Smnmary--We describe here an/n vitro technique to assess the estrogenic activity of chemicals. This technique is based on rainbow trout hepatocytes incubated in a basic medium free of any additional growth factors or estrogenic chemicals and uses the production of vitellogenin (VTG) as a marker for the estrogenic potency of the compounds tested. The system allows at least some of the metabolic transformations which are undertaken by the liver cells/n vivo and could therefore be used for xenobiotic compounds which exhibit estrogenic activities after liver metabolic transformation. A dose-response curve was always consistently obtained using estradiol-17fl (E2), with a mid point at around 100 nME 2 and a maximum response at around 1000nM. Established estrogens such as 17al ethynylestradiol (EE2) or diethylstilboestrol (DES) were also tested. EE 2 appeared to be equipotent with E2 and DES slightly less potent. E2 conjugates were, perhaps surprisingly, also very potent. Estradiol-3-sulfate was equipotent with E2 and estradiol-17fl-glucuronide approx. 10% as potent. Other steroids such as androgens and progesterone, though active in the bioassay, were 3 orders of magnitude less potent than E 2. Of the various steroids tested, only cortisol, at concentrations up to 50 #M, was completely inactive. Six different phytoestrogens were tested in the assay. All were weakly estrogenic, possessing approximately one thousanth the potency of E 2 (they were as potent as the androgens and progesterone). All six phytoestrogens, as well as the androgens and progesterone, were tested in the presence of tamoxifen. In all cases tamoxifen reduced the production of VTG significantly, demonstrating that the estrogenic action of all of these compounds was most likely mediated by the E 2 receptor. The potencies determined here may not reflect the situation/n vivo but can provide complementary results about the activity of chemicals which need an hepatic metabolization to be estrogenic. Hepatocyte cultures would profitably be developed in other species to sustain these results.

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