Origin and fate of cholesterol in rat plasma lipoproteins in vivo. I. Qualitative analysis.

Rats were conditioned to ingest a 15-gram dietary mixture, in a single daily meal, between 6:00 and 9:00 a.m. Labeled cholesterol was introduced into the organism through four different physiological routes: ingestion of 14C-cholesterol, injection of 14C-acetate, injection of red cells or plasma whose cholesterol was tritium-labeled. The specific radioactivities of free and esterified cholesterol, in the different plasma lipoproteins and in the major organs, were measured at various time spans after the introduction. The results revealed a different source of cholesterol in chylomicrons and VLDL discharged by the intestine: the chylomicrons would principally carry esterified cholesterol coming from the intestinal contents, while esterified cholesterol coming from the intestinal wall was found mainly in VLDL. The free cholesterol of these lipoproteins would initially have the same origin as esterified cholesterol but rapid free cholesterol exchanges would introduce precociously cholesterol coming from other structures. The results demonstrated the significant role of the intestine in cholesterol synthesis and revealed that of VLDL in the internal secretion of cholesterol by the intestine. Chylomicrons also appeared to play a significant role in this secretion, by way of exchanges between intestinal cells and chylomicrons in the process of formation. Finally, the results suggested the existence of esterified cholesterol transfers from HDL to lower density lipoproteins (chylomicrons, VLDL and possibly LDL).