Progesterone receptors in the rat uterus: variations in cytosol and nuclei during the oestrous cycle and pregnancy.

The concentration of progesterone receptors in rat uterine cytosol and nuclei was measured during the oestrous cycle and pregnancy. The method used allowed the measurement of the total concentration of binding sites (unbound and hormone-bound). During the oestrous cycle, the concentration of receptors in the cytosol peaked at prooestrus, low concentrations were observed at oestrus and metoestrus, and in increase was seen at dioestrus. In the nuclei, maximum concentrations occurred at pro-oestrus and dioestrus. The number of receptors in the cytosol was very low during the first half of pregnancy, but the concentration increased progressively after day 15 to attain a very high level (about 26 000 binding sites/cell) on day 22. In the nuclei, the concentration of receptors was low at the beginning of pregnancy. On day 5 (day of implantation) there was a slight increase, which corresponded to a decrease in the number of cytosolic receptors and a small peak in the level of progesterone in the plasma. Maximum concentrations were attained during a 'plateau' period between days 9 and 15. Thereafter, there was very low; in some animals, probably on the verge of parturition, no receptors were detectable in the nuclei.