In vitro steroid production by isolated ovarian follicles of the striped trumpeter

Ovarian follicles from striped trumpeter Latris lineata were incubated in L15 medium alone, or medium supplemented with gonadotropin (GtH) preparations (human chorionic GtH, carp maturational GtH or partially purified salmon GtH), testosterone (T) or 17‐hydroxyprogesterone (17P). Levels of oestrone (E 1 ), 17 β ‐oestradiol (E 2 ), T, and 17,20 β ‐dihydroxy‐4‐pregnen‐3‐one (17,20 β P) in the medium after incubation were measured by radioimmunoassay. Basal production of E 2 was high from previtellogenic follicles, whereas little T was produced. Both T and E 2 production increased in response to treatment with GtH or steroid precursors. Vitellogenic follicles showed basal production of both T and E 2 , and T but not E 2 levels generally increased in response to hormone treatment. Preparations containing follicles nearing final maturation showed low basal production of E 2 but high production of T. Treatment with steroids resulted in little change in E 2 but often very large increases in T production, whereas GtH stimulated lesser increases. 17,20 β P production was detectable from incubations of maturing follicles from two out of five fish, and in those two incubations, increased in response to treatment with 17P. E 1 was not detectable in any incubations. The results indicate that there is a shift in steroidogenesis from E 2 to T production during oocyte development, and provide further evidence that steroid biosynthesis in non‐salmonids is principally regulated by substrate availability.