The major excretory route of cholesterol (CH) from the body is by biliary secretion. Biliary CH secretion depends upon the availability and coordinated secretion of phosphatidylcholines (PCs) that have a particular molecular species composition. Both the secretion of CH and PCs are stimulated by bile salts that not only promote lipid secretion in bile but also promote the secretion of these same lipids from the liver in nascent HDL. A number of the key steps in the process of hepatic CH and PC secretion are not understood and will be the focus of proposed studies. These most prominently include: (1) a clear definition of the pathway of intrahepatic CH transport into bile and nascent HDL, and (2) the mechanism by which PCs are synthesized and transported into bile and HDL in conjunction with CH. Because intrahepatic CH pools rapidly equilibrate, it is not possible to directly track the uptake and transport of CH from the plasma into bile or HDL. Preliminary studies do, however, show that it is possible to use sitostanol, a plant sterol that is structurally similar sterol to CH, as an analog to (1) quantitate the net uptake by the liver and contribution to bile and nascent HDL of sterol from different plasma lipoproteins; (2) determine the effect of modifications in the composition of lipoproteins on hepatic sterol uptake; (3) localize by lobular zone and membrane distribution the pathway(s) of sterol that is taken up and transported from the liver into bile and HDL; and (4) determine the effect of amphipathic bile salts and apoproteins on the mobilization of sterol and selective PCs from hepatocytes and the effect of changes in the composition of hepatocyte (membrane) PCs on the magnitude of amphipath-induced sterol efflux. Past studies suggest that the kinds of PCs that are secreted in bile may derive from a special pool of hepatic triglycerides that are extensively remodeled. Proposed studies are also designed to localize the site of origin and mechanism of formation of the PCs that are secreted in bile and nascent HDL and will be largely conducted by comparing the specific activities of individual, newly-synthesizd bile PCs with the specific activities of the hepatic acylglyceride precursors of these PCs. This work will be performed using HPLC methods to isolate individual molecular species of PCs and precursors in conjunction with liver fractionation to obtain different kinetic pools of triglycerides, different hepatocyte organelles, and hepatocytes from different lobular zones. All studies will be initiated using the isolated, perfused rat liver that has been extensively utilized in the past to define mechanisms of lipid transport. Understanding the process by which specific PCs are synthesized and promote CH mobilization from the liver can provide more specific direction for reducing whole body CH stores and the risk of atherosclerosis.