Mechanisms and modification of chlorine-induced lung injury in animals

Abstract

Chlorine (Cl(2)) is a reactive oxidant gas used extensively in industrial processes. Exposure of both humans and animals to high concentrations of Cl(2) results in acute lung injury, which may resolve spontaneously or progress to acute respiratory failure. Injury to airway and alveolar epithelium may result from chemical reactions of Cl(2), from HOCl (the hydrolysis product of Cl(2)), and/or from the various reaction products, such as chloramines, that are formed from the reactions of these chlorinating species with biological molecules. Subsequent reactions may initiate self-propagating reactions and induce the production of inflammatory mediators compounding injury to pulmonary surfactant, ion channels, and components of lung epithelial and airway cells. Low-molecular-weight antioxidants, such as ascorbate, glutathione, and urate, present in the lung epithelial lining fluid and tissue, remove Cl(2) and HOCl and thus decrease injury to critical target biological targets. However, levels of lung antioxidants of animals exposed to Cl(2) in concentrations likely to be encountered in the vicinity of industrial accidents decrease rapidly and irreversibly. Our measurements show that prophylactic administration of a mixture containing ascorbate and desferal N-acetyl-cysteine, a precursor of reduced glutathione, prevents Cl(2)-induced injury to the alveolar epithelium of rats exposed to Cl(2). The clinical challenge is to deliver sufficient quantities of antioxidants noninvasively, after Cl(2) exposure, to decrease morbidity and mortality.

Figure 1.

Schematic representation of lung epithelial targets and reactive intermediates formed in the lung epithelial fluids during inhalation of Cl₂. Diagram on top shows sites of penetration of Cl₂ (upper airways for inhaled Cl₂ < 50 ppm; distal lung regions for Cl₂ > 50 ppm). The depths of the airway (in blue color) and alveolar (yellow color) epithelial lining fluids are not in scale (thickness of ALF and epithelial lining fluid [ELF] = 5–10 and 0.1–0.24 μm, respectively; reviewed in Reference 61). Some potential cellular targets such as cilia, surfactant monolayer, tubular myelin, surfactant protein A and D, ion channels, Na⁺,K⁺-ATPase, airway and alveolar cells, alveolar macrophages, and NF-κB are shown in the diagram. GSH = reduced glutathione; ASC = ascorbate; X = secondary reactive intermediates (such as chloramines) formed by the interaction of HOCl and Cl₂ with proteins and lipids. A sensory nerve that contains TRPA1 channels (red dots) is also shown. Cl₂ molecules (prior to being hydrolyzed to HOCl) may attack components of the surfactant monolayer at the surface of the alveolar epithelial lining fluid, tubular myelin, surfactant proteins, and proteins on the surface of alveolar type II cells because of the closer proximity of these targets to the air–liquid interface (5).

Figure 2.

Systemic administration of antioxidants decreases lung injury. Rats were injected with a mixture of antioxidants (ascorbate, NAC, and deferoxamine) or an equivalent amount of vehicle (saline), 18 hours and 1 hour before being exposed to 184 ppm Cl₂ for 30 minutes, returned to room air, and killed 1 hour later. Values are means ± 1 SEM. In each case, open bars represent values from rats exposed to air, solid bars from rats exposed to Cl₂ and pretreated with saline, and hatched bars from rats pretreated with antioxidants and then exposed to Cl₂. Pa_O2 was measured in arterial samples from the carotid artery drawn during anesthesia; protein and ascorbate concentrations were measured in BAL samples. *P < 0.05 compared with the corresponding saline values. Adapted by permission from Reference .

Figure 3.

Augmentation of lung ascorbate levels by post-exposure, intravenous administration of ascorbate. Rats were exposed to 300 ppm Cl₂ for 30 minutes and then returned to room air. Within 10 minutes after exposure, they received a single injection of 20 mg ascorbic acid in 0.2 ml of normal saline. They were killed 1 hour later, their lungs were lavaged, and a blood sample was drawn from the left ventricle. Urea concentrations were measured in both the bronchoalveolar lavage (BAL) fluid and the plasma and ELF volume was calculated from the dilution of urea as previously described (60). The amounts of ascorbate in the BAL fluid and lung tissues were then measured by HPLC as previously described (35). Values are means ± 1 SEM for n = 6 rats in each group. *Significantly different from the corresponding saline group under the same conditions; ^#significantly different from the corresponding variable in the air group.