Regulation of Thyrotropin-Releasing Hormone Receptors and Responses by L-Triiodothyronine in Dispersed Rat Pituitary Cell Cultures*

Abstract

The effects of physiological concentrations of L-T₃ (T₃) were examined in dispersed cell cultures of pituitaries obtained from 10- to 12-day-old rats. T₃ inhibited TSH secretion by 50% and blunted the TSH response to TRH. The PRL response to TRH was also inhibited by T₃, and GH secretion was increased 2-fold. These responses were half-maximal at 0.1 nM added T₃ in medium supplemented with 10% hypothyroid calf serum, corresponding to a free T₃ concentration of 5 pM. In the presence or absence of added T₃, TRH effects were halfmaximal at 0.5–3 nM, and T₃ suppression was not overcome by high concentrations of TRH (up to 1 μM). Maximal inhibition of TSH responses to TRH occurred when cultures were preincubated with thyroid hormone for 24 h; a significant effect was observed after 8 h. The specific binding of [³H]TRH to dispersed rat pituitary cells was decreased 55–70% by T₃ in a dose-dependent manner. Inhibition of TSH secretion by T₃ was reversible within 24 h, and the fraction of thyrotrophs in the cultures (0.22) was not altered by T₃ over the course of the experiments. The results demonstrate that physiological concentrations of T₃ regulate TSH and PRL responses to TRH and control TRH receptor levels by a direct action on normal rat pituitary cells.