Progesterone action in hamster uterus: rapid inhibition of 3H-estradiol retention by the nuclear fraction.

We reported previously a rapid inhibitory effect of progesterone (P) on nuclear estrogen receptor (Re) concentrations in the hamster uterus. In the present study, the effect of P on uterine uptake and retention of 3H-estradiol (E2) was examined. Hamsters were ovariectomized and given Silastic E2 implants. At the time of 3H-E2 injection (0.1 microgram/100 g BW), the E2 implants were removed and either P (2.5mg/100g BW) or corn oil vehicle (control) were injected SC. At 1, 2 and 4 h after treatment, blood, uteri and segments of the small intestine (control tissue) were collected. Cytosol and nuclear KCl-extract fractions were prepared, and bound radioactivity was measured by Dextran-charcoal adsorption of free steroid. Specifically bound 3H-E2 was limited to the cytosol and nuclear fractions of the uterus. P had no influence on blood or cytosol radioactivity, or the initial (1--2h) uptake of radioactivity by the uterine nuclear fraction. However, at 4 h after treatment the amount of radioactivity retained by the uterine nuclear fraction was reduced in P-treated animals. Bound radioactivity lost from the nuclear fraction did not appear in the cytosol fraction. KCl-resistant nuclear radioactivity was monitored concurrently and, though present at much lower levels (ca. 10% of total nuclear radioactivity), it followed changes in KCl-extractable radioactivity. These results indicate that P reduction of uterine nuclear Re results from the P-stimulated loss of Re from the nuclear fraction and not by the inhibition of cytosol Re translocation.