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Articles in PresS, published online ahead of print November 15, 2002
Am J Physiol Lung Cell Mol Physiol, 10.1152/ajplung.00237.2002
Submitted on July 19, 2002
Accepted on November 9, 2002
1 Department of Medicine, University of Southern California School of Medicine, Los Angeles, CA, USA; Department of Physiology and Biophyscis, University of Southern California School of Medicine, Los Angeles, CA, USA; Will Rogers Institute Pulmonary Research Center, Los Angeles, CA, USA
2 Department of Pharmaceutical Sciences, University of Southern California School of Pharmacy, Los Angeles, CA, USA
3 Department of Biochemistry, University of Southern California School of Medicine, Los Angeles, CA, USA
4 Department of Pharmaceutical Sciences, University of Southern California School of Pharmacy, Los Angeles, CA, USA; Department of Ophthalmology, University of Southern California School of Medicine, Los Angeles, CA, USA
5 Department of Medicine, University of Southern California School of Medicine, Los Angeles, CA, USA; Department of Pathology, University of Southern California School of Medicine, Los Angeles, CA, USA; Will Rogers Institute Pulmonary Research Center, Los Angeles, CA, USA
* To whom correspondence should be addressed. E-mail: kjkim{at}usc.edu.
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 (Kt) of ~ 1.6 µM and maximal transport rate (Jmax) of ~ 0.15 fmoles/cm2/sec. 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.
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