Oestrogen receptors and protein synthesis in caruncular and intercaruncular endometrium of sheep before implantation

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Abstract
Summary. The rate of protein synthesis was higher in caruncular (P < 0·05) and intercaruncular endometrium (P < 0·01) of pregnant compared to non-pregnant ewes on Day 11 (Day 0 = day of oestrus), but not on Days 9, 13 and 15. Cytosolic oestrogen receptor concentrations (nmol/mg protein) were lower in caruncular endometrium of pregnant ewes than in non-pregnant ewes on Days 9 (P < 0·02), 13 (P < 0·05) and 15 (P < 0·001) but not on Day 11. In intercaruncular endometrium, the receptor values were lower on Day 15 (P < 0·02) of pregnancy compared to non-pregnancy, but not on the other days. The dissociation constant (0·75–2·55 × 10−10 m) of oestradiol-17β for cytosolic receptors was similar in both tissues, regardless of pregnancy status or day of sampling. The rate of protein synthesis was generally higher in intercaruncular endometrium whereas cytosol receptor concentrations were higher in caruncular endometrium. When the blastocyst was confined to one uterine horn on Day 2 in another group of ewes, there was a decrease in nuclear (P > 0·05), cytosolic (P < 0·05) and total (P < 0·05) oestrogen receptors only in caruncular endometrium in the pregnant horn on Day 15. There were no differences in the proportions of receptors localized in nuclear fractions or in the rate of protein synthesis in either endometrial tissue from the pregnant or the non-pregnant horn. We conclude that the ovine blastocyst is capable of influencing oestrogen receptor levels in caruncular and intercaruncular endometrium by a local action as early as Day 9, well before endocrine recognition of pregnancy on Day 12 and attachment on Day 15.

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