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 I U ml(-1) of human chorionic gonadotropin (hCG) resulted in a dose-dependent increase (ED50 = 60 IU ml(-1)) in the maximal maturation and volume respon se 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 t o endogenous and exogenous gonadotropins was similar (t(1/2) similar or equ al 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 (2 0 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), respe ctively); 17 alpha,20 alpha-dehydroxy-4-pregnen-3-one (17,20 alpha P) was i neffective. 17,20 beta P was more potent at all doses tested. The stimulato ry actions of gonadotropin and maturation-inducing steroid (MIS) involved b oth transcription and translation since maturation was significantly inhibi ted 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 d ose- and time-dependent manner in follicle incubations as compared to 20 be ta S suggesting that this steroid may in fact be the MIS in this species. ( C) 1999 The Fisheries Society of the British Isles.