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Journal of Toxicology and Environmental Health: Current Issues Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uteh19
Silica‐induced pulmonary inflammation and fibrosis in mice is altered by acute exposure to nitrogen dioxide a
b
Karen M. Vetrano , John B. Morris & Andrea K. Hubbard
b
a
School of Pharmacy , University of Connecticut , U‐92, Storrs, CT, 06269 b
School of Pharmacy , University of Connecticut , Storrs, Connecticut Published online: 15 Oct 2009.
To cite this article: Karen M. Vetrano , John B. Morris & Andrea K. Hubbard (1992) Silica‐induced pulmonary inflammation and fibrosis in mice is altered by acute exposure to nitrogen dioxide, Journal of Toxicology and Environmental Health: Current Issues, 37:3, 425-442, DOI: 10.1080/15287399209531681 To link to this article: http://dx.doi.org/10.1080/15287399209531681
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
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SILICA-INDUCED PULMONARY INFLAMMATION AND FIBROSIS IN MICE IS ALTERED BY ACUTE EXPOSURE TO NITROGEN DIOXIDE Karen M. Vetrano, John B. Morris, Andrea K. Hubbard
Downloaded by [Monash University Library] at 10:56 04 May 2015
School of Pharmacy, University of Connecticut, Storrs, Connecticut
The biologic impact of consecutive exposures to two environmental pollutants was examined in mice exposed to silica crystals (SI) by intratracheal (IT) injection followed by an inhalation exposure to nitrogen dioxide (NO2). C57Bl/6 mice received an IT injection of 2 mg SI or sterile saline (SAL) followed by a 2-h inhalation exposure to NO2 at 20 ppm either within 2 h of or 24 h after SI instillation. During acute inflammation (d 3 postsilica), mice exposed to NO2 at either time showed a dramatic and significant reduction in the number of lavaged alveolar neutrophils (PMN) when compared to silical air-exposed mice. Animals exposed to NO2 24 h after silica also evidenced significant decreases in levels of lavage albumin and lactate dehydrogenase (LDH) 3 d after silica, as well as significant decreases in hydroxyproline content of the lung 30 and 60 d postsilica injection when compared to silicalair-exposed animals. NO2 administration 24 h after silica appeared to shift the appearance of PMN in the lung from d 3 to d 14, but did not otherwise alter chronic cellular inflammation. These data suggest that the marked neutrophil response and collagen deposition induced by SI can be modulated by NO2 exposure and that the time of oxidant gas exposure after silica administration is critical to this modulation.
INTRODUCTION Environmental air pollutants to which humans are exposed may include combinations of aerosolized particles and oxidant gases. Successive exposure to particulates (silica) and oxidant gases (nitrogen dioxide) could occur from photochemical smog or from occupation and lifestyle (coal mining, smoking). Nitrogen dioxide and silica appear to produce pulmonary dysfunction through both similar and dissimilar mechanisms. Following silica exposure, there is crystal deposition at the alveolar duct bifurcations, phagocytosis of particles (Brody et al., 1982), and a rapid influx of neutrophils and monocytes into the alveoli (Adamson and Bowden, 1984). During cell death or phagocytosis, recruited inflammatory Presented in part at the annual meeting of the 30th Annual Meeting of the Society of Toxicology, Dallas, Tex. The authors thank Dr. Roger Thrall for his helpful discussions and insight. This work was supported by the Connecticut Lung Association and the University of Connecticut Research Foundation. Requests for reprints should be sent to Dr. A. K. Hubbard, School of Pharmacy U-92, University of Connecticut, Storrs, CT 06269.
425 Journal of Toxicology and Environmental Health, 37:425-442,1992 Copyright © 1992 by Hemisphere Publishing Corporation
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cells release lysosomal enzymes and generate free radicals to elicit focal damage to type I epithelial cells (Snider, 1983). In addition, silica may exert its pulmonary toxicity directly through the generation of reactive oxygen radicals (Shi et al., 1988; Vallyathan et al., 1988). Particle-induced activation of alveolar macrophages and the subsequent release of fibrogenic factors (Davis, 1986) may ultimately lead to pulmonary fibrosis (Parkes, 1982). NO2 is thought to exert its biological action in part by initiating lipid peroxidation of cell membranes (Menzel, 1976) and/or by oxidizing lowmolecular-weight reducing substances and proteins, which subsequently leads to cell injury or death (Sagai and Ichinose, 1987). Inhalation of NO2 is known to cause alveolar type I cell damage, proliferation of alveolar type II cells, desquamation of endothelium, and an interstitial fibrotic response (Crapo et al., 1984; Evans et al., 1976), as well as centrilobular emphysema (Glasgow et al., 1987). The goal of this study was to examine the impact of oxidant gas exposure on the outcome of silica-induced inflammation and collagen deposition. This work also contributes to understanding the role of the acute and chronic inflammatory response in the outcome of injury following exposure to two physically and chemically different environmental toxicants. Although there are no reports of altered lung injury following a combined exposure to silica particles and NO2, investigators have examined animals after a combined exposure to ozone, another oxidant gas, and silica or asbestos. Shiotsuka et al. (1986) did not detect any changes in silica-induced fibrosis in rats injected IT with silica and subsequently exposed to 0.8 ppm ozone for 37 d. Pinkerton et al. (1990) exposed rats continuously for 6 wk to 0.06-0.25 ppm ozone and then to aerosolized asbestos fibers for a single 5-h period. They assessed these rats 30 d later for septal tissue volume of the first alveolar bifurcation and found no potentiation by this combined exposure. The current work reports that acute NO2 exposure modulates silicainduced pulmonary inflammation and fibrosis by decreasing levels of silica-induced alveolar neutrophils and collagen content of the lungs. Not only can these changes in particle-induced outcome be modulated by NO2 exposure, but the time of the oxidant gas exposure after silica is also critical to this modulation. METHODS Animals
Adult female C57BI/6 mice (8 wk; 20-25 g) were purchased from Jackson Animal Laboratories, Bar Harbor, Me. The C57BI/6 mouse was chosen because previous work suggested this strain to be a moderate to high responder to silica (Callis et al., 1985). The mice were housed in cages on
EFFECT OF NO 2 ON SILICA-INDUCED INJURY
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a horizontal laminar air flow rack and received food and water ad libitum. Animals were housed 5 per cage on hardwood Sani-chips and maintained on a 12-h light/dark cycle in a temperature (25°C) controlled animal care facility. Instillation of Particles Silica crystals (alpha quartz; Min-U-Sil 5; -
>\ y
FIGURE 2. Effect of NO 2 exposure on silica-induced cellular inflammation—histopathological analysis. Mice were injected IT with silica and subsequently exposed to air (/AIR) (a) or to NO 2 24 h after (/NO2-24) IT injection (fa). Three days after IT injection, nonlavaged lungs were removed, inflated with fixative, and embedded in paraffin. Tissue sections were stained with hematoxylin and eosin stain and photomicrographs (400x) were taken under light microscopy or representative fields.
Modulation of Acute and Chronic Pulmonary Inflammation
A time course (1,2,3, 7,14, 30, and 60 d postinjection) was conducted to determine if NO2 exposure 24 h postsilica altered the silica-induced neutrophil response and biochemical markers of injury throughout the 60 d or merely shifted the time and length of their presence in the lung. Only SI/AIR and SI/NO2-24 were compared, because NO2 exposure 24 h after silica injection elicited the most dramatic changes (Figs. 1-4). Figure 5a illustrates that NO2 exposure 24 h after silica injection (Sl/NO2-24) did eliminate the increased PMN response on d 2 and 3 postsilica and shifted the peak in this response from d 3 to d 7. After this time, the neutrophil response in the silica/NO2-exposed animals (SI/NO2-24) was similar to levels seen in the SI/AIR-exposed mice. At no time did silica exposure cause an increase in the number of lavaged epithelial cells (Fig. 56). However, when NO2 exposure was preceded by silica injection (SI/ NO2-24), there were significant increases in the number of epithelial cells at d 2, 3, and 7. The increase in lavage albumin (Fig. 5c) on d 1 and 3 postsilica in silica/air-exposed mice parallels the biphasic influx of PMNs
EFFECT OF NO 2 ON SILICA-INDUCED INJURY
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into the airways. Moreover, the increase in albumin normally seen 3 d after silica instillation was significantly reduced by the 24-h NO2 exposure (SI/NO2-24). Instead, the peak in lavage albumin in silica-injected NO2-exposed animals was on d 2 postsilica. Neither silica injection nor silica/NO2 exposure elicited any chronic changes in lavage albumin. As another marker of pulmonary injury, lavage LDH (Fig. 5d) was measured and found to be reduced at d 3 and 14 by NO2 exposure (SI/NO2-24). NO2 exposure 24 h after silica did not affect the number or kinetics of the PAM response or the lavage protein content over the 60 d (data not shown). DISCUSSION AND CONCLUSIONS The goal of this study was to examine the impact of oxidant gas exposure on the outcome of silica-induced inflammation and collagen deposition. Human exposure to these two environmental air pollutants could be anticipated to occur from photochemical smog or from occupation and lifestyle (coal mining, smoking). Moreover, resulting information
FIGURE 2. (Continued) Effect of NO 2 exposure on silica-induced cellular inflammation— histopathological analysis. Mice were injected IT with silica and subsequently exposed to air (/AIR) (a) or to NO 2 24 h after (/NO2-24) IT injection (6). Three days after IT injection, nonlavaged lungi were removed, inflated with fixative, and embedded in paraffin. Tissue sections were stained with hematoxylin and eosin stain and photomicrographs (400x) were taken under light microscopy or representative fields.
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K. M. VETRANO ET AL.
/AIR
/NO2-2 (a)
/NO2-24
FIGURE 3. Effect of NO 2 exposure on biochemical markers of silica-induced acute inflammation. Mice were injected IT with either silica (solid bar) or saline (open bar) and subsequently exposed to air (/AIR) or to NO 2 within 2 h of (/NO2-2) or to NO 2 24 h after (/NO2-24) IT injection. Three days after IT injection, cell-free lavage fluid from these mice was assessed for the (a) total protein content, (b) total albumin content, and (c) total LDH. Data are expressed as means ± SEM of 5-6 mice/group with significance at p < .05. Groups with the asterisk superscript are significantly different from comparable saline-injected mice. Groups with the plus superscript are significantly different from SI/AIR-exposed animals and groups with the number slash superscript are significantly different from SAL/AIR-exposed mice.
will increase our understanding of the mechanisms that determine outcome of lung injury following exposure to two physically and chemically different environmental toxicants. This work indicates that acute NO2 exposure modulates silicainduced pulmonary inflammation and fibrosis by decreasing levels of silica-induced airway neutrophils, altering markers of lung or cell injury (lavage albumin and LDH), and reducing silica-induced increases in collagen content of the lungs. In addition, the timing of NO2 exposure following silica injection appears to influence the outcome of silica-induced chronic injury. NO2 administered 24 h after silica injection elicited more changes in parameters of lung injury and collagen deposition than did NO2 exposure within 2 h of silica. Although the administered doses of silica (2 mg/mouse) and NO2 (20 ppm) represent higher levels than might be encountered in the environment or in the workplace, the effects on
Downloaded by [Monash University Library] at 10:56 04 May 2015
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outcome from these toxicants at these doses were chosen to mimic the maximum potential for injury from such an exposure. In silica/air-exposed animals, there appeared to be a biphasic influx of neutrophils into the airways on d 1 and 3 (Fig. 5a), which is paralleled by increases in lavage fluid albumin (Fig. 5c). Moreover, silica/NO2exposed animals evidenced a marked reduction in lavage albumin on d 3, which coincided with decreased alveolar neutrophils. These coincident increases and decreases suggest a relationship between the presence of neutrophils and alterations in the alveolar/capillary barrier. However, increased alveolar/capillary permeability in these animals may also be the result of PMN-independent mechanisms, because silica/NO2exposed mice evidenced increased lavage albumin on d 2 (1 d post NO2 exposure; 2 d post SI injection) and yet decreased PMN. Adamson and Bowden (1984) also examined the cellular composition of lavage fluid from mice injected IT with 2 mg silica and reported the peak increase in 12OO
Si
dl
Sal
960
720
480
ŒI
Journal of Toxicology and Environmental Health: Current Issues Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uteh19
Silica‐induced pulmonary inflammation and fibrosis in mice is altered by acute exposure to nitrogen dioxide a
b
Karen M. Vetrano , John B. Morris & Andrea K. Hubbard
b
a
School of Pharmacy , University of Connecticut , U‐92, Storrs, CT, 06269 b
School of Pharmacy , University of Connecticut , Storrs, Connecticut Published online: 15 Oct 2009.
To cite this article: Karen M. Vetrano , John B. Morris & Andrea K. Hubbard (1992) Silica‐induced pulmonary inflammation and fibrosis in mice is altered by acute exposure to nitrogen dioxide, Journal of Toxicology and Environmental Health: Current Issues, 37:3, 425-442, DOI: 10.1080/15287399209531681 To link to this article: http://dx.doi.org/10.1080/15287399209531681
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
Downloaded by [Monash University Library] at 10:56 04 May 2015
Terms & Conditions of access and use can be found at http://www.tandfonline.com/ page/terms-and-conditions
SILICA-INDUCED PULMONARY INFLAMMATION AND FIBROSIS IN MICE IS ALTERED BY ACUTE EXPOSURE TO NITROGEN DIOXIDE Karen M. Vetrano, John B. Morris, Andrea K. Hubbard
Downloaded by [Monash University Library] at 10:56 04 May 2015
School of Pharmacy, University of Connecticut, Storrs, Connecticut
The biologic impact of consecutive exposures to two environmental pollutants was examined in mice exposed to silica crystals (SI) by intratracheal (IT) injection followed by an inhalation exposure to nitrogen dioxide (NO2). C57Bl/6 mice received an IT injection of 2 mg SI or sterile saline (SAL) followed by a 2-h inhalation exposure to NO2 at 20 ppm either within 2 h of or 24 h after SI instillation. During acute inflammation (d 3 postsilica), mice exposed to NO2 at either time showed a dramatic and significant reduction in the number of lavaged alveolar neutrophils (PMN) when compared to silical air-exposed mice. Animals exposed to NO2 24 h after silica also evidenced significant decreases in levels of lavage albumin and lactate dehydrogenase (LDH) 3 d after silica, as well as significant decreases in hydroxyproline content of the lung 30 and 60 d postsilica injection when compared to silicalair-exposed animals. NO2 administration 24 h after silica appeared to shift the appearance of PMN in the lung from d 3 to d 14, but did not otherwise alter chronic cellular inflammation. These data suggest that the marked neutrophil response and collagen deposition induced by SI can be modulated by NO2 exposure and that the time of oxidant gas exposure after silica administration is critical to this modulation.
INTRODUCTION Environmental air pollutants to which humans are exposed may include combinations of aerosolized particles and oxidant gases. Successive exposure to particulates (silica) and oxidant gases (nitrogen dioxide) could occur from photochemical smog or from occupation and lifestyle (coal mining, smoking). Nitrogen dioxide and silica appear to produce pulmonary dysfunction through both similar and dissimilar mechanisms. Following silica exposure, there is crystal deposition at the alveolar duct bifurcations, phagocytosis of particles (Brody et al., 1982), and a rapid influx of neutrophils and monocytes into the alveoli (Adamson and Bowden, 1984). During cell death or phagocytosis, recruited inflammatory Presented in part at the annual meeting of the 30th Annual Meeting of the Society of Toxicology, Dallas, Tex. The authors thank Dr. Roger Thrall for his helpful discussions and insight. This work was supported by the Connecticut Lung Association and the University of Connecticut Research Foundation. Requests for reprints should be sent to Dr. A. K. Hubbard, School of Pharmacy U-92, University of Connecticut, Storrs, CT 06269.
425 Journal of Toxicology and Environmental Health, 37:425-442,1992 Copyright © 1992 by Hemisphere Publishing Corporation
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K. M. VETRANO ET AL.
cells release lysosomal enzymes and generate free radicals to elicit focal damage to type I epithelial cells (Snider, 1983). In addition, silica may exert its pulmonary toxicity directly through the generation of reactive oxygen radicals (Shi et al., 1988; Vallyathan et al., 1988). Particle-induced activation of alveolar macrophages and the subsequent release of fibrogenic factors (Davis, 1986) may ultimately lead to pulmonary fibrosis (Parkes, 1982). NO2 is thought to exert its biological action in part by initiating lipid peroxidation of cell membranes (Menzel, 1976) and/or by oxidizing lowmolecular-weight reducing substances and proteins, which subsequently leads to cell injury or death (Sagai and Ichinose, 1987). Inhalation of NO2 is known to cause alveolar type I cell damage, proliferation of alveolar type II cells, desquamation of endothelium, and an interstitial fibrotic response (Crapo et al., 1984; Evans et al., 1976), as well as centrilobular emphysema (Glasgow et al., 1987). The goal of this study was to examine the impact of oxidant gas exposure on the outcome of silica-induced inflammation and collagen deposition. This work also contributes to understanding the role of the acute and chronic inflammatory response in the outcome of injury following exposure to two physically and chemically different environmental toxicants. Although there are no reports of altered lung injury following a combined exposure to silica particles and NO2, investigators have examined animals after a combined exposure to ozone, another oxidant gas, and silica or asbestos. Shiotsuka et al. (1986) did not detect any changes in silica-induced fibrosis in rats injected IT with silica and subsequently exposed to 0.8 ppm ozone for 37 d. Pinkerton et al. (1990) exposed rats continuously for 6 wk to 0.06-0.25 ppm ozone and then to aerosolized asbestos fibers for a single 5-h period. They assessed these rats 30 d later for septal tissue volume of the first alveolar bifurcation and found no potentiation by this combined exposure. The current work reports that acute NO2 exposure modulates silicainduced pulmonary inflammation and fibrosis by decreasing levels of silica-induced alveolar neutrophils and collagen content of the lungs. Not only can these changes in particle-induced outcome be modulated by NO2 exposure, but the time of the oxidant gas exposure after silica is also critical to this modulation. METHODS Animals
Adult female C57BI/6 mice (8 wk; 20-25 g) were purchased from Jackson Animal Laboratories, Bar Harbor, Me. The C57BI/6 mouse was chosen because previous work suggested this strain to be a moderate to high responder to silica (Callis et al., 1985). The mice were housed in cages on
EFFECT OF NO 2 ON SILICA-INDUCED INJURY
427
Downloaded by [Monash University Library] at 10:56 04 May 2015
a horizontal laminar air flow rack and received food and water ad libitum. Animals were housed 5 per cage on hardwood Sani-chips and maintained on a 12-h light/dark cycle in a temperature (25°C) controlled animal care facility. Instillation of Particles Silica crystals (alpha quartz; Min-U-Sil 5; -
>\ y
FIGURE 2. Effect of NO 2 exposure on silica-induced cellular inflammation—histopathological analysis. Mice were injected IT with silica and subsequently exposed to air (/AIR) (a) or to NO 2 24 h after (/NO2-24) IT injection (fa). Three days after IT injection, nonlavaged lungs were removed, inflated with fixative, and embedded in paraffin. Tissue sections were stained with hematoxylin and eosin stain and photomicrographs (400x) were taken under light microscopy or representative fields.
Modulation of Acute and Chronic Pulmonary Inflammation
A time course (1,2,3, 7,14, 30, and 60 d postinjection) was conducted to determine if NO2 exposure 24 h postsilica altered the silica-induced neutrophil response and biochemical markers of injury throughout the 60 d or merely shifted the time and length of their presence in the lung. Only SI/AIR and SI/NO2-24 were compared, because NO2 exposure 24 h after silica injection elicited the most dramatic changes (Figs. 1-4). Figure 5a illustrates that NO2 exposure 24 h after silica injection (Sl/NO2-24) did eliminate the increased PMN response on d 2 and 3 postsilica and shifted the peak in this response from d 3 to d 7. After this time, the neutrophil response in the silica/NO2-exposed animals (SI/NO2-24) was similar to levels seen in the SI/AIR-exposed mice. At no time did silica exposure cause an increase in the number of lavaged epithelial cells (Fig. 56). However, when NO2 exposure was preceded by silica injection (SI/ NO2-24), there were significant increases in the number of epithelial cells at d 2, 3, and 7. The increase in lavage albumin (Fig. 5c) on d 1 and 3 postsilica in silica/air-exposed mice parallels the biphasic influx of PMNs
EFFECT OF NO 2 ON SILICA-INDUCED INJURY
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into the airways. Moreover, the increase in albumin normally seen 3 d after silica instillation was significantly reduced by the 24-h NO2 exposure (SI/NO2-24). Instead, the peak in lavage albumin in silica-injected NO2-exposed animals was on d 2 postsilica. Neither silica injection nor silica/NO2 exposure elicited any chronic changes in lavage albumin. As another marker of pulmonary injury, lavage LDH (Fig. 5d) was measured and found to be reduced at d 3 and 14 by NO2 exposure (SI/NO2-24). NO2 exposure 24 h after silica did not affect the number or kinetics of the PAM response or the lavage protein content over the 60 d (data not shown). DISCUSSION AND CONCLUSIONS The goal of this study was to examine the impact of oxidant gas exposure on the outcome of silica-induced inflammation and collagen deposition. Human exposure to these two environmental air pollutants could be anticipated to occur from photochemical smog or from occupation and lifestyle (coal mining, smoking). Moreover, resulting information
FIGURE 2. (Continued) Effect of NO 2 exposure on silica-induced cellular inflammation— histopathological analysis. Mice were injected IT with silica and subsequently exposed to air (/AIR) (a) or to NO 2 24 h after (/NO2-24) IT injection (6). Three days after IT injection, nonlavaged lungi were removed, inflated with fixative, and embedded in paraffin. Tissue sections were stained with hematoxylin and eosin stain and photomicrographs (400x) were taken under light microscopy or representative fields.
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K. M. VETRANO ET AL.
/AIR
/NO2-2 (a)
/NO2-24
FIGURE 3. Effect of NO 2 exposure on biochemical markers of silica-induced acute inflammation. Mice were injected IT with either silica (solid bar) or saline (open bar) and subsequently exposed to air (/AIR) or to NO 2 within 2 h of (/NO2-2) or to NO 2 24 h after (/NO2-24) IT injection. Three days after IT injection, cell-free lavage fluid from these mice was assessed for the (a) total protein content, (b) total albumin content, and (c) total LDH. Data are expressed as means ± SEM of 5-6 mice/group with significance at p < .05. Groups with the asterisk superscript are significantly different from comparable saline-injected mice. Groups with the plus superscript are significantly different from SI/AIR-exposed animals and groups with the number slash superscript are significantly different from SAL/AIR-exposed mice.
will increase our understanding of the mechanisms that determine outcome of lung injury following exposure to two physically and chemically different environmental toxicants. This work indicates that acute NO2 exposure modulates silicainduced pulmonary inflammation and fibrosis by decreasing levels of silica-induced airway neutrophils, altering markers of lung or cell injury (lavage albumin and LDH), and reducing silica-induced increases in collagen content of the lungs. In addition, the timing of NO2 exposure following silica injection appears to influence the outcome of silica-induced chronic injury. NO2 administered 24 h after silica injection elicited more changes in parameters of lung injury and collagen deposition than did NO2 exposure within 2 h of silica. Although the administered doses of silica (2 mg/mouse) and NO2 (20 ppm) represent higher levels than might be encountered in the environment or in the workplace, the effects on
Downloaded by [Monash University Library] at 10:56 04 May 2015
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outcome from these toxicants at these doses were chosen to mimic the maximum potential for injury from such an exposure. In silica/air-exposed animals, there appeared to be a biphasic influx of neutrophils into the airways on d 1 and 3 (Fig. 5a), which is paralleled by increases in lavage fluid albumin (Fig. 5c). Moreover, silica/NO2exposed animals evidenced a marked reduction in lavage albumin on d 3, which coincided with decreased alveolar neutrophils. These coincident increases and decreases suggest a relationship between the presence of neutrophils and alterations in the alveolar/capillary barrier. However, increased alveolar/capillary permeability in these animals may also be the result of PMN-independent mechanisms, because silica/NO2exposed mice evidenced increased lavage albumin on d 2 (1 d post NO2 exposure; 2 d post SI injection) and yet decreased PMN. Adamson and Bowden (1984) also examined the cellular composition of lavage fluid from mice injected IT with 2 mg silica and reported the peak increase in 12OO
Si
dl
Sal
960
720
480
ŒI
