Morphometry of the airways during late responses to antigen challenge in the rat

To quantitate the structural changes in the airways that contribute to the late bronchial response (LR) to antigen challenge we killed six Brown-Norway rats, sensitized to ovalbumin (OA) and challenged by aerosol, during the LR and compared the dimensions of the intraparenchymal airways with those of six control animals. Lungs were rapidly frozen with liquid nitrogen and fixed in Carnoy's solution. Paraffin sections were stained with hematoxylin-phloxine-saffron. At the time of the LR (382 +/- 39 min after OA challenge), RL increased from the baseline value (0.067 +/- 0.034 cm H2O.ml-1.s) by 0.107 +/- 0.03 cm H2O.ml-1.s (p less than 0.05). RL did not change significantly in the control rats. The lumen size and the wall area of all membranous airways were measured and were corrected for airway size by dividing by the basement membrane length squared (BM2). There was no increase in airway wall area in OA-challenged animals. However, the lumen of large airways (BM: 2.0 to 2.99 mm) was significantly less for the OA-challenged animals (0.039 +/- 0.0055 mm2) than for the control animals (0.058 +/- 0.0063 mm2; p less than 0.05). In six additional rats, the distribution of mast cells (MC) in the bronchial tree was determined. Tissues were fixed with Carnoy's solution and stained with a modified May-Grunwald-Giemsa stain. There were significantly more MC in the large airways than in medial or small airways. We conclude that smooth muscle constriction of large airways and not airway wall edema accounts for the LR in the rat. The distribution of the mast cells corresponds closely to the site of bronchoconstriction.