Prior heavy exercise elevates pyruvate dehydrogenase activity and speeds O2 uptake kinetics during subsequent moderate‐intensity exercise in healthy young adults

The adaptation of pulmonary oxygen uptake during the transition to moderate‐intensity exercise (Mod) is faster following a prior bout of heavy‐intensity exercise. In the present study we examined the activation of pyruvate dehydrogenase (PDHa) during Mod both with and without prior heavy‐intensity exercise. Subjects (n= 9) performed a Mod1–heavy‐intensity–Mod2 exercise protocol preceded by 20 W baseline. Breath‐by‐breath kinetics and near‐infrared spectroscopy‐derived muscle oxygenation were measured continuously, and muscle biopsy samples were taken at specific times during the transition to Mod. In Mod1, PDHa increased from baseline (1.08 ± 0.2 mmol min−1 (kg wet wt)−1) to 30 s (2.05 ± 0.2 mmol min−1 (kg wet wt)−1), with no additional change at 6 min exercise (2.07 ± 0.3 mmol min−1 (kg wet wt)−1). In Mod2, PDHa was already elevated at baseline (1.88 ± 0.3 mmol min−1 (kg wet wt)−1) and was greater than in Mod1, and did not change at 30 s (1.96 ± 0.2 mmol min−1 (kg wet wt)−1) but increased at 6 min exercise (2.70 ± 0.3 mmol min−1 (kg wet wt)−1). The time constant of was lower in Mod2 (19 ± 2 s) than Mod1 (24 ± 3 s). Phosphocreatine (PCr) breakdown from baseline to 30 s was greater (P < 0.05) in Mod1 (13.6 ± 6.7 mmol (kg dry wt)−1) than Mod2 (6.5 ± 6.2 mmol (kg dry wt)−1) but total PCr breakdown was similar between conditions (Mod1, 14.8 ± 7.4 mmol (kg dry wt)−1; Mod2, 20.1 ± 8.0 mmol (kg dry wt)−1). Both oxyhaemoglobin and total haemoglobin were elevated prior to and throughout Mod2 compared with Mod1. In conclusion, the greater PDHa at baseline prior to Mod2 compared with Mod1 may have contributed in part to the faster kinetics in Mod2. That oxyhaemoglobin and total haemoglobin were elevated prior to Mod2 suggests that greater muscle perfusion may also have contributed to the observed faster kinetics. These findings are consistent with metabolic inertia, via delayed activation of PDH, in part limiting the adaptation of pulmonary and muscle O2 consumption during the normal transition to exercise.