Possibility of anti-estrogen action from estriol specific binding sites in rabbit uterus.

Abstract 1. This study was designed to investigate the clomiphene or tamoxifen binding to receptor sites for estradiol-17β (E 2 R) and estriol (E 3 R) in the rabbit uterus. 2. Those so-called anti-estrogenic compounds tended to inhibit E 2 -E 2 R and E 3 -E 3 R bindings equally. 3. The inhibitor constant of clomiphene for E 2 R was approximately 3.8 × 10 −8 M at 4°C and that for E 3 R approximately 1.8 × 10 −8 M at 4°C in a given case, determined by charcoal assay. 4. It is suggested that the anti-estrogenic compounds demonstrate their effects after binding either to E 2 R or to E 3 R. 5. There were some tissue differences of the contents between E 2 R and E 3 R. For example, the uterus and the cortex contained E 2 R, and the pituitary E 3 R more than the other.