Different effects on oestrogen binding sites and anti-oestrogenic action of danazol and progesterone

Rabbit uterus contains type I and II oestrogen binding sites in the cytosol, and nuclear fractions. Oestrogen-stimulated increase in uterine weight was inhibited by concurrent treatment with progesterone or danazol. Oestrogen-induced type I binding sites were decreased by progesterone and to a lesser extent by danazol, while oestrogen-induced type II sites were decreased almost equally in the two groups. Neither progesterone nor danazol alone caused any detectable changes in uterine weight. These findings may suggest that the anti-oestrogenic effect on uterine weight is more correlated with the change in type II binding sites of oestrogen than that in type I sites. Thus, danazol may exert anti-oestrogenic actions through a pathway independent from that of progesterone.