Progesterone regulation of the occupied form of nuclear estrogen receptor.

Total concentrations of estrogen receptor in the uterine nuclear fraction are reduced rapidly after progesterone treatment of the proestrous hamster. Progesterone acts selectively on the occupied form of the nuclear estrogen receptor, with no effect on the concentration of an unoccupied form. This observation indicates that progesterone modulates the action of estrogen by controlling nuclear retention of the estrogen-receptor complex.