Sodium-glucose cotransporters display sodium- and phlorizin-dependent water permeability

Expression of Na+-glucose cotransporters of the SGLT-1 type by Xenopus laevis oocytes increased the osmotic water permeability (P-f) of oocytes by a factor of 1.9-2.8, in the presence and in the absence of 5 mM extracellular glucose. The P-f increase was correlated with the amount of SGLT-1 cRNA injected. In oocytes expressing SGLT-1, either addition of phlorizin to the medium or the replacement of Na+ by choline inhibited the uptake of methyl-alpha-D-glucopyranoside, a specific substrate for SGLT-1, and returned oocyte P-f to its level in uninjected oocytes. Phlorizin inhibited the SGLT-1-attributable increase in P-f with an inhibition constant (K-i) of 6.1 mu M, a value analogous to the K-i for phlorizin inhibition of sugar uptake. However, neither the presence of phlorizin nor the absence of extracellular Na+ significantly affected the increase in P-f elicited in oocytes expressing GLUT-1, a facilitative glucose transporter. These findings suggest that SGLT-1 forms a pore that allows the transmembrane passage of water and that water and glucose traverse the protein through this pore. The finding that removal of extracellular Na+ abolishes the increase in P-f attributable to SGLT-1 suggests that extracellular Na+ is required to maintain patency of this transporter's water-permeable transmembrane pore.