Recent studies have shown that close intra-arterial infusion of low doses of orexin-A markedly increases duodenal electrolyte secretion. However, stimulation occurs only in animals that have had continuous access to food. Overnight fasting abolishes the secretory response to orexin-A, and also markedly decreases sensitivity to the muscarinic agonist bethanechol. In contrast, food deprivation does not affect secretory responses to VIP or cholecystokinin (CCK-8). Our aim was to further elucidate the secretory action of orexin-A. Methods: Lewis x Dark Agouti rats were anesthetized and a 12-mm segment of proximal duodenum with intact blood supply cannulated in situ. Mucosal bicarbonate secretion (pH stat) and mean arterial blood pressure were continuously recorded. Total RNA was extracted from duodenal mucosa of either continuously fed or overnight (16 hours) fasted animals and expression levels of prepro-orexin (PPO), orexin receptor 1 (OX1) and orexin receptor 2 (OX2) measured by quantitative real-time PCR (normalized to ß-actin). Results: Intra-arterial infusion of orexin-A (240 – 600 pmoles/kg,h) caused a dose-dependent increase in duodenal bicarbonate secretion. The OX1-receptor antagonist SB-334867 (6 nmol/kg) had no effect on basal bicarbonate secretion, but markedly decreased the orexin-induced secretory response. Atropine (0.5 mg/kg followed by 0.5 mg/kg,h) had no effect. Orexin-A administered to the luminal perfusate (0.1-100 nM) or by intracerebroventricular infusion (2-20 nmoles/kg,h) did not affect secretion. Fasting significantly reduced expression levels of PPO, OX1 and OX2. Conclusion: The secretory response to orexin A is mediated by mucosal OX1-receptors and expression of these receptors is related to food intake.