GROWTH-HORMONE RELEASING HEXAPEPTIDE (GHRP-6) ACTIVATES THE INOSITOL (1,4,5)-TRISPHOSPHATE DIACYLGLYCEROL PATHWAY IN RAT ANTERIOR-PITUITARY-CELLS

Growth hormone-releasing hexapeptide (GHRP-6) is known to stimulate se cretion of growth hormone (GH) in vivo and in vitro in a variety of sp ecies. However, the cellular effects of GHRP-6 remain largely unknown. We have tested the influence of GHRP-6 on the inositol phospholipid s econd messenger system in cultured anterior pituitary cells. Cultured pituitary cells responded upon challenge with GHRP-6 with a dose-depen dent release of GH. Moreover, incubation of GHRP-6 with pituitary cell cultures labelled with myo-[H-3]inositol resulted in a dose-dependent rise in [H-3]inositol phosphates. Brief stimulation of pituitary cell s with GHRP-6 increased phosphorylation of MBP(4-14), a specific prote in kinase C substrate, when incubated with the cytosol- or plasma memb rane fraction from the stimulated cells. Furthermore, introduction of MBP(4-14) into the cytosol in digitonin permeabilized pituitary cells caused increased phosphorylation of this substrate. GHRP-6 induced a r ise in intracellular Ca2+ in individual somatotrophs loaded with the C a2+ indicator, Fura-2. Preincubation (3 min) with somatostatin (SRIF) diminished the Ca2+ spike elicited by GHRP-6, while no effect of SRIF was observed when added simultaneously with GHRP-6. These results indi cate that GHRP-6-stimulated GH-secretion involves the diacylglycerol/i nositol(1,4,5)trisphosphate pathway with a resulting rise in cytosolic Ca2+.