Sodium coupled bicarbonate transporters in the kidney, an update

Recently five genes have been cloned, which code for sodium dependent bicarbonate transport proteins. These genes belong to the SLC4A gene family. This short review summarizes our knowledge of these gene products with respect to their renal distribution and function. The best characterized members are the SLC4A4 and SLC4A7. SLC4A4 codes for an electrogenic Na+, HCO3--cotransporter (NBCe1), which is present in the basolateral membranes of proximal tubules and is responsible for the bicarbonate efflux here, and thus about 80% of the renal bicarbonate reabsorption. SLC4A7 codes for an electroneutral NBC (called NBC3 and NBCn1), which is present basolaterally in the thick ascending limb and the distal part of the collecting ducts and in intercalated cells (either apically or basolaterally) in the connecting and collecting tubules. In the thick ascending limb NBCn1 may be important for NH4+ reabsorption. SLCA5 codes for an electrogenic NBC (called NBC4 and NBCe2), which based on RT-PCR is located to the kidney but the exact localization awaits a good antibody. This is also the case for the SLC4A8 and SLC4A10 gene products, which are sodium dependent Cl-, HCO3- exchangers. The recent development in this field substantially increases our understanding of the complex renal regulation of acid base status.