Unique Estrogen-Binding Properties of Rat Pregnancy Plasma

Abstract

Estrone and estradiol-17β were shown to displace 3H-estradiol-17β from binding sites in plasma of the pregnant rat. The ligand specificity of this plasma differed from that of human pregnancy plasma. In the rat preparation, neither testosterone nor estradiol-17α competed with 3H-estradiol-17β, while estrone and estradiol-17β were equivalent in this activity. With plasma from nonpregnant rats, the binding of radioactive hormone was not affected by increasing amounts of nonradioactive estradiol- 17β. The binding specificity of rat pregnancy plasma was dissimilar to that of estrophilic substances of rat uterine cytosol. With uterine receptor, certain synthetic estrogens and antiuterotrophic agents competed with 3H-estradiol- 17β for affinity sites. However, these compounds were not effective in altering the binding of isotope by rat pregnancy plasma. The present observations demonstrate that rat pregnancy plasma contains an entity or entities which bind estrone and estradiol-17β, and that estrogenic or anti-estrogenic activity, per se, is not dependent upon the association of hormone or drug with these plasma constituents. (Endocrinology88: 427, 1971)