Pathophysiology and puzzles of the volume-sensitive outwardly rectifying anion channel

The COX-2 selective blocker etodolac inhibits TNFα-induced apoptosis in isolated rabbit articular chondrocytes

Chondrocyte apoptosis contributes to the disruption of cartilage integrity in osteoarthritis (OA). Recently, we reported that activation of volume-sensitive Cl⁻ current (I_Cl,vol) mediates cell shrinkage, triggering apoptosis in rabbit articular chondrocytes. A cyclooxygenase (COX) blocker is frequently used for the treatment of OA. In the present study, we examined in vitro effects of selective blockers of COX on the TNFα-induced activation of I_Cl,vol in rabbit chondrocytes using the patch-clamp technique. Exposure of isolated chondrocytes to TNFα resulted in an obvious increase in membrane Cl⁻ conductance. The TNFα-evoked Cl⁻ current exhibited electrophysiological and pharmacological properties similar to those of I_Cl,vol. Pretreatment of cells with selective COX-2 blocker etodolac markedly inhibited I_Cl,vol activation by TNFα as well as subsequent apoptotic events such as apoptotic cell volume decrease (AVD) and elevation of caspase-3/7 activity. In contrast, a COX-1 blocker had no effect on the decrease in cell volume or the increase in caspase-3/7 activity induced by TNFα. Thus, the COX-2-selective blocker had an inhibitory effect on TNFα-induced apoptotic events, which suggests that this drug would have efficacy for the treatment of OA.

Slc26a11 is prominently expressed in the brain and functions as a chloride channel: expression in Purkinje cells and stimulation of V H⁺-ATPase

SLC26A11 (human)/Slc26a11 (mouse), also known as kidney brain anion transporter (KBAT), is a member of the SLC26 anion transporter family and shows abundant mRNA expression in the brain. However, its exact cellular distribution and subcellular localization in the brain and its functional identity and possible physiological roles remain unknown. Expression and immunostaining studies demonstrated that Slc26a11 is abundantly expressed in the cerebellum, with a predominant expression in Purkinje cells. Lower expression levels were detected in hippocampus, olfactory bulb, cerebral cortex, and subcortical structures. Patch clamp studies in HEK293 cells transfected with mouse cDNA demonstrated that Slc26a11 can function as a chloride channel that is active under basal conditions and is not regulated by calcium, forskolin, or co-expression with cystic fibrosis transmembrane regulator. Single and double immunofluorescent labeling studies demonstrated the localization of vacuolar (V) H⁺-ATPase and Slc26a11 (KBAT) in the plasma membrane in Purkinje cells. Functional studies in HEK293 cells indicated that transfection with Slc26a11 stimulated acid transport via endogenous V H⁺-ATPase. We conclude that Slc26a11 (KBAT) is prominently distributed in output neurons of various subcortical and cortical structures in the central nervous system, with specific expression in Purkinje cells and that it may operate as a chloride channel regulating acid translocation by H⁺-ATPase across the plasma membrane and in intracellular compartments.

Electrophysiological characteristics of enteric neurons isolated from the immortomouse

BACKGROUND

Recently, two enteric neuronal cell lines, one fetal and the other post-natal (IM-PEN), have been developed from the H-2K(b)-tsA58 transgenic mouse (immortomouse). However, their electrophysiological properties are not known. The goal of this study was to determine the electrical excitability and ionic conductance of the immortalized postnatal enteric neuronal (IM-PEN) cell line.

METHODS

Whole cell patch clamp studies, immunohistochemistry and RT-PCR were performed on differentiated IM-PEN cells following propagation at 33 °C and differentiation at 37 °C.

RESULTS

Differentiated IM-PEN cells stained positively for the neuron specific markers βIII-tubulin and PGP9.5. The mRNA for several ion channels expressed in enteric neurons were detected by RT-PCR. In current clamp, the resting membrane potential was -24.6 ± 2.1 mV (n = 6) for IM-FEN and -29.8 ± 0.9 mV (n = 30) for IM-PEN. Current injections from Vh -80 mV resulted in passive responses but not action potentials. Depolarizing pulses in the whole cell voltage clamp configuration from Vh -80 mV elicited small nifedipine-sensitive inward currents. Additionally, outward currents with slow deactivating tail currents were blocked by niflumic acid and low chloride solution. A volume-regulated anion current was elicited by hypo-osmotic solution and inhibited by 10 μM DCPIB. Growth with rabbit gastrointestinal smooth muscle did not yield significant differences in the active properties of the IM-PEN cell line. Transient expression of L-type Ca(2+) channels produced large inward currents demonstrating a working mechanism for protein folding and transport.

CONCLUSION

The electrophysiological characteristics of IM-PEN cells suggest that chloride channels in IM-PEN cells play an important role in their resting state, and membrane trafficking of some of the ion channels may preclude their electrical excitability.

Volume-sensitive anion channels mediate osmosensitive glutathione release from rat thymocytes

Glutathione (GSH) is a negatively charged tripeptide, which is a major determinant of the cellular redox state and defense against oxidative stress. It is assembled inside and degraded outside the cells and is released under various physiological and pathophysiological conditions. The GSH release mechanism is poorly understood at present. In our experiments, freshly isolated rat thymocytes were found to release GSH under normal isotonic conditions at a low rate of 0.82±0.07 attomol/cell/min and that was greatly enhanced under hypoosomotic stimulation to reach a level of 6.1±0.4 attomol/cell/min. The swelling-induced GSH release was proportional to the cell density in the suspension and was temperature-dependent with relatively low activation energy of 5.4±0.6 kcal/mol indicating a predominant diffusion mechanism of GSH translocation. The osmosensitive release of GSH was significantly inhibited by blockers of volume-sensitive outwardly rectifying (VSOR) anion channel, DCPIB and phloretin. In patch-clamp experiments, osmotic swelling activated large anionic conductance with the VSOR channel phenotype. Anion replacement studies suggested that the thymic VSOR anion channel is permeable to GSH(-) with the permeability ratio P(GSH)/P(Cl) of 0.32 for influx and 0.10 for efflux of GSH. The osmosensitive GSH release was trans-stimulated by SLCO/OATP substrates, probenecid, taurocholic acid and estrone sulfate, and inhibited by an SLC22A/OAT blocker, p-aminohippuric acid (PAH). The inhibition by PAH was additive to the effect of DCPIB or phloretin implying that PAH and DCPIB/phloretin affected separate pathways. We suggest that the VSOR anion channel constitutes a major part of the γ-glutamyl cycle in thymocytes and, in cooperation with OATP-like and OAT-like transporters, provides a pathway for the GSH efflux from osmotically swollen cells.

17β-Oestradiol inhibits doxorubicin-induced apoptosis via block of the volume-sensitive Cl(-) current in rabbit articular chondrocytes

BACKGROUND AND PURPOSE Chondrocyte apoptosis contributes to disruption of cartilage integrity in osteoarthritis. Recent evidence suggested that the volume-sensitive organic osmolyte/anion channel [volume-sensitive (outwardly rectifying) Cl(-) current (I(Cl,vol) )] plays a functional role in the development of cell shrinkage associated with apoptosis (apoptotic volume decrease) in several cell types. In this study, we investigated the cellular effects of 17β-oestradiol on doxorubicin-induced apoptotic responses in rabbit articular chondrocytes. EXPERIMENTAL APPROACH Whole-cell membrane currents and cross-sectional area were measured from chondrocytes using a patch-clamp method and microscopic cell imaging, respectively. Caspase-3/7 activity was assayed as an index of apoptosis. KEY RESULTS Addition of doxorubicin (1 µM) to isosmotic bath solution rapidly activated the Cl(-) current with properties similar to those of I(Cl,vol) in chondrocytes. Doxorubicin also gradually decreased the cross-sectional area of chondrocytes, followed by enhanced caspase-3/7 activity; both of these responses were totally abolished by the I(Cl,vol) blocker DCPIB (20 µM). Pretreatment of chondrocytes with 17β-oestradiol (1 nM) for short (approximately 10 min) and long (24 h) periods almost completely prevented the doxorubicin-induced activation of I(Cl,vol) and subsequent elevation of caspase-3/7 activity. These effects of 17β-oestradiol were significantly attenuated by the oestrogen receptor blocker ICI 182780 (10 µM), as well as the phosphatidyl inositol-3-kinase (PI3K) inhibitors wortmannin (100 nM) and LY294002 (20 µM). Testosterone (10 nM) had no effect on the doxorubicin-induced Cl(-) current. CONCLUSIONS AND IMPLICATIONS 17β-Oestradiol prevents the doxorubicin-induced cell shrinkage mediated through activation of I(Cl,vol) and subsequent induction of apoptosis signals, through a membrane receptor-dependent PI3K pathway in rabbit articular chondrocytes.

The apoptotic volume decrease is an upstream event of MAP kinase activation during Staurosporine-induced apoptosis in HeLa cells

Persistent cell shrinkage, called apoptotic volume decrease (AVD), is a pivotal event of apoptosis. Activation of the volume-sensitive outwardly rectifying Cl⁻ channel (VSOR) is involved in the AVD induction. On the other hand, activation of the MAP kinase (MAPK) cascade is also known to play a critical role in apoptosis. In the present study, we investigated the relationship between the AVD induction and the stress-responsive MAPK cascade activation during the apoptosis process induced by staurosporine (STS) in HeLa cells. STS was found to induce AVD within 2–5 min and phosphorylation of c-Jun N-terminal kinase (JNK) and p38 MAPK after over 20–30 min. VSOR blockers suppressed not only STS-induced AVD but also phosphorylation of JNK and p38 as well as activation of caspase-3/7. Moreover, a p38 inhibitor, SB203580, and a JNK inhibitor, SP600125, failed to affect STS-induced AVD, whereas these compounds reduced STS-induced activation of caspase-3/7. Also, treatment with ASK1-specific siRNA suppressed STS-induced caspase-3/7 activation without affecting the AVD induction. Furthermore, sustained osmotic cell shrinkage per se was found to trigger phosphorylation of JNK and p38, caspase activation, and cell death. Thus, it is suggested that activation of p38 and JNK is a downstream event of AVD for the STS-induced apoptosis of HeLa cells.

A Lys49-PLA2 myotoxin of Bothrops asper triggers a rapid death of macrophages that involves autocrine purinergic receptor signaling

Lys49-PLA(2) myotoxins, an important component of various viperid snake venoms, are a class of PLA(2)-homolog proteins deprived of catalytic activity. Similar to enzymatically active PLA(2) (Asp49) and to other classes of myotoxins, they cause severe myonecrosis. Moreover, these toxins are used as tools to study skeletal muscle repair and regeneration, a process that can be very limited after snakebites. In this work, the cytotoxic effect of different myotoxins, Bothrops asper Lys49 and Asp49-PLA(2), Notechis scutatus notexin and Naja mossambica cardiotoxin, was evaluated on macrophages, cells that have a key role in muscle regeneration. Only the Lys49-myotoxin was found to trigger a rapid asynchronous death of mouse peritoneal macrophages and macrophagic cell lines through a process that involves ATP release, ATP-induced ATP release and that is inhibited by various purinergic receptor antagonists. ATP leakage is induced also at sublytical doses of the Lys49-myotoxin, it involves Ca(2+) release from intracellular stores, and is reduced by inhibitors of VSOR and the maxi-anion channel. The toxin-induced cell death is different from that caused by high concentration of ATP and appears to be linked to localized purinergic signaling. Based on present findings, a mechanism of cell death is proposed that can be extended to other cytolytic proteins and peptides.

Vesicular and conductive mechanisms of nucleotide release

Extracellular nucleotides and nucleosides promote a vast range of physiological responses, via activation of cell surface purinergic receptors. Virtually all tissues and cell types exhibit regulated release of ATP, which, in many cases, is accompanied by the release of uridine nucleotides. Given the relevance of extracellular nucleotide/nucleoside-evoked responses, understanding how ATP and other nucleotides are released from cells is an important physiological question. By facilitating the entry of cytosolic nucleotides into the secretory pathway, recently identified vesicular nucleotide and nucleotide-sugar transporters contribute to the exocytotic release of ATP and UDP-sugars not only from endocrine/exocrine tissues, but also from cell types in which secretory granules have not been biochemically characterized. In addition, plasma membrane connexin hemichannels, pannexin channels, and less-well molecularly defined ATP conducting anion channels have been shown to contribute to the release of ATP (and UTP) under a variety of conditions.

Volume-activated chloride currents in fetal human nasopharyngeal epithelial cells

Volume-activated chloride channels have been studied by us extensively in human nasopharyngeal carcinoma cells. However, the chloride channels in the counterpart of the carcinoma cells have not been investigated. In this study, volume-activated chloride currents (I(cl,vol)) were characterized in normal fetal human nasopharyngeal epithelial cells using the whole-cell patch-clamp technique. Under isotonic conditions, nasopharyngeal epithelial cells displayed only a weak background current. Exposure to 47% hypotonic solution activated a volume-sensitive current. The reversal potential of the current was close to the calculated equilibrium potential for Cl(-). The peak values of the hypotonicity-activated current at +80 mV ranged from 0.82 to 2.71 nA in 23 cells. Further analysis indicated that the density of the hypotonicity-activated current in most cells (18/23) was smaller than 60 pA/pF. Only five cells presented a current larger than 60 pA/pF. The hypotonicity-activated current was independent of the exogenous ATP. Chloride channel inhibitors ATP, tamoxifen and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), inhibited the current dramatically. The anion permeability of the hypotonicity-activated chloride channels was I(-) > Br(-) > Cl(-) > gluconate. Unexpectedly, in isotonic conditions, ATP (10 mM) activated an inward-rectified current, which had not been observed in the nasopharyngeal carcinoma cells. These results suggest that, under hypotonic challenges, fetal human nasopharyngeal epithelial cells can produce I(cl,vol), which might be involved in cell volume regulation.

Distinct expression/function of potassium and chloride channels contributes to the diverse volume regulation in cortical astrocytes of GFAP/EGFP mice

Recently, we have identified two astrocytic subpopulations in the cortex of GFAP-EGFP mice, in which the astrocytes are visualized by the enhanced green-fluorescent protein (EGFP) under the control of the human glial fibrillary acidic protein (GFAP) promotor. These astrocytic subpopulations, termed high response- (HR-) and low response- (LR-) astrocytes, differed in the extent of their swelling during oxygen-glucose deprivation (OGD). In the present study we focused on identifying the ion channels or transporters that might underlie the different capabilities of these two astrocytic subpopulations to regulate their volume during OGD. Using three-dimensional confocal morphometry, which enables quantification of the total astrocytic volume, the effects of selected inhibitors of K⁺ and Cl⁻ channels/transporters or glutamate transporters on astrocyte volume changes were determined during 20 minute-OGD in situ. The inhibition of volume regulated anion channels (VRACs) and two-pore domain potassium channels (K(2P)) highlighted their distinct contributions to volume regulation in HR-/LR-astrocytes. While the inhibition of VRACs or K(2P) channels revealed their contribution to the swelling of HR-astrocytes, in LR-astrocytes they were both involved in anion/K⁺ effluxes. Additionally, the inhibition of Na⁺-K⁺-Cl⁻ co-transporters in HR-astrocytes led to a reduction of cell swelling, but it had no effect on LR-astrocyte volume. Moreover, employing real-time single-cell quantitative polymerase chain reaction (PCR), we characterized the expression profiles of EGFP-positive astrocytes with a focus on those ion channels and transporters participating in astrocyte swelling and volume regulation. The PCR data revealed the existence of two astrocytic subpopulations markedly differing in their gene expression levels for inwardly rectifying K⁺ channels (Kir4.1), K(2P) channels (TREK-1 and TWIK-1) and Cl⁻ channels (ClC2). Thus, we propose that the diverse volume changes displayed by cortical astrocytes during OGD mainly result from their distinct expression patterns of ClC2 and K(2P) channels.

Swelling-activated anion channels are essential for volume regulation of mouse thymocytes

Channel-mediated trans-membrane chloride movement is a key process in the active cell volume regulation under osmotic stress in most cells. However, thymocytes were hypothesized to regulate their volume by activating a coupled K-Cl cotransport mechanism. Under the patch-clamp, we found that osmotic swelling activates two types of macroscopic anion conductance with different voltage-dependence and pharmacology. At the single-channel level, we identified two types of events: one corresponded to the maxi-anion channel, and the other one had characteristics of the volume-sensitive outwardly rectifying (VSOR) chloride channel of intermediate conductance. A VSOR inhibitor, phloretin, significantly suppressed both macroscopic VSOR-type conductance and single-channel activity of intermediate amplitude. The maxi-anion channel activity was largely suppressed by Gd(3+) ions but not by phloretin. Surprisingly, [(dihydroindenyl)oxy] alkanoic acid (DIOA), a known antagonist of K-Cl cotransporter, was found to significantly suppress the activity of the VSOR-type single-channel events with no effect on the maxi-anion channels at 10 μM. The regulatory volume decrease (RVD) phase of cellular response to hypotonicity was mildly suppressed by Gd(3+) ions and was completely abolished by phloretin suggesting a major impact of the VSOR chloride channel and modulatory role of the maxi-anion channel. The inhibitory effect of DIOA was also strong, and, most likely, it occurred via blocking the VSOR Cl(-) channels.

The molecular and functional interaction between ICln and HSPC038 proteins modulates the regulation of cell volume

Identifying functional partners for protein/protein interactions can be a difficult challenge. We proposed the use of the operon structure of the Caenorhabditis elegans genome as a “new gene-finding tool” (Eichmüller, S., Vezzoli, V., Bazzini, C., Ritter, M., Fürst, J., Jakab, M., Ravasio, A., Chwatal, S., Dossena, S., Bottà, G., Meyer, G., Maier, B., Valenti, G., Lang, F., and Paulmichl, M. (2004) J. Biol. Chem. 279, 7136–7146) that could be functionally translated to the human system. Here we show the validity of this approach by studying the predicted functional interaction between ICln and HSPC038. In C. elegans, the gene encoding for the ICln homolog (icln-1) is embedded in an operon with two other genes, Nx (the human homolog of Nx is HSPC038) and Ny. ICln is a highly conserved, ubiquitously expressed multifunctional protein that plays a critical role in the regulatory volume decrease after cell swelling. Following hypotonic stress, ICln translocates from the cytosol to the plasma membrane, where it has been proposed to participate in the activation of the swelling-induced chloride current (ICl_swell). Here we show that the interaction between human ICln and HSPC038 plays a role in volume regulation after cell swelling and that HSPC038 acts as an escort, directing ICln to the cell membrane after cell swelling and facilitating the activation of ICl_swell. Assessment of the NMR structure of HSPC038 showed the presence of a zinc finger motif. Moreover, NMR and additional biochemical techniques enabled us to identify the putative ICln/HSPC038 interacting sites, thereby explaining the functional interaction of both proteins on a molecular level.

Regulation of bradykinin-induced activation of volume-sensitive outwardly rectifying anion channels by Ca2+ nanodomains in mouse astrocytes

Volume-sensitive outwardly rectifying (VSOR) anion channels play a key role in a variety of essential cell functions including cell volume regulation, cell death induction and intercellular communications. We previously demonstrated that, in cultured mouse cortical astrocytes, VSOR channels are activated in response to an inflammatory mediator, bradykinin, even without an increase in cell volume. Here we report that this VSOR channel activation must be mediated firstly by 'nanodomains' of high [Ca2+]i generated at the sites of both Ca2+ release from intracellular Ca2+ stores and Ca2+ entry at the plasma membrane. Bradykinin elicited a [Ca2+]i rise, initially caused by Ca2+ release and then by Ca2+ entry. Suppression of the [Ca2+]i rise by removal of extracellular Ca2+ and by depletion of Ca2+ stores suppressed the VSOR channel activation in a graded manner. Quantitative RT-PCR and suppression of gene expression with small interfering RNAs indicated that Orai1, TRPC1 and TRPC3 channels are involved in the Ca2+ entry and especially the entry through TRPC1 channels is strongly involved in the bradykinin-induced activation of VSOR channels. Moreover, Ca2+-dependent protein kinases Cα and β were found to mediate the activation after the [Ca2+]i rise through inducing generation of reactive oxygen species. Intracellular application of a slow Ca2+ chelator, EGTA, at 10 mM or a fast chelator, BAPTA, at 1 mM, however, had little effect on the VSOR channel activation. Application of BAPTA at 10 mM suppressed significantly the activation to one-third. These suggest that the VSOR channel activation induced by bradykinin is regulated by Ca2+ in the vicinity of individual Ca2+ release and entry channels, providing a basis for local control of cell volume regulation and intercellular communications.

Dysfunction of volume-sensitive chloride channels contributes to cisplatin resistance in human lung adenocarcinoma cells

Cisplatin-based chemotherapy is the standard therapy used to treat non-small-cell lung cancer. However, its efficacy is largely limited due to the development of drug resistance. The exact mechanism in which cancer cells develop resistance to the drug is not yet fully understood. The purpose of the present study is to test the role of volume-sensitive Cl(-) channels in cisplatin resistance in human lung adenocarcinoma cells (A549 cells) using patch-clamp recording, cell volume measurement and apoptosis assay. The results showed that cisplatin treatment induced an apoptotic volume decrease (AVD) and activated a Cl(-) current that showed properties similar to the volume-sensitive outward rectifying (VSOR) Cl(-) current in wild-type A549 cells. Both the AVD process and VSOR Cl(-) current were blocked by the chloride channel blocker 4,4'-diisothiocyanostilbene-2,2' disulfonic acid. However, the A549/CDDP cells, a model of acquired cisplatin resistance cells, on the other hand, had almost no AVD process and VSOR Cl(-) current when treated with cisplatin. Treatment of A549/CDDP cells with trichostatin A (TSA), a drug that inhibits histone deacetylases, partially restored the VSOR Cl(-) current and increased cisplatin-induced cell apoptosis rate. These results suggest that impaired activity of VSOR Cl(-) channels contributes to the cisplatin resistance in A549/CDDP cells.

Balance of unidirectional monovalent ion fluxes in cells undergoing apoptosis: why does Na+/K+ pump suppression not cause cell swelling?

Cells dying according to the apoptotic program, unlike cells dying via an unprogrammed mode, are able to avoid swelling and osmotic bursting with membrane disruption.There are indications that apoptosis is accompanied by suppression of the Na+/K+ pump and changes in the K+ and Cl− channels. It remains unclear how ion fluxes through individual ion pathways are integrated so as to induce loss of intracellular ions and concomitant apoptotic volume decrease. A decrease in activity of the sodium pump during apoptosis should cause cell swelling rather than shrinkage. We have made the first systemic analysis of the monovalent ion flux balance in apoptotic cells. Experimental data were obtained for human U937 cells treated with staurosporine for 4–5 h, which is known to induce apoptosis. The data include cellular Cl− content and fluxes, K+, Na+, water content and ouabain-sensitive and -resistant Rb+ fluxes.Unidirectional monovalent ion fluxeswere calculated using these data and a cell model comprising the double Donnan system with the Na+/K+ pump, Cl−, K+, Na+ channels, the Na+–K+–2Cl−cotransporter (NKCC), the Na+–Cl− cotransporter (NC), and the equivalent Cl−/Cl− exchange.Apoptotic cell shrinkage was found to be caused, depending on conditions, either by an increase in the integral channel permeability of membrane for K+ or by suppression of the pump coupledwith a decrease in the integral channel permeability of membrane for Na+. The decrease in the channel permeability of membrane for Na+ plays a crucial role in cell dehydration in apoptosis accompanied by suppression of the pump. Supplemental Table S1 is given for easy calculating flux balance under specified conditions.

Swelling activated Cl- channels in microglia: Biophysics, pharmacology and role in glutamate release

Microglia have a swelling-activated Cl- current (which we call IClswell), and while some of its biophysical properties and functional roles have been elucidated, its molecular identity is unknown. To relate this current to cell functions and determine whether it is regulated by mechanisms other than cell swelling, it is important to establish both biophysical and pharmacological fingerprints. Here, we used rat microglia and a cell line derived from them (MLS-9) to study biophysical, regulatory and pharmacological properties of IClswell. The whole-cell current was activated in response to a hypo-osmotic bath solution, but not by voltage, and was time-independent during long voltage steps. The halide selectivity sequence was I->Br->Cl- (Eisenman sequence I) and importantly, the excitatory amino acid, glutamate was permeant. Current activation required internal ATP, and was not affected by the guanine nucleotides, GTPS or GDPS, or physiological levels of internal Mg2+. The same current was activated by a low intracellular ionic strength solution without an osmotic gradient. IClswell was reversibly inhibited by known Cl- channel blockers (NPPB, flufenamic acid, glibenclamide, DCPIB), and by the glutamate release inhibitor, riluzole. Cell swelling evoked glutamate release from primary microglia and MLS-9 cells, and this was inhibited by the blockers (above), and by IAA-94, but not by tamoxifen or the Na+/K+/Cl- symport inhibitor, bumetanide. Together, these results confirm the similarity of IClswell in the two cell types, and point to a role for this channel in inflammation-mediated glutamate release in the CNS.

Receptor regulation of osmolyte homeostasis in neural cells

The capacity of cells to correct their volume in response to hyposmotic stress via the efflux of inorganic and organic osmolytes is well documented. However, the ability of cell-surface receptors, in particular G-protein-coupled receptors (GPCRs), to regulate this homeostatic mechanism has received much less attention. Mechanisms that underlie the regulation of cell volume are of particular importance to cells in the central nervous system because of the physical restrictions of the skull and the adverse impact that even small increases in cell volume can have on their function. Increases in brain volume are seen in hyponatraemia, which can arise from a variety of aetiologies and is the most frequently diagnosed electrolyte disorder in clinical practice. In this review we summarize recent evidence that the activation of GPCRs facilitates the volume-dependent efflux of osmolytes from neural cells and permits them to more efficiently respond to small, physiologically relevant, reductions in osmolarity. The characteristics of receptor-regulated osmolyte efflux, the signalling pathways involved and the physiological significance of receptor activation are discussed. In addition, we propose that GPCRs may also regulate the re-uptake of osmolytes into neural cells, but that the influx of organic and inorganic osmolytes is differentially regulated. The ability of neural cells to closely regulate osmolyte homeostasis through receptor-mediated alterations in both efflux and influx mechanisms may explain, in part at least, why the brain selectively retains its complement of inorganic osmolytes during chronic hyponatraemia, whereas its organic osmolytes are depleted.

Control of volume-sensitive chloride channel inactivation by the coupled action of intracellular chloride and extracellular protons

The volume-sensitive chloride current (I(ClVol)) exhibit a time-dependent decay presumably due to channel inactivation. In this work, we studied the effects of chloride ions (Cl(-)) and H(+) ions on I(ClVol) decay recorded in HEK-293 and HL-60 cells using the whole-cell patch clamp technique. Under control conditions ([Cl(-)](e) = [Cl(-)](i) = 140 mM and pH(i) = pH(e) = 7.3), I(ClVol) in HEK cells shows a large decay at positive voltages but in HL-60 cells I(ClVol) remained constant independently of time. In HEK-293 cells, simultaneously raising the [Cl(-)](e) and [Cl(-)](i) from 25 to 140 mM (with pH(e) = pH(i) = 7.3) increased the fraction of inactivated channels (FIC). This effect was reproduced by elevating [Cl(-)](i) while keeping the [Cl(-)](e) constant. Furthermore, a decrease in pH(e) from 7.3 to 5.5 accelerated current decay and increased FIC when [Cl(-)] was 140 mM but not 25 mM. In HL-60 cells, a slight I(ClVol) decay was seen when the pH(e) was reduced from 7.3 to 5.5. Our data show that inactivation of I(ClVol) can be controlled by changing either the Cl(-) or H(+) concentration or both. Based on our results and previously published data, we have built a model that explains VRAC inactivation. In the model the H(+) binding site is located outside the electrical field near the extracellular entry whilst the Cl(-) binding site is intracellular. The model depicts inactivation as a pore constriction that happens by simultaneous binding of H(+) and Cl(-) ions to the channel followed by a voltage-dependent conformational change that ultimately causes inactivation.

Extracellular osmolarity modulates G protein-coupled receptor-dependent ATP release from 1321N1 astrocytoma cells

We previously reported that ATP release from 1321N1 human astrocytoma cells could be stimulated either by activation of G protein-coupled receptors (GPCR) or by hypotonic stress. Cheema et al. (Cheema TA, Ward CE, Fisher SK. J Pharmacol Exp Ther 315: 755-763, 2005) have demonstrated that thrombin activation of protease-activated receptor 1 (PAR1) in 1321N1 cells and primary astrocytes acts synergistically with hypotonic stress to gate the opening of volume-sensitive organic osmolyte and anion channels (VSOAC) and that hypertonic stress strongly inhibits PAR1 gating of VSOAC. We tested the hypothesis that a VSOAC-type permeability might comprise a GPCR-regulated pathway for ATP export by determining whether PAR1-sensitive ATP release from 1321N1 cells is similarly potentiated by hypotonicity but suppressed by hypertonic conditions. Strong hypotonic stress by itself elicited ATP release and positively modulated the response to thrombin. Thrombin-dependent ATP release was also potentiated by mild hypotonic stress that by itself did not stimulate ATP export. Notably, PAR1-sensitive ATP export was greatly inhibited in hypertonic medium. Neither the potency nor efficacy of thrombin as an activator of proximal PAR1 signaling was affected by hypotonicity or hypertonicity. 1,9-Dideoxyforskolin and carbenoxolone similarly attenuated PAR1-dependent ATP release and suppressed the PAR1-independent ATP elicited by strong hypotonic stress. Probenecid attenuated PAR1-stimulated ATP release under isotonic but not mild hypotonic conditions and had no effect on PAR1-independent release stimulated by strong hypotonicity. PAR1-dependent ATP export under all osmotic conditions required concurrent signaling by Ca(2+) mobilization and Rho-GTPase activation. In contrast, PAR1-independent ATP release triggered by strong hypotonicity required neither of these intracellular signals. Thus, we provide the new finding that GPCR-regulated ATP release from 1321N1 astrocytoma cells is remarkably sensitive to both positive and negative modulation by extracellular osmolarity. This supports a model wherein GPCR stimulation and osmotic stress converge on an ATP release pathway in astrocytes that exhibits several features of VSOAC-type channels.

Regulatory role of tyrosine phosphorylation in the swelling-activated chloride current in isolated rabbit articular chondrocytes

Articular chondrocytes are exposed in vivo to the continually changing osmotic environment and thus require volume regulatory mechanisms. The present study was designed to investigate (i) the functional role of the swelling-activated Cl(-) current (I(Cl,swell)) in the regulatory volume decrease (RVD) and (ii) the regulatory role of tyrosine phosphorylation in I(Cl,swell), in isolated rabbit articular chondrocytes. Whole-cell membrane currents were recorded from chondrocytes in isosmotic, hyposmotic and hyperosmotic external solutions under conditions where Na(+), K(+) and Ca(2+) currents were minimized. The cell surface area was also measured using microscope images from a separate set of chondrocytes and was used as an index of cell volume. The isolated chondrocytes exhibited a RVD during sustained exposure to hyposmotic solution, which was mostly inhibited by the I(Cl,swell) blocker 4-(2-butyl-6,7-dichloro-2-cyclopentyl-indan-1-on-5-yl)oxobutyric acid (DCPIB) at 20 microM. Exposure to a hyposmotic solution activated I(Cl,swell), which was also largely inhibited by 20 microM DCPIB. I(Cl,swell) in rabbit articular chondrocytes had a relative taurine permeability (P(tau)/P(Cl)) of 0.21. Activation of I(Cl,swell) was significantly reduced by the protein tyrosine kinase (PTK) inhibitor genistein (30 microM) but was only weakly affected by its inactive analogue daidzein (30 microM). Intracellular application of protein tyrosine phosphatase (PTP) inhibitor sodium orthovanadate (250 and 500 microM) resulted in a gradual activation of a Cl(-) current even in isosmotic solutions. This Cl(-) current was almost completely inhibited by 4,4-diisothiocyanatostilbene-2,2-disulfonate (DIDS, 500 microM) and was also largely suppressed by exposure to hyperosmotic solution, thus indicating a close similarity to I(Cl,swell). Pretreatment of chondrocytes with genistein significantly prevented the activation of the Cl(-) current by sodium orthovanadate, suggesting that the basal activity of endogenous PTK is required for the activation of this Cl(-) current. Our results provide evidence to indicate that activation of I(Cl,swell) is involved in RVD in isolated rabbit articular chondrocytes and is facilitated by tyrosine phosphorylation.