139 Cyclic changes in uterine epithelial sodium channel in mares

Abstract

Epithelial sodium channel (ENaC) has been localized within many organs of various species, including the uterus of mice, rats, and humans. Additionally, ENaC expression within the rodent uterus was differentially influenced by 17β-estradiol (E2) and progesterone (P4), and has been proposed to be the major mechanism regulating uterine fluid absorption and secretion. In the equine, ENaC has been observed in the trachea, hooves, and endometrium; however, the potential relationship between E2, P4, and ENaC in the mare endometrium has yet to be elucidated. The aim of this study was to investigate possible interactions between E2, P4, and ENaC from cycling mares using immunohistochemistry (IHC). Estrus was synchronized for all mares (n = 13) by daily administration of oral altrenogest (ALT) for 15 consecutive days. On the last day of ALT treatment, all mares received 5 mg of dinoprost IM. Following synchronization, all mares were teased daily with a stallion. Transrectal ultrasonography (US) for observation of ovarian activity and day of ovulation (OV; d0) were completed every other day until OV. On d10, US, blood collection via jugular venipuncture, and a uterine biopsy sample were collected. Biopsy samples were fixed to later complete ENaC evaluation by IHC. A uterine biopsysample and blood collection had also been similarly collected and processed for each mare during estrus when she simultaneously had a ≥ 30mm Graafian follicle present and was displaying estrual behaviors. For statistical comparison, a paired t-test was applied. Mares averaged an estrus duration of 4.85 ± 1.86 d. Plasma E2 was 2.27 ± 0.45 pg/mL on d10 and 3.58 ± 0.48 pg/mL on estrus sample day. Plasma P4 concentration was significantly higher (P < 0.01) on d10 when compared with estrus sample day (8.53 ± 1.17 ng/mL vs 0.04 ± 0.02 ng/mL, respectively). The percentage of positively stained ENaC cells was significantly higher (P < 0.01) on d10 when compared with estrus sample day (81.27 ± 3.19% vs 58.84 ± 3.23%). In conclusion, our findings indicate that sex steroids, specifically P4, may regulate ENaC within the mare endometrium, with ENaC being increased during diestrus and decreased during estrus. Collectively, the changes of ENaC concentrations during the estrous cycle might impact the critically sensitive fluid environment within the mare uterus.