A Possible Role of Cyclic AMP in Mediating the Effects of Thyrotropin-Releasing Hormone on Prolactin Release and on Prolactin and Growth Hormone Synthesis in Pituitary Cells in Culture

Abstract

Thyrotropin-releasing hormone (TRH) has 3 effects on clonal strains of rat pituitary cells in culture (GH-cells). Two long-term effects of TRH on GH-cells, measurable after 3 h or longer, were previously reported; these are an increase in prolactin synthesis and a decrease in growth hormone production. TRH also stimulates the rapid release of stored intracellular prolactin. The role of c(cyclic)AMP as a possible mediator of the effects of TRH on GH-cells was investigated. cAMP concentrations are higher in cells treated with TRH compared with paired controls; a maximum difference of greater than 150% of control values is detected at 15 min if the incubation is performed in serum-free medium in the presence of 1 mM theophylline. The concentration of TRH required to give half-maximum increases in both prolactin release and cAMP accumulation is 0.3 nM; half-maximal increases in prolactin synthesis occur at 3 nM TRH. Exogenous cAMP (1 mM) causes only a slight increase in prolactin release; 8-bromo-cAMP and 8-methylthio-cAMP (1 mM) do not cause significant release. Phosphodiesterase inhibitors (0.3 mM theophylline, 0.03 isobutylmethylxanthine) increase prolactin release, but their effects on hormone synthesis are more complicated. Isobutylmethylxanthine, 8-bromo-cAMP and 8-methylthio-cAMP (0.4 mM) increase prolactin synthesis but do not significantly affect growth hormone synthesis. Theophylline increases the synthesis of both hormones. Dibutyryl cAMP (0.5 mM or more) increases prolactin release and both growth hormone and prolactin synthesis, but equivalent amounts of sodium butyrate have the same effects. In GH-cells under carefully defined experimental conditions TRH causes an increase in intracellular cAMP concentrations; the increase in endogenous cAMP and the effects of phosphodiesterase inhibitors are consistent with a model with cAMP as a mediator of the effects of TRH on prolactin release; however, they do not prove this model, because the interpretation of these results depends on assumptions which may not all be valid; and none of the analogs of cAMP or the phosphodiesterase inhibitors tested mimic the decrease in growth hormone production caused by TRH.