H2O2-induced chloride currents are indicative of an endogenous Na(+)-Ca2+ exchange mechanism in Xenopus oocytes

The direct permeation of nanoparticles through the plasma membrane transiently modifies its properties

The exposure to metal nanoparticles (NPs) has increased with their widespread use in industry, research and medicine. It is well known that NPs may enter cells and that this mechanism is crucial to exert both the therapeutic and toxicity effects. The main cellular entrance route is endocytosis-based, however, recent experimental studies, have reported that NPs can also enter the cell crossing directly the plasma membrane, it is thus important to investigate this alternative internalization mechanism. Size, surface chemistry, solubility and shape play a role in NP ability of entering the cell, but it is still to be elucidated how these properties act on cell membrane. We have demonstrated that a direct permeation of metal oxide NPs through the lipid bilayer of the cell membrane can occur, giving direct access to the cytoplasm. In this paper, using the powerful tool of Xenopus laevis oocytes and two electrode Voltage Clamp, we have investigated several parameters that can influence the direct crossing. The most significant of them is the NP hydrodynamic size as clearly shown by the comparison of the behaviour between Co₃O₄ and NiO NPs. By collecting biophysical membrane parameters in different conditions, we have shown that NPs that are able to cross the membrane share the ability to maintain a hydrodynamic size lower than 200 nm. The presence of this route of entrance must be considered for a better comprehension of the effect at intracellular level considering possible mechanism in order to a safer design of engineered NPs.

On the mechanism of the electrophysiological changes and membrane lesions induced by asbestos fiber exposure in Xenopus laevis oocytes

The so-called amphibole asbestos fibers are enriched with mineral iron ions, able to stimulate ROS production. We recently reported that crocidolite asbestos was able to interact with the cell membranes of Xenopus laevis oocytes, to alter their electrical membrane properties. Here, we found that applied iron ions (Fe³⁺) or H₂O₂ (for ROS generation) mimicked these effects, suggesting that at least one effect of iron-containing asbestos fiber exposure was mediated by ROS production. Furthermore, combined Fe³⁺ and H₂O₂ acted synergistically, producing a membrane effect stronger than that induced by these factors alone. Similar to crocidolite, these changes peaked within 30 minutes of incubation and vanished almost completely after 120 min. However, in the presence of cytochalasin D, which inhibits membrane actin repair mechanisms, crocidolite or applied Fe³⁺/H₂O₂ invariably produced oocyte cell death. While the electrophysiological modifications induced by crocidolite suggested a modification of an intrinsic chloride ion channel, the morphological appearance of the treated oocytes also indicated the formation of membrane “pores”; the effects of asbestos exposure may therefore consist of multiple (not necessarily exclusive) underlying mechanisms. In conclusion, using Xenopus oocytes allowed us for the first time, to focus on a specific membrane effect of crocidolite asbestos exposure, which deserves to be tested also on human lung cell lines. Much available evidence suggests that asbestos fibers damage cells through the production of ROS. Our present data confirm that crocidolite fibers can indeed trigger ROS-mediated damaging effects in the oocyte cell membrane, provided iron ions and H₂O₂ are available for ROS production.

Dual Oxidase 2 (Duox2) Regulates Pannexin 1-mediated ATP Release in Primary Human Airway Epithelial Cells via Changes in Intracellular pH and Not H2O2 Production

Human airway epithelial cells express pannexin 1 (Panx1) channels to release ATP, which regulates mucociliary clearance. Airway inflammation causes mucociliary dysfunction. Exposure of primary human airway epithelial cell cultures to IFN-γ for 48 h did not alter Panx1 protein expression but significantly decreased ATP release in response to hypotonic stress. The IFN-γ-induced functional down-regulation of Panx1 was due to the up-regulation of dual oxidase 2 (Duox2). Duox2 suppression by siRNA led to an increase in ATP release in control cells and restoration of ATP release in cells treated with IFN-γ. Both effects were reduced by the pannexin inhibitor probenecid. Duox2 up-regulation stoichiometrically increases H2O2 and proton production. H2O2 inhibited Panx1 function temporarily by formation of disulfide bonds at the thiol group of its terminal cysteine. Long-term exposure to H2O2, however, had no inhibitory effect. To assess the role of cellular acidification upon IFN-γ treatment, fully differentiated airway epithelial cells were exposed to ammonium chloride to alkalinize the cytosol. This led to a 2-fold increase in ATP release in cells treated with IFN-γ that was also inhibited by probenecid. Duox2 knockdown also partially corrected IFN-γ-mediated acidification. The direct correlation between intracellular pH and Panx1 open probability was shown in oocytes. Therefore, airway epithelial cells release less ATP in response to hypotonic stress in an inflammatory environment (IFN-γ exposure). Decreased Panx1 function is a response to cell acidification mediated by IFN-γ-induced up-regulation of Duox2, representing a novel mechanism for mucociliary dysfunction in inflammatory airway diseases.

Direct voltage control of endogenous lysophosphatidic acid G-protein-coupled receptors in Xenopus oocytes

Lysophosphatidic acid (LPA) G-protein-coupled receptors (GPCRs) play important roles in a variety of physiological and pathophysiological processes, including cell proliferation, angiogenesis, central nervous system development and carcinogenesis. Whilst many ion channels and transporters are recognized to be controlled by a change in cell membrane potential, little is known about the voltage dependence of other proteins involved in cell signalling. Here, we show that the InsP(3)-mediated Ca(2+) response stimulated by the endogenous LPA GPCR in Xenopus oocytes is potentiated by membrane depolarization. Depolarization was able to repetitively stimulate transient [Ca(2+)](i) increases after the initial agonist-evoked response. In addition, the initial rate and amplitude of the LPA-dependent Ca(2+) response were significantly modulated by the steady holding potential over the physiological range, such that the response to LPA was potentiated at depolarized potentials and inhibited at hyperpolarized potentials. Enhancement of LPA receptor-evoked Ca(2+) mobilization by membrane depolarization was observed over a wide range of agonist concentrations. Importantly, the amplitude of the depolarization-evoked intracellular Ca(2+) increase displayed an inverse relationship with agonist concentration such that the greatest effect of voltage was observed at near-threshold levels of agonist. Voltage-dependent Ca(2+) release was not induced by direct elevation of InsP(3) or by activation of heterotrimeric G-proteins in the absence of agonist, indicating that the LPA GPCR itself represents the primary site of action of membrane voltage. This novel modulation of LPA signalling by membrane potential may have important consequences for control of Ca(2+) signals both in excitable and non-excitable tissues.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Time resolved kinetics of the guinea pig Na–Ca exchanger (NCX1) expressed in Xenopus oocytes: voltage and Ca2+ dependence of pre-steady-state current investigated by photolytic Ca2+concentration jumps

Kinetic properties of the Na-Ca exchanger (guinea pig NCX1) expressed in Xenopus oocytes were investigated by patch clamp techniques and photolytic Ca(2+) concentration jumps. Current measured in oocyte membranes expressing NCX1 is almost indistinguishable from current measured in patches derived from cardiac myocytes. In the Ca-Ca exchange mode, a transient inward current is observed, whereas in the Na-Ca exchange mode, current either rises to a plateau, or at higher Ca(2+) concentration jumps, an initial transient is followed by a plateau. No comparable current was observed in membrane patches not expressing NCX1, indicating that photolytic Ca(2+) concentrations jumps activate Na-Ca exchange current. Electrical currents generated by NCX1 expressed in Xenopus oocytes are about four times larger than those obtained from cardiac myocyte membranes enabling current recording with smaller concentration jumps and/or higher time resolution. The apparent affinity for Ca(2+) of nonstationary exchange currents (0.1 mM) is much lower than that of stationary currents (6 muM). Measurement of the Ca(2+) dependence of the rising phase provides direct evidence that the association rate constant for Ca(2+) is about 5 x 10(8) M(-1) s(-1) and voltage independent. In both transport modes, the transient current decays with a voltage independent but Ca(2+)-dependent rate constant, which is about 9,000 s(-1) at saturating Ca(2+) concentrations. The voltage independence of this relaxation is maintained for Ca(2+) concentrations far below saturation. In the Ca-Ca exchange mode, the amount of charge translocated after a concentration jump is independent of the magnitude of the jump but voltage dependent, increasing at negative voltages. The slope of the charge-voltage relation is independent of the Ca(2+) concentration. Major conclusions are: (1) Photolytic Ca(2+) concentration jumps generate current related to NCX1. (2) The dissociation constant for Ca(2+) at the cytoplasmic transport binding site is about 0.1 mM. (3) The association rate constant of Ca(2+) at the cytoplasmic transport sites is high (5 x 10(-8) M(-1)s(-1)) and voltage independent. (4) The minimal five-state model (voltage independent binding reactions, one voltage independent conformational transition and one very fast voltage dependent conformational transition) used before to describe Ca(2+) translocation at saturating Ca(2+) concentrations is valid for Ca(2+) concentrations far below saturation.

Angiotensin II (AT1) receptors and NADPH oxidase regulate Cl- current elicited by beta1 integrin stretch in rabbit ventricular myocytes

Direct stretch of β1 integrin activates an outwardly rectifying, tamoxifen-sensitive Cl⁻ current (Cl⁻ SAC) via focal adhesion kinase (FAK) and/or Src. The characteristics of Cl⁻ SAC resemble those of the volume-sensitive Cl⁻ current, I_Cl,swell. Because myocyte stretch releases angiotensin II (AngII), which binds AT1 receptors (AT1R) and stimulates FAK and Src in an autocrine-paracrine loop, we tested whether AT1R and their downstream signaling cascade participate in mechanotransduction. Paramagnetic beads coated with mAb for β1-integrin were applied to myocytes and pulled upward with an electromagnet while recording whole-cell anion current. Losartan (5 μM), an AT1R competitive antagonist, blocked Cl⁻ SAC but did not significantly alter the background Cl⁻ current in the absence of integrin stretch. AT1R signaling is mediated largely by H₂O₂ produced from superoxide generated by sarcolemmal NADPH oxidase. Diphenyleneiodonium (DPI, 60 μM), a potent NADPH oxidase inhibitor, rapidly and completely blocked both Cl⁻ SAC elicited by stretch and the background Cl⁻ current. A structurally unrelated NADPH oxidase inhibitor, 4-(2-aminoethyl) benzenesulfonyl fluoride (AEBSF, 0.5 and 2 mM), also rapidly and completely blocked Cl⁻ SAC as well as a large fraction of the background Cl⁻ current. With continuing integrin stretch, Cl⁻ SAC recovered upon washout of AEBSF (2 mM). In the absence of stretch, exogenous AngII (5 nM) activated an outwardly rectifying Cl⁻ current that was rapidly and completely blocked by DPI (60 μM). Moreover, exogenous H₂O₂ (10, 100, and 500 μM), the eventual product of NADPH oxidase activity, also activated Cl⁻ SAC in the absence of stretch, whereas catalase (1,000 U/ml), an H₂O₂ scavenger, attenuated the response to stretch. Application of H₂O₂ during NADPH oxidase inhibition by either DPI (60 μM) or AEBSF (0.5 mM) did not fully reactivate Cl⁻ SAC, however. These results suggest that stretch of β1-integrin in cardiac myocytes elicits Cl⁻ SAC by activating AT1R and NADPH oxidase and, thereby, producing reactive oxygen species. In addition, NADPH oxidase may be intimately coupled to the channel responsible for Cl⁻ SAC, providing a second regulatory pathway.

Cross-talk between Native Plasmalemmal Na+/Ca2+ Exchanger and Inositol 1,4,5-Trisphosphate-sensitive Ca2+ Internal Store in Xenopus Oocytes

Because the presence of a native plasmalemmal Na+/Ca2+ exchange (NCX) activity in Xenopus laevis oocytes remains controversial, its possible functional role in these cells is poorly understood. Here, in experiments on control oocytes and oocytes overexpressing a cloned NCX1 cardiac protein, confocal microscopy combined with electrophysiological techniques reveal that these cells express an endogenous NCX protein forming a functional microdomain with inositol 1,4,5-trisphosphate receptors (InsP3R) that controls intracellular Ca2+ in a restricted subplasmalemmal space. The following data obtained in control denuded oocytes are consistent with this view: (i) reverse transcription-PCR revealed that the oocyte expresses two transcripts for the NCX1 and NCX3 isoforms; (ii) immunofluorescence experiments showed that native NCX1 and InsP3Rs are largely codistributed in discrete areas of the plasma membrane in close apposition to the cortical endoplasmic reticulum shell; (iii) when stimulated by rabbit serum, which elevates intracellular Ca2+ mediated by InsP3, voltage-clamped oocytes display a large and transient inward Ca2+ -activated chloride current, IClCa, as a result of the Ca2+ rise at the inner surface membrane; (iv) this current is significantly enhanced by KB-R7943 and by an extracellular sodium-depleted medium, two maneuvers that prevent "Ca2+ extrusion" via NCX; and (v) blocking NCX enhanced the IClCa elicited by InsP3 but not by Ca2+ photolysis in oocytes injected with the respective caged compounds. Moreover, overexpression of cardiac NCX1, confirmed by confocal microscopy, has functional consequences for the "Ca2+ influx" but not for the serum-elicited "Ca2+ efflux" mode of basal exchange activity and does not alter the number of endogenous NCX/InsP3Rs colocalization sites. Our results suggest that native NCX, because of its strategic position, may regulate InsP3-mediated Ca2+ signaling during the early phases of oocyte maturation and/or fertilization, and furthermore foreign cardiac protein is excluded from the Ca2+ microdomains surrounding the native NCX/InsP3Rs complex in the oocyte.

Hydrogen peroxide-induced current in Xenopus oocytes: current characteristics similar to those induced by the removal of extracellular calcium

The effects of hydrogen peroxide (H(2)O(2)) exposure on Xenopus oocytes were examined. An application of 1 microL of 10% H(2)O(2) to oocytes voltage-clamped in 1mL of Modified Barth Saline (MBS: final concentration of 0.01% H(2)O(2)) induced a transient ionic current. This H(2)O(2)-induced current, however, was not transient but long-lasting in a Ca(2+)-free medium. The H(2)O(2)-induced current was independent of increases in intracellular calcium. Intriguingly, the H(2)O(2)-induced current was similar in signature to one stimulated by the removal of extracellular calcium (Ca(o)(2+)-inactivated current). Both currents (a) were inactivated by 1.5mM LaCl(3), GdCl(3), CdCl(2), NiCl(2), CaCl(2), or MgCl(2), but not by LiCl or KCl, (b) exhibited reversal potential shifts to more positive values with increasing external NaCl, (c) showed linear voltage-current (I-V) relationships, and (d) were reversibly inhibited by two chloride channel blockers, 200 microM 5-nitro-2-(3-phenylpropylamino)-benzoic acid and 250 microM niflumic acid. Additionally, H(2)O(2) was still able to induce current in oocytes loaded with either catalase or N-acetyl-L-cysteine, H(2)O(2) scavengers. These results imply that H(2)O(2) induces this ionic current possibly through the activation of Ca(o)(2+)-inactivated channels by an extracellular mechanism.

Fibronectin induces pseudopod formation and cell migration by mobilizing internal Ca(2+) in blastoderm cells from medaka embryos

During vertebrate embryogenesis, blastoderm cells at the gastrula stage migrate to new locations for subsequent development. The cellular mechanism of migration was studied in medaka (Oryzias latipes) embryos at the early gastrula stage. When fibronectin was applied iontophoretically or by the puff method, cell surface protrusion known as pseudopods and a local [Ca(2+)](i) rise at the site of application were observed in approximately half of the isolated blastoderm cells. When the pseudopod adhered to the substrate, the cell body moved toward the direction of the pseudopod as [Ca(2+)](i) declined and the pseudopod was withdrawn. Local puff application of ionomycin, a Ca(2+) ionophore, in the presence of external Ca(2+) induced protrusions of the plasma membrane similar to pseudopods, suggesting that the [Ca(2+)](i) rise itself is causing pseudopod formation. On the other hand, fibronectin induced pseudopods even in the absence of external Ca(2+), suggesting the mobilization of Ca(2+) from internal stores. In accordance with this interpretation, fibronectin failed to induce [Ca(2+)](i) rises after pretreatment with thapsigargin, a blocker of Ca(2+)-ATPase in the endoplasmic reticulum. Furthermore, chelating internal Ca(2+) with BAPTA prevented fibronectin from inducing pseudopods. U-73122, a blocker of phospholipase C, completely suppressed both the [Ca(2+)](i) rise and morphological changes accompanied with fibronectin application, suggesting involvement of the inositol phosphate pathway. On the other hand, caffeine evoked a [Ca(2+)](i) rise in a great majority of the fibronectin-responsive cells and the percentage of fibronectin-responsive cells was greatly reduced by a blocking dose of ryanodine. These results suggest that fibronectin activates phospholipase C and the initial [Ca(2+)](i) rise through IP(3) receptors further activates ryanodine receptors, achieving the local [Ca(2+)](i) rise. The decay time course of [Ca(2+)](i) after fibronectin application was prolonged in the absence of external Na(+). DCB, an inhibitor of Na(+)/Ca(2+) exchangers, also prolonged the time course of the [Ca(2+)](i) decay, suggesting the contribution of Na(+)/Ca(2+) exchangers. Cytochalasin D, an inhibitor of actin polymerization by binding to the barbed end of F-actin, induced swelling in fibronectin-responsive cells and prevented fibronectin from inducing pseudopod formation without suppressing the [Ca(2+)](i) rise. These results support the hypothesis that fibronectin facilitates cell migration via pseudopod formation during gastrulation.

Stretch-independent activation of the mechanosensitive cation channel in oocytes of Xenopus laevis

Oocytes of the South African clawed toad Xenopus laevis possess in their plasma membrane a so-called stretch-activated cation channel (SAC) which is activated by gently applying positive or negative pressure (stretch) to the membrane patch containing the channels. We show here that this mechanosensitive channel acted as a spontaneously opening, stretch-independent non-selective cation channel (NSCC) in more than half of the oocytes that we investigated. In 55% of cell-attached patches (total number of patches, 58) on 30 oocytes from several different donors, we found NSCC opening events. These currents were increased by elevating the membrane voltage or raising the temperature. NSCC and SAC currents shared some properties regarding the relative conductances of Na+>Li+>Ca2+, gating behaviour and amiloride sensitivity. Stretch-independent currents could be clearly distinguished from stretch induced SAC currents by their voltage and temperature dependence. Open events of NSCC increased strongly when temperature was raised from 21 to 27 degrees C. NSCC currents could be partly inhibited by high concentrations of extracellular Gd3+ and amiloride (100 and 500 microM, respectively). We further show exemplarily that NSCC can seriously hamper investigations when oocytes are used for the expression of foreign ion channels. In particular, NSCC complicated investigations on cation channels with small conductance as we demonstrate for a 4 pS epithelial Na+ channel (ENaC) from guinea pig distal colon. Our studies on NSCCs suggest the involvement of these channels in oocyte temperature response and ion transport regulation. From our results we suggest that NSCC and SAC currents are carried by one protein operating in different modes.

Single-channel properties of IKs potassium channels

Expressed in Xenopus oocytes, KvLQT1 channel subunits yield a small, rapidly activating, voltage- dependent potassium conductance. When coexpressed with the minK gene product, a slowly activating and much larger potassium current results. Using fluctuation analysis and single-channel recordings, we have studied the currents formed by human KvLQT1 subunits alone and in conjunction with human or rat minK subunits. With low external K+, the single-channel conductances of these three channel types are estimated to be 0.7, 4.5, and 6.5 pS, respectively, based on noise analysis at 20 kHz bandwidth of currents at +50 mV. Power spectra computed over the range 0.1 Hz-20 kHz show a weak frequency dependence, consistent with current interruptions occurring on a broad range of time scales. The broad spectrum causes the apparent single-channel current value to depend on the bandwidth of the recording, and is mirrored in very "flickery" single-channel events of the channels from coexpressed KvLQT1 and human minK subunits. The increase in macroscopic current due to the presence of the minK subunit is accounted for by the increased apparent single-channel conductance it confers on the expressed channels. The rat minK subunit also confers the property that the outward single-channel current is increased by external potassium ions.

Ca2+-dependent cytotoxicity of H2O2 in L929 cells: the role of H2O2-induced Na+-influx

We investigated the mechanism by which H2O2 mediates an increase in [Na+]i in L929 cells and the relevance of this Na+ load for H2O2-induced cell injury. [Na+]i increased early after exposure to H2O2 as monitored by fluorescence spectrophotometry of cells loaded with SBFI. The omission of Na+ from the incubation buffer significantly reduced H2O2-cytotoxicity. This protection could not be mimicked by inhibition of either the Na+/H+-antiporter, the Na+/HCO3- -cotransporter, or the Na+/K+/2Cl- -cotransporter by using Hoechst 694 (0.02 mM) or 4-acetamido-4'-isothio-cyanatostilbene-2,2'-disulfonic acid (SITS) (0.02 mM) or furosemide (1 mM) and bumetanide (0.5 mM). Only the blocker of the Na+/Ca2+-exchanger bepridil (0.2 mM) significantly reduced H2O2-cytotoxicity but without interfering with the increase in [Na+]i. H2O2 caused a rapid and sustained increase in [Ca2+]i, which was significantly reduced in bepridil pretreated cells and after replacing extracellular Na+ by choline. H2O2 was found to initiate a cellular uptake of unphysiological Ni2+ by using Newport Green diacetate as fluorescent dye. Our data suggest that H2O2 mediates Na+-influx across the plasma membrane rather unspecifically than through specific transporters. The protective effect of bepridil against H2O2-cytotoxicity occurs as a consequence of a reduced cellular Ca2+-uptake. We conclude that H2O2-mediated unspecific accumulation of Na+ seems to favor a Ca2+-influx into the cells, which takes place on the Na+/Ca2+-exchanger operating in reverse mode in exchange for Na+-efflux. Therefore, H2O2-induced cellular Na+ accumulation appears to play a permissive rather than a triggering role in H2O2-mediated cell injury.

Hydrogen peroxide relaxes porcine coronary arteries by stimulating BKCa channel activity

It has been known for a number of years that neutrophils and macrophages secrete H2O2 while fighting disease, and the levels obtained within the vasculature under these conditions can reach several hundred micromolar. Because the effect of H2O2 on vascular smooth muscle is not fully understood, the present study examined the cellular effects of H2O2 on coronary arteries. Under normal ionic conditions, H2O2 relaxed arteries that were precontracted with prostaglandin F2alpha or histamine (EC50 = 252 +/- 22 microM). The effect of H2O2 was concentration dependent and endothelium independent. In contrast, H2O2 did not relax arteries contracted with 80 mM KCl, suggesting involvement of K+ channels. Single-channel patch-clamp recordings revealed that H2O2 increased the activity of the large-conductance (119 pS), Ca2+- and voltage-activated K+ (BKCa) channel. This response was mimicked by arachidonic acid and inhibited by eicosatriynoic acid, a lipoxygenase blocker, suggesting involvement of leukotrienes. Further studies on intact arteries demonstrated that eicosatriynoic acid not only blocked the vasodilatory response to H2O2 but unmasked a vasoconstrictor effect that was reversed by blocking cyclooxygenase activity with indomethacin. These findings identify a novel effector molecule, the BKCa channel, which appears to mediate the vasodilatory effect of H2O2, and suggest that a single signaling pathway, arachidonic acid metabolism, can mediate the vasodilatory and vasoconstrictor effects of H2O2 and possibly other reactive oxygen species.

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.