Gonadotropin-induced steroidogenic shift towards maturation-inducing hormone in Japanese yellowtail during final oocyte maturation

Intact ovarian follicles, obtained from untreated and human chorionic gonad otropin (HCG) treated Japanese yellowtail Seriola quinyueradiata during dif ferent maturational stages, were incubated With radioactive [H-3]pregnenolo ne, [H-3]17-hydroxypropesterone or [C-14] androstenedione and steroid metab olites identified by thin layer chromatography (TLC) followed by recrystall ization to constant specific activity. In untreated late vitellogenic (0 h) follicles. androstenedione was the major product with smaller amounts of t estosterone and oestradiol-17 beta. In post-vitellogenic (12h post-injectio n) intact follicles, androstenedione pre dominated, and although testostero ne and oestradiol-17 beta were not produced, there were small amounts of 17 .20 beta -dihydroxy-4-pregnen-3-one (17,20 beta -P) and 17,21-dihydroxy-4-p regnene 3,20-dione (11-deoxycortisol). In HCG-treated fish, a steroidogenic shift resulted in the disappearance of testosterone and oestradiol-17 beta coinciding with the appearance of 17,20 beta -P. During early and late fin al oocyte maturation FOM (24 and 36 h post-injection), there was a five- to seven-fold increase in the production of 17,20 beta -P, whereas production of 11-deoxycortisol remained almost the same. During FOM, in addition to 1 7,20 beta -P, its 5 beta -reduced metabolite, 17,20 beta -dihydroxy-5 beta -pregnan-3-one (5 beta -17,20 beta -P) was synthesized, suggesting a decrea se in maturation-inducing 17,20 beta -P activity. 17,20 beta ,21 -Trihydrox y-4-pregnen-3-one (20 beta -S) was not synthesized by ovarian fragments in Japanese yellowtail at any maturational stage. The metabolites identified o n TLC during FOM were tested to evaluate their maturation-inducing activity in an in vitro bioassay. Of the steroids tested, 17,20 beta -P was the mos t effective inducer of germinal vesicle breakdown (GVBD), followed by 5 bet a -17,20 beta -P. Timely synthesis of 17,20 beta -P immediately prior to an d during FOM as well as its great potency in inducing GVBD in vitro support s the evidence for a physiological role of 17,20 beta -P as a maturation-in ducing hormone in Japanese yellowtail. (C) 2001 The Fisheries Society of th e British Isles.