Nitric oxide levels and nitric oxide synthase expression in uterine samples from mares susceptible and resistant to persistent breeding-induced endometritis

Problem: Breeding-induced endometritis (BIE) in the mare is resolved by 36 hr after insemination in resistant mares. However, 10-15% susceptible broodmares fail to do so because of impaired uterine contractility between 7 and 19 hr after exposure to seminal or bacterial challenge, which reduces their fertility. Method of study: Nitric oxide (NO) in uterine secretions, and expression of nitric oxide synthase (NOS) in uterine biopsies were compared between susceptible and resistant groups 13 hr after insemination. Results: Susceptible mares had a higher NO in their uterine secretions and greater inducible NOS (iNOS) expression in their biopsies compared with resistant mares. Conclusions: The NO mediates smooth muscle relaxation, but its role in persistent BIE has not been determined. Our data suggests a possible role of NO, either directly or in a NO-associated pathway, in delayed uterine clearance.