Estrogen and progestogen binding in the myometrium of the ewe. I. During the estrous cycle

Estradiol, progesterone, estradiol binding, and promegestone (R5020, a progestogen) binding were measured in the cytoplasmic an nuclear fractions of sheep myometrium on days O (estrus), 3, 6, 9, 12 and 15 of an estrous cycle. Estradiol and the numbers of unfilled estrogen binding sites were greatest at estrus in both the cytoplasm (.33 and 7.5 pmole/mg of DNA, respectively) and nuclei (.58 and 3.9 pmoles/mg DNA) and declined to low values by days 12 and 15 of the estrous cycle. The number of R5020 binding sites was greatest in both the cytoplasm (13.5 pmoles/mg of DNA) and nuclei (4.2 pmoles/mg of DNA) on days 0 and 3 of the estrous cycle). Progesterone in the tissues peaked on day 9 in the cytoplasm (2.0 pmoles/mg of DNA) and on day 6 in nuclei (1.2 pmoles/mg of DNA). High levels of estrogen binding and estradiol in the myometrium at estrus suggest that the marked stimulation or contractions at estrus by ovarian estrogens is mediated via myometrial binding of estrogen. High levels of progesterone were found when contractile activity was known to be relatively quiescent. High levels of progestogen binding found at estrus might play a role in progesterone-induced uterine quiescence during the luteal phase of the estrous cycle.