Abstract
Substructural specializations of the endothelial caveolae of rat lung capillaries are described. Dense knobs occur at the place where caveola membrane, diaphragm and plasma membrane fuse. Similar structures are seen where caveolae fuse to form complex groups and in preparations of isolated caveolae. The appearance of these knobs in transverse sections may represent a circular skeletal structure, possibly functioning to maintain the patency of the stoma and the integrity of the diaphragm. In addition, the inner concave surface of the caveola membrane contains regularly spaced globular structures, possibly enzyme clusters or binding sites. This interpretation is supported by the finding that the distribution of lead phosphate deposits (formed on reaction of caveolar 5'-nucleotidase with AMP in the presence of Pb(N0 3) 2) corresponds with the spacing of globular structures. These substructural features of caveolae are considered with reference to the ability of caveolae to metabolize circulating vasoactive substances.
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