An Investigation of Uterine Nitric Oxide Production in Mares Susceptible and Resistant to Persistent Breeding‐Induced Endometritis and the Effects of Immunomodulation

Abstract

The first objective of this study was to evaluate intrauterine nitric oxide (NO) and endometrial inducible NO synthase (iNOS) in mares susceptible or resistant to persistent breeding-induced endometritis (PBIE) within 24h after breeding. Mares susceptible (n=6) or resistant (n=6) to PBIE were inseminated over five cycles, and uterine secretions and endometrial biopsies were collected before and 2, 6, 12 and 24h after insemination. Uterine secretions were analysed for NO and biopsies were analyzed for iNOS expression. A second experiment evaluated the effect of treatment with dexamethasone or mycobacterial cell wall extract (MCWE) on uterine NO production and endometrial iNOS mRNA expression. Six susceptible mares were inseminated over three cycles with (i) killed spermatozoa without treatment (control), (ii) killed spermatozoa with 50mg of dexamethasone IV or (iii) MCWE IV 24h prior to insemination with killed spermatozoa. Six resistant mares were inseminated with killed spermatozoa as a control. Six hours after breeding, uterine biopsies and secretions were collected and evaluated for NO and iNOS mRNA. In Experiment 1, resistant mares had an increase in iNOS mRNA expression 2h post-breeding compared to baseline (p=0.045), 12h (p=0.014) and 24h (p=0.001). Susceptible mares had higher expression 2h compared to 6h (p=0.046). No differences were observed in mRNA or protein expression of iNOS between resistant and susceptible mares. Resistant mares had a relatively steady amount of total intrauterine NO over 24h, while susceptible mares had an increase over time, with a significantly higher increase in total NO than resistant mares at 6 (p=0.04) and 12h (p=0.032). In Experiment 2, no differences were observed for iNOS mRNA expression. Susceptible mares had increased NO when compared to resistant mares (p=0.008) and MCWE decreased NO (p=0.047).