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Inhibition of growth hormone secretion by activin A in human growth hormone-secreting tumour cells.

Effect of activin A on growth hormone secretion was studied in primary culture of 8 human GH-secreting adenomas, which were responsive to TRH in vivo. When studied in vitro, basal GH secretion was reduced in all cases when cells were pre-incubated for 48 h with activin A at a concentration of 5 X 10(-9) mol/l or greater. Pretreatment of GH-secreting cells with 1 X 10(-9) mol/l activin A did not affect either basal secretion or cellular content of GH. These tumour cells also responded to TRH in vitro and the GH response to TRH was completely blocked in cells pretreated with activin A. Activin A slightly reduced the increase in cytoplasmic free calcium concentration induced by TRH. Furthermore, pretreatment of the cells with activin A attenuated GH secretion induced by A23187 or 12-O-tetradecanoyl phorbol-4-acetate, agents which bypass receptor-mediated generation of second messengers. These results indicate that activin A inhibits GH secretion by directly acting on human GH-secreting cells and that activin A inhibits the action of TRH by acting on multiple steps in the messenger system.