Metabolic responses to ammonium acetate infusion in exercising horses.

This study explored the relationship between elevated plasma ammonia levels, fatigue development, and muscle metabolism in exercising horses. The findings did not show any evidence that increased plasma ammonia levels play a role in fatigue development but did shed light on other aspects of nitrogen metabolism during exercise in horses.

Study Design and Primary Findings

• The study tested the metabolic responses of eight Quarter Horse mares during a submaximal fatigue test. The horses were either intravenously infused with sodium acetate (control group) or ammonium acetate (experimental group) prior to performing a graded treadmill exercise while carrying 27 kg.
• Time to fatigue—the primary measure of exercise tolerance—did not significantly differ between the two groups.
• As an expected result of exertion, both intramuscular ammonia and lactate increased during exercise, but the type of infusion received (sodium or ammonium acetate) did not make any difference.
• Change in blood and muscle levels of alanine and glutamate was reciprocal, indicating the activation of the glucose-alanine cycle—a crucial process that ensures stable blood sugar levels during exercise.
• Even though the plasma level of glutamine, another amino acid, increased with exercise, the muscle level of the same remained unaffected.

Secondary Findings

• Exercise led to a decrease in the excretion of urea-N—a byproduct of protein metabolism—while the intensity of physical activity did not change the ratio of orotic acid to creatinine, two markers of muscle use and kidney function, respectively.
• Overall, other amino acids and urinary metabolites showed no significant alteration in response to exercise or the type of infusion.
• The increased level of plasma ammonia observed during exercise did not exhibit any correlation with fatigue development, leading the researchers to conclude that such an increase may not be responsible for fatigue development in horses. However, this research provided valuable insights into nitrogen metabolism during exercise, contributing to the understanding of equine physiology.