1 Along with arterial smooth muscle cells, they give rise to lipid-laden arterial plaque cells, termed foam cells.2 " 4 The intracellular neutral lipid within foam cells is cholesteryl ester, largely synthesized from lipoprotein-derived cholesterol. A great deal of attention has been focused on the mechanism by which lipoprotein cholesterol enters cells in such excess, especially in view of the stringent regulation of low density lipoprotein (LDL) receptor activity by cell cholesterol content. 5 The existence of a distinct receptor for certain species of modified LDL, termed the scavenger receptor, has been described in human macrophages, 67 macrophage cell lines, 8 and mouse peritoneal macrophages. 9 The activity of this receptor is regulated by a number of factors including cell density, 10 modified LDL uptake led to the suggestion that cholesteryl ester overload of macrophages in vivo could be accounted for by modification of LDL within the arterial wall and subsequent uptake by the scavenger receptor. 15 In line with this, it has already been shown 15 ' 16 that cultured endothelial cells and arterial smooth muscle cells can modify LDL in vitro to allow its recognition by the macrophage scavenger receptor. In addition, a protein has been isolated from vascular wall plaque lesions which is recognized by this binding site.

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The response of macrophages and smooth muscle cells to culture in free fatty acid has been compared. Because oleate and linoleate promoted triacylglycerol enrichment of smooth muscle cells, whereas palmitate had little effect, oleate was used for these studies. The kinetics of the accumulation of triacylglycerol produced by oleate was comparable between smooth muscle cells and macrophages. When grown in increasing concentrations of oleic acid at various fatty acid to albumin molar ratios, the extent of triacylglycerol accumulation in both cell types was dependent on the concentration of oleate, the concentration of albumin, and the oleate to albumin molar ratio. However, macrophages contained 2.6-fold more triacylglycerol than smooth muscle cells in the presence of oleate at 0.36 mM or greater and at levels of albumin higher than 0.15 mM. The cellular triacylglycerol content of macrophages was linearly related to the oleate to albumin molar ratio at both a constant albumin concentration and a constant oleate concentration, whereas the accumulation of triacylglycerol in smooth muscle cells showed a curvilinear relationship. When cells were preloaded with triacylglycerol, smooth muscle cells showed a greater loss of lipid when exposed to albumin than macrophages did. Over a two-hr time period, macrophages incorporated twice as much labeled fatty acid as smooth muscle cells. Thus, while smooth muscle cells and macrophages showed similar responses to exogenous fatty acid and albumin, there were also significant quantitative distinctions.

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