Steroidogenic capacity of coho salmon ovarian follicles throughout the periovulatory period.

Coho salmon follicles obtained at various times throughout the periovulatory period were incubatedin vitro with graded amounts of partially purified salmon gonadotropin (SG-G100) for 24 h and the amounts of 17β-estradiol, testosterone and 17α20β dihydroxy-4-pregnen-3-one (17α20βP) released into the media determined by radioimmunoassay. By this approach, the pattern of steroid secretion by ovarian follicles was shown to change in relation to the developmental status of the oocyte. Full-grown immature follicles produced large amounts of 17β-estradiol but negligible amounts of testosterone and 17α20βP. Both basal and gonadotropin-stimulated 17β-estradiol production was subsequently reduced with advancing oocyte development. In contrast, the production of testosterone and 17α20βP increased during the course of ovarian development with testosterone production highest in follicles with a peripheral germinal vesicle and 17α20βP production highest in matured and postovulatory follicles. These data are discussed in relation to information on the preovulatory changes in circulating levels of steriod hormones in salmonids.