In vitro oocyte maturation in the sea bass:: effects of hCG, pituitary extract and steroids

Exposure of sea bass Dicentrarchus labrax oocytes to 25, 50, 100, and 300 IU ml(-1) of human chorionic gonadotropin (hCG) resulted in a dose-dependent increase (ED50 = 60 IU ml(-1)) in the maximal maturation and volume response rate over a 120-h incubation period. Oocytes responded in a similar dose-dependent manner to graded doses of homologous sea bass pituitary extract (ED50 = 3.3 x 10(-2) PE ml(-1)). The response latency of sea bass oocytes to endogenous and exogenous gonadotropins was similar (t(1/2) similar or equal to 45 and 48 h, respectively). Both 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta P) and 17 alpha,20 beta,21-trihydroxy-4-pregnen-3-one (20 beta S) stimulated maturation in a dose- and time-dependent manner with t(1/2) values less than those of hCG (15 h and 7.5 h at 100 ng ml(-1), respectively); 17 alpha,20 alpha-dehydroxy-4-pregnen-3-one (17,20 alpha P) was ineffective. 17,20 beta P was more potent at all doses tested. The stimulatory actions of gonadotropin and maturation-inducing steroid (MIS) involved both transcription and translation since maturation was significantly inhibited in the presence of actinomycin D and cycloheximide. Moreover, pituitary extract and hCG stimulated release of higher levels of 17,20 beta P in a dose- and time-dependent manner in follicle incubations as compared to 20 beta S suggesting that this steroid may in fact be the MIS in this species. (C) 1999 The Fisheries Society of the British Isles.