Abstract

Administration of human chorionic gonadotrophin (hCG) to immature rats with luteinized ovaries caused a time- and dose-related decrease in ovarian content of receptors for luteinizing hormone (LH). The marked reduction in LH/hCG receptor concentration was not accompanied by a change in binding affinity for hCG. Subcutaneous injection of 2 µg of hCG produced a maximum serum hCG concentrations of 10 ng/ml, and was followed by a marked decrease in ovarian binding capacity. Measurement of receptor-bound hCG by radioimmunoassay after acid elution showed that the initial loss of binding capacity was due to occupancy of receptors by the administered hormone. After several hours occupancy no longer accounted for the decrease in binding capacity, and by 24 hr there was a 90% loss of total receptor sites. The ovarian content of receptors remained low for several days after the desensitizing dose of hCG had been cleared from the circulation. The lowest dose of hCG to cause a measurable loss of receptors was 20 ng/l00 g of body weight, suggesting that regulation of receptors may occur with physiological concentrations of gonadotrophins. Small degrees of receptor occupancy were followed by major losses of binding capacity, indicating that an active or cooperative process might be involved in the mechanism of receptor loss. Solubilization of control and desensitized ovaries with a nonionic detergent showed the same degree of receptor loss observed in particulate ovarian fractions, with no change in the sedimentation properties of the residual binding sites. Gonadotrophin-induced loss of LH/hCG receptors was associated with rapid, substantial loss of the ability of LH to stimulate adenylate cyclase activity in ovarian homogenates. This effect was hormone-specific, since the responses of adenylate cyclase to sodium fluoride and prostaglandin E₁ were not impaired when the LH response was completely abolished. Furthermore, the response of adenylate cyclase to LH was not restored by the GTP analogue 5'-guanylylimidodiphosphate. Administration of actinomycin D and cycloheximide before and during desensitization did not affect the extent or duration of receptor loss or the LH responsiveness of adenylate cyclase. These studies provide further evidence for the regulation of ovarian gonadotrophin receptors by the homologous hormone, and demonstrate that receptor loss is a major factor in the prolonged desensitization of adenylate cyclase by elevated gonadotrophin concentrations.