MUSCLE ENERGETICS AND PULMONARY OXYGEN-UPTAKE KINETICS DURING MODERATE EXERCISE

The present study tested whether, during moderate exercise, 1) the dynamic responses of ADP and changes in free energy of ATP hydrolysis (Delta G(ATP)) were similar to those of phosphocreatine [PCr; as would be expected for a simple controller of muscle respiration (Qo(2))] and 2) the rise in pulmonary O-2 uptake (VO2) during cycle exercise would reflect the rise in muscle Qo(2) indicated by the calf PCr kinetics. The responses of PCr, P-i, ADP, and Delta G(ATP) were measured from the calf in five subjects during supine treadle exercise using P-31-magnetic resonance spectroscopy and compared with those for Vo(2), measured breath by breath during upright cycle exercise. The time constants for Delta G(ATP) [24.2 +/- 14.2 (SE) s] were not significantly different from those for PCr (26.3 +/- 17.3 s) and P-i, (30.7 +/- 22.5 s) (P > 0.05). The time constants for phase 2 Vo(2) (29.9 +/- 16.8 s) were also similar to those of PCr. In contrast, the dynamics of ADP were distorted from those of PCr due to dynamic changes in pH. These results are consistent with mechanisms of respiratory control that feature substrate control by PCr or thermodynamic control through changes in Delta G(ATP). However, these results are not consistent with substrate control by ADP in a simple fashion. Furthermore, the similarity of time constants for phase 2 Vo(2) and muscle PCr suggests that phase 2 Vo(2) kinetics reflect those of muscle Qo(2) in healthy subjects during moderate exercise.