Human pancreatic growth hormone-releasing factor-44 differentially stimulates release of stored and newly synthesized rat growth hormone in Vitro

Effects of synthetic human pancreatic GH-releasing factor-44 (hpGRF-44) on synthesis and release of rat pituitary GH and PRL were examined in vitro in a static incubation system. A double label, specific immunoprecipitation protocol permitted simultaneous study of hormone synthesis as well as release of both stored and newly synthesized hormone. Synthetic hpGRF-44 (0.3 and 3.0 nM) stimulated the release of stored GH 240% beyond the basal level, while simultaneously stimulating the release of newly synthesized GH by 610%. Despite the stimulation of release, hpGRF-44 did not alter GH synthesis (102% of control value). A small but statistically significant increase in release of stored PRL occurred in response to hpGRF-44, while release of newly synthesized PRL and PRL synthesis were unaffected. In contrast, 1 mM (Bu)₂cAMP stimulated the release of both newly synthesized and stored GH and PRL. We conclude that hpGRF-44 differentially stimulates GH release from separate intracellular compartments and that the lactotroph may also, under certain conditions, respond to this secretagogue.