Ultrastructural changes in rat Clara cells induced by a single dose of O,S,S-trimethyl phosphorodithioate

Eosinophilic Inclusions in Rat Clara Cells and the Effect of an Inhaled Corticosteroid

Large eosinophilic cytoplasmic inclusions (ECIs) are occasionally seen in untreated rat Clara cells. Following inhalation exposure to a corticosteroid, the number of ECIs was increased. This is the first histopathological description of rat ECIs and attempted characterization by immunohistochemistry, in situ hybridization, and electron microscopy. ECIs were strongly positive for surfactant protein D (SP-D) and weakly positive for Clara cell specific protein (CCSP). Clara cell cytoplasm was positive for CCSP mRNA regardless of ECIs, but not within ECIs. Corticosteroid treatment and ECI presence did not affect the immunohistochemistry and in situ hybridization staining intensities. Electron microscopy revealed large intracytoplasmic granules with an irregular limiting membrane. The ECI number was microscopically quantified in rats from three-, six-, and twenty-four-month studies. The mean ECI counts in treated rats increased from three- to fifty-four-fold with a positive dose-related trend, when compared with vehicle controls. Although the mechanism is unclear, SP-D and to a lesser extent CCSP accumulate in the ECIs. As human bronchial epithelium does not appear to contain structures analogous to the ECI, it is suggested that the observation of an increased number of ECIs in the treated rats is not likely to be relevant for human clinical risk assessment.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Effects of injury on [3H]putrescine uptake by types I and II cells in rat lung slices

The capacity of different lung parenchymal cells to accumulate putrescine was investigated by incubating slices of rat lung in a medium containing the tritiated compound. Quantitative examination of autoradiographs, by electron microscopy, indicated that accumulation of putrescine occurred in both the Type I and Type II cells of the alveolar epithelium. Putrescine uptake was abolished by the addition of spermidine to the medium or by incubating at 0 degrees C. Lung samples from rats dosed with the pneumotoxin O,S,S-trimethyl phosphorodithioate (OSSMeO), which selectively damages Type I pneumocytes, showed a large reduction in the uptake of label by both Type I and Type II cells. This treatment also resulted in an increase in the labeling of alveolar macrophages. Control samples, from undosed rats, were incubated in medium containing tritiated 5-hydroxytryptamine; this compound did not accumulate in epithelial cells but it was concentrated in the endothelium of the alveolar capillaries and in the blood cells within these vessels. The demonstration of putrescine uptake by both Type I and Type II pneumocytes, together with its reduction by dosing with OSSMeO, has vindicated the use of this activity, in lung slices, as an index of damage to the alveolar epithelium.

Protection against chemical-induced lung injury by inhibition of pulmonary cytochrome P-450

Protection afforded by trialkyl phosphorothionates against the lung injury caused by trialkyl phosphorothiolates probably results from the inhibition by the P = S moiety of the thionates, of one or more pulmonary cytochrome P-450 isozymes. The aromatic hydrocarbons p-xylene and pseudocumene also protect against this injury and inhibit some P-450 isozymes, but by a different mechanism. OOS-Trimethylphosphorothionate and p-xylene were compared as protective agents against the effect of OOS-trimethylphosphorothiolate and two other lung toxins ipomeanol and 1-nitronaphthalene that are known to be activated by cytochrome P-450. The effects of these protective compounds, in vivo, on pulmonary cytochrome P-450 activity were also determined. Both compounds inhibited pentoxyresorufin O-deethylase activity, but not ethoxyresorufin O-deethylase. The phosphorothionate was most effective against lung injury caused by the phosphorothiolates and 1-nitronaphthalene, whereas p-xylene was much more effective against ipomeanol. beta-Naphthoflavone, which induces pulmonary ethoxyresorufin O-deethylase activity, did not protect against phosphorothiolate or 1-nitronaphthalene injury, and it was only marginally effective in decreasing the toxicity of ipomeanol.

Ultrastructural effects of phosphorothionates and other inhibitors of lung monooxygenases: protection against trialkylphosphorothiolate-induced lung injury

An oral dose (25 mg/kg) of O,O,S-triethylphosphorothiolate (OOSEtO) to rats results in selective injury of type I pneumocytes, degranulation of Clara cells, and pronounced increase in lung weight. A dose (12.5 mg/kg) of the related compound O,O,S-trimethylphosphorothionate (OOSMeS) causes neither injury nor degranulation but, when administered 2 h before OOSEtO (25 mg/kg), protects against all the signs of lung injury that would otherwise result from this dose of the compound. The administration of OOSMeS also results in the formation of large, electron-lucent granules within the apical cytoplasm of the Clara cells. The granules are not birefringent, and histochemical procedures indicate that they do not contain carbohydrate but may consist of lipid accumulated around a proteinaceous core. Similar granules are also observed after administration of p-xylene, pseudocumene, and the pesticide bromophos. These compounds, like OOSMeS, inhibit 7-ethoxycoumarin O-deethylase activity in the lung and are capable of protecting against trialkylphosphorothiolate toxicity. This inhibition of 7-ethoxycoumarin O-deethylase activity suggests loss of pulmonary cytochrome P-450. This loss may account for both the protective action of these compounds and the formation of abnormal granules within Clara cells.

The interaction between phosphorothionate insecticides, pneumotoxic trialkyl phosphorothiolates and effects on lung 7-ethoxycoumarin O-deethylase activity

A number of phosphorothionate (P = S) insecticides, including bromophos and fenitrothion, prevent trialkyl phosphorothiolate (P = O)-induced lung toxicity and the resulting increase in lung weight normally observed at 3 days in the rat. Measurement of 7-ethoxycoumarin O-deethylase (7-EC) activity after both phosphorothionate and phosphorothiolate dosing revealed differing patterns of loss of enzyme activity. Depletion of 7-EC activity by phosphorothionates was maximal between 2 and 10 h after dosing, with recovery between 24 and 72 h. Phosphorothiolates, however, appear to cause two phases of loss of 7-EC activity, an initial fall of approximately 30% observed at 2 h and a secondary fall, maximal on day 3, with loss of 97% of activity, apparently associated with the pathological changes in the lung. It is suggested that oxidative metabolism of phosphorothionates known to occur at the P = S moiety, with suicidal loss of P-450, may then prevent oxidative activation of an S-methyl on the phosphorothiolates, the most likely site for production of a reactive intermediate capable of damaging the lung. Lung 7-EC in rat is sensitive to concentrations of the phosphorothionates bromophos and fenitrothion at 5-25 times less than those causing loss of liver 7-EC activity and at doses 125-600 times less than their LD50s. If repeated in man this may have implications for personnel occupationally exposed to these compounds.

Ultrastructural changes in the respiratory tract of rats following methyl isocyanate inhalation

The static exposure of rats to 0.25 mg/l methyl isocyanate for 1 h resulted in damage to the epithelium of the proximal bronchioles and upper airways. Bronchiolar cells exhibited both nuclear and cytoplasmic damage; many epithelial cells, particularly in the bronchi and trachea, were killed and/or dislodged from the basement membrane. A "raft" of cell debris and fibrin lined most of the airways during the 1st week after exposure but repair to the underlying epithelium was well advanced within 2-3 days. The majority of airways were lined by a normal epithelium within 3 weeks of exposure, but isolated foci of hyperplasia and occluded airways probably accounted for continued respiratory impairment.

O,S,S,-trimethyl phosphorodithioate-induced lung damage in rats and mice

O,S,S,-Trimethyl phosphorodithioate (OSS) is a contaminant of various organophosphorus insecticides which induces delayed damage to rat lung bronchiolar and alveolar epithelial cells. Whether lung damage occurs in mice has not been tested. Changes in DNA synthesis, an index of cell division after the induction of damage, were monitored by measuring thymidine incorporation into pulmonary DNA. Mice, treated with 45 mg/kg OSS, exhibited a significant increase in pulmonary thymidine incorporation on Day 5. Maximal increases occurred on Days 7-10 and were followed by a gradual decline to control levels by Day 15. The labeling index of mouse lung cells, determined following autoradiography, exhibited a similar time course. Differential cell counts indicated that maximal division of type II cells occurred before that of interstitial cells, although interstitial cells were the predominant type labeled at all times. Pulmonary DNA synthesis was significantly increased in rats 2 days after treatment with 90 mg/kg OSS. Maximal thymidine incorporation was measured on Day 3, followed by a decline to control levels on Day 5. Thymidine incorporation into total lung DNA was dose related in both species. Maximal increases appeared after 45 and 90 mg/kg OSS in mice and rats, respectively. The histopathological changes in mouse lung tissue were similar, but somewhat less severe than those seen in rats. Rats exhibited a severe interstitial pneumonitis with type I alveolar cell destruction followed by type II cell proliferation. Mice exhibited a mild to moderate alveolitis with only slight damage to type I cells. Necrosis of bronchiolar Clara cells was evident in both species but was more extensive in rats. SKF 525a and piperonyl butoxide prevented OSS-induced increases in pulmonary DNA synthesis in rat lung suggesting that metabolic activation was necessary to elicit damage. Piperonyl butoxide treatments had no effect, however, on thymidine incorporation after OSS in mouse lung tissue, and the highest dose of SKF 525a had only a moderate inhibitory effect on this parameter while increasing animal mortality. These data indicate that systemic treatment with OSS results in damage to mouse, as well as rat, lung tissue at both the alveolar and bronchiolar levels.