Journal
AMERICAN JOURNAL OF PHYSIOLOGY-LUNG CELLULAR AND MOLECULAR PHYSIOLOGY
Volume 284, Issue 3, Pages L458-L465Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/ajplung.00237.2002
Keywords
protein transport; air-blood barrier; saturable transcytosis; albumin-binding sites; alveolar fluid balance
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Funding
- NHLBI NIH HHS [HL-38578, HL-38658, HL-38621, HL-64365] Funding Source: Medline
- NIGMS NIH HHS [GM-59297] Funding Source: Medline
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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 similar to1.6 muM and maximal transport rate of similar to0.15 fmol.cm(-2).s(-1). At 15 and 4degreesC, 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 similar to60-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.
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