POTENTIATING EFFECT OF GROWTH-HORMONE ON VITELLOGENIN SYNTHESIS INDUCED BY 17-BETA-ESTRADIOL IN PRIMARY CULTURE OF FEMALE SILVER EEL (ANGUILLA-ANGUILLA L) HEPATOCYTES

Previous in vivo experiments have indicated a potentiating role of gro wth hormone (GH) during experimentally induced vitellogenesis by 17 be ta-estradiol (E2) in the female silver eel (Anguilla anguilla L.). To investigate whether GH has direct hepatic actions, the effects of hypo physial-purified and recombinant GH on vitellogenin (Vg) synthesis in response to E2 were tested on primary cultures of hepatocytes. Hepatoc ytes were prepared from control or E2-primed eels. Addition of E2 alon e into the culture medium induced both Vg synthesis and secretion in a dose- and time-related fashion. Bovine growth hormone (bGH) alone had no effect on the induction of Vg synthesis or secretion. Bovine GH en hanced the in vitro effects of E2 on both Vg synthesis and secretion, an effect attenuated by an in vive E2 priming which was dose-dependent with an ED(50) of 5 ng/ml. To investigate the specificity of GH actio n, purified eel and salmon GH and salmon, trout, and tilapia prolactin s (PRL), as well as recombinant trout and tilapia GH, were tested, and the responses were compared to bGH. Purified salmon and homologous ee l GH potentiated the vitellogenic response to E2. Recombinant GH were highly efficacious, excluding the presence of active contaminants in t he potentiating effect of GH preparations. The potentiating effect of recombinant trout GH on the vitellogenic response was reduced at high doses (above 20 ng/ml), suggesting a down-regulation of GH binding sit es by GH itself. Salmon PRL has minimal activity, but not trout and ti lapia PRL, indicating that PRL is not an important potentiating factor on Vg synthesis in our model. It is concluded that GH acts directly o n the liver to potentiate E2 induction of eel hepatic Vg synthesis. Th e potentiating effect of GH appears to be time- and dose-dependent and modulated as a function of hormonal status (E2 priming) of the eel. ( C) 1996 Academic Press, Inc.