Intact alveolar epithelial permeability and transalveolar fluid absorption after thoracic irradiation in rats

We have addressed the question of how the alveolar space stays relatively free of fluid when thoracic irradiation injures the pulmonary capillary endothelium and plasma fluid leaks into the interstitium. A single dose of 15 Gy to the thorax of rats significantly increased the pulmonary capillary filtration coefficient and the lung wet/dry weight ratio 2 h after irradiation. However, there was no significant increase in the release of lactose dehydrogenase or leaking of Evans blue dye into the alveolar space, indicating that alveolar epithelial permeability remained intact. We found no significant difference in the basal alveolar fluid clearance between control and irradiated animals. There was also no significant difference in blockage of alveolar fluid clearance by amiloride. This indicates that the function of the alveolar epithelial Na⁺ channels is not impaired and that alveolar epithelium absorbs fluid normally. Examination of lung tissue by light microscopy demonstrated accumulation of fluid in the perivascular region but not in the alveolar space. Our data appear to indicate that the alveolar epithelial barrier function is more resistant to radiation than that of the pulmonary capillary endothelium. We conclude that intact alveolar epithelial permeability and normal transalveolar epithelial fluid absorption ability are of critical importance in keeping the alveolar space relatively free of fluid during acute radiation lung injury.