Free Amino Acid Content of Equine Muscle Changes in Response to Graded Levels of Lysine Intake

Abstract

In typical equine diets lysine is the first limiting amino acid (AA) making it of particular interest in growing horses. Six thoroughbred colts (401±5 d) were used to study six levels of lysine intake: 76, 90, 104, 118, 127, & 136 mg/kg/d. Diets were isonitrogenous and isocaloric. Each horse was fed each diet in a random order for 6 d utilizing a 6×6 Latin square design. On d 6 venipuncture samples were taken into heparinized vacutainers before the concentrate portion of the feeding and 90 min post‐feeding. Muscle biopsies were taken ~100 min post‐feeding from the gluteus medius and flash frozen. Muscle samples were freeze dried and homogenized. Plasma and homogenized muscle were analyzed via HPLC. Plasma lysine concentrations increased in a dose dependent manner with increasing lysine intake (P<0.0001). Muscle AAs had different response patterns. Taurine concentrations were greatest for horses receiving the greatest lysine intake (P=0.01). Arginine (P=0.03) and ornithine (P=0.10) concentrations were greatest in the horses receiving the highest and the lowest levels of lysine intake. Lysine tended (P=0.10) to respond, at first decreasing and then increasing with lysine intake. These results indicate that dietary lysine intake affects AAs that are structurally related to lysine, namely ornithine and arginine, and the mechanisms governing this effect require additional investigation. This project was supported by Agriculture and Food Research Initiative Competitive Grant no. 2010–65206‐20638 from the USDA National Institute of Food and Agriculture.