Chemical and isotopic measurement of cholesterol absorption in the rat

Data are presented on cholesterol absorption in 6 non-fasted lymph ductcannulated rats during continuous infusion of oil or oil plus cholesterol (mass + isotope). Endogenous thoracic duct lymph cholesterol averaged 2 μmoles/h during infusion of triolein. Lymph cholesterol flux increased to 3.3 μmoles/h during infusion of oil plus a low dose of cholesterol (1.7 μmoles/h), to 4.8 μmoles/h at a medium dose (4.4 μmoles/h) and to 8.9 μmoles/h during infusion of a high dose of cholesterol (30 μmoles/h). The transport of endogenous cholesterol was significantly decreased by the high infusion rate of cholesterol. At each level of cholesterol infusion, about 90% of the increase in lymph cholesterol was due to cholesteryl ester. Radioactive cholesterol significantly underestimated the percent of cholesteryl ester in the increased cholesterol flux due to cholesterol infusion, especially at the low dose. The percent of infused radioactive cholesterol absorbed was a fairly accurate monitor of the increase in lymph cholesterol due to the exogenous loads. However, small numerical discrepancies at the medium and high doses suggest that the use of isotopic cholesterol to measure cholesterol absorption in careful balance studies may have limitations.