Oestrogen administration induces progesterone receptors in lactating rat mammary gland

Displaceable binding of the synthetic progestin [3H]ORG 2058 (16 alpha-ethyl-21-hydroxy-19-nor-pregn-4-ene-3,20 dione) was found in ammonium sulphate treated cytosols from uterus and mammary gland from lactating rats. Specificity studies, however, showed that in mammary gland but not uterus this binding was exclusively to glucocorticoid receptors, and that specific progesterone receptors were undetectable. In contrast, after in vivo administration of oestradiol benzoate in oil (4 micrograms at -48, -24h), [3H]ORG 2058 bound predominantly to specific, high affinity progesterone receptors in lactating mammary gland. Thus though in lactation progesterone receptors are normally absent from the mammary gland, they can be induced by oestrogen administration.