Expression of vitamin D receptor in the porcine uterus and effect of 1,25(OH)D on progesterone and estradiol-17β secretion by uterine tissues in vitro.

Vitamin D acting via its nuclear receptor (VDR) was shown to target many reproductive tissues and regulate their function. Nevertheless, little is known about the role of vitamin D and VDR in the uterus. We hypothesized that VDR expression profile varies in the porcine uterus throughout the course of the estrous cycle, and 1,25(OH)D influences uterine steroidogenic activity. The aim of this study was to investigate VDR mRNA expression, VDR protein abundance and immunolocalization in the porcine endometrium and myometrium harvested on Days 2-5, 12-13, 15-16 and 18-20 of the estrous cycle. Additionally, in studied pigs, 25OHD concentration in plasma and uterine flushings was determined by RIA. The effect of 1,25(OH)D (10, 50 and 100 ng/mL) in vitro on progesterone (P) and estradiol-17β (E) release by endometrial and myometrial slices obtained on Days 12-13 of the estrous cycle was also examined. Nuclear VDR immunostaining was found in endometrial (luminal and glandular epithelium, stromal cells) and myometrial cells throughout examined days of the estrous cycle. In the endometrium, the highest VDR mRNA expression was observed on Days 12-13 and 18-20, whereas the greatest VDR protein abundance was noted only on Days 12-13 of the estrous cycle. In the myometrium, either VDR transcript or protein level was the greatest on Days 12-13. Interestingly, the highest 25OHD concentration in plasma and uterine flushings was shown also on Days 12-13 of the estrous cycle. 1,25(OH)D did not affect P release by uterine slices while myometrial release of E was significantly increased in response to 1,25(OH)D (10 and 50 ng/mL). Overall, obtained results indicate that porcine uterus is a target tissue for vitamin D throughout the entire estrous cycle. VDR mRNA expression and protein abundance altered within uterine tissues depending on studied days of the estrous cycle with the greatest protein abundance during mid-luteal phase of the estrous cycle in both uterine tissues. In addition, 1,25(OH)D significantly increased myometrial release of E on Days 12-13 of the estrous cycle. These results suggest the role of vitamin D-VDR system in the uterus, especially as a regulator of myometrial estrogenic activity in pigs during mid-luteal phase of the estrous cycle.