SGLT1 and Osmotic Water Flow across the Small Intestine

Abstract

Passive water flow is important in the physiology of the small intestine in response to osmotic gradients across the epithelium following the ingestion of food and water; and malabsorption syndromes. However, the mechanism of osmotic water flow across the small intestine is poorly understood. Questions arise on the relative contributions of the cellular and paracellular pathways; and the identity of water channels in the apical and basolateral membrane of the epithelium. It is often assumed that the tight junctions are highly permeable to water simply because they are permeable to small ions; and orthodox aquaporins are not expressed in the small intestine. We have previously postulated that the intestinal Na+/glucose cotransporter SGLT1 plays a key role in fluid transport based on studies of SGLT1 expressed in Xenopus laevis oocytes (see Loo et al 1999. J. Physiology 518:195–202; and Loo et al 1996 PNAS 93:13367–13370). The osmotic water permeability of SGLT1 in oocytes (Pos) is independent of the magnitude and direction of the osmotic gradient, and the presence of ligands (Na+ and sugar), but is inhibited by phlorizin in the presence of Na+ (Ki 5 μM). Furthermore, osmotic water flow has a low temperature dependence (5 kcal/mole) relative to Na+/glucose cotransport (>20 kcal/mole).To investigate the importance of SGLT1 in osmotic flow we measured the effect of phlorizin on osmotic flow. Our approach was to prepare everted sacs of mouse jejunum and measure water flow by recording the change in weight of the sacs with time in response to osmotic gradients. The osmotic gradients were generated by the addition of mannitol to the mucosal buffer, and the contribution of SGLT1 to the osmotic flow was determined by the addition of phlorizin. The experiments were conducted at 21–23°C to maintain the viability of the tissue, a long standing practice in studies of mammalian tissues in vitro. In the absence of an osmotic gradient between the mucosal and serosal solutions there was no significant change in the weight of the sac but the addition of 100 mM mannitol to the mucosal fluid produced a loss of weight equivalent to 16 ± 2 nl min−1 cm−1 and a Pos of 0.012 cm s−1. Phlorizin (250 μM) added to the mucosal fluid reduced the flow to 6.5 ± 2 nl min−1 cm−1.These results indicate that 65% of the osmotic flow across the mouse small intestine occurs through SGLT1, and less than 35% of the total flow occurs through the paracellular pathway.Support or Funding InformationNIH NIDDK DK19567