Characterization of the nuclear progesterone receptor in normal human endometrium after progesterone injection

Nuclear progesterone receptors (RPN) were characterized in the 0.4 M KCl nuclear extract of the endometrium using [3H]R5020 as a ligand, in women in the proliferative phase of the cycle (n = 5), in the midcycle (n = 5), in the secretory phase (n = 4), and in premarin-treated women (n = 3), 1-3 h following progesterone injection. The receptor character of the binding was demonstrated by high specificity for progestin binding, high saturable affinity for R5020 and a sedimentation constant of about 3S in sucrose density gradient. The RPN characteristics were similar in all 4 groups and were also similar to those of the remaining cytosolic RP. Despite the fact that total RP levels differed in all 4 groups, the percentage of the receptor that was measured in the nuclear extract was similar in all groups and ranged from 52 to 61% of the total RP levels.