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-Regulation of the uterine expression of estrogen and progesterone receptors was studied in 20 three-month-old lambs that were not treated or treated with estradiol-17β. Determinations of receptors were performed by binding assays in the nuclear and cytosolic fractions, receptor mRNAs by solution hybridization, and estrogen receptor protein by an enzyme-immunoassay. Estradiol treatment decreased the receptor binding capacity of both receptors and the levels of immunoreactive estrogen receptor 12 h after injection in the absence of decreased receptor mRNAs, suggesting that the initial decrease is due to degradation of the proteins or that mRNAs are translated into new receptor proteins at a reduced rate. The mRNA levels increased after estradiol treatment suggesting that the replenishment phase consists of synthesis of new receptors rather than recycling of inactivated receptors.estrogen / progesterone / receptor / uterus / lamb Résumé -Action biphasique de l'oestradiol sur l'expression des récepteurs aux oestrogènes et à la progestérone dans l'utérus de l'agnelle. La régulation de l'expression des récepteurs aux oestrogènes et à la progestérone au niveau de l'utérus a été étudiée chez des agnelles immatures âgées de 3 mois (n = 20), traitées et non traitées avec oestradiol-17β. Les déterminations des récepteurs ont été faites par des essais de liaison dans les fractions nucléaires et cytosoliques, les ARNm des récepteurs par la méthode d'hybridation en solution et la protéine du récepteur aux oestrogènes par un essai immuno-enzymatique. Le traitement à l'oestradiol réduit la capacité de liaison des deux récepteurs et les niveaux des récepteurs aux oestrogènes immunoréactifs 12 h après l'injection en absence d'une diminution des ARNm des récepteurs, suggèrent que la réduction initiale est due à la dégradation des Reprod. Nutr. Dev. 40 (2000) 283-293 283

Cell responsiveness to steroid hormones is related to the number and affinity of its receptors, thus factors affecting steroid expression will influence tissue sensitivity and functionality. The present review discusses the role of oestrogen and progesterone receptors in sheep female reproductive physiology. The mechanism of steroid hormone action in the target cell is introduced first; the tissue distribution, physiological functions and regulation of oestrogen receptor subtypes and progesterone receptor isoforms in ruminants are reported. The role of steroid receptors in target tissues (with emphasis on the uterus and pituitary gland) during different physiological events is addressed in an attempt to clarify oestrogen and progesterone actions in different developmental and reproductive stages: prepubertal period, oestrous cycle, pregnancy, post-partum period and seasonal anoestrus. The present review shows how the distinct reproductive stages are accompanied by dramatic changes in uterine receptor expression. The role of oestrogen and progesterone receptors in the molecular mechanism responsible for premature luteolysis that results in subnormal luteal function is discussed. Finally, the effect of nutrition on sex steroid receptor expression and the involvement on reproductive performance is reported.

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