Chloride is a critical component of all living cells. It is also the single most dominant diffusible anion inside of most cells - the others are mostly impermeable organic anions. Since cytoplasmic electroneutrality is maintained under normal physiological environment, changes in cellular chloride level is accompanied by total cell solute content. Because of high water permeability of cell membranes, changes in cell solute content are accompanied by changes in cellular volume. In order to maintain constant cell volume and prevent swelling, cells must expend energy through primary and secondary active transport mechanisms. The cation chloride cotransporters (CCC) protein family is involved in the electroneutral movement of ions across the cell membrane.

Three groups of proteins have been identified in CCC family based upon their structures, ligands, and inhibitors. These are the thiazide-sensitive Na+-Cl-cotransporters (NCC or TSC), the loop diuretics-sensitive Na+-K+-Cl- (NKCC) cotransporters (NKCC1/CCC1/BSC2 and NKCC2/CCC2/BSC1), and the K+-Cl--cotransporters (KCC1-4). These co-transporters share a common predicted membrane topology, with 12 TM domains (~500 aa), and long hydrophilic, intracellular N-and C-termini containing regulatory phosphorylation sites. NKCC transport Na, K, and Cl ions into and out of a wide variety of epithelial and non-epithelial cells. The transport process is characterized by electroneutrality (almost always with stoichiometry of 1Na:1K:2Cl) and inhibition by the loop diuretic bumetanide, benzametanide, and furosemide.

NKCC1/BSC2 (human 1212 aa, rat 1203 aa, and mouse 1205 aa; calculated mol. wt of ~115 kDa; actual size is ~145kDa) is strongly expressed in the kidney and many other tissues. In mouse kidney, NKCC1 is detected in basolateral membrane of IMCD and in mesangial cells in the glomerulus. Within the secretory epithelia, BSC2 protein has been localized to the basolateral membrane of acinar cells in rat submandubular gland. In the brain, NKCC1 mRNA is most prominent in choroid plexus, and lower levels were found in cerebellum, and brain stem.

The renal thick ascending limb of the loop of Henle (TALH) is the main pharmacological target of the loop diuretics (e.g., bumetanide) that inhibit NKCC2/BSC1 (human 1099 aa, rat/mouse 1095 aa; calculated mol size of ~121 kDa; actual size ~150 kDa; ~45% identity with NKCC1). Unlike NKCC1, NKCC2 is most strongly expressed in the kidney (inner and outer stripes of medulla and cortical thick ascending limb), and macula densa. NKCC2 plays a critical role in transcellular absorption of Na+-Cl- by the medullary and cortical TALH, and a secondary role in the paracellular transport of Na-Ca and Mg. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characterized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting, and volume depletion.

In the distal convoluted tubule (DCT) of the kidney, the primary apical entry pathway for Na is via the thiazide-sensitive Na-Cl transporter (TSC or NCC; human 1021 aa, rat/mouse 1002 aa; ~60 % identity with NKCC). TSC is specifically detected at the apical cell membrane of DCT.

ADI has produced highly specific rabbit antibodies to NKCC1, NKCC2, and TSC using peptide sequences specific to each transporter. These antibodies should be useful in studying the physiological roles of various transporters.

m=mouse; r=rat; h=human; ch=chicken; f=frog; ~CT or ~NT=near C or N-terminus. EC=Extracellular; CP=Cytoplasmic domain;