A high-affinity binding site for the antioestrogens, tamoxifen and CI 628, in immature rat uterine cytosol which is distinct from the oestrogen receptor.

A high-affinity, saturable antioestrogen binding site, which does not bind oestradiol, has been reported to exist in a number of oestrogen target tissues but not in the immature rat uterus. This study reports the results of a more thorough search for this site in immature rat uterine cytosol. When concentrations of uterine cytoplasmic receptor were selectively depleted by translocation of 90-95% of the cytoplasmic oestrogen receptor to the nucleus, saturation analysis studies revealed that the antioestrogens, tamoxifen and CI 628, were bound to high-affinity, saturable binding sites which were present at about 2.5 times the concentration of the residual oestrogen receptor sites. Oestradiol could only partially inhibit the binding of tritiated antioestrogens to their saturable binding sites in this material indicating that a significant proportion of these sites were distinct from the oestrogen receptor sites. This was confirmed in experiments where oestrogen receptor sites were saturated in vitro with oestradiol and high-affinity, saturable sites for CI 628 and tamoxifen were still present. The CI 628 and tamoxifen had high affinity for these sites with dissociation constants of 1.0-1.6 nmol/l. These specific antioestrogen binding sites were present at about 5% of the concentration of oestrogen receptors in normal immature rat uterine cytosol which probably explains their previous lack of detections in this material.