Hormonal regulation of cytoplasmic estrogen and progesterone receptors in the beagle uterus and oviduct

The hormonal regulation of uterine and oviductal cytoplasmic estrogen and progesterone receptors was studied in immature beagles that were untreated, treated with estradiol-17β, or treated sequentially with estradiol and progesterone. Estradiol treatment increased the concentration of estrogen receptors in both tissues. Progesterone receptors were not detectable in the reproductive tract of untreated animals, but increased dramatically under the influence of estradiol. Estrogen withdrawal following estrogen stimulation, concomitant estrogen plus progesterone administration, and estrogen withdrawal plus progesterone administration all caused significant reductions in both estrogen and progesterone receptors in uterine and oviductal cytosols when compared to estrogen treatment alone. Estrogen withdrawal resembled estrogen plus progesterone administration in reducing both estrogen and progesterone receptor levels, although estrogen withdrawal plus progesterone administration resulted in a further reduction in both receptor concentrations. The same positive and negative relationships between estrogen and progesterone receptor content were observed in uterine cytosols from cycling and ovariectomized adults. These data suggest that estrogen and progesterone regulate their respective receptors and that tissue sensitivity to both steroids may be controlled by mechanisms involving fluctuations in receptor concentration in the reproductive tract of the beagle.