Effects of 17 alpha-ethinyl estradiol on the serum lipoproteins of cholesterol-fed diabetic rats.

Abstract

Large doses of 17 alpha-ethinyl estradiol were administered to rats made diabetic by administration of streptozotocin and fed a diet containing 2% cholesterol and 1% cholic acid. Before estrogen treatment, these rats were severely hypercholesterolemic and had high concentrations of lipoproteins of d < 1.006 g/ml (very low density lipoproteins, VLDL), d = 1.006 to 1.03 g/ml (intermediate density lipoproteins, IDL), and d = 1.03 to 1.063 g/ml (low density lipoproteins, LDL), and low concentrations of lipoproteins of d = 1.063 to 1.21 g/ml (high density lipoproteins, HDL). The VLDL contained two populations of particles with a mean diameter of 1150 and 350 A, respectively. The IDL contains only the latter particles. These particles are unusual in that they contain apolipoprotein A-I in addition to their usual complement of apolipoproteins. After estrogen treatment, the concentrations of both large and smaller particles in VLDL were greatly decreased as were the concentrations of IDL and LDL; apo A-I was no longer present in the residual VLDL and IDL. HDL phospholipid, protein, and cholesterol increased 7-fold, 4-fold, and 2-fold, respectively. Studies by others have shown that large doses of estrogen enhance uptake of LDL by the liver and this is associated with an increase in the saturable binding sites for LDL on liver membranes. The present studies suggest that these binding sites recognize lipoproteins other than LDL. Furthermore, the enhanced uptake of these lipoproteins appears to result in transfer of their surface components to HDL.