Substrate Exchange during Aerobic Exercise and Provision of Amino Acids in Older and Younger Men

Abstract

Increased bulk blood flow or redistribution between nutritive and non-nutritive flow routes may influence skeletal muscle metabolism. Exercise increases blood flow to meet the metabolic demand of contracting skeletal muscle but also affects other known mediators of skeletal muscle metabolism. PURPOSE: We sought to measure leg glucose uptake (GU), lactate release (LR) and phenylalanine utilization in response to endurance-type exercise during provision of exogenous amino acids (AA). METHODS:Healthy younger [Y, 29.8 ± 1.7 (SE) yr; n = 9] and older [O, 67 ± 1.6 (SE) yr; n = 8] men were studied at rest and during an amino acid infusion at rest (AA) and during treadmill walking (40% of VO2peak) (EX+AA). Arteriovenous blood and muscle biopsies were collected to measure substrate metabolism across the leg. Local nutritive exchange was measured via the microdialysis ethanol technique (MET) after microdialysis probes were inserted into the vastus lateralis for collection of interstitial fluid. MET-measured outflow-to-inflow ratio is inversely related to local nutritive exchange. RESULTS:Local nutritive exchange increased from rest (Y = 0.44 ± 0.02; O = 0.55 ± 0.04) to AA (Y = 0.39 ± 0.02; O = 0.43 ± 0.03) and was further increased during EX+AA (Y = 0.15 ± 0.02; O = 0.17 ± 0.03) with no difference between the younger and older men. Glucose uptake and lactate release were similar in the younger and older men at rest (GU: Y = 64±7; O = 63±4 and LR: Y = 34±6; O = 40±9 μmol·min-1·leg-1) and during AA (GU: Y = 90±17; O = 113±32 and LR: Y = 38±12; O = 26±2 μmol·min-1·leg-1). However, during EX+AA glucose uptake was greater (Y = 316±32; O = 478±60 μmol·min-1·leg-1) and lactate release was lower (Y = 527±72; O = 236±47 μmol·min-1·leg-1) in the older than the younger men. Changes in intracellular phenylalanine concentrations were marginally higher in the older group (Y = 32±7%; O = 15±3%, P=0.06). Skeletal muscle phenylalanine utilization for protein synthesis was similar in young and older men and reflected a minority (<20%) of total leg phenylalanine disappearance. CONCLUSIONS: These results demonstrate that moderate aerobic exercise during ongoing provision of exogenous amino acids enhances nutritive flow for substrate exchange between blood and leg tissues, thus allowing for enhanced substrate utilization in younger and older men.