Na plus -dependent intestinal glucose absorption mechanisms and its luminal Na plus homeostasis across metamorphosis from tadpoles to frogs

The abrupt morphological changes of the intestine during metamorphosis have been detailed in frogs. The features of intestinal metamorphosis are shortening of the intestine and remodeling of the intestinal epithelium. It is believed that the purpose of the morphological changes of the intestine is adaptation from aquatic herbivorous to carnivorous life. However, little is known about the physiological importance of these morphological changes. To elucidate the functional changes during metamorphosis, we measured luminal Na+ concentrations and Na+-dependent glucose uptake in tadpoles and adult African clawed frogs Xenopus laevis. The small intestine was isolated and divided into four segments in length, the luminal contents collected for analysis of ion concentration by ion chromatography. Phlorizin-sensitive glucose-induced short-circuit current (DIsc) was measured in intesti-nal preparations mounted in Ussing chambers. Although dietary sodium intake was extremely low in tadpoles, luminal Na+ con-centration gradually increased along the proximal to the middle part of the intestine (>70 mM), and this Na + concentration was comparable with that of carnivorous adult frogs. The increment of glucose-induced DIsc was observed in tadpole intestine. We also measured the DIsc induced by acetic acid, which is the major short-chain fatty acid produced by fermentation. The expres-sion levels of mRNA for Na+-dependent glucose transporter 1 and tight junction protein claudin-15 in each intestinal segment was measured. These results suggest that luminal Na+ homeostasis is important and luminal Na+ is kept at a high concentra-tion for Na+-dependent nutrient absorption mechanisms.