Review Article The Nitric Oxide System in Equine Reproduction: Current Status and Future Directions Firdous A. Khan, Elizabeth L. Scholtz * , Tracey S. Chenier Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada article info Article history: Received 2 December 2014 Received in revised form 5 February 2015 Accepted 20 February 2015 Available online xxxx Keywords: Nitric oxide Stallion Mare Reproduction abstract The nitric oxide (NO) system is fairly ubiquitous and is involved in a wide range of physiological and pathologic processes. Besides its well-established roles in vasodilation, platelet aggregation, neurotransmission, and cytotoxicity, it has been shown to regulate reproduction in several animal species. In horses, the NO system in reproduction is a relatively recent research area. Information available in the mare is limited to the presence of the NO system in the ovaries and uterus, a possible role of the system in the regulation of normal ovarian and uterine functions, and its association with various reproductive abnormalities. Little is known about the NO system in the stallion, except for expression of nitric oxide synthase isoforms in the testis and epididymis and the effects of NO on cry- opreserved semen. However, there are clear indications from research to date that NO may be an important regulator of reproduction in both stallions and mares. This review is aimed at summarizing the available information on the NO system in equine reproduction and identifying gaps in the literature that need to be addressed in the future. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction Nitric oxide (NO) is synthesized in biological systems from L-arginine [1]. The reaction is catalyzed by nitric oxide synthase (NOS), an enzyme that exists in multiple isoforms in nature. Three of these isoforms have been identified in mammals, namely neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS). The former two are constitutively expressed and collectively referred to as constitutive NOS [2]. The enzyme activities of nNOS and eNOS are regulated by calcium (Ca 2þ ) and calmodulin, whereas the enzyme activity of iNOS is largely or com- pletely Ca 2þ independent [3]. Nitric oxide is a versatile molecule with an established role in a wide range of bio- logical processes including, but not limited to, vasodilation [4], platelet aggregation [5], neurotransmission [6], and cytotoxicity [7]. The biological actions of NO are mediated either through the guanosine cyclic 3 0 ,5 0 -monophosphate pathway or occur as a result of the direct effect of NO [8]. Since the initial implication of NO in regulation of penile erection in rabbits by Ignarro et al [9], the molecule has been shown to be involved in regulation of several repro- ductive processes in different species [10,11]. In horses, NO in reproduction is a relatively recent research area, in which NO synthesis and regulation have not been fully elucidated (Fig. 1). Studies so far have mainly focused on investigating the presence of the NO system and its role in the ovaries and uterus in the female and the expression of the NO system in the testis and epididymis and its effects on cry- opreserved semen in the male. To date, a review of the available literature on the NO system in equine reproduc- tion is lacking. This review is aimed at summarizing the literature and identifying important gaps that need to be addressed in the future. 2. The NO System in Female Reproduction The role of NO as a regulator of female reproduction has been investigated extensively in several laboratory animal * Corresponding author at: Elizabeth L. Scholtz, Department of Popu- lation Medicine, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada. E-mail address: escholtz@uoguelph.ca (E.L. Scholtz). Contents lists available at ScienceDirect Journal of Equine Veterinary Science journal homepage: www.j-evs.com 0737-0806/$ – see front matter Ó 2015 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jevs.2015.02.009 Journal of Equine Veterinary Science xx (2015) 1–7