The natural steroids estradiol-17beta (E2) and estrone (E1) and the synthetic steroid
ethynylestradiol-17alpha (EE2) have frequently been measured in waters receiving domestic
effluents. All of these steroids bind to the estrogen receptor(s) and have been shown
to elicit a range of estrogenic responses in fish at environmentally relevant concentrations.
At present, however, no relative potency estimates have been derived for either the
individual steroidal estrogens or their mixtures in vivo. In this study the estrogenic
activity of E2, E1, and EE2, and the combination effects of a mixture of E2 and EE2
(equi-potent fixed-ratio mixture), were assessed using vitellogenin induction in a
14-day in vivo juvenile rainbow trout screening assay. Median effective concentrations,
relative to E2, for induction of vitellogenin were determined from the concentration-response
curves and the relative estrogenic potencies of each of the test chemicals calculated.
Median effective concentrations were between 19 and 26 ng L(-1) for E2, 60 ng L(-1)
for E1, and between 0.95 and 1.8 ng L(-1) for EE2, implying that EE2 was approximately
11 to 27 times more potent than E2, while E2 was 2.3 to 3.2 times more potent than
E1. The median effective concentration, relative to E2, for the binary mixture of
E2 and EE2 was 15 ng L(-1) (comprising 14.4 ng L(-1) E2 and 0.6 ng L(-1) EE2). Using
the model of concentration addition it was shown that this activity of the binary
mixture could be predicted from the activity of the individual chemicals. The ability
of each individual steroid to contribute to the overall effect of a mixture, even
at individual no-effect concentrations, combined with the high estrogenic potency
of the steroids, particularly the synthetic steroid EE2, emphasizes the need to consider
the total estrogenic load of these chemicals in our waterways.