The kinetics of the in vitro cholesterol uptake at the endothelial cell surface of the rabbit aorta

Mechanism of inhibition of cholesterol uptake by the arterial wall

Experiments have been described dealing with lipid synthesis and cholesterol uptake in perfused human and pig coronary arteries, rabbit aortas, and with the inhibitory effect of 7-ketocholesterol on cholesterol uptake in these preparations and in rabbits in vivo. Human and pig coronary arteries failed to synthesize cholesterol in vitro. 7-ketocholesterol inhibited cholesterol uptake in human coronary arteries and aortas of pigs and rabbits in vitro and by rabbit aortas in vivo. The inhibitory effect in vivo could only be shown after repeated i.v. injections of 7-ketocholesterol after solubilizing the steroid with bile sale (Na-glycocholate). Although 7-ketocholesterol was absorbed from the G.I. tract, gastric feeding of the bile salt steroid complex was ineffective, probably because of inadequate blood levels of 7-ketocholesterol achieved. The metabolic fate of 7-ketocholesterol and the nature of its effect on cholesterol are discussed. It is not likely that inhibition of HMG-CoA reductase is responsible for the inhibition of cholesterol uptake. The possibility was discussed that both cholesterol and 7-ketocholesterol actively compete for identical and specific binding sites or that an increase in 7-ketocholesterol in plasma leads to an increase in intracellular concentrations of this steroid thus inhibiting cholesterol transfer across the cell membrane. However definite conclusions on the nature of inhibition must await further experimentation.

Mechanisms of cholesterol accumulation in the arterial wall

The rate of cholesterol accumulation is a function of three separate processes: the transfer of lipid or lipoprotein from blood plasma to the artery, the binding and sequestering of lipids in the arterial wall and the solubilization and removal of lipid from the artery. These processes have been studied with lipids or lipoproteins labeled with radioisotopes by autoradiographic and quantitative chemical procedures. More recently immunochemical procedures have been applied to this problem. Studies have been performed with intact animals, isolated organs and cell cultures. In addition, homogenates have been used successfully to study intraarterial transformations of lipids, (for example, cholesterol esterification). Although epidemiologic and clinical studies, as well as animal experiments, have provided evidence that the concentration of plasma low or very low density lipoproteins parallels the rate of atherogenesis, the nature of the causal chain linking plasma lipoproteins to atherosclerosis is as yet unclear. A possible link between plasma lipoproteins, arterial liproprotein lipase and atherogenesis has been postulated.