Role of LTD4 in the regulatory volume decrease response in Ehrlich ascites tumor cells

CysLT1 receptor antagonists pranlukast and zafirlukast inhibit LRRC8-mediated volume regulated anion channels independently of the receptor

Volume-regulated anion channels (VRACs) encoded by the leucine-rich repeat containing 8 (LRRC8) gene family play critical roles in myriad cellular processes and might represent druggable targets. The dearth of pharmacological compounds available for studying VRAC physiology led us to perform a high-throughput screen of 1,184 of US Food and Drug Administration-approved drugs for novel VRAC modulators. We discovered the cysteinyl leukotriene receptor 1 (CysLT1R) antagonist, pranlukast, as a novel inhibitor of endogenous VRAC expressed in human embryonic kidney 293 (HEK293) cells. Pranlukast inhibits VRAC voltage-independently, reversibly, and dose-dependently with a maximal efficacy of only ~50%. The CysLT1R pathway has been implicated in activation of VRAC in other cell types, prompting us to test whether pranlukast requires the CysLT1R for inhibition of VRAC. Quantitative PCR analysis demonstrated that CYSLTR1 mRNA is virtually undetectable in HEK293 cells. Furthermore, the CysLT1R agonist leukotriene D4 had no effect on VRAC activity and failed to stimulate G_q-coupled receptor signaling. Heterologous expression of the CysLT1R reconstituted LTD4-CysLT1R- G_q-calcium signaling in HEK293 cells but had no effect on VRAC inhibition by pranlukast. Finally, we show the CysLT1R antagonist zafirlukast inhibits VRAC with an IC₅₀ of ~17 µM and does so with full efficacy. Our data suggest that both pranlukast and zafirlukast are likely direct channel inhibitors that work independently of the CysLT1R. This study provides clarifying insights into the putative role of leukotriene signaling in modulation of VRAC and identifies two new chemical scaffolds that can be used for development of more potent and specific VRAC inhibitors.

Water Homeostasis and Cell Volume Maintenance and Regulation

From early unicellular organisms that formed in salty water environments to complex organisms that live on land away from water, cells have had to protect a homeostatic internal environment favorable to the biochemical reactions necessary for life. In this chapter, we will outline what steps were necessary to conserve the water within our cells and how mechanisms have evolved to maintain and regulate our cellular and organismal volume. We will first examine whole body water homeostasis and the relationship between kidney function, regulation of blood pressure, and blood filtration in the process of producing urine. We will then discuss how the composition of the lipid-rich bilayer affects its permeability to water and salts, and how the cell uses this differential to drive physiological and biochemical cellular functions. The capacity to maintain cell volume is vital to epithelial transport, neurotransmission, cell cycle, apoptosis, and cell migration. Finally, we will wrap up the chapter by discussing in some detail specific channels, cotransporters, and exchangers that have evolved to facilitate the movement of cations and anions otherwise unable to cross the lipid-rich bilayer and that are involved in maintaining or regulating cell volume.

Activation of ATP secretion via volume-regulated anion channels by sphingosine-1-phosphate in RAW macrophages

We report the activation of outwardly rectifying anion currents by sphingosine-1-phosphate (S1P) in the murine macrophage cell line RAW 264.7. The S1P-induced current is mainly carried by anions, because the reversal potential of the current was shifted by replacement of extracellular Cl(-) by glutamate(-) but not when extracellular Na(+) was substituted by Tris(+). The inhibition of the current by hypertonic extracellular or hypotonic intracellular solution as well as the inhibitory effects of NPPB, tamoxifen, and glibenclamide indicates that the anion current is mediated by volume-regulated anion channels (VRAC). The S1P effect was blocked by intracellular GDPβS and W123, which points to signaling via the S1P receptor 1 (S1PR1) and G proteins. As cytochalasin D diminished the action of S1P, we conclude that the actin cytoskeleton is involved in the stimulation of VRAC. S1P and hypotonic extracellular solution induced secretion of ATP from the macrophages, which in both cases was blocked in a similar way by typical VRAC blockers. We suppose that the S1P-induced ATP secretion in macrophages via activation of VRAC constitutes a functional link between sphingolipid and purinergic signaling in essential processes such as inflammation and migration of leukocytes as well as phagocytosis and the killing of intracellular bacteria.

Ion channels involved in cell volume regulation: effects on migration, proliferation, and programmed cell death in non adherent EAT cells and adherent ELA cells

This mini review outlines studies of cell volume regulation in two closely related mammalian cell lines: nonadherent Ehrlich ascites tumour cells (EATC) and adherent Ehrlich Lettre ascites (ELA) cells. Focus is on the regulatory volume decrease (RVD) that occurs after cell swelling, the volume regulatory ion channels involved, and the mechanisms (cellular signalling pathways) that regulate these channels. Finally, I shall also briefly review current investigations in these two cell lines that focuses on how changes in cell volume can regulate cell functions such as cell migration, proliferation, and programmed cell death.

Cell volume homeostatic mechanisms: effectors and signalling pathways

Cell volume homeostasis and its fine-tuning to the specific physiological context at any given moment are processes fundamental to normal cell function. The understanding of cell volume regulation owes much to August Krogh, yet has advanced greatly over the last decades. In this review, we outline the historical context of studies of cell volume regulation, focusing on the lineage started by Krogh, Bodil Schmidt-Nielsen, Hans-Henrik Ussing, and their students. The early work was focused on understanding the functional behaviour, kinetics and thermodynamics of the volume-regulatory ion transport mechanisms. Later work addressed the mechanisms through which cellular signalling pathways regulate the volume regulatory effectors or flux pathways. These studies were facilitated by the molecular identification of most of the relevant channels and transporters, and more recently also by the increased understanding of their structures. Finally, much current research in the field focuses on the most up- and downstream components of these paths: how cells sense changes in cell volume, and how cell volume changes in turn regulate cell function under physiological and pathophysiological conditions.

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.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

On the role of G-protein coupled receptors in cell volume regulation

Cell volume is determined genetically for each cell lineage, but it is not a static feature of the cell. Intracellular volume is continuously challenged by metabolic reactions, uptake of nutrients, intracellular displacement of molecules and organelles and generation of ionic gradients. Moreover, recent evidence raises the intriguing possibility that changes in cell volume act as signals for basic cell functions such as proliferation, migration, secretion and apoptosis. Cells adapt to volume increase by a complex, dynamic process resulting from the concerted action of volume sensing mechanisms and intricate signaling chains, directed to initiate the multiple adaptations demanded by a change in cell volume, among others adhesion reactions, membrane and cytoskeleton remodeling, and activation of the osmolyte pathways leading to reestablish the water balance between extracellular/intracellular or intracellular/intracellular compartments. In multicellular organisms, a continuous interaction with the external milieu is fundamental for the dynamics of the cell. It is in this sense that the recent surge of interest about the influence on cell volume control by the most extended family of signaling elements, the G proteins, acquires particular importance. As here reviewed, a large variety of G-protein coupled receptors (GPCRs) are involved in this interplay with cell volume regulatory mechanisms, which amplifies and diversifies the volume-elicited signaling chains, providing a variety of routes towards the multiple effectors related to cell volume changes.

Permissive role of calcium on regulatory volume decrease in freshly isolated mouse cholangiocytes

Calcium (Ca2+) pathways are important in cell volume regulation in many cells, but its role in volume regulatory processes in cholangiocytes is unclear. Thus, we have investigated the role of Ca2+ in regulatory volume decrease (RVD) in cholangiocytes using freshly isolated bile duct cell clusters (BDCCs) from normal mouse. No significant increase in [Ca2+]i was observed during RVD, while ionomycin and ATP showed significant increases. Confocal imaging also showed no significant changes in the levels or distributions of intracellular Ca2+ during RVD. Cell volume study by quantitative videomicroscopy indicated that removal and chelation of extracellular Ca2+ by ethylene glycol-bis (beta-aminoethyl ether)-N,N,N-tetraacetic acid (EGTA) or administration of nifedipine did not affect RVD but verapamil significantly inhibited the RVD. Moreover, Ca2+ agonists or inhibitors of Ca2+ release from intracellular stores had no significant effect on RVD. However, 1,2-bis (2-aminophenoxy) ethane-N,N,N'N'-tetraacetic acid-AM (BAPTA-AM) showed significant decreases in [Ca2+]i and significantly inhibited RVD, which was reversed with coadministration of valinomycin, suggesting that BAPTA-AM-induced inhibition is due to potassium conductance or other cellular processes requiring permissive [Ca2+](i. These findings indicate that an increase in [Ca2+]i or extracellular Ca2+ is not required for RVD but Ca2+ has a permissive role in RVD of mouse cholangiocytes.

Swelling-activated ion channels: functional regulation in cell-swelling, proliferation and apoptosis

Cell volume regulation is one of the most fundamental homeostatic mechanisms and essential for normal cellular function. At the same time, however, many physiological mechanisms are associated with regulatory changes in cell size meaning that the set point for cell volume regulation is under physiological control. Thus, cell volume is under a tight and dynamic control and abnormal cell volume regulation will ultimately lead to severe cellular dysfunction, including alterations in cell proliferation and cell death. This review describes the different swelling-activated ion channels that participate as key players in the maintenance of normal steady-state cell volume, with particular emphasis on the intracellular signalling pathways responsible for their regulation during hypotonic stress, cell proliferation and apoptosis.

Volume regulation following hyposmotic shock in isolated turbot (Scophthalmus maximus) hepatocytes

Regulatory volume decrease (RVD) following hyposmotic stimulation was studied in isolated turbot, Scophthalmus maximus, hepatocytes. Exposed to a reduced osmolality (from 320 to 240 mosm kg(-1)), cells first swelled and then exhibited a RVD. Volume regulation was significantly inhibited in presence of NPPB, 9-AC, acetazolamide, DIDS and barium. Taken together, these results could suggest that RVD operated via separate K+ and Cl- channels and probably Cl-/HCO3(-) exchanger in turbot hepatocytes. The K+/Cl- cotransporter could also be involved as furosemide and DIOA strongly inhibited the process whereas NEM, a K+/Cl- cotransporter activator, added under isosmotic conditions, led to cell shrinkage. RVD in turbot hepatocytes appeared also to depend on proteins p38 MAP kinase and tyrosine kinase but not on proteins ERK 1/2. Arachidonic acid and leukotrienes could also be involved since inhibition of synthesis of both these compounds by quinacrine and NDGA, respectively, inhibited the volume regulation. Likewise, Ca2+ has been proved to be an essential messenger as RVD was prevented in absence of Ca2+. Finally, this work provides bases for novel studies on cell volume regulation in marine teleosteans.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Chloride channels and hepatocellular function: prospects for molecular identification

Hepatocytes possess chloride channels at the plasma membrane and in multiple intracellular compartments. These channels are required for cell volume regulation and acidification of intracellular organelles. Evidence also supports a role of chloride channels in modulation of apoptosis and cell growth. Swelling- and Ca(2+)-activated chloride channels have been identified in hepatocyte plasma membranes, and chloride channels have been observed in the membranes of lysosomes, endosomes, Golgi, endoplasmic reticulum, mitochondria, and the nucleus. This review summarizes the functions of these channels and discusses the specific channel molecules they may represent. Chloride channel molecules shown to be expressed in hepatocytes include members of the ClC channel family (ClC-2, ClC-3, ClC-5, and ClC-7), members of the newly identified CLIC family of intracellular chloride channels (CLIC-1 and CLIC-4), the mitochondrial voltage-dependent anion channel, and a newly identified intracellular channel, MCLC (Mid-1 related chloride channel). Current understanding does not include a molecular identification of most of the observed channel functions, but details of the molecular properties of these channel molecules should allow future identification and further understanding of chloride channel function in hepatocytes.

Intracellular signalling involved in activation of the volume-sensitive K+ current in Ehrlich ascites tumour cells

The cell swelling-activated K+ channel in Ehrlich ascites tumour cells has a conductance of 5 pS estimated from noise analysis of the volume-sensitive whole-cell K+ current (I(K,vol)). I(K,vol) exhibits Goldman-Hodgkin-Katz type behaviour and is insensitive to clotrimazole, apamin and charybdotoxin (ChTX), but inhibited by clofilium. Its small conductance, lack of intrinsic voltage-dependence and peculiar pharmacological profile are similar to properties described for the two-pore domain background K+ TASK channels. Neither Ca2+ nor ATP work as initiators in the activation of I(K,vol). In contrast, several investigations in Ehrlich cells suggest an important role for leukotriene D4 (LTD4) in the activation of I(K,vol). Under isotonic conditions, LTD4 activates Ca2+-dependent, ChTX-sensitive K+ channels as well as Ca2+-independent. ChTX-insensitive K+ channels. The LTD4-activated, ChTX-insensitive K+ current exhibits a current-voltage relation, pharmacological profile and single channel conductance similar to that of I(K,vol), indicating that LTD4 is the signalling molecule responsible for activation of the volume-sensitive K+ channels in Ehrlich cells. Hypotonic swelling of Ehrlich cells results in translocation of the 85-kDa cytosolic (c) PLA2alpha to the nucleus where it is activated. This activation leads to an increase in arachidonic acid release followed by an increased release of leukotrienes, and is essential in cell swelling-induced activation of I(K,vol) and of the organic osmolyte channels.

Receptor-mediated control of regulatory volume decrease (RVD) and apoptotic volume decrease (AVD)

A fundamental property of animal cells is the ability to regulate their own cell volume. Even under hypotonic stress imposed by either decreased extracellular or increased intracellular osmolarity, the cells can re-adjust their volume after transient osmotic swelling by a mechanism known as regulatory volume decrease (RVD). In most cell types, RVD is accomplished mainly by KCl efflux induced by parallel activation of K+ and Cl- channels. We have studied the molecular mechanism of RVD in a human epithelial cell line (Intestine 407). Osmotic swelling results in a significant increase in the cytosolic Ca2+ concentration and thereby activates intermediate-conductance Ca2+-dependent K+ (IK) channels. Osmotic swelling also induces ATP release from the cells to the extracellular compartment. Released ATP stimulates purinergic ATP (P2Y2) receptors, thereby inducing phospholipase C-mediated Ca2+ mobilization. Thus, RVD is facilitated by stimulation of P2Y2 receptors due to augmentation of IK channels. In contrast, stimulation of another G protein-coupled Ca2+-sensing receptor (CaR) enhances the activity of volume-sensitive outwardly rectifying Cl- channels, thereby facilitating RVD. Therefore, it is possible that Ca2+ efflux stimulated by swelling-induced and P2Y2 receptor-mediated intracellular Ca2+ mobilization activates the CaR, thereby secondarily upregulating the volume-regulatory Cl- conductance. On the other hand, the initial process towards apoptotic cell death is coupled to normotonic cell shrinkage, called apoptotic volume decrease (AVD). Stimulation of death receptors, such as TNF receptor and Fas, induces AVD and thereafter biochemical apoptotic events in human lymphoid (U937), human epithelial (HeLa), mouse neuroblastoma x rat glioma hybrid (NG108-15) and rat phaeochromocytoma (PC12) cells. In those cells exhibiting AVD, facilitation of RVD is always observed. Both AVD induction and RVD facilitation as well as succeeding apoptotic events can be abolished by prior treatment with a blocker of volume-regulatory K+ or Cl- channels, suggesting that AVD is caused by normotonic activation of ion channels that are normally involved in RVD under hypotonic conditions. Therefore, it is likely that G protein-coupled receptors involved in RVD regulation and death receptors triggering AVD may share common downstream signals which should give us key clues to the detailed mechanisms of volume regulation and survival of animal cells. In this Topical Review, we look at the physiological ionic mechanisms of cell volume regulation and cell death-associated volume changes from the facet of receptor-mediated cellular processes.

Infertile spermatozoa of c-ros tyrosine kinase receptor knockout mice show flagellar angulation and maturational defects in cell volume regulatory mechanisms

Homozygous c-ros knockout male mice that lack prepubertal differentiation of the epididymal initial segment are healthy but sterile, despite normal sperm production and mating. Detailed computerized analysis of the motility of spermatozoa maturing in the epididymis revealed only minor defects. However, the majority of motile mature sperm released from the cauda epididymidis showed various extents of flagellar angulation that could not be corrected by raising extracellular osmolality. Measurement of the osmolality of cauda epididymal fluid showed no difference from the wild type. Studies in wild-type mice indicated a maturational change in the ability of motile sperm to maintain straight flagella during incubation, but angulation was induced in cauda sperm by the volume-sensitive ion channel blockers quinine, 5-nitro-2-(3-phenylpropylamino)-benzoic acid and BaCl(2), or by exposure to hypotonic media. Flagellar angulation, induced in the wild type or intrinsic to the knockout, was relieved upon demembranation by Triton X-100, confirming that it was a cell swelling phenomenon. A lack of response of immature wild-type sperm and mature knockout sperm to the channel blockers suggests that there is normally a development of the volume regulatory mechanisms upon maturation that is defective in sperm from the knockout animal. The resultant flagellar angulation may account for the reduction in sperm numbers in the oviduct of mated females and the failure to fertilize in vivo.

Increases in [Ca2+]i and changes in intracellular pH during chemical anoxia in mouse neocortical neurons in primary culture

The effect of chemical anoxia (azide) in the presence of glucose on the free intracellular Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) in mouse neocortical neurons was investigated using Fura-2 and BCECF. Anoxia induced a reversible increase in [Ca2+]i which was significantly inhibited in nominally Ca2+-free medium. A change in pHo (8.2 or 6.6), or addition of NMDA and non-NMDA receptor antagonists (D-AP5 and CNQX) in combination, significantly reduced the increase in [Ca2+]i, pointing to a protective effect of extracellular alkalosis or acidosis, and involvement of excitatory amino acids. An initial anoxia-induced acidification was observed under all experimental conditions. In the control situation, this acidification was followed by a recovery/alkalinization of pHi in about 50% of the cells, a few cells showed no recovery, and some showed further acidification. EIPA, an inhibitor of Na+/H+ exchangers, prevented alkalinization, pointing towards anoxia-induced activation of a Na+/H+ exchanger. In a nominally Ca2+-free medium, the initial acidification was followed by a significant alkalinization. At pHo 8.2, the alkalinization was significantly increased, while at pHo 6.2, the initial acidification was followed by further acidification in about 50% of the cells, and by no further change in the remaining cells.

Mechanical stress induces release of ATP from Ehrlich ascites tumor cells

The supernatant from a suspension of Ehrlich cells exposed to centrifugation at 700xg for 45 s induced a transient increase in the intracellular concentration of free, cytosolic Ca2+, [Ca2+]i, as well as activation of an outwardly rectifying whole-cell current when added to a suspension of non-stimulated cells. These effects were inhibited by suramin, a non-specific P2 receptor antagonist, and mimicked by ATP. Reversed phase HPLC analysis revealed that the supernatant from Ehrlich cells exposed to centrifugation contained 2. 6+/-0.2 microM ATP, and that the mechanical stress-induced release of ATP was inhibited by glibenclamide and verapamil, non-specific inhibitors of the cystic fibrosis transmembrane conductance regulator and P-glycoprotein, respectively. After trypan blue staining, less than 0.5% of the cells were unable to extrude the dye. Addition of extracellular ATP induced a suramin-sensitive, transient, concentration-dependent increase in [Ca2+]i, activation of an outwardly rectifying whole-cell current and a hyperpolarization of the plasma membrane. The ATP-induced hyperpolarization of the plasma membrane was strongly inhibited in the presence of charybdotoxin (ChTX), an inhibitor of several Ca2+-activated K+ channels, suggesting that stimulation of P2 receptors in Ehrlich cells evokes a Ca2+-activated K+ current. The relative potencies of several nucleotides (ATP, UTP, ADP, 2-MeSATP, alpha,beta-MeATP, bzATP) in eliciting an increase in [Ca2+]i, as well as the effect of repetitive addition of nucleotides were investigated. The results lead us to conclude that mechanical stimulation of Ehrlich cells leads to release of ATP, which in turn stimulates both P2Y1 and P2Y2 receptors, resulting in Ca2+ influx as well as release and activation of an outwardly rectifying whole-cell current.

Thrombin-, bradykinin-, and arachidonic acid-induced Ca2+ signaling in Ehrlich ascites tumor cells

Stimulation of single Ehrlich ascites tumor cells with agonists (bradykinin, thrombin) and with arachidonic acid (AA) induces increases in the free intracellular Ca2+ concentration ([Ca2+]i) in the presence and absence of extracellular Ca2+, measured using the Ca2+-sensitive probe fura 2. Sequential stimulation with two agonists elicits sequential increases in [Ca2+]i, unlike addition of the same agonist twice. Bradykinin and thrombin have additive effects on [Ca2+]i in Ca2+-free medium. The phosphoinositidase C inhibitor U-73122 inhibits the agonist-induced increases in [Ca2+]i, whereas ryanodine has no effect. Pretreatment of cells in Ca2+-free medium with thapsigargin abolishes the bradykinin-induced increase in [Ca2+]i but not the response to thrombin. The AA-induced response is not inhibited by U-73122 and cannot be mimicked by the inactive structural analog trifluoromethylarachidonyl ketone. Pretreatment of the cells with 50 microM AA (but not with 10 microM AA) abolishes the agonist-induced increase in [Ca2+]i. Thus bradykinin, thrombin, and AA induce increases in [Ca2+]i in Ehrlich cells due to Ca2+ entry and release from intracellular stores. Thrombin causes release of Ca2+ from an intracellular store that is insensitive to bradykinin and is not depleted by thapsigargin but is depleted by AA.

P2 receptor-mediated signal transduction in Ehrlich ascites tumor cells

The mechanisms, by which the P2 receptor agonists adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) evoke an increase in the free cytosolic calcium concentration ([Ca2+]i) and in intracellular pH (pHi), have been investigated in Ehrlich ascites tumor cells. The increase in [Ca2+]i evoked by ATP or UTP is abolished after depletion of intracellular Ca2+ stores with thapsigargin in Ca2+-free medium, and is inhibited by U73122, an inhibitor of phospholipase C (PLC), indicating that the increase in [Ca2+]i is primarily due to release from intracellular, Ins(1,4,5)P3-sensitive Ca2+ stores. ATP also activates a capacitative Ca2+-entry pathway. ATP as well as UTP evokes a biphasic change in pHi, consisting of an initial acidification followed by alkalinization. Suramin and 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid (DIDS) inhibit the biphasic change in pHi, apparently by acting as antagonists at P2 receptors. The alkalinization evoked by the P2 receptor agonists is found to be due to activation of a 5'-(N-ethyl-N-isopropyl)amiloride (EIPA)-sensitive Na+/H+ exchanger. ATP and UTP elicit rapid cell shrinkage, presumably due to activation of Ca2+ sensitive K+ and Cl- efflux pathways. Preventing cell shrinkage, either by incubating the cells at high extracellular K+ concentration, or by adding the K+-channel blocker, charybdotoxin, does not affect the increase in [Ca2+]i, but abolishes the activation of the Na+/H+ exchanger, indicating that activation of the Na+/H+ exchanger is secondary to the Ca2+-induced cell shrinkage.

Na+-K+-2Cl- cotransport in Ehrlich cells: regulation by protein phosphatases and kinases

To identify protein kinases (PK) and phosphatases (PP) involved in regulation of the Na+-K+-2Cl- cotransporter in Ehrlich cells, the effect of various PK and PP inhibitors was examined. The PP-1, PP-2A, and PP-3 inhibitor calyculin A (Cal-A) was a potent activator of Na+-K+-2Cl- cotransport (EC50 = 35 nM). Activation by Cal-A was rapid (<1 min) but transient. Inactivation is probably due to a 10% cell swelling and/or the concurrent increase in intracellular Cl- concentration. Cell shrinkage also activates the Na+-K+-2Cl- cotransport system. Combining cell shrinkage with Cal-A treatment prolonged the cotransport activation compared with stimulation with Cal-A alone, suggesting PK stimulation by cell shrinkage. Shrinkage-induced cotransport activation was pH and Ca2+/calmodulin dependent. Inhibition of myosin light chain kinase by ML-7 and ML-9 or of PKA by H-89 and KT-5720 inhibited cotransport activity induced by Cal-A and by cell shrinkage, with IC50 values similar to reported inhibition constants of the respective kinases in vitro. Cell shrinkage increased the ML-7-sensitive cotransport activity, whereas the H-89-sensitive activity was unchanged, suggesting that myosin light chain kinase is a modulator of the Na+-K+-2Cl- cotransport activity during regulatory volume increase.

Swelling-activated potassium currents of Ehrlich ascites tumour cells

The K+ and Cl- currents activated by Ca2+-ionophore treatment or by hypotonic cell swelling have been studied in Ehrlich ascites tumour cells by the patch-clamp technique. A charybdotoxin-inhibitable K+ current was activated by increasing intracellular Ca2+ concentration. In contrast, the K+ current activated by cell swelling was insensitive to charybdotoxin as well as to apamin, suggesting that channels different from those sensitive to Ca2+ are responsible for regulatory volume adjustments in these cells. The magnitude of the K+ and Cl- currents activated by hypotonic challenge was markedly temperature-dependent, possibly reflecting the temperature-dependence of enzymes involved in the intracellular signalling of cell volume regulation.

Swelling-induced arachidonic acid release via the 85-kDa cPLA2 in human neuroblastoma cells

Arachidonic acid or its metabolites have been implicated in the regulatory volume decrease (RVD) response after hypotonic cell swelling in some mammalian cells. The present study investigated the role of arachidonic acid (AA) during RVD in the human neuroblastoma cell line CHP-100. During the first nine minutes of hypo-osmotic exposure the rate of 3H-arachidonic acid (3H-AA) release increased to 250 +/- 19% (mean +/- SE, n = 22) as compared with cells under iso-osmotic conditions. This release was significantly inhibited after preincubation with AACOCF3, an inhibitor of the 85-kDa cytosolic phospholipase A2 (cPLA2). This indicates that a PLA2, most likely the 85-kDa cPLA2 is activated during cell swelling. In contrast, preincubation with U73122, an inhibitor of phospholipase C, did not affect the swelling-induced release of 3H-AA. Swelling-activated efflux of 36Cl and 3H-taurine were inhibited after preincubation with AACOCF3. Thus the swelling-induced activation of cPLA2 may be essential for stimulation of both 36Cl and 3H-taurine efflux during RVD. As the above observation could result from a direct effect of AA or its metabolite leukotriene D4 (LTD4), the effects of these agents were investigated on swelling-induced 36Cl and 3H-taurine effluxes. In the presence of high concentrations of extracellular AA, the swelling-induced efflux of 36Cl and 3H-taurine were inhibited significantly. In contrast, addition of exogenous LTD4 had no significant effect on the swelling-activated 36Cl efflux. Furthermore, exogenous AA increased cytosolic calcium levels as measured in single cells loaded with the calcium sensitive dye Fura-2. On the basis of these results we propose that cell swelling activates phospholipase A2 and that this activation via an increased production of AA or some AA metabolite(s) other than LTD4 is essential for RVD.

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.

Volume expansion-sensing outward-rectifier Cl- channel: fresh start to the molecular identity and volume sensor

The maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbation is essential for the normal function and survival of animal cells. Osmotically swollen cells restore their volume, exhibiting a regulatory volume decrease by releasing intracellular K+, Cl-, organic solutes, and obligated water. In many cell types, the volume regulatory effluxes of Cl- and some organic osmolytes are known to be induced by swelling-induced activation of anion channels that are characterized by their moderate outward rectification, cytosolic ATP dependency, and intermediate unitary conductance (10-100 pS). Recently, simultaneous measurements of cell size by light microscopy and whole cell Cl- current have shown that the Cl- current density is proportionally increased with an increase in the outer surface area, which is mainly achieved through unfolding of membrane invaginations by volume expansion. Thus this anion channel can somehow sense volume expansion and can be called the volume expansion-sensing outwardly rectifying (VSOR) anion channel. Its molecular identity and activation mechanism are yet to be elucidated. Three cloned proteins, ClC-2, P-glycoprotein, and pIcln, have been proposed as candidates for the VSOR anion channel. The unitary conductance, voltage dependency, anion selectivity, pH dependency, and pharmacology of the VSOR anion channel are distinct from the ClC-2 Cl- channel, which is also known to be sensitive to volume changes. Recent patch-clamp studies in combination with molecular biological techniques have shown that P-glycoprotein is not itself the channel protein but is a regulator of its volume sensitivity. Although there is still debate about another candidate protein, pIcln, the most recent study has suggested that this is likely to be a regulator of some other distinct Cl- channel. Identification of the VSOR anion channel protein per se, its volume-sensing mechanism, and its accessory/regulatory proteins at the molecular level is currently a subject of utmost physiological importance.