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Chronic Inhalation Exposure to Coal Dust and/or Diesel Exhaust: Effects on the Alveolar Macrophages of Rats

The use of diesel-powered equipment in underground mines has raised questions regarding possible toxic interactions between coal dust and diesel emissions. The National Institute for Occupational Safety and Health (NIOSH) has studied in rats the effects of coal dust, alone and in combination with diesel engine exhausts, on physiologic properties of the alveolar macrophages (the pulmonary cell that provides the first line of defense against inhaled particulates) (1). These pneumocytes were obtained after the animals were subjected to chronic exposure to aerosols containing coal dust and/or diesel exhausts.

The rats were exposed by inhalation for 7 hours a day, 5 days a week, for 2 years. In addition to a control group (exposed only to filtered air), groups of rats were exposed to atmospheres containing 2 mg/m((3)) coal dust, 2 mg/m((3)) diesel particulates, or 1 mg/m((3)) coal dust plus 1 mg/m((3)) diesel particulates. At the end of exposure, alveolar macrophages were obtained at necropsy by lavage of the rat lungs with phosphate-buffered saline, pH = 7.4 (2).

The following physiologic parameters of these cells were measured: (1) membrane integrity, as indicated by maintenance of a constant cell volume and absence of protein and lysosomal enzymes in acellular fluid from the pulmonary lavage; (2) viability, as indicated by lack of affect on cellular protein content, cellular lysosomal enzyme activity, or oxygen consumption; (3) metabolic activity, i.e., enhanced secretion of reactive forms of oxygen (e.g., superoxide, hydrogen peroxide, and hydroxyl radical); and (4) morphologic changes indicating phagocytic activity, such as "spreading" of the cell and "ruffling" of its surface, as observed by scanning electron microscopy.

The results of these tests suggested that chronic exposure to coal dust and/or diesel exhaust did not alter the membrane integrity of the alveolar macrophages. However, exposure to coal dust activates alveolar macrophages, while diesel exhaust depresses them. In other words, exposure to coal dust in situ increased the number of macrophages obtained by lavage, enhanced the secretion of reactive forms of oxygen, and increased cellular spreading. In contrast, exposure to diesel-engine exhausts resulted in decreased secretion of reactive forms of oxygen and less formation of surface ruffling. The combination of coal dust and diesel-engine exhausts resulted in degrees of secretory activity and surface morphology intermediate between the effects of separate exposures. Reported by Div of Respiratory Disease Studies, NIOSH, CDC.

Editorial Note

Editorial Note: The data reported here may have implications for understanding the development of emphysema in coal miners, i.e., hypersecretion of reactive forms of oxygen may act to destroy pulmonary tissue. Furthermore, exposure to diesel-engine exhaust may adversely affect the lungs by decreasing the phagocytic capacity of alveolar macrophages. These laboratory investigations point up the complexity of predicting the pulmonary response to combined exposures, thus emphasizing the need for careful epidemiologic investigations of workers exposed to combined coal-diesel emissions.

References

  1. Green GM. The J. Burns Amberson Lecture--In defense of the lung. Am Rev Respir Dis 1970;102:691-703.

  2. Myrvik QN, Leake ES, Fariss B. Lysozyme content of alveolar and peritoneal macrophages from the rabbit. J Immunol 1961;86:133-6.



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