Follicle-Stimulating Hormone Receptors in Rat Ovaries: Decrease in Numbers of Binding Sites Associated with Luteinization

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Abstract
Specific binding of human follicle-stimulating hormone (hFSH) to receptors in a particulate fraction of immature rat ovaries was studied. The binding of human [125I]iodo-FSH is both time- and temperature-dependent and is a saturable process. Total binding amounted to 21% of the total radioactivity with 10 mg equivalent wt [weight] of ovary used. Nonspecific binding was very low (5% of total binding). Binding was competitively inhibited by unlabeled hFSH. Purified hLH [luteinizing hormone] and hLH-.alpha. inhibited binding only at high concentrations, which was compatible with FSH contamination. Ovine [O] prolactin and ovalbumin were without effect. No human [125I]iodo-FSH was specifically bound by rat liver, intestine, spleen and kidney. The apparent Kd of the hFSH-receptor complex was found to be 3.8 .times. 10-10 M. The effect of luteinization upon FSH binding activity of rat ovaries was also studied. The FSH binding capacity for the immature ovary was found to be 8.2 .times. 10-16 mol/mg wt. During luteinization following a series of primings with oFSH and hCG, FSH receptor sites progressively decreased to 5.2 .times. 10-16 mol/mg wt and 2.2 .times. 1016 mol/mg wt, respectively, after 5 and 7 injections. The receptor sites decreased further to undetectable levels in heavy luteinized ovaries following PMSG-hCG [pregnant mare''s serum gonadotropin-human chorionic gonadotropin] priming. The apparent Kd of the human [125I]iodo-FSH-binding site complex was not affected by the gonadotropin treatments. hCG-Induced luteinization is associated with a decrease in number of FSH receptor sites and a loss of FSH-sensitive adenylate cyclase activity. These changes may be important in the control of granulosa cell differentiation to luteal cells.