Atherosclerosis alters the composition, structure and function of arterial smooth muscle cell plasma membranes

Abstract

The object of this study was to examine changes in plasma membranes of arterial smooth muscle (ASM) during atherogenesis obtained from cholesterol-fed (2%) rabbits. A microsomal fraction highly enriched with plasma membrane markers was prepared by subcellular organelle fractionation from ASM freshly isolated from the thoracic aorta. The membranes were analyzed for unesterified (free) cholesterol (FC) content, membrane bilayer structural parameters (X-ray diffraction), phospholipid (PL) composition, and Na +/K +-ATPase activity and kinetics. Following 8 weeks on diet, membrane FC content increased 67.1%. Small angle X-ray diffraction demonstrated an increase in membrane hydrocarbon core electron density and an increase in overall lipid bilayer width (56–62 Å). This increase in bilayer width was highly correlated with the membrane FC content ( r = 0.992). Both membrane FC content And bilayer width independently correlated with time on cholesterol diet. The phospholipid profile of the membrane revealed a 16.4% increase in phosphatidylcholine (PC), 19.3% decrease in phosphatidylethanolamine (PE) and 62.8% increase in sphingomyelin (SM) content with no change in total PL content. Na +/K +-ATPase activity was decreased 52.2% ( P < 0.005), and [ 3H]ouabain binding kinetics demonstrated a 27.6% decrease in maximum binding 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 atherosclerotic ASM membranes including an increase in membrane FC content, an increase in bilayer width, and a decrease in Na +/K +-ATPase activity with decreased ouabain B max. These data demonstrate marked compositional, structural and functional changes in ASM cell membrane characteristics in dietary atherosclerosis. These changes were highly correlated with cholesterol accumulation in the plasma membrane bilayer and were observed before the appearance of visible lesions. We suggest that these membrane defects may be linked with early atherogenesis.