Transcapillary movement of cationized ferritin in the isolated perfused rat lung

We have identified the ultrastructural localization of anionic sites on the luminal surface of the pulmonary microvascular endothelium by perfusing isolated rat lungs with polycationized ferritin (CF). The ligand decorated preferentially the luminal plasmalemma, coated pits, intercellular clefts, and about half of the plasmalemmal vesicles open to the lumen. Decoration of the plasmalemma was not uniform particularly in the nonvesiculated regions of the endothelium. Perfusion of the lung with high salt solution completely abolished binding of CF to the plasmalemma, coated pits, and intercellular clefts but did not significantly decrease binding of CF to the diaphragm of the luminal plasmalemmal vesicles. This indicates the presence of highly charged anionic sites on these regions of the endothelium. CF was taken up by vesicles and discharged on the capillary membrane. Perfusion of the lungs at 4 degrees C completely abolished transport of CF across the endothelium but did not modify the pattern of binding to the luminal endothelial surface. These findings are regarded as evidence for a functional subspecialization of plasmalemmal vesicles in the pulmonary microvascular endothelium.