Temporal relationships between endometrial RNA polymerase activities and steroid hormone receptors following estradiol administration during the midluteal phase of the ovine estrous cycle

The relationships of estradiol-induced changes in nuclear-bound estrogen and RNA polymerase activities in the ovine endometrium during the midluteal phase of the estrous cycle were studied. Concomitantly, changes in cytoplasmic progesterone receptors were also monitored. Mature ewes received a single i.m. injection of 500 μg estradiol-17β on each of Days 11 and 12 of the cycle and were necropsied at 0, 1, 6, 12, and 24 hr following the second injection of hormone. Activities of RNA polymerase I and II were determined by [³H]-UTP incorporation into RNA during incubation of endometrial nuclei in the absence and presence of α-amanitin, respectively. Total nuclear and cytoplasmic steroid receptors were estimated by exchange assay using [³H]-progestin (R5020) and [³H]-estradiol. Treatment of ewes with estradiol resulted in a transient increase in the concentration of neclear-bound estradiol at 1 hr (P<0.05) and enhanced activity of RNA polymerase I (P<0.01) at 6, 12, and 24 hr post-treatment. The activity of RNA polymerase II did not differ among intervals. Concentrations of cytoplasmic progesterone receptors also increased with time and at 24 hr were greater than levels at 0 hr (P<0.05). These data suggest that during the midluteal phase of the cycle, when progesterone secretion is near maximal, exogenous estradiol is able to stimulate RNA polymerase activities. The estradiol-induced increases in RNA polymerase I activity may result in RNA synthesis essential for promoting the observed increases in cytoplasmic progesterone receptors.