CELLULAR UPTAKE OF ALBUMIN FROM LUNGS OF ANESTHETIZED RABBITS

Resolution of alveolar edema depends on clearance of serum protein, as well as liquid from the alveolar space. Protein clearance is slower t han liquid clearance and may take days to weeks. Our earlier studies p resented evidence for the importance of paracellular removal of solubl e protein from the air spaces. However, long-term protein clearance ma y also depend on uptake by alveolar epithelial cells or macrophages. T his study examined cellular uptake of soluble human albumin and insolu ble colloidal gold-albumin from the lungs of anesthetized rabbits. Nat ive albumin was endocytosed by both alveolar type I and type II cells and appeared in vesicles and endosomes. Neither cell type took up coll oidal gold-albumin over periods as long as 8 h. Alveolar macrophages t ook up native albumin and colloidal gold-albumin to a greater extent a nd more rapidly than alveolar epithelial cells. The tracer proteins we re found in vesicles, endosomes, and phagolysosomes. Similarly, cultur ed alveolar macrophages took up native albumin more rapidly than cultu red type II cells. Thus macrophages may be important in clearing preci pitated protein from the air spaces, and they may have a role in compl eting the clearance of soluble protein. The potential for transepithel ial transport of soluble alveolar protein exists, but based on this wo rk and our prior studies, it appears to be a low-capacity pathway.