Tissue-specific concentration changes of estrone and estradiol during spontaneous and ACTH-induced parturition in sheep

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Abstract
Changes in estrogen production are considered important in the sequence of events leading to parturition. We sought tissue-specific changes in the concentration of unconjugated estrone (E1) and estradiol (E2) in intrauterine fetal (amnion, chorion) and maternal (endometrium, myometrium) tissues during normal pregnancy, labour, and ACTH-induced labour in sheep. The mean concentrations of E1 and E2 in the fetal membranes were higher than in endometrium and myometrium. In amnion there were no consistent changes in estrone concentrations with gestation, although estradiol concentrations increased between day 130 and term. In the endometrium there were increases in both estrone and estradiol between day 100 and term, whereas in the myometrium increases in the concentrations of E1 and E2 occurred between days 130–135 and term. Animals showing a labourlike pattern of uterine contractions after intrafetal ACTH administration did not show significant differences in estrone or estradiol concentrations in amnion, chorion, or endometrium compared with saline-infused controls. However, there was a progressive increase in the concentration of estrone and estradiol in the myometrium during ACTH-induced labour. We conclude that changes in the concentrations of estrone and estradiol in intrauterine tissues vary between the tissues studied and the two estrogens. In general, estrogen concentrations increased towards term, but this trend was more marked in the maternal than fetal tissues. The changes in estrone concentrations in myometrium, but not in the other tissues, were replicated during ACTH-induced labour. Our results would be compatible with the suggestion that tissue-specific changes in estrogen concentrations may contribute to the local intrauterine steroid milieu during pregnancy and at term.