Changes in serum steroid hormones and steroidogenic ability of ovarian follicles during artificial maturation of cultivated Japanese eel, Anguilla japonica

Serum levels of oestradiol-17 beta (E(2)), testosterone (T) and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-DHP) were assessed by radioimmunoassay during induction of sexual maturation of female Japanese eel (Anguilla japonica) by chum salmon pituitary homogenate (SPH) treatment. SPH stimulated vitellogenic growth in about 50% of the eels. Serum E(2) levels in eels which responded to SPH remained low for the first 10 weeks after the start of SPH injections, and tended to increase after 12 weeks, Levels increased further after completion of vitellogenesis. Serum levels of T increased gradually as SPH treatment progressed, and remained higher than E(2) levels during the first 12 weeks. A further increase in T levels occurred after completion of vitellogenesis. 17 alpha,20 beta-DHP was not detected in serum during the experimental period. In vitro production of E(2), T and 17 alpha,20 beta-DHP by ovarian follicles in response to forskolin (an activator of adenylate cyclase), and changes in the ability of follicles to convert exogenous pregnenolone of 17 alpha-hydroxyprogesterone (17 alpha-OHP) to E(2), T and 17 alpha,20 beta-DHP, and exogenous T to E(2), were examined using 18 h incubations. Forskolin had no effect on the production of any measured steroid by follicles at any stage of development. Production of E(2), T and 17 alpha,20 beta-DHP in the absence of exogenous substrates was low. The ability of follicles to produce E(2) from pregnenolone, 17 alpha-OHP or T was low during vitellogenesis, followed by an increase during the migratory nucleus stage. The ability of follicles to produce T from pregnenolone and 17 alpha-OHP gradually increased during vitellogenesis, followed by a further increase or a slight decrease during the migratory nucleus stage. The conversion of pregnenolone or 17 alpha-OHP to 17 alpha,20 beta-DHP increased during vitellogenesis, followed by either a further increase or a decrease during the migratory nucleus stage. A good correlation existed between serum levels of E(2) and aromatase activity (the ability of follicles to convert T to E(2)). E(2) production by follicles was shown to depend largely on aromatase activity, while T production by follicles appeared to be depend largely on SPH-induced pregnenolone production. Increased aromatase activity at the migratory nucleus stage may inhibit 17 alpha,20 beta-DHP production and spontaneous final oocyte maturation and ovulation.