Evidence for a decrease in aromatase activity in the ovarian granulosa cells of amago salmon (Oncorhynchus rhodurus) associated with final oocyte maturation.

Aromatase activity in the isolated granulosa layers of amago salmon (Oncorhynchus rhodurus) ovarian follicles was examined during the course of vitellogenesis and final oocyte maturation and ovulation. Estradiol-17 beta production by isolated granulosa layers incubated with exogenous testosterone increased during the period of vitellogenesis to reach a peak in late vitellogenesis, and then declined rapidly, in association with the ability of the oocyte to mature in response to partially purified salmon gonadotropin (SG-G100). Extremely low levels of estradiol-17 beta were produced by granulosa layers from follicles which had undergone final oocyte maturation in vivo and by post-ovulatory follicles. SG-G100 had no discernible effect on estradiol-17 beta production. These results are discussed in relation to other studies on the endocrine control of steroidogenesis in this species.