Olof Nylander

Melatonin decreases duodenal epithelial paracellular permeability via a nicotinic receptor-dependent pathway in rats in vivo.

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 51Cr‐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.

Adherent surface mucus gel restricts diffusion of macromolecules in rat duodenum in vivo

The aim of this study was to investigate the permeability of the adherent mucus gel layer in rat duodenum in vivo to macromolecules applied in the lumen. Rats were anesthetized with thiobarbiturate, and the duodenum was perfused with isotonic NaCl solution containing large-molecular-size secretagogues. Effects on mucosal HCO(-)(3) secretion and blood-to-lumen (51)chromium-labeled EDTA clearance were used as indexes that compounds had migrated across the mucus layer. Exposure to a low concentration of papain (10 U/100 ml) for 30 min removed the mucus layer without damage to the epithelium and induced or markedly enhanced HCO(-)(3) secretory responses to cholera toxin (molecular mass of 85 kDa) or glucagon (3.5 kDa). Water extracts from a VacA cytotoxin (89 kDa) producing Helicobacter pylori strain, but not from a toxin-negative isogenic mutant, caused a small increase in HCO(-)(3) secretion but only after the mucus layer had been removed by papain. The duodenal surface mucus gel thus significantly restricts migration of macromolecules to the duodenal surface. Release of bacterial toxins at the cell-mucus interface may enhance or be a prerequisite for their effects on the gastrointestinal mucosa.The aim of this study was to investigate the permeability of the adherent mucus gel layer in rat duodenum in vivo to macromolecules applied in the lumen. Rats were anesthetized with thiobarbiturate, and the duodenum was perfused with isotonic NaCl solution containing large-molecular-size secretagogues. Effects on mucosal [Formula: see text]secretion and blood-to-lumen51chromium-labeled EDTA clearance were used as indexes that compounds had migrated across the mucus layer. Exposure to a low concentration of papain (10 U/100 ml) for 30 min removed the mucus layer without damage to the epithelium and induced or markedly enhanced [Formula: see text]secretory responses to cholera toxin (molecular mass of 85 kDa) or glucagon (3.5 kDa). Water extracts from a VacA cytotoxin (89 kDa) producing Helicobacter pylori strain, but not from a toxin-negative isogenic mutant, caused a small increase in[Formula: see text] secretion but only after the mucus layer had been removed by papain. The duodenal surface mucus gel thus significantly restricts migration of macromolecules to the duodenal surface. Release of bacterial toxins at the cell-mucus interface may enhance or be a prerequisite for their effects on the gastrointestinal mucosa.

Gastroduodenal bicarbonate secretion in mucosal protection. Possible role of vasoactive intestinal peptide and opiates.

HCO3− secretion by surface epithelium in duodenum devoid of Brunners glands was titratedin situ in anesthetized rats. Intravenous injection of small amounts (20 ng/kg) of the endogenous opioid peptide β-endorphin significantly increased secretion. Naloxone prevented this effect, suggesting that stimulation is mediated by μ-opiate receptors. Morphine 50 μg/kg had a similar stimulatory action. Vasoactive intestinal peptide (VIP) 0.5–100 μg/kg dose-dependently increased secretion and this response was independent of simultaneous cholinergic stimulation. The HCO3− secretion maintained pH in the mucus gel adherent to the luminal surface at neutrality for long periods of time (≧60 min); even when the pH in the terminal bulk solution was as low as 2.0. Mucosal HCO3− secretion is thus very probably important in mucosal protection and VIP and endogenous opioid peptides may have a role in its control.

β-Endorphin and enkephalins stimulate duodenal mucosal alkaline secretion in the rat in vivo

Secretion of HCO3- by duodenum just distal to the Brunners glands area and devoid of pancreatic HCO3- was titrated in situ in anesthetized rats. Secretion increased significantly after intravenous injection of small amounts (10-20 ng/kg) of the opioid peptides beta-endorphin, methionine-enkephalin, and leucine-enkephalin. Maximum (approximately twofold) stimulation by beta-endorphin and leucine-enkephalin occurred at 20 ng/kg. Morphine (50 micrograms/kg) caused a similar stimulation and the mu-selective opiate antagonist naloxone prevented the stimulation by beta-endorphin and morphine. The synthetic analogue [D-Ala2,D-Leu5]-enkephalin (500 ng/kg), which is an agonist primarily at delta-opiate receptors, had no effect, further suggesting that the stimulation of duodenal HCO3- secretion is mediated by mu-receptors. Naloxone alone did not affect basal HCO3- secretion but reduced the duration of the rise in secretion in response to a 5-min exposure to luminal acid (pH 2.00). Endogenous opioid peptides may thus have a role in the humoral or neural control, or both, of duodenal surface epithelial HCO3- secretion and mucosal protection.

Renomedullary and intestinal hyaluronan content during body water excess: a study in rats and gerbils

Our previous studies in rats have suggested a role for renomedullary hyaluronan (HA) in water homeostasis. The gerbil is known for its unique ability to conserve water. In the present study renal papillary and intestinal HA were compared between groups of anaesthetized gerbils and rats before and after up to 6 h of i.v. water loading. Baseline papillary HA in gerbils was only 37 % of that in the rat. Water loading in rats increased the papillary HA content. Elevation was maximal (+27 %, P < 0.05) after 2 h of water loading and then declined to control levels after 6 h of water loading (+3 %, n.s.). In contrast, the gerbil responded with a decreased papillary HA content during water loading. The depression was maximal after 2 h (‐49 %, P < 0.05) and was still 41 % below the control values after 6 h (P < 0.05). The urine flow rate increased rapidly in the rat and its maximum, 21 times above the control level (P < 0.05), occurred at the HA peak, i.e. after 2 h of water loading while in the gerbil, the urine flow rate increased slowly and slightly and was only six times above control values after 6 h of water loading (P < 0.05). The HA content along the intestine was similar in the two species: lowest in the duodenum and jejunum and highest in the distal colon. To conclude, in the rat, the elevation of papillary interstitial HA during acute water loading would counteract water reabsorption by changing the physico‐chemical characteristics of the interstitial matrix favouring rapid water diuresis. This would work as a complement to the powerful regulation by ADH. The gerbil has a diametrically different regulation of papillary HA turnover during water loading. The decreased papillary HA level during water loading and the slow and small diuretic response may represent a genetic difference in adaptation to enhance the ability to conserve water in an arid environment.

Melatonin inhibits alcohol-induced increases in duodenal mucosal permeability in rats in vivo

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 ~30-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 ⁵¹Cr-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 HCl-induced 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.

Luminal hypotonicity increases duodenal mucosal permeability by a mechanism involving 5-hydroxytryptamine

Aim:  To investigate whether 5‐hydroxytryptamine (5‐HT) participates in the mediation of the hypotonicity‐induced increase in duodenal mucosal permeability.

Interaction between neurokinin A, VIP, prostanoids, and enteric nerves in regulation of duodenal function

Neurokinin A (NKA) induces duodenal motility and increases mucosal permeability and bicarbonate secretion in the in situ perfused duodenum in anesthetized rats. In the present study, the NKA-induced increase in mucosal permeability was potentiated by luminal perfusion with lidocaine and diminished by vasoactive intestinal peptide (VIP) but unaltered by elevated intraluminal pressure. Elevation of intraluminal pressure, however, potentiated the stimulatory effect of NKA on bicarbonate secretion. In contrast, the tachykinin decreased the rate of alkalinization in rats subjected to elevated intraluminal pressure and treated with indomethacin. Similarly, NKA partially inhibited the VIP-stimulated bicarbonate secretion. Luminal lidocaine did not affect the secretory response to NKA. The motility induced by NKA was unaffected by VIP or lidocaine but decreased by elevated intraluminal pressure. It is concluded that the NKA-induced increase in duodenal mucosal bicarbonate secretion is independent of neurons and possibly mediated by prostanoids. The increase in mucosal permeability in response to NKA may be suppressed by mucosal nerves, perhaps utilizing VIP as one of the transmitters.Neurokinin A (NKA) induces duodenal motility and increases mucosal permeability and bicarbonate secretion in the in situ perfused duodenum in anesthetized rats. In the present study, the NKA-induced increase in mucosal permeability was potentiated by luminal perfusion with lidocaine and diminished by vasoactive intestinal peptide (VIP) but unaltered by elevated intraluminal pressure. Elevation of intraluminal pressure, however, potentiated the stimulatory effect of NKA on bicarbonate secretion. In contrast, the tachykinin decreased the rate of alkalinization in rats subjected to elevated intraluminal pressure and treated with indomethacin. Similarly, NKA partially inhibited the VIP-stimulated bicarbonate secretion. Luminal lidocaine did not affect the secretory response to NKA. The motility induced by NKA was unaffected by VIP or lidocaine but decreased by elevated intraluminal pressure. It is concluded that the NKA-induced increase in duodenal mucosal bicarbonate secretion is independent of neurons and possibly mediated by prostanoids. The increase in mucosal permeability in response to NKA may be suppressed by mucosal nerves, perhaps utilizing VIP as one of the transmitters.

Products of cyclooxygenase-2 depress duodenal function in rats subjected to abdominal surgery

Aim:  Abdominal surgery evokes powerful biological responses that affect gastrointestinal functions. Here we investigate the role of the cyclooxygenase‐1 (COX‐1) and cyclooxygenase‐2 (COX‐2) isoforms in post‐operative duodenal ileus.

Exposure of the Duodenum to High Concentrations of Hydrochloric Acid: Effects on Mucosal Permeability, Alkaline Secretion, and Blood Flow

Proximal duodenum was perfused with HCl for 5 min and the effects on blood-to-lumen clearance of 51Cr-EDTA (ED-Cl), morphology, luminal alkalinization, and blood flow determined in anesthetized rats. The rate of alkalinization was determined by back titration and blood flow assessed by laser Doppler flowmetry or by ultrasonic transit time flowmetry. Perfusion of duodenum with 30, 50 or 100 mM HCl for 5 min increased ED-Cl in a concentration-dependent manner and induced a small increase in alkalinization but had no effect on blood flow. At 55 min after cessation of perfusion with 100 mM HCl ED-Cl was 2.2-fold higher than control whereas the ED-Cl values in animals perfused with 30 or 50 mM HCl were not different from pre-acid control values. 100 mM HCl also induced an increase in 14C-mannitol and 14C-polyethylene glycol 4000 clearance, suggesting that HCl does indeed increase mucosal permeability. The 100 mM HCl-induced rise in mucosal permeability most probably reflects disturbance of mucosal integrity because three of five animals exhibited villous tip damage. The increases in ED-Cl in response to 100 mM HCl were the same in control rats as in rats with the renal pedicles ligated, indicating that the acid susceptibility is not affected by acute functional nephrectomy.