Fig. 1.

Response of arterial (A), venous (V), and intramuscular free (M) amino acid pools to an amino acid drink. Initially, the drink causes an expansion in the arterial pool, which leads to both increased shunting into the venous pool and increased transport of amino acids into muscle and other cells, which leads to elevated muscle free amino acid levels. The plasma amino acid pool sizes will eventually return to predrink levels, which will cause an increased concentration gradient favoring outward transportation of amino acids from the muscle cell to the circulation until muscle amino acid pool sizes also return to predrink levels. The inward arrow (toward muscle) represents the “rate of release of amino acids from muscle protein and amino acid synthesis,” and the outward arrow (from the muscle) represents the “rate of incorporation of amino acids into muscle protein and irreversible loss” (26). And since phenylalanine and threonine do not metabolize in the muscle so that there is no amino acid synthesis and irreversible loss, thus these arrows represent incorporation and release of amino acids to and from muscle protein for these 2 particular amino acids. These dynamic changes need to be taken into account when measuring the amino acid and protein turnover using tracer kinetic methods.