Demonstration of fatty acid domains in membranes produced by lipolysis in mouse adipose tissue. A freeze-fracture study.

Fatty acids produced by isoproterenol-stimulated lipolysis in mouse adipose tissue incubated at pH 7.4 formed myelin figures when the tissue was processed at pH 9.0. Myelin figures, visualized with freeze-fracture electron microscopy, were found in intracellular channels of adipocytes, extracellular space, intracellular channels of endothelial cells, and capillary lumen. The E-fracture face of plasma membranes of adipocytes and endothelial cells and intracellular membranes of adipocytes contained areas that were free of particles. These areas, which were continuous with particle-studded areas of the E-fracture faces, were irregular in shape, sometimes circular or oblong, other times long and narrow. The surfaces of particle-free areas were flat, concave, convex, and often corrugated, with multiple folds that sometimes abutted on myelin figures. We conclude that the particle-free areas are composed of partially ionized fatty acids located in the external leaflets of plasma and intracellular membranes of adipocytes and endothelium. They were formed by fatty acids that entered leaflets at pH 9.0, probably from lipolyzed lipid droplets in adipocytes, moved in a continuum of membrane leaflets between and within cells, overcrowded the leaflets, and subsequently produced corrugations and lamellar extensions (myelin figures) of leaflets.