Caveolin and its cellular and subcellular immunolocalisation in lung alveolar epithelium: implications for alveolar epithelial type I cell function

Caveolae are flask-shaped invaginations of the plasmalemma which pinch off to form discrete vesicles within the cell cytoplasm. Biochemically, caveola e may be distinguished by the presence of a protein, caveolin, that is the principal component of filaments constituting their striated cytoplasmic co at. Squamous alveolar epithelial type I (ATI) cells, comprising approximate ly 95% of the surface area of lung alveolar epithelium, possess numerous pl asma-lemmal invaginations and cytoplasmic vesicles ultrastructurally indica tive of caveolae. However, an ultrastructural appearance does not universal ly imply the biochemical presence of caveolin. This immunocytochemical stud y has utilised a novel application of confocal laser scanning and electron microscopy unequivocally to localise caveolin-1 to ATI cells. Further, cyto plasmic vesicles and flask-shaped membrane invaginations in the ATI cell we re morphologically identified whose membranes were decorated with anti-cave olin-1 immunogold label. Coexistent with this, however, in both ATI and cap illary endothelial cells could be seen membrane invaginations morphological ly characteristic of caveolae, but which lacked associated caveolin immunog old label. This could reflect a true biochemical heterogeneity in populatio ns of morphologically similar plasmalemmal invaginations or an antigen thre shold requirement for labelling. The cuboidal alveolar epithelial type II c ell (ATII) also displayed specific label for caveolin-1 but with no ultrast ructural evidence for the formation of caveolae. The biochemical associatio n of caveolin with ATI cell vesicles has broad implications for the assignm ent and further study of ATI cell function.