The high affinity Na⁺/glucose cotransporter: Re-evaluation of function and distribution of expression

We report the primary structure, functional characterization, and tissue distribution of the high affinity Na⁺/glucose cotransporter SGLT1 from rat kidney. Rat SGLT1 (665 amino acid residues) is 86-87% identical to SGLT1 from rabbit, pig, and human. High stringency Northern analysis demonstrated that SGLT1 is strongly expressed in small intestine and at lower levels in kidney, liver, and lung. In situ hybridization performed on kidney sections revealed that SGLT1 is predominantly present in S3 segments of the proximal tubule. In small intestine, SGLT1 message was located in cells of the lower two-thirds of intestinal villi. Expression of rat SGLT1 in Xenopus oocytes resulted in a large Na⁺-dependent uptake of [¹⁴C]-α-methyl-D-glucopyranoside (αMeGlc). Overall, the transport characteristics were similar to those of rabbit SGLT1. High affinity Na⁺/ glucose cotransport in membrane vesicles was 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). Previous kinetic analysis of rat and rabbit SGLT1, however, demonstrated between second and first order dependence of sugar uptake on extracellular Na⁺ concentration, suggesting the existence of Na⁺-binding sites with different affinities. Here, we directly compared the initial rates of the αMeGlc uptake with αMeGlc-induced inward currents as an indicator of the Na⁺ flux. This analysis clearly revealed a Na⁺ to glucose coupling ratio of 2:1. In summary, our data provide important insights into the function and tissue distribution of the high affinity Na⁺/glucose cotransporter SGLT1 and clarify its role in the reabsorption mechanism of D-glucose in the kidney.