Gestrinone has a biophysical antiestrogenic effect. But the mechanism of its antiestrogenic effect is not clear. Gestrinone blocked the increase of estrogen binding of nuclear type II sites and uterine weight in the estrogen-treated immature rabbit. Competitive assays indicated that gestrinone, at low concentrations (0.4 ∼ 4 nM), inhibited [3H]-estradiol binding to nuclear type II sites. This inhibitory effect was the same as the addition of 20 µM diethylstilbestrol. This inhibition with gestrinone on [3H]-estradiol binding to nuclear type II sites appeared only when used at concentrations less than an equivalent molar of [3H]-estradiol. Time course analysis of the gestrinone binding inhibition showed that within the first 6 min gestrinone did not inhibit specific [3H]-estradiol binding to nuclear type II sites. This gestrinone-mediated inhibition was not observed in soluble fractions such as the cytosolic and KCl-extracted nuclear binding sites. These results suggest that gestrinone acts primarily on the nuclear fraction and then it operates the inhibitory mechanism on estradiol binding to nuclear type II sites.

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