The transport of exogenous cholesterol in the rabbit. I. Role of cholesterol ester of lymph chylomicra and lymph very low density lipoproteins in absorption.

Thoracic lymph duct cannulations were performed shortly after a meal in rabbits trained to ingest a moderate fat, low cholesterol diet. A tracer dose of cholesterol-(3)H was administered to label exogenous (dietary) cholesterol during absorption. Sequential lymph samples were collected up to 24 hr postprandially, after which ultracentrifugal fractionation of lymph lipoproteins was carried out. The d < 1.006 lipoproteins were separated into two classes, chylomicra and very low density lipoproteins (VLDL).A comparison was made between chylomicra and VLDL of lymph in the transport of exogenous cholesterol after ingestion of a single meal. The per cent of exogenous cholesterol present in VLDL of sequential lymph collections progressively increased with time after a meal and by 18 hr had reached a value of 80% or greater. In chylomicra the per cent of exogenous cholesterol of sequential lymph collections progressively decreased. Therefore, exogenous cholesterol was preferentially transported in VLDL compared with chylomicra. Cholesterol ester specific activity (CESA) of lymph chylomicra and VLDL increased at a more rapid rate than free cholesterol specific activity (FCSA). CESA of VLDL was three times higher than FCSA at the maximum. Exogenous cholesterol which appeared in both chylomicra and VLDL was consistently 80% esterified. while the per cent of total cholesterol esterified decreased with time and was significantly lower than that for exogenous cholesterol from 6 to 24 hr postprandially. These results demonstrate preferential esterification of exogenous cholesterol during absorption and indicate that a mechanism exists within the intestinal mucosal cell to maintain both free and esterified exogenous cholesterol in a chemically distinct pool from endogenous cholesterol during incorporation into both chylomicra and VLDL.