Aquaporin-6 is expressed along the rat gastrointestinal tract and upregulated by feeding in the small intestine

Background: Several aquaporins (a family of integral membrane proteins) have been recently identified in the mammalian gastrointestinal tract, and their involvement in the movement of fluid and small solutes has been suggested. In this direction we investigated, in some regions of the rat gastrointestinal tract, the presence and localization of aquaporin-6, given its peculiar function as an ion selective channel. Results: RT-PCR and immunoblotting experiments showed that aquaporin-6 was expressed in all the investigated portions of the rat gastrointestinal tract. The RT-PCR experiments showed that aquaporin-6 transcript was highly expressed in small intestine and rectum, and less in stomach, caecum and colon. In addition, jejunal mRNA expression was specifically stimulated by feeding. Immunoblotting analysis showed a major band with a molecular weight of about 55 kDa corresponding to the aquaporin-6 protein dimer; this band was stronger in the stomach and large intestine than in the small intestine. Immunoblotting analysis of brush border membrane vesicle preparations showed an intense signal for aquaporin-6 protein. The results of in situ hybridization experiments demonstrate that aquaporin-6 transcript is present in the isthmus, neck and basal regions of the stomach lining, and throughout the crypt-villus axis in both small and large intestine. In the latter regions, immunohistochemistry revealed strong aquaporin-6 labelling in the apical membrane of the surface epithelial cells, while weak or no labelling was observed in the crypt cells. In the stomach, an intense staining was observed in mucous neck cells and lower signal in principal cells and some parietal cells. Conclusion: The results indicate that aquaporin-6 is distributed throughout the gastrointestinal tract. Aquaporin-6 localization at the apical pole of the superficial epithelial cells and its upregulation by feeding suggest that it may be involved in movements of water and anions through the epithelium of the villi

Guanidine transport across the apical and basolateral membranes of human intestinal Caco-2 cells is mediated by two different mechanisms

The functional characteristics of the intestinal absorption and secretion of guanidine as a model of a nutritionally and metabolically essential organic cation were examined in the Caco-2 human intestinal cell line. Both apical and basolateral transport of [14C]-guanidine were studied using Caco-2 cells grown on polycarbonate permeable membranes. The basolateral-to-apical flux of [14C]-guanidine (i.e., its secretion) was quantitatively higher than the apical-to-basolateral transport (i.e., its absorption). When Na was replaced by K or Li, both apical and basolateral accumulation were significantly inhibited. Studies using the cell monolayers and apical membrane vesicles obtained from Caco-2 cells showed a potential-independent mechanism of guanidine apical uptake and efflux. Conversely, basolateral uptake and efflux were membrane potential dependent. Kinetic analysis revealed that both saturable and nonsaturable mechanisms accounted for the apical and basolateral accumulations. The [14C]-guanidine efflux from cells through the apical and basolateral membranes was significantly reduced at 4°C, suggesting carrier-mediated mechanisms. Moreover, the apical efflux was stimulated by an inwardly directed H gradient. Influx and efflux of [14C]-guanidine were unaffected by the presence of tetraethylammonium, cimetidine or decynium-22 in the donor compartment. Only quinine significantly reduced [14C]- guanidine entrance through apical and basolateral membranes and its exit through the basolateral membrane. In conclusion, our results suggest that the influx and the efflux through the apical membrane is mediated by different transporters, whereas transport across the basolateral membrane is mediated by a member of the organic cation transporter family with high affinity for guanidine

PYY-Tag transgenic mice displaying abnormal (H+-K+)ATPase activity and gastric mucosal barrier impairment

The mechanism by which the gastrointestinal hormones peptide YY and glucagon inhibit gastric acid secretion is largely unknown. PYY-Tag transgenic mice develop endocrine tumors in the colon that are composed mainly of peptide YY/enteroglucagon-producing L type cells. Therefore we studied the functional activity of such tumors and the gastric functions of PYY-Tag mice. Fasting and fed PYY-Tag transgenic mice and CD1 controls were assayed for circulating levels of peptide YY, glucagon, insulin, and gastrin. The gastric pH was determined and gastric samples were examined for (a) histologic appearance; (b) K(+)-stimulated p-nitrophenylphosphatase activity and [(14)C]aminopyrine accumulation of apical and tubulovesicle membranes; (c) adherent mucus determination by Alcian blue recovery; and (d) DNA/RNA/protein epithelial content and in vivo incorporation of [(3)H]thymidine into DNA. Transgenic mice showed high serum levels of peptide YY and glucagon, increased gastric pH, and a high incidence of gastric ulcers after fasting. p-Nitrophenylphosphatase activity, [(14)C] aminopyrine accumulation, and proton pump redistribution from cytoplasmic tubulovesicles to apical membranes were significantly lower in the gastric mucosa of transgenic mice compared with the controls. In addition, the adherent mucus was thinner, and [(3)H]thymidine incorporation into the DNA was decreased. The abnormal and unregulated levels of circulating peptide YY and glucagon led to gastric acid inhibition and an impairment of gastric barrier function as a result of a striking reduction in epithelial proliferation