Short ghrelin peptides neither displace ghrelin binding in vitro nor stimulate GH release in vivo

Ghrelin is an acylated peptide recently isolated from rat stomach that potently stimulates GH release in vitro and in vivo in rat and man. Ghrelin specifically activates the receptor for the growth hormone secretagogues (GHS-Rla), and it has been proposed as the endogenous ligand mimicked by these synthetic compounds. Very recently, it was shown in cells transfected with the GHS-Rla that short acylated peptides encompassing the first 4-5 residues of ghrelin were capable of increasing intracellular calcium almost as efficiently as the full-length ghrelin. In the present study, we demonstrate that truncated analogs of ghrelin are ineffective in stimulating GH release in neonatal rats and do not displace radiolabelled ghrelin from binding sites in membranes from human hypothalamus and pituitary. In conclusion, our data demonstrate that the ability of short ghrelins to stimulate the GHS-Rla in transfected cells is not predictive of their capability to stimulate GH secretion in vivo.