Amino acid utilization by the hindlimb of warmblood horses at rest and following low intensity exercise
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Background: In particular branched-chain amino acids might limit muscle protein loss in pathological conditions. Little is known on basic amino acid utilization of muscle in horses. Objective: To assess amino acid utilization by the hindlimb of horses at rest and following low intensity exercise. Animals & Methods: Amino acid uptake by the hindlimb was investigated using the arteriovenous difference technique. Blood from six warmblood mares (mean age 12±3 (SD) years and weighing 538±39 kg) was collected simultaneously from the (transverse) facial artery and from the caudal vena cava. Food was withheld for 12 hours prior to exercise. Exercise comprised of a standardized treadmill protocol consisting of 5 minutes of walk, 20 minutes of trot, and thereafter another 5 minutes of walk. Amino acids were determined quantitatively by means of anion exchange chromatography. Statistical analysis was performed using a general linear mixed model. Results: Amino acids with the largest average extraction at rest were citrulline (11.1±9%), cystine (8.3±36%), serine (7.9±11%), and leucine (5.9±9%). Of the 25 amino acids studied, none showed a significant difference following exercise. Glycine (485±65 µmol/L), glutamine (281±40 µmol/L), valine (183±26 µmol/L), and serine (165±22 µmol/L) showed highest plasma concentrations. The average extraction for α-aminobutyric acid at rest was 18.2±26%. Arterial plasma citrulline concentration was higher than venously. Conclusion: Citrulline, cystine, serine, and leucine might be regarded as most important amino acids at rest in warmblood mares. Clinical importance: Further investigation is necessary into the specific role of leucine supplementation to preserve or restore body protein in horses.