Role of membrane transport in interorgan amino acid flow between muscle and small intestine.

In the fasting state, amino acids are released from the periphery to be used in splanchnic tissues. To understand the mechanism of such interorgan substrate exchange at the tissue level, we have determined the relationships between inward and outward amino acid transport and intracellular amino acid kinetics in the small intestine and skeletal muscle of postabsorptive anesthetized dogs. In the gut, amino acids appearing intracellularly (from inward transport, protein degradation, and absorption from the lumen) were used for protein synthesis more efficiently (P < .05) than in muscle (phenylalanine, 55% +/- 5% v 13% +/- 3%; lysine, 70% +/- 7% v 28% +/- 3%). In contrast, in muscle, amino acids appearing intracellularly (from inward transport and protein degradation) were preferentially (P < .05) released into the bloodstream, as opposed to being incorporated into protein (phenylalanine, 87% +/- 4%; lysine, 72% +/- 3%). Inward transport accounted for a greater (P < .05) proportion of total intracellular amino acid appearance in the gut than in muscle (leucine, 63% +/- 3% v 37 +/- 3%; valine, 75% +/- 5% v 53% +/- 3%; phenylalanine, 66% +/- 1% v 50% +/- 4%; lysine, 52% +/- 2% v 31% +/- 2%). We conclude that differences in transmembrane amino acid transport kinetics in both the inward and outward directions contribute to the net flow of amino acids from the muscle to the gut in the fasting state.