Inpocketings of the cell membrane (caveolae) in the rat myocardium

Abstract

The surface membrane of cardiac muscle cells of the rat papillary muscle shows, in addition to the openings of the T tubules, many small inpocketings (caveolae). Some caveolae are flask shaped, but many others are composite in shape and nonuniform in size; they all open into the intercellular space through a single, narrow neck. Their packing density is 6.1/ μ m 2 of cell surface and it has been culculated that they increase the cell membrane area by about 27%. A network of sarcoplasmic reticulum tubules lies close to the caveolae. Intramembranous particles, 9 nm in diameter, are fewer in the membrane of the caveolae than in the surface membrane proper and have no special arrangement around the caveolar necks. By comparison, the caveolae of endothelial cells are smaller and uniform in size, and have a greater packing density (67.5/ μ m 2 ). The caveolae of vascular smooth muscle cells are small and uniform in size, are arranged in rows, are associated with sarcoplasmic reticulum, and have a characteristic distribution of intramembranous particles around their necks.