In vivo and in vitro Effects of Ghrelin/Motilin-Related Peptide on Growth Hormone Secretion in the Rat

Abstract

Ghrelin (Ghr), a 28 amino acid gastric peptide with an n-octanoylation on Ser 3, has recently been identified as an endogenous ligand of the growth hormone secretagogue (GHS) receptor. A cDNA was also isolated from a mouse stomach library encoding a protein named prepromotilin-related peptide (ppMTLRP) which shares sequence similarities with prepromotilin. Mouse and rat ppMTLRP sequences (rGhr) are identical and show 89% identity with human ghrelin (hGhr). By analogy with promotilin, cleavage of proMTLRP into an 18 amino acid endogenous processed peptide can be assumed on the basis of a conserved dibasic motif in position 9–10 of its sequence. In the present work, we compared the GH-releasing activity of rGhr28/MTLRP and of hGhr28/MTRLP with that of a shorter form of the peptide, hGhr18. A short peptide devoid of Ser-3 n-octanoylation hGhr18[–] was also tested. Addition of rGhr28, hGhr28 and hGhr18 stimulated GH release to the same extent from superfused pituitaries. The effect was dose dependent in a 10^–8 to 10^–6M concentration range. In contrast, hGhr 18[–] was inactive. In freely moving animals, both rGhr28 and hGhr28 (10 µg, i.v.) stimulated GH release, whereas the same dose of hGhr18 or of hGhr18[–] was ineffective. After rGhr28, GH plasma levels increased as early as 5 min after injection and returned to basal values within 40–60 min. Expressed as percent stimulation, administration of rGhr28 was equally effective when injected during troughs or peaks of GH. Plasma concentrations of prolactin, adrenocorticotropin and leptin were not modified. Spontaneous GH secretory episodes were no longer observed within 3 h of rGhr28 treatment, but repeated administration of the secretagogue at 3- to 4-hour intervals resulted in a similar GH response. Activation of somatostatin (SRIH) release by ether stress did not blunt the GH response to rGhr28. This suggests that the secretagogue acts in part by inhibiting endogenous SRIH, as further substantiated by the ability of rGhr28 (10^–6M), to decrease the amplitude of 25 mM K⁺-induced SRIH release from perifused hypothalami. In conclusion, (1) n-octanoylation of Ghrs and the shorter form hGhr18 is essential for the direct pituitary GH-releasing effect of this new family of endogenous GHSs; (2) only the longer forms are active in vivo and (3) inhibition of SRIH release appears involved in the mechanism of Ghr action.

Ghrelin: Ghrelin as a Regulatory Peptide in Growth Hormone Secretion

“…The amino acid sequence at positions 11 and 12 is different in rat and human ghrelin and they are equally potent [2,23] n-octanoylated ghrelin and the shorter form human Ghrelin (hGhr18) is necessary for the GH-releasing effect of ghrelin and the longer forms of Ghrelin are active in vivo [24].…”

Interactions of ghrelin signaling pathways with the GH neuroendocrine axis: a new and experimentally tested model

“…Two different sites of ghrelin synthesis are involved -the stomach (original site of discovery) and the hypothalamus (Kojima et al 1999, Sato et al 2005. Ghrelin shows potent GH-release activity when administered either centrally or peripherally (Kojima et al 1999, Date et al 2000, Tolle et al 2001, Tannenbaum et al 2003 and it is also a powerful orexigenic hormone stimulating feeding and body weight gain (Tschöp et al 2000). However, its in vivo interaction with the GH neuroendocrine axis to generate the GH rhythm remains to be elucidated.…”

La ghréline, un exemple saisissant de pléïotropie des peptides neuroendocriniens

“…Il est admis que la ghréline agit majoritairement par l'intermédiaire de récepteurs intrahypothalamiques exprimés par les neurones à GHRH [17], pour stimuler la libération de GH et, de manière indépendante, sur les neurones à neuropeptide Y/AgRP (Agouti-related protein), pour augmenter la prise alimentaire [18]. Cependant, elle agit aussi directement au niveau hypophysaire sur la sécrétion de GH [19]. La sécrétion gastrique et le complexe dorsovagal sont également impliqués [20].…”