Nuclear rceptor-estrogen complex: In vivo and in vitro binding of estradiol and estriol as influenced by serum albumin

The influence of serum albumin (SA) on the ability of estradiol (E2) and estriol (E3) to promote the translocation of the receptor-estrogen complex to the nucleus of uterine cells was examined. Uteri from immature rats were incubated with E2 or E3in vitro in the presence or absence of SA and the translocation of the receptor-estrogen complex was measured by the [3H]estradiol exchange assay. In the absence of SA estradiol is more potent in causing the translocation of the receptor to the nucleus (50% saturation: E2 at 2 nM, E3 at 6 nM). However, the reverse is true when SA is present in the incubation medium (50% saturation: E2 at 18 nM, E3 at 9nM). Thus the presence of SA in the incubation medium strongly influences the amount of estrogen available for receptor binding and may explain why the ability of E3 to cause transloeation in vivo is equal to or greater than that of E2. These results suggest that SA may play an important role in determining the relative potencies of estrogens in vivo with regard to the transloeation process and the subsequent induction of uterine responses. Furthermore, these data indicate that extrapolation of relative potencies for estrogens based on in vitro determinations in the absence of SA must be viewed with caution.