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1 Department of Surgery, The University of Texas Medical Branch, Galveston, Texas, USA; Shriners Hospitals for Children, Galveston, Texas, USA
2 Department of Surgery, The University of Texas Medical Branch, Galveston, Texas, USA; Department of Anesthesiology, The University of Texas Medical Branch, Galveston, Texas, USA; Shriners Hospitals for Children, Galveston, Texas, USA
* To whom correspondence should be addressed. E-mail: djpaddon{at}utmb.edu.
We recently demonstrated that muscle protein synthesis was stimulated to a similar extent in young and elderly subjects during a 3hr amino acid infusion. We sought to determine if a more practical bolus oral ingestion would also produce a similar response in young (34±4yrs) and elderly (67±2yrs) individuals. Arterio-venous blood samples and muscle biopsies were obtained during a primed (2.0µmol.kg-1) constant infusion (0.05µmol.kg.min-1) of L-[ring-2H5]phenylalanine. Muscle protein kinetics and mixed muscle fractional synthetic rate (FSR) were calculated before and after the bolus ingestion of 15g of essential amino acids (EAA) in young (n=6) and elderly (n=7) subjects. Following EAA ingestion, the rate of increase in femoral artery phenylalanine concentration was slower in elderly subjects but remained elevated for a longer period. EAA ingestion increased FSR in both age groups by approximately 0.04%.hr-1 (p<0.05). However, muscle intracellular (IC) phenylalanine concentration remained significantly higher in elderly subjects at the completion of the study (young: 115.6±5.4 nmol.ml-1; elderly:150.2±19.4 nmol.ml-1). Correction for the free phenylalanine retained in the muscle IC pool resulted in similar net phenylalanine uptake values in the young and elderly respectively. EAA ingestion increased plasma insulin levels in young (6.1±1.2 to 21.3±3.1µIU.ml-1), but not in elderly subjects (3.0±0.6 to 4.3±0.4µIU.ml-1). Despite differences in the time course of plasma phenylalanine kinetics and a greater residual IC phenylalanine concentration, amino acid supplementation acutely stimulated muscle protein synthesis in both young and elderly individuals.
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