Molecular cloning, primary structure, and characterization of two members of the mammalian electroneutral sodium-(potassium)-chloride cotransporter family expressed in kidney

The membrane transporters regulating epithelial NaCl secretion

Ten years ago, the basic principles operating in one specific, albeit non-mammalian, exocrine gland, the rectal gland of Squalus acanthias, were described in detail. The concept emerging from these studies appeared applicable to almost any other exocrine gland, because it involved membrane transporters which are also present in mammalian epithelial cells. Meanwhile, it has become clear that the mechanisms of NaCl secretion are diverse: the mechanisms of NaCl uptake; the ion channels involved; and also the mechanisms of hormonal control. Nevertheless, several steps in NaCl secretion still appear to be uniform: (1) several signalling pathways converge and act cooperatively, (2) one primary regulatory step is the upregulation of the luminal Cl- conductance, (3) secondarily active NaCl uptake mechanisms are upregulated, (4) increasing evidence links NaCl secretion to membrane trafficking and (5) the entire machinery seems to be primed to secure cellular homeostasis in terms of cytosolic ion concentrations. This brief review summarizes the mechanisms of control of NaCl secretion. The major issues addressed are the NaCl uptake mechanisms, the ion channels involved and the cellular mechanisms coordinating secretion. The major NaCl secreting cells discussed here will be the respiratory epithelial cells, the exocrine cells of pancreatic acini and the cells of colonic crypts.

Na(+)-K+(NH4+)-2Cl- cotransport in medullary thick ascending limb: control by PKA, PKC, and 20-HETE

Cell pH was monitored in suspensions of medullary thick ascending limbs (MTALs) of rat kidney to determine possible effects of various transduction pathways on apical Na(+)-K+ (NH4+)-2Cl- cotransport, the activity of which was measured as the bumetanide-sensitive component of cell acidification caused by abrupt exposure to 4 mM NH4Cl. 8-Bromoadenosine 3',5'-cyclic monophosphate stimulated cotransport activity through activation of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA), since the cAMP effect was abolished by N-[2-(p- bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89); stimulation by cAMP (P < 0.02) was observed even when other Na+, Cl-, and K+ carriers were blocked by ouabain, diphenylamine-2-carboxylate, and barium, which indicates that cotransport was directly affected by PKA. Phorbol 12,13-dibutyrate also stimulated cotransport activity (P < 0.03), which was abolished by protein kinase C (PKC) blockade by staurosporine. In contrast, cotransport activity was reduced (P < 0.001) by arachidonic acid or 20-hydroxyeicosatetraenoic acid (20-HETE), as well as by an ionomycin-induced rise in cytosolic Ca2+ ([Ca2+]i). Inhibition by arachidonic acid or ionomycin was abolished by econazole and SKF-525A that inhibit cytochrome P-450-dependent monoxygenase, which produces 20-HETE from arachidonic acid in the MTAL, and the ionomycin effect was prevented when phospholipase A2 (PLA2) was blocked by 4-bromophenacyl bromide or oleyloxyethyl phosphorylcholine. The results demonstrate that MTAL apical Na(+)-K+(NH4+)-2Cl- cotransport is stimulated by PKA and PKC and inhibited by 20-HETE that may be produced after a rise in [Ca2+]i through PLA2 activation.

Molecular cloning and functional expression of the K-Cl cotransporter from rabbit, rat, and human. A new member of the cation-chloride cotransporter family

We report the cloning, sequence analysis, tissue distribution, and functional expression of the K-Cl cotransport protein, KCC1. KCC1 was identified by searching the human expressed sequence tag data base, based on the expectation that it would be distantly related to the Na-K-Cl cotransporter. Rabbit KCC1 (rbKCC1) and rat KCC1 (rtKCC1) were cloned by screening rabbit kidney and rat brain cDNA libraries using homologous cDNA probes. Human KCC1 (hKCC1) was obtained from I.M.A.G.E. clones and in part by reverse transcription-polymerase chain reaction; it exhibits 97% identity with rbKCC1. KCC1 encodes a 1085-residue polypeptide with substantial sequence homology (24-25% identity) to the bumetanide-sensitive Na-K-Cl cotransporter (NKCC or BSC) and the thiazide-sensitive Na-Cl cotransporter (NCC or TSC). Hydropathy analysis of KCC1 indicates structural homology to NKCC, including 12 transmembrane domains, a large extracellular loop with potential N-linked glycosylation sites, and cytoplasmic N- and C-terminal regions. Northern blot analysis revealed a ubiquitously expressed 3. 8-kilobase transcript. Much of the genomic sequence of hKCC1 is in the data base, and the gene has been previously localized to 16q22.1 (Larsen, F., Solhein, J., Kristensen, T., Kolsto, A. B., and Prydz, H.(1993) Hum. Mol. Genet. 2, 1589-1595). Epitope-tagged rbKCC1 was stably expressed in human embryonic kidney (HEK 293) cells, resulting in production of a approximately150-kDa glycoprotein. The initial rate of 86Rb efflux from cells expressing rbKCC1 was more than 7 times greater than efflux from control cells and was inhibited by 2 mM furosemide; 86Rb efflux was stimulated by cell swelling. Uptake of 86Rb into rbKCC1 cells after a 15-min pretreatment with 1 mM N-ethylmaleimide was dependent on external chloride but not on external sodium, and was inhibited by furosemide with a Ki of approximately 40 microM and by bumetanide with a Ki of approximately 60 microM. These data demonstrate that the KCC1 cDNAs encode a widely expressed K-Cl cotransporter with the characteristics of the K-Cl transporter that has been characterized in red cells.

Molecular characterization of a putative K-Cl cotransporter in rat brain. A neuronal-specific isoform

Using a combination of data base searching, polymerase chain reaction, and library screening, we have identified a putative K-Cl cotransporter isoform (KCC2) in rat brain that is specifically localized in neurons. A cDNA of 5566 bases was obtained from overlapping clones and encoded a protein of 1116 amino acids with a deduced molecular mass of 123.6 kDa. Over its full length, the amino acid sequence of KCC2 is 67% identical to the widely distributed K-Cl cotransporter isoform (KCC1) identified in rat brain and rabbit kidney (Gillen, C., Brill, S., Payne, J.A., and Forbush, B., III(1996) J. Biol. Chem. 271, 16237-16244) but only approximately25% identical to other members of the cation-chloride cotransporter gene family, including "loop" diuretic-sensitive Na-K-Cl cotransport and thiazide-sensitive Na-Cl cotransport. Based on analysis of the primary structure as well as homology with other cation-chloride cotransporters, we predict 12 transmembrane segments bounded by N- and C-terminal cytoplasmic regions. Four sites for N-linked glycosylation are predicted on an extracellular intermembrane loop between putative transmembrane segments 5 and 6. Northern blot analysis using a KCC2-specific cDNA probe revealed a very highly expressed approximately5.6-kilobase transcript only in brain. Reverse transcriptase-polymerase chain reaction revealed that KCC1 was present in rat primary astrocytes and rat C6 glioma cells but that KCC2 was completely absent from these cells, suggesting KCC2 was not of glial cell origin. In situ hybridization studies demonstrated that the KCC2 transcript was expressed at high levels in neurons throughout the central nervous system, including CA1-CA4 pyramidal neurons of the hippocampus, granular cells and Purkinje neurons of the cerebellum, and many groups of neurons throughout the brainstem.

Localization of the thiazide sensitive Na-Cl cotransporter, rTSC1 in the rat kidney

A thiazide sensitive Na-Cl cotransporter, rTSC1, has recently been cloned from a rat kidney cortex cDNA library. The molecular regulation and nephron localization of this protein is unknown. The purpose of this study was to examine the nephron distribution and subcellular localization of the rTSC1 protein in the rat kidney. In situ hybridization showed rTSC1 transcripts were localized to short, convoluted tubule segments in the kidney cortex. Polyclonal antibodies raised against a 110 amino acid segment from the amino terminus of rTSC1 recognized three major bands of 135, 140 and 155 kDa on Western blotting of membrane protein from cortex but not outer medulla of the rat kidney. Immunofluorescence studies using the antibody alone and in double labeling experiments with antibodies against the H+ ATPase and calbindin D28, showed intense labeling of apical membranes in the distal nephron beginning in the initial distal convoluted tubule and terminating within the connecting tubule. The intensity of labeling diminished from proximal to distal sites along the distal tubule. Ultrastructural studies by immunoelectron microscopy showed the cotransporter protein to be localized predominately on apical microvilli of the distal convoluted tubule cells. These results are consistent with rTSC1 encoding the apical thiazide sensitive Na-Cl cotransporter in the distal tubule.

Bartter's syndrome, hypokalaemic alkalosis with hypercalciuria, is caused by mutations in the Na-K-2Cl cotransporter NKCC2

Inherited hypokalaemic alkalosis with low blood pressure can be divided into two groups-Gitelman's syndrome, featuring hypocalciuria, hypomagnesaemia and milder clinical manifestations, and Bartter's syndrome, featuring hypercalciuria and early presentation with severe volume depletion. Mutations in the renal Na-Cl cotransporter have been shown to cause Gitelman's syndrome. We demonstrate linkage of Bartter's syndrome to the renal Na-K-2Cl cotransporter gene NKCC2, and identify frameshift or non-conservative missense mutations for this gene that co-segregate with the disease. These findings demonstrate the molecular basis of Bartter's syndrome, provide the basis for molecular classification of patients with inherited hypokalaemic alkalosis, and suggest potential phenotypes in heterozygous carriers of NKCC2 mutations.

Whys and wherefores of juxtaglomerular apparatus function

The juxtaglomerular apparatus is an anatomic structure which links the distal end of the thick ascending limb at the macula densa with the glomerular vascular pole. Specialized interstital cells and renin containing granular cells are located in the vascular hilum at this site. Evidence has accumulated that this connection is critical for local regulation of renin secretion and glomerular vascular tone via the tubuloglomerular feedback mechanism. The tubuloglomerular feedback mechanism maintains a constant chloride concentration at the macula densa at a set point determined by the volume state of the animal, a effect probably important for adjustment of renin secretion to changing salt balance. Evidence supporting these two regulatory roles is reviewed here.

Structure, regulation and physiological roles of urea transporters

Urea is the major constituent of the urine and the principal means for disposal of nitrogen derived from amino acid metabolism. Specialized phloretin-inhibitable urea transporters are expressed in kidney medulla and play a central role in urea excretion and water balance. These transporters allow accumulation of urea in the medulla and enable the kidney to concentrate urine to an osmolality greater than systemic plasma. Recently, expression cloning with Xenopus oocytes has led to the isolation of a novel phloretin-inhibitable urea transporter (UT2) from rabbit, and subsequently from rat kidney. UT2 from both species has the characteristics of the phloretin-sensitive urea transporter previously defined in kidney by in vitro perfused tubule studies. Based on these advances, Ripoche and colleagues cloned a homologous urea transporter (HUT11) from erythrocytes. UT2 and HUT11 predict 43 kDa polypeptides and exhibit 64% amino acid sequence identity. Since regulation of urea transport in the kidney plays an important role in the orchestration of the antidiuretic response, we have studied the regulation of urea transporter in rat kidney at the mRNA level. On Northern blots probed at high stringency, rat UT2 hybridized to two transcripts of 2.9 kb and 4.0 kb, which have spatially distinct distributions within the kidney. Northern analysis and in situ hybridization of kidneys from rats maintained at different physiologic states revealed that the 2.9 and 4.0 kb transcripts are regulated by separate mechanisms. The 4 kb transcript was primarily responsive to changes in the dietary protein content, whereas the 2.9 kb transcript was highly responsive to changes in the hydration state of the animal. We propose that the two UT2 transcripts are regulated by distinct mechanisms to allow optimal fluid balance and urea excretion.

The electroneutral Na(+)-(K+)-Cl- cotransport family

Recently the molecular identification of the major electroneutral sodium-potassium-chloride entry mechanisms present on apical membranes of distal nephron segments of the mammalian kidney, on basolateral membranes of many non-renal epithelial cells and on certain non-epithelial tissues has been achieved. These transporters represent a major pathway for cellular uptake of chloride critical for chloride absorptive and secretory processes and for cell volume regulation following cell shrinkage. In the mammalian kidney, these sodium-coupled chloride cotransporters represent the major target sites for clinically useful diuretics including the "loop" diuretics [furosemide (Lasix) and bumetanide (Bumex)] and thiazides (such as, chlorothiazide, hydrochlorothiazide and metolazone). Although these Na-(K)-Cl cotransporters exhibit functional and pharmacological differences, they clearly evolved from a common ancestral gene and thus form a new gene family. This information is already advancing our understanding of the evolution, structure and function of these transporters both in renal handling of sodium and in hypertension.

Molecular genetics of human blood pressure variation

Hypertension is a common multifactorial vascular disorder of largely unknown cause. Recognition that hypertension is in part genetically determined has motivated studies to identify mutations that confer susceptibility. Thus far, mutations in at least 10 genes have been shown to alter blood pressure; most of these are rare mutations imparting large quantitative effects that either raise or lower blood pressure. These mutations alter blood pressure through a common pathway, changing salt and water reabsorption in the kidney. These findings demonstrate the utility of molecular genetic approaches to the understanding of blood pressure variation and may provide insight into the physiologic mechanisms underlying common forms of hypertension.

Cloning and kidney cell-specific activity of the promoter of the murine renal Na-K-C1 cotransporter gene

The murine Nkcc2/Slcl2a1 gene encodes a bumetanide-sensitive Na-K-Cl cotransporter that is expressed exclusively in the kidney in the thick ascending limb of the loop of Henle. Nuclear run-off assays demonstrated that kidney-specific expression of Nkcc2 was due, at least in part, to kidney-specific gene transcription. To begin study of the gene promoter, a genomic clone that contained 13.5 kilobases of the 5'-flanking region of Nkcc2 was isolated. A single transcription initiation site was located 1330 base pairs (bp) upstream of the start codon. The sequence of the proximal 5'-flanking region contained typical eukaryotic promoter elements including a TATA box, two CCAAT boxes, and an initiator. A (G-A)28.(C-T)28 microsatellite and consensus binding sites for hepatocyte nuclear factor 1, cAMP-response element binding protein, CCAAT/enhancer-binding proteins, and basic helix-loop-helix proteins, were also identified. To functionally express the promoter, 2255 bp of the proximal 5'-flanking region was ligated to a luciferase reporter gene and transfected into thick ascending limb (TAL) cells, a stable cell line derived from microdissected loops of Henle of the Tg(SV40E)Bri7 mouse. TAL cells exhibited furosemide-sensitive Na-K((NH4)+)-Cl cotransport activity and endogenously expressed the 5.0-kilobase Nkcc2 transcript. Luciferase activity was 130-fold greater following transfection into TAL cells compared with transfection into cells that did not express Nkcc2 (NIH 3T3 fibroblasts). Deletion analysis revealed that promoter activity in TAL cells was similar in constructs extending from the transcription initiation site to -1529 to -469, whereas further deletion to -190 resulted in a 76% decrease in activity. We conclude that the Nkcc2 promoter exhibits kidney cell-specific activity. Regulatory elements required for maximal promoter activity are located in a 280-bp DNA segment that contains consensus binding sites for several transcription factors expressed in the kidney.

Crypt base cells show forskolin-induced Cl- secretion but no cation inward conductance

Whole-cell patch-clamp studies in base cells of isolated colonic crypts of rats pretreated with dexamethasone were performed to examine the effects of stimulation by forskolin (10 micromol/l). The experiments were designed in order to distinguish between two postulated effector mechanisms: the activation of a non-selective cation channel and the activation of Cl- channels. As shown in an accompanying report, forskolin depolarizes the membrane voltage (Vm) by some 40-50 mV and enhances the whole-cell membrane conductance (Gm) substantially in these cells. In this report all experiments were performed in the presence of forskolin. A reduction of the bath Na+ concentration from 145 to 2 mmol/l led to a hyperpolarization of Vm by some 20-30 mV. This hyperpolarization occurred very slowly suggesting that the hyperpolarization produced by the low-Na+ solution was caused indirectly and not by a change in the equilibrium potential for Na+, ENa+. A complete kinetic analysis of the effect on voltage of bath Na+ revealed a saturation-type relation with a high apparent affinity for Na+ of around 5-10 mmol/l. A reduction in bath Cl- concentration from 145 to 32 mmol/l caused a depolarization of Vm from -34 +/- 3 to -20 +/- 4 mV (n = 13) in the presence of a high bath Na+ concentration, but had the opposite effect at low (5 mmol/l) Na+ concentrations: Vm was hyperpolarized from -46 +/- 4 to -62 +/- 6 mV (n = 13). If the effect of Na+ on Vm was caused by a non-selective cation channel the opposite would have been expected. To test directly whether the Na+2Cl-K+ cotransporter was responsible for the effects of changes in bath Na+ on Vm, the effects of increasing concentrations of several loop diuretics were examined. Furosemide, piretanide, torasemide and bumetanide (up to 0.1-0.5 mmol/l) all hyperpolarized Vm, albeit only by less than 10 mV. Another subclass of loop diuretics containing a tetrazolate in position 1 [e.g. azosemide, no. 19A and no. 20A from Schlatter E, Greger R, Weidtke C (1983) Pflüger Arch 396: 210-217] were much more effective. Azosemide hyperpolarized Vm from -46 +/- 3 to -74 +/- 2 mV (n = 18) and reduced Gm from 11 +/- 1 to 4 +/- 1 nS (n = 14). These data indicate that forskolin stimulates Cl- secretion in these cells by a mechanism fully compatible with the current scheme for exocrine secretion involving the Na+2Cl-K+ cotransporter.

Apical localization of the Na-K-Cl cotransporter, rBSC1, on rat thick ascending limbs

A bumetanide-sensitive Na-K-Cl cotransporter (rBSC1) was recently cloned from a rat renal outer medulla (OM) cDNA library and shown to be expressed predominantly in the kidney. The purpose of the present study was to examine the nephron distribution of cotransporter transcripts and protein in rat kidney. In situ hybridization showed an intense signal only in the outer medulla and extending along cortical medullary rays consistent with expression of rBSC1 transcripts in medullary (MTAL) and cortical (CTAL) thick ascending limbs. Polyclonal antibodies raised in rabbits against a unique 67 amino acid segment from the carboxyl terminus of rBSC1 identified a broad major band of 130 to 160 (midpoint of 150) kDa and at least two minor bands of 50 to 70 kD on Western blotting of homogenates from cortex (C) and outer medulla (OM), but not inner medulla (IM), of rat kidney. Thus the Na-K-Cl cotransporter protein detected by the polyclonal rBSC1 antibody in rat kidney was similar in size to the major approximately 150 kD bumetanide binding protein detected by others in mouse and dog kidneys. Immunofluorescence studies using the anti-rBSC1 polyclonal antibody on rat kidney sections showed an intense signal limited to apical surfaces of MTAL and CTAL segments. Colocalization with anti-Tamm-Horsfall antibody which is present in all TABA cells except macula densa cells confirmed the absence of anti-rBSC1 fluorescence in the macula densa cells. These results are consistent with rBSC1 encoding the, or the major isoform of the, apical Na-K-Cl cotransporter in the thick ascending limb. The Na-K-Cl cotransporter functionally detected in macula densa cells may be encoded by a different BSC isoform.

Gitelman's variant of Bartter's syndrome, inherited hypokalaemic alkalosis, is caused by mutations in the thiazide-sensitive Na-Cl cotransporter

Maintenance of fluid and electrolyte homeostasis is critical for normal neuromuscular function. Bartter's syndrome is an autosomal recessive disease characterized by diverse abnormalities in electrolyte homeostasis including hypokalaemic metabolic alkalosis; Gitelman's syndrome represents the predominant subset of Bartter's patients having hypomagnesemia and hypocalciuria. We now demonstrate complete linkage of Gitelman's syndrome to the locus encoding the renal thiazide-sensitive Na-Cl cotransporter, and identify a wide variety of non-conservative mutations, consistent with loss of function alleles, in affected subjects. These findings demonstrate the molecular basis of Gitelman's syndrome. We speculate that these mutant alleles lead to reduced sodium chloride reabsorption in the more common heterozygotes, potentially protecting against development of hypertension.

Molecular characterization of the epithelial Na-K-Cl cotransporter isoforms

Recent advances in the molecular characterization of specific isoforms of the Na-K-Cl cotransporter have allowed rapid progress in the study of the structure, function, and regulation of these members of a family of Cl-dependent cation cotransporters. Two distinct isoforms have been identified, one from Cl(-)-secretory epithelia and another found specifically in the diluting segment of the vertebrate kidney, a Cl(-)-absorptive epithelium. The discovery of three alternatively spliced variants of the absorptive isoform, which differ only by 31 amino acids and which appear to be differentially distributed within the mammalian thick ascending limb of the loop of Henle, highlight this spliced region as an important functional component of the protein.

Primary structure, functional expression, and chromosomal localization of the bumetanide-sensitive Na-K-Cl cotransporter in human colon

By moving chloride into epithelial cells, the Na-K-Cl cotransporter aids transcellular movement of chloride across both secretory and absorptive epithelia. Using cDNA probes from the recently identified elasmobranch secretory Na-K-Cl cotransporter (sNKCC1) (Xu, J. C., Lytle, C. Zhu, T. T., Payne, J. A., Benz, E., and Forbush, B., III (1994) Proc. Natl. Acad. Sci. 91, 2201-2205), we have identified the human homologue. By screening cDNA libraries of a human colonic carcinoma line, T84 cell, we identified a sequence of 4115 bases from overlapping clones. The deduced protein is 1212 amino acids in length, and analysis of the primary structure indicates 12 transmembrane segments. The primary structure is 74% identical to sNKCC1, 91% identical to a mouse Na-K-Cl cotransporter (mNKCC1), 58% identical to rabbit and rat renal Na-K-Cl cotransporters (NKCC2), and 43% identical to the thiazide-sensitive Na-Cl cotransporters from flounder urinary bladder and rat kidney. Similar to sNKCC1 and mNKCC1, the 5'-end of the human colonic cotransporter is rich in G + C content. Interestingly, a triple repeat (GCG)7 occurs within the 5'-coding region and contributes to a large alanine repeat (Ala15). Two sites for N-linked glycosylation are predicted on an extracellular loop between putative transmembrane segments 7 and 8. A single potential site for phosphorylation by protein kinase A is present in the predicted cytoplasmic C-terminal domain. Northern blot analysis revealed a 7.4-7.5-kilobase transcript in T84 cells and shark rectal gland and a approximately 7.2-kilobase transcript in mammalian colon, kidney, lung, and stomach. Metaphase spreads from lymphocytes were probed with biotin-labeled cDNA and avidin fluorescein (the cotransporter gene was localized to human chromosome 5 at position 5q23.3). Human embryonic kidney cells stably transfected with the full-length cDNA expressed a approximately 170-kDa protein recognized by anti-cotransporter antibodies. Following treatment with N-glycosidase F, the molecular mass of the expressed protein was similar to that predicted for the core protein from the cDNA sequence (132-kDa) and identical to that of deglycosylated T84 cotransporter (approximately 135-kDa). The stably transfected cells exhibited a approximately 15-fold greater bumetanide-sensitive 86Rb influx than control cells, and this flux required external sodium and chloride. Flux kinetics were consistent with an electroneutral cotransport of 1Na:1K:2Cl. Preincubation in chloride-free media was necessary to activate fully the expressed cotransporter, suggesting a [Cl]-dependent regulatory mechanism.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Molecular cloning and characterization of the renal diuretic-sensitive electroneutral sodium-(potassium)-chloride cotransporters

cDNAs encoding the two major electroneutral sodium-chloride transporters present in the mammalian kidney, the bumetanide-sensitive Na(+)-K(+)-Cl- symporter and thiazide-sensitive Na(+)-Cl- cotransporter, were isolated and their functional activity characterized in Xenopus laevis oocytes [2]. Although they differ in sensitivities to bumetanide and thiazides and have different requirements for potassium, these approximately 115-kDa proteins share about 60% sequence similarity and exhibit a topology featuring 12 potential membrane-spanning helices flanked by large hydrophilic domains at the NH2- and COOH-termini. These molecules, together with the Na-Cl cotransporter from the flounder urinary bladder, which exhibits a significant homology suggestive of common ancestry, define a new family of electroneutral Na(+)-(K+)-Cl- cotransporters. Northern blot analysis and in situ hybridization indicate that these transporters are expressed predominantly in kidney with an intrarenal distribution consistent with their recognized functional localization. The kidney-specific distribution of transcripts encoding these cotransporters suggest that other, probably related, genes encode non-renal Na(+)-(K+)-Cl- cotransporters.