Binding of human high density lipoproteins to membranes of luteinized rat ovaries

Abstract

Highly luteinized ovaries of immature PMS-hCG primed rats treated with 4-aminopyrazolopyrimidine to reduce endogenous lipoprotein levels accumulated 125I-labeled human HDL administered intravenously. Ovarian uptake of labeled HDL was reduced by prior treatment of rats with a bolus of unlabeled human HDL but not by injection of human LDL. Membranes prepared from the ovaries were found to possess saturable binding sites for the 125I-labeled HDL. Scatchard analysis revealed an apparent single class of binding sites with half maximal binding occurring at an HDL concentration of 56 μg protein/ml. Human and rat HDL competed for binding of 125I-labeled HDL to the ovarian membranes whereas human LDL was much less effective in this regard. Bovine serum albumin, lutropin and follitropin had no marked effects upon 125I-labeled HDL binding. Specific 125I-labeled HDL binding to rat adrenal membranes, and to a lesser extent, liver membranes was observed under similar assay conditions, but rat erythrocytes exhibited no specific HDL binding. Unlike the binding of LDL to its receptor, binding of human 125I-labeled HDL to ovarian membranes did not require divalent cations. Heparin, which releases LDL from its receptor, did not displace 125I-labeled HDL bound to the ovarian membranes. We conclude that luteinized rat ovaries have specific binding sites from HDL. These binding sites probably facilitate the uptake of blood cholesterol for steroidogenesis.