Regulation of Uterine Steroid Receptor Levels and Peroxidase Activity During the Puerperium of the Nonlactating Rat1

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Abstract
Uterine nuclear and cytosol levels of receptors for progesterone (PR) and estrogen (ER) were measured and compared with serum levels of these 2 hormones in the rat immediately after birth and during the first 25 days of the puerperium. Both nuclear and cytosol ER were elevated at birth, decreased extensively 24 h following parturition, then gradually increased until Day 5, postpartum. Serum estrogens were elevated at parturition, gradually decreased until Day 4, then increased again on Day 5. Nuclear PR content was low during the first 3 days postpartum before increasing to a maximum on Day 5, while cytosol PR concentration was increased at birth, then declined 1 day later before rising again by Day 5 of the puerperium. Serum progesterone was very low at birth and during the 4 days following, but started to rise on Day 5 and was significantly increased on Day 10 before declining again 15-25 days after parturition. Parturient rats were administered estrogen and progesterone to examine the hormonal response of the uterus during the puerperium. Estradiol treatment (100 .mu.g/animal) on Days 0, 1 and 2 or Days 7, 8 and 9 increased cytosol PR content 1.9-fold on Day 3 and 2.5-fold on Day 10, while peroxidase activity increased 4.2-fold on Day 3 and 45-fold on Day 10. Morphological examination of the uterus revealed that the luminal epithelium had changed in appearance from cuboidal to tall columnar on both Days 3 and 10. Uterine uptake of eosinophils also increased significantly on Days 3 and 10 and was proportional to the increase in peroxidase activity. Uterine wet weight was also increased on both days. Immediately following birth, administration of estradiol (100 .mu.g) to parturient rats resulted in translocation of the ER to the nucleus. Cytosol PR also translocated at this time after injection of 1.0 mg of progesterone. Uterine steroid receptor concentrations after parturition may be correlated with serum hormone levels. The uterus is able to respond to esterogen administration during the periparturient period as shown by increases in uterine wet weight, cytosol progesterone receptor, peroxidase activity and degree of eosinophil infiltration, and the increase in uterine peroxidase activity after 3 days of estrogen treatment is derived rom eosinophil infiltration and the estrogen-induced enzyme present in the epithelium.