Inhibition of 125 I-Labeled Human Chorionic Gonadotropin Binding to Gonadal Receptors by a Factor Obtained From Rat Testicular Tissue

Abstract

The ability of a factor obtained from a 105,000 .times. g supernatant of rat testis to inhibit the hCG [human chorionic gonadotropin] binding to gonadal receptors was studied. This factor was partially heat stable, not steroid in nature and presented a MW under 12,000. The hCG binding inhibitor interfered with the formation of the hormone-receptor complex mainly by competing with hCG for the same binding sites. The inhibitor prevented the interaction of hCG and its receptor in both testicular and ovarian tissues. Under in vitro binding conditions, as the quantities of the inhibitor increased during the 1st incubation with testicular homogenate, and after removing the inhibitor and washing the pellet, less of the added labeled hCG was bound during the 2nd incubation step. The hormone already bound to the tissue during the 1st incubation could not be displaced by increasing amounts of the inhibitor in the 2nd incubation. Thus, binding of the inhibitor, or hormone, to the tissue in the 1st step was not readily exchangeable with the subsequent inhibitor, or hormone, added in the 2nd step. Binding to LH[lutropin]/hCG receptors may proceed in an irreversible manner. Although the hCG binding inhibitor activity was detected in many of the tissue extracts and in serum of the rat, only testicular extracts showed the ability to retain a significant part of this activity (.apprx. 40%) after heating for 30 min in a boiling water bath, under equivalent experimental conditions. The content of hCG binding inhibitor in the testis was also examined during the gonadotropin-induced reduction in the availability of LH/hCG receptors following in vivo administration 100 IU of hCG. The receptor depletion and replenishment processes proceeded without significant changes in the hCG binding inhibitor activity in the testis. The phenomenon of down-regulation of LH/hCG receptors and the quantity of testicular inhibitor were not necessarily related to one another.