Relationships between dietary, plasma, and muscle amino acids in horses

Muscle and plasma amino acids were evaluated in horses of various functions. Eight horses were used for exercise (EH) and maintenance (MH) with 5 pregnant mares (PH) that continued into lactation (LH). Five weanlings were used for growth (WH). The EH, MH, and PH groups were used for 8 wk and the LH and WH groups for 4 wk. Blood samples and muscle biopsies were taken at the end of the study and analyzed for amino acids. There were no differences in total muscle amino acids between groups except histidine, which was lower for the EH group. Postfeeding amino acids were greater in the MH, EH, PH, and LH groups for lysine, arginine, and histidine, whereas threonine increased in the MH, EH, and PH groups. The MH group had postfeeding increases in plasma valine, methionine, isoleucine, leucine, and phenylalanine. Correlations within function were found between total and free muscle amino acids as well as plasma for several amino acids. The greatest number of correlations occurred within the weanling group. The WH group had correlations between intake and plasma threonine, lysine, valine, isoleucine, leucine, methionine, and phenylalanine; between intake and total muscle threonine, methionine, leucine, and phenylalanine; and between plasma and total muscle lysine, phenylalanine, and leucine. These data suggest that the balance between supply and demand of amino acids was in excess for the MH group but close to balance in the WH group. These data suggest potential limiting amino acids for various functions.