Endometrium estradiol receptors type I and type II during early pregnancy of rat

By centrifugation in a sucrose density gradient we studied the citosol 17β-estradiol binding sites of blastocyst receptive and non-receptive endometrial zones, as well as uterine horn endometrium whose ovary was extirpated three weeks before pregnancy. The cytosol was prelabelled with {3H}-17β-estradiol 2 and 25 nM. In this work two incubation temperatures were studied. On the other hand, at 4 °C unoccupied receptors were identified as different from the classic receptor 8S type I. At the same time, we found that 25 °C is the optimal temperature for the assay of total receptors to achieve complete exchange of {3H}-17β-estradiol by 17β-estradiol in the binding sites. In these conditions, the major component was the 4S type II receptor, mainly in the endometrium from ovariectomized uteri. Furthermore, 17β-estradiol content was determined in the total homogenized by radioimmunoassay and the results were: 1.42 ± 0.16, 1.22 ± 0.15 and 1.75 ± 0.27 pmol/g wet tissue for receptive, non-receptive and ovariectomized uteri, respectively.