Internalization of receptor-bound human chorionic gonadotrophin in preovulatory rat granulosa cells in vivo

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Abstract
The subcellular distribution of 125I-labeled human chorionic gonadotropin (hCG) in preovulatory rat granulosa cells was studied in vivo. Pregnant mare serum gonadotropin-pretreated immature female rats received an i.v. injection of [125I]hCG a few hours before the endogenous preovulatory gonadotropin surge. The animals were killed at 2 or 6 h after the [125]hCG injections. Light microscope autoradiographs showed that the mural granulosa cells of large follicles were the most highly labeled cells in the ovaries. EM autoradiography was used to study the subcellular distribution of radioactivity in the mural granulosa cells. At 2 h 45% of the counted Ag grains were associated with the plasma membrane and 10% with the lysosomes, at 6 h the values were 51% and 9%, respectively. The distribution of the observed Ag grains was compared with the generated expected source to grain pairs by computerized linear multiple regression analysis. The magnitudes of the regression coefficients revealed that the plasma membrane and the lysosomes were the only specifically 125I-labeled organelles, that a few radioactive molecules were located diffusely over the cytoplasm at 2 h and that the 125I-radioactivity of the nuclei was negligible. Apparently preovulatory rat granulosa cells are in vivo able to internalize into lysosomes [125I]hCG initially bound to LH [luteinizing hormone]/hCG receptors of the plasma membrane.

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