Distribution of labeled cholesterol in animal tissues

Rats were fed single doses of cholesterol-4-C14 and sacrificed at various time intervals. In another experiment, rabbits were injected intravenously with lipoproteins labeled with cholesterol-4-04 obtained from a donor animal and sacrificed at various time intervals. The ratios between specific radioactivities of cholesterol in several tissues and that in the serum were determined and followed as a function of time. For some tissues, this ratio reached values considerably above 1 when a sufficiently long time had elapsed between the administra tion of label and the time of sacrifice. Time curves for specific radioactivities in various tissues, best fitting the experimental data, were calculated with the aid of a digital computer by assuming that each tissue studied represented a single homogeneous compartment of cholesterol and that there was a flow of radioactive cholesterol from plasma into the tissues. Similar time curves, based on the additional assumption that no significant synthesis of cholesterol took place in the tissues, were also constructed. The results indicate that the treatment of cholesterol in each tissue as a single homogeneous compartment in exchange with plasma cholesterol, with a provision of some cholesterol synthesis in the various organs, is adequate to explain the data in most cases examined.