Myometrial estradiol and progesterone receptor changes in preterm and term pregnancies

To determine if labor is associated with changes in myometrial estradiol (E2) and progesterone receptors.

Lower myometrial segments were obtained from women undergoing cesarean deliveries at term in labor (n = 10), term not in labor (n = 10), preterm in labor (n = 9), and preterm not in labor (n = 11). Western immunoblotting was used to determine the presence and molecular size of E2 and progesterone receptor proteins. Immunocytochemistry was used to determine E2 and progesterone receptor changes in preterm and term pregnancies.

Myometrium from pregnant women contained 74-kilodalton (kDa) E2 receptor and 94- and 110-kDa progesterone receptor proteins. These receptors are present in both myometrial smooth muscle and myometrial blood vessels. The nuclei of myometrial smooth muscle cells primarily contain both receptors. The immunostaining for progesterone receptors was less in patients in labor compared with those not in labor in preterm and term pregnancies. In comparing patients not in labor, the immunostaining for progesterone receptors was less at term compared with preterm pregnancy. Unlike the differences in progesterone receptors, there are no obvious differences in E2 receptor immunostaining in myometrial samples from all four groups of women.

A myometrial decrease in progesterone receptors, rather than an increase in E2 receptors, may play a role in the onset of labor in women with term or preterm pregnancies.