Effects of danazol and medroxyprogesterone acetate on estrogen-(estradiol and estriol) specific binding sites in rabbit uterus

Abstract

In rabbit uterus, the presence of separate specific binding sites for not only estradiol but also estriol has been proposed. These sites may be correlated with an antiestradiol effect. Therefore, this study was designed to investigate the effect of antiestrogenic agents such as danazol and medroxyprogesterone acetate (MPA), especially on the estriol binding sites. Danazol and MPA in combination with estradiol were administered subcutaneously to immature female rabbits daily for 10 days, and resulted in a significant (p < 0.05) decrease in uterine weight and estradiol binding sites in the uterus. Treatment with MPA significantly (p < 0.05) decreased the level of estriol binding sites, but treatment with danazol resulted in this to a minimal extent in the uterus primed by estradiol. MPA did not bind to estradiol and estriol binding sites, while danazol at a high concentration bound to estriol binding sites with some affinity, but not to estradiol binding sites in the uterine cytosol of estrogen-primed rabbits. These results suggest that within the antiproliferative effect of danazol and MPA (an antiestrogenic action on estrogen-stimulated uterine growth) there are likely to be specific differences between some of the possible mechanisms of danazol and MPA in their action at the estriol binding site.