Triiodothyronine inhibits phorbol ester-induced thyrotropin release from elutriated bovine thyrotrophs

The inhibitory effect of T3 on TSH release was studied on a population of thyrotroph-enriched cells prepared from bovine pituitary glands by centrifugal elutriation. The cells (2.0 X 10(5)/ml) were cultured for 2 days and then exposed to TRH, phorbol-12 myristate-13 acetate (PMA), or calcium ionophore (A23187) with or without 100 nM T3 for two different preincubation periods, 3 h and 24 h. Cytosolic TSH and release of TSH into the medium were measured by a specific RIA for bovine TSH. TRH (10 nM, 100 nM), PMA (100 nM, 1 microM, 10 microM), and A23187 (100 nM, 1 microM, 10 microM) increased TSH release in a dose-dependent manner. One-hundred nanomolar TRH, 10 microM PMA, and 10 microM A23187 increased TSH release maximally from 176 +/- 6 microU/ml (mean +/- SD, n = 4) to 240 +/- 40, 308 +/- 39, and 228 +/- 16, respectively. PMA and A23187 interacted synergistically in the release of TSH. Cytosolic TSH was not affected by TRH or A23187. PMA (100 nM) together with A23187 resulted in a decrease in cytosolic TSH. PMA alone (1 and 10 microM) decreased cytosolic TSH content to 84% and 77%, respectively, of the control level, suggesting that PMA enhances release of TSH. One-hundred nanomolar T3 had no effect on the basal release of TSH when given for 3 h, but resulted in a 47% decrease when administered for 24 h. The inhibitory effect of T3 on TRH-induced TSH release was found when the cells were preincubated with T3 for 24 h, but not for 3 h. In contrast, PMA-induced TSH release was significantly inhibited to 74% of induced levels by preincubation with T3 even for 3 h, and further inhibition occurred with an increase in preincubation time. These data suggest that the effectiveness of T3 depends on the mode of stimulation, and that the more immediate reaction observed with PMA induction may result from the interaction of T3 with the protein kinase C pathway.