Internalization and degradation of human chorionic gonadotropin in ovine luteal cells: Kinetic studies

Ovine luteal cells grown in suspensions and/or monolayer culture were used to study the rates of internalization and degradation of [125I]hCG. At specified times after a 5- to 7-min exposure to [125I]hCG, cells were treated with acidic buffer (pH 3.9) to elute membrane-bound hormone, which left the internalized radioactivity associated with the cell pellet. Radioactivity released into the medium during the incubation periods was subjected to 20% trichloroacetic acid and/or thin layer chromatography to monitor the extent of degradation of the radioactive hormone. Secretion of progesterone into the medium and exclusion of trypan blue were used to monitor the viability of the cells in each experiment. Radioactivity was lost from the plasma membrane with a tsub1/2 of 9.6 h, with approximately 85% of the radioactivity being lost within 24 h. Cell-associated radioactivity increased linearly with time to a plateau at 4 h, remained stable until 12 h, and then decreased between 12-24 h. The plateau between 4-12 h reflected an equilibrium between the [125I]hCG which was internalized and degraded and the [125I]hCG which was released into the medium. The degraded [125I]hCG increased essentially linearly up to 24 h. These data suggest that the majority of [125I]hCG bound to receptors in luteal cells is internalized and degraded. Less than 20% of the radioactivity bound initially to cells dissociated into the incubation medium and was trichloroacetic acid precipitable within 24 h. The internalization and degradation of [125I]hCG was temperature dependent, with essentially no hCG internalized and/or degraded at 4C