Catecholamine-stimulated ion transport in duck red cells. Gradient effects in electrically neutral [Na + K + 2Cl] Co-transport

Regulation of erythrocyte Na+/K+/2Cl- cotransport by an oxygen-switched kinase cascade

Many erythrocyte processes and pathways, including glycolysis, the pentose phosphate pathway (PPP), KCl cotransport, ATP release, Na⁺/K⁺-ATPase activity, ankyrin-band 3 interactions, and nitric oxide (NO) release, are regulated by changes in O₂ pressure that occur as a red blood cell (RBC) transits between the lungs and tissues. The O₂ dependence of glycolysis, PPP, and ankyrin-band 3 interactions (affecting RBC rheology) are controlled by O₂-dependent competition between deoxyhemoglobin (deoxyHb), but not oxyhemoglobin (oxyHb), and other proteins for band 3. We undertook the present study to determine whether the O₂ dependence of Na⁺/K⁺/2Cl^- cotransport (catalyzed by Na⁺/K⁺/2Cl^- cotransporter 1 [NKCC1]) might similarly originate from competition between deoxyHb and a protein involved in NKCC1 regulation for a common binding site on band 3. Using three transgenic mouse strains having mutated deoxyhemoglobin-binding sites on band 3, we found that docking of deoxyhemoglobin at the N terminus of band 3 displaces the protein with no lysine kinase 1 (WNK1) from its overlapping binding site on band 3. This displacement enabled WNK1 to phosphorylate oxidative stress-responsive kinase 1 (OSR1), which, in turn, phosphorylated and activated NKCC1. Under normal solution conditions, the NKCC1 activation increased RBC volume and thereby induced changes in RBC rheology. Because the deoxyhemoglobin-mediated WNK1 displacement from band 3 in this O₂ regulation pathway may also occur in the regulation of other O₂-regulated ion transporters, we hypothesize that the NKCC1-mediated regulatory mechanism may represent a general pattern of O₂ modulation of ion transporters in erythrocytes.

Cytosolic sodium regulation in mouse cortical astrocytes and its dependence on potassium and bicarbonate

Sodium plays a major role in different astrocytic functions, including maintenance of ion homeostasis and uptake of neurotransmitters and metabolites, which are mediated by different Na⁺ -coupled transporters. In the current study, the role of an electrogenic sodium-bicarbonate cotransporter (NBCe1), a sodium-potassium-chloride transporter 1 (NKCC1) and sodium-potassium ATPase (Na⁺ -K⁺ -ATPase) for the maintenance of [Na⁺ ]_i was investigated in cultured astrocytes of wild-type (WT) and of NBCe1-deficient (NBCe1-KO) mice using the Na⁺ -sensitive dye, asante sodium green-2. Our results suggest that cytosolic Na⁺ was higher in the presence of CO₂ /HCO₃ ^- (15 mM) than CO₂ /HCO₃ ^- -free, HEPES-buffered solution in WT, but not in NBCe1-KO astrocytes (12 mM). Surprisingly, there was a strong dependence of cytosolic [Na⁺ ] on the extracellular [HCO₃ ^- ] attributable to NBCe1 activity. Pharmacological blockage of NKCC1 with bumetanide led to a robust drop in cytosolic Na⁺ in both WT and NBCe1-KO astrocytes by up to 6 mM. There was a strong dependence of the cytosolic [Na⁺ ] on the extracellular [K⁺ ]. Inhibition of the Na⁺ -K⁺ -ATPase led to larger increase in cytosolic Na⁺ , both in the absence of K⁺ as compared with the presence of ouabain and in NBCe1-KO astrocytes as compared with WT astrocytes. Our results show that cytosolic Na⁺ in mouse cortical astrocytes can vary considerably and depends greatly on the concentrations of HCO₃ ^- and K⁺ , attributable to the activity of the Na⁺ -K⁺ -ATPase, of NBCe1 and NKCC1.

Carriers, exchangers, and cotransporters in the first 100 years of the Journal of General Physiology

Jennings reviews the many contributions of JGP articles to our current understanding of solute transporter mechanisms.

Transporters, pumps, and channels are proteins that catalyze the movement of solutes across membranes. The single-solute carriers, coupled exchangers, and coupled cotransporters that are collectively known as transporters are distinct from conductive ion channels, water channels, and ATP-hydrolyzing pumps. The main conceptual framework for studying transporter mechanisms is the alternating access model, which comprises substrate binding and release events on each side of the permeability barrier and translocation events involving conformational changes between inward-facing and outward-facing conformational states. In 1948, the Journal of General Physiology began to publish work that focused on the erythrocyte glucose transporter—the first transporter to be characterized kinetically—followed by articles on the rates, stoichiometries, asymmetries, voltage dependences, and regulation of coupled exchangers and cotransporters beginning in the 1960s. After the dawn of cDNA cloning and sequencing in the 1980s, heterologous expression systems and site-directed mutagenesis allowed identification of the functional roles of specific amino acid residues. In the past two decades, structures of transport proteins have made it possible to propose specific models for transporter function at the molecular level. Here, we review the contribution of JGP articles to our current understanding of solute transporter mechanisms. Whether the topic has been kinetics, energetics, regulation, mutagenesis, or structure-based modeling, a common feature of these articles has been a quantitative, mechanistic approach, leading to lasting insights into the functions of transporters.

A brief history of the study of fish osmoregulation: the central role of the Mt. Desert Island Biological Laboratory

The Mt. Desert Island Biological Laboratory (MDIBL) has played a central role in the study of fish osmoregulation for the past 80 years. In particular, scientists at the MDIBL have made significant discoveries in the basic pattern of fish osmoregulation, the function of aglomerular kidneys and proximal tubular secretion, the roles of NaCl cotransporters in intestinal uptake and gill and rectal gland secretion, the role of the shark rectal gland in osmoregulation, the mechanisms of salt secretion by the teleost fish gill epithelium, and the evolution of the ionic uptake mechanisms in fish gills. This short review presents the history of these discoveries and their relationships to the study of epithelial transport in general.

Regulation of Na-K-2Cl cotransport in cultured bovine corneal endothelial cells

We have previously demonstrated the presence of a Na(+)-K(+)-2Cl cotransporter in cultured bovine corneal endothelial cells (CBCEC) and determined that this cotransporter is located in the basolateral membrane. This transporter may contribute to volume regulation and transendothelial fluid transport. We have now investigated factors regulating the activity of the cotransporter. This activity was assessed by measuring the bumetanide-sensitive (86)Rubidium ((86)Rb) uptake in (86)Rb-containing solutions. Data were normalized to protein content determined with a Lowry protein assay. We investigated the regulation by extracellular and intracellular ion concentrations, by osmotic gradients, and by second messengers. Our results indicate that extracellular Na+ and K+ each are required for activation of the cotransporter and activate with first-order kinetics at half-maximally effective concentrations (k(1/2)) of 21.1 and 1.33 mM, respectively. Extracellular Cl- is also required for cotransport activation, but shows higher order kinetics; the k(1/2) for Cl- is 28.1 mM and the Hill coefficient 2.1. HCO(3)(-) exerts a modulating effect on cotransporter activity; at 0 HCO(3)(-) the bumetanide-sensitive K(+) uptake is reduced by 30% compared to that at 26 mm HCO(3)(-). Manipulations of the intracellular [Cl-] by preincubation in Cl- -free solution or inhibition of Cl- efflux resulted in increased uptake at low [Cl-](i) and decreased uptake at high [Cl-](i). To assess the role of protein kinases in the regulation of cotransport, we have determined the effect of protein kinase inhibitors. H-89 and KT5270, inhibitors of PKA, inhibit cotransport almost completely, while calphostin C, an inhibitor of PKC, produces a small activation of cotransport. The tyrosine kinase inhibitor genistein reduced K+ uptake while its inactive analog daidzein was without effect. The calmodulin kinase inhibitor KN-93 was without effect. We also investigated the effects of phosphatase inhibitors. Calyculin A (k(1/2)=21 nM) and okadaic acid (k(1/2)=915 nM) produced approximate doubling of K+ uptake, suggesting that phosphatase 1 is dominant. We also investigated the role of the cytoskeleton and its activation. Reduction of Ca(i)(2+) by preincubation in Ca2+ -free medium as well as by exposure to W-7, an inhibitor of the binding of Ca(2+) to calmodulin, reduced K+ uptake. Consistent with this, ML-7, a relatively specific inhibitor of the Ca2+ -calmodulin activated myosin light chain kinase, inhibited cotransport by 40%. The Ca2+ -calmodulin activated myosin light chain kinase contributes to the modulation of the cytoskeleton by regulating the actin-myosin interaction. Consistent with the above, disruption of the actin polymerization by cytochalasin D led to a decrease in K+ uptake. We conclude that extracellular Na+, K+ and Cl- are requirements for the function of the CBCEC Na(+)-K(+)-2Cl(-) cotransporter, while intracellular Cl- and extracellular HCO(3)(-) modulate its activity. Several protein kinases, including PKA, PKC, tyrosine kinase, and myosin light chain kinase, modulate the K+ uptake. Another modulating pathway for cotransport involves the state of the cytoskeleton.

Isoosmotic shrinkage by self-stimulated outward Na-K-Cl cotransport in quail erythrocytes

In mammalian erythrocytes, outward fluxes by the Na-K-Cl cotransporter NKCC have been clearly characterized, but NKCC fluxes are small and their physiological role, if any, is poorly understood. Avian erythrocytes are nucleated cells, in which a physiologically relevant NKCC acts as a cell volume regulator. Therefore, we further investigated outward cotransport and its relation to cell volume by using quail erythrocytes. Unlike human or rat erythrocytes, quail erythrocytes exhibit outward cotransport fluxes: (1) of high magnitude [maximal rate of bumetanide-sensitive Li+ efflux=12.3+/-1.1 mmol (l cells x h)(-1), mean +/-SEM, n=23] and (2) strongly stimulated by hyperosmotic media (by 100-200% in 500 mosmol/l media). Na+- or Li+-loaded quail erythrocytes exhibited rapid cell shrinkage when incubated in K+-free media. Thus, cell volume remained stationary up to 5-10 min and then started to shrink. Shrinkage was first slow, but progressively accelerated, finally reaching a new stationary state where cell volume had decreased by about 20%. Such rapid cell shrinkage was fully inhibited by bumetanide and was associated with outward cotransport stimulation (self-stimulated or an auto-catalytic process, i.e. a reaction stimulated by its product). External K+ reduced all these phenomena, but significant cell shrinkage was still observed at an external K+ concentration of 2.8 mM. K+ removal failed to stimulate outward cotransport in hypotonic media (250 mosmol/l). Finally, reincubation of shrunken erythrocytes in physiological saline revealed that inward cotransport was stimulated more than outward cotransport. In conclusion, isoosmotic hypokalaemia drives a rapid shrinkage of quail erythrocytes, due to auto-catalytic net outward cotransport stimulation. Whether this is an experimental curiosity or indicates that outward cotransport can have some physiological role deserves further investigation.

Molecular cloning of NHE1 from winter flounder RBCs: activation by osmotic shrinkage, cAMP, and calyculin A

In this report, we describe the cloning, cellular localization, and functional characteristics of Na(+)/H(+) exchanger 1 (NHE1) from red blood cells of the winter flounder Pseudopleuronectes americanus (paNHE1). The paNHE1 protein localizes primarily to the marginal band and exhibits a 74% similarity to the trout beta-NHE, and 65% to the human NHE1 (hNHE1). Functionally, paNHE1 shares characteristics of both beta-NHE and hNHE1 in that it is activated both by manipulations that increase cAMP and by cell shrinkage, respectively. In accordance, the paNHE1 protein exhibits both protein kinase A consensus sites as in beta-NHE and a region of high homology to that required for shrinkage-dependent activation of hNHE1. After shrinkage-dependent activation of paNHE1 and resulting activation of a Cl(-)/HCO(3)(-) exchanger, their parallel operation results in net uptake of NaCl and osmotically obliged water. Activation of paNHE1 by cAMP is at least additive to that elicited by osmotic shrinkage, suggesting that these stimuli regulate paNHE1 by distinct mechanisms. Finally, exposure to the serine/threonine phosphatase inhibitor calyculin A potently activates paNHE1, and this activation is also additive to that induced by shrinkage or cAMP.

Regulation of Na-K-2Cl cotransport by phosphorylation and protein-protein interactions

The Na-K-2Cl cotransporter plays important roles in cell ion homeostasis and volume control and is particularly important in mediating the movement of ions and thus water across epithelia. In addition to being affected by the concentration of the transported ions, cotransport is affected by cell volume, hormones, growth factors, oxygen tension, and intracellular ionized Mg(2+) concentration. These probably influence transport through three main routes acting in parallel: cotransporter phosphorylation, protein-protein interactions and cell Cl(-) concentration. Many effects are mediated, at least in part, by changes in protein phosphorylation, and are disrupted by kinase and phosphatase inhibitors, and manoeuvres that reduce cell ATP content. In some cases, phosphorylation of the cotransporter itself on serine and threonine (but not tyrosine) is associated with changes in transport rate, in others, phosphorylation of associated proteins has more influence. Analysis of the stimulation of cotransport by calyculin A, arsenite and deoxygenation suggests that the cotransporter is phosphorylated by several kinases and dephosphorylated by several phosphatases. These kinases and phosphatases may themselves be regulated by phosphorylation of residues including tyrosine, with Src kinases possibly playing an important role. Protein-protein interactions also influence cotransport activity. Cotransporter molecules bind to each other to form high molecular weight complexes, they also bind to other members of the cation-chloride cotransport family, to a variety of cytoskeletal proteins, and to enzymes that are part of regulatory cascades. Many of these interactions affect transport and may override the effects of cotransporter phosphorylation. Cell Cl(-) may also directly affect the way the cotransporter functions independently of its role as substrate.

Coordinate modulation of Na-K-2Cl cotransport and K-Cl cotransport by cell volume and chloride

Na-K-2Cl cotransporter (NKCC) and K-Cl cotransporter (KCC) play key roles in cell volume regulation and epithelial Cl(-) transport. Reductions in either cell volume or cytosolic Cl(-) concentration ([Cl(-)](i)) stimulate a corrective uptake of KCl and water via NKCC, whereas cell swelling triggers KCl loss via KCC. The dependence of these transporters on volume and [Cl(-)](i) was evaluated in model duck red blood cells. Replacement of [Cl(-)](i) with methanesulfonate elevated the volume set point at which NKCC activates and KCC inactivates. The set point was insensitive to cytosolic ionic strength. Reducing [Cl(-)](i) at a constant driving force for inward NKCC and outward KCC caused the cells to adopt the new set point volume. Phosphopeptide maps of NKCC indicated that activation by cell shrinkage or low [Cl(-)](i) is associated with phosphorylation of a similar constellation of Ser/Thr sites. Like shrinkage, reduction of [Cl(-)](i) accelerated NKCC phosphorylation after abrupt inhibition of the deactivating phosphatase with calyculin A in vivo, whereas [Cl(-)] had no specific effect on dephosphorylation in vitro. Our results indicate that NKCC and KCC are reciprocally regulated by a negative feedback system dually modulated by cell volume and [Cl(-)]. The major effect of Cl(-) on NKCC is exerted through the volume-sensitive kinase that phosphorylates the transport protein.

Effect of radiologic contrast media on cell volume regulatory mechanisms in human red blood cells

RATIONALE AND OBJECTIVES

The authors performed this study to evaluate cell volume regulation in human red blood cells (RBCs) after incubation in solutions of three contrast media: iohexol (830 mOsm), ioxaglate (520 mOsm), and iodixanol (300 mOsm).

MATERIALS AND METHODS

Whole blood sampled from six healthy subjects was exposed to Ringer solutions containing 25% or 5% vol/vol iohexol (final osmolality, 440 or 340 mOsm, respectively), ioxaglate (final osmolality, 395 or 335 mOsm, respectively), iodixanol (final osmolality, 330 or 315 mOsm, respectively), or NaCl (control solutions with the same osmolality as that of the contrast media). In some experiments, control RBCs were subjected to a hyposmotic solution (100 mOsm). RBC volumes were obtained with a Coulter counter.

RESULTS

The RBCs showed normal regulatory cell shrinkage after hyposmotically induced swelling. All 25% vol/vol contrast material solutions and their control solutions induced RBC shrinkage (range, 6% +/- 1 [standard error] to 22% +/- 3). The same was true for cells exposed to 5% vol/vol contrast material (range, 4% +/- 1 to 7% +/- 1). The shrinkage phase was followed by cell swelling (10% +/- 2 to 20% +/- 2 for 25% contrast material and their control solutions and 8% +/- 1 to 15% +/- 2 for 5% contrast material and their control solutions). No contrast material-exposed RBCs increased their volumes to the level reached with their control solutions.

CONCLUSION

RBCs exposed to hyperosmotic iohexol, ioxaglate, or iodixanol solutions shrank and then swelled. The degree of shrinkage and subsequent swelling could not be explained simply with the osmolality of the test solutions. Physicochemical properties of the contrast media must be involved, putatively affecting electrolyte fluxes over the RBC membrane. Possible targets of these effects are the K+/Cl- symporter, K+ channels, and the Na+/K+/Cl- symporter.

Cl(-)-dependent secretory mechanisms in isolated rat bile duct epithelial units

Cholangiocytes absorb and secrete fluid, modifying primary canalicular bile. In several Cl(-)-secreting epithelia, Na(+)-K(+)-2Cl(-) cotransport is a basolateral Cl(-) uptake pathway facilitating apical Cl(-) secretion. To determine if cholangiocytes possess similar mechanisms independent of CO2/HCO, we assessed Cl(-)-dependent secretion in rat liver isolated polarized bile duct units (IBDUs) by using videomicroscopy. Without CO2/HCO, forskolin (FSK) stimulated secretion entirely dependent on Na(+) and Cl(-) and inhibited by Na(+)-K(+)-2Cl(-) inhibitor bumetanide. Carbonic anhydrase inhibitor ethoxyzolamide had no effect on FSK-stimulated secretion, indicating negligible endogenous CO2/HCO transport. In contrast, FSK-stimulated secretion was inhibited approximately 85% by K(+) channel inhibitor Ba(2+) and blocked completely by bumetanide plus Ba(2+). IBDU Na(+)-K(+)-2Cl(-) cotransport activity was assessed by recording intracellular pH during NH4Cl exposure. Bumetanide inhibited initial acidification rates due to NH entry in the presence and absence of CO2/HCO. In contrast, when stimulated by FSK, a 35% increase in Na(+)-K(+)-2Cl(-) cotransport activity occurred without CO2/HCO. These data suggest a cellular model of HCO-independent secretion in which Na(+)-K(+)-2Cl(-) cotransport maintains high intracellular Cl(-) concentration. Intracellular cAMP concentration increases activate basolateral K(+) conductance, raises apical Cl(-) permeability, and causes transcellular Cl(-) movement into the lumen. Polarized IBDU cholangiocytes are capable of vectorial Cl(-)-dependent fluid secretion independent of HCO. Bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransport, Cl(-)/HCO exchange, and Ba(2+)-sensitive K(+) channels are important components of stimulated fluid secretion in intrahepatic bile duct epithelium.

Chloride in smooth muscle

Interest in the functions of intracellular chloride expanded about twenty years ago but mostly this referred to tissues other than smooth muscle. On the other hand, accumulation of chloride above equilibrium seems to have been recognised more readily in smooth muscle. Experimental data is used to show by calculation that the Donnan equilibrium cannot account for the chloride distribution in smooth muscle but it can in skeletal muscle. The evidence that chloride is normally above equilibrium in smooth muscle is discussed and comparisons are made with skeletal and cardiac muscle. The accent is on vascular smooth muscle and the mechanisms of accumulation and dissipation. The three mechanisms by which chloride can be accumulated are described with some emphasis on calculating the driving forces, where this is possible. The mechanisms are chloride/bicarbonate exchange, (Na+K+Cl) cotransport and a novel entity, "pump III", known only from own work. Their contributions to chloride accumulation vary and appear to be characteristic of individual smooth muscles. Thus, (Na+K+Cl) always drives chloride inwards, chloride/bicarbonate exchange is always present but does not always do it and "pump III" is not universal. Three quite different biophysical approaches to assessing chloride permeability are considered and the calculations underlying them are worked out fully. Comparisons with other tissues are made to illustrate that low chloride permeability is a feature of smooth muscle. Some of the functions of the high intracellular chloride concentrations are considered. This includes calculations to illustrate its depolarising influence on the membrane potential, a concept which, experience tells us, some people find confusing. The major topic is the role of chloride in the regulation of smooth muscle contractility. Whilst there is strong evidence that the opening of the calcium-dependent chloride channel leads to depolarisation, calcium entry and contraction in some smooth muscles, it appears that chloride serves a different function in others. Thus, although activation and inhibition of (Na+K+Cl) cotransport is associated with contraction and relaxation respectively, the converse association of inhibition and contraction has been seen. Nevertheless, inhibition of chloride/bicarbonate exchange and "pump III" and stimulation of (K+Cl) cotransport can all cause relaxation and this suggests that chloride is always involved in the contraction of smooth muscle. The evidence that (Na+K+Cl) cotransport more active in experimental hypertension is discussed. This is a common but not universal observation. The information comes almost exclusively from work on cultured cells, usually from rat aorta. Nevertheless, work on smooth muscle freshly isolated from hypertensive rats confirms that (Na+K+Cl) cotransport is activated in hypertension but there are several other differences, of which the depolarisation of the membrane potential may be the most important.Finally, a simple calculation is made which indicates as much as 40% of the energy put into the smooth muscle cell membrane by the sodium pump is necessary to drive (Na+K+Cl) cotransport. Notwithstanding the approximations in this calculation, this suggests that chloride accumulation is energetically expensive. Presumably, this is related to the apparently universal role of chloride in contraction.

The Na-K-Cl cotransporter of secretory epithelia

The Na-K-Cl cotransporters are a class of ion transport 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. To date, two Na-K-Cl cotransporter isoforms have been identified: NKCC1, which is present in a wide variety of secretory epithelia and non-epithelial cells; and NKCC2, which is present exclusively in the kidney, in the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Both NKCC isoforms represent part of a diverse family of cation-chloride cotransport proteins that share a common predicted membrane topology; this family also includes Na-Cl cotransporters and multiple K-Cl cotransporter isoforms. In secretory epithelia, the regulation of NKCC1, which is typically present on the basolateral membrane, is tightly coordinated with that of other transporters, including apical Cl channels, to maintain cell volume and integrity during active salt and fluid secretion. Changes in intracellular [Cl] ([Cl]i) appear to be involved in this regulation of NKCC1, which is directly phosphorylated by an unknown protein kinase in response to various secretagogues as well as reductions in [Cl]i and cell volume. This review focuses on structure-function relationships within NKCC1 and on recent developments pertaining to NKCC1 regulation at cellular and molecular levels.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Protein kinase C activation downregulates the expression and function of the basolateral Na+/K+/2Cl(-) cotransporter

The basolateral Na+/K+/2Cl(-) cotransporter (NKCC1) has been shown to be an independent regulatory site for electrogenic Cl(-) secretion. The proinflammatory phorbol ester, phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C (PKC), inhibits basal and cyclic adenosine monophosphate (cAMP)-stimulated NKCC1 activity in T84 intestinal epithelial cells and decreases the steady state levels of NKCC1 mRNA in a time- and dose-dependent manner. The levels of NKCC1 protein also fall in accordance with the NKCC1 mRNA transcript and these levels are unaffected by 4alpha-phorbol, which does not activate PKC. Inhibition of maximal (cAMP-stimulated) NKCC1 functional activity by PMA was first detected by 1 h, whereas decreases in the steady state levels of NKCC1 mRNA were not detectable until 4 h. NKCC1 mRNA expression recovers toward control levels with extended treatment of cells with PMA suggesting that the PMA effects on NKCC1 expression are mediated through activation of PKC. Although NKCC1 mRNA and protein levels return to control values after extended PMA exposure, NKCC1 functional activity does not recover. Immunofluorescence imaging suggest that the absence of functional recovery is due to failure of newly synthesized NKKC1 protein to reach the cell surface. We conclude that NKCC1 has the capacity to be regulated at the level of de novo expression by PKC, although decreased NKCC1 expression alone cannot account for either early or late loss of NKCC1 function.

A volume-sensitive protein kinase regulates the Na-K-2Cl cotransporter in duck red blood cells

When Na-K-2Cl cotransport is activated in duck red blood cells by either osmotic cell shrinkage, norepinephrine, fluoride, or calyculin A, phosphorylation of the transporter occurs at a common set of serine/threonine sites. To examine the kinetics and regulation of the activating kinase, phosphatase activity was inhibited abruptly with calyculin A and the subsequent changes in transporter phosphorylation and activity were determined. Increases in fractional incorporation of 32P into the transporter and uptake of 86Rb by the cells were closely correlated, suggesting that the phosphorylation event is rate determining in the activation process. Observed in this manner, the activating kinase was 1) stimulated by cell shrinkage, 2) inhibited by cell swelling, staurosporine, or N-ethylmaleimide, and 3) unaffected by norepinephrine or fluoride. The inhibitory effect of swelling on kinase activity was progressively relieved by calyculin A, suggesting that the kinase itself is switched on by phosphorylation. The kinetics of activation by calyculin A conformed to an autocatalytic model in which the volume-sensitive kinase is stimulated by a product of its own reaction (e.g., via autophosphorylation).

A model of Na-K-2Cl cotransport based on ordered ion binding and glide symmetry

In the duck red blood cell, Na-K-2Cl cotransport exhibits two modes of ion movement: net cotransport and obligate cation exchange. In high-K cells, the predominant exchange is K/K (or K/Rb). In high-Na cells, it becomes Na/Na (or Na/Li). Both represent partial reactions in which a fully loaded carrier releases part of its cargo, rebinds fresh ions, and returns back across the membrane fully loaded. Net cotransport occurs when the carrier unloads completely and returns empty. This mode has a fixed stoichiometry of 1Na:1K:2Cl under all conditions tested. The ion requirements of the two exchanges differ: K/K exchange requires only K and Cl outside but all three ions inside. Na/Na exchange requires all three ions outside but only Na inside. We propose a simple model in which the carrier can only move when either fully loaded or completely empty and in which the ions bind in a strictly ordered sequence. For example, externally, a Na binds first and then a Cl, followed by a K and a second Cl. Internally, the first on is the first off (glide symmetry), so the Na is released first and then the first Cl, followed by the K and finally by the second Cl. Only then can the empty form return to the outside to start a new cycle.

Activation of the avian erythrocyte Na-K-Cl cotransport protein by cell shrinkage, cAMP, fluoride, and calyculin-A involves phosphorylation at common sites

Na-K-Cl cotransport activity in duck erythrocytes increases approximately 10-fold in response to osmotic cell shrinkage, norepinephrine, fluoride, or calyculin-A (an inhibitor of type-1 and -2a phosphatases). To assess whether all four stimuli promote phosphorylation of the cotransport protein and whether this phosphorylation is catalyzed by the same kinase, the cotransporter was isolated from erythrocytes by immunoprecipitation and its pattern of phosphorylation was evaluated. Each stimulus evoked proportionate increases in cotransporter activity and phosphorylation. No two stimuli in combination evoked greater activation and phosphorylation than did the more potent of the two stimuli acting alone. Phosphoamino acid analysis of the cotransport protein indicated that phosphorylation occurs at serine and threonine residues. Phosphopeptide mapping revealed a distinctive pattern of 8 major tryptic phosphopeptides, none of which were significantly phosphorylated in the unstimulated state. Maps of cotransporters activated by the four different stimuli were indistinguishable. Measurements of phosphorylation stoichiometry indicated that each cotransporter acquires approximately 5 phosphates on going from an inactive state in swollen cells to an active state in shrunken cells. Staurosporine, a kinase inhibitor with broad selectivity, inhibited each stimulus equipotently (IC50 approximately 0.7 microM). Staurosporine promptly reversed cotransporter activity and phosphorylation when added to shrinkage-stimulated but not to calyculin-stimulated cells, indicating that it enters the cell rapidly and blocks phosphorylation. These results suggest that cell shrinkage, cAMP, fluoride, and calyculin-A promote the phosphorylation of the Na-K-Cl cotransport protein at a similar constellation of serine and threonine residues. It is proposed that all modes of stimulation ultimately involve the same protein kinase.

[Cl-]i-dependent phosphorylation of the Na-K-Cl cotransport protein of dog tracheal epithelial cells

Basolateral Na-K-Cl cotransport activity in primary cultures of dog tracheal epithelial cells is stimulated by beta-adrenergic agents, such as isoproterenol, and by apical UTP, which acts through an apical P2-purinergic receptor. While at least part of the stimulatory effect of isoproterenol appears to involve direct activation of the cotransporter via cAMP-dependent protein kinase, cotransport stimulation by apical UTP is entirely secondary to apical Cl- efflux and a resultant decrease in intracellular [Cl-] ([Cl-]i) and/or cell shrinkage (Haas, M., and McBrayer, D. G. (1994) Am. J. Physiol. 266, C1440-C1452). In the secretory epithelia of the shark rectal gland and avian salt gland, Na-K-Cl cotransport activation by both cAMP-dependent and cAMP-independent secretagogues has been shown to be accompanied by phosphorylation of the cotransport protein itself (Lytle, C., and Forbush, B., III (1992) J. Biol. Chem. 267, 25438-25443; Torchia, J., Lytle, C., Pon, D. J., Forbush, B., III, and Sen, A. K. (1992) J. Biol. Chem. 267, 25444-25450). In the present study, we immunoprecipitate the approximately 170-kDa Na-K-Cl cotransport protein of dog tracheal epithelial cells with a monoclonal antibody against the cotransporter of the intestinal cell line T84. Incubation of confluent primary cultures of tracheal cells with isoproterenol and apical UTP increases basolateral-to-apical 36Cl- flux 3.4- and 2.6-fold, respectively, and produces similar increases (3.2- and 2.8-fold, respectively) in 32P incorporation into the approximately 170-kDa cotransport protein. Decreasing [Cl-]i (without concomitant cell shrinkage) by incubating cultures with apical nystatin and reduced apical [Cl-] ([Cl-]alpha) likewise increases both cotransport activity and cotransport protein phosphorylation. These effects become more pronounced with greater reductions in [Cl-]alpha; after 20 min of incubation with nystatin and 32 mM [Cl-]alpha, cotransport activity and 32P incorporation into the cotransport protein are increased 2.8- and 2.7-fold, respectively, similar to increases seen with apical UTP. 2-3-fold increases in cotransporter activity and phosphorylation are also seen in nystatin-treated cells under hypertonic conditions (50 mM sucrose added apically and basolaterally). These findings suggest a close correlation between Na-K-Cl cotransport activity and phosphorylation of the approximately 170-kDa cotransport protein. The latter is phosphorylated in response to both reduced [Cl-]i and cell shrinkage, either or both of which are likely to be involved in secondary cotransport activation in response to apical UTP.

Urea inhibits NaK2Cl cotransport in human erythrocytes

We examined the effect of urea on NaK2Cl cotransport in human erythrocytes. In erythrocytes from nine normal subjects, the addition of 45 mM urea, a concentration commonly encountered in uremic subjects, inhibited NaK2Cl cotransport by 33 +/- 7%. Urea inhibited NaK2Cl cotransport reversibly, and in a concentration-dependent fashion with half-maximal inhibition at 63 +/- 10 mM. Acute cell shrinkage increased, and acute cell swelling decreased NaK2Cl cotransport in human erythrocytes. Okadaic acid (OA), a specific inhibitor of protein phosphatase 1 and 2A, increased NaK2Cl cotransport by nearly 80%, suggesting an important role for these phosphatases in the regulation of NaK2Cl cotransport. Urea inhibited bumetanide-sensitive K influx even when protein phosphatases were inhibited with OA, suggesting that urea acted by inhibiting a kinase. In cells subjected to shrinking and OA pretreatment, maneuvers expected to increase the net phosphorylation, urea inhibited cotransport only minimally, suggesting that urea acted by causing a net dephosphorylation of the cotransport protein, or some key regulatory protein. The finding that concentrations of urea found in uremic subjects inhibited NaK2Cl cotransport, a widespread transport pathway with important physiological functions, suggests that urea is not only a marker for accumulation of other uremic toxins, but may be a significant uremic toxin itself.

Functional coupling of Na(+)-K(+)-2Cl- cotransport and Ca(2+)-dependent K+ channels in vascular endothelial cells

To determine whether the activation of Na(+)-K(+)-2Cl- cotransport by Ca(2+)-mobilizing agonists is a direct effect of Ca2+ or is secondary to activation of Ca(2+)-dependent K+ channels [via cell shrinkage or decreased intracellular Cl- concentration ([Cl-]), we measured K+ fluxes in aortic endothelial cells in response to ATP and bradykinin. With either agonist there was an immediate bumetanide-insensitive efflux inhibitable by the K+ channel blockers tetrabutylammonium (TBA, 23 mM) and quinidine (1 mM), followed several minutes later by increased bumetanide-sensitive efflux or influx (Na(+)-K(+)-2Cl- cotransport). ATP induced a loss of cell K+ that was prevented by TBA and augmented by bumetanide. Both TBA and quinidine prevented the stimulation of cotransport by agonists but not by hypertonic shrinkage. Raising medium [K+] to prevent K+ loss also blocked activation of cotransport by agonists. The results indicate that the stimulation of Na(+)-K(+)-2Cl- cotransport by Ca2+ is not direct but instead is indirect via activation of Ca(2+)-dependent K+ channels and a resulting decrease in cell volume and intracellular [Cl-]. This suggests that at least one role of Na(+)-K(+)-2Cl- cotransport in endothelial cells is to maintain cell volume and intracellular [Cl-] during agonist stimulation.

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.

pH regulatory Na/H exchange by Amphiuma red blood cells

In Amphiuma red blood cells, the Na/H exchanger has been shown to play a central role in the regulation of cell volume following cell shrinkage (Cala, P. M. 1980. Journal of General Physiology. 76:683-708.) The present study was designed to evaluate the existence of pH regulatory Na/H exchange in the Amphiuma red blood cell. The data illustrate that when the intracellular pHi was decreased below the normal value of 7.00, Na/H exchange was activated in proportion to the degree of acidification. Once activated, net Na/H exchange flux persisted until normal intracellular pH (6.9-7.0) was restored, with a half time of approximately 5 min. These observations established a pHi set point of 7.00 for the pH-activated Na/H exchange of Amphiuma red blood cell. This is in contrast to the behavior of osmotically shrunken Amphiuma red blood cells in which no pHi set point could be demonstrated. That is, when activated by cell shrinkage the Na/H exchange mediated net Na flux persisted until normal volume was restored regardless of pHi. In contrast, when activated by cell acidification, the Na/H exchanger functioned until pHi was restored to normal and cell volume appeared to have no effect on pH-activated Na/H exchange. Studies evaluating the kinetic and inferentially, the molecular equivalence of the volume and pHi-induced Amphiuma erythrocyte Na/H exchanger(s), indicated that the apparent Na affinity of the pH activated cells is four times greater than that of shrunken cells. The apparent Vmax is also higher (two times) in the pH activated cells, suggesting the involvement of two distinct populations of the transporter in pH and volume regulation. However, when analyzed in terms of a bisubstrate model, the same data are consistent with the conclusion that both pH and volume regulatory functions are mediated by the same transport protein. Taken together, these data support the conclusion that volume and pH are regulated by the same effector (Na/H exchanger) under the control of as yet unidentified, distinct and cross inhibitory volume and pH sensing mechanisms.

Ion transport asymmetry and functional coupling in bovine pigmented and nonpigmented ciliary epithelial cells

The solute and water transport properties of the bovine ciliary epithelium were studied using isolated pigmented (PE) and nonpigmented (NPE) cells. It was shown that these cells were functionally coupled by demonstrating dye diffusion between paired PE and NPE cells after microinjection of lucifer yellow. Electronic cell sizing was used to measure cell volume changes of isolated PE and NPE cells in suspension after anisosmotic perturbations and after transport inhibition under isosmotic conditions. The PE cells showed the presence of a regulatory volume increase when subjected to osmotic shrinkage with NaCl, whereas the NPE cells did not demonstrate a regulatory volume increase under these conditions. In contrast, the NPE cells exhibited a regulatory volume decrease when subjected to osmotic swelling, whereas the PE cells did not recover from swelling. The regulatory volume decrease in NPE cells was inhibited by increased bath K or pretreatment with quinine (1 mM). The presence of a bumetanide-sensitive mechanism capable of moving measurable amounts of solute and water, probably Na-K-2Cl cotransport, was demonstrated in the PE cells but absent in the NPE cells. Bumetanide produced a dose-dependent shrinkage of PE cells at concentrations as low as 1 microM. Isosmotically reducing bath Cl, Na, or K concentration caused a rapid shrinkage of PE cells that was bumetanide inhibitable. The asymmetry of transport properties in PE and NPE cells supports a functional syncytium model of aqueous humor formation (39) across the two layers of the ciliary epithelium wherein ion uptake from the blood is carried out by the PE cells and ion extrusion by the NPE cells. Gap-junction coupling between the cells allows the ions taken up by the PE cells to move into the NPE cells. Extrusion of Na by the Na-K pump across the aqueous facing (basolateral) membranes of the NPE cells, most likely accompanied by Cl, determines the formation of the aqueous humor.

Characterization of the endogenous Na(+)-K(+)-2Cl- cotransporter in Xenopus oocytes

Over time, Xenopus laevis changed from producing stage V and VI oocytes with little native Na(+)-K(+)-2Cl- cotransport activity to those with substantial activity. In oocytes with high endogenous activity, K+ uptake, using the tracer 86Rb+ was approximately 20 pmol.min-1.oocyte-1 in the presence of blockers of Na(+)-K(+)-ATPase and conductive K+ transport. Bumetanide (10 microM) inhibited > 90% of this uptake, suggesting involvement of Na(+)-K(+)-2Cl- cotransport. This was confirmed by two observations that are found in this cotransporter in other tissues: 1) The related diuretics, thiobenzmetanide [50% inhibitory concentration (IC50), 2 x 10(-11) M] > bumetanide (IC50, 7 x 10(-8) M) > furosemide (IC50, 2.5 x 10(-6) M) inhibited the cotransporter in a dose-dependent manner. 2) There was little uptake of K+ in the absence of extracellular Na+ or Cl-. Halving medium osmolarity to 92 mosM decreased bumetanide-sensitive K+ uptake by approximately 75%, whereas a doubling of medium osmolarity increased it by approximately 50%. The cotransport activity was increased fourfold by the phosphatase inhibitor calyculin A (200 nM) but was unaffected by 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, 8-bromoguanosine 3',5'-cyclic monophosphate, ATP, ionomycin, or okadaic acid. Both the photoaffinity bumetanide analogue, 4-[3H]benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid, and an antiserum raised against Ehrlich ascites cell cotransporter specifically labeled an approximately 140-kDa oocyte membrane protein. These results demonstrated that, in addition to the Na+ pump and K+ channels, K+ uptake in Xenopus oocytes occurs via a loop-diuretic-sensitive Na(+)-K(+)-2Cl- cotransporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory interaction of ATP Na+ and Cl- in the turnover cycle of the NaK2Cl cotransporter

To probe the mechanism by which intracellular ATP, Na+, and Cl- influence the activity of the NaK2Cl cotransporter, we measured bumetanide-sensitive (BS) 86Rb fluxes in the osteosarcoma cell line UMR-106-01. Under physiological gradients of Na+, K+, and Cl-, depleting cellular ATP by incubation with deoxyglucose and antimycin A (DOG/AA) for 20 min at 37 degrees C reduced BS 86Rb uptake from 6 to 1 nmol/mg protein per min. Similar incubation with 0.5 mM ouabain to inhibit the Na+ pump had no effect on the uptake, excluding the possibility that DOG/AA inhibited the uptake by modifying the cellular Na+ and K+ gradients. Loading the cells with Na+ and depleting them of K+ by a 2-3-h incubation with ouabain or DOG/AA increased the rate of BS 86Rb uptake to approximately 12 nmol/mg protein per min. The unidirectional BS 86Rb influx into control cells was approximately 10 times faster than the unidirectional BS 86Rb efflux. On the other hand, at steady state the unidirectional BS 86Rb influx and efflux in ouabain-treated cells were similar, suggesting that most of the BS 86Rb uptake into the ouabain-treated cells is due to K+/K+ exchange. The entire BS 86Rb uptake into ouabain-treated cells was insensitive to depletion of cellular ATP. However, the influx could be converted to ATP-sensitive influx by reducing cellular Cl- and/or Na+ in ouabain-treated cells to impose conditions for net uptake of the ions. The BS 86Rb uptake in ouabain-treated cells required the presence of Na+, K+, and Cl- in the extracellular medium. Thus, loading the cells with Na+ induced rapid 86Rb (K+) influx and efflux which, unlike net uptake, were insensitive to cellular ATP. Therefore, we suggest that ATP regulates a step in the turnover cycle of the cotransporter that is required for net but not K+/K+ exchange fluxes. Depleting control cells of Cl- increased BS 86Rb uptake from medium-containing physiological Na+ and K+ concentrations from 6 to approximately 15 nmol/mg protein per min. The uptake was blocked by depletion of cellular ATP with DOG/AA and required the presence of all three ions in the external medium. Thus, intracellular Cl- appears to influence net uptake by the cotransporter. Depletion of intracellular Na+ was as effective as depletion of Cl- in stimulating BS 86Rb uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of Na-K-2Cl cotransport and bumetanide binding by ethacrynic acid, its analogues, and adducts

The inhibitory effect of ethacrynic acid (EA) and a variety of its derivatives on Na-K-2Cl cotransport in avian erythrocytes was investigated. The most potent compound tested was the adduct of EA with L-cysteine, with an IC50 of 7.2 x 10(-7) M. EA itself, dihydro-EA, EA-D-cysteine, and adducts of EA with other sulfhydryl (-SH) compounds were much less potent. The mechanism of action of EA and EA-L-cysteine differed in several respects: 1) EA-L-cysteine acted more rapidly than EA (half times of < 1 and 4 min, respectively, at 37 degrees C); 2) the action of EA-L-cysteine was reversible by washing, whereas that of EA was not; and 3) the degree of inhibition by EA-L-cysteine varied with medium [K], whereas that of EA did not. The inhibitory effects of both EA-L-cysteine and EA were affected by medium [Na] and [Cl]. We conclude that EA-L-cysteine does not "deliver" EA to transport-related -SH residues or act as an alkylating agent but has some stereospecific effect on cotransport that is a property of the entire molecule. EA does appear to inhibit cotransport by alkylating -SH residues, as closely related compounds lacking the ability to covalently react with such groups were reversible, and other -SH reagents (e.g., N-ethylmaleimide) also inhibited cotransport. EA, EA-L-cysteine, and EA-D-cysteine all inhibited [3H]bumetanide binding to membranes from activated avian erythrocytes at concentrations similar to those that inhibited cotransport. It is possible that the EA and bumetanide types of diuretics interact with closely apposed sites on the Na-K-2Cl cotransporter.

Cold activation of Na influx through the Na-H exchange pathway in guinea pig red cells

Previous work showed that amiloride partially inhibits the net gain of Na in cold-stored red cells of guinea pig and that the proportion of unidirectional Na influx sensitive to amiloride increases dramatically with cooling. This study shows that at 37 degrees C amiloride-sensitive (AS) Na influx in guinea pig red blood cells is activated by cytoplasmic H+, hypertonic incubation, phorbol ester in the presence of extracellular Ca2+ and is correlated with cation-dependent H+ loss from acidified cells. Cytoplasmic acidification increases AS Na efflux into Na-free medium. These properties are consistent with the presence of a Na-H exchanger with a H+ regulatory site. Elevation of cytoplasmic free Mg2+ above 3 mM greatly increases AS Na influx: this correlates with a Na-dependent loss of Mg2+, indicating the presence of a Na-Mg exchanger. At 20 degrees C activators of Na-H exchange have little or no further stimulatory effect on the already elevated AS Na influx. AS Na influx is much larger than either Na-dependent H+ loss or AS Na efflux at 20 degrees C. The affinity of the AS Na influx for cytoplasmic H+ is greater at 20 degrees C than at 37 degrees C. Depletion of cytoplasmic Mg2+ does not abolish the high AS Na influx at 20 degrees C. Thus, elevation of AS Na influx with cooling appears to be due to increased activity of a Na-H exchanger (operating in a "slippage" mode) caused by greater sensitivity to H+ at a regulatory site.

Delayed shortening and shrinkage of cochlear outer hair cells

Slow shortening of cochlear outer hair cells has been speculated to modify cochlear sensitivity. Tetanic electrical field stimulation of isolated outer hair cells from guinea pigs shortened the cells for 2-3 min. Electrical stimulation reduced cell length and volume (-13.5 +/- 1.5 and -37.3 +/- 3.0% of initial values, respectively, n = 16) and decreased the intracellular Cl- concentration. Cytochalasin B (100 microM) inhibited electrical stimulation-induced shortening but not volume reduction. The following chemicals or manipulations inhibited the responses: 10 microM furosemide, 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 1 mM anthracene-9-carboxylic acid (AC9), 25 mM tetraethylammonium, 2.3 microM charybdotoxin (ChTX), 250 nM omega-conotoxin, and Ca(2+)-free medium. These findings suggest that both electrical stimulation-induced shortening and shrinkage of outer hair cells result not only from an actin-mediated contractile force, but also from Cl- efflux through furosemide-, DIDS-, and AC9-sensitive Cl- channels, and K+ efflux through ChTX-sensitive K+ channels.

Erythrocyte K-Cl cotransport: properties and regulation

Erythrocytes possess a Cl-dependent, Na-independent K transport system cotransporting K and Cl in a 1:1 stoichiometry that is membrane potential independent. This K-Cl cotransporter is stimulated by cell swelling, acidification, Mg depletion, and thiol modification. Cell shrinkage, elevation of cellular divalent ions, thiol alkylation, phosphatase inhibitors, and derivatives of certain loop diuretics and stilbenes are inhibitory. Thus regulation of K-Cl cotransport at the membrane and cytoplasmic levels is highly complex. Basal K-Cl cotransport decreases with cellular maturation, whereas its modes of stimulation and inhibition are variable between species. The physiological inactivation appears to be prevented in low-K animal erythrocytes. In certain human hemoglobinopathies, K-Cl cotransport may be the cause of cellular dehydration and volume decrease. K-Cl cotransport occurs also in nonerythroid cells, such as in epithelial and liver cells of other species. At the threshold of molecular characterization, this comprehensive review places our present understanding of the mechanisms modulating K-Cl cotransport physiologically and pathophysiologically into kinetic and thermodynamic perspectives.

Internal magnesium, 2,3-diphosphoglycerate, and the regulation of the steady-state volume of human red blood cells by the Na/K/2Cl cotransport system

This study is concerned with the relationship between the Na/K/Cl cotransport system and the steady-state volume (MCV) of red blood cells. Cotransport rate was determined in unfractionated and density-separated red cells of different MCV from different donors to see whether cotransport differences contribute to the difference in the distribution of MCVs. Cotransport, studied in cells at their original MCVs, was determined as the bumetanide (10 microM)-sensitive 22Na efflux in the presence of ouabain (50 microM) after adjusting cellular Na (Nai) and Ki to achieve near maximal transport rates. This condition was chosen to rule out MCV-related differences in Nai and Ki that might contribute to differences in the net chemical driving force for cotransport. We found that in both unfractionated and density-separated red cells the cotransport rate was inversely correlated with MCV. MCV was correlated directly with red cell 2,3-diphosphoglycerate (DPG), whereas total red cell Mg was only slightly elevated in cells with high MCV. Thus intracellular free Mg (Mgifree) is evidently lower in red cells with high 2,3-DPG (i.e., high MCV) and vice versa. Results from flux measurements at their original MCVs, after altering Mgifree with the ionophore A23187, indicated a high Mgi sensitivity of cotransport: depletion of Mgifree inhibited and an elevation of Mgifree increased the cotransport rate. The apparent K0.5 for Mgifree was approximately 0.4 mM. Maximizing Mgifree at optimum Nai and Ki minimized the differences in cotransport rates among the different donors. It is concluded that the relative cotransport rate is regulated for cells in the steady state at their original cell volume, not by the number of copies of the cotransporter but by differences in Mgifree. The interindividual differences in Mgifree, determined primarily by differences in the 2,3-DPG content, are responsible for the differences in the relative cotransport activity that results in an inverse relationship with in vivo differences in MCV. Indirect evidence indicates that the relative cotransport rate, as indexed by Mgifree, is determined by the phosphorylated level of the cotransport system.

Regulatory volume increase in Ehrlich ascites tumor cells is mediated by the 1Na:1K:2Cl cotransport system

After swelling in hyposmotic solution, Ehrlich ascites tumor cells shrink towards their original volume. Upon restoration of isosmolality (300 mOsM) the cells initially shrink but subsequently recover volume. This regulatory volume increase (RVI) is completely blocked when [Na+]o or [Cl-]o is reduced by 50% in the presence of normal [K+]o. With normal [NaCl]o but less than 2 mM [K+]o, not only is volume recovery blocked but the cells lose KCl and shrink. When [K+]o is increased to 5 mM there is a rapid net uptake of K+ and Cl- which results in volume recovery. This suggests that the reswelling phase requires the simultaneous presence of Na+, K+, and Cl-. Although ouabain has no effect on volume recovery, bumetanide completely blocks RVI by inhibiting a cotransport pathway that mediates the net uptake of Na+, K+ and Cl- in the ratio of 1Na:1K:2Cl. Na+ that accumulates is then replaced by K+ via the Na/K pump.

Hemisodium, a novel selective Na ionophore. Effect on normal human erythrocytes

Hemisodium is a novel Na ionophore that belongs to the class of compounds called cryptands. These compounds possess an electron-rich cavity for binding of cations and are conformationally organized during synthesis to favor the selective binding of one cation over another. In media containing 145 mM NaCl and 5 mM KCl, hemisodium (10(-5) M) increased erythrocyte Na content from 23 to 345 mmol/kg.dry cell solid (dcs) over 4 h and increased water content from 1.8 to 3.5 liter/kg.dcs over the same period. K content decreased somewhat over the same time period, but this fall in K content was prevented entirely by incubation in either low Na media (to prevent net Na entry) or in Cl free media. Thus, the decrease in K content in high NaCl media was due to cell swelling, which activated KCl cotransport, and not due to a direct action of hemisodium on K permeability. Hemisodium-mediated Na transport was conductive, because erythrocyte membrane potential (Vm), determined by diS-C3-5 fluorescence, changed from -9 to +22 mV in high Na media in the presence of hemisodium and DIDS. In cells equilibrated with sulfamate, an anion with low conductive permeability, Vm changed 54 mV per 10-fold change in external Na concentration with the addition of hemisodium. In contrast, a 10-fold change in the external concentration of K, Rb, Cs, or T1 failed to alter Vm in the presence of hemisodium, suggesting a high Na specificity of the ionophore. Na conductance determined from net fluxes increased from 0.04 to 5.2 microS/cm2 with 10 microM hemisodium, and with that concentration the ratio of Na to K conductance was 45:1. Among the Na ionophores available so far, hemisodium appears to have the greatest specificity. Hemisodium may be a valuable tool in membrane transport studies.

The effects of metabolism on Na(+)-K(+)-Cl- co-transport in ferret red cells

1. The effects of altering metabolism on Na(+)-K(+)-Cl- co-transport were studied in ferret red cells. Na(+)-K(+)-Cl- co-transport was measured as the bumetanide-sensitive uptake of 86Rb. 2. Glucose, but not inosine or adenosine, sustained metabolism and maintained cell ATP content ([ATP]i) at the physiological level. [ATP]i could be reduced by prolonged incubation of cells in a substrate-free medium or more quickly by incubating cells with 2-deoxyglucose or with a mixture of iodoacetamide and glucose. 3. Na(+)-K(+)-Cl- co-transport activity was inhibited when [ATP]i was reduced to below 100 mumol (1 cell)-1 by starvation or by treatment with 2-deoxyglucose. However, a unique relationship between [ATP]i and activity could not be found. [ATP]i and the method and time course of ATP depletion all influenced activity. The inhibition of Na(+)-K(+)-Cl- co-transport, caused by reducing [ATP]i could be partially reversed by restoring [ATP]i to normal. 4. Increasing the concentration of intracellular ionized magnesium [( Mg2+]i) did not stimulate co-transport activity in ATP-depleted cells. This contrasts with the substantial stimulation seen in cells with normal [ATP]i. 5. Vanadate stimulated Na(+)-K(+)-Cl- co-transport activity in ATP-depleted cells but not in cells with normal [ATP]i. Fluoride did not affect activity at any [ATP]i. 6. The effects of some sulphydryl reagents on Na(+)-K(+)-Cl- co-transport were also examined. n-Ethylmaleimide (1 mM) inhibited Na(+)-K(+)-Cl- co-transport while it stimulated bumetanide-resistant potassium transport. Dithiothreitol (1 mM) did not affect activity. Iodoacetamide (6 mM) appeared to reduce the inhibition of cotransport activity seen at low [ATP]i but also greatly increased cell fragility. 7. The data suggest that activity of the Na(+)-K(+)-Cl- co-transport system is controlled by a cycle of phosphorylation and dephosphorylation with the phosphorylated form being active. Phosphorylation and transport appear to be almost maximal in ferret red cells with normal [ATP]i. Reduction of [ATP]i may allow changes in phosphatase activity to manifest as changes in transport rate. Differences in the balance between phosphorylation and dephosphorylation may explain tissue-dependent variations in the response of the system to various stimuli.

Inability of Ehrlich ascites tumor cells to volume regulate following a hyperosmotic challenge

Ehrlich cells shrink when the osmolality of the suspending medium is increased and behave, at least initially, as osmometers. Subsequent behavior depends on the nature of the hyperosmotic solute but in no case did the cells exhibit regulatory volume increase. With hyperosmotic NaCl an osmometric response was found and the resultant volume maintained relatively constant. Continuous shrinkage was observed, however, with sucrose-induced hyperosmolality. In both cases increasing osmolality from 300 to 500 mOSM initiated significant changes in cellular electrolyte content, as well as intracellular pH. This was brought about by activation of the Na+/H+ exchanger, the Na/K pump, the Na+ + K+ + 2Cl cotransporter and by loss of K+ via a Ba-sensitive pathway. The cotransporter in response to elevated [Cl-]i (approximately 100 mM) and/or the increase in the outwardly directed gradient of chemical potential for Na+, K+ and Cl-, mediated net loss of ions which accounted for cell shrinkage in the sucrose-containing medium. In hyperosmotic NaCl, however, the net Cl- flux was almost zero suggesting minimal net cotransport activity. We conclude that volume stability following cell shrinkage depends on the transmembrane gradient of chemical potential for [Na+ + K+ + Cl-], as well as the ratio of intra- to extracellular [Cl-]. Both factors appear to influence the activity of the cotransport pathway.

Rapid co-transport of sodium and chloride ions in giant salivary gland cells of the leech Haementeria ghilianii

1. Double-barrelled Cl(-)-selective microelectrodes were used to measure the apparent intracellular Cl- activity (aiCl) and membrane potential (Em) of leech salivary gland cells. In standard physiological solution buffered with HEPES (10 mM), intracellular Cl- activity (corrected for interference) was 38 +/- 8 mM (n = 11) compared to a value of 12.8 mM expected for passive Cl- distribution. The mean Em was -49.4 +/- 8.2 mV (n = 21) which was about 27 mV negative to the Cl- equilibrium potential. 2. Removal of external Cl- led to a slow fall in aiCl until a steady-state level of 4-11 mM was reached in 30-60 min. Recovery of aiCl on readdition of external Cl- took only 2-3 min. The uptake followed an exponential time course having a single rate constant of 1.73 +/- 0.1 min-1 (n = 5) whereas the loss appeared to occur in two phases. Changes in external Cl- produced immediate changes in Em which were the opposite of those expected for a high Cl- permeability, i.e. Cl- removal produced an immediate hyperpolarization (3-18 mV) and readdition of Cl- produced a transient depolarization (5-22 mV). 3. The intracellular accumulation of Cl- was dependent on the external Cl- activity. Even when the external Cl- concentration was reduced to 3%, the cells accumulated Cl- against an electrochemical gradient. 4. Cl- accumulation was temperature sensitive (Q10 approximately 2). 5. On removal of external Na+, aiCl fell to a level which was close to that expected for passive distribution. The active reaccumulation of Cl-, after intracellular Cl- depletion, was abolished in the absence of external Na+; aiCl slowly increased to its passive level. Steady-state aiCl or its recovery by Cl(-)-depleted cells was not affected by the absence of K+ in the bathing solution. 6. The reaccumulation of Cl- was not affected by furosemide (1-5 mM), bumetanide (10(-4) M), amiloride (10(-3) M) or 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS, 10(-4) M). 7. Removal of external Cl- caused a fall in intracellular Na+ activity (aiNa, measured with Na(+)-selective microelectrodes) from 15.9 +/- 6.8 mM (n = 9) to 2.5 +/- 1.3 mM (n = 3). When external Cl- was readded, aiNa rose to 46.5 +/- 6.6 mM (n = 3) before slowly recovering towards its original value. The maximal change in aiNa was 41.7 +/- 4.5 mM (n = 3) and the rate constant for Na+ uptake was 1.8 +/- 0.4 min-1 (n = 3).(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic mechanism of ATP action in Na(+)-K(+)-Cl- cotransport of HeLa cells determined by Rb+ influx studies

The kinetics of Na(+)-K(+)-Cl- cotransport were studied by measuring ouabain-insensitive furosemide-sensitive Rb+ influx (JRb) into HeLa cells while varying the cellular ATP and the extracellular Rb+ and Na+ concentrations. Results reveal that ATP stimulates JRb by increasing the affinity of the cotransporter for Rb+ (K+), and the apparent Michaelis constant (Km) for ATP was 0.95 +/- 0.03 mmol/l cell water. Two ATP molecules may relate to the uptake of one Rb+ by the cotransport pathway, as examined by the nonlinear least-squares method for goodness-of-fit and a Hill plot, JRb was strengthened by an increase in the inward chemical gradient associated with cell swelling on preincubation in a low-Na+ high-K+ medium, accompanying an increase in the affinity of the transporter for ATP. JRb was apparently activated by extracellular Na+, and the activation was enhanced by an increase in the cellular ATP concentration. Lactate production stimulated by 2 microM carbonylcyanide m-chlorophenyl hydrazone (CCCP) was reduced by 10 microM ouabain but not altered by further addition of 0.1 mM furosemide. Increases in cellular adenosine 3',5'-cyclic monophosphate (cAMP) caused by treatment with 0.1 mM isoproterenol plus 0.5 mM 3-isobutyl-1-methylxanthine or with 0.1 mM dibutyryl cAMP did not influence JRb. From this and previous studies, we propose a general and a specific model of Na(+)-K(+)-Cl- cotransport, which elucidate the order of binding of extracellular ions and reaction of cellular ATP.

Regulatory volume increase in Ehrlich ascites tumor cells

Ehrlich ascites tumor cells, shrunken as a result of KCl-depletion and Na+ loading, re-establish normal ionic concentrations by the combined activity of the Na+/K+ pump and the (2Cl- + K+ + Na+) cotransport system. Restoration of cell volume, however, correlates only with the increase in intracellular Cl-. This along with the finding that the equilibrium volume is linearly related to the steady state [Cl-] suggests that the extent to which cell volume increases is determined by Cl- transport. Net Cl- uptake, which is mediated almost exclusively by the cotransport system, is ultimately responsible for establishing the steady-state intracellular Cl- concentration. Transport mediated by this pathway ceases when the sum of the chemical potentials for Na+, K+ and Cl- approaches zero and corresponds with the establishment of a steady state for Cl-. These findings suggest that Cl- plays a key role in the regulation of net cotransport activity and thereby cell volume.

Kinetics of red cell Na+ and K+ transport in Prague hypertensive rats

Kinetics of ouabain-sensitive, furosemide-sensitive (FS), bumetanide-sensitive (BS) and -resistant Na+ and K+ transport were studied in erythrocytes of Prague hypertensive rats (PHR) and Prague normotensive rats (PNR). Maximal transport rates (Vmax) and apparent affinities for either intracellular Na+ or extracellular K+ (replaced by Rb+) were determined in red cells in which Na+ content varied around the physiological range and that were incubated in Na+ media. No major differences between PHR and PNR were disclosed in the kinetics of ion transport mediated by the Na(+)-K+ pump or BS inward Na(+)-K+ cotransport. FS Rb+ uptake was higher (due to a greater Vmax) in red cells of PHR as compared to PNR. In cells with a lowered Na+ content this elevation of FS Rb+ uptake was largely due to an augmented K(+)-Cl- cotransport which exhibits a low affinity for Rb+o and is blocked by 1 mM furosemide but not by 10 microM bumetanide. Red cells of PHR and PNR strains did not differ in either Na+ or Rb+ leaks. A slight increase of red cell Na+ content in PHR was evaluated in terms of the pump-leak concept. The present study did not reveal any obvious kinetic abnormalities of red cell cation transport the presence of which in tissues involved in blood pressure regulation would favor the development or the maintenance of genetic hypertension in PHR.

Characteristics of the volume- and chloride-dependent K transport in human erythrocytes homozygous for hemoglobin C

In human red cells homozygous for hemoglobin C (CC), cell swelling and acid pH increase K efflux and net K loss in the presence of ouabain (0.1 mM) and bumetanide. We report herein, that K influx is also dependent on cell volume in CC cells: cell swelling induces a marked increase in the maximal rate (from 6 to 18 mmol/liter cell X hr) and in the affinity for external K (from 77 +/- 16 mM to 28 +/- 3 mM) of K influx. When the external K concentration is varied from 0 to 140 mM. K efflux from CC and normal control cells is unaffected. Thus, K/K exchange is not a major component of this K movement. K transport through the pathway of CC cells is dependent on the presence of chloride or bromide; substitution with nitrate, acetate or thiocyanate inhibits the volume- and pH-dependent K efflux. When CC cells are separated according to density, a sizable volume-dependent component of K efflux can be identified in all the fractions and is the most active in the least dense fraction. N-ethylmaleimide (NEM) markedly stimulates K efflux from CC cells in chloride but not in nitrate media, and this effect is present in all the fractions of CC cells separated according to density. The persistence of this transport system in denser CC cells suggests that not only cell age, but also the presence of the positively charged C hemoglobin is an important determinant of the activity of this system. These data also indicate that the K transport pathway of CC cells is not an electrodiffusional process and is coupled to chloride.

Transport pathways for Na+ and Br- (Cl-) in noradrenaline-stimulated frog skin (Rana temporaria)

(1) Transport pathways for Na+ and Br- across noradrenaline-stimulated frog skin, in which the sodium channels in the apical membrane had been blocked with amiloride, were examined by pre-steady-state flux ratio experiments. (2) To analyse the experiments, equations are derived from which the fluxes through two parallel pathways can be determined if these differ with respect to the flux ratio as well as the mean passage time for the ion in question. (3) The fluxes of Na+ and Cl- were followed over a period of 3 h and it was found that about 1 h after addition of noradrenaline time-invariant fluxes could be achieved. In that period inward and outward tracer fluxes of Na+ and Br- (used as a substitute for Cl-) were followed during the build-up of isotope in the tissue until steady-state fluxes were attained. (4) The experiments showed that when the Na channels of the capital cells are blocked the glands provide the major pathway for sodium transport across the skin. The efflux of Br- can be separated into two components. A fast one through the mitochondria-rich cells, which is the pathway for passive anion fluxes, and a slower one through the glands. No influx through the slow transport pathway could be detected. In some skins exchange of Br- through the mitochondria-rich cells is revealed. An anion exchange mechanism also appears to be present in the basolateral membrane of the gland cells.

Regulation of Na+ and K+ contents in rat thymocytes

A modified nystatin technique allowed the investigation of the initial rate of Na+ efflux as a function of internal Na+ content under steady-state conditions in rat thymocytes. This kinetic study showed that 1) ouabain-sensitive Na+ efflux as a function of internal Na+ can be adjusted by a three-sites kinetic model, with a maximal pump rate of 581 +/- 79 mmol.l cells-1.h-1 and an apparent dissociation constant for internal Na+ of 10.0 +/- 1.0 mmol/l cells (mean +/- SE of 3 experiments), 2) bumetanide-sensitive Na+ efflux was extremely low compared with the pump efflux (approximately 1%), and 3) ouabain- and bumetanide-resistant Na+ efflux was almost a linear function of internal Na+ content (as expected for a Na+ leak). This "all-pump" mechanism of thymocyte Na+ regulation was confirmed by non-steady-state experiments showing that 1) ouabain induced a rapid net Na+ gain and K+ depletion in fresh thymocytes and completely blocked the recovery of normal cation contents in Na+-loaded-K+-depleted thymocytes, and 2) bumetanide was unable to modify thymocyte Na+ and K+ contents. Na+ extrusion by Na+-loaded thymocytes was unaffected by prostaglandin E2, isoproterenol, or platelet-aggregating factor (PAF) and was slightly impaired in the adult spontaneously hypertensive rat of the Okamoto strain (10% lower rate constant for net Na+ extrusion, P less than 0.05). Concerning cell Na+ regulation, our results do not support the concept that rat thymocytes are more representative of vascular cells than enucleated erythrocytes.

Kinetics of volume-sensitive K transport in human erythrocytes: evidence for asymmetry

The kinetic properties of volume-sensitive K fluxes in swollen human erythrocytes were investigated. Swelling-activated Cl-dependent K influx was a saturable function of external K concentration with a low affinity (apparent Km of 115-130 mM) and high capacity [maximal velocity (Vmax) = 20-30 mmol.l original cells-1.h-1 (mmol.loc-1.h-1)]. The Vmax and apparent Km for Cl-dependent K efflux were lower (Km = 47 mM; Vmax = 2.2 mmol.loc-1.h-1). The Hill coefficients for both K efflux and influx were close to unity, suggesting a single binding site for K. The increase of external K trans-stimulated K efflux, but the increase of intracellular K had no effect on Cl-dependent K influx in swollen cells. Under zero trans conditions, the Vmax (18 vs. 3 mmol.loc-1.h-1) and Km (138 vs. 32) were markedly different for influx and efflux, respectively. These results provide evidence for intrinsic functional asymmetry, such that the transporter is more prevalent and stable in the outward-facing conformation. The mean ratio of Km to Vmax for efflux (12.1) was 1.56 times larger than the same ratio for influx (7.8), but the difference between the means did not reach statistical significance. These kinetic observations are analyzed in terms of the simple carrier and the cotransport models.

Role of chloride in potassium transport through a K-Cl cotransport system in human red blood cells

In this paper, we report experiments demonstrating the coupling of Cl and K movements in a volume-dependent K-Cl cotransport system in human red blood cells. We show that an outwardly directed Cl gradient can promote net K efflux against an inwardly directed K gradient at constant membrane potential. Red blood cell membrane potential was kept constant by using anions that are not transported through the K-Cl cotransport system but that are more permeable than Cl (NO3 and SCN). Under these conditions, when the activities of band 3 (capnophorin)-mediated anion exchange and of the carbonic anhydrase have been inhibited, it is possible to maintain a Cl gradient at constant membrane potential. Similar data were obtained in human red blood cells (least dense fraction from normal subjects and whole blood from patients with homozygous hemoglobin S disease), in rabbit red blood cells, and in low-K sheep red blood cells. These data confirm that the volume-dependent Cl-dependent K movement in these cells operates through coupled K-Cl cotransport.