Differentiated distribution of the cell surface charge on the alveolar-capillary unit. Characteristic paucity of anionic sites on the air-blood barrier.

The distribution of the surface charge on the cells which constitute the air-blood barrier was investigated by perfusing cationized ferritin (CF) into the vasculature or into the airways of the mouse lung. Binding of CF is selective and defines highly differentiated domains. The most salient finding is that the air-blood barrier proper that includes type I epithelial cell and part of the corresponding avesicular area of the endothelial cell has very few or lacks anionic sites. In the vesicular area of endothelial cells the plasma membrane binds CF homogenously, with the exception of the membrane of plasmalemmal vesicles and transendothelial channels and their associated diaphragms. In contradistinction to the luminal surface of the type I epithelial cell, which is virtually devoid of anionic sites, CF decorates heavily the luminal surface of type II epithelial cells, up to the level of the junction with type I epithelial cells. Extruded lamellar bodies and tubular myelin also binds CF, presumably due to the phosphate groups of the dipalmitoyl phosphatidylcholine. The cell surface of the alveolar macrophages has relatively few binding sites for CF, but significant internalization of the latter occurs at early time intervals. The preferential distribution of anionic sites on cell surfaces of the alveolar-capillary unit may be influential in the transport of molecules and gases across various regions of the air-blood barrier.