718 | wileyonlinelibrary.com/journal/rda Reprod Dom Anim. 2018;53:718–724. © 2018 Blackwell Verlag GmbH 1 | INTRODUCTION A mild transient endometritis after breeding is a normal, physiolog- ical response in mares (Liu & Troedsson, 2008). In some mares, the inflammation and intrauterine fluid retention after breeding per- sist and have a significant negative impact on fertility (Newcombe, 1997). The incidence of persistent breeding-induced endometritis (PBIE) has been reported to range between 15% in a Thoroughbred broodmare population (Zent, Troedsson, & Xue, 1998) to 43% in a mixed population of mares (Newcombe, 1997). It has also been reported that mares show variability in their susceptibility to persistent endometritis. Hughes and Loy (1969) demonstrated a difference between mares in their ability to resolve endometritis after experimental inoculation of the uterus with Streptococcus zooepidemicus. Similar findings have been reported in more re- cent studies using intrauterine infusion of live (Alghamdi, Foster, Carlson, & Troedsson, 2005) or killed (Woodward et al., 2013) spermatozoa. This variability in susceptibility to persistent endo- metritis has been attributed mostly to impaired myometrial con- tractility (Troedsson, Liu, Ing, Pascoe, & Thurmond, 1993) and delayed uterine clearance (LeBlanc et al., 1994; Troedsson & Liu, 1991) in susceptible mares. Received: 6 November 2017 | Accepted: 11 February 2018 DOI: 10.1111/rda.13162 ORIGINAL ARTICLE Endometrial nitric oxide synthase activity in mares susceptible or resistant to persistent breeding-induced endometritis and the effect of a specific iNOS inhibitor in vitro FA Khan 1 | TS Chenier 1 | RA Foster 2 | J Hewson 3 | EL Scholtz 1 1 Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada 2 Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada 3 Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada Correspondence Firdous A. Khan, Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada. Email: firdous@uoguelph.ca Present address FA Khan, Department of Large Animal Medicine and Surgery, School of Veterinary Medicine, St. George’s University, True Blue, Grenada Funding information Equine Guelph, Grant/Award Number: 051648 Contents Emerging research suggests that the nitric oxide system may play a role in persistent breeding-induced endometritis (PBIE) in the mare. Differences in uterine nitric oxide (NO) levels between mares susceptible or resistant to PBIE and a dose-dependent inhibitory effect of NO on uterine contractility have been demonstrated. The objec- tives of this study were to investigate the difference in total nitric oxide synthase (NOS) activity of the endometrium between susceptible and resistant mares and the effect of a specific inducible nitric oxide synthase (iNOS) inhibitor on the endometrial NOS activity in vitro. Six susceptible and six resistant mares were selected based on preset criteria and the results of an intrauterine challenge with killed spermatozoa during oestrus. Endometrial biopsy samples were collected 24 hr post-challenge and cultured at 37°C for 24 hr in L-arginine supplemented minimum essential medium with or without a specific iNOS inhibitor (1,400 W dihydrochloride, 1 mM). The me- dium and the cultured endometrial tissue were collected after 24 hr of culture and assayed for NO and total protein, respectively. Total NO content of the medium, normalized to endometrial tissue wet weight or total protein, was used as a measure of endometrial NOS activity. Non-parametric tests were applied for statistical analy- sis. Susceptible mares had significantly greater endometrial NOS activity than resist- ant mares. The iNOS inhibitor treatment significantly reduced NOS activity in endometrial samples derived from susceptible and resistant mares. These findings provide a basis for in vivo testing of specific iNOS inhibitors as preventative or thera- peutic options for PBIE in mares.