Growth hormone releasing activity of small peptides: effect of pyroglutamyl-seryl-glycinamide and thyrotropin releasing hormone on rat pituitaries.

Growth hormone (GH) releasing activity of pyroglutamyl-seryl-glycinamide (PSG) and of thyrotropin-releasing hormone (TRH) was assayed on rat pituitaries in vitro, by radioimmunoassay of the GH released into the incubation media. TRH was found to be a partial agonist, producing maximal effects (at ca. 600 ng/ml) that were below the maximum response of the system to hypothalamic extract. The responses to a higher TRH concentration (1500 ng/ml) were smaller than that obtained at 600 ng/ml. PSG also behaved as a partial agonist producing maximum response at 150 ng/ml. The results are discussed in relation to possible structural similarities between the physiological GHRF and the tripeptides TRH, PSG and MIF (MSH release inhibiting hormone).