Abstract
The structural relationship between the motilin and the growth hormone secretagogue receptor (GHS-R), and between their respective ligands, motilin and ghrelin, prompted us to investigate whether ghrelin and the GHS-R agonist growth hormone-releasing peptide-6 (GHRP-6), could interact with the motilin receptor. The interaction was evaluated in the rabbit gastric antrum with binding studies on membrane preparations and with contraction studies on muscle strips in the presence of selective antagonists under conditions of electrical field stimulation (EFS) or not. Binding studies indicated that the affinity (pKd) for the motilin receptor was in the order of ghrelin (4.23 ± 0.07) < GHRP-6 (5.54 ± 0.08) < motilin (9.13 ± 0.03). The interaction of ghrelin with the motilin receptor requires the octanoyl group. Motilin induced smooth muscle contractile responses but ghrelin and GHRP-6 were ineffective. EFS elicited on- and off-responses that were increased by motilin already at 10−9 M, but not by 10−5 M ghrelin. In contrast, GHRP-6 also enhanced the on- and off-responses. The motilin antagonist Phe-cyclo[Lys-Tyr(3-tBu)-βAla-] trifluoroacetate (GM-109) blocked the effect of GHRP-6 on the off-responses but not on the on-responses. Under nonadrenergic noncholinergic conditions, the effects of motilin and GHRP-6 on the on-responses were abolished; those on the off-responses were preserved. All responses were blocked by neurokinin (NK)1 and NK2 antagonists. In conclusion, ghrelin is unable to induce contractions via the motilin receptor. However, GHRP-6 enhances neural contractile responses, partially via interaction with the motilin receptor on noncholinergic nerves with tachykinins as mediator, and partially via another receptor that may be a GHS-R subtype on cholinergic nerves that corelease tachykinins.
Footnotes
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This study was supported by grants from the Fund for Scientific Research-Flanders (FWO) (Belgium) (FWO Grant Number G 0109.00) and the Belgian Ministry of Science (GOA 98/011 and IUAP P5/20).
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DOI: 10.1124/jpet.102.047563
- Abbreviations:
- GHS-R
- growth hormone secretagogue receptor, GHS, growth hormone secretagogue
- GHRP-6
- growth hormone releasing peptide 6
- GH
- growth hormone
- oct
- octanoylated
- EFS
- electrical field stimulation
- NK
- neurokinin
- NANC
- nonadrenergic noncholinergic
- RT-PCR
- reverse transcription-polymerase chain reaction
- PCR
- polymerase chain reaction
- bp
- base pair(s)
- ANOVA
- analysis of variance
- MK 06777
- N-[1(R)-[(1,2-dihydro-1-methanesulfonyl-spiro[3H-indole-3,4′-piperiolin]-1′-yl)carbonyl]2-(phenyl-methoxy)ethyl]-2-amino-2-methylpropanamide methanesulfonate
- EM-523
- de(N-methyl)-N-ethyl-8,9-anhydroerythromycin A 6,9-hemiacetal
- Received December 5, 2002.
- Accepted February 4, 2003.
- The American Society for Pharmacology and Experimental Therapeutics
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