Uterine responses to estradiol in the neonatal rat.

Although single doses of estrogens are known to be ineffective in stimulating complete uterine responses in newborn rats, repeated doses elicit toxic responses that become evident in adulthood. In this study, neonates injected daily from birth with 10 micrograms 17 beta-estradiol (E2) demonstrated significant uterine wet weight gain by day 3 and near-maximum growth (230% of control) by day 5. Elevated uterine weight can be maintained by repeated daily injections at least through day 13. Other uterine growth responses after 5 days of E2 (as percent of control) are: dry weight, 163%; DNA content, 193%; protein content, 211%; and nuclear estrogen receptor, 890%. Ornithine decarboxylase activity increased to 520% of control when measured 6 h after a single E2 injection on day 5. Histological examination of uteri from animals treated for 5 days reveals an altered stroma with evidence of circular muscle differentiation, while the lumenal epithelium, which is cuboidal in controls, becomes columnar after E2 injections. These data suggest that estrogens act in fundamentally the same manner in the neonatal uterus as in the adult uterus, although the appearance of the complete response appears to e slower in the neonate. The estrogen-induced precocious development we describe suggests that an estrogen may be involved in the normal postnatal development of uterine estrogen responsiveness. Adult toxicity, resulting from repeated neonatal estrogen dosing, may partly be a consequence of continual hormone action inducing developmentally inappropriate responses.