REDUCTION OF HUMAN AND RAT TESTICULAR FOLLICLE STIMULATING HORMONE RECEPTORS BY HUMAN MENOPAUSAL GONADOTROPHIN IN VIVO AND IN VITRO

Reduction of human and rat testicular FSH receptors by hMG was studied in vivo and in vitro. After a single injection of hMG, the number of high affinity FSH receptors were significantly reduced for 5 and 3 d in human and rat testes, respectively, without affecting their affinity. FSH receptor numbers recovered to pretreatment values by 14 d after the injection. Radioactivity in rat testes found 48 h after a s.c. injection of 125I-labelled human FSH was less than 10% of the maximum found 10 h after the injection, showing that the prolonged reduction of FSH receptors after hMG injection was not due to occupancy of the binding sites. Occupied FSH receptors measured in rat testes accounted for about 15% of all receptors on the day after an injection of hMG and for less than 5% after the third day. In experiments in vitro using organ culture techniques, an exposure to hMG for 24 h induced a dose-related significant loss of the specific FSH binding sites for 7 and 5 d in human and rat testes, respectively. Thereafter, the loss was gradually recovered. These findings suggest that the reduction in FSH receptors in human and rat testes by hMG is mainly due to down-regulation of the binding sites and that the testicular organ culture method used in the present study is useful to study hormonal regulation of testicular function, especially in human testes.