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Departments of 1Medicine, 3Kinesiology and Biological Sciences, University of Southern California, Los Angeles, California; and Departments of 2Internal Medicine and 4Physical Therapy, University of Texas Medical Branch, Galveston, Texas
Submitted 10 April 2006 ; accepted in final form 2 August 2006
Sexual dimorphism in skeletal muscle mass is apparent, with men having more muscle mass and larger individual muscle cells. However, no sex-based differences have been detected in blood forearm phenylalanine turnover, although whole body leucine oxidation has been reported to be greater in men than in women. We hypothesized that sex differences in intracellular amino acid turnover may account for these discrepancies, with men having a higher intracellular turnover than women. We studied young, healthy women (women, n = 8) and men (men, n = 10) following an overnight fast. Phenylalanine, leucine, and alanine muscle intracellular kinetics were assessed using stable isotope methodologies, femoral arteriovenous blood sampling, and muscle biopsies. Muscle intracellular amino acid kinetics were reported relative to both leg volume and lean leg mass because of sex differences in leg volume and in muscle and fat distribution. When expressed per leg volume (nmol·min–1·100 ml leg volume–1), phenylalanine net balance (women: –16 ± 4, men: –31 ± 5), release from proteolysis in the blood (women: 46 ± 9, men: 75 ± 10) and intracellular availability (women: 149 ± 23, men: 241 ± 35), and alanine production, utilization, and intracellular availability were higher in men (P < 0.05). However, when the kinetic parameters were normalized per unit of lean leg mass, all differences disappeared. Muscle fractional synthetic rate was also not different between women and men. We conclude that there are no sex-based differences in basal muscle intracellular amino acid turnover when the data are normalized by lean mass. It remains to be determined if there are sex differences in intracellular amino acid metabolism following anabolic or catabolic stimuli.
protein metabolism; muscle; sex; stable isotopes; gender
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