Gastric motor effects of peptide and non-peptide ghrelin agonists in mice in vivo and in vitro

Background and aims: The gastroprokinetic activities of ghrelin, the natural ligand of the growth hormone secretagogue receptor (GHS-R), prompted us to compare the effect of ghrelin with that of synthetic peptide ( growth hormone releasing peptide 6 (GHRP-6)) and non-peptide (capromorelin) GHS-R agonists both in vivo and in vitro. Methods: In vivo, the dose dependent effects ( 1 - 150 nmol/kg) of ghrelin, GHRP-6, and capromorelin on gastric emptying were measured by the C-14 octanoic breath test which was adapted for use in mice. The effect of atropine, N-G-nitro-L-arginine methyl ester hydrochloride (L-NAME), or D-Lys(3)-GHRP-6 (GHS-R antagonist) on the gastroprokinetic effect of capromorelin was also investigated. In vitro, the effect of the GHS-R agonists ( 1 mu M) on electrical field stimulation (EFS) induced responses was studied in fundic strips in the absence and presence of L-NAME. Results: Ghrelin, GHRP-6, and capromorelin accelerated gastric emptying in an equipotent manner, with bell-shaped dose-response relationships. In the presence of atropine or L-NAME, which delayed gastric emptying, capromorelin failed to accelerate gastric emptying. D-Lys(3)-GHRP-6 also delayed gastric emptying but did not effectively block the action of the GHS-R agonists, but this may be related to interactions with other receptors. EFS of fundic strips caused frequency dependent relaxations that were not modified by the GHS-R agonists. L-NAME turned EFS induced relaxations into cholinergic contractions that were enhanced by ghrelin, GHRP-6, and capromorelin. Conclusion: The C-14 octanoic breath test is a valuable technique to evaluate drug induced effects on gastric emptying in mice. Peptide and non-peptide GHS-R agonists accelerate gastric emptying of solids in an equipotent manner through activation of GHS receptors, possibly located on local cholinergic enteric nerves.