Growth hormone-releasing peptide-2 (GHRP-2) does not act via the human growth hormone-releasing factor receptor in GC cells

Effect of growth hormone-releasing peptide-2 (GHRP-2) on ovine somatotrophs is abolished by a growth hormone-releasing factor (GRF) receptor antagonist, which raises the possibility that GHRP-2 may act on GRF receptors. In the present study, we used rat pituitary GC cells with or without stable transfection of cDNA coding for the human GRF receptor (GC/R ⁺ or GC/R ^-) to determine whether or not GHRP-2 acts via the GRF receptor. Northern blot analysis indicated that GRF receptor mRNA was undetectable in GC/R ^- cells, whereas a high level of expression occurred in GC/R ⁺ cells that were transfected by GRF receptor cDNA. In GC/R ^- cells, incubation with up to 10 ^-7M of either hGRF or GHRP-2 did not alter the intracellular cAMP, [Ca ²⁺]i, or GH secretion. In GC/R ⁺ cells, hGRF (10 ^-11-10 ^-7M) increased cAMP levels in a concentration-dependent manner up to 20-fold. This increase in cAMP levels was blocked by a GRF receptor antagonist, [Ac-Tyr ¹, D-Arg ²]-GRF 1-29, but not by a Ca ²⁺ channel blocker, NiCl ₂ (0.5 mM). GH secretion and [Ca ²⁺]i were, however, not increased by hGRF. Incubation of the transfected cells with 10 ^-11-10 ^-8M GHRP-2 did not modify intracellular cAMP levels. This result suggests that GHRP-2 does not act through the GRF receptor.