We plan to investigate the role of cholesterol in chylomicron structure and metabolism. The hypothesis that a fixed proportion of free cholesterol is essential to the chylomicron for the maintenance of a stable membrane (and that esterified cholesterol is "excess") will be tested in unanesthetized rats deprived of all luminal cholesterol by bile diversion and tomatine administration. Animals will receive intraduodenal infusions of triglyceride at high rates for several hours, to stress the mucosal capacity for chylomicron synthesis and release. The effect of rate and duration of triglyceride infusion on the extent of cholesterol esterification in intestinal chylomicrons and on the ability of the intestine to maintain high rates of triglyceride transport will be studied. The mucosal cholesterol esterification system will be characterized and localized within the mucosal cell, using homogenates and subcellular fractions of isolated epithelial cells from pancreatic juice-diverted rats. We will investigate whether the rate of de novo cholesterol synthesis by the mucosa is related to triglyceride transport, using both in vivo and isolated tissue preparations. The effect of triglyceride or fatty acid composition and of the model lipid, epi-cholesterol, which is not esterified by the mucosa, on mucosal cholesterol and triglyceride metabolism will be studied. We will also determine whether clearance rates of intestinal chylomicrons from the circulation vary with chylomicron composition. The effects of triglyceride, total cholesterol, or cholesterol ester content, and of triglyceride composition, on the kinetics of hepatic chylomicron uptake and on biliary cholesterol and bile acid output will be studied in unanesthetized rats receiving continuous intravenous infusions of chylomicrons of known composition. The aim of all these studies is to provide basic information about those interrelationships between cholesterol and triglycerides that may be obligatory for their absorption and metabolism, in order to allow improved methods for the prevention and control of arterial lipid deposition to be developed.