ATHEROSCLEROSIS ALTERS THE COMPOSITION, STRUCTURE AND FUNCTION OF ARTERIAL SMOOTH-MUSCLE CELL PLASMA-MEMBRANES

The object of this study was to examine changes in plasma membranes of arterial smooth muscle (ASM) during atherogenesis obtained from chole sterol-fed (2%) rabbits. A microsomal fraction highly enriched with pl asma membrane markers was prepared by subcellular organelle fractionat ion from ASM freshly isolated from the thoracic aorta. The membranes w ere analyzed for unesterified (free) cholesterol (FC) content, membran e bilayer structural parameters (X-ray diffraction), phospholipid (PL) composition, and Na+/K+-ATPase activity and kinetics. Following 8 wee ks on diet, membrane FC content increased 67.1%. Small angle X-ray dif fraction demonstrated an increase in membrane hydrocarbon core electro n density and an increase in overall lipid bilayer width (56-62 Angstr om). This increase in bilayer width was highly correlated with the mem brane FC content (r = 0.992). Both membrane FC content and bilayer wid th independently correlated with time on cholesterol diet. The phospho lipid profile of the membrane revealed a 16.4% increase in phosphatidy lcholine (PC), 19.3% decrease in phosphatidylethanolamine (PE) and 62. 8% increase in sphingomyelin (SM) content with no change in total PL c ontent. Na+/K+-ATPase activity was decreased 52.2% (P < 0.005), and [H -3]ouabain binding kinetics demonstrated a 27.6% decrease in maximum b inding sites (B-max) (P < 0.01) while the dissociation constant (K-d) remained unaltered. Membranes obtained from control ASM cells enriched with FC in culture demonstrated changes similar to those in atheroscl erotic ASM membranes including an increase in membrane FC content, an increase in bilayer width, and a decrease in Na+/K+-ATPase activity wi th decreased. These data demonstrate marked compositional, structural and functional changes in ASM cell membrane characteristics in dietary atherosclerosis. These changes were highly correlated with cholestero l accumulation in the plasma membrane bilayer and were observed before the appearance of visible lesions. We suggest that these membrane def ects may be linked with early atherogenesis.