O2 uptake kinetics, pyruvate dehydrogenase activity, and muscle deoxygenation in young and older adults during the transition to moderate-intensity exercise

The adaptation of pulmonary O-2 uptake (Vo(2P)) kinetics is slowed in older compared with young adults during the transition to moderate-intensity exercise. In this study, we examined the relationship between Vo(2P) kinetics and mitochondrial pyruvate dehydrogenase (PDH) activity in young (n = 7) and older (n = 6) adults. Subjects performed cycle exercise to a work rate corresponding to similar to 90% of estimated lactate threshold. Phase 2 Vo(2P) kinetics were slower (P < 0.05) in older (tau = 40 +/- 17 s) compared with young (tau = 21 +/- 6 s) adults. Relative phosphocreatine (PCr) breakdown was greater (P < 0.05) at 30 s in older compared with young adults. Absolute PCr breakdown at 6 min was greater (P < 0.05) in older compared with young adults. In young adults, PDH activity increased (P < 0.05) from baseline to 30 s, with no further change observed at 6 min. In older adults, PDH activity during baseline exercise was similar to that seen in young adults. During the exercise transition, PDH activity did not increase (P < 0.05) at 30 s of exercise but was elevated (P < 0.05) after 6 min. The change in deoxyhemoglobin (HHb) was greater for a given Vo(2P) in older adults, and there was a similar time course of HHb accompanying the slower Vo(2P) kinetics in the older adults, suggesting a slower adaptation of bulk O-2 delivery in older adults. In conclusion, the slower adaptation of Vo(2P) in older adults is likely a result of both an increased metabolic inertia and lower O-2 availability.