106 Maternal recognition of pregnancy (MRP) and mares: Is the epithelial sodium channel (ENaC) involved?

Abstract

Maternal recognition of pregnancy (MRP) describes the prevention of luteolysis via the early conceptus. By prolonging the presence of the corpus luteum (CL), the conceptus ensures an environment capable of supporting pregnancy. In mares, MRP is enigmatic; the horse is one of few domesticated species wherein the MRP signal has yet to be discovered. Equine conceptus mobility throughout the uterine horns, termed migration, is known to be essential for pregnancy establishment. However, while the MRP signal in several other species is conceptus-derived, such a signal has yet to be observed from the equine embryo. We hypothesize that the migration of the equine conceptus itself activates MRP, potentially through epithelial sodium channel (ENaC). ENaC has been previously detected in multiple equine organs, observed in the uteri of various mammalian species, and also found to be upregulated during preimplantation in mice. However, neither the presence of ENaC within the mare uterus nor its potential involvement with the equine pregnancy has been researched. The aim of this study was 1) to determine if ENaC was present within the endometrium of the mare uterus and 2) to describe the location of ENaC if observed. Estrus synchronization was completed in mares (n = 14) during peak breeding season. Mares were subsequently blocked by age and breed and then placed into one of 2 groups: control mares (C; n = 7) and treatment mares (T; n = 7). Transrectal ultrasonography (US) was completed in all mares daily when one or more ovarian follicles reached 25 mm. When a mare had a 35 mm or larger follicle present, she received 2,000 international units of human chorionic gonadotropin intramuscularly to induce ovulation (OV) and US continued daily. Additionally, T mares were artificially inseminated (AI) with fresh, extended semen every day until the day after OV and were subsequently checked for pregnancy 12 d post-OV. C mares had the cervix manually manipulated the day after OV to replicate physical stimulation that occurs to the cervix when AI; however, no semen was deposited in C mare reproductive tracts. Endometrial biopsies were collected from all mares before OV, after OV, and during early pregnancy (when applicable) and samples were then processed for ENaC immunohistochemistry analysis. ENaC protein was immunodetected in all C and T mare endometrial samples and was localized to both uterine luminal epithelium and uterine glands. While further analysis will be conducted to quantify endometrial ENaC expression in C versus T (open versus pregnant) mares, the present histological data confirms that ENaC is present within the equine uterus.