Clara cell protein (CC16) is a 16-17-kDa protein secreted by Clara cells in the bronchial lining fluid of the lung from which it passively diffuses into serum before being eliminated by the kidneys. The concentration of CC16 in serum has recently been proposed as a peripheral marker of the integrity of Clara cells and/or of the bronchoalveolar/blood barrier. To evaluate the sensitivity of this new lung marker to acute epithelial damage induced by ozone (O(3)), CC16 was determined in the serum of rats after a single 3-h exposure to 0.3, 0.6, or 1 ppm O(3). The urinary excretion of the protein was also studied in rats repeatedly exposed to 1 ppm O(3), 3 h/day, for up to 10 days. The concentrations of CC16 in the lung or trachea homogenates, the lung CC16 mRNA levels, and classical markers of lung injury in bronchoalveolar lavage fluid (BALF) were also determined. O(3) produced a transient increase of CC16 concentration in serum that reached values on average 13 times above normal 2 h after exposure to 1 ppm O(3). The intravascular leakage of CC16 was dose-dependent and correlated with the extent of lung injury as assessed by the levels of total protein, LDH, and inflammatory cells in BALF. This effect was most likely responsible for the concomitant marked reduction of CC16 concentrations in BALF and lung homogenate, since the CC16 mRNA levels in the lungs were unchanged and the absolute amounts of CC16 leaking into serum or lost from the respiratory tract were similar. These changes were paralleled by an elevation of the urinary excretion of CC16 resulting from an overloading of the tubular reabsorption process. These results demonstrate that the assay of CC16 in serum and even in urine represents a new noninvasive test to detect the increased lung epithelial permeability induced by O(3).
Copyright 1999 Academic Press.