In Vivo Skeletal Muscle Oxidative and Glycolytic ATP Synthesis in Young and Older Adults

Abstract

We have recently demonstrated that older adults rely more on oxidative production of adenosine triphosphate (ATP) during maximal intensity muscle contractions in comparison to young adults, who rely more on glycolytic ATP production. It is unclear whether glycolytic ATP synthesis is impaired with old age or whether older adults are simply able to adequately meet the energetic demands of muscular work via oxidative phosphorylation. PURPOSE: To compare the sources of ATP supply in young vs. older adults during free-flow and ischemic muscle contractions, in order to assess whether older adults can effectively increase glycolytic ATP production under conditions in which oxidative ATP synthesis is restricted. METHODS: 10 young (27±4 yr, mean±SD; 5M, 5F) and 5 older (75±4 yr; 3M, 2F) healthy adults performed isometric ankle dorsiflexion under free-flow (FF) and ischemic (ISC) conditions while intracellular phosphorous metabolites were measured using magnetic resonance spectroscopy. Protocol: six 12s maximal voluntary isometric contractions (MVC) with 12s rest periods between contractions. The protocol was repeated on the contralateral leg with ischemia maintained continuously via thigh cuff inflated to 220 torr. The rates of ATP synthesis from net phosphocreatine hydrolysis, oxidative phosphorylation, and anaerobic glycolysis were calculated from changes in phosphorus metabolites and pH. RESULTS: During FF, there were trends for higher rates of oxidative phosphorylation (P = 0.11) and lower glycolytic flux (P= 0.10) in older compared to young. During ISC, oxidative phosphorylation was suppressed to a similar level in young and older subjects (P=0.27) and glycolytic rates increased in both age groups. The trend for lower glycolytic flux in older during FF was eliminated in ISC (P=0.91). CONCLUSIONS: These data suggest that the apparent reliance of older adults on oxidative phosphorylation, which can be observed during intermittent MVCs in which blood flow is not externally restricted, does not appear to be due to impairment in glycolytic ATP production.