Early markers of lung injury

The goal of this study was to develop an early marker of lung injury that might change in response to exposure to a mobile source emission. Nitrogen dioxide (NO2)2 was chosen as an example of an atmospheric pollutant that is related to automobile emissions. Since reorganization of the connective tissue matrix of the lung occurs in response to injury, markers of connective tissue metabolism were selected as targets. Hydroxylysine became the marker of choice. It is an amino acid that is virtually exclusive to collagen, although it does occur in minimal amounts in other proteins. Furthermore, it is excreted in the urine, which makes it readily available for analysis using noninvasive techniques. Other markers evaluated as part of the study included angiotensin-converting enzyme as a marker of lung injury, desmosine as a marker of elastin degradation, and hydroxyproline as another marker of collagen metabolism. Male Fischer-344 rats were exposed in whole-body chambers to controlled concentrations of NO2 for various doses and periods of time. The concentrations of NO2 ranged from 0.5 to 30 parts per million (ppm); the rats were exposed for six hours per day for periods of two days to four weeks. Urine and bronchoalveolar lavage samples were collected and analyzed for the appropriate marker. In addition, pulmonary function studies and histologic examinations of the lungs were completed at selected time points. Urinary hydroxylysine concentration increased as a function of NO2 concentration during six-hour-per-day exposures for two days. This short-term exposure required relatively high doses to achieve significant changes in the hydroxylysine output. During one-week exposures to either 25 or 30 ppm NO2, there was an increase in urinary hydroxylysine associated with changes in lavage concentrations of angiotensin-converting enzyme and hydroxylysine. The lungs of these animals demonstrated histologic changes typical of oxidant injury. Four-week exposure protocols using 0.5 and 1 ppm NO2 were most interesting in terms of the sensitivity of the marker. There was minimal damage revealed by the histology and function studies, yet there were significant increases in the excretion of hydroxylysine. It appears that hydroxylysine can be indicative of exposure when other parameters are normal. It will require long-term follow-up of exposed rats to determine whether or not the change in marker concentration is predictive of damage. Hydroxylysine may be an excellent marker of exposure to oxidants in the human population. Controlled studies to establish base-line values are needed, followed by carefully controlled studies in individuals with connective tissue abnormalities of the lung.