Effects of short-term food deprivation on orexin-A-induced intestinal bicarbonate secretion in comparison with related secretagogues

Studies of gastrointestinal physiology in humans and intact animals are usually conducted after overnight fast. We compared the effects of orexin-A, vasoactive intestinal polypeptide (VIP), melatonin, serotonin, uroguanylin, ghrelin and prostaglandin E(2) (PGE(2)) on duodenal bicarbonate secretion in fed and overnight fasted animals. This review is a summary of our findings. Secretagogues were administered by intra-arterial infusion or luminally (PGE(2)). Enterocyte intracellular calcium ([Ca(2+)](i)) signalling was studied by fluorescence imaging. Total RNA was extracted, reverse transcripted to cDNA and expression of orexin receptors measured by quantitative real-time PCR. Orexin-A stimulates the duodenal secretion in continuously fed animals but not in food-deprived animals. Similarly, short-term fasting causes a 100-fold decrease in the amount of the muscarinic agonist bethanechol required for stimulation of secretion. In contrast, fasting does not affect secretory responses to intra-arterial VIP, melatonin, serotonin, uroguanylin and ghrelin, or that to luminal PGE(2). Orexin-A induces [Ca(2+)](i) signalling in enterocytes from fed rats but no significant [Ca(2+)](i) responses occur in enterocytes from fasted animals. In addition, overnight fasting decreases the expression of mucosal orexin receptors. Short-term food deprivation thus decreases duodenal expression of orexin receptors and abolishes the secretory response to orexin-A as well as orexin-A-induced [Ca(2+)](i) signalling. Fasting, furthermore, decreases mucosal sensitivity to bethanechol. The absence of declines in secretory responses to other secretagogues tested strongly suggests that short-term fasting does not affect the secretory capacity of the duodenal mucosa in general. Studies of intestinal secretion require particular evaluation with respect to feeding status.