Slow internalization of human chorionic gonadotropin by cultured granulosa cells

Kinetic studies were performed on two-day cultures of rat ovarian granulosa cells to follow the fate of surface-bound 125I-labeled human chorionic gonadotropin (125I-hCG). Low pH was used to release hCG from its surface receptor, allowing us to distinguish between surface-bound and internalized hormone. Because our results indicated that hormone is lost from the cell surface by dissociation as well as internalization, equations were derived to determine independent rate constants for each process. We calculate that if hormone binding were irreversible, the t 1/2 for internalization would be 8.5 hour. Morphometric studies on the uptake of horseradish peroxidase indicate that the t 1/2 for internalization of bulk membrane in granulosa cells is 55 to 77 minutes. Thus, the rate of uptake of surface-bound hCG appears to be seven to nine times slower than the rate of uptake of bulk plasma membrane, which suggests that the LH/hCG receptor may be selectively excluded from the endocytic vesicles of granulosa cells.