Swelling activation of K-Cl cotransport in LK sheep erythrocytes: a three-state process

Should I shrink or should I grow: cell size changes in tissue morphogenesis

Cells change shape, move, divide, and die to sculpt tissues. Common to all these cell behaviours are cell size changes, which have recently emerged as key contributors to tissue morphogenesis. Cells can change their mass-the number of macromolecules they contain-or their volume-the space they encompass. Changes in cell mass and volume occur through different molecular mechanisms and at different timescales, slow for changes in mass and rapid for changes in volume. Therefore, changes in cell mass and cell volume, which are often linked, contribute to the development and shaping of tissues in different ways. Here, we review the molecular mechanisms by which cells can control and alter their size, and we discuss how changes in cell mass and volume contribute to tissue morphogenesis. The role that cell size control plays in developing embryos is only starting to be elucidated. Research on the signals that control cell size will illuminate our understanding of the cellular and molecular mechanisms that drive tissue morphogenesis.

Role of WNK Kinases in the Modulation of Cell Volume

Ion Transport across the cell membrane is required to maintain cell volume homeostasis. In response to changes in extracellular osmolarity, most cells activate specific metabolic or membrane-transport pathways to respond to cell swelling or shrinkage and return their volume to its normal resting state. This process involves the rapid adjustment of the activities of channels and transporters that mediate flux of K⁺, Na⁺, Cl^-, and small organic osmolytes. Cation chloride cotransporters (CCCs) NKCCs and KCCs are a family of membrane proteins modulated by changes in cell volume and/or in the intracellular chloride concentration ([Cl^-]_i). Cell swelling triggers regulatory volume decrease (RVD), promoting solute and water efflux to restore normal cell volume. Swelling-activated KCCs mediate RVD in most cell types. In contrast, cell shrinkage triggers regulatory volume increase (RVI), which involves the activation of the NKCC1 cotransporter of the CCC family. Regulation of the CCCs during RVI and RVD by protein phosphorylation is a well-characterized mechanism, where WNK kinases and their downstream kinase substrates, SPAK and OSR1 constitute the essential phospho-regulators. WNKs-SPAK/OSR1-CCCs complex is required to regulate cell shrinkage-induced RVI or cell swelling-induced RVD via activating or inhibitory phosphorylation of NKCCs or KCCs, respectively. WNK1 and WNK4 kinases have been established as [Cl^-]_i sensors/regulators, while a role for WNK3 kinase as a cell volume-sensing kinase has emerged and is proposed in this chapter.

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.

Molecular features and physiological roles of K+-Cl- cotransporter 4 (KCC4)

A K⁺-Cl^- cotransport system was documented for the first time during the mid-seventies in sheep and goat red blood cells. It was then described as a Na⁺-independent and ouabain-insensitive ion carrier that could be stimulated by cell swelling and N-ethylmaleimide (NEM), a thiol-reacting agent. Twenty years later, this system was found to be dispensed by four different isoforms in animal cells. The first one was identified in the expressed sequence tag (EST) database by Gillen et al. based on the assumption that it would be homologous to the Na⁺-dependent K⁺-Cl^- cotransport system for which the molecular identity had already been uncovered. Not long after, the three other isoforms were once again identified in the EST databank. Among those, KCC4 has generated much interest a few years ago when it was shown to sustain distal renal acidification and hearing development in mouse. As will be seen in this review, many additional roles were ascribed to this isoform, in keeping with its wide distribution in animal species. However, some of them have still not been confirmed through animal models of gene inactivation or overexpression. Along the same line, considerable knowledge has been acquired on the mechanisms by which KCC4 is regulated and the environmental cues to which it is sensitive. Yet, it is inferred to some extent from historical views and extrapolations.

Coordinated control of volume regulatory Na+/H+ and K+/H+ exchange pathways in Amphiuma red blood cells

The Na(+)/H(+) and K(+)/H(+) exchange pathways of Amphiuma tridactylum red blood cells (RBCs) are quiescent at normal resting cell volume yet are selectively activated in response to cell shrinkage and swelling, respectively. These alkali metal/H(+) exchangers are activated by net kinase activity and deactivated by net phosphatase activity. We employed relaxation kinetic analyses to gain insight into the basis for coordinated control of these volume regulatory ion flux pathways. This approach enabled us to develop a model explaining how phosphorylation/dephosphorylation-dependent events control and coordinate the activity of the Na(+)/H(+) and K(+)/H(+) exchangers around the cell volume set point. We found that the transition between initial and final steady state for both activation and deactivation of the volume-induced Na(+)/H(+) and K(+)/H(+) exchange pathways in Amphiuma RBCs proceed as a single exponential function of time. The rate of Na(+)/H(+) exchange activation increases with cell shrinkage, whereas the rate of Na(+)/H(+) exchange deactivation increases as preshrunken cells are progressively swollen. Similarly, the rate of K(+)/H(+) exchange activation increases with cell swelling, whereas the rate of K(+)/H(+) exchange deactivation increases as preswollen cells are progressively shrunken. We propose a model in which the activities of the controlling kinases and phosphatases are volume sensitive and reciprocally regulated. Briefly, the activity of each kinase-phosphatase pair is reciprocally related, as a function of volume, and the volume sensitivities of kinases and phosphatases controlling K(+)/H(+) exchange are reciprocally related to those controlling Na(+)/H(+) exchange.

Current knowledge about the functional roles of phosphorylative changes of membrane proteins in normal and diseased red cells

With the advent of proteomic techniques the number of known post-translational modifications (PTMs) affecting red cell membrane proteins is rapidly growing but the understanding of their role under physiological and pathological conditions is incompletely established. The wide range of hereditary diseases affecting different red cell membrane functions and the membrane modifications induced by malaria parasite intracellular growth represent a unique opportunity to study PTMs in response to variable cellular stresses. In the present review, some of the major areas of interest in red cell membrane research have been considered as modifications of erythrocyte deformability and maintenance of the surface area, membrane transport alterations, and removal of diseased and senescent red cells. In all mentioned research areas the functional roles of PTMs are prevalently restricted to the phosphorylative changes of the more abundant membrane proteins. The insufficient information about the PTMs occurring in a large majority of the red membrane proteins and the general lack of mass spectrometry data evidence the need of new comprehensive, proteomic approaches to improve the understanding of the red cell membrane physiology.

ROS activate KCl cotransport in nonadherent Ehrlich ascites cells but K+ and Cl- channels in adherent Ehrlich Lettré and NIH3T3 cells

Addition of H(2)O(2) (0.5 mM) to Ehrlich ascites tumor cells under isotonic conditions results in a substantial (22 +/- 1%) reduction in cell volume within 25 min. The cell shrinkage is paralleled by net loss of K(+), which was significant within 8 min, whereas no concomitant increase in the K(+) or Cl(-) conductances could be observed. The H(2)O(2)-induced cell shrinkage was unaffected by the presence of clofilium and clotrimazole, which blocks volume-sensitive and Ca(2+)-activated K(+) channels, respectively, and is unaffected by a raise in extracellular K(+) concentration to a value that eliminates the electrochemical driving force for K(+). On the other hand, the H(2)O(2)-induced cell shrinkage was impaired in the presence of the KCl cotransport inhibitor (dihydro-indenyl)oxyalkanoic acid (DIOA), following substitution of NO(3)(-) for Cl(-), and when the driving force for KCl cotransport was omitted. It is suggested that H(2)O(2) activates electroneutral KCl cotransport in Ehrlich ascites tumor cells and not K(+) and Cl(-) channels. Addition of H(2)O(2) to hypotonically exposed cells accelerates the regulatory volume decrease and the concomitant net loss of K(+), whereas no additional increase in the K(+) and Cl(-) conductance was observed. The effect of H(2)O(2) on cell volume was blocked by the serine-threonine phosphatase inhibitor calyculin A, indicating an important role of serine-threonine phosphorylation in the H(2)O(2)-mediated activation of KCl cotransport in Ehrlich cells. In contrast, addition of H(2)O(2) to adherent cells, e.g., Ehrlich Lettré ascites cells, a subtype of the Ehrlich ascites tumor cells, and NIH3T3 mouse fibroblasts increased the K(+) and Cl(-) conductances after hypotonic cell swelling. Hence, H(2)O(2) induces KCl cotransport or K(+) and Cl(-) channels in nonadherent and adherent cells, respectively.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Activation of Na+/H+ and K+/H+ exchange by calyculin A in Amphiuma tridactylum red blood cells: implications for the control of volume-induced ion flux activity

Alteration in cell volume of vertebrates results in activation of volume-sensitive ion flux pathways. Fine control of the activity of these pathways enables cells to regulate volume following osmotic perturbation. Protein phosphorylation and dephosphorylation have been reported to play a crucial role in the control of volume-sensitive ion flux pathways. Exposing Amphiuma tridactylu red blood cells (RBCs) to phorbol esters in isotonic medium results in a simultaneous, dose-dependent activation of both Na(+)/H(+) and K(+)/H(+) exchangers. We tested the hypothesis that in Amphiuma RBCs, both shrinkage-induced Na(+)/H(+) exchange and swelling-induced K(+)/H(+) exchange are activated by phosphorylation-dependent reactions. To this end, we assessed the effect of calyculin A, a phosphatase inhibitor, on the activity of the aforementioned exchangers. We found that exposure of Amphiuma RBCs to calyculin-A in isotonic media results in simultaneous, 1-2 orders of magnitude increase in the activity of both K(+)/H(+) and Na(+)/H(+) exchangers. We also demonstrate that, in isotonic media, calyculin A-dependent increases in net Na(+) uptake and K(+) loss are a direct result of phosphatase inhibition and are not dependent on changes in cell volume. Whereas calyculin A exposure in the absence of volume changes results in stimulation of both the Na(+)/H(+) and K(+)/H(+) exchangers, superimposing cell swelling or shrinkage and calyculin A treatment results in selective activation of K(+)/H(+) or Na(+)/H(+) exchange, respectively. We conclude that kinase-dependent reactions are responsible for Na(+)/H(+) and K(+)/H(+) exchange activity, whereas undefined volume-dependent reactions confer specificity and coordinated control.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Signalling pathways involved in the control of sperm cell volume

The ability to maintain cellular volume is an important general physiological function, which is achieved by specific molecular mechanisms. Hypotonically induced swelling results in the opening of K+ and Cl- ion channels, through which these ions exit with accompanying water loss. This process is known as regulatory volume decrease (RVD). The molecular mechanisms that control the opening of the ion channels in spermatozoa are as yet poorly understood. The present study investigated pathways of osmo-signalling using boar spermatozoa as a model. Spermatozoa were diluted into isotonic and hypotonic Hepes-buffered saline in the presence or absence of effector drugs, and at predetermined intervals volume measurements were performed electronically. Treatment with protein kinase C (PKC) inhibitors staurosporine, bismaleimide I and bismaleimide X led to dose-dependent increases of both isotonic and hypotonic volumes (P<0.05). However, as the isotonic volume was affected more than the hypotonic volume, the kinase inhibitors appeared to improve RVD, whereas activation of PKC with phorbol dibutyrate blocked RVD. The increase in isotonic cell volume induced by bismaleimide X was observed in chloride-containing medium but not in the medium in which chloride was replaced by sulphate, implying that PKC was involved in the control of chloride channel activity, e.g. by closing the channel after volume adjustment. The protein phosphatase PP1/PP2 inhibitors calyculin and okadaic acid increased the isotonic volume only slightly but they greatly increased the relative cell volume and blocked RVD. The activation of RVD processes was found to be cAMP-dependent; incubation with forskolin and papaverine improved volume regulation. Moreover, papaverine was able to overcome the negative effect of protein phosphatase inhibitors. The mechanism of sperm RVD appears to involve (a) alterations in protein phosphorylation/dephosphorylation balance brought about by PKC and PP1 and (b) a cAMP-dependent activating pathway.

Urea stimulation of KCl cotransport induces abnormal volume reduction in sickle reticulocytes

KCl cotransport (KCC) activity contributes to pathologic dehydration in sickle (SS) red blood cells (RBCs). KCC activation by urea was measured in SS and normal (AA) RBCs as Cl-dependent Rb influx. KCC-mediated volume reduction was assessed by measuring reticulocyte cellular hemoglobin concentration (CHC) cytometrically. Urea activated KCC fluxes in fresh RBCs to levels seen in swollen cells, although SS RBCs required lower urea concentrations than did normal (AA) RBCs. Little additional KCC stimulation by urea occurred in swollen AA or SS RBCs. The pH dependence of KCC in "euvolemic" SS RBCs treated with urea was similar to that in swollen cells. Urea triggered volume reduction in SS and AA reticulocytes, establishing a higher CHC. Volume reduction was Cl dependent and was limited by the KCC inhibitor, dihydro-indenyl-oxyalkanoic acid. Final CHC depended on urea concentration, but not on initial CHC. Under all activation conditions, volume reduction was exaggerated in SS reticulocytes and produced higher CHCs than in AA reticulocytes. The sulfhydryl-reducing agent, dithiothreitol, normalized the sensitivity of KCC activation to urea in SS RBCs and mitigated the urea-stimulated volume decrease in SS reticulocytes, suggesting that the dysfunctional activity of KCC in SS RBCs was due in part to reversible sulfhydryl oxidation.

Regulation of K-Cl cotransport by protein phosphatase 1alpha in mouse erythrocytes

The K-Cl cotransport (KCC) is an electroneutral-gradient-driven-membrane transport system, which is involved in regulation of red cell volume. Although the regulatory cascade of KCC is largely unknown, a signaling pathway involving phosphatases and kinases has been proposed. Here, we investigated the expression and the activity of protein phosphatase 1(PP-1) isoforms in mouse red cells, focusing on two models of abnormally activated KCC: mice genetically lacking the two Src-family tyrosine kinases, Hck and Fgr, (hck-/-fgr-/-) and the SAD transgenic sickle-cell-mice. The PP-1alpha, PP-1gamma, PP-1delta isoforms were expressed at similar levels in wild-type, hck-/-fgr-/- and SAD mouse erythrocytes and in each case were predominantly localized to cytoplasm. The PP-1alpha activity was significantly higher in both membrane and cytosol fractions of hck-/-fgr-/- and of SAD erythrocytes than in those of wild-type red cells, suggesting PP-1alpha as a target of the Hck and Fgr kinases. The PP2, a specific inhibitor of Src-family kinase, significantly increased KCC activity in wild-type mouse red cells, but failed to modify the already increased KCC activity in SAD erythrocytes. The lag-time for activation of KCC was considerably reduced in both hck-/-fgr-/- and SAD erythrocytes, suggesting that the rate limiting activation steps in both strains are freed from their tonic inhibition. Sulfhydryl reduction by dithiothreitol (DTT) lowered KCC activity only in SAD red cells, but did not affect the PP2-treated erythrocytes. These data suggest up-regulation of KCC in SAD red cells is mainly secondary to oxidative damage, which most likely reduces or removes the tonic KCC inhibition resulting from PP-1alpha activity controlled in turn by Src-family kinases.

Protein phosphatase 1alpha is tyrosine-phosphorylated and inactivated by peroxynitrite in erythrocytes through the src family kinase fgr

Protein serine/threonine phosphorylation is a significant component of the intracellular signal that together with tyrosine phosphorylation regulates several processes, including cell-cycle progression, muscle contraction, transcription, and neuronal signaling. Cross-talk between phosphoserine/threonine- and phosphotyrosine-mediated pathways is not yet well understood. In this study we found that peroxynitrite, a physiological oxidant formed by the fast radical-radical reaction between the nitric oxide and the superoxide anion, induced tyrosine phosphorylation of the serine/threonine protein phosphatase 1alpha (PP1alpha) in human erythrocytes through activation of src family kinases. We have previously shown in mouse red cells that upregulation of the src kinase fgr phosphorylates PP1alpha, acting as an upstream negative regulator of PP1alpha, and downregulates K-Cl cotransport. Here we found that PP1alpha is a selective substrate of peroxynitrite-activated fgr and that tyrosine phosphorylation of PP1alpha corresponds to an inhibition of its enzymatic activity. Despite fgr activation and PP1alpha downregulation, peroxynitrite stimulated in a dose-dependent fashion the function of the K-Cl cotransporter. In an attempt to understand the mechanism of K-Cl cotransport activation, we found that the effect of peroxynitrite is completely reversed by dithriothreitol, suggesting that peroxynitrite acts as an oxidizing agent by an SH-dependent and PP1alpha-independent mechanism. These findings highlight a novel function of peroxynitrite in regulating the intracellular signal transduction pathways involving serine/threonine phosphorylation and the functional role of proteins that are targets of these phosphatases.

Effect of Peroxynitrite on Passive K+ Transport in Human Red Blood Cells

Peroxynitrite is generated in vivo by the reaction between nitric oxide, from endothelial and other cells, and the superoxide anion. It is therefore pertinent to examine its effects on the membrane permeability of red blood cells. Treatment of human red blood cells with peroxynitrite (nominally 1 mM) markedly stimulated passive K+ permeability. The main effect was on a Cl(-)-independent K+ pathway, which remains unidentified. Although K+-Cl- cotransport (KCC) was stimulated, this was dependent on saline composition, being inhibited by physiological levels of glucose (IC50 4 mM), and also by sucrose and MOPS. Effects on the Cl(-)-independent K+ pathway were less dependent on saline composition, and were not inhibited by amiloride, ethylisopropylamiloride, dimethylamiloride or gadolinium. Na+-K+-2Cl- cotransporter was inhibited whilst there was little effect on the Gardos channel (Ca2+-activated K+ channel). Peroxynitrite was markedly more effective in oxygenated cells than deoxygenated ones. Treatment with peroxynitrite per se did not affect initial cell volume. Anisotonic swelling modestly increased the Cl(-)-independent K+ influx, but did not affect peroxynitrite-stimulated KCC. Decreasing extracellular pH from 7.4 to 7.2 or 7.0 increased KCC stimulation, whilst the Cl(-)-independent component of K+ transport was lowest at pH 7.2. Finally, protein phosphatase inhibition with calyculin A (100 nM) inhibited KCC, implying that, as with other KCC stimuli, peroxynitrite acts via decreased protein phosphorylation; pre-treatment with calyculin A also inhibited the Cl(-)-independent component of K+ transport. These findings are relevant to the actions of peroxynitrite in vivo.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

Ion transport pathology in the mechanism of sickle cell dehydration

Polymers of deoxyhemoglobin S deform sickle cell anemia red blood cells into sickle shapes, leading to the formation of dense, dehydrated red blood cells with a markedly shortened life-span. Nearly four decades of intense research in many laboratories has led to a mechanistic understanding of the complex events leading from sickling-induced permeabilization of the red cell membrane to small cations, to the generation of the heterogeneity of age and hydration condition of circulating sickle cells. This review follows chronologically the major experimental findings and the evolution of guiding ideas for research in this field. Predictions derived from mathematical models of red cell and reticulocyte homeostasis led to the formulation of an alternative to prevailing gradualist views: a multitrack dehydration model based on interactive influences between the red cell anion exchanger and two K(+) transporters, the Gardos channel (hSK4, hIK1) and the K-Cl cotransporter (KCC), with differential effects dependent on red cell age and variability of KCC expression among reticulocytes. The experimental tests of the model predictions and the amply supportive results are discussed. The review concludes with a brief survey of the therapeutic strategies aimed at preventing sickle cell dehydration and with an analysis of the main open questions in the field.

Regulation of K-Cl cotransport during reticulocyte maturation and erythrocyte aging in normal and sickle erythrocytes

The age/density-dependent decrease in K-Cl cotransport (KCC), PP1 and PP2A activities in normal and sickle human erythrocytes, and the effect of urea, a known KCC activator, were studied using discontinuous, isotonic gradients. In normal erythrocytes, the densest fraction (d approximately 33.4 g/dl) has only about approximately 5% of the KCC and 4% of the membrane (mb)-PP1 activities of the least-dense fraction (d approximately 24.7 g/dl). In sickle and normal erythrocytes, density-dependent decreases for mb-PP1 activity were similar (d50% 28.1 +/- 0.4 vs. 27.2 +/- 0.2 g/dl, respectively), whereas those for KCC activity were not (d50% 31.4 +/- 0.9 vs. 26.8 +/- 0.3 g/dl, respectively, P = 0.004). Excluding the 10% least-dense cells, a very tight correlation exists between KCC and mb-PP1 activities in normal (r2 = 0.995) and sickle erythrocytes (r2 = 0.93), but at comparable mb-PP1 activities, KCC activity is higher in sickle erythrocytes, suggesting a defective, mb-PP1-independent KCC regulation. In normal, least-dense but not in densest cells, urea stimulates KCC (two- to fourfold) and moderately increases mb-PP1 (20-40%). Thus mb-PP1 appears to mediate part of urea-stimulated KCC activity.

Effect of 1-chloro-2,4-dinitrobenzene on K+ transport in normal and sickle human red blood cells

1-Chloro-2,4-dinitrobenzene (CDNB), which causes oxidative stress through depletion of reduced glutathione (GSH), increases the passive K+ permeability of red cells. In this paper, we investigated the effects of CDNB (1 mM) on the activities of the K+-Cl- cotransporter (KCC; measured as Cl--dependent K+ influx) and the Gardos channel (taken as clotrimazole-sensitive K+ influx, 5 microM) in human red cells, using 86Rb+ as a K+ congener. 45Ca2+ was used to study passive Ca2+ entry and active Ca2+ efflux via the plasma membrane Ca2+ pump. Both the Gardos channel and KCC were stimulated in both normal and sickle red cells. In sickle cells, stimulation of KCC was similar in oxygenated and deoxygenated cells; that of the Gardos channel was greater in deoxygenated cells. In normal red cells, stimulation of both pathways was greater in oxygenated cells (by 4 +/- 1-fold; all means +/- S.E.M., n = 3). The effects on the Gardos channel were dependent on extracellular Ca2+ and were associated with inhibition of the plasma membrane Ca2+ pump (by 29 +/- 3 %, P < 0.01) and increased Ca2+ sensitivity of the channel (EC50 for [Ca2+]i reduced from 260 +/- 26 to 175 +/- 15 nM; P < 0.05). Cell volume, pHi, ATP levels and passive Ca2+ entry were not affected by CDNB. The effects on KCC were inhibited (93 +/- 6 %) by prior treatment with the protein phosphatase inhibitor calyculin A (100 nM) and were not additive with stimulation by N-ethylmaleimide (1 mM), regardless of the order of addition. These findings are therefore consistent with inhibition of a regulatory protein kinase, although stimulation of the conjugate protein phosphatase(s) may also occur. KCC stimulation was also Ca2+ dependent. These findings are important for understanding how GSH depletion alters membrane permeability and how to protect against red cell dehydration.

Macromolecular crowding and its role as intracellular signalling of cell volume regulation

Macromolecular crowding has been proposed as a mechanism by means of which a cell can sense relatively small changes in volume or, more accurately, the concentration of intracellular solutes. According to the macromolecular theory, the kinetics and equilibria of enzymes can be greatly influenced by small changes in the concentration of ambient, inert macromolecules. A 10% change in the concentration of intracellular proteins can lead to changes of up to a factor of ten in the thermodynamic activity of putative molecular regulatory species, and consequently, the extent to which such regulator(s) may bind to and activate membrane-associated ion transporters. The aim of this review is to examine the concept of macromolecular crowding and how it profoundly affects macromolecular association in an intact cell with particular emphasis on its implication as a sensor and a mechanism through which cell volume is regulated.

Volume-sensitive K(+)/Cl(-) cotransport in rabbit erythrocytes. Analysis of the rate-limiting activation and inactivation events

The kinetics of activation and inactivation of K(+)/Cl(-) cotransport (KCC) have been measured in rabbit red blood cells for the purpose of determining the individual rate constants for the rate-limiting activation and inactivation events. Four different interventions (cell swelling, N-ethylmaleimide [NEM], low intracellular pH, and low intracellular Mg(2+)) all activate KCC with a single exponential time course; the kinetics are consistent with the idea that there is a single rate-limiting event in the activation of transport by all four interventions. In contrast to LK sheep red cells, the KCC flux in Mg(2+)-depleted rabbit red cells is not affected by cell volume. KCC activation kinetics were examined in cells pretreated with NEM at 0 degrees C, washed, and then incubated at higher temperatures. The forward rate constant for activation has a very high temperature dependence (E(a) approximately 32 kCal/mol), but is not affected measurably by cell volume. Inactivation kinetics were examined by swelling cells at 37 degrees C to activate KCC, and then resuspending at various osmolalities and temperatures to inactivate most of the transporters. The rate of transport inactivation increases steeply as cell volume decreases, even in a range of volumes where nearly all the transporters are inactive in the steady state. This finding indicates that the rate-limiting inactivation event is strongly affected by cell volume over the entire range of cell volumes studied, including normal cell volume. The rate-limiting inactivation event may be mediated by a protein kinase that is inhibited, either directly or indirectly, by cell swelling, low Mg(2+), acid pH, and NEM.

Molecular cloning and functional characterization of KCC3, a new K-Cl cotransporter

We isolated and characterized a novel K-Cl cotransporter, KCC3, from human placenta. The deduced protein contains 1,150 amino acids. KCC3 shares 75-76% identity at the amino acid level with human, pig, rat, and rabbit KCC1 and 67% identity with rat KCC2. KCC3 is 40 and 33% identical to two Caenorhabditis elegans K-Cl cotransporters and approximately 20% identical to other members of the cation-chloride cotransporter family (CCC), two Na-K-Cl cotransporters (NKCC1, NKCC2), and the Na-Cl cotransporter (NCC). Hydropathy analysis indicates a typical KCC topology with 12 transmembrane domains, a large extracellular loop between transmembrane domains 5 and 6 (unique to KCCs), and large NH(2) and COOH termini. KCC3 is predominantly expressed in kidney, heart, and brain, and is also expressed in skeletal muscle, placenta, lung, liver, and pancreas. KCC3 was localized to chromosome 15. KCC3 transiently expressed in human embryonic kidney (HEK)-293 cells fulfilled three criteria for increased expression of K-Cl cotransport: stimulation of cotransport by swelling, treatment with N-ethylmaleimide, or treatment with staurosporine.

Role of polyamine structure in inhibition of K+-Cl- cotransport in human red cell ghosts

1. K+-Cl- cotransport in human red cell ghosts is inhibited by divalent inorganic cations, soluble polycations and amphipathic organic cations. These findings suggest a common mechanism of inhibition, namely, binding of the cations to negative charges at the surface of a hydrophobic structure. 2. We have characterized the inhibitory capacity of a number of polyamines in order to obtain information about the nature of the charges with which they interact. Neomycin inhibited swelling-stimulated cotransport. The diquaternary amines dimethonium and decamethonium were relatively ineffective inhibitors. These compounds are thought to shield negative charges, but not bind to them. 3. Comparison of a homologous series of polyamines indicated that primary amines were better inhibitors than secondary amines, that inhibition increased with the charge of the polyamine, and that inhibition increased as the distance separating the amines increased. 4. The results indicate that the negative charges to which polycations bind are multiple and mobile. Since they must be associated with a hydrophobic environment, it is likely that they are negatively charged phospholipids located in the inner leaflet of the bilayer membrane. 5. Heating red cells or ghosts to 49 C denatures spectrin. Heating markedly increased K+ uptake in swollen ghosts but not in shrunken ghosts. The increase in uptake was reversed when swollen ghosts were shrunk even though denaturation of spectrin was not reversed. Polyamines, which inhibited swelling-activated K+ uptake in control ghosts, similarly inhibited the increased uptake in heated ghosts. 6. We speculate that spectrin, which is closely associated with the inner bilayer leaflet, shields negative charges in a volume-dependent manner and so regulates volume-sensitive K+ transport.

Inhibition of Na(+)-K(+)-2Cl(-) cotransport by mercury

Mercury alters the function of proteins by reacting with cysteinyl sulfhydryl (SH(-)) groups. The inorganic form (Hg(2+)) is toxic to epithelial tissues and interacts with various transport proteins including the Na(+) pump and Cl(-) channels. In this study, we determined whether the Na(+)-K(+)-Cl(-) cotransporter type 1 (NKCC1), a major ion pathway in secretory tissues, is also affected by mercurial substrates. To characterize the interaction, we measured the effect of Hg(2+) on ion transport by the secretory shark and human cotransporters expressed in HEK-293 cells. Our studies show that Hg(2+) inhibits Na(+)-K(+)-Cl(-) cotransport, with inhibitor constant (K(i)) values of 25 microM for the shark carrier (sNKCC1) and 43 microM for the human carrier. In further studies, we took advantage of species differences in Hg(2+) affinity to identify residues involved in the interaction. An analysis of human-shark chimeras and of an sNKCC1 mutant (Cys-697-->Leu) reveals that transmembrane domain 11 plays an essential role in Hg(2+) binding. We also show that modification of additional SH(-) groups by thiol-reacting compounds brings about inhibition and that the binding sites are not exposed on the extracellular face of the membrane.

Regulation of Na+-K+-2Cl- cotransport in turkey red cells: the role of oxygen tension and protein phosphorylation

1. Na+-K+-2Cl- cotransport (NKCC) was studied in turkey red cells using Na+ dependence or bumetanide sensitivity of 86Rb+ influx to monitor activity of the transporter. 2. Deoxygenation was the major physiological stimulus for NKCC activity: oxygen tensions (PO2) over the physiological range modulated the transporter, with a PO2 for half-maximal activation of about 41 mmHg (n = 3). In air, activity of NKCC was also stimulated by shrinkage and isoproteronol (isoprenaline, 5 microgr;M). By contrast, in deoxygenated cells, although the transporter activity was markedly elevated, it was no longer sensitive to volume or beta-adrenergic stimulation. 3. Calyculin A, a protein phosphatase inhibitor, stimulated cotransport with a lag of about 5 min. N-Ethylmaleimide (NEM) inhibited cotransport and also blocked the stimulatory effect of calyculin A if administered before calyculin A. Stimulation by calyculin A and deoxygenation were not additive. Staurosporine (2 microM) inhibited deoxygenated-stimulated K+ influxes, but not those stimulated by calyculin A. NEM added during calyculin A stimulation, i.e. during the 5 min lag, caused transport activity to be clamped at levels intermediate between maximal (calyculin A alone) and control. Cells treated with calyculin A alone or with calyculin A followed by NEM were no longer sensitive to volume, isoproteronol or PO2. 4. The results have characterized the interaction between deoxygenation and other stimuli of NKCC activity. They have also shown that it is possible to manipulate the transporter in a reciprocal way to that shown previously for K+-Cl- cotransport.

Swelling activation of transport pathways in erythrocytes: effects of Cl-, ionic strength, and volume changes

If swelling of a cell is induced by a decrease in external medium tonicity, the regulatory response is more complex than if swelling of similar magnitude is due to salt uptake. The present results provide an explanation. In fish erythrocytes, two distinct transport pathways were swelling activated: a channel of broad specificity and a K+-Cl- cotransporter. Each was activated by a specific signal: the channel by a decrease in intracellular ionic strength and the K+-Cl- cotransporter by cell enlargement. A decrease in ionic strength also affected K+-Cl- cotransport activity, but by acting as a negative modulator of the cotransport. Thus cells swollen by salt accumulation respond by activating exclusively the K+-Cl- cotransport, leading to a Cl--dependent K+ loss. By contrast, cells swollen by electrolyte dilution respond by activating both pathways, leading to a reduced loss of electrolytes and a large loss of taurine. Thus two swelling-sensitive pathways, differently regulated, would allow control of the ionic composition of a cell exposed to different volume perturbations.

K-Cl cotransporter expression in the human kidney

The K-Cl cotransporter protein KCC1 is a membrane transport protein that mediates the coupled, electroneutral transport of K and Cl across plasma membranes. The precise cell type(s) in the kidney that express the K-Cl cotransporter have remained unknown. The aim of the present investigation was to define the distribution of KCC1 mRNA in the human kidney. We used in situ hybridization with a nonradioactive digoxigenin-labeled riboprobe. We identified abundant KCC1 mRNA expression in the epithelial cells throughout the distal and proximal renal tubular epithelium. The transporter was also expressed in glomerular mesangial cells and endothelial cells of the renal vessels. These findings suggest that the K-Cl cotransporter may have an important role in transepithelial K and Cl reabsorption.

Cloning, characterization, and gene organization of K-Cl cotransporter from pig and human kidney and C. elegans

We isolated and characterized the cDNAs for the human, pig, and Caenorhabditis elegans K-Cl cotransporters. The pig and human homologs are 94% identical and contain 1,085 and 1,086 amino acids, respectively. The deduced protein of the C. elegans K-Cl cotransporter clone (CE-KCC1) contains 1,003 amino acids. The mammalian K-Cl cotransporters share approximately 45% similarity with CE-KCC1. Hydropathy analyses of the three clones indicate typical KCC topology patterns with 12 transmembrane segments, large extracellular loops between transmembrane domains 5 and 6 (unique to KCC), and large COOH-terminal domains. Human KCC1 is widely expressed among various tissues. This KCC1 gene spans 23 kb and is organized in 24 exons, whereas the CE-KCC1 gene spans 3.5 kb and contains 10 exons. Transiently and stably transfected human embryonic kidney cells (HEK-293) expressing the human, pig, and C. elegans K-Cl cotransporter fulfilled two (pig) or five (human and C. elegans) criteria for increased expression of the K-Cl cotransporter. The criteria employed were basal K-Cl cotransport; stimulation of cotransport by swelling, N-ethylmaleimide, staurosporine, and reduced cell Mg concentration; and secondary stimulation of Na-K-Cl cotransport.

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 a wide variety of epithelial and nonepithelial cells. The transport process mediated by Na-K-Cl cotransporters is characterized by electroneutrality (almost always with stoichiometry of 1Na:1K:2Cl) and inhibition by the "loop" diuretics bumetanide, benzmetanide, and furosemide. Presently, two distinct Na-K-Cl cotransporter isoforms have been identified by cDNA cloning and expression; genes encoding these two isoforms are located on different chromosomes and their gene products share approximately 60% amino acid sequence identity. The NKCC1 (CCC1, BSC2) isoform is present in a wide variety of tissues; most epithelia containing NKCC1 are secretory epithelia with the Na-K-Cl cotransporter localized to the basolateral membrane. By contrast, NKCC2 (CCC2, BSC1) is found only in the kidney, localized to the apical membrane of the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characterized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting, and volume depletion. The two Na-K-Cl cotransporter isoforms are also part of a superfamily of cation-chloride cotransporters, which includes electroneutral K-Cl and Na-Cl cotransporters. Na-K-Cl cotransporter activity is affected by a large variety of hormonal stimuli as well as by changes in cell volume; in many tissues this regulation (particularly of the NKCCI isoform) occurs through direct phosphorylation/dephosphorylation of the cotransport protein itself though the specific protein kinases involved remain unknown. An important regulator of cotransporter activity in secretory epithelia and other cells as well is intracellular [Cl] ([Cl]i), with a reduction in [Cl]i being the apparent means by which basolateral Na-K-Cl cotransport activity is increased and thus coordinated with that of stimulated apical Cl channels in actively secreting epithelia.

Fish red blood cells: characteristics and physiological role of the membrane ion transporters

Several membrane ion transporters playing a role in gas transport and exchanges, cell volume regulation and intracellular acid-base regulation have been identified in fish red blood cells (RBCs). This short review focuses on Na+/K+ATPase and its role in establishing the ionic gradients across the membrane, on the Cl-/HCO3- exchanger and its key role in respiration and possibly in inducing a chloride conductance, on the Na+/H+ exchanger and the recent advances on its molecular mechanisms of activation and regulation, on the different types of K-Cl cotransports, the different hypotheses and suggested models and their role in cell volume regulation. There is no evidence in the literature for ionic channels in fish RBCs. We present original data obtained with the patch-clamp technique that shows for the first time the existence of a DIDS-sensitive chloride anionic conductance measured in whole cell configuration and the presence of a stretch-activated nonselective cationic channel recorded in cell-attached and excised inside-out configuration. The part played by these ionic conductances is discussed in relation with their possible involvement in volume regulation.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

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.

L antigens of sheep red blood cell membranes and modulation of ion transport

Sheep are polymorphic with respect to the intracellular Na+ and K+ concentrations of their erythrocytes. Erythrocytes of sheep of the high-K+ (HK) phenotype have high K+ and low Na+ concentrations; erythrocytes from sheep of the allelic low-K+ (LK) phenotype have abnormally low K+ and high Na+ concentrations. The difference is due to differences in rates of cation transport: higher Na+-K+ pump flux in HK cells and higher K+-Cl- cotransport in LK cells. The HK/LK polymorphism is associated with a polymorphism of red blood cell antigens: the L antigen is only on LK cells, and HK cells have only the M antigen. There are two classes of L antigen that assort together: Lp, which is associated with Na+-K+ pumps, and Ll, which is associated with K+-Cl- cotransporters. There are functional consequences of these associations: anti-Lp antibody stimulates the pump and anti-Ll antibody inhibits cotransport. The use of these antibodies has permitted delineation of the roles of the antigens in modulating the function of the transporters. In this review, we summarize the evidence that these antigens are entities distinct from the pump. The Lp antigen reacts reversibly with the Na+-K+ pump; the antigen inhibits the pump, mainly by promoting nonspecific inhibition by intracellular K+. The antigen also modulates pump differentiation in immature cells. In contrast, the Ll antigen stimulates K+-Cl- cotransport. The evidence suggests that the two polymorphisms are controlled by a single genetic locus and that all of the distinct properties of ion transporters in LK cells are attributable to interactions with L antigens.

Protein tyrosine phosphorylation and the regulation of KCl cotransport in trout erythrocytes

Electroneutral salt transporters are activated and deactivated by changes to the phosphorylation status either of the transporter itself or of other, as yet unidentified, regulatory proteins. We have studied the effects of an inhibitor of protein tyrosine kinase (PTK), genistein, upon KCl cotransport in trout erythrocytes. We show that Cl-dependent K fluxes activated by physiological stimuli, i.e. oxygenation and beta-adrenergic agonists, are rapidly and completely blocked by genistein, whilst the inactive analogue of genistein, daidzein, had no effect. By contrast, the protein tyrosine phosphatase (PTP) inhibitor, vanadate (V), caused a slow but strong activation of an inactive cotransporter. This vanadate (V) activated flux was inhibited by genistein as well as by the serine/threonine phosphatase (PSP) inhibitor, calyculin A. However, genistein had no effect upon the activation of the cotransporter by the protein (serine/threonine) kinase (PSK) inhibitor, staurosporine, or by N-ethylmaleimide, which also appears to act by inhibiting a PSK. These results are consistent with a sequential scheme of at least two tyrosine phosphorylation events which lie upstream to the serine/threonine phosphorylation sites in the signal transduction pathway leading from stimulus to transporter activation. The regulation of the activity of KCl cotransporter appears to involve a complex series of phosphorylation reactions.

Role of protein kinases in regulating sheep erythrocyte K-Cl cotransport

K-Cl cotransport in sheep erythrocytes can be activated by treatment either with A-23187 and EDTA to reduce concentration of internal ionized Mg [Mg]i) to submicromolar levels, with staurosporine, a potent kinase inhibitor, or with N-ethylmaleimide (NEM). Activation by these maneuvers is prevented and reversed by genistein [inhibition constant (Ki) of 15 microM], which inhibits tyrosine kinases (TK). The related glycosidated compound genistin, which does not inhibit TK, does not inhibit transport, whereas another TK inhibitor, tyrphostin B46, inhibits both basal and stimulated transport (Ki of 28 microM). Cotransport activation by NEM is prevented and reversed by the phosphatase inhibitor, calyculin A, and activation by staurosporine occurs only if cells contain ATP. Increasing [Mg]i inhibits cotransport in the presence of calyculin A whether or not staurosporine is present as well. Our work suggests that genistein inhibits cotransport through a TK and that staurosporine and NEM activate cotransport, probably through inhibition of other kinases, causing stimulation through dephosphorylation of a protein (possibly the transporter itself) be a serine/threonine phosphatase. [Mg]i inhibits cotransport by activating a kinase (concentration for half-maximal activation of 10 microM) that phosphorylates this protein.

The effects of oxygenation upon the Cl-dependent K flux pathway in equine red cells

The effects of oxygen tension (PO2) upon the K influx pathways of equine red cells have been studied using 86Rb+ as congener for K. Equilibration of cells in 100% nitrogen led to a low and Cl-independent K flux. Change to an atmosphere of 100% air led to a rapid sixfold increase in K flux. The oxygen-activated flux was entirely Cl dependent and was maintained for up to 3 h. Oxygenation-evoked activation was dependent upon PO2 over the physiological range with little effect up to 70% saturation of haemoglobin with oxygen but significant effects between 70 and 100%. K flux at low PO2 was unaffected by acidification to pH 7 or by hypotonic cell swelling. By contrast, at high PO2 both manipulations caused a substantial increase in Cl-dependent K flux. N-Ethylmaleimide (NEM; 1 mM) caused a progressive activation of KCl cotransport in cells held under nitrogen. The protein phosphatase inhibitor, calyculin A (100 nM), applied during NEM-evoked activation caused a "clamping" of K influx at that level. This "clamped" activity was unaffected by subsequent oxygenation. We conclude that oxygenation exerts a primary control over cotransport activity and that acidification and cell swelling are secondary modulators. It appears that oxygenation-evoked activation of the Cl-dependent K flux involves a serine/threonine phosphorylation event. Regulating the PO2 of the solution before and during experiments is important in controlling the activity of the KCl cotransporter and cell volume.

Mechanism of swelling activation of K-Cl cotransport in inside-out vesicles of LK sheep erythrocyte membranes

Stimulation by swelling of K-Cl cotransport was studied in inside-out vesicles (IOVs) made from membranes of LK sheep erythrocytes. The purpose was to understand this stimulation in terms of the three-state process proposed for regulation of the cotransporter (P.B. Dunham, J. Klimczak, and P.J. Logue. J. Gen. Physiol. 101: 733-765, 1993). The first step in this process, A --> B, is rate limiting and controlled by transphosphorylation reactions. The second step, B --> C, is fast; its control is unknown. Predictions were that maximum velocity (Jmax) of cotransport increases with A --> B and concentration at one-half Jmax (K1/2) of K+ as a substrate decreases with B --> C. We tested the hypothesis that most transporters in IOVs are in the B state and that swelling activates cotransport in vesicles by the B --> C conversion. In accordance with this hypothesis, swelling should activate K+ influx with no discernable delay. It did. K1/2 for K+ should decrease with swelling and Jmax should not change. K1/2 decreased 10-fold, and Jmax did not change. Inhibitors of transphosphorylation, reactions of A --> B, should not affect K+ flux into IOVs, and they did not. The results support the hypothesis: swelling activation of K+ flux into IOVs corresponds to B --> C. A mechanical change in the membrane causes a specific change in the cotransporter: an increase in apparent affinity for K+.

Reconstitution of calyculin-inhibited K-Cl cotransport in dog erythrocyte ghosts by exogenous PP-1

Osmotic swelling of dog and other mammalian erythrocytes activates Cl-dependent K transport, K-Cl cotransport. This activation can be abolished by nanomolar concentrations of calyculin, a potent inhibitor of serine-threonine protein phosphatases. Therefore, K-Cl cotransport is probably activated by dephosphorylation by a type 1 and/or type 2A protein phosphatase (PP-1 and PP-2A, respectively). This was tested directly by incorporating exogenous protein phosphatases into resealed ghosts made from dog erythrocytes previously exposed to calyculin. K-Cl cotransport was nearly completely inhibited in the ghosts. Incorporation of PP-1 reconstituted K-Cl cotransport. Maximal reconstitution was up to 90% of the control flux in the ghosts and 0.1 U PP-1/ml lysate gave half-maximal reconstitution of cotransport. In contrast, PP-2A had no effect. This result with PP-1 provides direct evidence that K-Cl cotransport is activated by PP-1 in dog erythrocytes. Half-maximal activation of K-Cl cotransport required approximately 180 molecules of PP-1 per ghost.

KCl cotransport activation in human erythrocytes by high hydrostatic pressure

1. Pressure induced a 4- to 5-fold stimulation of the residual (i.e. oubain-bumetanide insensitive) 86Rb+ influx across the human red cell membrane. This enhancement showed a broad pHo dependence with a maximum stimulation around pHo 7. 2. At atmospheric pressure, the protein kinase inhibitors staurosporine and chelerythrine stimulated a normally silent component of 86Rb+ influx in a dose-dependent manner with a half-maximum stimulatory concentration at about 550 nM and 140 microM, respectively. The component stimulated by staurosporine was entirely Cl- dependent, but part of the chelerythrine effect was Cl- independent. 3. Staurosporine (3 microM), chelerythrine (200 microM) and N-ethylmaleimide (1 mM) stimulated further the increased residual 86Rb+ influx in cells at high pressure. 4. The serine/threonine protein phosphatase inhibitors okadaic acid, cantharidin and calyculin A inhibited the stimulatory pressure effect in a dose-dependent manner with half-maximum inhibitory concentrations of 70 nM, 2.5 microM and 3.3 nM, respectively. In contrast, deltamethrin, a specific protein phosphatase type 2B inhibitor, did not affect the stimulation by pressure, up to a concentration of 10 microM. 5. Decreasing the internal ionized magnesium concentration ([Mg2+]i) with A23187 and EDTA stimulated the increased residual 86Rb+ influx in cells at high pressure. On the other hand, increasing the [Mg2+]i nearly abolished the stimulatory pressure effect. 6. Decreasing the [Mg2+]i produced a marked change in the pHo dependence curve, with a linear increase of the 86Rb+ influx at higher pHo values. 7. We demonstrate that high pressure stimulates the normally silent component of 86Rb+ influx by modifying the phosphorylation/dephosphorylation ratio of the KCl cotransporter.

Resistance to osmotic lysis in BXD-31 mouse erythrocytes: association with upregulated K-Cl cotransport

The decreased osmotic fragility and reduced K+ content of BXD-31 mouse erythrocytes arise from variation at a single genetic locus. We compared ion transport in erythrocytes from BXD-31 mice and the parental strain, DBA/2J. The strains had similar rates for Na-K pump, Na/H exchange, Na-K-2Cl cotransport, Ca2+ activated K+ channel, or AE1-mediated SO4 transport. In contrast, K-Cl cotransport was twice as active in BXD-31 as in DBA/2J cells. Cl- dependent K+ efflux from BXD-31 cells displayed steep activation by acid pH (with maximal transport occurring at pH 6.75), whereas DBA/2J erythrocytes displayed a far less dramatic response to pH. Both strains displayed regulatory volume decrease in response to cell swelling. However, a 62% greater loss of cell K+ via K-Cl cotransport was observed in the BXD-31 strain. Furthermore the decreased osmotic fragility of BXD-31 red blood cells was normalized by treatment with nystatin to achieve normal cell K+ and water content. Thus upregulated K-Cl cotransport induces cell dehydration and K+ deficit in BXD-31 erythrocytes and causes their characteristic resistance to osmotic lysis.

Studies in red blood cell preservation 10. 51Cr recovery of red cells after liquid storage in a glycerol-containing additive solution

The purpose of the present study was to compare the 24-hour recovery of red blood cells stored for 9 weeks in a hypoosmolar additive solution containing 150 mM glycerol to cells stored in Adsol. Seven units of packed red cells were split into 2 aliquots. To one sample, 100 ml of the experimental additive solution (EAS 25) was added, and to the other, 50 ml of Adsol. At the end of the storage period the cells were labeled with 51Cr. A double chromium technique was used to make it possible to perform comparative autologous studies in the same donor. The 24-hour 51Cr recovery value for EAS 25 was 73.0 +/- (SD) 4.2% and for Adsol 60.9 +/- 7.1. At 9 weeks the adenosine triphosphate levels were not significantly better compared to Adsol but the other in vitro measurements were better. New approaches to the study of red cell preservation are suggested.

Sulfhydryl oxidation and activation of red cell K(+)-Cl- cotransport in the transgenic SAD mouse

The SAD mouse is characterized by the expression of human SAD hemoglobin (Hb), a super S Hb with a higher tendency to polymerize than HbS due to the presence of two additional mutations, Antilles beta 23Ile and D Punjab beta 121Glu. Monovalent cation transport was studied in erythrocytes from SAD-1 (Hb SAD = 19%) and beta-thal/SAD-1 (Hb SAD = 26%) mice. Erythrocytes containing Hb SAD exhibited dehydration, increased maximal rate of Na(+)-K+ pump, unchanged Rb+ flux via the Gardos channel, and increased K(+)-Cl- cotransport. K(+)-Cl- cotransport was defined as Cl(-)-dependent (substitution with sulfamate or methanesulfonate) okadaic acid-sensitive K+ efflux. Volume regulatory decrease via K(+)-Cl- cotransport was also increased in swollen SAD erythrocytes compared with controls. K(+)-Cl- cotransport was stimulated by staurosporine in all mouse strains, but the extent of stimulation was reduced in beta-thal/SAD-1 mice. Treatment with dithiothreitol reduced K(+)-Cl- cotransport activity in SAD-1 and beta-thal/SAD-1 mice to levels similar to that of control strains, indicating that reversible sulfhydryl oxidation contributes to the activated state of K(+)-Cl- cotransport in mouse erythrocytes that express transgenic human Hb SAD.

H2O2 activates red blood cell K-Cl cotransport via stimulation of a phosphatase

K-Cl cotransport is involved in volume regulation in a number of cell types. Cell swelling stimulates K-Cl cotransport, probably by inhibition of a volume-sensitive kinase. K-Cl cotransport can also be activated by oxidants and thiol reagents. We investigated the effect of H2O2 on K-Cl cotransport of LK sheep red blood cells in an attempt to identify the target of oxidants. H2O2 stimulated K-Cl cotransport. The stimulation was virtually abolished by subsequent incubation with calyculin, a protein phosphatase inhibitor. This suggests that H2O2 stimulates a calyculin-sensitive phosphatase and activates K-Cl cotransport by causing a decrease in phosphorylation of the transporter or a regulatory protein. The thiol reagent N-ethylmaleimide, which stimulates K-Cl cotransport, did not stimulate cotransport further in cells with cotransport activated by staurosporine but did stimulate cotransport further in cells with cotransport activated by H2O2. These results suggest that there are at least two distinct phosphorylation sites on the transporter or a regulator. The results also suggest that the phosphatase is associated with the membrane.

Glutathione removal reveals kinases as common targets for K-Cl cotransport stimulation in sheep erythrocytes

K-Cl cotransport is activated by swelling, lowering of cellular free Mg (Mgi), and thiol modification of erythrocytes. Direct actions by thiol reagents on the K-Cl cotransport complex were separated from indirect effects through nonoxidative changes in cellular glutathione (GSH). We used 1-chloro-2,4-dinitrobenzene (CDNB), which, conjugated to GSH, is extruded from the erythrocyte as a thioether. CDNB caused a small biphasic effect (inhibition and stimulation) on K-Cl cotransport and, at 1 mM, abolished its stimulation by N-ethylmaleimide (NEM), diazenedicarboxylic acid bis[N,N-dimethylamide], methyl methanethiosulfonate, and staurosporine, a kinase inhibitor, independent of the order of treatment. Hence, NEM and other activating-thiol reagents, and perhaps GSH removal itself, target unidentified kinases involved in activation of K-Cl cotransport. CDNB also abrogated K-Cl cotransport stimulation by Mgi depletion independent of the order of treatment, indicating inhibition at a second site nearer to the transporter. Furthermore, CDNB treatment elevated and rendered K-Cl cotransport insensitive to osmotic shrinkage, suggesting uncoupling from the regulator.

Effects of ionic strength on the regulation of Na/H exchange and K-Cl cotransport in dog red blood cells

Dog red cell membranes contain two distinct volume-sensitive transporters: swelling-activated K-Cl cotransport and shrinkage-activated Na/H exchange. Cells were prepared with intracellular salt concentration and weight percentage of cell water (%cw) varied independently by transient permeabilization of the cell membrane to cations. The dependence of transporter-mediated Na and K influxes upon %cw and upon extracellular salt concentration (c(ext)) was measured in cells so prepared. It was found that the critical value of %cw at which transporters are activated, called the set point, is similar for the two transporters, and that the set points for the two transporters decrease similarly with increasing extracellular salt concentration. These findings suggest a common mechanism of regulation of these two transporters. Cellular Na, K, and Cl concentrations were measured as functions of %cw and c(ext). Using these data together with data from the literature for other solute concentrations, empirical expressions were developed to describe the dependence of the intracellular concentrations of all significant small molecule electrolytes, and therefore the intracellular ionic strength, upon %cw and c(ext). A mechanistic model for the dependence of the set point of an individual transporter upon intracellular ionic strength is proposed. According to this model, the set point represents a critical extent of association between the transporter and a postulated soluble regulatory protein, called regulator. Model functions are presented for the calculation of the thermodynamic activity of regulator, and hence extent of regulator-transporter association, as a function of total intracellular protein concentration (or %cw) and ionic strength. The experimentally observed dependence of set point %cw on c(ext) are simulated using these functions and the empirical expressions described above, together with reasonable but not uniquely determined values of model parameters.

Cation transport in mouse erythrocytes: role of K(+)-Cl- cotransport in regulatory volume decrease

We investigated cation transport and cell volume regulation in erythrocytes of CD1 and C57/B6 mice. Swelling of cells from either strain stimulated K+ efflux that was insensitive to ouabain, bumetanide, and clotrimazole. Seventy-five percent of swelling-induced K+ efflux was Cl- dependent (inhibited by sulfamate or methanesulfonate, partially by NO3-, but not by SCN-) and was inhibited by okadaic acid (OA; 50% inhibitory concentration = 18 +/- 6 nM in CD1 and 10 +/- 4 nM in C57/B6). In both strains, K+ efflux into isotonic medium was stimulated by staurosporine or by N-ethylmaleimide, and the latter was partially blocked by pretreatment of cells with OA. When cells of either strain were incubated in hypotonic medium or preswollen isosmotically with nystatin, OA-sensitive regulatory volume decrease (RVD) and K+ loss were observed. RVD produced by hypotonic swelling was prevented by Cl- replacement with sulfamate or methanesulfonate. These properties suggest the presence in outbred and inbred mouse erythrocytes of RVD mediated by K(+)-Cl- cotransport.

Effects of urea on K-Cl cotransport in sheep red blood cells: evidence for two signals of swelling

The activation proceeds with a delay, like activation by swelling. Swelling of cells in urea activates K uptake further, but with no delay. Inactivation after removal of urea also proceeds without delay. With cotransport partially activated by reducing intracellular Mg concentration ([Mg]i) or with staurosporine, urea did not activate cotransport further. However, swelling activated cotransport further in these two types of cells. In terms of the three-state process for swelling-activation of K-Cl cotransport (P. B. Dunham, J. Klimczak, and P. J. Logue, J. Gen. Physiol. 101: 733-765, 1993), these results indicate that urea activates the first conversion, A-->B, and does so by inhibiting the reverse reaction promoted by a kinase, just as reducing [Mg]i does. Stimulation of cotransport by urea is nearly completely reversed by shrinkage, whereas activation by reducing [Mg]i is reversed only approximately 35%. Therefore urea inhibits the kinase indirectly, like swelling, by reducing macromolecular crowding of cytoplasmic proteins (A. P. Minton, G. C. Coleclasure, and J. C. Parker. Proc. Natl. Acad. Sci. USA 89: 10504-10506, 1992). Since swelling activates cotransport in two ways, one mimicked by urea and one not, there must be two signals of swelling, one a reduction of macromolecular crowding and the other probably a mechanical signal.

Stimulation of KCl co-transport in equine erythrocytes by hydrostatic pressure: effects of kinase/phosphatase inhibition

The effects of hydrostatic pressure on the KCl co-transporter of equine erythrocytes were studied to determine factors involved in its regulation. Pressure (0.1-40MPa) increased Cl-dependent K+ transport; in the presence of the putative kinase inhibitor N-ethylmaleimide (NEM) which stimulates the transporter, or the phosphatase inhibitor calyculin A, pressure had no significant effect. The sequential application of NEM and calyculin A clamped the transporter at about 30% of maximal flux compared to NEM alone; pressure also had no further effect. These results suggest that pressure acts on the phosphorylation status of the transporter or regulatory peptide, rather than on the ion flux per se. Since the activation of the KCl co-transporter by pressure occurs without an apparent change in cell volume these results have implications for any universal model for the regulation of KCl co-transport.