Plasma membrane cholesterol in myocardial muscle and capillary endothelial cells. Distribution of filipin-induced deformations in freeze-fracture.

Glutaraldehyde-fixed rabbit myocardium was treated with the polyene antibiotic, filipin, which binds specifically to sterols in membranes, producing distinct 25 nm-diameter deformations. These deformations (lesions) serve as labels for the presence of cholesterol-rich membranes and membrane domains. The distribution and density of filipin-induced lesions was examined in the plasma membranes of cardiac muscle and capillary endothelial cells by freeze-fracture electron microscopy. In the exposed fracture faces of endothelial plasma membranes, more than 99% of the surface area was covered with lesions, whereas about 90% of the plasma membrane area in the muscle cells remained unaffected by the treatment. Muscle cell plasma membranes revealed a patchy response; the lesions occurred in clusters which were separated by interconnecting expanses of unaffected membrane. Endothelial cell plasmalemmal vesicles, and muscle cell caveolae and transverse tubules displayed the same response to filipin as neighbouring areas of plasma membrane. However, gap junctions and desmosomes of the muscle intercalated disc membrane, and coated pits in both cell types were completely resistant to filipin treatment. These results provide evidence that the plasma membranes of the two cell types possess markedly different overall cholesterol levels, and that each has a characteristic pattern in the planar distribution of cholesterol.