TRH and GRF stimulate release of growth hormone through different mechanisms

Thyrotropin-releasing hormone (TRH) is an effective stimulator of growth hormone (GH) release from cultured adenohypophysial cells of chronically hypothyroid rats in vitro. The present study explored the question of cAMP and calcium mediation of the GH-stimulatory effect of TRH in this system. A maximally stimulatory concentration of TRH was added together with various concentrations of human GH-releasing factor 40 (hGRF-40) whose action is cAMP mediated, or of dibutyryl cAMP (DBcAMP), to primary monolayer cultures of adenohypophysial cells from thyroidectomized rats. The GH responses to the combined addition of TRH with all doses of GRF or DBcAMP were fully additive, causing parallel elevations of the dose-response curves. Whereas the GH response to maximally effective concentrations of hGRF-40 and DBcAMP, added together, was not greater than that to either secretagogue alone, the inclusion of TRH increased the response to a new Emax. The calcium inhibitors, verapamil, EGTA, and CoCl2, markedly suppressed basal GH release and virtually completely blocked the GH response to TRH, suggesting calcium mediation. In chronically hypothyroid, urethan-anesthetized rats, the in vivo effect of the combined administration of maximally effective doses of TRH and GRF on plasma GH levels was also additive. These findings indicate that TRH stimulates GH release in adenohypophysial cells of hypothyroid rats by a cAMP-independent, calcium-dependent mechanism.