Interaction of estradiol with rat uterus “receptors” has been studied using tissue incubation. The reversibility of estradiol binding has been confirmed and 2 types of binders have been observed. The first have great affinity, limited capacity and high specificity. The other, having smaller affinity and nonlimited capacity in the range of concentration used (up to 1 × 10-5 M), can be treated as a “partition” system. Nontarget organs, such as muscle or fat, also incorporate estradiol as a partition system, but with acoefficient smaller than that of the uterus. Different estrogen derivatives have been studied and there is some correlation between their affinity for limited capacity receptors and their uterotrophic activity. After incubation at 37 C, there is no “8 S” cytoplasmic estrogen binder, whereas there is more estradiol incorporated into nuclei than after 2 C incubation when the “8 S receptor” is preserved. (Endocrinology84: 108, 1969)

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