Steroid metabolism by ovarian follicles of the sea bass Dicentrarchus labrax

Ovarian samples from fear sea bass, Dicentrarchus labrax, were collected fo r the in vitro incubations during the spawning period. Follicles with fully developed vitellogenic oocytes showing central germinal Vesicle (stage I f ollicles) and follicles with oocytes showing initial germinal vesicle migra tion (stage II follicles) were treated with either (1) 20 mu g sea bass hyp ophysis plus 50 ng 17-hpdroxyprogesterone (17-P), (2) 20 mu g hypophysis al one, (3) 50 ng 17-P alone and (4) media alone. Structure-activity experimen ts used stage II follicles treated with several dosages (0.1, 1.0 and 10.0 ng/ml) of either 17-P, 17,20 beta-P, or 17,20 beta,21-P. Free and conjugate d (sulfates and glucuronides) levels of the established teleost oocyte matu ration inducing steroids (MIS), i.e. 17,20 beta-dihydroxy-4-pregnen-3-one ( 17,20 beta-P) and 17,20 beta,21-trihydroxy-4-pregnen-3-one (17,20 beta,21-P ) were measured in the incubation media by high performance liquid chromato graphy. Our results show that the synthesis of free and conjugated 17,20 be ta-P is constant (0.1-0.2 ng/ml) in all incubates. In contrast, the synthes is of free and conjugated 17,20 beta,21-P is higher in incubates containing stage II follicles (up to 5 ng/ml) than in those having stage I follicles (up to 3 ng/ml; P < 0.01). Structure-activity data reveal that 17-P is not effective at inducing in vitro germinal vesicle breakdown whereas both 17,2 0 beta-P and 17,20 beta,21-P are equally potent and highly effective. These results demonstrate that 17,20 beta-P and 17,20 beta,21-P are synthesized in vitro by follicles of sea bass and that sulfation is the main route for the metabolism of the C-21-steroids in riper follicles. The highest levels of 17,20 beta,21-P, found in incubates containing stage II follicles, point s at 17,20 beta,21-P, rather than 17,20 beta-P, as the most probable MIS in sea bass, nonetheless, this hypothesis requires further confirmation. (C) 2000 Elsevier Science Inc. All rights reserved.