Hormonal mechanisms regulating hepatic vitellogenin synthesis in the gilthead sea bream, Sparus aurata.

Experiments were carried out to study in vitro the effects of 17beta-estradiol (E(2)), homologous pituitary homogenate (HPH), and recombinant red sea bream growth hormone (sbGH) on vitellogenin (VTG) secretion from cultured sea bream liver fragments. Basal secretion of VTG was found to be significantly higher in the prespawning period, compared with sea bream liver in the spawning and postspawning periods. Similarly, the sea bream liver obtained during the prespawning period responded more significantly to treatments with E(2), HPH, or sbGH compared with sea bream liver during spawning. In the postspawning period, treatments with E(2), HPH, or sbGH were without significant effect on VTG secretion level in sea bream liver. The level of E(2) receptors was also analyzed by Western blot analysis. The result demonstrates a significantly higher level of E(2) receptors in the sea bream liver at the prespawning stage compared with those at the spawning and postspawning stages. The findings support the hypothesis that homologous upregulation of estrogen receptors plays an important role in the estrogen-sensitive control of VTG synthesis in the sea bream liver.