Extracellular ATP stimulates volume decrease in Necturus red blood cells

Finite Element Model of Oxygen Transport for the Design of Geometrically Complex Microfluidic Devices Used in Biological Studies

Red blood cells play a crucial role in the local regulation of oxygen supply in the microcirculation through the oxygen dependent release of ATP. Since red blood cells serve as an oxygen sensor for the circulatory system, the dynamics of ATP release determine the effectiveness of red blood cells to relate the oxygen levels to the vessels. Previous work has focused on the feasibility of developing a microfluidic system to measure the dynamics of ATP release. The objective was to determine if a steep oxygen gradient could be developed in the channel to cause a rapid decrease in hemoglobin oxygen saturation in order to measure the corresponding levels of ATP released from the red blood cells. In the present study, oxygen transport simulations were used to optimize the geometric design parameters for a similar system which is easier to fabricate. The system is composed of a microfluidic device stacked on top of a large, gas impermeable flow channel with a hole to allow gas exchange. The microfluidic device is fabricated using soft lithography in polydimethyl-siloxane, an oxygen permeable material. Our objective is twofold: (1) optimize the parameters of our system and (2) develop a method to assess the oxygen distribution in complex 3D microfluidic device geometries. 3D simulations of oxygen transport were performed to simulate oxygen distribution throughout the device. The simulations demonstrate that microfluidic device geometry plays a critical role in molecule exchange, for instance, changing the orientation of the short wide microfluidic channel results in a 97.17% increase in oxygen exchange. Since microfluidic devices have become a more prominent tool in biological studies, understanding the transport of oxygen and other biological molecules in microfluidic devices is critical for maintaining a physiologically relevant environment. We have also demonstrated a method to assess oxygen levels in geometrically complex microfluidic devices.

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

P2X and P2Y receptor signaling in red blood cells

Purinergic signaling involves the activation of cell surface P1 and P2 receptors by extracellular nucleosides and nucleotides such as adenosine and adenosine triphosphate (ATP), respectively. P2 receptors comprise P2X and P2Y receptors, and have well-established roles in leukocyte and platelet biology. Emerging evidence indicates important roles for these receptors in red blood cells. P2 receptor activation stimulates a number of signaling pathways in progenitor red blood cells resulting in microparticle release, reactive oxygen species formation, and apoptosis. Likewise, activation of P2 receptors in mature red blood cells stimulates signaling pathways mediating volume regulation, eicosanoid release, phosphatidylserine exposure, hemolysis, impaired ATP release, and susceptibility or resistance to infection. This review summarizes the distribution of P2 receptors in red blood cells, and outlines the functions of P2 receptor signaling in these cells and its implications in red blood cell biology.

Overcoming matrix effects in the chemiluminescence determination of extracellular adenosine triphosphate in erythrocyte suspensions

As erythrocyte-derived extracellular adenosine triphosphate (ATP) gains recognition as a key vasodilator, its accurate determination is critical. Erythrocytes' high hemoglobin content can act as an inner filter when measuring ATP concentrations by chemiluminescence. We evaluated two approaches to correct for this matrix effect: addition of cell-free hemoglobin to the ATP standards and standard addition of ATP to erythrocyte suspensions. In addition, we reduced sample hematocrit to minimize the absorbance. We conclude that extracellular ATP should be determined in erythrocyte suspensions at 0.06 to 0.004% hematocrit. This gives robust signals without matrix effects and requires only microliters of blood.

Stretch of atrial myocytes stimulates recruitment of macrophages via ATP released through gap-junction channels

Atrial inflammation is critical to atrial fibrillation initiation and progression. Although left atrial dilatation is a risk factor for atrial fibrillation, the mechanism linking atrial dilatation and inflammation is unclear. We evaluated the mechanisms underlying infiltration of macrophages induced by stretch of atrial myocytes. Murine macrophages were co-cultured with HL-1 murine atrial myocyte-derived cells. Mechanical stretch applied to atrial myocytes induced transwell macrophage migration. The gap junction-channel blocker carbenoxolone and the non-specific ATP-signaling modifiers apyrase and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate inhibited the enhanced migration. Mechanical stretch of atrial myocytes induced transient increase in extracellular ATP concentration, which was inhibited by carbenoxolone. siRNA knockdown of pannexin-2 inhibited ATP release and macrophage migration. Mice underwent transverse aortic constriction or sham procedure. Transverse aortic constriction procedure induced macrophage infiltration. Daily carbenoxolone administration significantly inhibited macrophage infiltration in the atrium. Thus, mechanical stretch of atrial myocytes induces macrophage migration by ATP released through gap-junction channels, at least in part, in vitro. Administering a gap junction family-channel blocker inhibited this inflammatory change in vivo.

Plasmalemmal VDAC controversies and maxi-anion channel puzzle

The maxi-anion channel has been observed in many cell types from the very beginning of the patch-clamp era. The channel is highly conductive for chloride and thus can modulate the resting membrane potential and play a role in fluid secretion/absorption and cell volume regulation. A wide nanoscopic pore of the maxi-anion channel permits passage of excitatory amino acids and nucleotides. The channel-mediated release of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression and physiological/pathophysiological significance, the molecular identity of the maxi-anion channel is still obscure. VDAC is primarily a mitochondrial protein; however several groups detected it on the cellular surface. VDAC in lipid bilayers reproduced the most important biophysical properties of the maxi-anion channel, such as a wide nano-sized pore, closure in response to moderately high voltages, ATP-block and ATP-permeability. However, these similarities turned out to be superficial, and the hypothesis of plasmalemmal VDAC as the maxi-anion channel did not withstand the test by genetic manipulations of VDAC protein expression. VDAC on the cellular surface could also function as a ferricyanide reductase or a receptor for plasminogen kringle 5 and for neuroactive steroids. These ideas, as well as the very presence of VDAC on plasmalemma, remain to be scrutinized by genetic manipulations of the VDAC protein expression. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Red blood cell dynamics: from cell deformation to ATP release

The mechanisms of red blood cell (RBC) deformation under both static and dynamic, i.e., flow, conditions have been studied extensively since the mid 1960s. Deformation-induced biochemical reactions and possible signaling in RBCs, however, were proposed only fifteen years ago. Therefore, the fundamental relationship between RBC deformation and cellular signaling dynamics i.e., mechanotransduction, remains incompletely understood. Quantitative understanding of the mechanotransductive pathways in RBCs requires integrative studies of physical models of RBC deformation and cellular biochemical reactions. In this article we review the physical models of RBC deformation, spanning from continuum membrane mechanics to cellular skeleton dynamics under both static and flow conditions, and elaborate the mechanistic links involved in deformation-induced ATP release.

Extracellular nucleotides and apyrases regulate stomatal aperture in Arabidopsis

This study investigates the role of extracellular nucleotides and apyrase enzymes in regulating stomatal aperture. Prior data indicate that the expression of two apyrases in Arabidopsis (Arabidopsis thaliana), APY1 and APY2, is strongly correlated with cell growth and secretory activity. Both are expressed strongly in guard cell protoplasts, as determined by reverse transcription-polymerase chain reaction and immunoblot analyses. Promoter activity assays for APY1 and APY2 show that expression of both apyrases correlates with conditions that favor stomatal opening. Correspondingly, immunoblot data indicate that APY expression in guard cell protoplasts rises quickly when these cells are moved from darkness into light. Both short-term inhibition of ectoapyrase activity by polyclonal antibodies and long-term suppression of APY1 and APY2 transcript levels significantly disrupt normal stomatal behavior in light. Stomatal aperture shows a biphasic response to applied adenosine 5'-[γ-thio]triphosphate (ATPγS) or adenosine 5'-[β-thio] diphosphate, with lower concentrations inducing stomatal opening and higher concentrations inducing closure. Equivalent concentrations of adenosine 5'-O-thiomonophosphate have no effect on aperture. Two mammalian purinoceptor inhibitors block ATPγS- and adenosine 5'-[β-thio] diphosphate-induced opening and closing and also partially block the ability of abscisic acid to induce stomatal closure and of light to induce stomatal opening. Treatment of epidermal peels with ATPγS induces increased levels of nitric oxide and reactive oxygen species, and genetically suppressing the synthesis of these agents blocks the effects of nucleotides on stomatal aperture. A luciferase assay indicates that treatments that induce either the closing or opening of stomates also induce the release of ATP from guard cells. These data favor the novel conclusion that ectoapyrases and extracellular nucleotides play key roles in regulating stomatal functions.

Regulatory volume decrease and P receptor signaling in fish cells: mechanisms, physiology, and modeling approaches

For animal cell plasma membranes, the permeability of water is much higher than that of ions and other solutes, and exposure to hyposmotic conditions almost invariably causes rapid water influx and cell swelling. In this situation, cells deploy regulatory mechanisms to preserve membrane integrity and avoid lysis. The phenomenon of regulatory volume decrease, the partial or full restoration of cell volume following cell swelling, is well-studied in mammals, with uncountable investigations yielding details on the signaling network and the effector mechanisms involved in the process. In comparison, cells from other vertebrates and from invertebrates received little attention, despite of the fact that e.g. fish cells could present rewarding model systems given the diversity in ecology and lifestyle of this animal group that may be reflected by an equal diversity of physiological adaptive mechanisms, including those related to cell volume regulation. In this review, we therefore present an overview on the most relevant aspects known on hypotonic volume regulation presently known in fish, summarizing transporters and signaling pathways described so far, and then focus on an aspect we have particularly studied over the past years using fish cell models, i.e. the role of extracellular nucleotides in mediating cell volume recovery of swollen cells. We, furthermore, present diverse modeling approaches developed on the basis of data derived from studies with fish and other models and discuss their potential use for gaining insight into the theoretical framework of volume regulation.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Activation of the MAPKs ERK1/2 by cell swelling in turbot hepatocytes

BACKGROUND INFORMATION

Activation of MAPKs (mitogen-activated protein kinases), in particular ERK1/2 (extracellular-signal-regulated kinase 1/2), has been reported to take place in a large variety of cell types after hypo-osmotic cell swelling. Depending on cell type, ERK1/2 phosphorylation can then serve or not the RVD (regulatory volume decrease) process. The present study investigates ERK1/2 activation after aniso-osmotic stimulations in turbot hepatocytes and the potential link between phosphorylation of these proteins and RVD.

RESULTS

In turbot hepatocytes, Western-blot analysis shows that a hypo-osmotic shock from 320 to 240 mOsm kg(-1) induced a rapid increase in ERK1/2 phosphorylation, whereas a hyper-osmotic shock from 320 to 400 mOsm kg(-1) induced no significant change in the phosphorylation of these proteins. The hypo-osmotic-induced ERK1/2 phosphorylation was significantly prevented when hypo-osmotic shock was performed in the presence of the specific MEK (MAPK/ERK kinase) inhibitor PD98059 (100 microM). In these conditions, the RVD process was not altered, suggesting that ERK1/2 did not participate in this process in turbot hepatocytes. Moreover, the hypo-osmotic-induced activation of ERK1/2 was significantly prevented by breakdown of extracellular ATP by apyrase (10 units ml(-1)), by inhibition of purinergic P2 receptors by suramin (100 microM) or by calcium depletion using EGTA (1 mM) and thapsigargin (1 microM).

CONCLUSIONS

In turbot hepatocytes, hypo-osmotic swelling but not hyper-osmotic shrinkage induced the activation of ERK1/2. However, these proteins do not seem to be involved in the RVD process. Their hypo-osmotic-induced activation is partially due to cascades of signalling events triggered by the binding of released ATP on purinergic P2 receptors and requires the presence of calcium.

A volume regulatory response can be triggered by nucleosides in human erythrocytes, a perfect osmometer no longer

Human erythrocytes have been regarded as perfect osmometers, which swell or shrink as dictated by their osmotic environment. In contrast, in most other cells, swelling elicits a regulatory volume decrease (RVD) modulated by the activation of purinic and pyrimidinic receptors (P receptors). For human erythrocytes this modulation has not been tested, and we thus investigated whether P receptor activation can induce RVD in these cells. Further, because ectonucleotidases may scavenge ATP or ADP or act as a source for extracellular adenosine and therefore modulate P receptor activation and RVD, we also determined their activity in intact erythrocytes. We found relatively low ectoATPase but significant ectoADPase and ectoAMPase activities. When erythrocytes were exposed to hypotonic medium alone, they swelled as expected for an osmometric response and showed no RVD. Activation of P2 receptors by exogenous ATP or ADP did not trigger RVD, whereas P1 agonists adenosine and adenosine-5'-N-ethylcarboxamide induced significant RVD. The effect of adenosine-5'-N-ethylcarboxamide was dose-dependent (maximal RVD of 27%; apparent K((1/2)) of 1.6 +/- 1.7 microM). The RVD induced by adenosine was blocked 80% with the non-selective P1 antagonist 8-(p-sulfophenyl theophylline) or the P1-A(2B) inhibitor MRS1754, but not by inhibitors of P1 subtypes A(1), A(2A), and A(3). In addition, forskolin (an inducer of intracellular cAMP formation) could mimic the effect of adenosine, supporting the idea of P1-A(2B) receptor activation. In conclusion, we report a novel P1-A(2B) receptor-mediated RVD activation in mature human erythrocytes and thus indicate that these long held perfect osmometers are not so perfect after all.

Dynamics of shear-induced ATP release from red blood cells

Adenosine triphosphate (ATP) is a regulatory molecule for many cell functions, both for intracellular and, perhaps less well known, extracellular functions. An important example of the latter involves red blood cells (RBCs), which help regulate blood pressure by releasing ATP as a vasodilatory signaling molecule in response to the increased shear stress inside arterial constrictions. Although shear-induced ATP release has been observed widely and is believed to be triggered by deformation of the cell membrane, the underlying mechanosensing mechanism inside RBCs is still controversial. Here, we use an in vitro microfluidic approach to investigate the dynamics of shear-induced ATP release from human RBCs with millisecond resolution. We demonstrate that there is a sizable delay time between the onset of increased shear stress and the release of ATP. This response time decreases with shear stress, but surprisingly does not depend significantly on membrane rigidity. Furthermore, we show that even though the RBCs deform significantly in short constrictions (duration of increased stress <3 ms), no measurable ATP is released. This critical timescale is commensurate with a characteristic membrane relaxation time determined from observations of the cell deformation by using high-speed video. Taken together our results suggest a model wherein the retraction of the spectrin-actin cytoskeleton network triggers the mechanosensitive ATP release and a shear-dependent membrane viscosity controls the rate of release.

Purinergic activation of anion conductance and osmolyte efflux in cultured rat hippocampal neurons

The majority of mammalian cells demonstrate regulatory volume decrease (RVD) following swelling caused by hyposmotic exposure. A critical signal initiating RVD is activation of nucleotide receptors by ATP. Elevated extracellular ATP in response to cytotoxic cell swelling during pathological conditions also may initiate loss of taurine and other intracellular osmolytes via anion channels. This study characterizes neuronal ATP-activated anion current and explores its role in net loss of amino acid osmolytes. To isolate anion currents, we used CsCl as the major electrolyte in patch electrode and bath solutions and blocked residual cation currents with NiCl(2) and tetraethylammonium. Anion currents were activated by extracellular ATP with a K(m) of 70 microM and increased over fourfold during several minutes of ATP exposure, reaching a maximum after 9.0 min (SD 4.2). The currents were blocked by inhibitors of nucleotide receptors and volume-regulated anion channels (VRAC). Currents showed outward rectification and inactivation at highly depolarizing membrane potentials, characteristics of swelling-activated anion currents. P2X agonists failed to activate the anion current, and an inhibitor of P2X receptors did not block the effect of ATP. Furthermore, current activation was observed with extracellular ADP and 2-(methylthio)adenosine 5'-diphosphate, a P2Y(1) receptor-specific agonist. Much less current activation was observed with extracellular UTP, suggesting the response is mediated predominantly by P2Y(1) receptors. ATP caused a dose-dependent loss of taurine and alanine that could be blocked by inhibitors of VRAC. ATP did not inhibit the taurine uptake transporter. Thus extracellular ATP triggers a loss of intracellular organic osmolytes via activation of anion channels. This mechanism may facilitate neuronal volume homeostasis during cytotoxic edema.

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.

Calcium signaling in lizard red blood cells

The ion calcium is a ubiquitous second messenger, present in all eukaryotic cells. It modulates a vast number of cellular events, such as cell division and differentiation, fertilization, cell volume, decodification of external stimuli. To process this variety of information, the cells display a number of calcium pools, which are capable of mobilization for signaling purposes. Here we review the calcium signaling on lizards red blood cells, an interesting model that has been receiving an increasing notice recently. These cells possess a complex machinery to regulate calcium, and display calcium responses to extracellular agonists. Interestingly, the pattern of calcium handling and response are divergent in different lizard families, which enforces the morphological data to their phylogenetic classification, and suggest the radiation of different calcium signaling models in lizards evolution.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

ATP-conducting maxi-anion channel: a new player in stress-sensory transduction

The regulated release of ATP is a fundamental process in cell-to-cell signaling. The electrogenic translocation of ATP via an anion channel has been suggested as one possible mechanism of the release. In this review, we survey possible candidate channels for this pathway. The maxi-anion channel characterized by an exceedingly large unitary conductance has been a stray channel with regard to its function. A newly discovered property, its ATP conductivity and its activation in response to stress signals, indicates that this channel has a central role in stress-sensory transduction for cell volume regulation and tubuloglomerular feedback.

Cell swelling-induced ATP release is tightly dependent on intracellular calcium elevations

Mechanical stresses release ATP from a variety of cells by a poorly defined mechanism(s). Using custom-designed flow-through chambers, we investigated the kinetics of cell swelling-induced ATP secretion, cell volume and intracellular calcium changes in epithelial A549 and 16HBE14o- cells, and NIH/3T3 fibroblasts. Fifty per cent hypotonic shock triggered transient ATP release from cell confluent monolayers, which consistently peaked at around 1 min 45 s for A549 and NIH/3T3, and at 3 min for 16HBE14o- cells, then declined to baseline within the next 15 min. Whereas the release time course had a similar pattern for the three cell types, the peak rates differed significantly (294 +/- 67, 70 +/- 22 and 17 +/- 2.8 pmol min(-1) (10(6) cells)(-1), for A549, 16HBE14o- and NIH/3T3, respectively). The concomitant volume changes of substrate-attached cells were analysed by a 3-dimensional cell shape reconstruction method based on images acquired from two perpendicular directions. The three cell types swelled at a similar rate, reaching maximal expansion in 1 min 45 s, but differed in the duration of the volume plateau and regulatory volume decrease (RVD). These experiments revealed that ATP release does not correlate with either cell volume expansion and the expected activation of stretch-sensitive channels, or with the activation of volume-sensitive, 5-nitro-2-(3-phenylpropylamino) benzoic acid-inhibitable anion channels during RVD. By contrast, ATP release was tightly synchronized, in all three cell types, with cytosolic calcium elevations. Furthermore, loading A549 cells with the calcium chelator BAPTA significantly diminished ATP release (71% inhibition of the peak rate), while the calcium ionophore ionomycin triggered ATP release in the absence of cell swelling. Lowering the temperature to 10 degrees C almost completely abolished A549 cell swelling-induced ATP release (95% inhibition of the peak rate). These results strongly suggest that calcium-dependent exocytosis plays a major role in mechanosensitive ATP release.

Mechanisms of the ATP potentiation of hyposmotic taurine release in Swiss 3T3 fibroblasts

Reducing osmolarity by 35% increased (3)H-taurine efflux from Swiss 3T3 fibroblasts from 0.5% to a peak of 5.7%. The presence of ATP (10-100 microM; EC(50) 1.5 microM) increased taurine efflux up to 10%, and decreased the set point for hyposmotically stimulated taurine release (HTR). ATP potentiation was mimicked by UTP, reduced by addition of suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and unaffected by ADP, beta,gamma-methylene-ATP (beta,gamma-ATP) or 2-methylthio-ATP (Me-ATP), suggesting its mediation by purinergic P2Y(2) and P2Y(4) metabotropic receptors. Under isosmotic conditions ATP increased the cytosolic [Ca(2+)] ([Ca(2+)](i)) markedly, but did not increase taurine release. HTR was independent of external Ca(2+) but was reduced (by 56-59%) by BAPTA-AM, thapsigargin-induced depletion of intracellular Ca(2+) stores, or phospholipase C (PLC) inhibition. Blockade of calmodulin (CaM) or calmodulin kinase II (CaMKII) reduced HTR by 54% and 76%, respectively. The ATP-mediated potentiation was prevented fully by all these treatments. HTR was reduced by 30-50% by blockers of protein tyrosine kinases (AG18), phosphoinositide 3-kinase (PI3K) (wortmannin), p21rho (toxin B), p21rho-kinase (Y27632) and the stress-activated kinase p38 (PD169316). ATP-mediated potentiation was reduced similarly by these blockers. Simultaneous inhibition of PI3K and CaMKII abolished HTR. Altogether, these results suggest a modulatory effect of ATP, probably exerted by a potentiation of the Ca(2+)-dependent fraction of HTR. This fraction has as signalling elements a PLC-dependent [Ca(2+)](i) increase, resulting from Ca(2+) released from thapsigargin-sensitive internal stores, followed by activation of CaM/CaMKII reactions. The Ca(2+)/ATP effect operates only when the Ca(2+)-independent, tyrosine kinase-mediated pathway is already activated. Suggested elements of cross-talk between the two pathways are PLC, PI3K and CaMKII.

Extracellular ATP increases cation fluxes in human erythrocytes by activation of the P2X7 receptor

Canine erythrocytes are known to undergo a reversible increase in cation permeability when incubated with extracellular ATP. We have examined the expression and function of P2X receptors on human erythrocytes using confocal microscopy and a panel of anti-P2X(1-7) antibodies and have measured monovalent cation fluxes in the presence of various nucleotide agonists. Human erythrocytes expressed P2X7 receptors on all cells examined from eight of eight subjects, as well as P2X2 at a far lower staining intensity in six of eight subjects. ATP stimulated the efflux of 86Rb+ (K+) from human erythrocytes in a dose-dependent fashion with an EC50 of approximately 95 microM. Other nucleotides also induced an efflux of 86Rb+ from erythrocytes with an order of agonist potency of 2'- and 3'-O(4-benzoylbenzoyl) ATP (BzATP) > ATP > 2-methylthio-ATP (2MeSATP) > adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), whereas ADP or UTP had no effect. ATP-induced efflux of 86Rb+ from erythrocytes was inhibited by extracellular Na+ and oxidized ATP, as well as by KN-62, an antagonist specific for the human P2X7 receptor. When erythrocytes were incubated in isotonic KCl medium, the addition of ATP stimulated an 86Rb+ influx approximately equal in magnitude to ATP-stimulated 86Rb+ efflux from the same cells. BzATP also stimulated the influx of 22Na+ into erythrocytes incubated in isotonic NaCl medium. Both ATP-induced efflux and influx of 86Rb+ and 22Na+ were impaired in erythrocytes from subjects who had inherited loss-of-function polymorphisms in the P2X7 receptor. These results suggest that the reversible permeabilization of erythrocytes by extracellular ATP is mediated by the P2X7 receptor.

Volume-regulated anion conductance in cultured rat cerebral astrocytes requires calmodulin activity

We examined the calmodulin dependence of anion channel activation during hypo-osmotic swelling in rat cerebral astrocytes. Control cells bathed in iso-osmotic (290 mOsm) phosphate-buffered saline (PBS) and recorded using a patch electrode containing 140 mM KCl increased membrane conductance threefold over basal levels after 12 min in hypo-osmotic (200 mOsm) PBS. Cells injected with monoclonal anticalmodulin antibody demonstrated no increase in membrane conductance during a subsequent exposure to hypo-osmotic PBS. In contrast, cells iontophoretically injected with monoclonal antiglial fibrillary acidic protein antibody or with anticalmodulin antibody absorbed with an excess of free calmodulin demonstrated an increase in conductance during hypo-osmotic exposure similar to that of control cells. Conductance in iso-osmotic conditions was unchanged by antibody injection. Similar results were obtained when using patch electrode and bath solutions containing chloride as the only cell permeant ion, indicating a calmodulin-dependent anion current is activated with this degree of hypo-osmotic treatment. Western blots confirmed the specificity of the anticalmodulin and antiglial fibrillary acidic protein antibodies used in this study for proteins of 17 and 51 kD, respectively. In addition, in vitro studies demonstrated inhibition of the calmodulin-dependent activation of phosphodiesterase by the anticalmodulin antibody. Thus, binding of this antibody to calmodulin causes functional inhibition of calmodulin activity. No change in the intensity or cellular distribution of calmodulin immunostaining was observed during 30 min of hypo-osmotic exposure. However, increased immunostaining for activated calmodulin kinase IIalpha was observed after 10 min of hypo-osmotic exposure, suggesting initiation of calmodulin-dependent processes by cell swelling. The data indicate calmodulin activity is critical for activation of volume-regulated anion channels in rat cerebral astrocytes.

Effects of extracellular nucleotides and their hydrolysis products on regulatory volume decrease of trout hepatocytes

In trout hepatocytes, hypotonic swelling is followed by a compensatory shrinkage called regulatory volume decrease (RVD). It has been postulated that extracellular ATP and other nucleotides may interact with type 2 receptors (P(2)) to modulate this response. In addition, specific ectoenzymes hydrolyze ATP sequentially down to adenosine, which may bind to type 1 receptors (P(1)) and also influence RVD. Accordingly, in this study, we assessed the role of extracellular nucleoside 5'-tri- and diphosphates and of adenosine on RVD of trout hepatocytes. The extent of RVD after 40 min of maximum swelling was denoted as RVD(40), whereas the initial rate of RVD was called v(RVD). In the presence of hypotonic medium (60% of isotonic), hepatocytes swelled 1.6 times followed by v(RVD) of 1.7 min(-1) and RVD(40) of 60.2%. ATP, UTP, UDP, or ATPgammaS (P(2) agonists; 5 microM) increased v(RVD) 1.5-2 times, whereas no changes were observed in the values of RVD(40). Addition of 100 microM suramin or cibacron blue (P(2) antagonists) to the hypotonic medium produced no effect on v(RVD) but a 53-58% inhibition of RVD(40). Incubation of hepatocytes in the presence of either 5 microM [gamma-(32)P]ATP or [alpha-(32)P]ATP induced the extracellular release of [gamma-(32)P]P(i) (0.21 nmol.10(-6) cells(-1).min(-1)) and [alpha-(32)P]P(i) ( approximately 8 x 10(-3) nmol.10(-6) cells(-1).min(-1)), suggesting the presence of ectoenzymes capable of fully dephosphorylating ATP. Concerning the effect of P(1) activation on RVD, 5 microM adenosine, both in the presence and absence of 100 microM S-(4-nitrobenzil)-6-tioinosine (a blocker of adenosine uptake), decreased RVD(40) by 37-44%, whereas 8-phenyl theophylline, a P(1) antagonist, increased RVD(40) by 15%. Overall, results indicate that ATP, UTP, and UDP, acting via P(2), are important factors promoting RVD of trout hepatocytes, whereas adenosine binding to P(1) inhibits this process.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Increased vesicle recycling in response to osmotic cell swelling. Cause and consequence of hypotonicity-provoked ATP release

Osmotic swelling of Intestine 407 cells leads to an immediate increase in cell surface membrane area as determined using the fluorescent membrane dye FM 1-43. In addition, as measured by tetramethylrhodamine isothiocyanate (TRITC)-dextran uptake, a robust (>100-fold) increase in the rate of endocytosis was observed, starting after a discrete lag time of 2-3 min and lasting for approximately 10-15 min. The hypotonicity-induced increase in membrane surface area, like the cell swelling-induced release of ATP (Van der Wijk, T., De Jonge, H. R., and Tilly, B. C. (1999) Biochem. J. 343, 579-586), was diminished after 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester loading or cytochalasin B treatment. Uptake of TRITC-dextrans, however, was not affected. Treatment of the cells with the vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptor-specific protease Clostridium botulinum toxin F not only nearly eliminated the hypotonicity-induced increase in membrane surface area but also strongly diminished the release of ATP, indicating the involvement of regulated exocytosis. Both the ATP hydrolase apyrase and the MEK inhibitor PD098059 diminished the osmotic swelling-induced increase in membrane surface area as well as the subsequent uptake of TRITC-dextrans. Taken together, the results indicate that extracellular ATP is required for the hypotonicity-induced vesicle recycling and suggest that a positive feedback loop, involving purinergic activation of the Erk-1/2 pathway, may contribute to the release of ATP from hypo-osmotically stimulated cells.

Hypotonic treatment evokes biphasic ATP release across the basolateral membrane of cultured renal epithelia (A6)

In renal A6 epithelia, an acute hypotonic shock evokes a transient increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) through a mechanism that is sensitive to the P2 receptor antagonist suramin, applied to the basolateral border only. This finding has been further characterized by examining ATP release across the basolateral membrane with luciferin-luciferase (LL) luminescence. Polarized epithelial monolayers, cultured on permeable supports were mounted in an Ussing-type chamber. We developed a LL pulse protocol to determine the rate of ATP release (R(ATP)) in the basolateral compartment. Therefore, the perfusion at the basolateral border was repetitively interrupted during brief periods (90 s) to measure R(ATP) as the slope of the initial rise in ATP content detected by LL luminescence. Under isosmotic conditions, 1 microl of A6 cells released ATP at a rate of 66 +/- 8 fmol min(-1). A sudden reduction of the basolateral osmolality from 260 to 140 mosmol (kg H(2)O)(-1) elevated R(ATP) rapidly to a peak value of 1.89 +/- 0.11 pmol min(-1) (R(ATP)(peak)) followed by a plateau phase reaching 0.51 +/- 0.07 pmol min(-1) (R(ATP)(plat)). Both R(ATP)(peak) and R(ATP)(plat) values increased with the degree of dilution. The magnitude of R(ATP)(plat) remained constant as long as the hyposmolality was maintained. Similarly, a steady ATP release of 0.78 +/- 0.08 pmol min(-1) was recorded after gradual dilution of the basolateral osmolality to 140 mosmol (kg H(2)O)(-1). This R(ATP) value, induced in the absence of cell swelling, is comparable to R(ATP)(plat). Therefore, the steady ATP release is unrelated to membrane stretching, but possibly caused by the reduction of intracellular ionic strength during cell volume regulation. Independent determinations of dose-response curves for peak [Ca(2+)](i) increase in response to exogenous ATP and basolateral hyposmolality demonstrated that the exogenous ATP concentration, required to mimic the osmotic reduction, was linearly correlated with R(ATP)(peak). The link between the ATP release and the fast [Ca(2+)](i) transient was also demonstrated by the depression of both phenomena by Cl(-) removal from the basolateral perfusate. The data are consistent with the notion that during hypotonicity, basolateral ATP release activates purinergic receptors, which underlies the suramin-sensitive rise of [Ca(2+)](i) during the hyposmotic shock.

Volume-dependent ATP-conductive large-conductance anion channel as a pathway for swelling-induced ATP release

In mouse mammary C127i cells, during whole-cell clamp, osmotic cell swelling activated an anion channel current, when the phloretin-sensitive, volume-activated outwardly rectifying Cl(-) channel was eliminated. This current exhibited time-dependent inactivation at positive and negative voltages greater than around +/-25 mV. The whole-cell current was selective for anions and sensitive to Gd(3)+. In on-cell patches, single-channel events appeared with a lag period of approximately 15 min after a hypotonic challenge. Under isotonic conditions, cell-attached patches were silent, but patch excision led to activation of currents that consisted of multiple large-conductance unitary steps. The current displayed voltage- and time-dependent inactivation similar to that of whole-cell current. Voltage-dependent activation profile was bell-shaped with the maximum open probability at -20 to 0 mV. The channel in inside-out patches had the unitary conductance of approximately 400 pS, a linear current-voltage relationship, and anion selectivity. The outward (but not inward) single-channel conductance was suppressed by extracellular ATP with an IC(50) of 12.3 mM and an electric distance (delta) of 0.47, whereas the inward (but not outward) conductance was inhibited by intracellular ATP with an IC(50) of 12.9 mM and delta of 0.40. Despite the open channel block by ATP, the channel was ATP-conductive with P(ATP)/P(Cl) of 0.09. The single-channel activity was sensitive to Gd(3)+, SITS, and NPPB, but insensitive to phloretin, niflumic acid, and glibenclamide. The same pharmacological pattern was found in swelling-induced ATP release. Thus, it is concluded that the volume- and voltage-dependent ATP-conductive large-conductance anion channel serves as a conductive pathway for the swelling-induced ATP release in C127i cells.

ATP transduces signals from ASGM1, a glycolipid that functions as a bacterial receptor

The flagella of the Gram-negative bacterium Pseudomonas aeruginosa serve not only for motility but also to bind bacteria to the host cell glycolipid asialoGM1 (ASGM1) through the protein flagellin. This interaction triggers defensive responses in host cells. How this response occurs is unclear because ASGM1 lacks transmembrane and cytoplasmic domains and there is little information about the downstream effectors that connect ASGM1 ligation to the initiation of host defense responses. Here, we show that ASGM1 ligation promotes ATP release from the host cell, followed by autocrine activation of a nucleotide receptor. This response links ASGM1 to cytoplasmic signaling molecules and results in activation of phospholipase C, Ca(2+) mobilization, phosphorylation of a mitogen-activated protein kinase (Erk 1/2), and activation of mucin transcription. These results indicate that bacterial interaction with host cells can trigger autocrine nucleotide signaling and suggest that agents affecting nucleotide receptors may modulate host responses to bacteria.

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.

Receptor-mediated facilitation of cell volume regulation by swelling-induced ATP release in human epithelial cells

Osmotic swelling induces the release of intracellular ATP in a number of cell types. In the immediate vicinity of the cell surface, released ATP has been shown to reach a concentration high enough to stimulate P2-purinergic receptors in a human epithelial cell line, Intestine 407. The role of released ATP in the regulatory volume decrease (RVD) after cell swelling was thus studied in Intestine 407 cells. The RVD was suppressed by an ATP hydrolyzing enzyme, apyrase, or by a purinergic receptor antagonist, suramin. Extracellular application of ATP accelerated the RVD rate in a concentration-dependent manner. An increase in the cytosolic free-Ca(2+) concentration was induced by a hypotonic challenge, and the swelling-induced Ca(2+) response was partially suppressed by apyrase or suramin. A rise in cytosolic Ca(2+) was also induced by extracellular application of ATP or UTP, but not ADP, 2-methylthio-ATP or alpha,beta-methylene ATP. The ATP-induced Ca(2+) response was blocked by suramin. Therefore, it is concluded that RVD is facilitated by ATP, which is released upon cell swelling, by augmenting intracellular Ca(2+) rise via the stimulation of purinergic (P2Y(2)) receptors in the human epithelial cell.