Summary. The secretion and synthesis of protein in vitro by explants of endometrium were examined in entire ewes during the first 10 days of the oestrous cycle and during an equivalent interval in ovariectomized ewes which received injections of oestradiol and progesterone. The schedule of steroid injections given was designed to simulate endogenous ovarian secretion of progesterone during the luteal phase before oestrus, of oestradiol around oestrus and of progesterone during the luteal phase after oestrus.
The rate of protein synthesis and tissue RNA: DNA and protein: DNA ratios in intercaruncular and caruncular endometrium were generally higher in entire than in ovariectomized ewes. In ovariectomized ewes oestradiol increased these activities at 2– 4 days after oestrus, whereas progesterone preceding oestradiol caused increases at oestrus, but not thereafter. In entire ewes and in ovariectomized ewes receiving the full steroid treatment regimen, protein secretion was high at oestrus and declined markedly during the next 4–6 days. In ovariectomized ewes not receiving progesterone before oestradiol, secretion increased between 4 and 6 days after oestrus, or during the equivalent stage of treatment in ewes which did not show oestrus. The omission of this progesterone did not modify secretion by caruncular endometrium. Oestradiol increased protein secretion by both tissues.
The data suggest that progesterone given before oestradiol (or its equivalent in entire ewes) inhibits the secretion, at about 4–7 days after oestrus, of uterine proteins which may impair embryo development in ovariectomized ewes which do not receive this progesterone.