Developmental changes in the properties of gonadotropin receptors in the ovarian follicles of amago salmon (Oncorhynchus rhodurus) during oogenesis.

ABSTRACT

The presence of specific, saturable, high-affinity gonadotropin receptors was demonstrated in membrane preparations from preovulatory ovarian follicles of amago salmon (Oncorhynchus rhodurus), including intact follicles, isolated thecal layers, and isolated granulosa cells. Optimum conditions for the binding study using the amago salmon receptor system were similar to those previously reported for postovulatory ovaries of the same species (A. Kanamori, H. Kagawa, and Y. Nagahama, 1987, Gen. Comp. Endocrinol. 66, 210-217). Scatchard analysis of chum salmon gonadotropin (CSG-SII) binding to the membrane fraction suggested the presence of high-affinity binding sites in the intact follicles, isolated thecal layers, and isolated granulosa cells at all stages of development. The dissociation constant is consistent with those reported for gonadotropin receptors in several teleost gonads and in other vertebrate classes (about 0.1-1 nM). During oogenesis, the number of binding sites per follicle increased from about 20 to about 60 pg. Similarly an increase in binding sites was observed with granulosa cells and thecal layers during oogenesis. These findings show an increase in the number of gonadotropin receptors in the follicles and are in good temporal agreement with the developmental changes in follicular steroidogenesis in response to gonadotropin. The increase in gonadotropin receptors in the thecal layer was associated with the increased capacity for production of testosterone, whereas the increase in gonadotropin receptors in the granulosa cells was associated with an increase in gonadotropin sensitivity in terms of 20 beta-hydroxysteroid dehydrogenase activation.