Effects of acid aspiration on pulmonary alveolar epithelial membrane permeability

Employing a modification of the in vivo model of a liquid-filled canine lung, we measured the movement of substances of specific sizes (albumin, 69,000 daltons with a molecular radius of 35 A; and dextran with a molecular weight of 150,000 to 170,000 and an approximate molecular radius of 100 A) from the pulmonary capillary blood to the liquid-filled lung. A solution with a specific pH (1.5 to 4.5) was instilled into the left lung of the animals at a dosage of 3 to 5 ml/kg of body weight. For both albumin and dextran with a molecular weight of 150,000 to 170,000, the time for 50 percent equilibration between the specific substance in the blood and the same substance in the pulmonary liquid decreased significantly with instillation of pulmonary liquid with a pH of 1.5 and 2.5 but did not with a pH of 3.5 or above (P less than 0.05). In addition, since histamine has been implicated as a possible humoral mediator leading to increased permeability of alveolar membranes, the levels of histamine were measured in pulmonary liquids and blood in all groups. Levels of histamine in the pulmonary liquid (but not blood) were significantly higher in animals with instillation of liquids with a pH of 1.5 and 2.5 compared to all other groups.