Mechanisms of postprandial protein accretion in human skeletal muscle. Insight from leucine and phenylalanine forearm kinetics.

Abstract

The relative role of protein synthesis and degradation in determining postprandial net protein deposition in human muscle is not known. To this aim, we studied forearm leucine and phenylalanine turnover by combining the arteriovenous catheterization with tracer infusions, before and following a 4 h administration of a mixed meal in normal volunteers. Forearm amino acid kinetics were assessed in both whole blood and plasma. Fasting forearm protein degradation exceeded synthesis ( P Ͻ 0.01) using either tracer, indicating net muscle protein loss. The net negative forearm protein balance was quantitatively similar in whole blood and in plasma. After the meal, forearm proteolysis was suppressed ( P Ͻ 0.05-Ͻ 0.03), while forearm protein synthesis was stimulated ( P Ͻ 0.05-Ͻ 0.01). However, stimulation of protein synthesis was greater ( P Ͻ 0.05-Ͻ 0.01) in whole blood (leucine data: ϩ 50.4 Ϯ 7.8 nmol/min ϫ 100 ml of forearm; phenylalanine data: ϩ 30.4 Ϯ 11.6) than in plasma (leucine data: ϩ 17.8 Ϯ 5.6 nmol/min ϫ 100 ml of forearm; phenylalanine data: ϩ 5.7 Ϯ 2.1). Consequently, the increment of net amino acid balance was approximately two to fourfold greater ( P Ͻ 0.04-Ͻ 0.03) in whole blood than in plasma.

Method for the determination of the arteriovenous muscle protein balance during non-steady-state blood and muscle amino acid concentrations

“…We conclude that the protein NB in non-steady-state blood phenylalanine concentrations can be determined from the arteriovenous phenylalanine NB by accounting for changes in free phenylalanine within its volume of distribution. methodology; phenylalanine net balance; intracellular phenylalanine; volume of distribution THE ARTERIOVENOUS NET BALANCE (NB) technique has been used extensively in humans to evaluate rates of protein synthesis and breakdown in forearm and leg muscles (1,3,7,9,15,25,29,31,33) as well as other tissues (17) under a variety of physiological conditions. In the above studies, as well as others (18,28,30,32), the amino acid phenylalanine has been used to trace muscle protein balance.…”

Regulation of Muscle Protein Synthesis in Neonatal Pigs During Prolonged Endotoxemia

“…Healthy neonates have relatively high muscle protein synthetic rates that are markedly stimulated by the postprandial rise in both amino acids and insulin (17)(18)(19)(20)(21)(22). This response decreases rapidly with development (17,21,23). This maturational change occurs in part as a consequence of developmental changes in the insulin signaling pathway that modulates translation initiation in muscle, with decreased activation of specific eukaryotic initiation factors (eIF) involved in 43S ribosomal complex assembly as the neonate develops (24,25).…”

Postprandial body protein synthesis and amino acid catabolism measured with leucine and phenylalanine-tyrosine tracers

“…A PRECISE ESTIMATE of postprandial whole body protein synthesis is essential to determine the anabolic effects of meal ingestion, i.e., the key physiological stimulus of net protein deposition in body tissues (13,14,20,27). By use of amino acid tracer techniques, whole body protein synthesis is commonly calculated from the portion of amino acid flux (e.g., disposal) that is not irreversibly catabolized and therefore must be incorporated into the proteins (3,27,31).…”

“…By use of amino acid tracer techniques, whole body protein synthesis is commonly calculated from the portion of amino acid flux (e.g., disposal) that is not irreversibly catabolized and therefore must be incorporated into the proteins (3,27,31). Protein synthesis is stimulated by a meal of adequate energy and protein content (13,18,27). An index of such an adequacy is the stimulation of postprandial amino acid catabolism (33).…”

“…Leucine and phenylalanine tracers are extensively used for this purpose (3,13,14,20,27,31). The first step of leucine irreversible catabolism is oxidation (3,24,31); that of phenylalanine is hydroxylation to tyrosine (7,21).…”