Fish Physiology and Biochemistry vol. I no. 4 pp 179-186 (1986) Kugler Publications, Amsterdam/Berkeley Steroidogenic capacity of coho salmon ovarian follicles throughout the periovulatory period Glen Van Der Kraak I and Edward M. Donaldson 2 I Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada, and z Department of Fisheries and Oceans, Fisheries Research Branch, West Vancouver Laboratory, 4160 Marine Drive, West Vancouver, British Columbia, Canada Keywords: teleost, reproduction, Pacific salmon, gonadotropin, oocyte maturation, ovulation, ovarian steroidogenesis Abstract Coho salmon follicles obtained at various times throughout the periovulatory period were incubated in vitro with graded amounts of partially purified salmon gonadotropin (SG-G 100) for 24 h and the amounts of 17/3- estradiol, testosterone and 17~20/3 dihydroxy-4-pregnen-3-one (17~20/3P) 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 17/3-estradiol but negligible amounts of testosterone and 17~20/3P. Both basal and gonadotropin- stimulated 17/3-estradiol production was subsequently reduced with advancing oocyte development. In con- trast, the production of testosterone and 17o~20~3Pincreased during the course of ovarian development with testosterone production highest in follicles with a peripheral germinal vesicle and 17c~20/3Pproduction highest in matured and postovulatory follicles. These data are discussed in relation to information on the preovulatory changes in circulating levels of steroid hormones in salmonids. Introduction Oocyte development in salmonid fishes is regu- lated by the action of a maturational gonadotro- pin on ovarian steroidogenesis with vitellogenin synthesis and oocyte maturation mediated by 17/3- estradiol and 17a20/3 dihydroxy-4-pregnen-3-one (17c~20/3P), respectively (Goetz 1983; Ng and Idler 1983). There are characteristic changes in the pat- tern of steroid secretion during the preovulatory period which signal the transition from vitello- genesis to oocyte maturation. For example, during the preovulatory period in rainbow trout (Fostier and Jalabert 1982; Scott et al. 1983) and coho salmon (Van Der Kraak et al. 1984), plasma levels of 17/3-estradiol decline prior to increases in 17~20/3P levels coincident with oocyte maturation. Studies on amago salmon have shown that these changes result from a progressive loss of aromatase and increased amounts of 20/3-hydroxysteroid dehydrogenase, the key enzymes mediating 17/3- estradiol and 17a20/3P production, respectively (Young et aL 1983a; Nagahama 1984). In an effort to define the time course of changes in ovarian responsiveness to gonadotropin during the pre- ovulatory period, the present studies were co.n- ducted to determine the effects of salmon gonado- tropin on in vitro 17~3-estradiol, testosterone and Address correspondence to: Dr Glen Van Der Kraak, Departmentof Zoology,University of Alberta, Edmonton,Alberta, Canada, T6G 2E9.