Molecular evidence for two renal Na+/glucose cotransporters

Previous studies have shown that two kinetically and genetically distinct Na+/glucose cotransporters exist in mammalian kidney. We have recently cloned and sequenced one of the rabbit renal Na+/glucose cotransporters (SGLT1) and have found that it is identical in sequence to the intestinal Na+/glucose cotransporter. Northern blots showed that SGLT1 mRNA was found predominantly in the outer medulla of rabbit kidney. Injection of mRNA from outer medulla and outer cortex into Xenopus oocytes resulted in equal expression of Na(+)-dependent sugar uptake, indicating that the outer cortex sample contained mRNA encoding both SGLT1 and a second Na+/glucose cotransporter. Western blots using antipeptide antibodies against SGLT1 showed that the SGLT1 protein is more abundant in outer medulla than outer cortex. However, brush border membrane vesicles prepared from outer cortex had a greater capacity for Na(+)-dependent glucose transport, indicating the presence of a second transporter in the vesicles from outer cortex. It appears that the cloned renal Na+/glucose cotransporter, SGLT1, is the 'high affinity, low capacity' transporter found predominantly in outer medulla. There is evidence that a second transporter, the 'low affinity, high capacity' transporter, is in outer cortex. Finally, the cDNA and protein sequences of the two renal Na+/glucose cotransporters are predicted to differ by more than 20%.