The postovulatory cat oviduct and uterus: correlation of morphological features with progesterone receptor levels

Oviducts and uteri were obtained from cats at estrus and up to 14 days after coitus. Progesterone binding sites were estimated by Scatchard analysis, serum levels of progesterone were determined by radioimmunoassay, nd key morphological parameters of the lining of the epithelium of the oviduct and the deep uterine glands were quantitated. Progesterone cytosol binding was highest at estrus, significantly depleted by day 4, and continued a gradual decline out to day 14 in both tissues. Nuclear binding was markedly elevated at day 4; with time the binding decreased until by day 14 it was equal to or lower than that detected at estrus. The initial depletion of cytosol binding and rise in nuclear binding coincided with the initial rise in serum progesterone levels. Within the oviduct, the increase in nuclear binding coincided with the onset of regressive changes within the epithelium. These changes included atrophy, deciliation, and apoptosis. Within the uterine glands, the increase in nuclear binding coincided with the onset of the progestational response. This response included cell hypertrophy, glycogen synthesis and deposition, and with longer exposure to elevated levels of progesterone, the disappearance of these glycogen deposits. These results clearly illustrate the immediate effect of low levels of progesterone on the subcellular compartmentalization of the progesterone receptor system, and the down regulation of the total receptor system when target tissues are exposed to a continuous elevation of serum progesterone. These results also illustrate that similar changes in receptor compartmentalization induce different morphological responses within target tissues from the same animal.