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      Role of Nitric Oxide and Reactive Oxygen Species in Platelet-Activating Factor-Induced Microvascular Leakage

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          Platelet-activating factor (PAF), released during inflammatory responses, increases microvascular permeability to fluid and macromolecules. Previous studies in the hamster cheek pouch microcirculation have shown that PAF-induced increases in permeability can be diminished by pretreatment with a nitric oxide synthase inhibitor indicating that nitric oxide is required for PAF to cause leakage, although nitric oxide itself does not cause leakage. We evaluated the hypothesis that PAF stimulates the production of reactive oxygen species (ROS) that then react with nitric oxide to form a new species that signals the increase in vascular permeability. The hamster cheek pouch microcirculation was used to quantify the leakage of FITC-dextran following topical application of PAF. PAF-induced leakage was markedly inhibited (70%) by prior superfusion of the cheek pouch with superoxide dismutase and catalase. Superfusing the cheek pouch with ROS generated by xanthine oxidase and hypoxanthine produced leakage similar to that observed with PAF. Pretreating the cheek pouch with a nitric oxide synthase inhibitor (N<sup>ω</sup>-nitro- L-arginine, L-NA) inhibited ROS-induced leakage by 59% and PAF-induced leakage by 64%. The effects of L-NA and superoxide dismutase plus catalase on PAF-induced leakage were not additive. Systemic administration of mercaptoethylguanidine, a peroxynitrite scavenger, inhibited PAF-induced leakage by 60%. These results suggest that PAF-induced leakage may be mediated by an interaction between ROS and NO, perhaps through the formation of peroxynitrite or one of its products.

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          Mercaptoethylguanidine and guanidine inhibitors of nitric-oxide synthase react with peroxynitrite and protect against peroxynitrite-induced oxidative damage.

          Nitric oxide (NO) produced by the inducible nitric-oxide synthase (iNOS) is responsible for some of the pathophysiological alterations during inflammation. Part of NO-related cytotoxicity is mediated by peroxynitrite, an oxidant species produced from NO and superoxide. Aminoguanidine and mercaptoethylguanidine (MEG) are inhibitors of iNOS and have anti-inflammatory properties. Here we demonstrate that MEG and related compounds are scavengers of peroxynitrite. MEG caused a dose-dependent inhibition of the peroxynitrite-induced oxidation of cytochrome c2+, hydroxylation of benzoate, and nitration of 4-hydroxyphenylacetic acid. MEG reacts with peroxynitrite with a second-order rate constant of 1900 +/- 64 M-1 s-1 at 37 degrees C. In cultured macrophages, MEG reduced the suppression of mitochondrial respiration and DNA single strand breakage in response to peroxynitrite. MEG also reduced the degree of vascular hyporeactivity in rat thoracic aortic rings exposed to peroxynitrite. The free thiol plays an important role in the scavenging effect of MEG. Aminoguanidine neither affected the oxidation of cytochrome c2+ nor reacted with ground state peroxynitrite, but inhibited the peroxynitrite-induced benzoate hydroxylation and 4-hydroxyphenylacetic acid nitration, indicating that it reacts with activated peroxynitrous acid or nitrogen dioxide. Compounds that act both as iNOS inhibitors and peroxynitrite scavengers may be useful anti-inflammatory agents.
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            Platelet activating factor modulates microvascular permeability through nitric oxide synthesis.

            Biochemical signaling determines the specific action of vasomediators in the control of microvascular permeability and tone. We tested the hypothesis that nitric oxide (NO) synthesis is involved in the biochemical signaling pathway of platelet activating factor (PAF). The cheek pouch of anesthetized male Syrian hamsters was used as a microvascular model. Vessel diameter [expressed as the ratio of the experimental to the control (e/c) diameter, with control diameter normalized to 1] and extravasation of FITC-dextran 150 by integrated optical intensity (IOI) were determined using intravital fluorescent microscopy and computer-assisted digital image analysis. N-Nitro-L-arginine methyl ester (L-NAME) at 10(-5) and 10(-6) M and N-nitro-L-mono-methyl arginine (L-NMMA) at 10(-4) and 10(-5) M were used as inhibitors of NO synthase (NOS). Acetylcholine (ACh) and bradykinin were used as indirect indices of NOS activation. L-NAME and L-NMMA attenuated both ACh and bradykinin vasodilatory effects as well as the bradykinin-induced increase in vascular permeability. Topical PAF (10(-7) M) caused vasoconstriction (mean +/- SEM e/c ratio = 0.3 +/- 0.1) and increased IOI from a normalized baseline of 0 to 67.4 +/- 12.8. Topical administration of L-NAME produced differential effects on the series-arranged arterioles but had no effect on postcapillary venular permeability. L-NMMA did not influence the basal arteriolar diameter, but at 10(-5) M it caused a small increase in permeability (IOI = 14.3 +/- 4.2). In the presence of NOS inhibitors, PAF caused a reduced arteriolar constriction (e/c ratio = 0.6 +/- 0.1) relative to PAF alone. Both NOS inhibitors reduced the PAF-stimulated increase in vasopermeability. At 10(-5) M L-NMMA, the PAF-stimulated IOI mean value was 26.1 +/- 5.2, while at 10(-4) M L-NMMA the PAF-stimulated IOI was 15.2 +/- 2.6 compared to 10(-7) M PAF (67.4 +/- 12.8). These results support our hypothesis that NO synthesis is a step in the biochemical signaling pathway of the postcapillary cellular responses to PAF.
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              Role of superoxide and nitric oxide in platelet-activating factor-induced acute lung injury, hypotension, and mortality in rats.

              To investigate the role of superoxide and nitric oxide in platelet-activating factor-induced acute lung injury, hypotension, and mortality. Prospective, randomized, controlled, experimental study. University research laboratory. Anesthetized male Wistar rats (180 to 220 g) were studied. In the first set of experiments, animals were divided into three groups. Group 1 received platelet-activating factor (2 microg/kg i.v.). Group 2 received recombinant human superoxide dismutase (50,000 U/kg i.v.) 30 mins before platelet-activating factor injection. Group 3 received vehicle agents. In the second set of experiments, animals were divided into six groups that received N(G)-nitro-L-arginine (L-NNA), a selective inhibitor of nitric oxide synthesis, or L-arginine, the physiologic precursor of nitric oxide synthesis: a) vehicles (i.v.); b) vehicle plus L-arginine (100 mg/kg i.v.); c) vehicle plus L-NNA (10 mg/kg i.v.); d) vehicle plus platelet-activating factor (2 microg/kg i.v.); e) L-arginine plus platelet-activating factor; and f) L-NNA plus platelet-activating factor. The first intravenous administration was given 5 mins before the second intravenous injection for each group. In the first set of experiments, vascular labeling with Monastral blue B demonstrated diffuse microvascular injury in the alveolar capillary beds 2 hrs after platelet-activating factor challenge. Thiobarbituric acid-reactive substances in the lung significantly increased at 2 hrs after platelet-activating factor injection. Platelet-activating factor treatment also resulted in an increased concentration of total protein, albumin, and Evans blue dye in bronchoalveolar lavage fluid at 2 hrs after administration, suggesting platelet-activating factor induction of increased alveolar permeability. The platelet-activating factor-induced alveolar microvascular injury, lipid peroxidation, and increased alveolar permeability were inhibited by pretreatment with recombinant human superoxide dismutase. Although L-NNA alone did not affect alveolar permeability in the second set of experiments, L-NNA treatment before platelet-activating factor challenge significantly aggravated platelet-activating factor-induced increased alveolar permeability 2 hrs after platelet-activating factor challenge. Platelet-activating factor also produced a rapid decrease in blood pressure that was not ameliorated by treatment with L-NNA. However, L-NNA pretreatment was associated with a significant increase in platelet-activating factor-caused mortality within 6 hrs. All rats survived with L-arginine treatment before platelet-activating factor challenge. L-NNA treatment decreased nitrate/nitrite concentration, an index of total nitric oxide production, in plasma. These results indicate that superoxide, the derived active oxygen species, and lipid peroxidation are implicated in the pathogenesis of platelet-activating factor-induced acute lung injury. Nitric oxide does not play a major role in platelet-activating factor-induced hypotension. Nitric oxide appears to play a protective role in the acute lung injury and mortality induced by platelet-activating factor.

                Author and article information

                J Vasc Res
                Journal of Vascular Research
                S. Karger AG
                June 2002
                22 August 2008
                : 39
                : 3
                : 238-245
                Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, Ohio, USA
                63689 J Vasc Res 2002;39:238–245
                © 2002 S. Karger AG, Basel

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                Page count
                Figures: 6, References: 45, Pages: 8
                Research Paper


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