid rats without apparent changes in Ka. The relative affinities of various thyroid hormone analogues for the nuclear receptor were also measured in the adult animal. The affinities of these analogues are in the following order: TRIAC greater than L-T3 greater than D-T3 greater than L-T4 greater than D-T4 greater than DIMIT greater than L-T2 greater than rT3. The present findings demonstrate the presence of high affinity nuclear T3 binding sites in the pituitary of neonatal rat and could thus account for the effects of thyroid hormones on GH synthesis and TSH secretion observed in these animals.