Apelin is the endogenous ligand of the G protein-coupled receptor APJ and apelin peptide as well as APJ mRNA are expressed in several tissues. Proposed actions include involvement in the control of appetite and body metabolism. The orexogenic hormone orexin-A and the incretin GIP stimulate the bicarbonate secretion by the duodenal mucosa, but stimulation occurs only in fed animals. Our aim was to study effects of apelin on the duodenal secretion in fed and overnight food deprived animals.

Methods: 

Lewis x Dark Agouti rats had free access to water and, unless fasted overnight, free access to food. Animals were anesthetized and a segment of proximal duodenum with intact blood supply cannulated in situ. Mucosal bicarbonate secretion was titrated (pH stat) and apelin-13 was administered to the duodenum by close intra-arterial infusion. Total RNA was extracted from mucosal specimens, reverse transcripted to cDNA and expression of APJ receptor measured by quantitative real-time PCR.

Results: 

Apelin caused a 30–40 % rise in secretion at the lowest dose infused (6 pmol/kg × h). A 10-fold higher dose (60 pmol/kg × h) caused an additional slight increase in secretion but a 100-folder higher dose had no further effect. Stimulation occurred only in fed animals. No stimulation was thus observed in overnight fasted animals, even with the highest dose of apelin tested (600 pmol/kg × h). Pretreatment with atropine did not affect the secretory response to apelin in fed animals. Overnight fasting caused a 8-fold decrease in the expression of APJ receptor mRNA. Discussion. Very low doses of apelin stimulate the bicarbonate secretion by the duodenal mucosa. Stimulation does not involve muscarinergic pathways and is, as found with orexin-A and GIP, markedly dependent on feeding status.