Nitric oxide synthase inhibition in thoroughbred horses augments O2 extraction at rest and submaximal exercise, but not during short-term maximal exercise.

The research observed the effects of nitric oxide synthase inhibition on oxygen extraction and anaerobic metabolism in Thoroughbred horses, at rest and during exercise. Findings showed that nitric oxide synthase inhibition increased oxygen extraction at rest and during submaximal exercise, but not during short-term maximal exercise. Furthermore, the inhibition did not affect anaerobic metabolism regardless of the horse’s physical state.

Research Objective and Methodology

• Researchers aimed to understand the role of endogenous nitric oxide (NO) in the metabolism of resting and exercising Thoroughbred horses.
• The experiment used the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) administered at 20 mg/kg to healthy, exercise-trained Thoroughbred horses.
• The experiments were conducted at rest and during a series of exercises culminating with 120 seconds of maximal exertion at a speed of 14 m/s on a 3.5% uphill grade.
• The study was placebo-controlled, with the inhibitor and placebo studies carried out 7 days apart.

Main Findings

• The results showed that NO synthase inhibition significantly increased the arterial to mixed-venous blood oxygen content gradient and oxygen extraction in resting horses. This increase was due to a decrease in mixed-venous blood oxygen tension and saturation.
• During submaximal exercise, NO synthase inhibition resulted in a more substantial decrease in mixed-venous blood oxygen tension and hemoglobin-oxygen saturation, leading to an increase in oxygen extraction.
• However, during short-term maximal exercise, the changes in oxygen extraction and markers of anaerobic metabolism were not different from the placebo group.
• The inhibition of NO synthase did not affect arterial blood-gas tensions in exercising horses. Additionally, it attenuated the exercise-induced increment in hemoglobin concentration and arterial oxygen content.

Conclusions

• The study concluded that endogenous nitric oxide production augments oxygen extraction at rest and during submaximal exertion, but not during short-term maximal exercise.
• It was also found that nitric oxide synthase inhibition does not impact anaerobic metabolism in resting or exercising horses.
• This suggests that it is unlikely that endogenous nitric oxide release modifies aerobic or anaerobic metabolism in horses performing short-term maximal exertion.