Dominant expression of porcine Calbindin-D9k in the uterus during a luteal phase.

Calbindin-D9k (CaBP-9k) is a member of intracellular calcium binding proteins, which have a high affinity to calcium. CaBP-9k is mainly expressed in the mammalian intestine, uterus and placenta, and is regulated in tissue- and species-specific manners. Previous studies have shown that CaBP-9k expression is mainly controlled by steroid hormones and their receptors. Thus, we further investigated the expression and regulation of CaBP-9k during an estrus cycle in the pig uterus by Northern blot and immunoblot analysis in this study. In addition, serum levels of estrogen (E2) and progesterone (P4) were measured using ELISA. The CaBP-9k mRNA is highly expressed in the porcine uterus during a luteal phase compared to a follicular phase, and its mRNA level in a luteal phase is increased up to 10-fold compared to a follicular phase. In parallel to the level of CaBP-9k mRNA, the CaBP-9k protein is also dominantly expressed in the porcine uterus, and strongly expressed in the epithelium and glands of the porcine uterus during a luteal phase. Although, the localization of the CaBP-9k protein is scarcely detected at follicular phase, it is dominantly expressed in the porcine uterus during a luteal phase. In addition, the serum P4 level was significantly increased during a luteal phase compared to a follicular phase, whereas no difference was observed in E2 levels between follicular and luteal phases, indicating that the ratio of P4/E2 is remarkably increased in porcine uterus during a luteal phase compared to a follicular phase. In conclusion, these results suggest that P4 may play an important role in the up-regulation of CaBP-9k gene in the porcine uterus in a luteal phase, which is unlike the condition in the rat uterus. In addition, the porcine CaBP-9k may be dominantly expressed in the epithelium and glandular structure of pig uterus during a luteal phase. It may also be differentially regulated during this cycle presumably by steroid hormones, especially up-regulated P4 levels in this tissue.