Intestinal epithelial intercellular tight junctions (TJs) provide a rate-limiting barrier restricting passive transepithelial movement of solutes. TJs are highly dynamic areas, and their permeability is changed in response to various stimuli. Defects in the intestinal epithelial TJ barrier may contribute to intestinal inflammation or leaky gut. The gastrointestinal tract may be the largest extrapineal source of endogenous melatonin. Melatonin released from the duodenal mucosa is a potent stimulant of duodenal mucosal bicarbonate secretion (DBS). The aim of this study was to elucidate the role of melatonin in regulating duodenal mucosal barrier functions, including mucosal permeability, DBS, net fluid flux, and duodenal motor activity, in the living animal. Rats were anesthetized with thiobarbiturate, and a ~30-mm segment of the proximal duodenum with an intact blood supply was perfused in situ. Melatonin and the selective melatonin receptor antagonist luzindole were perfused luminally or given intravenously. Effects on permeability (blood-to-lumen clearance of (51)Cr-EDTA), DBS, mucosal net fluid flux, and duodenal motility were monitored. Luminal melatonin caused a rapid decrease in paracellular permeability and an increase in DBS, but had no effect on duodenal motor activity or net fluid flux. Luzindole did not influence any of the basal parameters studied, but significantly inhibited the effects of melatonin. The nonselective and noncompetitive nicotinic acetylcholine receptor antagonist mecamylamine abolished the effect of melatonin on duodenal permeability and reduced that on DBS. In conclusion, these findings provide evidence that melatonin significantly decreases duodenal mucosal paracellular permeability and increases DBS. The data support the important role of melatonin in the neurohumoral regulation of duodenal mucosal barrier.

Sommansson A, Saudi WSW, Nylander O, Sjöblom M. Melatonin inhibits alcohol-induced increases in duodenal mucosal permeability in rats in vivo. Am J Physiol Gastrointest Liver Physiol 305: G95-G105, 2013. First published May 2, 2013; doi:10.1152/ajpgi.00074.2013.-Increased intestinal permeability is often associated with epithelial inflammation, leaky gut, or other pathological conditions in the gastrointestinal tract. We recently found that melatonin decreases basal duodenal mucosal permeability, suggesting a mucosal protective mode of action of this agent. The aim of the present study was to elucidate the effects of melatonin on ethanol-, wine-, and HCl-induced changes of duodenal mucosal paracellular permeability and motility. Rats were anesthetized with thiobarbiturate and a ϳ30-mm segment of the proximal duodenum was perfused in situ. Effects on duodenal mucosal paracellular permeability, assessed by measuring the blood-to-lumen clearance of 51 Cr-EDTA, motility, and morphology, were investigated. Perfusing the duodenal segment with ethanol (10 or 15% alcohol by volume), red wine, or HCl (25-100 mM) induced concentration-dependent increases in paracellular permeability. Luminal ethanol and wine increased, whereas HCl transiently decreased duodenal motility. Administration of melatonin significantly reduced ethanol-and wine-induced increases in permeability by a mechanism abolished by the nicotinic receptor antagonists hexamethonium (iv) or mecamylamine (luminally). Signs of mucosal injury (edema and beginning of desquamation of the epithelium) in response to ethanol exposure were seen only in a few villi, an effect that was histologically not changed by melatonin. Melatonin did not affect HClinduced increases in mucosal permeability or decreases in motility. Our results show that melatonin reduces ethanol-and wine-induced increases in duodenal paracellular permeability partly via an enteric inhibitory nicotinic-receptor dependent neural pathway. In addition, melatonin inhibits ethanol-induced increases in duodenal motor activity. These results suggest that melatonin may serve important gastrointestinal barrier functions.

Although further studies are needed, our data demonstrate that melatonin administration markedly improves duodenal barrier functions, suggesting its utility in clinical applications when intestinal barrier functions are compromised.

Alcohol may induce metabolic and functional changes in gastrointestinal epithelial cells, contributing to impaired mucosal barrier function. Duodenal mucosal bicarbonate secretion (DBS) is a primary epithelial defense against gastric acid and also has an important function in maintaining the homeostasis of the juxtamucosal microenvironment. The aim in this study was to investigate the effects of the luminal perfusion of moderate concentrations of ethanol in vivo on epithelial DBS, fluid secretion and paracellular permeability. Under thiobarbiturate anesthesia, a ∼30-mm segment of the proximal duodenum with an intact blood supply was perfused in situ in rats. The effects on DBS, duodenal transepithelial net fluid flux and the blood-to-lumen clearance of 51Cr-EDTA were investigated. Perfusing the duodenum with isotonic solutions of 10% or 15% ethanol-by-volume for 30 min increased DBS in a concentration-dependent manner, while the net fluid flux did not change. Pre-treatment with the CFTR inhibitor CFTRinh172 (i.p. or i.v.) did not change the secretory response to ethanol, while removing Cl− from the luminal perfusate abolished the ethanol-induced increase in DBS. The administration of hexamethonium (i.v.) but not capsazepine significantly reduced the basal net fluid flux and the ethanol-induced increase in DBS. Perfusing the duodenum with a combination of 1.0 mM HCl and 15% ethanol induced significantly greater increases in DBS than 15% ethanol or 1.0 mM HCl alone but did not influence fluid flux. Our data demonstrate that ethanol induces increases in DBS through a mechanism that is critically dependent on luminal Cl− and partly dependent on enteric neural pathways involving nicotinic receptors. Ethanol and HCl appears to stimulate DBS via the activation of different bicarbonate transporting mechanisms.

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