Does lower oxidative capacity influence the relative contributions of ATP-producing pathways during muscular work in aging?

Although the capacity of skeletal muscle to produce ATP via oxidative phosphorylation may decrease in some muscles in older age, the influence of a lower capacity on relative use of oxidative and non-oxidative ATP production pathways in vivo during contractions is unclear. To test the hypothesis that lower oxidative capacity would yield greater non-oxidative ATP production, 19 young (10F) and 17 older (9F) adults performed knee extensor muscle contractions in a 3-tesla magnetic resonance system. Phosphorus metabolites were used to calculate oxidative capacity (rate constant of phosphocreatine recovery; kPCr, s-1) and estimate the maximal rate of oxidative ATP production (Vmax, mM·s-1) following a 24-s dynamic contraction protocol. Next, ATP production (mM·s-1) by the creatine kinase reaction (ATPCK), glycolysis (ATPGLY), and oxidative phosphorylation (ATPOX) was determined during 4 min of dynamic muscle contractions. Proton spectroscopy of deoxymyoglobin was also acquired in a subset (n = 12) and used to calculate the cytosolic partial pressure of oxygen (PO2). Young muscle had a greater kPCr (0.023 ± 0.005 s-1, mean ± SD) than older muscle (0.020 ± 0.003 s-1, P = .033). ATPCK, ATPGLY, and ATPOX were not different by group (P ≥ .129), but ATPOX as %Vmax was lower in younger than older muscle (55 ± 14%, 71 ± 10%, respectively, P < .001). Intracellular oxygen availability (PO2) was not different by group (young: 2.4 ± 0.7 Torr, n = 7; older: 3.2 ± 1.6 Torr, n = 5, P = .371). These new findings suggest a bioenergetic rigidity in older muscle, such that it adapts to the energetic demand by using oxidative ATP production at a greater percentage of capacity rather than switching to an increased use of non-oxidative ATP production.