Oxidized glutathione mediates cation channel activation in calf vascular endothelial cells during oxidant stress

Regulation of ion transport by oxidants

Ion channels perform a variety of cellular functions in lung epithelia. Oxidant- and antioxidant-mediated mechanisms (that is, redox regulation) of ion channels are areas of intense research. Significant progress has been made in our understanding of redox regulation of ion channels since the last Experimental Biology report in 2003. Advancements include: 1) identification of nonphagocytic NADPH oxidases as sources of regulated reactive species (RS) production in epithelia, 2) an understanding that excessive treatment with antioxidants can result in greater oxidative stress, and 3) characterization of novel RS signaling pathways that converge upon ion channel regulation. These advancements, as discussed at the 2013 Experimental Biology Meeting in Boston, MA, impact our understanding of oxidative stress in the lung, and, in particular, illustrate that the redox state has profound effects on ion channel and cellular function.

G6PD deficiency: global distribution, genetic variants and primaquine therapy

Glucose-6-phosphate dehydrogenase (G6PD) is a potentially pathogenic inherited enzyme abnormality and, similar to other human red blood cell polymorphisms, is particularly prevalent in historically malaria endemic countries. The spatial extent of Plasmodium vivax malaria overlaps widely with that of G6PD deficiency; unfortunately the only drug licensed for the radical cure and relapse prevention of P. vivax, primaquine, can trigger severe haemolytic anaemia in G6PD deficient individuals. This chapter reviews the past and current data on this unique pharmacogenetic association, which is becoming increasingly important as several nations now consider strategies to eliminate malaria transmission rather than control its clinical burden. G6PD deficiency is a highly variable disorder, in terms of spatial heterogeneity in prevalence and molecular variants, as well as its interactions with P. vivax and primaquine. Consideration of factors including aspects of basic physiology, diagnosis, and clinical triggers of primaquine-induced haemolysis is required to assess the risks and benefits of applying primaquine in various geographic and demographic settings. Given that haemolytically toxic antirelapse drugs will likely be the only therapeutic options for the coming decade, it is clear that we need to understand in depth G6PD deficiency and primaquine-induced haemolysis to determine safe and effective therapeutic strategies to overcome this hurdle and achieve malaria elimination.

On the role of endothelial TRPC3 channels in endothelial dysfunction and cardiovascular disease

In endothelium, calcium (Ca(2+)) influx through plasma membrane Ca(2+)-permeable channels plays a fundamental role in several physiological functions and in the pathogenesis of cardiovascular disease. Current knowledge on the influence of Ca(2+) influx in signaling events associated to endothelial dysfunction has grown significantly over recent years, particularly after identification of members of the Transient Receptor Potential Canonical (TRPC) family of channel forming proteins as prominent mediators of Ca(2+) entry in endothelial cells. Among TRPC members TRPC3 has been at the center of many of these physiopathological processes. Progress in elucidating the mechanism/s underlying regulation of endothelial TRPC3 and characterization of signaling events downstream TRPC3 activation are of most importance to fully appreciate the role of this peculiar cation channel in cardiovascular disease and its potential use as a therapeutic target. In this updated review we focus on TRPC3 channels, revising and discussing current knowledge on channel expression and regulation in endothelium and the roles of TRPC3 in cardiovascular disease in relation to endothelial dysfunction.

Understanding the mechanisms for metabolism-linked hemolytic toxicity of primaquine against glucose 6-phosphate dehydrogenase deficient human erythrocytes: evaluation of eryptotic pathway

Therapeutic utility of primaquine, an 8-aminoquinoline antimalarial drug, has been limited due to its hemolytic toxicity in population with glucose 6-phosphate dehydrogenase deficiency. Recent investigations at our lab have shown that the metabolites generated through cytochrome P(450)-dependent metabolic reactions are responsible for hemotoxic effects of primaquine, which could be monitored with accumulation of methemoglobin and increased oxidative stress. The molecular markers for succeeding cascade of events associated with early clearance of the erythrocytes from the circulation were evaluated for understanding the mechanism for hemolytic toxicity of primaquine. Primaquine alone though did not induce noticeable methemoglobin accumulation, but produced significant oxidative stress, which was higher in G6PD-deficient than in normal erythrocytes. Primaquine, presumably through redox active hemotoxic metabolites generated in situ in human liver microsomal metabolism-linked assay, induced a dose-dependent methemoglobin accumulation and oxidative stress, which were almost similar in normal and G6PD-deficient erythrocytes. Primaquine alone or in presence of pooled human liver microsomes neither produced significant effect on intraerythrocytic calcium levels nor affected the phosphatidyl serine asymmetry of the normal and G6PD-deficient human erythrocytes as monitored flowcytometrically with Annexin V binding assay. The studies suggest that eryptosis mechanisms are not involved in accelerated removal of erythrocytes due to hemolytic toxicity of primaquine.

Redox regulation of endothelial canonical transient receptor potential channels

The endothelium is a highly dynamic structure lining the inside of blood vessels that exhibits physical and chemical properties that are critical determinants of overall vascular function. Physically, the endothelium constitutes a semipermeable barrier. Chemically, the endothelium synthesizes numerous factors such as reactive oxygen species (ROS) that can act as autocrine and paracrine signaling molecules. Oxidative stress results when ROS levels increase to levels that cause cellular injury, and, in the endothelium oxidative stress leads to barrier disruption. Endothelial barrier disruption also results from increased cytosolic calcium through store-operated calcium (SOC) entry channels. Although it is known that ROS can interact with and regulate some ion channels, relatively little is known about the interaction of these species with components of endothelial SOC entry channels, the canonical transient receptor potential (TRPC) proteins. Here we review our current understanding of ROS-mediated TRPC channel function and how it affects SOC entry and endothelial barrier disruption.

The chemopreventive effect of dimethylthiourea against carmustine-induced myelotoxicity in rats

The possible chemopreventive role of dimethylthiourea (DMTU) against carmustine (1,3-bis(2-chloroethyl)-1-nitrosourea, BCNU)-induced myelotoxicity was assessed through evaluation of apoptosis, lipid peroxidation, glutathione (GSH) content and some antioxidant enzymes activities in bone marrow cells of rats. Thirty-six rats were randomly classified into four groups. The first group was injected i.p. with ethanol and served as a control. The second group was treated with BCNU. The third group was given DMTU, while the fourth group was co-administered with DMTU prior to BCNU administration. BCNU treatment in a single dose of 30 mg/kg significantly decreased the normal counts of RBCs, WBCs and platelets as well as hemoglobin level. In addition, BCNU exhibited marked apoptotic effect associated with significant alterations in the oxidative cascade parameters. Treatment of animals with DMTU in a single dose of 500 mg/kg 1h before BCNU injection, followed by 125 mg/kg twice daily for 5 consecutive days significantly mitigated the induced changes in the hematological parameters. The induced alterations in the oxidant and antioxidant parameters as well as apoptosis were also improved. Conclusively, DMTU treatment exhibited marked chemopreventive effect against BCNU-induced myelotoxicity; an effect which may be partially attributed to its inherently antioxidant potential.

Redox modulation of Ca2+ signaling in human endothelial and smooth muscle cells in pre-eclampsia

Pre-eclampsia (PE) is a leading cause of maternal hypertension in pregnancy and is associated with fetal growth restriction, premature birth, and fetal and maternal mortality. Activation and dysfunction of the maternal and fetal endothelium in PE appears to be a consequence of increased oxidative stress, resulting from elevated levels of circulating lipid peroxides. Accumulating evidence implicates reactive oxygen species (ROS) in the pathogenesis of vascular dysfunction in PE, perhaps involving a disturbance in intracellular Ca(2+) signaling. Several ion-transport pathways are highly sensitive to oxidative stress, and the resulting modulation of ion transport by ROS will affect intracellular Ca(2+) homeostasis. We review the evidence that changes in ion transport induced by ROS may be linked with abnormalities in Ca(2+)-mediated signal transduction, leading to endothelial and smooth muscle dysfunction in maternal and fetal circulations in PE. As dysregulation of Ca(2+) signaling in fetal umbilical endothelial cells is maintained in culture and embryonic, fetal, and postnatal development is affected by the cellular redox state, we hypothesize that impaired redox signaling in PE may influence "programming" of the fetal cardiovascular system and endothelial function in adulthood.

Effects of antioxidants on calcium influx through TRPM2 channels in transfected cells activated by hydrogen peroxide

Melastatin-like transient receptor potential 2 (TRPM2) channel is a redox sensitive Ca(2+)-permeable cation channel that can be gated by H(2)O(2) binding to the channel's enzymatic Nudix domain. Since the mechanisms that lead to TRPM2 action in response to H(2)O(2) are not understood, we examined the effects of various antioxidants on H(2)O(2)-induced TRPM2 cation channel currents in transfected Chinese hamster ovary (CHO) cells. The CHO cells were transfected with cDNA coding for TRPM2. Membrane currents were measured with the conventional whole cell patch-clamp technique. The intracellular solution contained ethylenediamine tetraacetic acid (EDTA) as a chelator for Ca(2+) and heavy metal ions instead of ethylene glycol tetraacetic acid (EGTA). Moreover, we chose an intracellular Ca(2+) concentration calculated to be in the range of 1 microM. H(2)O(2) (10 mM) was added extracellularly to the bath chamber. With these conditions, we were able to evoke TRPM2 currents consistently with H(2)O(2). We next tested whether vitamins C and E or glutathione (GSH) would prevent or attenuate the induction of TRPM2 currents by H(2)O(2) when applied extracellularly or intracellularly. Unexpectedly, administration of these antioxidants did not inhibit activation of TRPM2 by H(2)O(2). In conclusion, TRPM2 channels were constitutively activated by H(2)O(2) although we could not detect any inhibitory effect of the antioxidants on H(2)O(2)-induced TRPM2 cation channel currents in CHO cells.

TRPC3 and TRPC4 associate to form a redox-sensitive cation channel. Evidence for expression of native TRPC3-TRPC4 heteromeric channels in endothelial cells

Canonical transient receptor potential proteins (TRPC) have been proposed to form homo- or heteromeric cation channels in a variety of tissues, including the vascular endothelium. Assembly of TRPC multimers is incompletely understood. In particular, heteromeric assembly of distantly related TRPC isoforms is still a controversial issue. Because we have previously suggested TRPC proteins as the basis of the redox-activated cation conductance of porcine aortic endothelial cells (PAECs), we set out to analyze the TRPC subunit composition of endogenous endothelial TRPC channels and report here on a redox-sensitive TRPC3-TRPC4 channel complex. The ability of TRPC3 and TRPC4 proteins to associate and to form a cation-conducting pore complex was supported by four lines of evidence as follows: 1) Co-immunoprecipitation experiments in PAECs and in HEK293 cells demonstrated the association of TRPC3 and TRPC4 in the same complex. 2) Fluorescence resonance energy transfer analysis demonstrated TRPC3-TRPC4 association, involving close proximity between the N terminus of TRPC4 and the C terminus of TRPC3 subunits. 3) Co-expression of TRPC3 and TRPC4 in HEK293 cells generated a channel that displayed distinct biophysical and regulatory properties. 4) Expression of dominant-negative TRPC4 proteins suppressed TRPC3-related channel activity in the HEK293 expression system and in native endothelial cells. Specifically, an extracellularly hemagglutinin (HA)-tagged TRPC4 mutant, which is sensitive to blockage by anti-HA-antibody, was found to transfer anti-HA sensitivity to both TRPC3-related currents in the HEK293 expression system and the redox-sensitive cation conductance of PAECs. We propose TRPC3 and TRPC4 as subunits of native endothelial cation channels that are governed by the cellular redox state.

Aspirin inhibits NF-κB activation in a glycolysis-depleted lung epithelial cell line

Inhibition of glycolysis at the phosphofructo-1-kinase step slows cell growth. For this reason, overexpression of fructose-2,6-bisphosphatase is a potential target for antineoplasic treatments. However, therapeutic objectives may be compromised by side effects of glycolysis restriction, including enhanced resistance to oxidants and tumor necrosis factor-alpha (TNF-alpha), as well as increased activity of the nuclear factor kappa B (NF-kappaB). In this study we evaluated aspirin as an adjuvant drug for glycolysis restriction by overexpression of fructose-2,6-bisphosphatase. The effect of aspirin on antioxidant defences and NF-kappaB activity were evaluated both in control cells and in fructose-2,6-bisphosphatase-overexpressing cells. Interestingly, aspirin-induced inhibition of NF-kappaB activity was greater in transfectants with restricted glycolysis than in control cells. Our results indicate that aspirin is a suitable complement to therapy based on glycolysis restriction to overcome resistance associated with increased NF-kappaB activity and oxidative stress.

Electrophysiological effects of O2*- on the plasma membrane in vascular endothelial cells

Vascular dysfunction is a hallmark of many diseases, including coronary heart disease, stroke, and diabetes. The underlying mechanisms of these disorders are intimately associated with an increase in oxidative stress and excess generation of reactive oxygen species. Here, we report that the anionic free radical, superoxide (O2*- ), directly affects the function of ion channels in vascular endothelial cells. Vascular endothelial cells were exposed to O2*- under physiological, symmetrical chloride and chloride-free conditions. Superoxide was generated from the reaction of xanthine (0.2 mM) and xanthine oxidase (0.1, 1, and 10 mU/ml) while its effects were determined with the whole cell mode of the patch-clamp technique. Inhibitors of K+ and Cl- channels were used to determine the role of these ion channels in mediating the electrophysiological effects of superoxide. The addition of O2*- caused a dose-dependent depolarization of endothelial cells and activation of the whole cell current. Activation of superoxide-dependent current was observed in the presence of inhibitors of K+ channels, Ba2+ (100 microM) or iberiotoxin (100 nM), and was not affected by inhibitors of nonselective cation channels, La3+, or by inhibition of the Cl-/HCO3- transporter by bumetanide. The inhibitors of the Cl- channel, NPPB (0.1 mM) or DIDS (100 microM), partially prevented activation of superoxide-dependent current but were unable to reverse it. The effects of superoxide on the amplitude of whole cell current were prevented and reversed by superoxide dismutase. Taken together, these results suggest that superoxide directly affects the function of ion channels in vascular endothelium but the mechanisms of its modulatory effects remain unresolved.

Hydroxyl radical activation of a Ca(2+)-sensitive nonselective cation channel involved in epithelial cell necrosis

In a previous work the involvement of a fenamate-sensitive Ca(2+)-activated nonselective cation channel (NSCC) in free radical-induced rat liver cell necrosis was demonstrated (5). Therefore, we studied the effect of radical oxygen species and oxidizing agents on the gating behavior of a NSCC in a liver-derived epithelial cell line (HTC). Single-channel currents were recorded in HTC cells by the excised inside-out configuration of the patch-clamp technique. In this cell line, we characterize a 19-pS Ca(2+)-activated, ATP- and fenamate-sensitive NSCC nearly equally permeable to monovalent cations. In the presence of Fe(2+), exposure of the intracellular side of NSCC to H(2)O(2) increased their open probability (P(o)) by approximately 40% without affecting the unitary conductance. Desferrioxamine as well as the hydroxyl radical (.OH) scavenger MCI-186 inhibited the effect of H(2)O(2), indicating that the increase in P(o) was mediated by.OH. Exposure of the patch membrane to the oxidizing agent 5,5'-dithio-bis-2-nitrobenzoic acid (DTNB) had a similar effect to.OH. The increase in P(o) induced by.OH or DTNB was not reverted by preventing formation or by DTNB washout, respectively. However, the reducing agent dithiothreitol completely reversed the effects on P(o) of both.OH and DTNB. A similar increase in P(o) was observed by applying the physiological oxidizing molecule GSSG. Moreover, GSSG-oxidized channels showed enhanced sensitivity to Ca(2+). The effect of GSSG was fully reversed by GSH. These results suggest an intracellular site(s) of action of oxidizing agents on cysteine targets on the fenamate-sensitive NSCC protein implicated in epithelial cell necrosis.

Apoptosis versus necrosis in acute pancreatitis

Acute pancreatitis is a disease of variable severity in which some patients experience mild, self-limited attacks, whereas others manifest a severe, highly morbid, and frequently lethal attack. The events that regulate the severity of acute pancreatitis are, for the most part, unknown. It is generally believed that the earliest events in acute pancreatitis occur within acinar cells and result in acinar cell injury. Other processes, such as recruitment of inflammatory cells and generation of inflammatory mediators, are believed to occur subsequent to acinar cell injury, and these "downstream" events are believed to influence the severity of the disease. Several recently reported studies, however, have suggested that the acinar cell response to injury may, itself, be an important determinant of disease severity. In these studies, mild acute pancreatitis was found to be associated with extensive apoptotic acinar cell death, whereas severe acute pancreatitis was found to involve extensive acinar cell necrosis but very little acinar cell apoptosis. These observations led to the hypothesis that apoptosis could be a favorable response to acinar cells and that interventions that favor induction of apoptotic, as opposed to necrotic, acinar cell death might reduce the severity of an attack of acute pancreatitis. Indeed, in an experimental setting, the induction of pancreatic acinar cell apoptosis protects mice against acute pancreatitis. Little is known about the mechanism of apoptosis in the pancreatic acinar cell, although some early attempts have been made in that direction. Also, clinical relevance of these experimental studies remains to be investigated.

Ion dependence of cytotoxicity of carmustine against PC12 cells

Cytotoxicity is a major complication of carmustine (1,3-bis(2-chloroethyl)-1-nitrosourea, BCNU) therapy for treatment of brain tumors and lymphomas. Using the lactate dehydrogenase (LDH) cell death assay in PC12 cells, we studied the role in this phenomenon of transmembrane ion fluxes that could be activated following inhibition by carmustine of glutathione reductase. The cytotoxic effect of carmustine developed during 4-6 h, with the EC50 of 27 microM. It depended on the extracellular Ca2+ concentration and substantially decreased upon Ca2+ removal. An almost complete suppression of toxicity was achieved when, additionally, monovalent cations were also replaced with impermeant organic cations. A similar loss of toxicity occurred in the presence of Ca2+ when extracellular Cl- was replaced with impermeable gluconate. Various blockers of cation and Cl- channels, as well as antioxidants also protected cells from carmustine. We conclude that carmustine toxicity against PC12 cells requires an influx of Ca2+ ions, supposedly through redox-sensitive cation channels.

Cell death induced by acute renal injury: a perspective on the contributions of apoptosis and necrosis

In humans and experimental models of renal ischemia, tubular cells in various nephron segments undergo necrotic and/or apoptotic cell death. Various factors, including nucleotide depletion, electrolyte imbalance, reactive oxygen species, endonucleases, disruption of mitochondrial integrity, and activation of various components of the apoptotic machinery, have been implicated in renal cell vulnerability. Several approaches to limit the injury and augment the regeneration process, including nucleotide repletion, administration of growth factors, reactive oxygen species scavengers, and inhibition of inducers and executioners of cell death, proved to be effective in animal models. Nevertheless, an effective approach to limit or prevent ischemic renal injury in humans remains elusive, primarily because of an incomplete understanding of the mechanisms of cellular injury. Elucidation of cell death pathways in animal models in the setting of renal injury and extrapolation of the findings to humans will aid in the design of potential therapeutic strategies. This review evaluates our understanding of the molecular signaling events in apoptotic and necrotic cell death and the contribution of various molecular components of these pathways to renal injury.

LTRPC2 Ca2+-permeable channel activated by changes in redox status confers susceptibility to cell death

Redox status changes exert critical impacts on necrotic/apoptotic and normal cellular processes. We report here a widely expressed Ca2+-permeable cation channel, LTRPC2, activated by micromolar levels of H2O2 and agents that produce reactive oxygen/nitrogen species. This sensitivity of LTRPC2 to redox state modifiers was attributable to an agonistic binding of nicotinamide adenine dinucleotide (beta-NAD+) to the MutT motif. Arachidonic acid and Ca2+ were important positive regulators for LTRPC2. Heterologous LTRPC2 expression conferred susceptibility to death on HEK cells. Antisense oligonucleotide experiments revealed physiological involvement of "native" LTRPC2 in H2O2- and TNFalpha-induced Ca2+ influx and cell death. Thus, LTRPC2 represents an important intrinsic mechanism that mediates Ca2+ and Na+ overload in response to disturbance of redox state in cell death.

Necrotic volume increase and the early physiology of necrosis

Whether a lethally injured mammalian cell undergoes necrosis or apoptosis may be determined by the early activation of specific ion channels at the cell surface. Apoptosis requires K+ and Cl- efflux, which leads to cell shrinking, an active phenomenon termed apoptotic volume decrease (AVD). In contrast, necrosis has been shown to require Na+ influx through membrane carriers and more recently through stress-activated non-selective cation channels (NSCCs). These ubiquitous channels are kept dormant in viable cells but become activated upon exposure to free-radicals. The ensuing Na+ influx leads to cell swelling, an active response that may be termed necrotic volume increase (NVI). This review focuses on how AVD and NVI become conflicting forces at the beginning of cell injury, on the events that determine irreversibility and in particular, on the ion fluxes that decide whether a cell is to die by necrosis or by apoptosis.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

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.

Effects of cyanide and deoxyglucose on Ca2+ signalling in macrovascular endothelial cells

1. We have studied the effects of the metabolic inhibitors cyanide (CN) and deoxyglucose (DG) on the intracellular Ca2+ concentration ([Ca2+]i) in macrovascular endothelial cells derived from human umbilical vein (EA cells). 2. CN- and DG increased [Ca2-]i in non-voltage clamped cells. This effect was dependent on extracellular Ca2+ concentration and membrane potential, indicating that CN- induced a Ca2+ entry. 3. During expose to CN- and/or DG, EA cells depolarise. This depolarisation is sometimes preceded by a small, but transient hyperpolarisation due to activation of a big - conductance K+ channels, BKCa, present in EA cells. However, in approximately 90% of the cells tested, the CN- and/or DG induced elevation of [Ca2+]i was insufficient to activate BKCa. 4. CN- and/or DG enhanced BKCa currents preactivated by an elevation of [Ca2+]i via cell dialysis with 0.5 and 1 microM, respectively. Thus, metabolic inhibition sensitises BKCa. 5. The CN- induced depolarisation of EA cells occurs by activating a current that reversed at positive membrane potentials. Substituting extracellular cations abolished the inward component of this current by NMDG, indicating that CN- activated a non-selective cation channel, NSC. This current was reduced by extracellular Ca2+ and Mg2'+ but is partially carried by Ca2+. 6. It is concluded that CN elevates [Ca2+]i by activating Ca2+ permeable NSC channels. The properties of these channels are similar to those of the recently described trp3 channels expressed in endothelium.

Calcium signaling and oxidant stress in the vasculature

Recent evidence suggests that oxidant stress plays a major role in several aspects of vascular biology. Oxygen free radicals are implicated as important factors in signaling mechanisms leading to vascular pathologies such as postischemic reperfusion injury and atherosclerosis. The role of intracellular Ca(2+) in these signaling events is an emerging area of vascular research that is providing insights into the mechanisms mediating these complex physiological processes. This review explores sources of free radicals in the vasculature, as well as effects of free radicals on Ca(2+) signaling in vascular endothelial and smooth muscle cells. In the endothelium, superoxides enhance and peroxides attenuate agonist-stimulated Ca(2+) responses, suggesting differential signaling mechanisms depending on radical species. In smooth muscle cells, both superoxides and peroxides disrupt the sarcoplasmic reticulum Ca(2+)-ATPase, leading to both short- and long-term effects on smooth muscle Ca(2+) handling. Because vascular Ca(2+) signaling is altered by oxidant stress in ischemia-related disease states, understanding these pathways may lead to new strategies for preventing or treating arterial disease.

Functional interactions between oxidative stress, membrane Na(+) permeability, and cell volume in rat hepatoma cells

BACKGROUND & AIMS

Oxidative stress leads to a rapid initial loss of liver cell volume, but the adaptive mechanisms that serve to restore volume have not been defined. This study aimed to evaluate the functional interactions between oxidative stress, cell volume recovery, and membrane ion permeability.

METHODS

In HTC rat hepatoma cells, oxidative stress was produced by exposure to H(2)O(2) or D-alanine plus D-amino acid oxidase (40 U/mL).

RESULTS

Oxidative stress resulted in a rapid decrease in relative cell volume to 0.85 +/- 0.06. This was followed by an approximately 100-fold increase in membrane cation permeability and partial volume recovery to 0.97 +/- 0.05 of original values. The volume-sensitive conductance was permeable to Na(+) approximately K(+) >> Tris(+), and whole-cell current density at -80 mV increased from -1.2 pA/pF at 10(-5) mol/L H(2)O(2) to -95.1 pA/pF at 10(-2) mol/L H(2)O(2). The effects of H(2)O(2) were completely inhibited by dialysis of the cell interior with reduced glutathione, and were markedly enhanced by inhibition of glutathione synthase.

CONCLUSIONS

These findings support the presence of dynamic functional interactions between cell volume, oxidative stress, and membrane Na(+) permeability. Stress-induced Na(+) influx may represent a beneficial adaptive response that partially restores cell volume over short periods, but sustained cation influx could contribute to the increase in intracellular [Na(+)] and [Ca(2+)] associated with cell injury and necrosis.

Sensing of extracellular calcium by neurones

Transient changes in the intracellular concentration of Ca2+ provide a major signal for the regulation of many ion channels and enzymes in central neurones. In contrast, changes in extracellular Ca2+ are thought to play little or no signaling role. However, concentrations of extracellular calcium in the central nervous system do change dramatically during intense physiological and pathological stimulation, and recent studies have identified a number of membrane proteins that can sense and respond to changes in extracellular Ca2+. These include the recently cloned Ca2+-sensing receptor, hemi-gap-junction channels, and a potential Ca2+-sensing cation channel. Lowering extracellular Ca2+ strongly depolarizes and excites cultured hippocampal neurones. The excitation can be detected with decreases from physiological concentrations of as little as 100 µM. The depolarization results from activation of a nonselective cation current, which is sensitive to block by divalent and polyvalent cations. In outside-out patches, lowering Ca2+ induces a single-channel current with a conductance of 36 pS. Activation of this cation channel, in response to decreases in extracellular Ca2+, likely plays a key role in a positive feedback system of excessive neuronal depolarization, which accompanies intense excitatory activity in the hippocampus.Key words: nonselective cation channel, calcium-sensing receptor, calcium-sensing channel, hemi-gap channels, extracellular calcium.

The cardiac acetylcholine-activated, inwardly rectifying K+-channel subunit GIRK1 gives rise to an inward current induced by free oxygen radicals

Reactive oxygen species (ROS) play a crucial role in pathophysiology of the cardiovascular system. The present study was designed to analyze the redox sensitivity of G-protein-activated inward rectifier K+ (GIRK) channels, which control cardiac contractility and excitability. GIRK1 subunits were heterologously expressed in Xenopus laevis oocytes and the resulting K+ currents were measured with the two-electrode voltage clamp technique. Oxygen free radicals generated by the hypoxanthine/xanthine oxidase system led to a marked increase in the current through GIRK channels, termed superoxide-induced current (I(SO)). Furthermore, I(SO) did not depend on G-protein-dependent activation of GIRK currents by coexpressed muscarinic m2-receptors, but could also be observed when no agonist was present in the bathing solution. Niflumic acid at a concentration of 0.5 mmol/l did not abolish I(SO), whereas 100 micromol/l Ba2+ attenuated I(SO) completely. Catalase (10(6) i.u./l) failed to suppress I(SO), whereas H2O2 concentration was kept close to zero, as measured by chemiluminescence. Hence, we conclude that O2*- or a closely related species is responsible for I(SO) induction. Our results demonstrate a significant redox sensitivity of GIRK1 channels and suggest redox-activation of G-protein-activated inward rectifier K+ channels as a key mechanism in oxidative stress-associated cardiac dysfunction.

Redox agents regulate ion channel activity in vacuoles from higher plant cells

The ability of redox agents to modulate certain characteristics of voltage- and calcium-activated channels has been recently investigated in a variety of animal cells. We report here the first evidence that redox agents regulate the activation of ion channels in the tonoplast of higher plants. Using the patch-clamp technique, we have demonstrated that, in tonoplasts from the leaves of the marine seagrass Posidonia oceanica and the root of the sugar beet, a variety of sulphydryl reducing agents, added at the cytoplasmic side of the vacuole, reversibly favoured the activation of the voltage-dependent slow vacuolar (SV) channel. Antioxidants, like dithiothreitol (DTT) and the reduced form of glutathione, gave a reversible increase of the voltage-activated current and faster kinetics of channel activation. Other reducing agents, such as ascorbic acid, also increased the SV currents, although to a lesser extent in comparison with DTT and glutathione, while the oxidising agent chloramine-T irreversibly abolished the activity of the channel. Single channel experiments demonstrated that DTT reversibly increased the open probability of the channel, leaving the conductance unaltered. The regulation of channel activation by glutathione may correlate ion transport with other crucial mechanisms that in plants control turgor regulation, response to oxidative stresses, detoxification and resistance to heavy metals.

Reduced glutathione inhibits beta-NAD+-activated non-selective cation currents in the CRI-G1 rat insulin-secreting cell line

1. Whole-cell voltage-clamp recordings were used to study the characteristics of a non-selective cation current, activated by intracellular beta-NAD+, present in CRI-G1 insulin-secreting cells. The monovalent cations Na+, K+ and Cs+ were equally permeant through this channel. 2. The magnitude of the beta-NAD+ current was dependent on the concentration of both beta-NAD+ and Ca2+ in the cell. The properties of the beta-NAD+-activated macroscopic current are similar to those of the beta-NAD+-activated non-selective cation channel (NSNAD) examined at the single channel level in this cell line. 3. The presence of intracellular reduced glutathione (GSH) inhibited the beta-NAD+-activated macroscopic current and the activity of the NSNAD channel in inside-out patch recordings. 4. The inhibition of beta-NAD+-activated currents by GSH is mimicked by its analogue ophthalmic acid but not by another thiol reducing agent dithiothreitol, indicating the presence of a specific GSH binding site present on the NSNAD channel or associated protein.

Interaction of reactive oxygen species with ion transport mechanisms

The use of electrophysiological and molecular biology techniques has shed light on reactive oxygen species (ROS)-induced impairment of surface and internal membranes that control cellular signaling. These deleterious effects of ROS are due to their interaction with various ion transport proteins underlying the transmembrane signal transduction, namely, 1) ion channels, such as Ca2+ channels (including voltage-sensitive L-type Ca2+ currents, dihydropyridine receptor voltage sensors, ryanodine receptor Ca2+-release channels, and D-myo-inositol 1,4,5-trisphosphate receptor Ca2+-release channels), K+ channels (such as Ca2+-activated K+ channels, inward and outward K+ currents, and ATP-sensitive K+ channels), Na+ channels, and Cl- channels; 2) ion pumps, such as sarcoplasmic reticulum and sarcolemmal Ca2+ pumps, Na+-K+-ATPase (Na+ pump), and H+-ATPase (H+ pump); 3) ion exchangers such as the Na+/Ca2+ exchanger and Na+/H+ exchanger; and 4) ion cotransporters such as K+-Cl-, Na+-K+-Cl-, and Pi-Na+ cotransporters. The mechanism of ROS-induced modifications in ion transport pathways involves 1) oxidation of sulfhydryl groups located on the ion transport proteins, 2) peroxidation of membrane phospholipids, and 3) inhibition of membrane-bound regulatory enzymes and modification of the oxidative phosphorylation and ATP levels. Alterations in the ion transport mechanisms lead to changes in a second messenger system, primarily Ca2+ homeostasis, which further augment the abnormal electrical activity and distortion of signal transduction, causing cell dysfunction, which underlies pathological conditions.

Near-visible ultraviolet light induces a novel ubiquitous calcium-permeable cation current in mammalian cell lines

1. We studied the immediate and short-term effects of UV light in the near-visible range at the cellular and membrane level using the whole-cell patch-clamp technique in combination with digital fluorescence imaging. 2. Illumination with monochromatic UVA light (340-380 nm) induced a sustained non-saturable increase in membrane conductance dependent on wavelength and light intensity in several different mammalian cell types including RBL, mast, HEK, PC12 and 3T3 cells. 3. The current was non-selective for cations and permeable to Ca2+, but was inhibited by trivalent cations and was not due to the activation of an endogenous ion channel. We termed this novel current ILiNC for light-induced non-selective cation current. 4. A similar current was evoked by chemical peroxidants such as hydrogen peroxide and tertbutylhydroperoxide, but not by cytosolic oxidized glutathione. 5. The free-radical scavengers tocopherol (vitamin E) and ascorbic acid (vitamin C) significantly reduced the UV light effect. 6. The generation of the current was membrane delimited since it could be induced by the same UVA treatment in cell-free membrane patches showing a similar wavelength dependence. 7. These results suggest that ILiNC is activated by UVA light-induced generation of free radicals acting through lipid or protein peroxidation, and may represent a ubiquitous mechanism by which Na+ and Ca2+ can enter cells after phototoxic or free radical-induced membrane damage.

Activation of a novel non-selective cation channel by alloxan and H2O2 in the rat insulin-secreting cell line CRI-G1

1. Alloxan and its auto-oxidation product hydrogen peroxide (H2O2) irreversibly depolarize insulinoma cells by opening a non-selective cation channel. The channel opened is characterized by a linear current-voltage relation with a conductance of approximately 70 pS and very slow kinetics (of the order of seconds). 2. Cells are protected against the alloxan-induced channel opening and consequent cell depolarization by the presence of H2O2 and hydroxyl radical scavengers. 3. The free radical-activated non-selective cation channel is not operative in isolated patches but can be activated by the application of beta-NAD+ to the cytoplasmic aspect of the membrane.