THE EFFECTS OF GH-RELEASING PEPTIDE-6 (GHRP-6) AND GHRP-2 ON INTRACELLULAR ADENOSINE-3',5'-MONOPHOSPHATE (CAMP) LEVELS AND GH SECRETION IN OVINE AND RAT SOMATOTROPHS

The mechanism of action of GH-releasing peptide-6 (GHRP-6) and GHRP-2 on GH release was investigated in ovine and rat pituitary cells in vit ro. In partially purified sheep somatotrophs, GHRP-2 and GH-releasing factor (GRF) increased intracellular cyclic AMP (cAMP) concentrations and caused GH release in a dose-dependent manner; GHRP-6 did not incre ase cAMP levels. An additive effect of maximal doses of GRF and GHRP-2 was observed in both cAMP and GH levels whereas combined GHRP-6 and G HRP-2 at maximal doses produced an additive effect on GH release only. Pretreatment of the cells with MDL 12,330A, an adenylyl cyclase inhib itor, prevented cAMP accumulation and the subsequent release of GH tha t was caused by either GHRP-2 or GRF. The cAMP antagonist, Rp-cAMP als o blocked GH release in response to GHRP-2 and GRF. The cAMP antagonis t did not prevent the effect of GHRP-6 on GH secretion whereas MDL 12, 330A partially reduced the effect. An antagonist for the GRF receptor, [Ac-Tyr(1),D-Arg(2)]-GRF 1-29, significantly diminished the effect of GHRP-2 and GRF on cAMP accumulation and GH release, but did not affec t GH release induced by GHRP-6. Somatostatin prevented cAMP accumulati on and GH release responses to GHRP-2, GRF and GHRP-6. Ca2+ channel bl ockade did not affect the cAMP increase in response to GHRP-2 or GRF b ut totally prevented GH release in response to GHRP-2, GRF and GHRP-6. These results indicated that GHRP-2 acts on ovine pituitary somatotro phs to increase cAMP concentration in a manner similar to that of GRF; this occurs even during the blockade of Ca2+ influx. GHRP-6 caused GH release without an increase in intracellular cAMP levels. GH release in response to all three secretagogues was reduced by somatostatin and was dependent upon the influx of extracellular Ca2+. The additive eff ect of GHRP-2 and GRF or GHRP-6 suggested that the three peptides may act on different receptors. In rat pituitary cell cultures, GHRP-6 had no effect on cAMP levels, but potentiated the effect of GRF on cAMP a ccumulation. The synergistic effect of GRF and GHRP-6 on cAMP accumula tion did not occur in sheep somatotrophs. Whereas GHRP-2 caused cAMP a ccumulation in sheep somatotrophs, it did not do so in rat pituitary c ells. These data indicate species differences in the response of pitui tary somatotrophs to the GHRPs and this is probably due to different s ubtypes of GHRP receptor in rat or sheep.