The objective of the present study was to measure plasma amino acid concentrations in horses under the influence of feed intake and exercise. The study was carried out in two independent trials.
In the first trial, pre- and postprandial concentrations of plasma free amino acids were measured in 10 yearling horses. Blood samples were taken in the morning before feeding of hay, oats and soya meal and over an 8-hours postprandial period at 2 hour intervals on days 1 and 40 of the study. The plasma amino acids were measured by high pressure liquid chromatography after orthophthalaldehyde derivatization. Mean fasting concentrations of the amino acids were not significantly influenced by the individuum and sampling day. While the absolute postprandial AA concentrations differed between sampling days, the relative changes were comparable. All AA increased after feeding, reaching their maximum concentration at 2 hrs post feeding. Most AA approached the fasting concentrations at 8 hours, only glycine increased between 6 and 8 hours after the meal. 3-methyl-histidine concentrations remained constant throughout the entire period. The second trial focussed on changes of plasma free amino acid concentrations after short intense exercise. Blood samples were taken from 36 standardbred trotters, and sampling was repeated in 20 horses after 35 days. Exercise intensity was estimated from postexercise plasma lactate levels and horses were grouped in retrospect depending on plasma lactate concentrations between 4 and 15 mmol/l (group 1) or above 15 mmol/l (group 2). The plasma concentrations of alanine, aspartate, glutamate, isoleucine, leucine, lysine and taurine were increased and arginine, asparagine, citrulline, glutamine, glycine, histidine, methionine, serine and tryptophan were decreased after exercise.
Ornithine, threonine, tyrosine, phenylalanine and valine concentrations remained constant. Higher intensity of exercise significantly decreased tryptophan and increased taurine concentrations. Sampling day had a significant effect on the absolute pre- and postexercise amino acid concentrations. In conclusion these studies show that feed uptake and exercise have measurable effects on plasma amino acid concentrations in horses.
Futhermore exercise appeared to trigger some form of muscle protein catabolism, leading to the release of amino acids into the plasma.
Plasma amino acid concentrations should therefore only be interpreted under well defined conditions, especially regarding the feeding regimen.
Further studies are necessary to evaluate the clinical significance of these findings. Optimized amino acid supply could, however, be of significance to improve muscle integrity and shorten recovery times after strenuous exercise in competition horses. This requires further studies focussing on the relationship of net amino acid release by skeletal muscle and the onset of catabolic processes.