A cryptic peptide of TRH prohormone inhibits TRH-induced GH release.

The effects of two cryptic peptides from pro-TRH: Ps4 (160-169) and Ps5 (178-199) were investigated on basal and secretagogue (GRH and TRH)-induced releases of GH from perifused fragments of rat adenohypophysis. Validation of the perifusion system was done by measuring: (1) the dose-dependent effect of GRH and TRH on GH release; and (2) the stimulation of that release by forskolin (to mimic the adenylate cyclase pathway) or by phorbol ester (to mimic the protein kinase C pathway). We show that: (1) Ps4 and Ps5 (1 microM) do not modify basal GH release; (2) Ps4 (1 microM) changes neither GRH (10 nM)- nor TRH (100 nM)-induced release of GH; (3) Ps5 (100 nM and 1 microM) significantly decreases the release of GH induced by equimolar concentrations of TRH but not that induced by GRH.