Differences in the Rates of Internalization of125I-Labeled Human Chorionic Gonadotropin, Luteinizing Hormone, and Epidermal Growth Factor by Ovine Luteal Cells*

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Abstract
The rate of internalization and degradation of radioiodinated hCG [human chorionic gonadotropin], ovine LH [luteinizing hormone] (oLH), human LH (hLH) and mouse epidermal growth factor (mEGF) was studied in suspensions of ovine luteal cells. Hormone bound to the plasma membrane was quantified by treating the cells with acidic buffer (pH 3.0) and the radioactivity removed from the receptor was measured. The radioactivity remaining in the cell pellet was considered to have been internalized. The extent of degradation of the radioactive hormones was determined by subjecting the radioactivity that was released into the medium during the incubation periods to precipitation with 20% trichloroacetic acid. The non-precipitable radioactivity was considered to have been degraded. The required time for loss of half of the radioactivity that had been bound to the cell membrane at time zero was 22.8 .+-. 2.3 h for hCG, 23.3 .+-. 1.1 h for asialo-hCG, 15.1 .+-. 1.4 h for hLH, 0.4 .+-. 0.2 h for oLH and 0.3 .+-. 0.1 h for mEGF. The quantities of radioactivity that had been internalized (intracellular plus degraded) were greater at earlier times (15-120 min) for oLH or mEGF than for hCG, asialo-hCG or hLH. Apparently ligands bound to different receptors on ovine luteal cells are internalized and degraded at different rates by the same cell, and that ligands that bind to the same receptor (hCG, hLH and oLH) can also be internalized and degraded at very different rates.