In vivo effects of endotoxin on intraepithelial mucosubstances in rat pulmonary airways. Quantitative histochemistry.

Bacteria-induced bronchopneumonias are often characterized by an influx of neutrophils and excess mucus in pulmonary airways. This study determined how endotoxin, a component of gram-negative bacteria and a potent inflammatory agent, affects the ultrastructure of the mucociliary apparatus and the amount of stored intraepithelial mucosubstances in the main axial airways within the lung. Rats were intranasally instilled, once a day for 3 days, with endotoxin or saline (controls). Animals were sacrificed 1, 2, or 7 days after the last instillation. Microdissected intrapulmonary axial airways (generations 8-11) from the right caudal lobes of infusion-fixed lungs were processed for light and electron microscopy. Morphometric techniques were used to determine the volume densities (Vs) of histochemically stained intraepithelial mucosubstances and numerical densities of airway epithelial cells. There were marked increases, compared with controls, in the amount of intraepithelial mucosubstances in the intrapulmonary axial airways at generations 8 and 11 in the right caudal lobes from endotoxin-instilled rats sacrificed 1, 2, and 7 days after the last instillation. There were significantly greater numbers of surface epithelial cells per length of basal lamina (i.e., hyperplasia) in endotoxin-exposed airways compared with airways from controls. This endotoxin-induced hyperplasia was due primarily to an increase in the number of mucus-secretory cells, which in endotoxin-exposed epithelium were columnar and contained numerous, large confluent, electronlucent, secretory granules composed of acidic and neutral glycoproteins. In contrast, secretory cells in airway epithelium from controls were cuboidal and contained small discrete, electron-dense, granules composed of only neutral glycoproteins. The numbers of ciliated cells and basal cells were similar in both control and endotoxin-exposed epithelium. Only endotoxin-exposed epithelium, however, contained atypical epithelial cells with numerous basal bodies, few cilia, and few apical secretory granules. These results indicate that repeated airway instillations of endotoxin induce an increase in the amount of intraepithelial mucosubstances, secretory cell hyperplasia, and excess luminal mucus in pulmonary airways. Therefore, endotoxin released from gram-negative bacteria may be partially responsible for the structural alterations, in the airway surface epithelium, which result in the excess luminal mucus observed in bacteria-induced bronchopneumonias.