Vasodilator action of ghrelin
Ghrelin is a 28-amino acid peptide predominantly produced in the stomach and secreted into the circulation. Ghrelin is found in plasma and tissues in two major forms of n-octanoyl-modified at its N-terminal third serine residue and des-acyl ghrelin. The n-octanoyl group of ghrelin is essential for its growth hormone (GH)-releasing activity and appetite regulation mediated through growth hormone secretagogue receptor (GHS-R). We demonstrated that both ghrelin and des-acyl ghrelin evoke vasodilatation at remarkably low concentrations compared to acetylcholine (ACh) in phenylephrine (PE)-constricted perfused rat mesenteric vascular bed (MVB). This was abolished in endothelium-denuded preparations and in endothelium-intact preparations exposed to either a calcium-activated potassium channel (KCa) blocker or a depolarizing stimulus. While KATP channel blockade, nitric oxide synthase and cyclooxygenase inhibition had no effect, the responses were abolished in the presence of combinations of apamin and charybdotoxin, apamin and TRAM-34, and ouabain and Ba2+. The GHS-R antagonist, [D-Lys3]-GHRP-6, per se evoked vasodilatation. Inclusion of L-756867, a peptide antagonist of classical GHS-R, failed to evoke any vasodilator response or to affect vasodilatation evoked by ghrelin. Both non-peptide agonists of GHS-R, L-166446 and L-163255, demonstrated concentration-dependent decreases in perfusion pressure. All short peptides encompassing the first 20, 16, 10, 6, 4, and 3 residues of des-acyl ghrelin were able to evoke vasodilator responses to the same extent as des-acyl ghrelin. However, vasodilatation to single amino acids, L-serine and glycine, were significantly attenuated. Streptozotocin (STZ)-induced diabetes increased plasma ghrelin concentration. Diabetes for 4-weeks did not cause any significant reduction in ghrelin-evoked vasodilatation, whereas 8-weeks diabetes significantly reduced ghrelin-evoked vasodilatation. In contrast to ghrelin, there was a duration-dependent fall in vasodilator response to ACh from 4- to 8-weeks diabetes. These data suggest that the vasodilatation evoked by ghrelin is mediated by endothelium-dependent hyperpolarization (EDHF) by mechanism(s) that are independent of classical GHS-R activation. In addition, EDHF-dependent ghrelin-evoked vasodilator responses may not be affected, at least in the early stages of STZ diabetes, whereas the responses to ACh, predominantly mediated through nitric oxide, are progressively diminished right from the early stages of endothelial dysfunction in STZ diabetic rats.
Doctor of Philosophy (Ph.D.)