Progesterone Receptors in the Fetal Uterus and Ovary of the Guinea Pig: Evolution during Fetal Development and Induction and Stimulation In Estradiol-Primed Animals*

Specific binding sites for [3H]progesterone ([3H]P) and for the synthetic progestagen [3H]R-5020 (17α,21-dimethyl-19-nor-pregna-4,9-diene-3,20-dione) are found in the cytosol and nuclear fractions of the fetal uterus of the guinea pig. P, R-5020, and 5α-dihydroprogesterone compete for binding with [3H]P and [3H]R-5020. Estradiol (E2), estrone, testosterone, and cortisol do not compete. In the cytosol fraction, the dissociation constant (Kd) at 4 C is 3.3 ± 1.7 (sd) × 10−9m for [3H]P, and the concentration of sites (n) is 180 ± 28 fmol/mg protein. Two components are found in sucrose density gradients at 6–7S and 4S. Few if any [3H]P-specific sites can be detected in uterine cytosol until 37–42 days of gestation. By 50–52 days, the concentration of sites is 35.4 ± 6.34 fmol/mg protein. These sites increase at the end of gestation to 85.6 ± 15.7 fmol/mg protein. Similar specific binding sites are found in the fetal ovaries, and at the end of gestation reach concentrations of 25.6 ± 8.6 fmol/mg protein. In an...