Coordinate expression of piretanide receptors and Na+,K+,Cl- cotransport activity in Madin-Darby canine kidney cell mutants

Plant Cation-Chloride Cotransporters (CCC): Evolutionary Origins and Functional Insights

Genomes of unicellular and multicellular green algae, mosses, grasses and dicots harbor genes encoding cation-chloride cotransporters (CCC). CCC proteins from the plant kingdom have been comparatively less well investigated than their animal counterparts, but proteins from both plants and animals have been shown to mediate ion fluxes, and are involved in regulation of osmotic processes. In this review, we show that CCC proteins from plants form two distinct phylogenetic clades (CCC1 and CCC2). Some lycophytes and bryophytes possess members from each clade, most land plants only have members of the CCC1 clade, and green algae possess only the CCC2 clade. It is currently unknown whether CCC1 and CCC2 proteins have similar or distinct functions, however they are both more closely related to animal KCC proteins compared to NKCCs. Existing heterologous expression systems that have been used to functionally characterize plant CCC proteins, namely yeast and Xenopus laevis oocytes, have limitations that are discussed. Studies from plants exposed to chemical inhibitors of animal CCC protein function are reviewed for their potential to discern CCC function in planta. Thus far, mutations in plant CCC genes have been evaluated only in two species of angiosperms, and such mutations cause a diverse array of phenotypes-seemingly more than could simply be explained by localized disruption of ion transport alone. We evaluate the putative roles of plant CCC proteins and suggest areas for future investigation.

Endogenous and exogenous Na-K-Cl cotransporter expression in a low K-resistant mutant MDCK cell line

A low K-resistant mutant Madin-Darby canine kidney (MDCK) cell line, LK-C1, has been shown previously to lack functional Na-K-Cl cotransporter (NKCC) activity, indicating that it may be a useful NKCC “knockout” cell line for structure-function studies. Using immunological probes, we first characterized the defect in the endogenous NKCC protein of the LK-C1 cells and then fully restored NKCC activity in these cells by stably expressing the human secretory NKCC1 protein (hNKCC1). The endogenous NKCC protein of the LK-C1 cells was expressed at significantly lower levels than in wild-type MDCK cells and was not properly glycosylated. This latter finding indicated that the lack of functional NKCC activity in the LK-C1 cells may be due to the inability to process the protein to the plasma membrane. In contrast, exogenously expressed hNKCC1 protein was properly processed and fully functional at the plasma membrane. Significantly, the exogenous hNKCC1 protein was regulated in a manner similar to the protein in native secretory cells as it was robustly activated by cell shrinkage, calyculin A, and low-Cl incubation. Furthermore, when the LK-C1 cells formed an epithelium on permeable supports, the exogenous hNKCC1 protein was properly polarized and functional at the basolateral membrane. The low levels of endogenous NKCC protein expression, the absence of any endogenous NKCC transport activity, and the ability to form a polarized epithelium indicate that the LK-C1 cells offer an excellent expression system with which to study the molecular physiology of the cation Cl cotransporters.

Corneal endothelial NKCC: molecular identification, location, and contribution to fluid transport

Although Na(+)-K(+)-2Cl(-) cotransport has been demonstrated in cultured bovine corneal endothelial cells, its presence and role in the native tissue have been disputed. Using RT-PCR we have now identified a partial clone of the cotransporter protein in freshly dissected as well as in cultured corneal endothelial and epithelial cells. The deduced amino acid sequence of this protein segment is 99% identical to that of the bovine isoform (bNKCC1). [(3)H]bumetanide binding shows that the cotransporter sites are located in the basolateral membrane region at a density of 1.6 pmol/mg of protein, close to that in lung epithelium. Immunocytochemistry confirms the basolateral location of the cotransporter. We calculate the turnover rate of the cotransporter to be 83 s(-1). Transendothelial fluid transport, determined from deepithelialized rabbit corneal thickness measurements, is partially inhibited (30%) by bumetanide in a dose-dependent manner. Our results demonstrate that Na(+)-K(+)-2Cl(-) cotransporters are present in the basolateral domain of freshly dissected bovine corneal endothelial cells and contribute to fluid transport across corneal endothelial preparations.

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.

The effects of muzolimine and urine from muzolimine-treated rats on Na+K+Cl- cotransport in Madin-Darby canine kidney cells

Muzolimine is a loop diuretic with an original chemical structure devoid of the acidic or sulfonamide group known to be necessary for an interaction with Na+K+Cl- cotransport. We studied the effects of urine from muzolimine-treated rats on the Na+K+Cl- cotransport-dependent 86Rb influx in MDCK cells. Na+K+Cl- cotransport was inhibited by urine obtained 15 min (42% inhibition) and 60 min (49% inhibition) after muzolimine injection (50 mumol/kg i.v.). Muzolimine itself was not detectable in the urine. Probenecid (100 mumol/kg i.v.) suppressed both the diuretic effect of muzolimine and the inhibition of Na+K+Cl- cotransport by urine from muzolimine-treated rats. These results suggest that the diuretic effect of muzolimine is due to the metabolism of muzolimine into an active compound which inhibits Na+K+Cl- cotransport after its secretion into the tubular lumen via a proximal pathway. The direct effect of muzolimine on Na+K+Cl- cotransport in MDCK cells was also tested: surprisingly, the inhibition of 86Rb influx was significant in the presence of muzolimine (IC50 = 1.44 microM). We show that this effect was due to the metabolism of muzolimine by these cells into an active compound.

[3H]bumetanide binding to mouse kidney membranes: identification of corresponding membrane proteins

Crude plasma membranes from whole mouse kidneys have two classes of [3H]bumetanide binding sites. High-affinity sites (K1/2 approximately equal to 0.04 microM; Bmax = 1-2 pmol/mg protein) are similar to those identified on dog kidney membranes (B. Forbush and H.C. Palfrey. J. Biol. Chem. 258: 11787-11792, 1983) both with respect to affinity and in that Na, K, and Cl are required for [3H]bumetanide binding. Low-affinity sites (K1/2 approximately equal to 1 microM; Bmax = 7-14 pmol/mg) are unaffected by removal of these ions; such sites are not seen with dog kidney. When mouse kidney membranes are photolabeled with 4-[3H]benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid [( 3H]BSTBA), a photoreactive bumetanide analogue, specific incorporation of the label is seen in two regions. As with dog kidney [M. Haas and B. Forbush. Am. J. Physiol. 253 (Cell Physiol. 22): C243-C252, 1987], an approximately 150-kDa protein is labeled with high affinity (K1/2 approximately equal to 0.05 microM). This labeling also requires Na, K, and Cl and appears to correspond to the high-affinity [3H]bumetanide binding sites and to the Na-K-Cl cotransport system. A second peak of [3H]BSTBA photolabeling, centered at approximately 75 kDa, incorporates the label with lower affinity (K1/2 = 2-3 microM). The photolabeling at approximately 75 kDa is unaffected by Na, K, and Cl concentrations and thus may correspond, at least in part, to the low-affinity [3H]bumetanide binding sites. Western blot analysis of [3H]BSTBA-labeled mouse kidney membranes was performed using an antiserum raised to proteins of approximately 82 and approximately 39 kDa isolated from mouse Ehrlich ascites tumor cells using a bumetanide affinity gel (P. B. Dunham, F. Jessen, and E. K. Hoffmann. Proc. Natl. Acad. Sci. USA 87: 6828-6832, 1990). This antiserum cross-reacts with a approximately 150-kDa mouse kidney protein, the staining profile of which on Western blot corresponds very closely to the peak of specific [3H]BSTBA incorporation in this region. The antiserum also reacts with proteins in the range of 65-85 kDa, overlapping the low-affinity peak of [3H]BSTBA incorporation.

Rabbit distal colon epithelium: II. Characterization of (Na+,K+,Cl-)-cotransport and [3H]-bumetanide binding

Loop diuretic-sensitive (Na+,K+,Cl-)-cotransport activity was found to be present in basolateral membrane vesicles of surface and crypt cells of rabbit distal colon epithelium. The presence of gradients of all three ions was essential for optimal transport activity. (Na+,K+) gradient-driven 36Cl fluxes were half-maximally inhibited by 0.14 microM bumetanide and 44 microM furosemide. While 86Rb uptake rates showed hyperbolic dependencies on Na+ and K+ concentrations with Hill coefficients of 0.8 and 0.9, respectively, uptakes were sigmoidally related to the Cl concentration, Hill coefficient 1.8, indicating a 1 Na+:1 K+:2 Cl stoichiometry of ion transport. The interaction of putative (Na+,K+,Cl-)-cotransport proteins with loop diuretics was studied from equilibrium-binding experiments using [3H]-bumetanide. The requirement for the simultaneous presence of Na+,K+, and Cl-, saturability, reversibility, and specificity for diuretics suggest specific binding to the (Na+,K+,Cl-)-cotransporter. [3H]-bumetanide recognizes a minimum of two classes of diuretic receptor sites, high-affinity (KD1 = 0.13 microM; Bmax1 = 6.4 pmol/mg of protein) and low-affinity (KD2 = 34 microM; Bmax2 = 153 pmol/mg of protein) sites. The specific binding to the high-affinity receptor was found to be linearly competitive with Cl- (Ki = 60 mM), whereas low-affinity sites seem to be unaffected by Cl-. We have shown that only high-affinity [3H]-bumetanide binding correlates with transport inhibition raising questions on the physiological significance of diuretic receptor site heterogeneity observed in rabbit distal colon epithelium.

Chemical modification of cell proliferation and fluid secretion in renal cysts

We used an in vitro model, MDCK cyst, to determine the extent to which pharmacologic compounds known to inhibit plasma membrane solute transport mechanisms could alter the enlargement of renal epithelial cysts. Solitary MDCK cells cultured within collagen gel undergo clonal growth to form true epithelial cysts in which a single layer of polarized cells (apex toward lumen) encloses a fluid-filled cavity. Repeated observations by light microscopy were used to quantitate the rate of cyst growth in diameter, and demonstrated that cyst enlargement involved an increase in cell number (proliferation) and a net increase in intracystic volume (fluid secretion). Intracyst pressure was greater than the interstitium (6.7 mm H2O +/- 3.1 SD), indicating that fluid entry was secondary to net solute accumulation. Amiloride and seven amiloride analogs that inhibited to different degrees conductive Na+ transport, Na+-dependent H+ transport and Na+-dependent Ca++ transport reversibly decreased the rate of cyst enlargement. The effectiveness of these agents to retard cyst enlargement correlated with their relative potencies to inhibit Na+-dependent Ca++ transport. Morphologic examination indicated that amiloride and amiloride analogs decreased cell proliferation and fluid secretion to the same degree. Ouabain and vanadate (Na+K,ATPase inhibitors), and L-645,695 (Na+-dependent Cl-/HCO3- inhibitor) potently slowed cyst expansion. In contrast to amiloride and amiloride analogs, these agents caused an unusual degree of cellular stratification within the cyst walls, a finding consistent with the notion that fluid secretion was inhibited to a greater extent then cellular proliferation. We conclude that chemical inhibitors of primary and secondary active solute transport can diminish or halt the enlargement of epithelial cysts in vitro by decreasing the rate of cellular proliferation and/or net fluid secretion.

Stimulation of Na-K-Cl cotransport in cultured vascular endothelial cells by atrial natriuretic peptide

Vascular endothelial cells have been shown to contain atrial natriuretic peptide (ANP)-sensitive Na-K-Cl cotransport system whose activity is regulated by intracellular cGMP levels. Addition of ANP to culture medium stimulated 86Rb+ uptake in bovine endothelial cells with a concomitant increase in cGMP contents. This action of ANP was mimicked by 8-bromo-cGMP and completely diminished by furosemide. These results indicate that ANP selectively activates the Na-K-Cl cotransporter in vascular endothelial cells via cGMP and offer new insight into the physiological significance of endothelial ANP receptors.

A bumetanide-sensitive, potassium carrier-mediated transport system in excitable tissues

The binding of [3H]-bumetanide to rat brain synaptosomes revealed the existence of two binding sites. The high affinity site (R1 = 46.6 fmoles/mg protein) binds bumetanide and furosemide with Kd1 of 13 nM and 1.5 microM respectively, while the low affinity site (R2 = 1.37 nmoles/mg protein) is characterized by Kd2 of 200 microM and 680 microM for bumetanide and furosemide, respectively. Bumetanide sensitive 86Rb uptake was 34 +/- 14.5, 38.3 +/- 1.4, 18.6 +/- 1.3 and 29.0 +/- 6.1% of total 86Rb uptake in synaptic plasma membrane vesicles, rat brain synaptosomes, Neuroblastoma N1E115 cell line and chick chest muscle cells, respectively. Furosemide and bumetanide inhibited 86Rb uptake to rat brain SPM- vesicles in a dose dependent fashion. Half maximal inhibition (IC50) was observed at 20 nM and 4 microM for bumetanide and furosemide, respectively. Bumetanide-sensitive transport was dependent on extravesicular sodium and chloride concentrations with a Km of 21 and 25 mM for the two ions, respectively. These results demonstrate the existence of a "loop diuretic" sensitive carrier-mediated K+ transport system in brain and other excitable cells.

Binding of loop diuretics to their renal receptors: use as a screening model for potential diuretic activity

Loop diuretics of the benzoic acid and aryloxyacetic acid families inhibit Na+K+Cl- cotransport. The ranking order of potencies measured in the thick ascending limb of Henle's loop and the ranking order of affinities for [3H]piretanide receptors on renal plasma membranes are the same. Potencies and affinities correlate well (correlation coefficient r = 0.959 for the medulla and r = 0.951 for the cortex). Therefore, measurement of [3H]piretanide binding is proposed to facilitate screening for loop diuretic action.