Ontogeny of uterine responsiveness to estrogen during early development in the rat

1. 1. Uterine responsiveness to exogenous estradiol was studied in rats of 5-23 days of age in terms of several known estrogen-dependent parameters - induction of the synthesis of a specific uterine estrogen-induced protein, IP, and increased glucose metabolism, two temporally early responses to estrogen, and increased uterine weight at 1 day and at 3 days after hormone, two later estrogen-induced responses. 2. 2. Induced protein (IP) synthesis can be induced, and induced nearly maximally on a per cell basis in uteri of day 5 rats. The IP to DNA ratio remains nearly constant from day 5 through day 15-18 and drops at day 23. At all ages, IP represents 1.6-3.1% of the newly labeled soluble uterine protein. 3. 3. In contrast to the early uterine responsiveness in terms of IP synthesis, maximal stimulation of 2-deoxyglucose metabolism is not obtained until day 12-15 and maximal increases in the 24 h estradiol-induced uterine wet weight are not obtained until day 12. Likewise, a maximal 3 day uterine growth response cannot be induced until day 20 although some minimal response can be obtained to the 3 daily injections even in the day 5 uterus. 4. 4. These studies indicate that the uterus can respond partially to exogenous estrogen very early (by day 5), but that full responsiveness develops over a further period of approximately two weeks. Further, the induction of IP synthesis is itself not sufficient for induction of the full complement of responses culminating in uterine growth.