Role of Internalization and Degradation in the Removal of Receptor-Bound Human Chorionic Gonadotropin in Rat Luteal Cells in Vivo*

Abstract

The subcellular and catabolic fate of [125I]iodohCG in rat luteal cells after in vivo binding was studied to determine whether internalization and degradation of bound hormone are associated with the removal of hormone-receptor complexes. Pseudopregnant rats were injected iv with [125I]iodohCG, and the ovarian uptake of radioactivity was studied over 24 h. The radioactivity disappeared from the ovarian tissue after the maximal uptake at 1.5 h in a biphasic manner, with half-lives of 1.2 and 4.8 h. The decrease in the loss of radioactivity was not due to an accumulation of hormone catabolites in the tissue. Gel filtration of the radioactivity from the ovarian tissue 2, 6, 12, or 24 h after the injection of labeled hCG displayed only one radioactive peak corresponding to the hormone-receptor complex. Electron microscopic autoradiographs showed that a majority of the silver grains were associated with microvillous projections of the luteal cell plasma membrane directed toward the capillary regions. Multiple regression analysis of the grain distribution indicated that only the plasma membrane was significantly (P < 0.001) labeled both 2 and 6 h after the injection of labeled hCG. Analysis of the silver grains in relation to the plasma membrane showed no transition of the grains to the interior of the cell as a function of time. Gel filtration of plasma 6 or 12 h after the injection of labeled hormone revealed no accumulation of peptide fragments of the hormone in the circulation. The ovarian homogenate was found to be capable of splitting labeled hCG to fragments of sizes similar to those of hormone subunit fragments but not to those of amino acids fragments. The results of the present study show that only a small fraction of the hCGreceptor complex is internalized in the luteal cell of the pseudopregnant rat. The extent of internalization found in this study does not fully explain the rapid removal of hormone-receptor complexes from the luteal cells, and thus, it is likely that other mechanisms are involved in this process.