Effect of respiratory pattern on ozone injury to the airways of isolated rat lungs

Airway obstruction due to bronchial vascular injury after sulfur mustard analog inhalation

RATIONALE

Sulfur mustard (SM) is a frequently used chemical warfare agent, even in modern history. SM inhalation causes significant respiratory tract injury, with early complications due to airway obstructive bronchial casts, akin to those seen after smoke inhalation and in single-ventricle physiology. This process with SM is poorly understood because animal models are unavailable.

OBJECTIVES

To develop a rat inhalation model for airway obstruction with the SM analog 2-chloroethyl ethyl sulfide (CEES), and to investigate the pathogenesis of bronchial cast formation.

METHODS

Adult rats were exposed to 0, 5, or 7.5% CEES in ethanol via nose-only aerosol inhalation (15 min). Airway microdissection and confocal microscopy were used to assess cast formation (4 and 18 h after exposure). Bronchoalveolar lavage fluid (BALF) retrieval and intravascular dye injection were done to evaluate vascular permeability.

MEASUREMENTS AND MAIN RESULTS

Bronchial casts, composed of abundant fibrin and lacking mucus, occluded dependent lobar bronchi within 18 hours of CEES exposure. BALF contained elevated concentrations of IgM, protein, and fibrin. Accumulation of fibrin-rich fluid in peribronchovascular regions (4 h) preceded cast formation. Monastral blue dye leakage identified bronchial vessels as the site of leakage.

CONCLUSIONS

After CEES inhalation, increased permeability from damaged bronchial vessels underlying damaged airway epithelium leads to the appearance of plasma proteins in both peribronchovascular regions and BALF. The subsequent formation of fibrin-rich casts within the airways then leads to airways obstruction, causing significant morbidity and mortality acutely after exposure.

Gas Uptake in a Three-Generation Model Geometry During Steady Expiration: Comparison of Axisymmetric and Three-Dimensional Models

Mass transfer coefficients were predicted and compared for uptake of a formaldehyde-air gas system using an axisymmetric single path model (ASPM) and a three-dimensional computational fluid dynamics model (CFDM) in three-generation model geometry at steady expiratory flow. The flow and concentration fields in the ASPM were solved using Galerkin's finite-element method and in the CFDM using a commercial finite-element software, FIDAP. Numerical results were compared for two different inlet flow rates, wall mass transfer coefficients, and bifurcation angles. The mass transfer coefficients variation with bifurcation unit from the ASPM and CFDM compared qualitatively and quantitatively closely at all flows and lower wall mass transfer coefficients for both 40 degrees and 70 degrees bifurcation angles. However, at higher wall mass transfer coefficients, quantitatively they were within 40% for both the bifurcation angles. Also, at higher flow and wall mass transfer coefficients, they were off qualitatively for a 70 degrees bifurcation angle although the uptake compared qualitatively. This is due to the normalization of uptake within a bifurcation unit with the average of inlet and outlet average concentrations. Both CFDM and ASPM predict the same trends of increase in mass transfer coefficients with inlet flow and wall mass transfer coefficients. Also, the local values of the mass transfer coefficients compared closely at all conditions. These results validate the simplified ASPM and the complex CFDM. Mass transfer coefficients increase with bifurcation angles and with a flat inlet velocity profile compared to a parabolic velocity profile since the flow is non-fully developed and hence, the uptake increases.

Gas Uptake in a Three-Generation Model Geometry with a Flat Inlet Velocity During Steady Inspiration: Comparison of Axisymmetric and Three-Dimensional Models

Mass transfer coefficients were predicted and compared for uptake of reactive gas system using an axisymmetric single-path model (ASPM) with experimentally predicted values in a two-generation geometry and with a three-dimensional computational fluid dynamics model (CFDM) in a three-generation model geometry at steady inspiratory flow with a flat inlet velocity profile. The flow and concentration fields in the ASPM were solved using Galerkin's finite element method and in the CFDM using a commercial finite element software FIDAP. ASPM predicted average gas phase mass transfer coefficients within 25% of the experimental values. Numerical results in terms of overall mass transfer coefficients from the two models within each bifurcation unit were compared for two different inlet flow rates, wall mass transfer coefficients, and bifurcation angles. The overall mass transfer coefficients variation with bifurcation unit from the ASPM and CFDM compared qualitatively and quantitatively closely at lower wall mass transfer coefficients for both 40 degree and 70 degree bifurcation angles. But at higher wall mass transfer coefficients, quantitatively they were off in the range of 2-10% for 40 degree bifurcation angle and in the range of 4-15% for 70 degree bifurcation angle. Both CFDM and ASPM predict the same trends of increase in mass transfer coefficients with inlet flow, wall mass transfer coefficients, and during inspiration compared to expiration. Higher mass transfer coefficients were obtained with a flat velocity profile compared to a parabolic velocity profile using ASPM. These results validate the simplified ASPM and the complex CFDM.

Comparison of Axisymmetric and Three-Dimensional Models for Gas Uptake in a Single Bifurcation During Steady Expiration

Reactive gas uptake is predicted and compared in a single bifurcation at steady expiratory flow in terms of Sherwood number using an axisymmetric single-path model (ASPM) and a three-dimensional computational fluid dynamics model (CFDM). ASPM is validated in a two-generation geometry by comparing the average gas-phase mass transfer coefficients with the experimental values. ASPM predicted mass transfer coefficients within 20% of the experimental values. The flow and concentration variables in the ASPM were solved using Galerkin finite element method and in the CFDM using commercial finite element software FIDAP. The simulations were performed for reactive gas flowing at Reynolds numbers ranging from 60 to 350 in both symmetric bifurcation for three bifurcation angles, 30deg, 70deg, and 90deg, and in an asymmetric bifurcation. The numerical models compared with each other qualitatively but quantitatively they were within 0.4–8% due to nonfully developed flow in the parent branch predicted by the CFDM. The radially averaged concentration variation along the axial location matched qualitatively between the CFDM and ASPM but quantitatively they were within 32% due to differences in the flow field. ASPM predictions compared well with the CFDM predictions for an asymmetric bifurcation. These results validate the simplified ASPM and the complex CFDM. ASPM predicts higher Sherwood number with a flat velocity inlet profile compared to a parabolic inlet velocity profile. Sherwood number increases with the inlet average velocity, wall mass transfer coefficient, and bifurcation angle since the boundary layer grows slower in the parent and daughter branches.

Three-Dimensional Simulations of Reactive Gas Uptake in Single Airway Bifurcations

The pattern of lung injury induced by the inhalation of ozone (O(3)) depends on the dose delivered to different tissues in the airways. This study examined the distribution of O(3) uptake in a single, symmetrically branched airway bifurcation. Reaction in the epithelial lining fluid was assumed to be so rapid that O(3) concentration was negligible along the entire surface of the bifurcation wall. Three-dimensional numerical solutions of the continuity, Navier-Stokes and convection-diffusion equations were obtained for steady inspiratory and expiratory flows at Reynolds numbers ranging from 100 to 500. The total rate of O(3) uptake was found to increase with increasing flow rate during both inspiration and expiration. Hot spots of O(3) flux appeared at the carina of the bifurcation for virtually all inspiratory and expiratory Reynolds numbers considered in the simulations. At the lowest expiratory Reynolds number, however, the location of the maximum flux was shifted to the outer wall of the daughter branch. For expiratory flow, additional hot spots of flux were found on the parent branch wall just downstream of the branching region. In all cases, O(3) uptake in the single bifurcation was larger than that in a straight tube of equal inlet radius and wall surface area. This study provides insight into the effect of flow conditions on O(3) uptake and dose distribution in individual bifurcations.

Elevation of susceptibility to ozone-induced acute tracheobronchial injury in transgenic mice deficient in Clara cell secretory protein

Increases in Clara cell abundance or cellular expression of Clara cell secretory protein (CCSP) may cause increased tolerance of the lung to acute oxidant injury by repeated exposure to ozone (O3). This study defines how disruption of the gene for CCSP synthesis affects the susceptibility of tracheobronchial epithelium to acute oxidant injury. Mice homozygous for a null allele of the CCSP gene (CCSP-/-) and wild type (CCSP+/+) littermates were exposed to ozone (0.2 ppm, 8 h; 1 ppm, 8 h) or filtered air. Injury was evaluated by light and scanning electron microscopy, and the abundance of necrotic, ciliated, and nonciliated cells was estimated by morphometry. Proximal and midlevel intrapulmonary airways and terminal bronchioles were evaluated. There was no difference in airway epithelial composition between CCSP+/+ and CCSP-/- mice exposed to filtered air, and exposure to 0.2 ppm ozone caused little injury to the epithelium of both CCSP+/+ and CCSP-/- mice. After exposure to 1.0 ppm ozone, CCSP-/- mice suffered from a greater degree of epithelial injury throughout the airways compared to CCSP+/+ mice. CCSP-/- mice had both ciliated and nonciliated cell injury. Furthermore, lack of CCSP was associated with a shift in airway injury to include proximal airway generations. Therefore, we conclude that CCSP modulates the susceptibility of the epithelium to oxidant-induced injury. Whether this is due to the presence of CCSP on the acellular lining layer surface and/or its intracellular distribution in the secretory cell population needs to be defined.

Antioxidant-mediated augmentation of ozone-induced membrane oxidation

The pulmonary epithelial lining fluid (ELF) contains substrates, e.g., ascorbic acid (AH2), uric acid (UA), glutathione (GSH), proteins, and unsaturated lipids, which undergo facile reaction with inhaled ozone (O3). Reactions near the ELF gas/liquid interface likely provide the driving force for O3 absorption ("reactive absorption") and constrain O3 diffusion to the underlying epithelium. To investigate the potential mechanisms wherein O3/ELF interactions may induce cellular damage, we utilized a red cell membrane (RCM) model intermittently covered by an aqueous film to mimic the lung surface compartmentation, and evaluated exposure-mediated loss of acetylcholinesterase activity (AChE) and TBARS accumulation. In the absence of aqueous reactants, O3 exposure induced no detectable changes in AChE or TBARS. AH2 and GSH preferentially induced oxidative damage in a dose-dependent fashion. AH2-mediated RCM oxidation was not inhibited by superoxide dismutase, catalase, mannitol, or Fe chelators. O3 reaction with UA, Trolox, or albumin produced no RCM oxidation but oxidation occurred when AH2 was combined with UA or albumin. Rat bronchoalveolar lavage fluid (BALF) also induced RCM oxidation. However, in vivo O3 exposure dampened the extent of BALF-mediated RCM oxidation. Although we cannot completely rule out O3 diffusion to the RCM, product(s) derived from O3 + AH2/GSH reactions (possibly O3*- or 1O2) likely initiated RCM oxidation and may suggest that in vivo, such secondary species account for O3 permeation through the ELF leading to cellular perturbations.

Ozone-induced acute pulmonary injury in inbred mouse strains

To determine if host factors influence the time course and extent of lung injury after acute inhalation of ozone (O3), we evaluated the physiologic and biologic response of nine genetically diverse inbred strains of mice (C57BL/6J, 129/SvIm, BTBR, BALB/cJ, DBA/2J, A/J, FVB/NJ, CAST/Ei, and C3H/HeJ) exposed to O3 (2.0 ppm x 3 h). Whole lung lavage determined that 129/Svlm, BTBR, DBA/2J, and FVB/NJ had a peak increase in polymorphonuclear cells (PMNs) at 6 h, whereas C57BL/6J and CAST/Ei had a peak increase at 24 h after exposure; airway PMNs were minimally elevated in A/J and C3H/HeJ; BALB/cJ had a predominant lymphocytic influx. Interleukin-6 concentration in the lavage fluid was associated with the influx of PMNs, whereas the total protein in the lavage fluid did not always correlate with lavage cellularity. Respiratory responses were monitored using whole body plethysmography and enhanced pause index. C57BL/6J, BALB/cJ, 129/SvIm, and BTBR were highly sensitive to O3 and exhibited significant increases in enhanced pause to methacholine aerosol stimulation at 6 and 24 h after exposure to O3. In contrast, DBA/2J, A/J, FVB/NJ, CAST/Ei, and C3H/HeJ strains had demonstrated increases in sensitivity to MCh at 6 h after exposure, but responses had returned to near baseline by 24 h after exposure to O3. Epithelial cell proliferation as assessed by proliferating cell nuclear antigen staining was evident at 24 h after exposure to O3. C57BL/6J and A/J showed 4% proliferating cell nuclear antigen-positive cells; 129/SvIm, DBA/2J, and FVB/NJ had 1-3%; and BTBR, BALB/cJ, CAST/Ei, and C3H/HeJ had < 1%. Phenotypic measurements in six inbred strains were used for an in silico genome analysis based on the Roche mouse database. Consistent loci on chromosomes 1, 7, and 15 were among those identified to have a significant association with the phenotypes studied. In aggregate, our approach has identified O3-resistant (C3H/HeJ and A/J) and -vulnerable (C57BL/6J and 129/SvIm) strains of mice, and determined novel genomic loci, suggesting a clear genetic basis for the lung response to inhaled O3.

Interfacial phospholipids inhibit ozone-reactive absorption-mediated cytotoxicity in vitro

The intrapulmonary distribution of inhaled ozone (O(3)) and induction of site-specific cell injury are related to complex interactions among airflow patterns, local gas-phase concentrations, and the rates of O(3) flux into, and reaction and diffusion within, the epithelial lining fluid (ELF). Recent studies demonstrated that interfacial phospholipid films appreciably inhibited NO(2) absorption. Because surface-active phospholipids are present on alveolar and airway interfaces, we investigated the effects of interfacial films on O(3)-reactive absorption and acute cell injury. Compressed films of dipalmitoyl-glycero-3-phosphocholine (DPPC) and rat lung lavage lipids significantly reduced O(3)-reactive absorption by ascorbic acid, reduced glutathione, and uric acid. Conversely, unsaturated phosphatidylcholine films did not inhibit O(3) absorption. We evaluated O(3)-mediated cell injury using a human lung fibroblast cell culture system, an intermittent tilting exposure regimen to produce a thin covering layer, and nuclear fluorochrome permeability. Exposure produced negligible injury in cells covered with MEM. However, addition of AH(2) produced appreciable (<50%) cell injury. Film spreading of DPPC monolayers necessitated the use of untilted regimens. Induction of acute cell injury in untilted cultures required both AH(2) plus very high O(3) concentrations. Addition of DPPC films significantly reduced cell injury. We conclude that acute cell injury likely results from O(3) reaction with ELF substrates. Furthermore, interfacial films of surface-active, saturated phospholipids reduce the local dose of O(3)-derived reaction products. Finally, because O(3) local dose and tissue damage likely correlate, we propose that interfacial phospholipids may modulate intrapulmonary distribution of inhaled O(3) and the extent of site-specific cell injury.

Detection of viral infection in the respiratory tract of virus antibody free mice: advantages of high-resolution imaging for respiratory toxicology

Using a highly sensitive membrane permeability assay, a viral infection was discovered in the lungs of virus antibody free (VAF) Swiss-Webster mice purchased for respiratory toxicology studies. The assay is based on the uptake of a charged fluorescent compound by cells lacking an intact plasma membrane. Lungs from 74% of the untreated animals from a single vendor tested positive for injury in this assay. High-resolution histopathologic analysis of 1-microm epoxy resin sections from affected animals identified increased peribronchiolar lymphocytic infiltration and markers of epithelial cell injury. Viral particles were directly observed to be budding from the membranes of infiltrating lymphocytic cells by transmission electron microscopy. Standard histological analysis of paraffin-embedded tissues from lungs of the same mice failed to detect obvious pathology. Serological analyses failed to detect the presence of a virus in the affected mice. Therefore, we conclude that (1) a pathogenic condition was present in the respiratory systems of mice judged pathogen free by standard methodologies, (2) the observed condition produced a pattern of injury comparable to those caused by pulmonary toxicants, (3) high-resolution histopathology and advanced imaging techniques can increase the potential for detection of pathological conditions, and (4) apparently healthy animals can have unrecognized infections with the potential for confounding respiratory toxicology studies.

Neuroplasticity in nucleus tractus solitarius neurons after episodic ozone exposure in infant primates

Acute ozone exposure evokes adverse respiratory responses, particularly in children. With repeated ozone exposures, however, despite the persistent lung inflammation and increased sensory nerve excitability, the central nervous system reflex responses, i.e., rapid shallow breathing and decreased lung function, adapt, suggesting changes in central nervous system signaling. We determined whether repeated ozone exposures altered the behavior of nucleus tractus solitarius (NTS) neurons where reflex respiratory motor outputs are first coordinated. Whole cell recordings were performed on NTS neurons in brain stem slices from infant monkeys exposed to filtered air or ozone (0.5 ppm, 8 h/day for 5 days every 14 days for 11 episodes). Although episodic ozone exposure depolarized the membrane potential, increased the membrane resistance, and increased neuronal spiking responses to depolarizing current injections (P < 0.05), it decreased the excitability to vagal sensory fiber activation (P < 0.05), suggesting a diminished responsiveness to sensory transmission, despite overall increases in excitability. Substance P, implicated in lung and NTS signaling, contributed to the increased responsiveness to current injections but not to the diminished sensory transmission. The finding that NTS neurons undergo plasticity with repeated ozone exposures may help to explain the adaptation of the respiratory motor responses.

Methacholine responsiveness of proximal and distal airways of monkeys and rats using videomicrometry

Rat and monkey are species that are used in models of human airway hyperresponsiveness. However, the wall structures of rat and monkey airways are different from each other, with that of the monkey more closely resembling that of humans. We hypothesized that differences in wall structure would explain differences in airway responsiveness. Using videomicrometry, we measured airway luminal area in lung slices to compare proximal and distal airway responsiveness to methacholine in the rat and monkey. The airway type was then histologically identified. Proximal airways of the young rat and monkey were equally responsive to methacholine. In contrast, respiratory bronchioles of monkeys were less responsive than were their proximal bronchi, whereas the distal bronchioles of rats were more responsive than their proximal bronchioles. Both proximal and distal airways of younger monkeys were more responsive than those of older monkeys. Airway heterogeneity in young monkeys was greatest with regard to degree of airway closure of respiratory bronchioles. We conclude that responsiveness to methacholine varies with airway wall structure and location.

Effect of C-fiber-mediated, ozone-induced rapid shallow breathing on airway epithelial injury in rats

We examined the relationship between C-fiber-mediated, ozone-induced rapid shallow breathing and airway epithelial cell injury at different airway sites within the lower respiratory tract of conscious Wistar rats (n = 24). We combined an acute 8-h ozone inhalation with vagal perineural capsaicin treatment, a selective C-fiber conduction block, and 5-bromo-2'-deoxyuridine (BrdU) labeling as an index of epithelial injury. Vehicle-treated rats that inhaled ozone developed a rapid shallow breathing pattern during ozone inhalation, whereas the capsaicin-treated rats that inhaled ozone showed no changes in respiratory frequency. In vehicle-treated, ozone-exposed rats that developed rapid shallow breathing, a progressive increase in BrdU-labeling density (no. of BrdU-labeled cells/mm(2) airway) was observed starting at the bifurcation of the left main stem bronchi (central airway) and going down either a short or long airway path. In vehicle-treated, ozone-exposed rats, terminal bronchioles supplied by short and long airway paths had a similar degree of BrdU-labeling density that was significantly (P < 0.05) greater than the BrdU-labeling density of the proximal airways that supply them. In contrast, the attenuation of rapid shallow breathing produced by capsaicin treatment resulted in a significantly reduced BrdU-labeling density in the terminal bronchioles supplied by short airway paths compared with the terminal bronchioles supplied by long airway paths. Our data indicate that ozone-induced rapid shallow breathing protects large conducting airways while producing a more even distribution of injury to terminal bronchioles.

Hyperthyroidism Increases the Risk of Ozone-Induced Lung Toxicity in Rats

The risk of lung injury from ozone exposure has been well documented. It is also known that various factors may significantly influence the susceptibility of animals to the toxic effects of ozone. In the present study, we investigated the possibility that hyperthyroidism might be associated with increases in ozone-induced pulmonary toxicity. To create a hyperthyroid condition, mature male Sprague--Dawley rats were given injections of thyroxine (dose range: 0.1 to 1 mg/kg body wt daily for 7 days). Control rats received vehicle injections. The animals were then exposed to air or ozone (dose range: 0.5 to 3 ppm for 3 h). At 18 h postexposure, bronchoalveolar lavage fluid and cells were harvested. In hyperthyroid animals, ozone exposure was associated with three- to sixfold increases in bronchoalveolar lavage fluid lactate dehydrogenase activities and albumin levels as well as the number of polymorphonuclear leukocytes harvested by bronchoalveolar lavage above levels observed in ozone-exposed control rats. Additional results from the present study suggest that these thyroid hormone-linked effects cannot be fully explained by differences in whole-body metabolic rate or changes in the inhaled dose of ozone. These findings indicate that the risk of ozone-induced lung toxicity is substantially increased in a hyperthyroid state and suggest that the susceptibility of the lung to damage from ozone exposure may be significantly influenced by individual thyroid hormone status.