Differentiation of estradiol receptors in rat uterine cytosol by sensitivity to tamoxifen.

Analysis of rat uterine cytosol by means of immobilized antibody discloses the presence of two distinct high affinity, low capacity estradiol binding components. One of these is readily saturable by the antiestrogen tamoxifen (TS = tamoxifen sensitive), the other is tamoxifen insensitive (TI). Only TS-estradiol binding shows positive cooperativity at low ligand concentration. TS but not TI is lost when frozen tissue is thawed at 4°C then refrozen and stored for an additional 12 hours. Experiments with ovariectomized rats show that TI is formed only in the presence of estradiol. In the estrus cycle TS increases in the order: metestrus, diestrus, proestrus and estrus. The quantity of TI is the same in metestrus and diestrus and also the same but fractionally higher in proestrus and estrus.