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Binding Characteristics of Estrogen Receptor (ER) in Atlantic Croaker (Micropogonias undulatus) Testis: Different Affinity for Estrogens and Xenobiotics from that of Hepatic ER¹

^a Department of Marine Science, Marine Science Institute, University of Texas at Austin, Port Aransas, Texas 78373

An estrogen receptor (ER) was identified in cytosolic and nuclear fractions of the testis in a marine teleost, Atlantic croaker (Micropogonias undulatus). A single class of high affinity, low capacity, and displaceable binding sites was identified by saturation analysis, with a K_d of 0.40 nM in cytosolic extracts and a K_d of 0.33 nM in nuclear extracts. Competition studies demonstrated that the receptor was highly specific for estrogens (diethylstilbestrol > estradiol >> estriol = estrone) and also bound several antiestrogens. Testosterone and 5-dihydrotestosterone had much lower affinities for the receptor, whereas no displacement of specific binding occurred with 11-ketotestosterone or any of the C₂₁ maturation-inducing steroids. A variety of xenoestrogens, including o,p'-dichlorodiphenyltrichloroethane (DDT), chlordecone (Kepone), nonylphenol, hydroxylated polychlorinated biphenyls (PCBs), and the mycotoxin zearalenone, bound to the receptor with relatively low binding affinities, 10^-3 to 10^-5 that of estradiol. A comparison of the binding affinities of various ligands for the testicular ER and the hepatic ER in this species revealed that the testicular ER was saturated at a lower [³H]estradiol concentration (1 nM vs. 4 nM). The binding affinities of several compounds, including testosterone and nafoxidine, exhibited marked differences for the two ERs; and most of the estrogens and xenoestrogens tested had higher binding affinities for the testicular receptor. Minor amounts of estradiol (0.12 ng/g tissue/h) were produced by testicular tissue fragments incubated in vitro, and estradiol was detected in male Atlantic croaker plasma. The identification of a testicular ER and evidence that estradiol is produced by the testes in croaker suggest that estrogens participate in the hormonal control of testicular function in teleosts.

¹ This study was funded by Texas Sea Grant College Program, no. R/MBT-3 and EPA STAR grant no. R826125.

² Correspondence: Peter Thomas, University of Texas Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373. FAX: 512 749 6777; thomas{at}utmsi.utexas.edu

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