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Departments of 1 Medicine, 2 Physiology and Biophysics, 3 Molecular Pharmacology and Toxicology, 4 Biomedical Engineering, 5 Pharmaceutical Sciences, 6 Biochemistry, 7 Ophthalmology, 8 Pathology, and 9 Will Rogers Institute Pulmonary Research Center, Schools of Pharmacy, Medicine, and Engineering, University of Southern California, Los Angeles, California 90033
Transport characteristics of intact
albumin were investigated using primary cultured rat alveolar
epithelial cell monolayers. The apical-to-basolateral (ab)
flux of intact fluorescein isothiocyanate (FITC)-labeled albumin
(F-Alb) is greater than basolateral-to-apical (ba) flux at
the same upstream [F-Alb]. Net absorption of intact F-Alb
occurs with half-maximal concentration of ~1.6 µM and maximal transport rate of ~0.15
fmol · cm
2 · s
1.
At 15 and 4°C, both ab and ba F-Alb fluxes are
not different from zero, collapsing net absorption. The presence of
excess unlabeled albumin (but not other macromolecule species) in
either the apical or basolateral fluid significantly reduces both
ab and ba unidirectional F-Alb fluxes.
Photoaffinity labeling of apical cell membranes revealed an ~60-kDa
protein that exhibits specificity for albumin. These data indicate
that net absorption of intact albumin takes place via saturable
receptor-mediated transcellular endocytotic processes recognizing
albumin, but not other macromolecules, that may play an important role
in alveolar homeostasis in the mammalian lung.
protein transport; air-blood barrier; saturable transcytosis; albumin-binding sites; alveolar fluid balance
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