In vivo and in vitro effects of ghrelin/motilin-related peptide on growth hormone secretion in the rat

Ghrelin (Ghr), a 28 amino acid gastric peptide with an n-octanoylation on S er 3, has recently been identified as an endogenous ligand of the growth ho rmone secretagogue (GHS) receptor. A cDNA was also isolated from a mouse st omach library encoding a protein named prepromotilin-related peptide (ppMTL RP) which shares sequence similarities with prepromotilin. Mouse and rat pp MTLRP sequences (rGhr) are identical and show 89% identity with human ghrel in (hGhr). By analogy with promotilin, cleavage of proMTLRP into an 18 amin o acid endogenous processed peptide can be assumed on the basis of a conser ved 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 wit h 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 pituit aries. The effect was dose dependent in a 10(-8) to 10(-6) M concentration range. In contrast, hGhr18[-] was inactive. In freely moving animals, both rGhr28 and hGhr28 (10 mug, i.v.) stimulated GH release, whereas the same do se of hGhr18 or of hGhr18[-] was ineffective. After rGhr28, GH plasma level s increased as early as 5 min after injection and returned to basal values within 40-60 min. Expressed as percent stimulation, administration of rGhr2 8 was equally effective when injected during troughs or peaks of GH. Plasma concentrations of prolactin, adrenocorticotropin and leptin were not modif ied. Spontaneous GH secretory episodes were no longer observed within 3 h o f rGhr28 treatment, but repeated administration of the secretagogue at 3- t o Lt-hour intervals resulted in a similar GH response. Activation of somato statin (SRIH) release by ether stress did not blunt the GH response to rGhr 28. This suggests that the secretagogue acts in part by inhibiting endogeno us SRIH, as further substantiated by the ability of rGhr28 (10(-6) M), to d ecrease the amplitude of 25 mM K+-induced SRIH release from perifused hypot halami. In conclusion, (1) n-octanoylation of Ghrs and the shorter form hGh r18 is essential for the direct pituitary GH-releasing effect of this new f amily 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 act ion. Copyright (C) 2001 S. Karger AG, Basel.