The in vivo effect of combinations of octylphenol, butylbenzylphthalate and estradiol on liver estradiol receptor modulation and induction of zona radiata proteins in rainbow trout: no evidence of synergy

Juvenile rainbow trout were given intraperitoneal (IP) injections of (i) octylphenol (OP) and butylbenzylphthalate (BBP) or (ii) octylphenol and estradiol (E). Both pairs of compounds were administered singly and in combination, at two different dose levels. Modulation of the hepatic estradiol receptor (ER) binding capacity and induction of hepatic zona radiata proteins (Zrp) were utilised as indicators of estrogenic effect. In treatment (i) OP caused a significant (approx 2-fold) upregulation of the ER and a significant (approx 3-5 fold) increase in Zrp at both doses compared to control, vehicle-injected fish. In contrast, no effect on the ER was observed in BBP-treated fish, but plasma Zrp levels were significantly reduced. No synergistic effect was observed on ER modulation or Zrp induction in fish receiving OP and BBP in combination. In treatment (ii) both OP and E caused a significant (approx 2-fold) upregulation of the ER and a significant (approx 6-7 fold) increase in plasma Zrp at both dose levels compared to controls. The effect of OP and E in combination, on both ER modulation and induction of Zrp, was additive and not synergistic.