The measurement of the exchangeable pools of cholesterol in the baboon.

Five male baboons were fed cholesterol-1alpha-(3)H until an isotopic steady state was approached in which the ratio of serum to dietary cholesterol specific activity was constant. The animals were then given single intravenous injections of cholesterol-4-(14)C, and the disappearance curves of the (14)C from the circulation were followed for 70-85 days. The total exchangeable cholesterol pools under these conditions were determined by carcass analysis at the end of the experiment and were shown to agree closely with the sums of the two exchangeable cholesterol pools as predicted from analysis of the die-away curve in terms of a two exchangeable pool model, assuming that entry into and exit from the system occur predominantly through the more rapidly exchanging of the two pools. These results have been interpreted as support for the validity of a model consisting of two exchangeable pools and one nonexchangeable or very slowly exchangeable pool as an approximation of over-all cholesterol metabolism in this species. In addition, an anatomical and chemical dissection of these pools was performed. While the three pool model is not applicable to every tissue, it has been possible to show that its general applicability in the intact animal stems from the fact that the major portions of the three cholesterol pools of the body lie within a limited number of tissues.