Alterations in progesterone receptors in the rat uterus bearing an intra-uterine device during the oestrous cycle and early pregnancy.

The binding characteristics, content and intracellular distribution of cytosolic and nuclear progesterone receptors have been investigated, using [3H]progesterone as ligand, in the rat uterus bearing a unilateral intra-uterine device (IUD) during the oestrous cycle and from days 3 to 6 of pregnancy. The dissociation constants of nuclear and cytosolic progesterone-receptor complexes for IUD-containing and control uterine horns were similar. Cytosolic receptor concentrations in the IUD-containing uterus were always lower but changed in a manner similar to the control during the periods studied. Nuclear receptor concentrations in the control horn reflected changes in hormone levels during the oestrous cycle although concentrations measured were greater than previously reported. However, in IUD-containing uteri the pattern was completely reversed with minimal levels at pro-oestrus. Nuclear receptor concentrations were little different in both horns during early pregnancy. Total progesterone receptor synthesis determined between metoestrus and pro-oestrus in IUD-containing horns was significantly less than that of control horns. This correlated with the attenuated rise of nuclear oestrogen receptor levels previously observed between these times in IUD-containing uterine horns.