The degree of uterine responsiveness to exogenous estrogen was studied in 4-day cycling rats and in ovariectomized, estrogen-primed rats. Uterine tissue collected at different stages was incubated in medium-199 containing [3H]leucine at 37 C under 95% oxygen and 5% carbon dioxide for 2 hr. Tissue uptake and protein incorporation of the radioactivity were measured to assess uterine activity. In cycling rats, the uterus was able to respond to 1.0 μg of 17β-estradiol by increases in both uptake and incorporation of [3H] leucine during estrus, metestrus and in the morning of diestrus but not during proestrus nor in the afternoon of diestrus. In ovariectomized rats, estrogen priming (1.0 μg) seemed to cause an immediate loss in uterine sensitivity to further hormonal stimulation. Subsequently, the estrogen-primed uterus gradually regained its estrogen responsiveness and eventually reached a degree even higher than that of unprimed uterus. The data suggest that the ability of the uterus to respond to exogenous estrogen appears to correlate with the level of available estrogen receptor in the tissue.
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