The relationship between HDL-, LDL-, liposomes-free cholesterol, biliary cholesterol and bile salts in the rat

In order to study the relationship between bile cholesterol and free cholesterol carried by high and low density lipoproteins (HDL and LDL), 10 male Wistar rats, 11 weeks old and fed with a standard diet were divided into 3 groups which received an intravenous infusion (jugular vein) of either LDL, HDL or liposomes. Liposomes were used for comparison because they are assimilated by hepatocytes, but are not recognized by specific receptors. HDL isolated from rat sera were labeled with [14C]cholesterol by molecular exchange and LDL were labeled by exchange with [14C]cholesterol incorporated into phosphatidyl choline/cholesterol liposomes. The peaks of radioactivity appeared in bile 30 min after the HDL or liposome injection and after 210 min for the LDL injection. The kinetic behavior of the cholesterol carried by the liposomes was quite similar to that of cholesterol carried by HDL. Cholesterol carried by HDL was metabolized in bile salts faster than that carried by LDL: cholesterol-HDL or cholesterol-liposomes contributed to the same extent to the secretion of bile cholesterol (15 and 11%, respectively, of the injected dose), LDL (20% of the injected dose). However, the main part of [14C]cholesterol from HDL, LDL or liposomes was metabolized in bile salts. Thus, cholesterol from an exogenous source seemed to be used mainly as a substrate for bile salts. Our study revealed a difference between the hepatic metabolism of HDL, liposomes and LDL in the rat: the kinetic difference between the secretions of the radioactive compounds in bile may be explained by differences in assimilation, intracellular pathways or bile secretion.