Changes in cytosolic and nuclear progesterone receptors during pregnancy in rat placenta

These studies examine changes in placental growth and the abundance of progesterone receptors (Rp) in whole placentas between Days 9 and 22 of pregnancy. In addition, some placentas were dissected into decidual basalis, junctional zone, and labyrinth zone before assay of Rp. High affinity binding of 3H-progesterone to Rp was detected at all stages of pregnancy in whole placentas and in decidua basalis and the junctional zone of the placenta. Cytosolic and nuclear receptors exhibited similar affinity for progesterone in both tissues (Kd = 3.1 +/- 0.3 and 4.4 +/- 0.7 nM, respectively). Receptor binding in whole placentas increased from Day 9 to Day 12 (p less than 0.05), declined markedly at Day 16 (p less than 0.05), and returned to former levels on Days 19 and 22 (p less than 0.05). Decidua basalis contained 84% of total Rp on Day 14, which declined to 67% on Day 17 (p less than 0.05). The junctional zone contained 16% of total Rp on Day 14 and 33% on Day 17. After Day 17, junctional zone was the only source of Rp. The decline in Rp on Day 16 followed regression of decidua basalis; recovery of Rp thereafter was due to growth of the junctional zone. The labyrinth zone did not express significant amounts of Rp at any stage despite a 4-fold increase in growth in late pregnancy. Although the biologic role of the Rp in maintenance of pregnancy is poorly understood, these studies suggest that the maternal decidua basalis and fetal junctional zone are targets of progesterone action.