Gonadotropin receptor complexes and free receptors in porcine Leydig cell cultures during recovery from hCG stimulation

Reduction of testicular human chorionic gonadotropin receptors by human chorionic gonadotropin in infertile men

We measured testicular human chorionic gonadotropin (hCG) receptors in 30 infertile men either before or 1, 3, 5, 7, or 14 days after a single administration of 5000 IU of hCG. For 5 days the administration of hCG significantly reduced the testicular binding of 125I-hCG compared with that of the testes before administration. From the 7th day the binding capacity began to increase and returned to the preadministration level 14 days after the treatment. Occupied hCG binding sites accounted for about half of the reduction in binding sites on the day after administration of hCG. After this time, however, the occupancy did not contribute so much to the reduction in binding sites. These findings suggest that the reduction in testicular hCG binding sites after a single administration of 5000 IU of hCG is due to not only occupancy but also down-regulation of the binding sites.

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.

Evidence for the rapid internalization and recycling of lutropin receptors in rat testis Leydig cells

A study into the binding of 125I-human chorionic gonadotropin (hCG) to the lutropin (LH) receptor in rat testis Leydig cells, and subsequent internalization of the hormone-receptor complex, has been carried out. The results show that there is rapid internalization of the hormone-receptor complex; 240 receptors/cell (from a total of approx. 4000 receptors/cell) were internalized each minute in the first hour after exposure to hCG. Radioactivity was released from the cell 1 h after internalization and was found to be associated with highly degraded hCG. The endocytic process was found to have two temperature-sensitive steps. At 4 degrees C, movement of the hormone-receptor complex inside the cell did not occur, and at 21 degrees C hormone accumulated within the cytoplasm but was not degraded or released from the cell. At 34 degrees C, internalization, degradation and loss of the degraded hormone from the cell occurred. These processes appeared to reach a steady state after 2 h. Even though there is rapid internalization of the hormone-receptor complex following exposure to hCG, the binding sites on the cell surface were maintained for at least 4 h. The number of binding sites on the cell surface was not decreased by a protein synthesis inhibitor but was reduced to undetectable levels by monensin. This compound inhibits acidification of endocytic vesicles, which is known to be an important prerequisite to receptor cycling. It is concluded that, in the rat testis Leydig cells, following binding of hCG to the LH receptor there is rapid internalization of the complex and that recycling of the receptor occurs to the cell surface. This process may be essential in maintaining the capacity of the Leydig cell to bind fresh hormone.