A quantitative assay for a clara cell-specific protein and its application in the study of development of pulmonary airways in the rat

Rat lung lavage contains a 10 kDa protein that has been shown by immunocytochemistry to be specific for Clara cells. An inhibition enzyme-linked immunosorbent assay was established for this protein using rabbit antibody to the 10 kDa Clara cell protein. The assay has a sensitivity of about 3.0 ng/ml and a working range of about 5 to 50 ng/ml. Quantitation of the 10 kDa protein in amniotic fluid revealed an increase of about 4-fold at day 20 of gestation. The 10 kDa protein content of lung ho-mogenate increased steadily from day 18 of gestation to 1 wk after birth, after which a decline was observed. Nearly 60-fold increase in the concentration of the 10 kDa Clara cell protein in lungs was noted from day 18 of gestation to birth and a further about 7-fold increase was noted from the day of birth to 1 wk of age. A progressive increase in the 10 kDa protein, with increasing age, was also noted on immunoblot analysis of lung homogenates. As judged from the immunoblots of lung homogenates, stained with rabbit antirat 10 kDa protein antiserum, the content of an anti-genically similar 200 kDa Clara cell protein was negligible. The quantitative results for 10 kDa Clara cell protein parallel the results of immunocytochemistry and quantitation of the volume density of Clara cell granules indicating that quantitation for the 10 kDa protein could be used to monitor the development of Clara cells and that of the pulmonary airways.