THE NA-K-CL COTRANSPORTERS

The Na-K-Cl cotransporters are a class of membrane proteins that trans port Na, K, and Cl ions into and out of a wide variety of epithelial a nd nonepithelial cells. The transport process mediated by Na-K-CI cotr ansporters is characterized by electroneutrality (almost always with s toichiometry of 1Na:1K:2Cl) and inhibition by the ''loop'' diuretics b umetanide, benzmetanide, and furosemide. Presently, two distinct Na-K- Cl cotransporter isoforms have been identified by cDNA cloning and exp ression; genes encoding these two isoforms are located on different ch romosomes and their gene products share approximately 60% amino acid s equence identity. The NKCC1 (CCC1, BSC2) isoform is present in a wide variety of tissues; most epithelia containing NKCC1 are secretory epit helia with the Na-K-Cl cotransporter locaIized to the basolateral memb rane. By contrast, NKCC2 (CCC2, BSC1) is found only in the kidney, loc alized to the apical membrane of the epithelial cells of the thick asc ending limb of Henle's loop and of the macula densa. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characte rized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting , and volume depletion. The two Na-K-Cl cotransporter isoforms are als o part of a superfamily of cation-chloride cotransporters, which inclu des electroneutral K-CI and Na-CI cotransporters. Na-K-Cl cotransporte r activity is affected by a large variety of hormonal stimuli as well as by changes in cell volume; in many tissues this regulation (particu larly of the NKCC1 isoform) occurs through direct phosphorylation/deph osphorylation of the cotransport protein itself though the specific pr otein kinases involved remain unknown. An important regulator of cotra nsporter activity-in secretory epithelia and other cells as well is in tracellular [Cl] ([Cl](i)), with a reduction in [Cl](i) being the appa rent means by which basolateral Na-K-Cl cotransport activity is increa sed and thus coordinated with that of stimulated apical Cl channels in actively secreting epithelia.