MOLECULAR CHARACTERISTICS OF NA-COUPLED GLUCOSE TRANSPORTERS IN ADULTAND EMBRYONIC RAT-KIDNEY()

Two distinct Na+-coupled glucose transporters (SGLTs) with either a hi gh or a low affinity for glucose were shown to provide reabsorption of filtered glucose in the kidney. We have previously reported the chara cteristics of the high affinity Na+/glucose cotransporter SGLT1 hom ra bbit, rat, and human kidney and the low affinity Na+/glucose cotranspo rter SGLT2 from human kidney. Because the molecular identity of SGLT2 as the kidney cortical low affinity Na+/glucose cotransporter has been recently challenged based on studies of the porcine low affinity Na+/ glucose cotransporter SAAT-pSGLT2 (Mackenzie, B., Panayotova-Heiermann , M., Loo, D. D. F., Lever, J. E., and Wright, E. M. (1994) J. Biol. C hem. 269, 22488-22491), we have reevaluated the properties of SGLT2 in greater detail. We furthermore report new data on the regulation of S GLT1 and SGLT2 during kidney development. To analyze and compare SGLT1 and SGLT2 in adult and embryonic kidney, we have cloned and character ized SGLT2 from rat kidney and determined its tissue distribution base d on Northern analysis and in situ hybridization. When expressed in Xe nopus oocytes, rat SGLT2 stimulated transport of cu-methyl-D-glucopyra noside (2 mM) in oocytes up to 4.5-fold over controls with an apparent K-m of 3.0 mM. The transport properties (i.e. a Na+ to glucose coupli ng of 1:1 and lack of galactose transport) generally matched those of the kidney cortical low affinity system. We show that expression of ra t SGLT2 mRNA is kidney specific and that it is strongly and exclusivel y expressed in proximal tubule Si segments. Hybrid-depletion studies w ere performed to conclusively determine whether SGLT2 corresponds to t he kidney cortical low affinity system, injection of rat kidney superf icial cortex mRNA into oocytes stimulated the uptake of alpha-methyl-D -glucopyranoside (2 mM) 2-3-fold. We show that hybrid depletion of thi s kidney RNA using an SGLT2 antisense oligonucleotide completely suppr esses the uptake. These data strongly indicate that SGLT2 is the major kidney cortical low affinity glucose transporter. We therefore propos e that SAAT-pSGLT2 be renamed SGLT3. Experiments addressing the expres sion of SGLT1 and SGLT2 mRNAs in embryonic rat kidneys reveal that the two Na+/glucose cotransporters are developmentally regulated and that there may be a different splice variant for SGLT2 in embryonic kidney compared to the adult.