Involvement of adenosine signaling in controlling the release of ghrelin from the mouse stomach.

Ghrelin, a potent orexigenic hormone released from the stomach, is important in regulating energy metabolism. Abnormal ghrelin levels are associated with eating disorders and metabolic diseases. However, factors involved in the regulation of ghrelin release remain unclear. Here, we examined the involvement of adenosine signaling in the control of ghrelin release from the perfused mouse stomach. Adenosine stimulated ghrelin release concentration-dependently, and the A(2A) receptor-selective antagonists 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) and 2-(2-furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine (SCH 58261) abolished the increased release. The A(2A) receptor-selective agonist 2-p-(2-carboxyethyl)phenethylamino-5-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680) augmented ghrelin release concentration-dependently, whereas the A(1) receptor-selective agonist 2-chloro-N(6)-cyclopentyladenosine inhibited ghrelin release. In A(2A) receptor knockout mice, adenosine inhibited ghrelin release, and the A(1) receptor-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine blocked this inhibition. The adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride increased ghrelin release in wild-type and A(1) receptor knockout mice but not in A(2A) receptor knockout mice. Colocalization of ghrelin immunoreactivity with A(1) and A(2A) receptor immunoreactivities in the gastric nerve fibers were observed. Colocalization was also detected for ghrelin and A(1) receptor immunoreactivities in the gastric mucosa. Blockade of neural activities with tetrodotoxin abolished the stimulatory effect of adenosine on ghrelin release. In conclusion, adenosine exerts predominantly a tonic A(2A) receptor-mediated stimulatory action on gastric ghrelin release, whereas an A(1) receptor-mediated inhibitory action is also apparent when the tonic excitatory effect was removed.