Expression of membrane progestin receptors (mPRs) α, β and γ in the bovine uterus during the oestrous cycle and pregnancy.

Progesterone (P4) affects cell function through the nuclear progesterone receptor and membrane-bound progesterone binding proteins, including the membrane progestin receptors (mPRs) alpha (mPRα), beta (mPRβ) and gamma (mPRγ), which belong to the progestin and adipoQ receptor family (PAQR7, 8 and 5, respectively). The aim of this study was to determine the mRNA and protein expression levels of mPRα, mPRβ and mPRγ through real-time PCR and Western blot analyses, respectively, and to determine the cellular localization of these proteins in the bovine endometrium and myometrium on days 2-5, 6-10, 11-16 and 17-20 of the oestrous cycle and weeks 3-5, 6-8 and 9-12 of pregnancy (n = 5/each time period). The resulting data showed the highest (P < 0.05) mPRα and mPRβ mRNA expression in the endometrium on days 11-16 of the oestrous cycle compared to the other stages. In the myometrium, the level of mPRα mRNA was the lowest (P < 0.05) on days 6-16 of the oestrous cycle, while mPRβ was the lowest on days 11-16. There were no changes (P > 0.05) in mPRγ mRNA expression in the endometrium and myometrium during the oestrous cycle. During pregnancy, in the endometrium and myometrium, the levels of mPRα and mPRβ mRNA were comparable with those observed during the oestrous cycle. However, mPRγ mRNA expression was the highest (P < 0.001) during all stages of pregnancy compared with that observed during the oestrous cycle in both uterine tissues. The mPRα protein level only changed in the myometrium and was the highest (P < 0.05) during weeks 9-12 of pregnancy. However, in the endometrium, the expression of mPRβ protein was higher (P < 0.05) on days 6-10 of the oestrous cycle than during weeks 6-8 of pregnancy. Strong positive immunoreactions for all mPR proteins were observed in the luminal and glandular epithelium but were less evident in the stromal cells and myocytes. In addition, all proteins were also localized in the endothelial cells of blood vessels in the uterus, suggesting that P4 may affect blood flow in this organ through mPRs. The presence of mPR receptors in the uterus indicates their participation in the regulation of uterine functions.