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.
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