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Critical Care Medicine  |   September 2012
Sodium Nitrite Mitigates Ventilator-induced Lung Injury in Rats
Author Affiliations & Notes
  • Philipp A. Pickerodt, M.D.
    *
  • Michael J. Emery, Ph.D.
  • Rachel Zarndt, B.A.
  • William Martin, M.D.
    §
  • Roland C. E. Francis, M.D.
  • Willehad Boemke, M.D.
    #
  • Erik R. Swenson, M.D.
    **
  • *Research Fellow and Staff Anesthesiologist, Department of Anesthesiology and Intensive Care Medicine, Campus Virchow-Klinikum and Campus Charité Mitte, Charité–Universitätsmedizin, Berlin, Germany, and Research Fellow, Division of Pulmonary and Critical Care Medicine, University of Washington, Seattle, Washington. Research Fellow, Research Scientist, Pulmonary and Critical Care Medicine Section, Veterans Affairs Puget Sound Health Care System, Seattle, Washington. §Resident in Internal Medicine, Division of Pulmonary and Critical Care Medicine, University of Washington. Senior Staff Anesthesiologist, #Senior Staff Anesthesiologist and Professor of Anesthesia, Department of Anesthesiology and Intensive Care Medicine, Campus Virchow-Klinikum and Campus Charité Mitte, Charité–Universitätsmedizin. **Professor of Medicine, Division of Pulmonary and Critical Care Medicine, University of Washington, and Pulmonary and Critical Care Medicine Section, Veterans Affairs Puget Sound Health Care System.
Article Information
Critical Care Medicine / Critical Care / Respiratory System
Critical Care Medicine   |   September 2012
Sodium Nitrite Mitigates Ventilator-induced Lung Injury in Rats
Anesthesiology 9 2012, Vol.117, 592-601. doi:10.1097/ALN.0b013e3182655f80
Anesthesiology 9 2012, Vol.117, 592-601. doi:10.1097/ALN.0b013e3182655f80
What We Already Know about This Topic
  • Sodium nitrite decreases numerous organ injuries, including lung injury with chlorine inhalation.

What This Article Tells Us That Is New
  • In a rat model of ventilator-induced lung injury, IV sodium nitrite protects against lung injury and mortality by a nitric oxide-dependent mechanism. The benefits of sodium nitrite protection followed a U-shaped dose–response curve.

CURRENTLY, numerous therapeutic interventions have failed to improve the outcome of patients suffering from the acute respiratory distress syndrome,1 with the exception of lung protective ventilation using low tidal volumes.2 Considerable interest in nitric oxide as a potential therapeutic option has led to extensive research since the first description of inhaled nitric oxide for the treatment of adult acute respiratory distress syndrome.3 Despite the often-observed physiologic improvements in oxygenation and pulmonary artery pressure reduction during therapeutic use of inhaled nitric oxide, evidence is lacking for any reduction of mortality. This discrepancy may be explained by more recent advances in our understanding of the complete biology of nitric oxide, which may offer new strategies in the therapeutic use of this molecule in the critically ill.4 
As an alternative nitric oxide-based therapy, the common anion salts nitrite (NO2) and nitrate (NO3) have evolved from being viewed as inert oxidative end products of nitric oxide to being recognized as important physiologic storage pools of nitric oxide within the blood and tissues.5,6 The one electron reduction of nitrite to nitric oxide and subsequent increases in nitric oxide bioavailability are favored during hypoxia and acidosis, and several proteins have been identified as nitrite reductases. These include hemoglobin and myoglobin, components of the mitochondrial respiratory chain, and the molybdo-flavoenzymes xanthine oxidoreductase (XOR) and aldehyde oxidase. Controversy remains regarding the quantitative contributions of the proposed pathways involved in the reduction of NO2to nitric oxide in vivo.  7 
Studies mimicking tissue hypoxia and acidosis in animal models of ischemia-reperfusion (I/R) injuries show that nitrite exerts potent cytoprotective effects in multiple organs, including the heart,8,9 brain,10 liver,11 and kidney.12 Interestingly, Zuckerbraun et al.  recently reported that sodium nitrite potently prevents hypoxic and inflammatory pulmonary arterial hypertension and proliferation of pulmonary vascular smooth muscle cells in rodents.13 Moreover, Yadav et al.  have shown that administration of sodium nitrite after chlorine gas inhalation decreases airway necrosis, lung edema, and alveolar protein leak.14 
To our knowledge, studies investigating the possible beneficial effects of nitrite on other lung injuries are lacking. The purpose of this study was therefore to investigate the effects of sodium nitrite on ventilator-induced lung injury (VILI). We used a rat model of VILI to test whether nitrite is protective in this lung injury; to test if its protection is mediated by nitric oxide; and to identify the enzymic catalyst responsible for nitrite reduction in the lung under these conditions.
Materials and Methods
Animal Preparation
This study was approved by the Animal Care Committee of the Veterans Affairs Puget Sound Health Care System (Seattle, Washington), and all procedures were conducted in accordance with institutional guidelines. Healthy male Sprague-Dawley rats (438 ± 37 g; mean ± SD) were studied. Anesthesia was induced with sodium pentobarbital (70 mg/kg intraperitoneally) and maintained by continuous intravenous infusion of 20 mg/kg per hour. Animals were placed supine on a heating pad and body temperature was monitored with a rectal thermometer and maintained with a heating lamp. A tracheotomy was performed and the animals were mechanically ventilated (Inspira® ASVP; Harvard Apparatus, Holliston, MA): tidal volume (VT): 6 ml/kg; positive end-expiratory pressure (PEEP) 2 cm H2O; respiratory rate 70/min; I:E ratio = 1:1; and fraction of inspired oxygen (FIO2) = 0.21. The partial pressure of mixed exhaled carbon dioxide was monitored throughout the experiment (Datex Ohmeda Capnomac®; GE Healthcare, Waukesha, WI). The left femoral artery was catheterized for measurements of systemic arterial blood pressure, arterial blood gas analyses, and cohemoximetry (ABL 800 Flex® and OSM3 Hemoximeter®; Radiometer, Copenhagen, Denmark). The left femoral and both external jugular veins were catheterized (MicroRenathane®, Braintree Scientific Inc., Braintree, MA) and separately used for administration of intravenous anesthesia and infusion of study drugs and fluids.
Drugs
Sodium nitrite (NaNO2) (Sigma-Aldrich, St. Louis, MO) was dissolved in sterile filtered phosphate-buffered saline and administered intravenously. To first study the dose–response relationship of NaNO2in VILI, sodium nitrite was given at intravenous dosages of 0.25, 2.5, and 25 μmol/kg. To determine if the effects of NaNO2on VILI are mediated by nitric oxide, the nitric oxide scavenger C-PTIO (2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide, potassium salt; 2 mg/kg IV; Sigma-Aldrich) was dissolved in 0.9% NaCl and administered in a volume of 1 ml/kg before administration of NaNO2. In equal volumes, the nitric oxide synthase (NOS) inhibitor L-NAME (Nω-nitro-L-arginine methyl ester; 10 mg/kg IV; Sigma-Aldrich) was given to study the contribution of NOS to nitrite reduction under these conditions. To further elucidate the importance of xanthine oxidoreductase as a functional nitrite reductase, the XOR inhibitor allopurinol (25 mg/kg IV; Sigma-Aldrich) was dissolved in 1N NaOH. This solution was titrated with 2N HCL to pH 9 or 10 and 0.9% NaCl was added to reach a final volume of 1 ml/kg body weight.
VILI Model
During the first hour, all animals were ventilated with a low tidal volume (6 ml/kg VT+ PEEP = 2 cm H2O). Subsequently, rats were ventilated with either LVTventilation or high tidal volume (HVT) injurious ventilation for 2 h, followed by additional 20 min with LVTventilation in all groups. For HVTventilation, PEEP was set to 0 cm H2O and tidal volumes were adjusted during 5 min to reach a peak inspiratory pressure of 35 cm H2O at the end of the inspiratory cycle. These ventilatory settings are thought to result in a model of ventilator-induced lung injury caused by high tidal volume breathing and maximal alveolar derecruitment during expiration. The respiratory rate was adjusted to achieve normocapnia in all protocols without the addition of inspired carbon dioxide. Sodium nitrite was given 20 min before HVT-ventilation and all inhibitors were slowly injected intravenously 10 min before administration of sodium nitrite. Figure 1depicts the experimental protocol.
Fig. 1. Experimental time course and measured airway pressures during the experimental time course. Each value represents mean ± SD. Bars  below graph indicate changes in ventilatory settings and fraction of inspired oxygen during the experiment. FIO2= fraction of inspired oxygen; HVT= high-tidal volume; LVT= low-tidal volume; PAirway= airway pressure; PEEP = positive end-expiratory pressure; PIP = peak inspiratory pressure; VT= tidal volume.
Fig. 1. Experimental time course and measured airway pressures during the experimental time course. Each value represents mean ± SD. Bars 
	below graph indicate changes in ventilatory settings and fraction of inspired oxygen during the experiment. FIO2= fraction of inspired oxygen; HVT= high-tidal volume; LVT= low-tidal volume; PAirway= airway pressure; PEEP = positive end-expiratory pressure; PIP = peak inspiratory pressure; VT= tidal volume.
Fig. 1. Experimental time course and measured airway pressures during the experimental time course. Each value represents mean ± SD. Bars  below graph indicate changes in ventilatory settings and fraction of inspired oxygen during the experiment. FIO2= fraction of inspired oxygen; HVT= high-tidal volume; LVT= low-tidal volume; PAirway= airway pressure; PEEP = positive end-expiratory pressure; PIP = peak inspiratory pressure; VT= tidal volume.
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Eight groups of animals were studied:
LVT: low tidal volume (6 ml/kg; 2 cm H2O PEEP) (controls) (n = 10).
HVT: high tidal volume ventilation (peak inspiratory pressure = 35 cm H2O; 0 cm H2O PEEP) (n = 14).
HVT+ NaNO2: HVTventilation + sodium nitrite (0.25; 2.5 and 25 μmol/kg IV) (n = 6–8).
HVT+ NaNO2+ nitric oxide scavenging: HVTventilation + sodium nitrite (2.5 μmol/kg IV) + C-PTIO (2 mg/kg IV) (n = 6).
HVT+ NaNO2+ inhibition of NOS: HVTventilation + sodium nitrite (2.5 μmol/kg IV) + L-NAME (10 mg/kg IV) (n = 7).
HVT+ NaNO2+ XOR inhibition: HVTventilation + sodium nitrite (2.5 μmol/kg IV) + allopurinol (25 mg/kg IV) (n = 6).
In Vivo  Measurements
Mean arterial blood pressure (MAP), heart rate, and standard respiratory parameters were continuously monitored and recorded with a digital data acquisition system (ADInstruments, Colorado Springs, CO). During the first hour, blood gases were taken in 20-min intervals (time-point, or T = 20, 40, 60). After the first blood gas measurement, a brief recruitment maneuver was performed by increasing the peak inspiratory pressure to 20 cm H2O and PEEP to 4 cm H2O (approximately 10 breaths), followed by a reduction of VTto the initial settings and decreasing PEEP to 2 cm H2O. Thereafter (T = 20), FIO2was set to 0.4 and the animals were observed for 40 min (T = 40, 60). During the following 2h of LVTor HVTventilation, blood gases were measured every 30 min (T = 90, 120, 150, 180). Subsequently, all animals were ventilated with LVTventilation for 20 min (T = 200) and a final arterial blood gas was measured. For analysis of acid-base balance, the hydrogen ion concentrations corresponding to each individual arterial pH value were averaged in each group. Subsequently, the resulting mean H+concentrations were converted into pH.
Ex Vivo  Measurements
On completion of the experiments, the animals were euthanized by an overdose of IV pentobarbital followed by measurements of volume-pressure curves using the super-syringe technique. Briefly, after standardization of lung volume history, 1 ml aliquots of air were injected into the lungs (1025SL®; Hamilton, Reno, NV), and pressure measurements were obtained 3 s after each injection until a final airway pressure of 30 cm H2O was reached. Airway pressures were measured and recorded at the external opening of the endotracheal tube.
The right lower lung lobes were used for determination of lung wet-to-dry-ratios by use of the microwave technique.15 The right upper lobe was used for histologic analysis after staining with hematoxilin and eosin. Bronchoalveolar-lavage was performed on the whole left lung and the bronchoalveolar-lavage fluid (BALF) was centrifuged at 1,000 x g for 10 min at 4°C and stored at −80°C until cytokine differentiation assays were performed by a technician blinded to the experimental protocols. Measured cytokines in BALF using ELISA were: tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and macrophage inflammatory protein 2 (MIP-2) (R&D Systems Inc., Minneapolis, MN). Protein concentration in BALF was measured with a bicinchoninic acid assay (BCA Protein Assay; Fisher Scientific, Pittsburgh, PA).
Statistical Analysis
All values are given as means ± SD. Statistical analysis was performed using NCSS software (NCSS 2004; NCSS Statistical Software, Kaysville, UT). After testing for normal distribution of the data, a two-way ANOVA (repeated measure: time; between-subjects factor: group) and multiple comparison tests were applied to quantify the effects of treatment, time, and time-treatment interactions on respiratory, hemodynamic, and blood gas measurements. A one-way ANOVA (between-subject factor: group) was applied for comparisons of differences between groups for lung edema formation, total lung compliance, and cytokine measurements. All tests were two-tailed and adjusted for multiple comparisons using Bonferroni correction. When statistical significance was indicated, post hoc  analysis of intragroup differences over time (vs.  baseline) and between-group differences (at equal time-points) were performed with paired (intragroup) and unpaired (between-groups) Student t  tests and adjusted for multiple comparisons using the Holm–Bonferroni procedure. Statistical significance was assumed at P  < 0.05.
Results
Baseline Characteristics and VILI Model
Baseline values for systemic hemodynamics and blood gas data are presented in table 1. Figure 1shows the measured airway pressures during the whole experimental time course (for clarity of the figure, only the LVTand HVTgroups are depicted). Baseline values for peak inspiratory airway pressures and PEEP during low tidal volume ventilation were comparable among all protocols, despite a small, albeit statistically significant, difference between the peak inspiratory pressure in HVT+ NaNO22.5 μmol/kg + L-NAME-, C-PTIO-, and allopurinol-treated animals (10 ± 1.0, 9.2 ± 1.6, and 8.5 ± 0.6 cm H2O, respectively), as compared with untreated HVTanimals (11.7 ± 1.6 cm H2O; P  = 0.03, 0.018, and 0.001 respectively). During HVTventilation, values for peak inspiratory pressure (range: 34.5 to 35.4 cm H2O) and PEEP (range: 0 to 0.4 cm H2O) were similar in all animals subjected to HVTventilation. Baseline respiratory rates (range: 63–66/min) were comparable during LVTamong all groups with the exception of lower baseline respiratory rates in the HVT+ NaNO22.5 μmol/kg + XOR inhibition (58 ± 3/min; P  = 0.023) as compared with LVTanimals. Respiratory rate decreased in all HVTgroups (range: 11–13/min) as compared with LVTanimals during HVTventilation. All animals survived the LVTprotocol (n = 10), whereas mortality rate was 43% (6/14) in the untreated HVTinjury (table 2). In animals subjected to LVTventilation heart rate, MAP, PaO2, and PaO2/FIO2ratios, as well as PaCO2and acid-base parameters, were normal during the whole experimental time course (fig. 2A-B). In animals subjected to untreated high VTventilation, MAP decreased with HVTventilation and remained lower as compared with LVTanimals until the end of the experiment (fig. 2A). PaO2values in the HVTgroup decreased and were lower as compared to LVTanimals (p less than 0.01), until final PaO2/FIO2ratios of 233 ± 77 were measured at the end of the experiment (fig. 2B). Lung edema formation, as measured by lung wet/dry ratios, was increased in untreated HVTanimals (6.2 ± 1.3 HVTvs.  4.1 ± 0.3 LVT; P  < 0.001), whereas respiratory system compliance was reduced as compared with LVTventilation (0.31 ± 0.04 vs.  0.48 ± 0.08 ml/cm H2O; P  < 0.001) (figs. 3A, B).
Table 1. Baseline Characteristics of Systemic Hemodynamics and Blood Gas Analysis Parameters
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Table 1. Baseline Characteristics of Systemic Hemodynamics and Blood Gas Analysis Parameters
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Table 2. Mortality
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Table 2. Mortality
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Fig. 2. Dose–response effect of intravenous sodium nitrite on ventilator-induced lung injury. (A  ) Mean arterial pressure in rats ventilated with either low tidal volume (LVT) or high tidal volume (HVT) ventilation with and without IV administration of sodium nitrite. HVT+ sodium nitrite (0.25, 2.5, and 25) groups were given IV sodium nitrite at 0.25, 2.5, and 25 μmol/kilogram body weight, respectively. Values are means ± SD; P  < 0.05 and *versus  LVT. (B  ) Time course of PaO2and PaCO2in animals ventilated with LVTor untreated HVTinjurious ventilation as compared with animals subjected to HVTventilation with IV sodium nitrite pretreatment (0.25, 2.5, and 25 μmol/kilogram body weight). Values are means ± SEM; P  < 0.05 and *versus  LVT, †versus  baseline, #versus  HVT. HVT= high-tidal volume; LVT= low-tidal volume; MAP = mean arterial pressure; NaNO2= sodium nitrite; ns = not significant.
Fig. 2. Dose–response effect of intravenous sodium nitrite on ventilator-induced lung injury. (A 
	) Mean arterial pressure in rats ventilated with either low tidal volume (LVT) or high tidal volume (HVT) ventilation with and without IV administration of sodium nitrite. HVT+ sodium nitrite (0.25, 2.5, and 25) groups were given IV sodium nitrite at 0.25, 2.5, and 25 μmol/kilogram body weight, respectively. Values are means ± SD; P 
	< 0.05 and *versus 
	LVT. (B 
	) Time course of PaO2and PaCO2in animals ventilated with LVTor untreated HVTinjurious ventilation as compared with animals subjected to HVTventilation with IV sodium nitrite pretreatment (0.25, 2.5, and 25 μmol/kilogram body weight). Values are means ± SEM; P 
	< 0.05 and *versus 
	LVT, †versus 
	baseline, #versus 
	HVT. HVT= high-tidal volume; LVT= low-tidal volume; MAP = mean arterial pressure; NaNO2= sodium nitrite; ns = not significant.
Fig. 2. Dose–response effect of intravenous sodium nitrite on ventilator-induced lung injury. (A  ) Mean arterial pressure in rats ventilated with either low tidal volume (LVT) or high tidal volume (HVT) ventilation with and without IV administration of sodium nitrite. HVT+ sodium nitrite (0.25, 2.5, and 25) groups were given IV sodium nitrite at 0.25, 2.5, and 25 μmol/kilogram body weight, respectively. Values are means ± SD; P  < 0.05 and *versus  LVT. (B  ) Time course of PaO2and PaCO2in animals ventilated with LVTor untreated HVTinjurious ventilation as compared with animals subjected to HVTventilation with IV sodium nitrite pretreatment (0.25, 2.5, and 25 μmol/kilogram body weight). Values are means ± SEM; P  < 0.05 and *versus  LVT, †versus  baseline, #versus  HVT. HVT= high-tidal volume; LVT= low-tidal volume; MAP = mean arterial pressure; NaNO2= sodium nitrite; ns = not significant.
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Fig. 3. (A  ) Lung edema formation as measured by lung wet/dry ratio after 200 min of mechanical ventilation. (B  ) Whole respiratory system compliance after completion of the experiment (closed chest). Each value represents mean ± SD; P  < 0.05 and *versus  low-tidal volume ventilation, #versus  high-tidal volume ventilation. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Numbers within error bars  depict sample size. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; L-NAME = Nω-nitro-L-arginine methyl ester; NaNO2= sodium nitrite; NO = nitric oxide; VT= tidal volume.
Fig. 3. (A 
	) Lung edema formation as measured by lung wet/dry ratio after 200 min of mechanical ventilation. (B 
	) Whole respiratory system compliance after completion of the experiment (closed chest). Each value represents mean ± SD; P 
	< 0.05 and *versus 
	low-tidal volume ventilation, #versus 
	high-tidal volume ventilation. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Numbers within error bars 
	depict sample size. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; L-NAME = Nω-nitro-L-arginine methyl ester; NaNO2= sodium nitrite; NO = nitric oxide; VT= tidal volume.
Fig. 3. (A  ) Lung edema formation as measured by lung wet/dry ratio after 200 min of mechanical ventilation. (B  ) Whole respiratory system compliance after completion of the experiment (closed chest). Each value represents mean ± SD; P  < 0.05 and *versus  low-tidal volume ventilation, #versus  high-tidal volume ventilation. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Numbers within error bars  depict sample size. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; L-NAME = Nω-nitro-L-arginine methyl ester; NaNO2= sodium nitrite; NO = nitric oxide; VT= tidal volume.
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Total protein concentration in BALF was increased in rats ventilated with high tidal volume ventilation when compared with LVTventilated animals (3.2 ± 0.4 vs.  1.6 ± 0.5 g/L; P  < 0.001). TNF-α was not detected in BALF of animals ventilated with LVTor with HVTventilation. IL-1β and MIP-2 were not different between the LVTand the high tidal volume protocol (fig. 4).
Fig. 4. (A  ) Concentration of tumor necrosis factor (TNF)-α. (B  ) Concentration of interleukin-1β. (C  ) Concentration of macrophage inflammatory protein-2. (D  ) Total protein concentration in bronchoalveolar lavage fluid. Rats ventilated with either 6 ml/kg tidal volume or high tidal volume injurious ventilation for 2 h with and without administration of sodium nitrite and alterations of nitric oxide bioavailability. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Values are means ± SD; P  < 0.05 and *versus  low tidal volume ventilation, #versus  high-tidal volume ventilation. Numbers within error bars  depict sample size. Note that 100signifies a value below the detection threshold. Allo = allopurinol; BAL = bronchoalveolar lavage; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; IL = interleukin; L-NAME = Nω-nitro-L-arginine methyl ester; MIP = macrophage inflammatory protein; NaNO2= sodium nitrite; NO = nitric oxide; TNF = tumor necrosis factor; VT= tidal volume.
Fig. 4. (A 
	) Concentration of tumor necrosis factor (TNF)-α. (B 
	) Concentration of interleukin-1β. (C 
	) Concentration of macrophage inflammatory protein-2. (D 
	) Total protein concentration in bronchoalveolar lavage fluid. Rats ventilated with either 6 ml/kg tidal volume or high tidal volume injurious ventilation for 2 h with and without administration of sodium nitrite and alterations of nitric oxide bioavailability. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Values are means ± SD; P 
	< 0.05 and *versus 
	low tidal volume ventilation, #versus 
	high-tidal volume ventilation. Numbers within error bars 
	depict sample size. Note that 100signifies a value below the detection threshold. Allo = allopurinol; BAL = bronchoalveolar lavage; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; IL = interleukin; L-NAME = Nω-nitro-L-arginine methyl ester; MIP = macrophage inflammatory protein; NaNO2= sodium nitrite; NO = nitric oxide; TNF = tumor necrosis factor; VT= tidal volume.
Fig. 4. (A  ) Concentration of tumor necrosis factor (TNF)-α. (B  ) Concentration of interleukin-1β. (C  ) Concentration of macrophage inflammatory protein-2. (D  ) Total protein concentration in bronchoalveolar lavage fluid. Rats ventilated with either 6 ml/kg tidal volume or high tidal volume injurious ventilation for 2 h with and without administration of sodium nitrite and alterations of nitric oxide bioavailability. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Values are means ± SD; P  < 0.05 and *versus  low tidal volume ventilation, #versus  high-tidal volume ventilation. Numbers within error bars  depict sample size. Note that 100signifies a value below the detection threshold. Allo = allopurinol; BAL = bronchoalveolar lavage; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; IL = interleukin; L-NAME = Nω-nitro-L-arginine methyl ester; MIP = macrophage inflammatory protein; NaNO2= sodium nitrite; NO = nitric oxide; TNF = tumor necrosis factor; VT= tidal volume.
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Effects of Sodium Nitrite on Hemodynamics and Lung Injury
Administration of IV NaNO2at 0.25, 2.5, or 25 μmol/kg had no effects on MAP or heart rate during LVTventilation (table 1and fig. 2A). MAP was lower with injurious ventilation regardless of the dosage of sodium nitrite as compared with LVTanimals, but did not differ from the HVTgroup (fig. 2A).
Repeated measures ANOVA show a strong treatment effect of sodium nitrite on preservation of PaO2as compared with untreated HVTanimals. Yet, time-course data (fig. 2B) and post hoc  analysis show that this protection was lost with 0.25 and 25 μmol/kg sodium nitrite at the end of the experiment (PaO2: 118 ± 45 and 112 ± 41 mmHg vs.  86 ± 34 mmHg for untreated HVT; P  = 0.154 and 0.2, respectively). In contrast, PaO2was maintained with sodium nitrite at 2.5 μmol/kg (163 ± 41 mmHg; P  < 0.01). Here, PaO2values were entirely comparable with control LVTanimals during the whole experimental time course (fig. 2B). In line with these findings, 2.5 μmol/kg NaNO2reduced edema formation as compared with untreated HVTanimals (4.6 ± 0.4 vs.  6.2 ± 1.3; P  = 0.01) (fig. 3A). Whole respiratory system compliance was best preserved with sodium nitrite at IV 2.5 μmol/kg, whereas IV 0.25 and 25 μmol/kg NaNO2did not prevent the decrease in compliance as compared with the untreated high tidal volume ventilation (fig. 3B). These findings were mirrored by the histomorphological findings of lung tissues (hematoxilin and eosin staining of right upper lobes), demonstrating a hemorrhagic alveolar edema and atelectasis formation in the HVTgroup. Except for mild interstitial edema, the HVT+ NaNO22.5 and the LVTcontrol group were comparable (fig. 5). Mortality was reduced to 25% with sodium nitrite at 0.25 μmol/kg and was 0% both for NaNO2at 2.5 and 25 μmol/kg (table 2).
Fig. 5. Hematoxilin and eosin staining of right upper lobes, magnification: x200. Untreated injurious ventilation with high tidal volumes caused a hemorrhagic alveolar edema and atelectasis formation. Except for mild interstitial edema, animals treated with 2.5 μmol/kg sodium nitrite (high-tidal volume ventilation + sodium nitrite [2.5]) and the low-tidal volume ventilation control group are comparable. HVT= high tidal volume; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 5. Hematoxilin and eosin staining of right upper lobes, magnification: x200. Untreated injurious ventilation with high tidal volumes caused a hemorrhagic alveolar edema and atelectasis formation. Except for mild interstitial edema, animals treated with 2.5 μmol/kg sodium nitrite (high-tidal volume ventilation + sodium nitrite [2.5]) and the low-tidal volume ventilation control group are comparable. HVT= high tidal volume; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 5. Hematoxilin and eosin staining of right upper lobes, magnification: x200. Untreated injurious ventilation with high tidal volumes caused a hemorrhagic alveolar edema and atelectasis formation. Except for mild interstitial edema, animals treated with 2.5 μmol/kg sodium nitrite (high-tidal volume ventilation + sodium nitrite [2.5]) and the low-tidal volume ventilation control group are comparable. HVT= high tidal volume; LVT= low tidal volume; NaNO2= sodium nitrite.
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Total protein concentration in BALF as well as TNF-α (not detected), IL-1β, and MIP 2 were comparable with the untreated HVTinjury, regardless of the applied NaNO2dose (fig. 4).
Effects of Altering Nitric Oxide Bioavailability on Lung Injury with Sodium Nitrite
Acutely decreasing nitric oxide bioavailability by scavenging of nitric oxide (C-PTIO) or by inhibition of NOS (L-NAME) abolished the beneficial effects of NaNO2on gas exchange after VILI (fig. 6). Lung edema formation with NaNO2+ L-NAME was comparable with untreated HVTanimals and increased with NaNO2+ C-PTIO (fig. 3A). Figure 3B depicts the changes in compliance with these inhibitors. With L-NAME, respiratory system compliance was reduced to 0.27 ± 0.05 ml/cm H2O. C-PTIO elicited a similar reduction in whole respiratory system compliance (0.27 ± 0.03 ml/cm H2O), and with both agents compliance was comparable with the untreated HVTgroup (HVT: 0.31 ± 0.04 ml/cm H2O; P  = 0.18 and 0.12, respectively). When NaNO2(2.5μmol/kg) was administered after inhibition of xanthine oxidoreductase (allopurinol), the response to HVTventilation as measured by changes in PaO2and whole respiratory system compliance and lung edema formation as measured by lung wet-to-dry-ratios were comparable with untreated HVTanimals (fig. 6and figs. 3A and B, respectively). Mortality was 83% both with the nitric oxide scavenger C-PTIO and XOR inhibition by allopurinol. With L-NAME, mortality was comparable with the untreated HVTanimals (43%) (table 2).
Fig. 6. Effects of nitric oxide scavenger C-PTIO, unselective inhibition of nitric oxide synthases, and xanthine oxidoreductase inhibition on PaO2(fraction of inspired oxygen = 0.4) at baseline and after ventilator-induced lung injury. Values are means ± SD; P  < 0.05 and †versus  baseline, #versus  high-tidal volume ventilation. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; HVT= high tidal volume; L-NAME = Nω-nitro-L-arginine methyl ester; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 6. Effects of nitric oxide scavenger C-PTIO, unselective inhibition of nitric oxide synthases, and xanthine oxidoreductase inhibition on PaO2(fraction of inspired oxygen = 0.4) at baseline and after ventilator-induced lung injury. Values are means ± SD; P 
	< 0.05 and †versus 
	baseline, #versus 
	high-tidal volume ventilation. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; HVT= high tidal volume; L-NAME = Nω-nitro-L-arginine methyl ester; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 6. Effects of nitric oxide scavenger C-PTIO, unselective inhibition of nitric oxide synthases, and xanthine oxidoreductase inhibition on PaO2(fraction of inspired oxygen = 0.4) at baseline and after ventilator-induced lung injury. Values are means ± SD; P  < 0.05 and †versus  baseline, #versus  high-tidal volume ventilation. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; HVT= high tidal volume; L-NAME = Nω-nitro-L-arginine methyl ester; LVT= low tidal volume; NaNO2= sodium nitrite.
×
The concentrations of TNF-α (not detected), IL-1β, and MIP-2 in BALF of animals treated with NaNO2(2.5 μmol/kg) after alteration of nitric oxide bioavailability by C-PTIO, L-NAME, or XOR inhibition were comparable with the untreated high tidal volume injury (fig. 4). With NaNO2+ nitric oxide scavenging by C-PTIO, the total protein concentration in bronchoalveolar fluid was increased as compared with HVTanimals. This difference in BALF protein concentration was statistically nonsignificant with IV administration of 2.5 μmol/kg NaNO2after complete inhibition of endogenous NOS or XOR inhibition when compared with HVTanimals.
Discussion
Our key findings are that sodium nitrite reduces ventilator-induced lung injury in rats by a nitric oxide dependent mechanism and that nitric oxide synthases and xanthine oxidoreductase act as functional nitrite reductases in this injury model.
To test whether NO2induced protection in systemic organ I/R injuries successfully translates to a clinically relevant lung injury,16 we chose the ventilator-induced lung injury model. First, in dose–response experiments, NaNO2pretreatment limits gas exchange deterioration, edema formation, and histologic injury with HVTventilation (figs. 25). In this model of severe injury, nitrite reduced mortality from 43% to 25% at 0.25 μmol/kg and to 0% with 2.5 and 25 μmol/kg.
The benefits of nitrite on VILI did not follow a linear dose–response relationship, but rather a U-shaped curve similar to that demonstrated by Duranski et al.  in mouse hepatic and cardiac I/R injuries using comparable dosages.11 In rat brain I/R injury, neuroprotection was maximal at 10-fold higher nitrite dosing, but was reduced or absent at higher and lower dosages.10 As for the lung, we speculate that at higher nitrite dosages, some beneficial effects might be lost with increasing tissue nitric oxide generation. With increasing nitrite concentrations, NO2-derived nitric oxide may become an important source of injurious peroxynitrite-mediated oxidant damage.17 In line with these findings, numerous studies now implicate excessive nitric oxide with VILI pathogenesis.18,19 Therefore, nitric oxide may be cytotoprotective at prevailing physiologic levels, whereas supra-physiologic levels may mediate cellular necrosis and apoptosis.20 
We used the nitric oxide scavenger C-PTIO to test if injury reduction with nitrite was mediated by nitric oxide. As others have shown in I/R injury,8,9 the protection by NaNO2was completely blocked by C-PTIO (figs. 3, 4, 6), suggesting that it was not NO2per se  mediating the beneficial effects, but rather NaNO2-derived nitric oxide.
We hypothesized that nonselective inhibition of NOS (with L-NAME) would not alter injury reduction by nitrite, because nitrite-derived nitric oxide formation is thought to be independent of NOS activity.21 Surprisingly, gas exchange, edema formation, and respiratory system compliance were comparable with untreated HVTanimals, when NOS was inhibited before administration of NaNO2(figs. 3, 4, 6). In support of our findings, in vitro  studies have established endothelial NOS (eNOS) as a functional nitrite reductase under hypoxic conditions,22,23 thus providing a parallel pathway for endothelial cells to increase nitric oxide bioavailability when conventional L-arginine-derived nitric oxide synthesis is suppressed.24 Milsom et al.  showed eNOS acts protectively as a nitrite reductase in vivo  against renal I/R injury in mice.12 As to the role of NOS in VILI, experimental models have provided conflicting results.25 Vaporidi et al.  provided in vivo  evidence for eNOS uncoupling in VILI, leading to superoxide production and tissue injury.26 Because evaluating NOS pathways in VILI was not the main focus of our study, we did not quantify the contribution of eNOS in nitrite-mediated protection. However, the failure of higher nitrite concentrations to reduce injury in our experiments may have been due to uncoupled eNOS superoxide production.
Our finding that xanthine oxidoreductase inhibition with allopurinol abolished the nitrite-induced reduction of VILI supports evidence suggesting that XOR is a cardinal nitrite reductase.7,17,27 XOR has a similar structure to bacterial nitrite reductase28 and is present in mammalian lungs,29 vessels, and erythrocytes,22 and its enzymatic activity in the lung is activated by mechanical stress, as occurs in VILI.30 In rats, NO2-derived protection against in vivo  kidney I/R injury was mediated in part by XOR acting as a nitrite reductase.31 Casey et al.  found pulmonary vasodilating effects of NaNO2in rats to be abrogated by allopurinol.32 In addition, Zuckerbraun et al.  showed that antiproliferative effects of nitrite in hypoxia- and monocrotaline-induced pulmonary hypertension are mediated by XOR nitrite–reductase activity by nitric oxide-dependent mechanisms. Their results indicate that more than 70% of pulmonary nitrite bioactivation to nitric oxide is mediated by XOR in vivo  .13 In contrast to this beneficial role, XOR has been implicated in oxidative tissue damage in a variety of organs. In the lung, several studies report protection against injurious stimuli with allopurinol administration via  reduced radical oxygen species formation.33 Interestingly, in isolated enzyme studies the observed rate of nitric oxide generation from XOR decreases over time.7 This may be caused by a feedback inhibition of nitric oxide on XOR.34 More recently, Godber et al.  describe “suicide inactivation” of XOR during reduction of NO2to nitric oxide,35 a mechanism that would explain both enzymic NO2reduction via  XOR in nitrite-derived cytoprotection, while on the other hand contributing to cellular injury by promoting radical oxygen species formation.
With regards to the cytokines in lavage fluid, we could not detect statistically significant protective effects of nitrite. No TNF-α was detected regardless of the ventilatory regime. In addition, we found only moderate amounts of IL-1β and MIP-2 in rats subjected to HVTventilation for 2 h, comparable with that found in animals ventilated with 6 ml/kg. This is in contrast to several studies suggesting release of proinflammatory cytokines in response to excessive mechanical strain by high tidal volumes contributes to VILI.36 In a critical reappraisal of these data, Ricard et al.  pointed out that most of these studies were done in preinjured lungs or in ex vivo  lung preparations.37 Furthermore, they could not reproduce in vivo  the increase in cytokine concentrations measured ex vivo  in isolated, unperfused lungs.38 As in our study, Ricard et al.  did not detect TNF-α in BALF of rats ventilated in vivo  for 2 h with a comparable tidal volume of 42 ml/kg.37 
Our study has several limitations: First, we did not address whether nitrite protection in VILI occurs when administered after injury onset. This is important with regards to clinical significance in patients with established lung injury and merits further investigation.
Second, we used a VILI model caused by high tidal volumes and maximal alveolar derecruitment. Therefore, our results cannot separate nitrite-derived protection against large tidal volumes or against atelectasis-induced lung injury. Atelectasis is known to potentiate acute lung injury and contributes to acute lung injury during LVTbreathing in otherwise healthy lungs.39 Thus, further studies are needed in order to test whether NaNO2might reduce atelectrauma because of insufficient PEEP in healthy and injured lungs.
Third, we did not address another established possible nitrite reductase: red cell deoxyhemoglobin. Hemoglobin is an allosterically regulated nitrite reductase,40 with a maximal rate of nitrite reduction occurring at a hemoglobin saturation of 50%.41 We measured right ventricular hemoglobin–oxygen saturations as low as 57–70% in uninjured, anesthetized rats at FIO2= 0.4 (data not shown), well within the proposed range of deoxy-mediated nitrite reduction. Therefore, we cannot exclude a possible contribution of this mechanism to nitrite-derived protection. In contrast, we have shown that nitrite inhibits hypoxic pulmonary vasoconstriction in buffer-perfused lungs, but not when erythrocyte concentration approaches physiologic levels, despite enhanced nitrite degradation.42 These findings support one major criticism of the deoxy-hemoglobin nitrite reductase paradigm, namely whether autocapture by oxy-heme groups precludes nitric oxide efflux from erythrocytes.43,44 In addition, although XOR concentrations in blood are below μM (when hemoglobin concentration is approximately 2.5 mM), nitric oxide generation from nitrite is 100-fold higher in tissues than in blood.7 Further evidence showing that it is not hemoglobin-derived nitric oxide generation via  deoxyhemoglobin nitrite reductase activity, but rather tissue NO2reductase activity, is the finding that inhaled nitrite, but not infused nitrite, reverses hemolysis-induced pulmonary vasoconstriction, likely by hemoglobin autocapture of vascularly generated nitric oxide.45 
Fourth, we did not measure pulmonary artery pressure or vascular resistance and cannot exclude that nitrite-mediated reductions of pulmonary vascular resistance and pressure may have contributed to lessen the injury. However, with respect to the in vivo  pulmonary vasodilator response to nitrite, our protective dose in VILI is 12-fold lower than the half maximal dose for pulmonary vasodilation in normal rat lungs (2.5 vs.  30 μmol/kg).32 Limitation of pulmonary vascular pressures attenuates lung damage during HVTventilation,46,47,48 thus theoretically edema formation should decrease with increasing NO2dosages. Because we show a U-shaped dose–response curve for nitrite in reduction of edema formation, this mechanism would not fully account for the observed reduction of VILI by NaNO2. Because we did not assess indices of right or left heart dysfunction, we cannot exclude the possibility that the increase in transpulmonary pressure with HVTventilation – by increasing pulmonary vascular resistance, right ventricular afterload, and subsequent reduction of ventricular ejection – in conjunction with the reduction of MAP (fig. 2A) led to a critical imbalance between right ventricular oxygen consumption and demand. In human acute respiratory distress syndrome, mechanical augmentation of right ventricular afterload during inspiration leads to acute right ventricular dilatation, abnormal septal motion, and low cardiac output.49 Of note, during LVTventilation without PEEP maximal derecruitment and atelectasis formation also increases pulmonary vascular resistance, potentially leading to right ventricular failure in otherwise uninjured rats.39 These pathologic effects of atelectasis are mediated by alveolar hypoxia and increased hypoxic pulmonary vasoconstriction and are independent of direct mechanical effects of atelectasis, because they are attenuated by use of higher FIO2(more than 0.4) as applied in our study.50 
In aggregate, there is ample evidence for nitrite reduction to nitric oxide within the circulation and tissues to be a potent mechanism of reduction of I/R injuries. We show that nitrite-induced protection against VILI is mediated by nitric oxide and dependent on both xanthine oxidoreductase and nitric oxide synthases in the rat. Further studies are indicated to identify other potential pathways of nitrite reduction within the pulmonary circulation or tissue and to address the quantitative rates of nitric oxide formation from nitrite within the lung in health and disease.
The authors thank Rainer Mohnhaupt, Dipl.-Ing., Research Scientist, Charité-Universitätsmedizin Berlin, Berlin School of Public Health, Berline, Germany, for consulting in statistics.
References
Ware LB, Matthay MA: The acute respiratory distress syndrome. N Engl J Med 2000; 342:1334–49
Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000;342:1301–8
Rossaint R, Falke KJ, López F, Slama K, Pison U, Zapol WM: Inhaled nitric oxide for the adult respiratory distress syndrome. N Engl J Med 1993; 328:399–405
Creagh-Brown B, Griffiths M, Evans T: Bench-to-bedside review: Inhaled nitric oxide therapy in adults. Crit Care 2009; 13:221–9
Lundberg JO, Weitzberg E, Gladwin MT: The nitrate-nitrite-nitric oxide pathway in physiology and therapeutics. Nat Rev Drug Discov 2008; 7:156–67
Weitzberg E, Hezel M, Lundberg JO: Nitrate-nitrite-nitric oxide pathway: Implications for anesthesiology and intensive care. ANESTHESIOLOGY 2010; 113:1460–75
Li H, Cui H, Kundu TK, Alzawahra W, Zweier JL: Nitric oxide production from nitrite occurs primarily in tissues not in the blood: Critical role of xanthine oxidase and aldehyde oxidase. J Biol Chem 2008; 283:17855–63
Webb A, Bond R, McLean P, Uppal R, Benjamin N, Ahluwalia A: Reduction of nitrite to nitric oxide during ischemia protects against myocardial ischemia-reperfusion damage. Proc Natl Acad Sci U S A 2004; 101:13683–8
Dezfulian C, Shiva S, Alekseyenko A, Pendyal A, Beiser DG, Munasinghe JP, Anderson SA, Chesley CF, Vanden Hoek TL, Gladwin MT: Nitrite therapy after cardiac arrest reduces reactive oxygen species generation, improves cardiac and neurological function, and enhances survival via  reversible inhibition of mitochondrial complex I. Circulation 2009; 120:897–905
Jung KH, Chu K, Ko SY, Lee ST, Sinn DI, Park DK, Kim JM, Song EC, Kim M, Roh JK: Early intravenous infusion of sodium nitrite protects brain against in vivo  ischemia-reperfusion injury. Stroke 2006; 37:2744–50
Duranski MR, Greer JJ, Dejam A, Jaganmohan S, Hogg N, Langston W, Patel RP, Yet SF, Wang X, Kevil CG, Gladwin MT, Lefer DJ: Cytoprotective effects of nitrite during in vivo  ischemia-reperfusion of the heart and liver. J Clin Invest 2005; 115:1232–40
Milsom AB, Patel NS, Mazzon E, Tripatara P, Storey A, Mota-Filipe H, Sepodes B, Webb AJ, Cuzzocrea S, Hobbs AJ, Thiemermann C, Ahluwalia A: Role for endothelial nitric oxide synthase in nitrite-induced protection against renal ischemia-reperfusion injury in mice. Nitric Oxide 2010; 22:141–8
Zuckerbraun BS, Shiva S, Ifedigbo E, Mathier MA, Mollen KP, Rao J, Bauer PM, Choi JJ, Curtis E, Choi AM, Gladwin MT: Nitrite potently inhibits hypoxic and inflammatory pulmonary arterial hypertension and smooth muscle proliferation via  xanthine oxidoreductase-dependent nitric oxide generation. Circulation 2009; 121:98–109
Yadav AK, Doran SF, Samal AA, Sharma R, Vedagiri K, Postlethwait EM, Squadrito GL, Fanucchi MV, Roberts LJ 2nd, Patel RP, Matalon S: Mitigation of chlorine gas lung injury in rats by postexposure of sodium nitrite. Am J Physiol Lung Cell Mol Physiol 2011; 300:L362–9
Peterson BT, Brooks JA, Zack AG: Use of microwave oven for determination of postmortem water volume of lungs. J Appl Physiol 1982; 52:1661–3
Dreyfuss D, Saumon G: Ventilator-induced lung injury: Lessons from experimental studies. Am J Respir Crit Care Med 1998; 157:294–323
Li H, Samouilov A, Liu X, Zweier JL: Characterization of the magnitude and kinetics of xanthine oxidase-catalyzed nitrate reduction: Evaluation of its role in nitrite and nitric oxide generation in anoxic tissues. Biochemistry 2003; 42:1150–9
Kuebler WM, Uhlig U, Goldmann T, Schael G, Kerem A, Exner K, Martin C, Vollmer E, Uhlig S: Stretch activates nitric oxide production in pulmonary vascular endothelial cells in situ  . Am J Respir Crit Care Med 2003; 168:1391–8
Broccard AF, Feihl F, Vannay C, Markert M, Hotchkiss J, Schaller MD: Effects of L-NAME and inhaled nitric oxide on ventilator-induced lung injury in isolated, perfused rabbit lungs. Crit Care Med 2004; 32:1872–8
Calvert JW, Lefer DJ: Clinical translation of nitrite therapy for cardiovascular diseases. Nitric Oxide 2010; 22:91–7
Zweier JL, Wang P, Samouilov A, Kuppusamy P: Enzyme-independent formation of nitric oxide in biological tissues. Nat Med 1995; 1:804–9
Webb AJ, Milsom AB, Rathod KS, Chu WL, Qureshi S, Lovell MJ, Lecomte FM, Perrett D, Raimondo C, Khoshbin E, Ahmed Z, Uppal R, Benjamin N, Hobbs AJ, Ahluwalia A: Mechanisms underlying erythrocyte and endothelial nitrite reduction to nitric oxide in hypoxia: Role for xanthine oxidoreductase and endothelial nitric oxide synthase. Circ Res 2008; 103:957–64
Mikula I, Durocher S, Martasek P, Mutus B, Slama-Schwok A: Isoform-specific differences in the nitrite reductase activity of nitric oxide synthases under hypoxia. Biochem J 2009; 418:673–82
Giraldez RR, Panda A, Xia Y, Sanders SP, Zweier JL: Decreased nitric-oxide synthase activity causes impaired endothelium-dependent relaxation in the postischemic heart. J Biol Chem 1997; 272:21420–6
Ware LB, Summar ML: Understanding the role of NOS-3 in ventilator-induced lung injury: Don't take no for an answer. Am J Physiol Lung Cell Mol Physiol 2010; 299:L147–9
Vaporidi K, Francis RC, Bloch KD, Zapol WM: Nitric oxide synthase 3 contributes to ventilator-induced lung injury. Am J Physiol Lung Cell Mol Physiol 2010; 299:L150–9
Millar TM, Stevens CR, Benjamin N, Eisenthal R, Harrison R, Blake DR: Xanthine oxidoreductase catalyses the reduction of nitrates and nitrite to nitric oxide under hypoxic conditions. FEBS Lett 1998; 427:225–8
Mitchell PC: Molybdenum in enzymatic and heterogeneous catalysis. J Inorg Biochem 1986; 28:107–23
Saugstad OD: Role of xanthine oxidase and its inhibitor in hypoxia: Reoxygenation injury. Pediatrics 1996; 98:103–7
Abdulnour RE, Peng X, Finigan JH, Han EJ, Hasan EJ, Birukov KG, Reddy SP, Watkins JE 3rd, Kayyali US, Garcia JG, Tuder RM, Hassoun PM: Mechanical stress activates xanthine oxidoreductase through MAP kinase-dependent pathways. Am J Physiol Lung Cell Mol Physiol 2006; 291:L345–53
Tripatara P, Patel NS, Webb A, Rathod K, Lecomte FM, Mazzon E, Cuzzocrea S, Yaqoob MM, Ahluwalia A, Thiemermann C: Nitrite-derived nitric oxide protects the rat kidney against ischemia/reperfusion injury in vivo  : Role for xanthine oxidoreductase. J Am Soc Nephrol 2007; 18:570–80
Casey DB, Badejo AM Jr., Dhaliwal JS, Murthy SN, Hyman AL, Nossaman BD, Kadowitz PJ: Pulmonary vasodilator responses to sodium nitrite are mediated by an allopurinol-sensitive mechanism in the rat. Am J Physiol Heart Circ Physiol 2009; 296:H524–33
Boueiz A, Damarla M, Hassoun PM: Xanthine oxidoreductase in respiratory and cardiovascular disorders. Am J Physiol Lung Cell Mol Physiol 2008; 294:L830–40
Cote CG, Yu FS, Zulueta JJ, Vosatka RJ, Hassoun PM: Regulation of intracellular xanthine oxidase by endothelial-derived nitric oxide. Am J Physiol 1996; 271:L869–74
Godber BL, Doel JJ, Goult TA, Eisenthal R, Harrison R: Suicide inactivation of xanthine oxidoreductase during reduction of inorganic nitrite to nitric oxide. Biochem J 2001; 358:325–33
Frank JA, Parsons PE, Matthay MA: Pathogenetic significance of biological markers of ventilator-associated lung injury in experimental and clinical studies. Chest 2006; 130:1904–14
Ricard JD, Dreyfuss D, Saumon G: Production of inflammatory cytokines in ventilator-induced lung injury: A reappraisal. Am J Respir Crit Care Med 2001; 163:1176–80
Tremblay L, Valenza F, Ribeiro SP, Li J, Slutsky AS: Injurious ventilatory strategies increase cytokines and c-fos m-RNA expression in an isolated rat lung model. J Clin Invest 1997; 99:944–52
Duggan M, McCaul CL, McNamara PJ, Engelberts D, Ackerley C, Kavanagh BP: Atelectasis causes vascular leak and lethal right ventricular failure in uninjured rat lungs. Am J Respir Crit Care Med 2003; 167:1633–40
Cosby K, Partovi KS, Crawford JH, Patel RP, Reiter CD, Martyr S, Yang BK, Waclawiw MA, Zalos G, Xu X, Huang KT, Shields H, Kim-Shapiro DB, Schechter AN, Cannon RO 3rd, Gladwin MT: Nitrite reduction to nitric oxide by deoxyhemoglobin vasodilates the human circulation. Nat Med 2003; 9:1498–505
Gladwin MT, Raat NJ, Shiva S, Dezfulian C, Hogg N, Kim-Shapiro DB, Patel RP: Nitrite as a vascular endocrine nitric oxide reservoir that contributes to hypoxic signaling, cytoprotection, and vasodilation. Am J Physiol Heart Circ Physiol 2006; 291:H2026–35
Deem S, Min JH, Moulding JD, Eveland R, Swenson ER: Red blood cells prevent inhibition of hypoxic pulmonary vasoconstriction by nitrite in isolated, perfused rat lungs. Am J Physiol Heart Circ Physiol 2007; 292:H963–70
Joshi MS, Ferguson TB Jr, Han TH, Hyduke DR, Liao JC, Rassaf T, Bryan N, Feelisch M, Lancaster JR Jr: Nitric oxide is consumed, rather than conserved, by reaction with oxyhemoglobin under physiological conditions. Proc Natl Acad Sci U S A 2002; 99:10341–6
Mikulski R, Tu C, Swenson ER, Silverman DN: Reactions of nitrite in erythrocyte suspensions measured by membrane inlet mass spectrometry. Free Radic Biol Med 2010; 48:325–31
Blood AB, Schroeder HJ, Terry MH, Merrill-Henry J, Bragg SL, Vrancken K, Liu T, Herring JL, Sowers LC, Wilson SM, Power GG: Inhaled nitrite reverses hemolysis-induced pulmonary vasoconstriction in newborn lambs without blood participation. Circulation 2011; 123:605–12
Fu Z, Costello ML, Tsukimoto K, Prediletto R, Elliott AR, Mathieu-Costello O, West JB: High lung volume increases stress failure in pulmonary capillaries. J Appl Physiol 1992; 73:123–33
Broccard AF, Hotchkiss JR, Kuwayama N, Olson DA, Jamal S, Wangensteen DO, Marini JJ: Consequences of vascular flow on lung injury induced by mechanical ventilation. Am J Respir Crit Care Med 1998; 157:1935–42
Hotchkiss JR Jr, Blanch L, Murias G, Adams AB, Olson DA, Wangensteen OD, Leo PH, Marini JJ: Effects of decreased respiratory frequency on ventilator-induced lung injury. Am J Respir Crit Care Med 2000; 161:463–8
Feihl F, Broccard AF: Interactions between respiration and systemic hemodynamics. Part II: Practical implications in critical care. Intensive Care Med 2009; 35:198–205
Duggan M, McNamara PJ, Engelberts D, Pace-Asciak C, Babyn P, Post M, Kavanagh BP: Oxygen attenuates atelectasis-induced injury in the in vivo  rat lung. ANESTHESIOLOGY 2005; 103:522–31
Fig. 1. Experimental time course and measured airway pressures during the experimental time course. Each value represents mean ± SD. Bars  below graph indicate changes in ventilatory settings and fraction of inspired oxygen during the experiment. FIO2= fraction of inspired oxygen; HVT= high-tidal volume; LVT= low-tidal volume; PAirway= airway pressure; PEEP = positive end-expiratory pressure; PIP = peak inspiratory pressure; VT= tidal volume.
Fig. 1. Experimental time course and measured airway pressures during the experimental time course. Each value represents mean ± SD. Bars 
	below graph indicate changes in ventilatory settings and fraction of inspired oxygen during the experiment. FIO2= fraction of inspired oxygen; HVT= high-tidal volume; LVT= low-tidal volume; PAirway= airway pressure; PEEP = positive end-expiratory pressure; PIP = peak inspiratory pressure; VT= tidal volume.
Fig. 1. Experimental time course and measured airway pressures during the experimental time course. Each value represents mean ± SD. Bars  below graph indicate changes in ventilatory settings and fraction of inspired oxygen during the experiment. FIO2= fraction of inspired oxygen; HVT= high-tidal volume; LVT= low-tidal volume; PAirway= airway pressure; PEEP = positive end-expiratory pressure; PIP = peak inspiratory pressure; VT= tidal volume.
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Fig. 2. Dose–response effect of intravenous sodium nitrite on ventilator-induced lung injury. (A  ) Mean arterial pressure in rats ventilated with either low tidal volume (LVT) or high tidal volume (HVT) ventilation with and without IV administration of sodium nitrite. HVT+ sodium nitrite (0.25, 2.5, and 25) groups were given IV sodium nitrite at 0.25, 2.5, and 25 μmol/kilogram body weight, respectively. Values are means ± SD; P  < 0.05 and *versus  LVT. (B  ) Time course of PaO2and PaCO2in animals ventilated with LVTor untreated HVTinjurious ventilation as compared with animals subjected to HVTventilation with IV sodium nitrite pretreatment (0.25, 2.5, and 25 μmol/kilogram body weight). Values are means ± SEM; P  < 0.05 and *versus  LVT, †versus  baseline, #versus  HVT. HVT= high-tidal volume; LVT= low-tidal volume; MAP = mean arterial pressure; NaNO2= sodium nitrite; ns = not significant.
Fig. 2. Dose–response effect of intravenous sodium nitrite on ventilator-induced lung injury. (A 
	) Mean arterial pressure in rats ventilated with either low tidal volume (LVT) or high tidal volume (HVT) ventilation with and without IV administration of sodium nitrite. HVT+ sodium nitrite (0.25, 2.5, and 25) groups were given IV sodium nitrite at 0.25, 2.5, and 25 μmol/kilogram body weight, respectively. Values are means ± SD; P 
	< 0.05 and *versus 
	LVT. (B 
	) Time course of PaO2and PaCO2in animals ventilated with LVTor untreated HVTinjurious ventilation as compared with animals subjected to HVTventilation with IV sodium nitrite pretreatment (0.25, 2.5, and 25 μmol/kilogram body weight). Values are means ± SEM; P 
	< 0.05 and *versus 
	LVT, †versus 
	baseline, #versus 
	HVT. HVT= high-tidal volume; LVT= low-tidal volume; MAP = mean arterial pressure; NaNO2= sodium nitrite; ns = not significant.
Fig. 2. Dose–response effect of intravenous sodium nitrite on ventilator-induced lung injury. (A  ) Mean arterial pressure in rats ventilated with either low tidal volume (LVT) or high tidal volume (HVT) ventilation with and without IV administration of sodium nitrite. HVT+ sodium nitrite (0.25, 2.5, and 25) groups were given IV sodium nitrite at 0.25, 2.5, and 25 μmol/kilogram body weight, respectively. Values are means ± SD; P  < 0.05 and *versus  LVT. (B  ) Time course of PaO2and PaCO2in animals ventilated with LVTor untreated HVTinjurious ventilation as compared with animals subjected to HVTventilation with IV sodium nitrite pretreatment (0.25, 2.5, and 25 μmol/kilogram body weight). Values are means ± SEM; P  < 0.05 and *versus  LVT, †versus  baseline, #versus  HVT. HVT= high-tidal volume; LVT= low-tidal volume; MAP = mean arterial pressure; NaNO2= sodium nitrite; ns = not significant.
×
Fig. 3. (A  ) Lung edema formation as measured by lung wet/dry ratio after 200 min of mechanical ventilation. (B  ) Whole respiratory system compliance after completion of the experiment (closed chest). Each value represents mean ± SD; P  < 0.05 and *versus  low-tidal volume ventilation, #versus  high-tidal volume ventilation. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Numbers within error bars  depict sample size. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; L-NAME = Nω-nitro-L-arginine methyl ester; NaNO2= sodium nitrite; NO = nitric oxide; VT= tidal volume.
Fig. 3. (A 
	) Lung edema formation as measured by lung wet/dry ratio after 200 min of mechanical ventilation. (B 
	) Whole respiratory system compliance after completion of the experiment (closed chest). Each value represents mean ± SD; P 
	< 0.05 and *versus 
	low-tidal volume ventilation, #versus 
	high-tidal volume ventilation. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Numbers within error bars 
	depict sample size. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; L-NAME = Nω-nitro-L-arginine methyl ester; NaNO2= sodium nitrite; NO = nitric oxide; VT= tidal volume.
Fig. 3. (A  ) Lung edema formation as measured by lung wet/dry ratio after 200 min of mechanical ventilation. (B  ) Whole respiratory system compliance after completion of the experiment (closed chest). Each value represents mean ± SD; P  < 0.05 and *versus  low-tidal volume ventilation, #versus  high-tidal volume ventilation. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Numbers within error bars  depict sample size. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; L-NAME = Nω-nitro-L-arginine methyl ester; NaNO2= sodium nitrite; NO = nitric oxide; VT= tidal volume.
×
Fig. 4. (A  ) Concentration of tumor necrosis factor (TNF)-α. (B  ) Concentration of interleukin-1β. (C  ) Concentration of macrophage inflammatory protein-2. (D  ) Total protein concentration in bronchoalveolar lavage fluid. Rats ventilated with either 6 ml/kg tidal volume or high tidal volume injurious ventilation for 2 h with and without administration of sodium nitrite and alterations of nitric oxide bioavailability. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Values are means ± SD; P  < 0.05 and *versus  low tidal volume ventilation, #versus  high-tidal volume ventilation. Numbers within error bars  depict sample size. Note that 100signifies a value below the detection threshold. Allo = allopurinol; BAL = bronchoalveolar lavage; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; IL = interleukin; L-NAME = Nω-nitro-L-arginine methyl ester; MIP = macrophage inflammatory protein; NaNO2= sodium nitrite; NO = nitric oxide; TNF = tumor necrosis factor; VT= tidal volume.
Fig. 4. (A 
	) Concentration of tumor necrosis factor (TNF)-α. (B 
	) Concentration of interleukin-1β. (C 
	) Concentration of macrophage inflammatory protein-2. (D 
	) Total protein concentration in bronchoalveolar lavage fluid. Rats ventilated with either 6 ml/kg tidal volume or high tidal volume injurious ventilation for 2 h with and without administration of sodium nitrite and alterations of nitric oxide bioavailability. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Values are means ± SD; P 
	< 0.05 and *versus 
	low tidal volume ventilation, #versus 
	high-tidal volume ventilation. Numbers within error bars 
	depict sample size. Note that 100signifies a value below the detection threshold. Allo = allopurinol; BAL = bronchoalveolar lavage; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; IL = interleukin; L-NAME = Nω-nitro-L-arginine methyl ester; MIP = macrophage inflammatory protein; NaNO2= sodium nitrite; NO = nitric oxide; TNF = tumor necrosis factor; VT= tidal volume.
Fig. 4. (A  ) Concentration of tumor necrosis factor (TNF)-α. (B  ) Concentration of interleukin-1β. (C  ) Concentration of macrophage inflammatory protein-2. (D  ) Total protein concentration in bronchoalveolar lavage fluid. Rats ventilated with either 6 ml/kg tidal volume or high tidal volume injurious ventilation for 2 h with and without administration of sodium nitrite and alterations of nitric oxide bioavailability. The table below the figures depicts the experimental groups, ventilatory regime, and treatment strategies. Values are means ± SD; P  < 0.05 and *versus  low tidal volume ventilation, #versus  high-tidal volume ventilation. Numbers within error bars  depict sample size. Note that 100signifies a value below the detection threshold. Allo = allopurinol; BAL = bronchoalveolar lavage; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; IL = interleukin; L-NAME = Nω-nitro-L-arginine methyl ester; MIP = macrophage inflammatory protein; NaNO2= sodium nitrite; NO = nitric oxide; TNF = tumor necrosis factor; VT= tidal volume.
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Fig. 5. Hematoxilin and eosin staining of right upper lobes, magnification: x200. Untreated injurious ventilation with high tidal volumes caused a hemorrhagic alveolar edema and atelectasis formation. Except for mild interstitial edema, animals treated with 2.5 μmol/kg sodium nitrite (high-tidal volume ventilation + sodium nitrite [2.5]) and the low-tidal volume ventilation control group are comparable. HVT= high tidal volume; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 5. Hematoxilin and eosin staining of right upper lobes, magnification: x200. Untreated injurious ventilation with high tidal volumes caused a hemorrhagic alveolar edema and atelectasis formation. Except for mild interstitial edema, animals treated with 2.5 μmol/kg sodium nitrite (high-tidal volume ventilation + sodium nitrite [2.5]) and the low-tidal volume ventilation control group are comparable. HVT= high tidal volume; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 5. Hematoxilin and eosin staining of right upper lobes, magnification: x200. Untreated injurious ventilation with high tidal volumes caused a hemorrhagic alveolar edema and atelectasis formation. Except for mild interstitial edema, animals treated with 2.5 μmol/kg sodium nitrite (high-tidal volume ventilation + sodium nitrite [2.5]) and the low-tidal volume ventilation control group are comparable. HVT= high tidal volume; LVT= low tidal volume; NaNO2= sodium nitrite.
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Fig. 6. Effects of nitric oxide scavenger C-PTIO, unselective inhibition of nitric oxide synthases, and xanthine oxidoreductase inhibition on PaO2(fraction of inspired oxygen = 0.4) at baseline and after ventilator-induced lung injury. Values are means ± SD; P  < 0.05 and †versus  baseline, #versus  high-tidal volume ventilation. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; HVT= high tidal volume; L-NAME = Nω-nitro-L-arginine methyl ester; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 6. Effects of nitric oxide scavenger C-PTIO, unselective inhibition of nitric oxide synthases, and xanthine oxidoreductase inhibition on PaO2(fraction of inspired oxygen = 0.4) at baseline and after ventilator-induced lung injury. Values are means ± SD; P 
	< 0.05 and †versus 
	baseline, #versus 
	high-tidal volume ventilation. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; HVT= high tidal volume; L-NAME = Nω-nitro-L-arginine methyl ester; LVT= low tidal volume; NaNO2= sodium nitrite.
Fig. 6. Effects of nitric oxide scavenger C-PTIO, unselective inhibition of nitric oxide synthases, and xanthine oxidoreductase inhibition on PaO2(fraction of inspired oxygen = 0.4) at baseline and after ventilator-induced lung injury. Values are means ± SD; P  < 0.05 and †versus  baseline, #versus  high-tidal volume ventilation. Allo = allopurinol; C-PTIO = 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide; HVT= high tidal volume; L-NAME = Nω-nitro-L-arginine methyl ester; LVT= low tidal volume; NaNO2= sodium nitrite.
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Table 1. Baseline Characteristics of Systemic Hemodynamics and Blood Gas Analysis Parameters
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Table 1. Baseline Characteristics of Systemic Hemodynamics and Blood Gas Analysis Parameters
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Table 2. Mortality
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Table 2. Mortality
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