Abstract
The effect of ovarian steroids on the maintenance of pregnancy in rats on an isoleucine-free diet was investigated. A combination of 0.5 μg of estrone and 4 mg of progesterone or one of these steroids only was injected into the rats for various periods until mid pregnancy. Administration of estrone with progesterone from day 3 to 13 prevented fetal loss completely, and even when injected only from day 9 to 13 about 80 % of the rats maintained pregnancy. Pregnant rats fed an inadequate diet without steroids failed to maintain pregnancy. Pregnant rats receiving deficient diets with or without steroids were sacrificed at intervals during early and mid pregnancy and the nucleic acid and protein contents of the uterus and incorporation of 14C-leucine into uterine proteins were examined. On day 7 of pregnancy, growth of conception products, total RNA, DNA, and protein in the uterus were significantly less in rats that did not receive steroids than in those that did. Incorporation of 14C-leucine into uterine proteins was the same in both groups until day 11, but on day 14 the incorporation rate rose in the group in which pregnancy was maintained and fell in the group in which it failed. These results suggest that ovarian steroids, particularly progesterone, may act on the uterus to inhibit protein degradation, at least during mid pregnancy. The total amount of estrogen receptor proteins in the uterus was maintained until quite a late stage of pregnancy in rats on an inadequate diet.
