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Biology of Reproduction, Vol 25, 519-529, Copyright © 1981 by Society for the Study of Reproduction
1 Cátedra de Fisiopatología, Facultad de Medicina,
Universidad de los Andes,
Mérida, Venezuela The ability of a factor obtained from a 105,000 x g supernatant of rat testis to inhibit the hCG
binding to gonadal receptors was studied. This factor was partially heat stable, not steroid in
nature, and presented a molecular weight under 12,000. Binding studies indicated that the hCG
binding inhibitor interfered with the formation of the hormone-receptor complex mainly by competing with hCG for the same binding sites. Consequently, it was demonstrated that the inhibitor
prevented the interaction of hCG and its receptor in both testicular and ovarian tissues. Under in
vitro binding conditions, it was observed that as the quantities of the inhibitor increased during the
first incubation with testicular homogenate, and after removing the inhibitor and washing the pellet, less and less of the added labeled hCG was bound during the second incubation step. On the
other hand, the hormone already bound to the tissue during the first incubation could not be displaced by increasing amounts of the inhibitor in the second incubation. Thus, binding of the inhibitor, or hormone, to the tissue in the first step was not readily exchangeable with the subsequent
inhibitor, or hormone, added in the second step. These findings were consistent with the view that
binding to LH/hCG receptors may proceed in an irreversible manner. Although the hCG binding inhibitory 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 (
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 of
100 IU of hCG. Results indicated that the receptor depletion and replenishment processes proceeded without significant changes in the hCG binding inhibitory activity in the testis. It was concluded that the phenomenon of down-regulation of LH/hCG receptors and the quantity of testicular inhibitor were not necessarily related to one another.
Note:
ACKNOWLEDGMENTS
This work was supported by the Consejo de Desarrollo Cientifico y Humanístico de la Universidad de
Los Andes, Project M-164-79 (to F.J.R.). We wish to
thank Dr. Vinod K. Bhalla, Medical College of
Georgia, Augusta, for a generous gift of purified hCG
(CR 121). We gratefully acknowledge Drs. V. K.
Bhalla and T. G. Muldoon, Medical College of Georgia,
for helpful criticism and suggestions. We also wish to
thank Ms. Irlanda Márquez for secretarial assistance.
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