Employing an in vitro method, the content of available estrogen receptor in the 100, 000 Xg supernatant fraction of uterine homogenate has been studied in female rats during pubescence and the estrous cycle. In prepuberal rats (closed vagina), concentrations of 37, 37 and 26 femtomoles/mg (fmoles/mg) of wet uterus were found at 21, 27 and 33 days of age, respectively. In post-puberal rats (open vagina), the respective concentrations were 13, 10 and 6 fmoles/mg for animals 33, 39 and 45 days old. The concentration therefore decreases with age. Total receptor content per uterus was 940, 1310, 1700, 2260, 1810 and 1760 fmoles, respectively, for the same 6 groups. There was only a 2-fold change in total available receptor although uteri have increased in weight 10-fold from 21 to 45 days of age. In cycling rats, uterine receptor concentrations were: proestrus, 1.2; estrus, 4.8; metestrus, 3.2; and diestrus, 4.7 fmoles/mg of wet uterus. The corresponding total contents of uterine receptor were: 676, 2400, 1150 and 1610 fmoles. These data reveal a cyclic fluctuation of receptor concentration in the rat uterus during the estrous cycle, suggesting that uterine content of available estrogen receptor is directly or indirectly under hormonal control.
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