Uptake of plasma triacylglycerol by a muscular artery in the rat: an ultrastructural study

The uptake of plasma triacylglycerol by the dorsalis pedis artery in the rat was studied using intravenous infusion of an emulsion of triacylglycerol at a rate of 2.3 mumol per min for 1.5 or 5 h. Electron microscopy revealed lipid droplets in the arterial lumen near the endothelium and in the medial smooth muscle cells (SMC), but not in the endothelial cells or in the extracellular space. Lamellar structures with a periodicity of 40 A developed in the arterial tissue when glutaraldehyde-fixed specimens were incubated at +25 degrees C before postfixation in osmium. Lamellae were present at the luminal and basal surfaces and within endothelial cells, and also in the medial extracellular space associated with the plasma membrane of SMC, in the intracellular channels and near and inside the mitochondria of the medial SMC. No lipid droplets or lamellae were found in the arterial tissue of the control rats. The findings indicate that plasma triacylglycerol is not taken up by the arterial tissue as intact lipid particles, but that these are hydrolyzed at the luminal surface of the endothelium, the lipolytic products then being transferred to the medial SMC for re-esterification and storage in the form of triacylglycerol. The lamellar structures found in the fixed and incubated arterial tissue are thought to represent fatty acids produced by the lipolysis of triacylglycerol during incubation, and we suggest that the transport of fatty acids from the arterial lumen to the medial SMC occurs by lateral movement in a continuum of cell membranes.