Detection and comparison of nitric oxide in clinically healthy horses and those with naturally acquired strangulating large colon volvulus.

The objective of the study was to determine whether nitric oxide (NO) is present in clinically healthy horses (control) under basal conditions, and if it increases secondary to naturally acquired strangulating large colon volvulus (affected). Eleven affected horses and 10 controls were studied. Jugular venous blood, abdominal fluid, and urine were collected. The NO concentrations were standardized to the creatinine concentration in the respective samples. A biopsy specimen collected from the large colon pelvic flexure at surgery was divided into subsections for processing for inducible nitric synthase (iNOS) and nitrotyrosine (NT) immunohistochemical staining and reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemical staining. There were no significant differences in plasma, abdominal fluid, or urine NO concentrations between affected and control horses. There was a significant decrease in submucosal arteriolar and venular endothelium, submucosal plexus, mucosal leukocyte, mucosal and musclaris vasculature, and myenteric plexus NADPH diaphorase staining in affected versus control horses. There was a significant increase in iNOS staining in mucosal leukocytes and vasculature in affected versus control horses. Other than a greater number of positively stained mucosal leukocytes in affected horses, there were no significant differences between affected and control horses for NT staining. The presence of NADPH diaphorase staining in the endothelium and submucosal neurons suggests endothelial and neuronal NOS are present under basal conditions in the large colon of horses. Increased iNOS and NT staining in mucosal leukocytes of affected horses suggests involvement of the NO pathway in large colon volvulus. The reasons for the lack of a significant difference in plasma, abdominal fluid, and urine NO concentrations between affected and control horses are unknown.