Evidence for Gd(3+) inhibition of membrane ATP permeability and purinergic signaling

Evidence for Extracellular ATP as a Stress Signal in a Single-Celled Organism

ATP is omnipresent in biology and acts as an extracellular signaling molecule in mammals. Information regarding the signaling function of extracellular ATP in single-celled eukaryotes is lacking. Here, we explore the role of extracellular ATP in cell volume recovery during osmotic swelling in the amoeba Dictyostelium. Release of micromolar ATP could be detected during cell swelling and regulatory cell volume decrease (RVD) phases during hypotonic challenge. Scavenging ATP with apyrase caused profound cell swelling and loss of RVD. Apyrase-induced swelling could be rescued by 100 μM βγ-imidoATP. N-Ethylmalemide (NEM), an inhibitor of vesicular exocytosis, caused heightened cell swelling, loss of RVD, and inhibition of ATP release. Amoebas with impaired contractile vacuole (CV) fusion (drainin knockout [KO] cells) displayed increased swelling but intact ATP release. One hundred micromolar Gd(3+) caused cell swelling while blocking any recovery by βγ-imidoATP. ATP release was 4-fold higher in the presence of Gd(3+). Cell swelling was associated with an increase in intracellular nitric oxide (NO), with NO-scavenging agents causing cell swelling. Swelling-induced NO production was inhibited by both apyrase and Gd(3+), while NO donors rescued apyrase- and Gd(3+)-induced swelling. These data suggest extracellular ATP released during cell swelling is an important signal that elicits RVD. Though the cell surface receptor for ATP in Dictyostelium remains elusive, we suggest ATP operates through a Gd(3+)-sensitive receptor that is coupled with intracellular NO production.

CR1-mediated ATP release by human red blood cells promotes CR1 clustering and modulates the immune transfer process

Humans and other higher primates are unique among mammals in using complement receptor 1 (CR1, CD35) on red blood cells (RBC) to ligate complement-tagged inflammatory particles (immune complexes, apoptotic/necrotic debris, and microbes) in the circulation for quiet transport to the sinusoids of spleen and liver where resident macrophages remove the particles, but allow the RBC to return unharmed to the circulation. This process is called immune-adherence clearance. In this study we found using luminometric- and fluorescence-based methods that ligation of CR1 on human RBC promotes ATP release. Our data show that CR1-mediated ATP release does not depend on Ca(2+) or enzymes previously shown to mediate an increase in membrane deformability promoted by CR1 ligation. Furthermore, ATP release following CR1 ligation increases the mobility of the lipid fraction of RBC membranes, which in turn facilitates CR1 clustering, and thereby enhances the binding avidity of complement-opsonized particles to the RBC CR1. Finally, we have found that RBC-derived ATP has a stimulatory effect on phagocytosis of immune-adherent immune complexes.

Maxi-anion channel and pannexin 1 hemichannel constitute separate pathways for swelling-induced ATP release in murine L929 fibrosarcoma cells

The maxi-anion channel plays a classically recognized role in controlling the membrane potential through the chloride conductance. It also has novel functions as a regulated pathway for the release of the anionic signaling molecules ATP and excitatory amino acids from cells subjected to osmotic perturbation, ischemia, or hypoxia. Because hemichannels formed by pannexins and connexins have been reported to mediate ATP release from a number of cell types, these hemichannels may represent the molecular correlate of the maxi-anion channel. Here, we found that L929 fibrosarcoma cells express functional maxi-anion channels which mediate a major portion of swelling-induced ATP release, and that ATP released via maxi-anion channels facilitates the regulatory volume decrease after osmotic swelling. Also, it was found that the cells express the mRNA for pannexin 1, pannexin 2, and connexin 43. Hypotonicity-induced ATP release was partially suppressed not only by known blockers of the maxi-anion channel but also by several blockers of pannexins including the pannexin 1-specific blocking peptide (10)Panx1 and small interfering (si)RNA against pannexin 1 but not pannexin 2. The inhibitory effects of maxi-anion channel blockers and pannexin 1 antagonists were additive. In contrast, maxi-anion channel activity was not affected by pannexin 1 antagonists and siRNAs against pannexins 1 and 2. Although a connexin 43-specific blocking peptide, Gap27, slightly suppressed hypotonicity-induced ATP release, maxi-anion channel activity was not affected by Gap27 or connexin 43-specific siRNA. Thus, it is concluded that the maxi-anion channel is a molecular entity distinct from pannexin 1, pannexin 2, and connexin 43, and that the maxi-anion channel and the hemichannels constitute separate pathways for swelling-induced ATP release in L929 cells.

Evidence for sustained ATP release from liver cells that is not mediated by vesicular exocytosis

Extracellular ATP regulates many important cellular functions in the liver by stimulating purinergic receptors. Recent studies have shown that rapid exocytosis of ATP-enriched vesicles contributes to ATP release from liver cells. However, this rapid ATP release is transient, and ceases in ~30 s after the exposure to hypotonic solution. The purpose of these studies was to assess the role of vesicular exocytosis in sustained ATP release. An exposure to hypotonic solution evoked sustained ATP release that persisted for more than 15 min after the exposure. Using FM1-43 (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide) fluorescence to measure exocytosis, we found that hypotonic solution stimulated a transient increase in FM1-43 fluorescence that lasted ~2 min. Notably, the rate of FM1-43 fluorescence and the magnitude of ATP release were not correlated, indicating that vesicular exocytosis may not mediate sustained ATP release from liver cells. Interestingly, mefloquine potently inhibited sustained ATP release, but did not inhibit an increase in FM1-43 fluorescence evoked by hypotonic solution. Consistent with these findings, when exocytosis of ATP-enriched vesicles was specifically stimulated by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), mefloquine failed to inhibit ATP release evoked by NPPB. Thus, mefloquine can pharmacologically dissociate sustained ATP release and vesicular exocytosis. These results suggest that a distinct mefloquine-sensitive membrane ATP transport may contribute to sustained ATP release from liver cells. This novel mechanism of membrane ATP transport may play an important role in the regulation of purinergic signaling in liver cells.

Inhibition of neutrophil-mediated production of reactive oxygen species (ROS) by endothelial cells is not impaired in anti-neutrophil cytoplasmic autoantibodies (ANCA)-associated vasculitis patients

Leucocyte transendothelial migration is strictly regulated to prevent undesired inflammation and collateral damage of endothelial cells by activated neutrophils/monocytes. We hypothesized that in anti-neutrophil cytoplasmic autoantibodies (ANCA)-associated vasculitis (AAV) patients' dysregulation of this process might underlie vascular inflammation. Peripheral blood mononuclear cells (PBMC) and neutrophils from AAV patients (n = 12) and healthy controls (HC, n = 12) were isolated. The influence of human umbilical vein endothelial cells (HUVEC) on neutrophil/monocytes function was tested by N-formyl-methionyl-leucyl-phenyl-alanine (fMLP)- and phorbol 12-myristate 13-acetate (PMA)-mediated ROS production, degranulation and interleukin (IL)-8 production. In addition, the ability of lipopolysaccharide (LPS)-stimulated PBMC to produce tumour necrosis factor (TNF)-alpha in the presence or absence of HUVEC was tested. HUVEC inhibited ROS production dose-dependently by fMLP-stimulated neutrophils but did not influence degranulation. No differences between neutrophils from HC and AAV were found. However, in only one active patient was degranulation inhibited significantly by HUVEC only before cyclophosphamide treatment, but not 6 weeks later. Co-cultures of HUVEC with LPS-stimulated neutrophils/monocytes increased IL-8 production while TNF-alpha production was inhibited significantly. There was no apparent difference between AAV patients and HC in this respect. Our findings demonstrate that HUVEC are able to inhibit ROS and modulate cytokine production upon stimulation of neutrophils or monocytes. Our data do not support the hypothesis that endothelial cells inhibit ROS production of neutrophils from AAV patients inadequately. Impaired neutrophil degranulation may exist in active patients, but this finding needs to be confirmed.

ATP released by electrical stimuli elicits calcium transients and gene expression in skeletal muscle

ATP released from cells is known to activate plasma membrane P2X (ionotropic) or P2Y (metabotropic) receptors. In skeletal muscle cells, depolarizing stimuli induce both a fast calcium signal associated with contraction and a slow signal that regulates gene expression. Here we show that nucleotides released to the extracellular medium by electrical stimulation are partly involved in the fast component and are largely responsible for the slow signals. In rat skeletal myotubes, a tetanic stimulus (45 Hz, 400 1-ms pulses) rapidly increased extracellular levels of ATP, ADP, and AMP after 15 s to 3 min. Exogenous ATP induced an increase in intracellular free Ca(2+) concentration, with an EC(50) value of 7.8 +/- 3.1 microm. Exogenous ADP, UTP, and UDP also promoted calcium transients. Both fast and slow calcium signals evoked by tetanic stimulation were inhibited by either 100 mum suramin or 2 units/ml apyrase. Apyrase also reduced fast and slow calcium signals evoked by tetanus (45 Hz, 400 0.3-ms pulses) in isolated mouse adult skeletal fibers. A likely candidate for the ATP release pathway is the pannexin-1 hemichannel; its blockers inhibited both calcium transients and ATP release. The dihydropyridine receptor co-precipitated with both the P2Y(2) receptor and pannexin-1. As reported previously for electrical stimulation, 500 mum ATP significantly increased mRNA expression for both c-fos and interleukin 6. Our results suggest that nucleotides released during skeletal muscle activity through pannexin-1 hemichannels act through P2X and P2Y receptors to modulate both Ca(2+) homeostasis and muscle physiology.

Thick ascending limb: the Na(+):K (+):2Cl (-) co-transporter, NKCC2, and the calcium-sensing receptor, CaSR

The thick ascending limb of Henle's loop is a nephron segment that is vital to the formation of dilute and concentrated urine. This ability is accomplished by a consortium of functionally coupled proteins consisting of the apical Na(+):K(+):2Cl(-) co-transporter, the K(+) channel, and basolateral Cl(-) channel that mediate electroneutral salt absorption. In thick ascending limbs, salt absorption is importantly regulated by the calcium-sensing receptor. Genetic or pharmacological disruption impairing the function of any of these proteins results in Bartter syndrome. The thick ascending limb is also an important site of Ca(2+) and Mg(2+) absorption. Calcium-sensing receptor activation inhibits cellular Ca(2+) absorption induced by parathyroid hormone, as well as passive paracellular Ca(2+) transport. The present review discusses these functions and their genetic and molecular regulation.

Attenuated, flow-induced ATP release contributes to absence of flow-sensitive, purinergic Cai2+ signaling in human ADPKD cyst epithelial cells

Flow-induced cytosolic Ca2+ Ca(i)2+ signaling in renal tubular epithelial cells is mediated in part through P2 receptor (P2R) activation by locally released ATP. The ability of P2R to regulate salt and water reabsorption has suggested a possible contribution of ATP release and paracrine P2R activation to cystogenesis and/or enlargement in autosomal dominant polycystic kidney disease (ADPKD). We and others have demonstrated in human ADPKD cyst cells the absence of flow-induced Ca(i)2+ signaling exhibited by normal renal epithelial cells. We now extend these findings to primary and telomerase-immortalized normal and ADPKD epithelial cells of different genotype and of both proximal and distal origins. Flow-induced elevation of Ca(i)2+ concentration ([Ca2+](i)) was absent from ADPKD cyst cells, but in normal cells was mediated by flow-sensitive ATP release and paracrine P2R activation, modulated by ecto-nucleotidase activity, and abrogated by P2R inhibition or extracellular ATP hydrolysis. In contrast to the elevated ATP release from ADPKD cells in static isotonic conditions or in hypotonic conditions, flow-induced ATP release from cyst cells was lower than from normal cells. Extracellular ATP rapidly reduced thapsigargin-elevated [Ca2+](i) in both ADPKD cyst and normal cells, but cyst cells lacked the subsequent, slow, oxidized ATP-sensitive [Ca2+](i) recovery present in normal cells. Telomerase-immortalized cyst cells also exhibited altered CD39 and P2X7 mRNA levels. Thus the loss of flow-induced, P2R-mediated Ca(i)2+ signaling in human ADPKD cyst epithelial cells was accompanied by reduced flow-sensitive ATP release, altered purinergic regulation of store-operated Ca2+ entry, and altered expression of gene products controlling extracellular nucleotide signaling.

The maxi-anion channel: a classical channel playing novel roles through an unidentified molecular entity

The maxi-anion channel is widely expressed and found in almost every part of the body. The channel is activated in response to osmotic cell swelling, to excision of the membrane patch, and also to some other physiologically and pathophysiologically relevant stimuli, such as salt stress in kidney macula densa as well as ischemia/hypoxia in heart and brain. Biophysically, the maxi-anion channel is characterized by a large single-channel conductance of 300-400 pS, which saturates at 580-640 pS with increasing the Cl(-) concentration. The channel discriminates well between Na(+) and Cl(-), but is poorly selective to other halides exhibiting weak electric-field selectivity with an Eisenman's selectivity sequence I. The maxi-anion channel has a wide pore with an effective radius of approximately 1.3 nm and permits passage not only of Cl(-) but also of some intracellular large organic anions, thereby releasing major extracellular signals and gliotransmitters such as glutamate(-) and ATP(4-). The channel-mediated efflux 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, well-defined properties and physiological/pathophysiological significance of this classical channel, the molecular entity has not been identified. Molecular identification of the maxi-anion channel is an urgent task that would greatly promote investigation in the fields not only of anion channel but also of physiological/pathophysiological signaling in the brain, heart and kidney.

Interstitial adenosine triphosphate modulates muscle afferent nerve-mediated pressor reflex

Previous work has shown that muscle contraction elevates interstitial adenosine triphosphate concentration ([ATP]i), which is likely due to the release of ATP from active skeletal muscle. ATP activation of purinergic receptors P2X on thin muscle afferent fibers further enhances cardiovascular responses to contraction. Thus, the purposes of this study were: (1) to examine the mechanisms by which ATP is released from muscle in response to mechanical stimulation; and (2) to study the effects of interstitial ATP concentrations on modulating pressor response to muscle contraction. Static contraction of the triceps surae muscle was evoked by electrical stimulation (at 5 HZ and 2.5 times motor threshold) of the tibial nerve in 9 anesthetized cats. Muscle interstitial ATP samples were collected from microdialysis probes inserted into the muscles. Dialysate ATP concentrations were determined using the luciferin-luciferase assay. In a control experiment, contraction was induced after 0.5 ml of saline was injected into the arterial blood supply of the hindlimb muscles. This increased [ATP]i by 220% (P < 0.05 vs. baseline). After gadolinium (1 mM), a blocker of mechanically sensitive channels, was injected into the muscles, contraction increased [ATP]i by 112% (P < 0.05 vs. control). In contrast, glibenclamide (an inhibitor of the ATP-binding cassette protein), monensin, and brefeldin A, which interfere with vesicular formation (or trafficking) and inhibit exocytosis, did not significantly affect ATP release by muscle contraction. In addition, a regression analysis showed that [ATP]i was linearly related to the pressor response to muscle contraction. The data suggest that ATP release from skeletal muscle is mediated via involvement of mechanosensitive channels. These findings further support a physiological role for release of ATP in modulating cardiovascular responses during static muscle contraction.

Ecto-nucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1/CD39) regulates neutrophil chemotaxis by hydrolyzing released ATP to adenosine

Polymorphonuclear neutrophils release ATP in response to stimulation by chemoattractants, such as the peptide N-formyl-methionyl-leucyl-phenylalanine. Released ATP and the hydrolytic product adenosine regulate chemotaxis of neutrophils by sequentially activating purinergic nucleotide and adenosine receptors, respectively. Here we show that that ecto-nucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1, CD39) is a critical enzyme for hydrolysis of released ATP by neutrophils and for cell migration in response to multiple agonists (N-formyl-methionyl-leucyl-phenylalanine, interleukin-8, and C5a). Upon stimulation of human neutrophils or differentiated HL-60 cells in a chemotactic gradient, E-NTPDase1 tightly associates with the leading edge of polarized cells during chemotaxis. Inhibition of E-NTPDase1 reduces the migration speed of neutrophils but not their ability to detect the orientation of the gradient field. Studies of neutrophils from E-NTPDase1 knock-out mice reveal similar impairments of chemotaxis in vitro and in vivo. Thus, E-NTPDase1 plays an important role in regulating neutrophil chemotaxis by facilitating the hydrolysis of extracellular ATP.

Two pathways for ATP release from host cells in enteropathogenic Escherichia coli infection

We previously reported that enteropathogenic Escherichia coli (EPEC) infection triggered a large release of ATP from the host cell that was correlated with and dependent on EPEC-induced killing of the host cell. We noted, however, that under some circumstances, EPEC-induced ATP release exceeded that which could be accounted for on the basis of host cell killing. For example, EPEC-induced ATP release was potentiated by noncytotoxic agents that elevate host cell cAMP, such as forskolin and cholera toxin, and by exposure to hypotonic medium. These findings and the performance of the EPEC espF mutant led us to hypothesize that the CFTR plays a role in EPEC-induced ATP release that is independent of cell death. We report the results of experiments using specific, cell-permeable CFTR activators and inhibitors, as well as transfection of the CFTR into non-CFTR-expressing cell lines, which incriminate the CFTR as a second pathway for ATP release from host cells. Increased ATP release via CFTR is not accompanied by an increase in EPEC adherence to transfected cells. The CFTR-dependent ATP release pathway becomes activated endogenously later in EPEC infection, and this activation is mediated, at least in part, by generation of extracellular adenosine from the breakdown of released ATP.

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.

Calcium-sensing receptor regulation of PTH-inhibitable proximal tubule phosphate transport

Inorganic phosphate (Pi) is absorbed by proximal tubules through a cellular pathway that is inhibited by parathyroid hormone (PTH). The calcium-sensing receptor (CaSR) is expressed on apical membranes of proximal tubules. In the present studies, we determined the effect of luminal and/or basolateral PTH on phosphate absorption and tested the hypothesis that CaSR activation blocks PTH-inhibitable phosphate absorption. Single proximal S3 tubules were dissected from the kidneys of mice and studied by the Burg technique. Tubules were bathed with DMEM culture media supplemented with 6% BSA and perfused with an ultrafiltrate prepared from the bathing solution. 33P and FITC-inulin were added to the luminal perfusate to measure phosphate absorption (JPi) and fluid absorption (Jv), respectively. JPi averaged 2.9 pmol.min-1.mm-1 under control conditions and decreased by 20% upon addition of serosal PTH. PTH had no effect on Jv. Inclusion of PTH in the luminal perfusate reduced JPi to 2.1 pmol. min-1. mm-1. Combined addition of PTH to perfusate and bathing solutions reduced JPi to 1.5 pmol. min-1. mm-1 without affecting Jv. Indirect immunofluorescence studies revealed abundant PTH receptor (PTH1R) expression on brush-border membranes, with lower amounts on basolateral membranes. CaSRs were localized primarily, but not exclusively, to brush-border membranes. CaSR activation with luminal Gd3+ abolished the inhibitory action of PTH on JPi. Addition of Gd3+ to the serosal bathing solution had no effect on PTH-sensitive JPi. Gd3+ i.e., PTH-independent JPi. Gd3+ did not affect basal, had no effect on Jv when added to lumen or bath. Dopamine-inhibitable JPi was not affected by Gd3+. Experiments with proximal-like opossum kidney cells showed that elevated extracellular Ca2+ or NPS R467, a type II calcimimetic, inhibited PTH action on Pi uptake. In conclusion, PTH1Rs are expressed on apical and basolateral membranes of mouse proximal tubules. Stimulating apical or basolateral PTH1R inhibits phosphate absorption. CaSR activation specifically regulates PTH-suppressible phosphate absorption.

ATP concentrations and muscle tension increase linearly with muscle contraction

Previous studies have suggested that activation of ATP-sensitive P2X receptors in skeletal muscle play a role in mediating the exercise pressor reflex (Li J and Sinoway LI. Am J Physiol Heart Circ Physiol 283: H2636-H2643, 2002). To determine the role ATP plays in this reflex, it is necessary to examine whether muscle interstitial ATP (ATPi) concentrations rise with muscle contraction. Accordingly, in this study, muscle contraction was evoked by electrical stimulation of the L7 and S1 ventral roots of the spinal cord in 12 decerebrate cats. Muscle ATPi was collected from microdialysis probes inserted in the muscle. ATP concentrations were determined by the HPLC method. Electrical stimulation of the ventral roots at 3 and 5 Hz increased mean arterial pressure by 13 +/- 2 and 16 +/- 3 mmHg (P < 0.05), respectively, and it increased ATP concentration in contracting muscle by 150% (P < 0.05) and 200% (P < 0.05), respectively. ATP measured in the opposite control limb did not rise with ventral root stimulation. Section of the L7 and S1 dorsal roots did not affect the ATPi seen with 5-Hz ventral root stimulation. Finally, ventral roots stimulation sufficient to drive motor nerve fibers did not increase ATP in previously paralyzed cats. Thus ATPi is not largely released from sympathetic or motor nerves and does not require an intact afferent reflex pathway. We conclude that ATPi is due to the release of ATP from contracting skeletal muscle cells.

Luminal NaCl delivery regulates basolateral PGE2 release from macula densa cells

Macula densa (MD) cells express COX-2 and COX-2-derived PGs appear to signal the release of renin from the renal juxtaglomerular apparatus, especially during volume depletion. However, the synthetic machinery and identity of the specific prostanoid released from intact MD cells remains uncertain. In the present studies, a novel biosensor tool was engineered to directly determine whether MD cells release PGE2 in response to low luminal NaCl concentration ([NaCl]L). HEK293 cells were transfected with the Ca2+-coupled E-prostanoid receptor EP1 (HEK/EP1) and loaded with fura-2. HEK/EP1 cells produced a significant elevation in intracellular [Ca2+] ([Ca2+]i) by 29.6 +/- 12.8 nM (n = 6) when positioned at the basolateral surface of isolated perfused MD cells and [NaCl]L was reduced from 150 mM to zero. HEK/EP1 [Ca2+]i responses were observed mainly in preparations from rabbits on a low-salt diet and were completely inhibited by either a selective COX-2 inhibitor or an EP1 antagonist, and also by 100 microM luminal furosemide. Also, 20-mM graduated reductions in [NaCl]L between 80 and 0 mM caused step-by-step increases in HEK/EP1 [Ca2+]i. Low-salt diet greatly increased the expression of both COX-2 and microsome-associated PGE synthase (mPGES) in the MD. These studies provide the first direct evidence that intact MD cells synthesize and release PGE2 during reduced luminal salt content and suggest that this response is important in the control of renin release and renal vascular resistance during salt deprivation.

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.

Calcium-sensing receptor regulation of PTH-dependent calcium absorption by mouse cortical ascending limbs

Resting Ca(2+) absorption by cortical thick ascending limbs (CALs) is passive and proceeds through the paracellular pathway. In contrast, parathyroid hormone (PTH) stimulates active, transcellular Ca(2+) absorption (J(Ca)). The Ca(2+)-sensing receptor (CaSR) is expressed on serosal membranes of CALs. In the present study, we tested the hypothesis that activation of the CAL CaSR indirectly inhibits passive Ca(2+) transport and directly suppresses PTH-induced cellular J(Ca). To test this theory, we measured J(Ca) and Na absorption (J(Na)) by single perfused mouse CALs. Net absorption was measured microfluorimetrically in samples collected from tubules perfused and bathed in symmetrical HEPES-buffered solutions or those in which luminal Na(+) was reduced from 150 to 50 mM. We first confirmed that Gd(3+) activated the CaSR by measuring intracellular Ca(2+) concentration ([Ca(2+)](i)) in CALs loaded with fura 2. On stepwise addition of Gd(3+) to the bath, [Ca(2+)](i) increased, with a half-maximal rise at 30 microM Gd(3+). J(Ca) and transepithelial voltage (V(e),) were measured in symmetrical Na(+)-containing solutions. PTH increased J(Ca) by 100%, and 30 microM Gd(3+) inhibited this effect. V(e) was unchanged by either PTH or Gd(3+). Similarly, NPS R-467, an organic CaSR agonist, inhibited PTH-stimulated J(Ca) without altering V(e). Neither PTH nor Gd(3+) affected J(Na). Addition of bumetanide to the luminal perfusate abolished J(Na) and V(e). These results show that CaSR activation directly inhibited PTH-induced transcellular J(Ca) and that cellular Ca(2+) and Na(+) transport can be dissociated. To test the effect of CaSR activation on passive paracellular Ca(2+) transport, J(Ca) was measured under asymmetrical Na conditions, in which passive Ca(2+) transport dominates transepithelial absorption. PTH stimulated J(Ca) by 24% and was suppressed by Gd(3+). In this setting, Gd(3+) reduced V(e) by 32%, indicating that CaSR activation inhibited both transcellular and paracellular Ca(2+) transport. We conclude that the CaSR regulates both active transcellular and passive paracellular Ca(2+) reabsorption but has no effect on J(Na) by CALs.

Regulation of an ATP-conductive large-conductance anion channel and swelling-induced ATP release by arachidonic acid

Mouse mammary C127 cells responded to hypotonic stimulation with activation of the volume-dependent ATP-conductive large conductance (VDACL) anion channel and massive release of ATP. Arachidonic acid downregulated both VDACL currents and swelling-induced ATP release in the physiological concentration range with K(d) of 4- 6 microM. The former effect observed in the whole-cell or excised patch mode was more prominent than the latter effect observed in intact cells. The arachidonate effects were direct and not mediated by downstream metabolic products, as evidenced by their insensitivity to inhibitors of arachidonate-metabolizing oxygenases, and by the observation that they were mimicked by cis-unsaturated fatty acids, which are not substrates for oxygenases. A membrane-impermeable analogue, arachidonyl coenzyme A was effective only from the cytosolic side of membrane patches suggesting that the binding site is localized intracellularly. Non-charged arachidonate analogues as well as trans-unsaturated and saturated fatty acids had no effect on VDACL currents and ATP release, indicating the importance of arachidonate's negative charge and specific hydrocarbon chain conformation in the inhibitory effect. VDACL anion channels were inhibited by arachidonic acid in two different ways: channel shutdown (K(d) of 4- 5 microM) and reduced unitary conductance (K(d) of 13-14 microM) without affecting voltage dependence of open probability. ATP(4-)-conducting inward currents measured in the presence of 100 mM ATP in the bath were reversibly inhibited by arachidonic acid. Thus, we conclude that swelling-induced ATP release and its putative pathway, the VDACL anion channel, are under a negative control by intracellular arachidonic acid signalling in mammary C127 cells.

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.

ATP is released from guinea pig ureter epithelium on distension

Distension of the perfused guinea pig ureter at pressures from 20 to 700 cmH(2)O increased the amount of ATP released from the epithelium in a pressure-dependent manner. During basal perfusion (40 microl/min), the perfusate contained 10 pmol/ml ATP; this increased 10- to 50-fold at various distending pressures. ATP was released from epithelial cells during distension as mechanical removal of the urothelium blocked release. No lactate dehydrogenase was detected in the perfusate, and scanning electron microscopy confirmed an intact urothelium after distension. ATP was not released due to the activation of stretch-activated channels, as gadolinium (10 microM) failed to affect ATP release. Glibenclamide (10 microM), known to inhibit two members of the ATP-binding cassette (ABC) protein family, did not affect ATP release after distension; nor did verapamil (10 microM). In contrast, both monensin (100 microM) and brefeldin A (10 microM), which interfere with vesicular formation or trafficking, inhibited distension-evoked ATP release, which was Ca(2+)-dependent. This suggests that ATP release from the ureter epithelium might be mediated by vesicular exocytosis. The role of ATP released by distension of hollow visceral organs is discussed in relation to the concept of purinergic mechanosensory transductions, with special reference to nociception and the activation of P2X(3) receptors on the subepithelial sensory nerves.

Extracellular ATP signaling and P2X nucleotide receptors in monolayers of primary human vascular endothelial cells

ATP and its metabolites regulate vascular tone; however, the sources of the ATP released in vascular beds are ill defined. As such, we tested the hypothesis that all limbs of an extracellular purinergic signaling system are present in vascular endothelial cells: ATP release, ATP receptors, and ATP receptor-triggered signal transduction. Primary cultures of human endothelial cells derived from multiple blood vessels were grown as monolayers and studied using a bioluminescence detection assay for ATP released into the medium. ATP is released constitutively and exclusively across the apical membrane under basal conditions. Hypotonic challenge or the calcium agonists ionomycin and thapsigargin stimulate ATP release in a reversible and regulated manner. To assess expression of P2X purinergic receptor channel subtypes (P2XRs), we performed degenerate RT-PCR, sequencing of the degenerate P2XR product, and immunoblotting with P2XR subtype-specific antibodies. Results revealed that P2X(4) and P2X(5) are expressed abundantly by endothelial cell primary cultures derived from multiple blood vessels. Together, these results suggest that components of an autocrine purinergic signaling loop exist in the endothelial cell microvasculature that may allow for "self-regulation" of endothelial cell function and modulation of vascular tone.

Molecular characterization of volume-sensitive SK(Ca) channels in human liver cell lines

In human liver, Ca(2+)-dependent changes in membrane K(+) permeability play a central role in coordinating functional interactions between membrane transport, metabolism, and cell volume. On the basis of the observation that K(+) conductance is partially sensitive to the bee venom toxin apamin, we aimed to assess whether small-conductance Ca(2+)-sensitive K(+) (SK(Ca)) channels are expressed endogenously and contribute to volume-sensitive K(+) efflux and cell volume regulation. We isolated a full-length 2,140-bp cDNA (hSK2) highly homologous to rat brain rSK2 cDNA, including the putative apamin-sensitive pore domain, from a human liver cDNA library. Identical cDNAs were isolated from primary human hepatocytes, human HuH-7 hepatoma cells, and human Mz-ChA-1 cholangiocarcinoma cells. Transduction of Chinese hamster ovary cells with a recombinant adenovirus encoding the hSK2-green fluorescent protein fusion construct resulted in expression of functional apamin-sensitive K(+) channels. In Mz-ChA-1 cells, hypotonic (15% less sodium glutamate) exposure increased K(+) current density (1.9 +/- 0.2 to 37.5 +/- 7.1 pA/pF; P < 0.001). Apamin (10-100 nM) inhibited K(+) current activation and cell volume recovery from swelling. Apamin-sensitive SK(Ca) channels are functionally expressed in liver and biliary epithelia and likely contribute to volume-sensitive changes in membrane K(+) permeability. Accordingly, the hSK2 protein is a potential target for pharmacological modulation of liver transport and metabolism through effects on membrane K(+) permeability.

Cell swelling-induced ATP release and gadolinium-sensitive channels

ATP release induced by hypotonic swelling is an ubiquitous phenomenon in eukaryotic cells, but its underlying mechanisms are poorly defined. A mechanosensitive (MS) ATP channel has been implicated because gadolinium (Gd(3+)), an inhibitor of stretch-activated channels, suppressed ATP efflux monitored by luciferase bioluminescence. We examined the effect of Gd(3+) on luciferase bioluminescence and on ATP efflux from hypotonically swollen cells. We found that luciferase was inhibited by < or =10 microM Gd(3+), and this may have contributed to the previously reported inhibition of ATP release. In ATP efflux experiments, luciferase inhibition could be prevented by chelating Gd(3+) with EGTA before luminometric ATP determinations. Using this approach, we found that 10-100 microM Gd(3+), i.e., concentrations typically used to block MS channels, actually stimulated hypotonically induced ATP release from fibroblasts. Inhibition of ATP release required at least 500, 200, or 100 microM Gd(3+) for fibroblasts, A549 cells, and 16HBE14o(-) cells, respectively. Such biphasic and cell-specific effects of Gd(3+) are most consistent with its action on membrane lipids and membrane-dependent processes such as exocytosis.

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.

Key role for constitutive cyclooxygenase-2 of MDCK cells in basal signaling and response to released ATP

Madin-Darby canine kidney (MDCK) cells release ATP upon mechanical or biochemical activation, initiating P2Y receptor signaling that regulates basal levels of multiple second messengers, including cAMP (J Biol Chem 275: 11735--11739, 2000). Data shown here document inhibition of cAMP formation by Gd(3+) and niflumic acid, channel inhibitors that block ATP release. cAMP production is stimulated via Ca(2+)-dependent activation of cytosolic phospholipase A(2), release of arachidonic acid (AA), and cyclooxygenase (COX)-dependent production of prostaglandins, which activate prostanoid receptors coupled to G(s) and adenylyl cyclase. In the current investigation, we assessed the expression and functional role of the two known isoforms of COX, COX-1 and COX-2. Treatment of cells with either a COX-1-selective inhibitor, SC-560, or COX-2-selective inhibitors, SC-58125 or NS-398, inhibited basal and UTP-stimulated cAMP levels. COX inhibitors also decreased forskolin-stimulated cAMP formation, implying this response is in part attributable to an action of AA metabolites. These findings imply an important role for the inducible form of COX, COX-2, under basal conditions. Indeed, COX-2 expression was readily detectable by immunoblot, and treatments that induce or reduce COX-2 expression in other cells (interleukin-1beta, tumor necrosis factor-alpha, phorbol ester, or dexamethasone) had minimal or no effect on the levels of COX-2 immunoreactivity. RT-PCR using isoform-specific primers detected COX-2 mRNA. We conclude that COX-2 is constitutively expressed in MDCK-D(1) cells and participates in basal and P2Y(2)-mediated signaling, implying a key role for COX-2 in regulation of epithelial cell function.

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.

Extracellular nucleotide signaling along the renal epithelium

During the past two decades, several cell membrane receptors, which preferentially bind extracellular nucleotides, and their analogs have been identified. These receptors, collectively known as nucleotide receptors or "purinergic" receptors, have been characterized and classified on the basis of their biological actions, their pharmacology, their molecular biology, and their tissue and cell distribution. For these receptors to have biological and physiological relevance, nucleotides must be released from cells. The field of extracellular ATP release and signaling is exploding, as assays to detect this biological process increase in number and ingenuity. Studies of ATP release have revealed a myriad of roles in local regulatory (autocrine or paracrine) processes in almost every tissue in the body. The regulatory mechanisms that these receptors control or modulate have physiological and pathophysiological roles and potential therapeutic applications. Only recently, however, have ATP release and nucleotide receptors been identified along the renal epithelium of the nephron. This work has set the stage for the study of their physiological and pathophysiological roles in the kidney. This review provides a comprehensive presentation of these issues, with a focus on the renal epithelium.