Pulmonary toxicity of methylcyclopentadienyl manganese tricarbonyl: nonciliated bronchiolar epithelial (Clara) cell necrosis and alveolar damage in the mouse, rat, and hamster

The impact of environmental and occupational exposures of manganese on pulmonary, hepatic, and renal functions

Manganese (Mn) is an essential trace element for humans, but long-term environmental or occupational exposures can lead to numerous health problems. Although many studies have identified an association between Mn exposures and neurological abnormalities, emerging data suggest that occupationally and environmentally relevant levels of Mn may also be linked to multiple organ dysfunction in the general population. In this regard, many experimental and clinical studies provide support for a causal link between Mn exposure and structural and functional changes that are responsible for organ dysfunction in major organs like lung, liver, and kidney. The underlying mechanisms suggested to Mn toxicity include altered activities of the components of intracellular signaling cascades, oxidative stress, apoptosis, affected cell cycle regulation, autophagy, angiogenesis, and an inflammatory response. We further discussed the sources and possible mechanisms of Mn absorption and distribution in different organs. Finally, treatment strategies available for treating Mn toxicity as well as directions for future studies were discussed.

Methylcyclopentadienyl manganese tricarbonyl increases cell vulnerability to oxidative stress on rat thymocytes

Methylcyclopentadienyl manganese tricarbonyl (MMT) is used as a gasoline antiknock additive. However, the toxic effect of MMT is currently not well understood. In this study, we investigated the toxic effect of MMT on rat thymocytes using a flow cytometer and fluorescent probes. MMT at 100-300 µM significantly increased the population of cells exhibiting propidium fluorescence, i.e., the population of dead cells. The intensity of BES-So-AM fluorescence significantly increased when using 100 µM MMT. In addition, the intensity of oxonol fluorescence in rat thymocytes increased with the treatment with MMT in a concentration-dependent manner (10-100 µM). The toxic effect of MMT was inhibited by quercetin, antioxidant flavonoid. Moreover, co-treatment with 30-100 µM MMT and 100 µM H₂O₂ increased the cell lethality further. These results indicate that MMT increases cell vulnerability to oxidative stress on rat thymocytes. This study provides insight into the toxic effect of MMT on the immune system.

Alterations in the properties and isoforms of sciatic nerve Na(+), K(+)-ATPase in methylcyclopentadienyl manganese tricarbonyl-treated mice

The in vivo effect of methylcyclopentadienyl manganese tricarbonyl (MMT), an organic manganese-containing compound, on the mouse motor nerve was studied. The motor nerve conduction velocity was markedly decreased in MMT-treated mice. The Na(+),K(+)-ATPase activity of sciatic nerve isolated from MMT-treated mice was decreased; however, the sciatic nerve Na(+),K(+)-ATPase activity was not affected by the in vitro treatment of MMT. Moreover, [(3)H]ouabain binding of sciatic nerve isolated from MMT-treated mice was decreased. Using Western blot analysis, the amount of Na(+),K(+)-ATPase catalytic alpha1 subunit polypeptide in sciatic nerve of MMT-treated mice was also decreased. These results indicate that a causal relationship may exist between reduced nerve Na(+),K(+)-ATPase activity and motor nerve conduction velocity in MMT-treated mice and that a measurable decrease in alpha1 catalytic subunit isoform of Na(+),K(+)-ATPase may be necessary for the development of peripheral neuropathy by MMT.

Mechanisms of glucocorticoid involvement in mouse lung tumorigenesis

This report examines a possible mechanism of mouse lung tumor prophylaxis by glucocorticoids (GC). Adrenalectomy (Ax) increased, and corticosterone replacement decreased, lung tumor multiplicity when treatment was begun before administration of the carcinogen, urethan. Ax increased the 3H-thymidine labeling index of alveolar epithelial cells. Tumor multiplicity was also enhanced when urethan was administered during the period of compensatory hyperplasia that occurred in response to lung injury induced by methylcylopentadienyl manganese tricarbonyl. Thus, carcinogen-induced tumor development was amplified by stimulation of division of the target cell population. GC regulation of alveolar epithelial cell proliferation, and hence tumor susceptibility, may be mediated by the Ca++/phospholipid-dependent protein kinase (PKC).The tumor-resistant strain, C57BL/6J, has greater adrenal corticosterone content, higher epithelial cell PKC activity, and lower alveolar epithelial cell proliferation than the tumor-susceptible strain, A/J. In vitro, GC inhibit proliferation of a lung epithelial-derived cell line and increase PKC activity in that cell line. Thus, we hypothesize that GC protect against lung tumor development by increasing PKC content in the epithelial cells from which lung tumors arise; increased intracellular PKC results in decreased epithelial proliferation, and reduces the probability of induction of tumorigenesis by urethan.

Comparative pneumotoxicity of cyclopentadienyl manganese tricarbonyl and methylcyclopentadienyl manganese tricarbonyl

The acute pneumotoxic effects of cyclopentadienyl manganese tricarbonyl (CMT) and methylcyclopentadienyl manganese tricarbonyl (MMT) were compared to delineate the role of the methyl side chain in the toxicity of these organomanganese compounds and to further our understanding of the mechanisms by which these compounds act. Specifically, lung manganese (Mn) burdens and the pneumotoxic response, as measured by bronchoalveolar lavage parameters, were determined in male Sprague-Dawley rats 24 hr after sc administration of 0.5, 1.0, or 2.5 mg Mn/kg as CMT or MMT. The pneumotoxic response to either compound was characterized by large increases in lavage albumin and protein content with smaller increases in lactate dehydrogenase levels. CMT was approximately twice as potent as MMT. This difference in potency may be due to methyl side chain oxidation, a metabolic detoxification pathway unavailable to CMT. Lung Mn content was significantly elevated after treatment with either CMT or MMT. Heptane extraction studies revealed that Mn was accumulated in a nonlipid soluble form, suggesting the accumulation of metabolites rather than heptane soluble parent MMT or CMT. A strong correlation between pulmonary Mn content and toxicity was observed, suggesting a causal relationship between the accumulation of CMT or MMT metabolites and toxicity. Piperonyl butoxide diminished both the pneumotoxicity and Mn accumulation resulting from CMT or MMT, suggesting both phenomena are due to monooxygenase metabolites. Pulmonary nonprotein sulfhydryl (NPSH) levels were increased twofold 24 hr after administration of either CMT or MMT. Depletion of NPSH was not observed 1.5 or 6 hr after administration. The mechanisms of this response are unclear but may be due to the metabolism of CMT or MMT to unstable compounds which release inorganic Mn within pulmonary cells.

Toxic effects of mosquito coil (a mosquito repellent) smoke on rats. II. Morphological changes of the respiratory system

A group of 20 female albino rats was exposed to mosquito coil smoke, 8 h a day, 6 days per week, for 60 days. An additional group receiving air exposure served as control. The smoke-exposed animals had a lower body weight than the controls. Smoke-induced histopathological lesions, including an inflammation of the tracheal epithelium, atelectasis of the lung parenchyma, emphysema, increase of alveolar macrophages in the alveolar space and perivascular infiltration of polymorphonuclear cells were observed in the experimental rats. An elevation of enzyme activities of lactate dehydrogenase, glutamate pyruvate transaminase, glutamate oxoacetate transaminase and acid phosphatase were found in the serum of the smoke-exposed rats indicating the enzymes were released from the damaged tissues into the blood stream.

Neurotoxic effects of methylcyclopentadienyl manganese tricarbonyl (MMT) in the mouse: basis of MMT-induced seizure activity

Methylcyclopentadienyl manganese tricarbonyl (MMT) is an organic manganese-containing compound which is used as an additive in unleaded gasoline. One neurotoxic effect of MMT in mice is seizure activity. In this study, seizures were observed in mice treated with MMT in propylene glycol or corn oil. The LD50 associated with seizure activity was lower in mice receiving MMT in propylene glycol (152 mg/kg) than in those receiving MMT in corn oil (999 mg/kg). Manganese concentrations in the brains of mice which showed seizure activity due to MMT were higher than in those that did not (2.45 micrograms/g vs. 1.14 micrograms/g for MMT treated in propylene glycol and 3.25 micrograms/g vs. 1.63 micrograms/g for MMT in corn oil). Mice treated with manganese chloride (MnCl2) showed increases in brain manganese comparable to those of the mice showing seizure activity due to MMT, but exhibited no sign of seizure activity. MMT in non-lethal seizure-inducing doses had no effect on the accumulation of 4-aminobutyric acid (GABA) in mouse brain. However, MMT inhibited the binding of t-[3H]t-butylbicycloorthobenzoate [3H]-TBOB (a ligand for the GABA-A-receptor linked chloride channel) in mouse brain membranes with an IC50 value of 22.8 microM. The data suggest that MMT (organic manganese) or a closely related metabolite and not elemental manganese itself is responsible for the seizure activity observed. The seizure activity may be the result of an inhibitory effect of MMT at the GABA-A receptor linked chloride channel.

Disposition and toxicity of methylcyclopentadienyl manganese tricarbonyl in the rat

The disposition and toxicity of methylcyclopentadienyl manganese tricarbonyl (MMT) was studied in Sprague-Dawley rats after subcutaneous administration at a dose of 4 mg/kg. Blood, lung, liver and kidney Mn levels were increased between 1.5 and 96 h after MMT injection, with peak organ levels occurring at 3-6 h. At this time the MMT-derived Mn concentration in lung, liver and kidney averaged 13-, 4- and 4-fold higher, respectively, than in the blood, indicating the accumulation and retention of MMT (or metabolite) in these tissues. Maximal pulmonary toxicity, as assessed by pulmonary lavage protein levels, occurred 24-48 h after injection. Plasma urea and sorbitol dehydrogenase levels were not increased at any time after MMT, suggesting minimal or no hepatic or renal injury. That maximal pulmonary toxicity occurred after peak Mn accumulation, and that the organ-specific toxicity of MMT correlated with its accumulation and retention, suggests a causal relationship between tissue Mn accumulation and MMT-induced toxicity.

Systematically applied chemicals that damage lung tissue

A large, and increasing number of drugs and chemicals have been found which are toxic to lung following systemic administration. These agents damage lung tissue specifically, or in addition to damage to other tissues. Mechanisms explaining the pulmonary damage produced by some lung toxins have been uncovered. These include concentration of the agent within lung, the absence of adequate pulmonary detoxication systems, and bioactivation to a toxic species within specific lung cells or at distant sites followed by transport to the lung. The basic biochemical lesions underlying lung damage, responses of individual lung cells and pulmonary repair processes to the toxic agent, and species and age differences in susceptibility to lung damage have not, however, been well defined for most lung toxins. This review describes the information available on pulmonary biochemical and pathological changes associated with some of these lung-toxic agents. In addition, mechanisms proposed to explain the lung damage are discussed. The agents covered include: paraquat, the thioureas, butylated hydroxytoluene, the trialkylphosphorothioates, various lung-toxic furans and antineoplastic agents, the pyrrolizidine alkaloids, metals and organometallic compounds, amphiphilic agents, hydrocarbons, oleic acid, 3-methylindole, and diabetogenic agents. Detailed reviews on the overall toxicity of many of these agents have been published elsewhere. This review concentrates on their pulmonary toxicity. Information is presented as an overview to illustrate both the extensive literature that is available and the important questions that remain to be answered about systemic chemicals that damage lung tissue.

The role of metabolism in N-methylthiobenzamide-induced pneumotoxicity

N-Methylthiobenzamide (NMTB) produces pulmonary edema, hydrothorax, and death in rodents. The objectives of the present studies were to establish a relationship between the lethality of NMTB and its pneumotoxicity and to explore the role of S-oxidation in these events. Pulmonary injury was assessed by measuring [14C]thymidine incorporation into pulmonary DNA. Administration of NMTB resulted in increased pulmonary [14C]thymidine incorporation in both rats and mice. These increases were blocked in both species by pretreatment of animals with sublethal doses of NMTB. However, the lethality of NMTB was not blocked in mice by prior administration of NMTB even though this procedure has been shown to protect rats. 1-Methyl-1-phenyl-3-benzoylthiourea (MPBTU) protected both rats and mice from lethal doses of NMTB and blocked NMTB-induced increases in pulmonary [14C]thymidine incorporation. N-Methylthiobenzamide S-oxide (NMTBSO), a metabolite of NMTB, produced lung injury which was similar to that produced by NMTB. NMTBSO was more potent than NMTB when administered iv, but not when given ip. The role of hepatic metabolism in NMTB pneumotoxicity was examined by administering NMTB to rats which had either undergone partial hepatectomy or been pretreated with N-octylimidazole. Neither of these procedures diminished the lethality of NMTB. These data suggest that NMTB lethality is mediated by pulmonary injury resulting from NMTB S-oxidation in the lung.

The acute toxicity of cyclopentadienyl manganese tricarbonyl in the rat

The acute toxicity of cyclopentadienyl manganese tricarbonyl (CMT) was studied in Sprague-Dawley rats. CMT was found to produce convulsions and pulmonary edema. The ED50s for convulsion were 32 mg/kg (95% C.I. 24-42 mg/kg) p.o. and 20 mg/kg (95% C.I. 15-26 mg/kg) i.p. The LD50s for p.o. and i.p. administration were 22 mg/kg (95% C.I. 19-26 mg/kg) and 14 mg/kg (95% C.I. 10-20 mg/kg), respectively. Approximately 13-16% of the administered dose was recovered in the urine from 0 to 48 h post-dosing. The majority of this material was present as an organometallic form of manganese other than CMT. Phenobarbital pretreatment prevented the convulsions and pulmonary damage produced by a 50 mg/kg i.p. dose of CMT. Rats pretreated with CMT (5 mg/kg, i.p.) for 3 days exhibited convulsions but no deaths after treatment with a 34 mg/kg p.o. dose of CMT. These results suggest that CMT does not require metabolic activation to produce toxic effects, and that prior exposure to CMT produces tolerance.

Long-term effects of repeated exposure to 3-methylfuran in hamsters and mice

Male and female Syrian golden hamsters were exposed for 2 h once a week for 10 consecutive weeks to 344 mumol/l of 3-methylfuran (3MF), an agent known to produce acute Clara-cell necrosis. Ten months later their respiratory function was evaluated. No functional differences were found between control and treated animals, and histopathologic evaluation of the lungs did not reveal any major treatment-related alterations. Male and female BALB/c mice were exposed for 1 h to 26.8 mumol/l of 3MF once weekly for 10 wk. After 2 yr the tumor incidence in exposed animals was not increased when compared to controls. It is concluded that the acute Clara-cell necrosis produced by 3MF at the doses used is of little long-term consequence.

An improved method for calculating labeling indices of lung epithelial and interstitial cells

The sampling method was developed as an improvement in calculating labeling indices (LIs) of epithelial and interstitial cells in the airways and alveolar region of the lungs of rats. This method is more accurate and requires less time than the traditional cytokinetic methods, which involve counting all labeled and unlabeled cells. In the sampling method, all labeled cells are counted, but the total number of cells is estimated based on a representative sample count of airway and alveolar cells in each animal. The LIs as calculated by the two methods correlated very well (r greater than 0.97). The small amount of error introduced by estimating the total number of cells was more than compensated for by the increased accuracy associated with a larger sample size.

Acute target organ toxicity of 1-nitronaphthalene in the rat

1-Nitronaphthalene (1-NN) produced respiratory distress and centrilobular liver necrosis in male Sprague-Dawley rats after a single intraperitoneal injection (100 mg kg-1). Microscopic examination of the lungs of rats killed 24 h after the injection revealed a highly selective non-ciliated bronchiolar (Clara) cell necrosis as the only remarkable lesion. Pretreatment of animals with phenobarbital offered complete protection from the respiratory distress induced by 1-NN, but increased the severity of the hepatotoxicity. Pretreatment with SKF-525A protected against 1-NN-induced liver necrosis, but did not alter the incidence or severity of the respiratory distress. Under similar conditions, this pattern of toxicity was not seen with the structural analogue 2-nitronaphthalene.

Selective damage to nonciliated bronchiolar epithelial cells in relation to impairment of pulmonary monooxygenase activities by 1,1-dichloroethylene in mice

A single ip dose of 1,1-dichloroethylene (DCE) to mice (125 mg/kg) caused a reduction within 24 hr in cytochrome P-450 and related monooxygenases in lung microsomes, with no corresponding changes in liver and kidney microsomes. Light microscopy revealed that at 24 hr, DCE caused a highly selective and complete loss of the bronchiolar nonciliated (Clara) cells at all levels of the tracheobronchial tree. Electron microscopy showed that at this time, the bronchiolar luminal surface was covered by flattened, elongated ciliated cells. Within 24 hr total microsomal cytochrome P-450 and NADPH cytochrome c reductase were maximally reduced to about 50% of control and cytochrome P-450-dependent enzyme activities decreased to about 60% of control. By contrast, coumarin 7-hydroxylase was reduced to approximately 10% of control within 4 days. Since pulmonary coumarin 7-hydroxylase has been shown to reside almost exclusively in the Clara cells, this finding is in agreement with the observed extensive necrosis of the Clara cells. The return of lung microsomal P-450-linked enzyme activities took between 3 and 6 weeks and was paralleled by a corresponding slow reappearance of the bronchiolar Clara cells.

Metabolism of bromobenzene to glutathione adducts in lung slices from mice treated with pneumotoxicants

Recent studies showing that the bronchiolar Clara cell and alveolar Type II cell are major loci of cytochrome P-450 monooxygenases in the lung suggested that measurement of xenobiotic metabolizing enzyme activity might provide a useful and sensitive index of injury to these cell types. Accordingly, an assay has been developed for quantitating the rate of formation of reactive bromobenzene metabolites in lung slices which is based upon measuring the rate of formation of bromobenzene glutathione adducts. To demonstrate that monitoring adduct formation would yield quantitatively similar data to the traditional covalent binding assay for measuring the formation of reactive bromobenzene intermediates, covalent binding and conjugate formation were assayed in incubations of phenobarbital-induced hepatic microsomes conducted in the presence of various cytochrome P-450 monooxygenase inhibitors. Incubation conditions which decreased the rate of covalent binding (incubations done in the absence of glutathione) resulted in similar decreases in conjugate formation (incubations done in the presence of glutathione). In lung slices, the metabolism of bromobenzene to glutathione conjugates was linear for 20 min and continued to increase with time over the entire 160 min of the study. The formation of bromobenzene glutathione adducts in lung slices from piperonyl butoxide-treated animals occurred at a significantly lower rate than control. Likewise, lung slices from animals treated with butylated hydroxytoluene or carbon tetrachloride, agents known to injure alveolar epithelial cells, metabolized bromobenzene to glutathione conjugates at significantly slower rates than control. In contrast, treatment with naphthalene or dichloroethylene, agents which damage the bronchiolar epithelial cells, had little or no effect on conjugate formation. Similarly, there were no significant differences in the rate of bromobenzene glutathione conjugate formation between lungs of air- and ozone-exposed (1.0 ppm X 4 hr) mice killed 2, 24, 48, 72, or 120 hr after exposure. These studies suggest that monitoring the rate of bromobenzene glutathione conjugate formation in lung slices may be a useful and sensitive biochemical index of injury to certain cells of the lung but that severe damage to the nonciliated bronchiolar epithelial cells has little effect on the rate of metabolic activation of this aromatic hydrocarbon.

Acute inhalation toxicity of 3-methylfuran in the mouse: pathology, cell kinetics, and respiratory rate effects

The acute inhalation toxicity of 3-methylfuran (3MF) was investigated in male BALB/c mice by morphologic examination of animals killed at varying timepoints following a 1-hr exposure to an initial chamber concentration of 14 to 37 mumol/liter (343 to 906 ppm). In addition, respiratory rate measurements and cell kinetics were used to assess quantitatively pulmonary damage and repair. Necrosis of nonciliated bronchiolar epithelial (Clara) cells was seen 1 day following exposure and was followed by regeneration, which was virtually complete, within 21 days. Cell kinetic studies showed peak bronchiolar cell proliferation at 3 days with a labeling index (LI) of 5.0% compared to 0.4% in controls. An increase in parenchymal cell proliferation was also noted coincident with a mild interstitial pneumonitis. This parenchymal proliferation, peaking at 10 days with an LI of 1.4% compared to 0.2% in controls, consisted primarily of type II epithelial and endothelial cell proliferation indicating possible delayed damage and repair of type I epithelial and endothelial cells. The respiratory rate showed an initial transient increase followed by a more prolonged decrease with eventual return to control levels. 3MF toxicity was also evidenced by a necrotizing suppurative rhinitis, centrilobular hepatic necrosis, lymphocyte necrosis in the thymus and spleen, sialoadenitis, and otitis media.

Pathology of acute inhalation exposure to 3-methylfuran in the rat and hamster

The acute inhalation toxicity of 3-methylfuran (3MF) was investigated in SPF Fischer-derived and CD/CR rats, and golden Syrian hamsters by determination of the 2-week LC50, and by histologic examination of animals killed 1, 3, and 14 days following a 1-hr exposure to 148 and 322 mumole 3MF/liter for CD/CR rats and hamsters, respectively. The Fischer-derived rat was more sensitive to 3MF-induced lethality than the CD/CR rat with an LC50 in the male rat of 81 mumole/liter-1 hr as compared to 222 mumole/liter-1 hr. No sex difference was found. The hamster was relatively resistant with no lethality at 322 mumole 3MF/liter-2 hr. Pulmonary damage was present in both species. In the hamster, selective necrosis of nonciliated bronchiolar epithelial (Clara) cells was seen at 1 day with virtually complete regeneration by 14 days whereas in the rat the bronchiolar epithelial damage was more extensive and was followed by scattered peribronchiolar fibrosis and epithelial mucous metaplasia suggestive of "small airway disease" of man. Relatively selective 3MF-induced necrosis of olfactory epithelium occurred in the nasal mucosa of both species. Resolution of this lesion was seen by 14 days in the hamster. In the rat, however, the necrosis was much more extensive and was followed by partially occlusive fibrosis of the nasal cavity as seen at 14 days. 3MF also induced centrilobular hepatic necrosis in both species. In the rat, lymphocyte necrosis in the thymus and spleen, and esophageal necrosis was also seen.

Potentiating effects of oxygen in lungs damaged by methylcyclopentadienyl manganese tricarbonyl, cadmium chloride, oleic acid, and antitumor drugs

The intraperitoneal administration of methylcyclopentadienyl manganese tricarbonyl (MMT) and cyclophosphamide, exposure to an aerosol of cadmium chloride, intravenous administration of oleic acid, and intratracheal instillation of bleomycin to young female BALB/c mice or CD/CR rats result in acute lung injury. Pulmonary morphology and lung collagen content were examined in animals treated with these chemicals alone or in combination with an elevated oxygen concentration (80%) in the inspired air. In mice, the development of fibrosis could be significantly enhanced if animals treated with MMT, cadmium chloride, cyclophosphamide, or bleomycin were exposed to 80% oxygen immediately following exposure to these agents. In rats only cyclophosphamide- and bleomycin-induced acute lung injury was potentiated by hyperoxia, resulting in significant enhancement of lung collagen content. The pathogenesis responsible for this differential species response of pulmonary injury to hyperoxia remains to be investigated.