Thyroid hormone feedback regulation of the secretion of bioactive thyrotropin in the frog

Thyrotropin-releasing hormone (TRH), ovine corticotropin-releasing hormone (oCRH) (both 268 nM), and mammalian gonadotropin-releasing hormone (mGnRH) (268 and 2680 nM) stimulated the secretion of bioactive thyrotropin (TSH) by Rana esculenta pituitaries (pars distalis) in vitro. Preincubation of the pituitaries with 50 ng/ml (64 nM) thyroxine (T4) for 6 hr suppressed the TRH- and oCRH-induced (268 nM) secretion of bioactive TSH, but did not affect the response of the pituitaries to 268 nM mGnRH. Triiodothyronine (T3) (64 nM) reduced both the TRH- and mGnRH-stimulated release of bioactive TSH; the response of TSH to TRH even decreased toward basal levels while a significant TSH response to mGnRH remained. In a separate experiment, pituitaries were preincubated for 6 hr with different equimolar doses of T3 and T4 (6.4, 32, and 64 nM); neither treatment affected the mGnRH-stimulated secretion of bioactive TSH. On the other hand, T4 suppressed the TSH response to TRH in a dose-dependent manner. The inhibitory effects of thyroid hormones on the TRH-induced release of bioactive TSH was present for at least 4 hr after their removal from the incubation medium. These results suggest that thyroid hormones exert a negative feedback control on the secretion of bioactive TSH in adult frogs by a direct action on the pars distalis. There may also be differences in thyroid hormone sensitivities of the TSH responses to mGnRH and TRH.