Estrogen receptors and estrogen-induced gene expression in the rat mammary glands and uteri during pregnancy and lactation: Changes in progesterone receptor and RNA polymerase activity

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The modulations of estrogen (ER) and progesterone (PgR) receptors in the mammary glands and uteri have been studied in rats during pregnancy and at various time periods during lactation. The ability of estradiol in vivo to induce PgR synthesis in the lactating glands and uteri and to stimulate RNA polymerase activity of isolated nuclei from mammary glands during lactation was also investigated. A gradual increase in ER concentration in the lactating glands (compared to pregnancy) and uteri and a qualitative shift from 4S (in pregnancy) to 8S (in lactation) was observed in the mammary glands. On the other hand PgR levels (15 fmol/mg protein in the pregnant gland) declined at the onset of lactation and remained extremely low or undetectable (<1 fmol/mg protein) throughout lactation; uterine PgR was present throughout lactation (14-30 fmol/mg protein). While estradiol treatment (1 μg/day/rat, for 3 days) in rats during lactation induced a three-fold increase in PgR synthesis in the uteri, it failed to do so in the mammary glands. Lactating glands however responded to estradiol treatment as evidenced by a 4-5 fold increase in the activities of RNA polymerase I and II in isolated nuclei. © 1981.

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