O2 uptake kinetics, pyruvate dehydrogenase activity, and muscle deoxygenation in young and older adults during the...

The adaptation of pulmonary O(2) uptake (Vo(2)(p)) 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(2)(p) 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 approximately 90% of estimated lactate threshold. Phase 2 Vo(2)(p) 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(2)(p) in older adults, and there was a similar time course of HHb accompanying the slower Vo(2)(p) kinetics in the older adults, suggesting a slower adaptation of bulk O(2) delivery in older adults. In conclusion, the slower adaptation of Vo(2)(p) in older adults is likely a result of both an increased metabolic inertia and lower O(2) availability.