THE HIGH-AFFINITY NA+ GLUCOSE COTRANSPORTER - REEVALUATION OF FUNCTION AND DISTRIBUTION OF EXPRESSION/

We report the primary structure, functional characterization, and tiss ue distribution of the high affinity Na+/glucose cotransporter SGLT1 f rom rat kidney. Rat SGLT1 (665 amino acid residues) is 86-87% identica l to SGLT1 from rabbit, pig, and human. High stringency Northern analy sis demonstrated that SGLT1 is strongly expressed in small intestine a nd at lower levels in kidney, liver, and lung. In situ hybridization p erformed on kidney sections revealed that SGLT1 is predominantly prese nt in S3 segments of the proximal tubule. In small intestine, SGLT1 me ssage was located in cells of the lower two-thirds of intestinal villi . Expression of rat SGLT1 in Xenopus oocytes resulted in a large Na+-d ependent uptake of [C-14]-alpha-methyl-D-glucopyranoside (alpha MeGlc) . Overall, the transport characteristics were similar to those of rabb it SGLT1. High affinity Na+/glucose cotransport in membrane vesicles w as previously shown to be coupled to the cotransport of two Na+ ions ( Turner, R. J., and Moran, A. (1982) J. Membr. Biol. 70, 37-45). Previo us kinetic analysis of rat and rabbit SGLT1, however, demonstrated bet ween second and first order dependence of sugar uptake on extracellula r Na+ concentration, suggesting the existence of Na+-binding sites wit h different affinities. Here, we directly compared the initial rates o f the alpha MeGlc uptake with alpha MeGlc induced inward currents as a n indicator of the Na+ flux. This analysis clearly revealed a Na+ to g lucose coupling ratio of 2:1. In summary, our data provide important i nsights into the function and tissue distribution of the high affinity Na+/glucose cotransporter SGLT1 and clarify its role in the reabsorpt ion mechanism of D-glucose in the kidney.