Influence of follicular development on steroid production in the medaka (Oryzias latipes) ovarian follicle in response to exogenous substrates.

Changes in the capacity of medaka, Oryzias latipes, ovarian follicles to convert exogenous 17 alpha-hydroxyprogesterone (17 alpha-OHprog) or testosterone to testosterone, estradiol-17 beta, and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOHprog) were examined using 18-hr incubations. Under a constant long photoperiod (14 hr light-10 hr dark) at 26 degrees medaka spawn daily within 1 hr of the onset of light. Under these conditions, vitellogenesis and oocyte maturation of individual follicles occur within 72 hr, allowing accurate determination of the time of oocyte maturation and ovulation. In the absence of substrates, vitellogenic follicles isolated between 28 and 16 hr before spawning produced increased amounts of estradiol-17 beta, while postvitellogenic follicles between 14 and 8 hr produced a large amount of 17 alpha,20 beta-diOHprog. However, under the same conditions, testosterone levels were very low in follicles from all stages of development. The capacity of follicles to produce estradiol-17 beta in response to 17 alpha-OHprog or testosterone increased as follicles developed from the early to late vitellogenic stage, but declined during oocyte maturation. Maximum estradiol-17 beta production was observed in follicles at 20 hr before spawning. In contrast, the conversion of 17 alpha-OHprog to 17 alpha,20 beta-diOHprog was low in vitellogenic follicles and increased in follicles isolated immediately prior to or during oocyte maturation, with maximum 17 alpha,20 beta-diOHprog production occurring in follicles isolated at 14 hr before spawning. These results demonstrate a distinct shift in the activities of steroidogenic enzymes from C17-20 lyase, 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD), and aromatase to 20 beta-hydroxysteroid dehydrogenase (20 beta-HSD) occurring in the medaka ovarian follicles immediately prior to oocyte maturation.