Oestrogen and nuclear binding sites. Determination of specific sites by [3H]oestradiol exchange

A method was developed for the determination of the number of specific oestradiol-binding sites in the nuclear fraction of oestrogen-sensitive tissues. The method is based on the exchange of [ 3 H]oestradiol with non-labelled oestradiol that is bound to nuclear binding sites. The number of specific nuclear binding sites after the injection of 2.5μg of oestradiol, an amount sufficient to saturate all binding sites, is 1.6–1.7pmol per immature uterus. The number of sites occupied after an injection of physiological amounts of oestradiol (0.1μg) was 0.46pmol. The injection of oestradiol results in an increased number of nuclear binding sites in uterus and vagina, but has no effect on kidney or muscle. Injections of testosterone or progesterone failed to increase the number of uterine nuclear binding sites. This method permits an evaluation of the number of oestradiol-binding sites in the nuclear fraction of various tissues as a function of either endogenous oestradiol or non-labelled oestradiol administered by injection.