Assessing the biological potency of binary mixtures of environmental estrogens using vitellogenin induction in juvenile rainbow trout (Oncorhynchus mykiss)

Experiments were conducted to assess the in vivo potency of binary mixtures of estrogenic chemicals using plasma vitellogenin (VTG) concentrations in juvenile rainbow trout (Oncorhynchus mykiss) as the endpoint. The estrogeni c potencies of estradiol-17 beta (E2), 4-tert-nonylphenol (NP), and methoxy chlor (MXC) were determined following 14 day exposures to the individual ch emicals and binary mixtures of these chemicals. E2, NP, and MXC all induced concentration dependent increases in plasma VTG, with lowest observed effe ct concentrations of 4.7 and 7.9 ng L-1 for E2, 6.1 and 6.4 mug L-1 for NP, and 4.4 and 8.5 mug L-1 for MXC. Concentration-response curves for fixed r atio binary mixtures of E2 and NP (1:1000), E2 and MXC (1: 1000), and NP an d MXC (1:1) were compared to those obtained for the individual chemicals, u sing the model of concentration addition. Mixtures of E2 and NP were additi ve at the concentrations tested, but mixtures of E2 and MXC were less than additive. This suggests that while NP probably acts via the same mechanism as E2 in inducing VTG synthesis, MXC may be acting via a different mechanis m(s), possibly as a result of its conversion to HPTE which is an estrogen r eceptor alpha agonist and an estrogen receptor beta antagonist. It was not possible to determine whether mixtures of MXC and NP were additive using VT G induction, because the toxicity of MXC restricted the effect range for wh ich the expected response curve for the binary mixture could be calculated. The data presented illustrate that the model of concentration addition can accurately predict effects on VTG induction, where we know that both chemi cals act via the same mechanism in mediating a vitellogenic response.