Concentrations of Nuclear Progesterone Receptors in Endometrium of Virgin and Repeat Breeder Heifers after Embryo Transfer

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Abstract
Summary: The present study aimed to correlate the repeat breeder syndrome in the bovine with an impaired or suboptimal uterine progestational response. Concentrations of nuclear (i. e. transformed) receptors for progesterone (PRn) were determined with a binding and exchange method in endometrial samples from virgin (VH) and repeat breeder (RBH) heifers 15 days post oestrus. The heifers were recipients of Day 7 demi‐embryos collected from donors with normal fertility and transferred 8 days prior to tissue sampling. Results were compared with both the type of heifer, the condition of the embryo present within the uterus and the temporal relationship to the hormone plasma levels. The binding data for PRn indicated that a single class of high‐affinity, low‐capacity sites existed. The amount of PRn in VH endometria holding embryonic structures was significantly greater than in RBH, but no statistical differences were found in their plasma progesterone levels. PRn concentrations were also higher in the uterine horn in which an elongated (> 15 mm), morphologically normal embryo was present, when compared to cornua with small (lt; 5 mm) embryos, regardless of recipient group. Furthermore, in endometria with an elongated embryo in the lumen, the relative amount of PRn was significantly greater in VH than in RBH. The present results indicate that the RBH recipients of Day 7 demi‐embryos by Day 15 have fewer numbers of specific receptors for maternal progesterone, which could explain in part the poorer development of the transferred embryos compared with that in virgin heifers.