Two-week inhalation toxicity of polymeric diphenylmethane-4,4 '-diisocyanate (PMDI) in rats: Analysis of biochemical and morphological markers of early pulmonary response

The pulmonary response of Wistar rats to respirable polymeric diphenylmetha ne-4,4'-diisocyanate (PMDI) aerosol was examined in a 2-wk repeated nose-on ly inhalation exposure study. Exposure concentrations were 1.1, 3.3, and 13 .7 mg PMDI/m(3) (6 h/day, 15 exposures). The level of 13.7 mg/m(3) was actu ally a combination of an initial target concentration of 10 mg/m(3) in wk 1 , which was raised to 16 mg/m(3) in wk 2, due to a lack of signs suggestive of pulmonary irritation. An acute sensory irritation study on rats served as basis for selection of these concentrations. Shortly after the 2-wk expo sure period, rats were subjected to pulmonary function and arterial blood gas measurements. Lungs were examined by light and transmission electron mi croscopy, and labeling indices in terminal bronchioles were measured. Bronc hoalveolar lavage (BAL) was performed to assess various indicators of pulmo nary inflammation, including neutrophil and macrophage numbers, protein, la ctate dehydrogenase (LDH), gamma-glutamyltranspeptidase (gamma-GT), alkalin e phosphatase (APh), acid phosphatase (ACPh), and beta-N-acetylglucosaminid ase (beta-NAG). Phosphatidylcholine in BAL fluid and BAL cells was determin ed as aggregated endpoint suggestive of changes in pulmonary surfactant. Ra ts exposed to 3.3 and 13.7 mg/m(3) experienced concentration-dependent sign s of respiratory tract irritation. Determination of arterial blood gases, l ung mechanics, and carbon monoxide diffusing capacity did not demonstrate s pecific effects. Analysis of BAL fluid and BAL cells revealed changes indic ative of marked inflammatory response and/or cytotoxicity in rats exposed t o 13.7 mg/m(3), and the changes were characterized by statistically signifi cantly increased activities of LDH, beta-NAG, and protein. Phospholipid con centrations were increased in rats exposed to 1.1 mg/m(3) and above (elevat ed levels of lipid material in alveolar macrophages demonstrated by polychr ome stain) and 3.3 mg/m(3) and above (increased intracellular ACPh activity and intracellular phospholipids). In these groups, gamma-GT was statistica lly significantly increased. These findings suggest that changes in phospho lipid homeostasis appear to occur at lower levels than those eliciting infl ammation and cytotoxicity. Light and transmission electron microscopy sugge st that exposure to 3.3 and 13.7 mg/m(3) resulted in focal inflammatory les ions and an accumulation of refractile, yellowish-brownish material in alve olar macrophages with concomitant activation of type II pneumocytes. In the terminal bronchioles a concentration-dependent increase of bromodeoxyuridi ne (BrdU)-labeled epithelial cells was observed in ail PMDI exposure groups . in summary, it appears that respirable PMDI aerosol interacts with pulmon ary surfactant, which, in turn, may stimulate type Ii pneumocytes to increa se their production of surfactant and to proliferate.