PHYSICAL EFFECTS OF CHOLESTEROL ON ARTERIAL SMOOTH-MUSCLE MEMBRANES -EVIDENCE OF IMMISCIBLE CHOLESTEROL DOMAINS AND ALTERATIONS IN BILAYERWIDTH DURING ATHEROGENESIS

Small angle X-ray diffraction was used to examine arterial smooth musc le cell (SMC) plasma membranes isolated from control and cholesterol-f ed (2%) atherosclerotic rabbits. A microsomal membrane enriched with p lasma membrane obtained from animals fed cholesterol for up to 13 week s showed a progressive elevation in the membrane unesterified (free) c holesterol:phospholipid (C/PL) mole ratio. Beyond 9 weeks of cholester ol feeding, X-ray diffraction patterns demonstrated a lateral immiscib le cholesterol domain at 37 degrees C with a unit cell periodicity of 34 Angstrom coexisting within the liquid crystalline lipid bilayer, On warming, the immiscible cholesterol domain disappeared, and on coolin g it reappeared, indicating that the immiscible cholesterol domain was fully reversible. These effects were reproduced in a model C/PL binar y lipid system. In rabbits fed cholesterol for less than 9 weeks, less er increases in membrane C/PL mole ratio were observed. X-ray diffract ion analysis demonstrated an increase in membrane bilayer width that c orrelated with the C/PL mole ratio, This effect was also reproduced in a C/PL binary lipid system. Taken together, these findings demonstrat e that in vivo, feeding of cholesterol causes cholesterol-phospholipid interactions in the membrane bilayer that alter bilayer structure and organization. This interaction results in an increase in bilayer widt h peaking at a saturating membrane cholesterol concentration, beyond w hich lateral phase separation occurs resulting in the formation of sep arate cholesterol bilayer domains. These alterations in structure and organization in SMC plasma membranes may have significance in phenotyp ic modulation or aortic SMC during early atherogenesis.