The present study tested whether, during moderate exercise, 1) the dynamic responses of ADP and changes in free energy of ATP hydrolysis (delta GATP) were similar to those of phosphocreatine [PCr; as would be expected for a simple controller of muscle respiration (QO2)] and 2) the rise in pulmonary O2 uptake (VO2) during cycle exercise would reflect the rise in muscle QO2 indicated by the calf PCr kinetics. The responses of PCr, Pi, ADP, and delta GATP were measured from the calf in five subjects during supine treadle exercise using 31P-magnetic resonance spectroscopy and compared with those for VO2, measured breath by breath during upright cycle exercise. The time constants for delta GATP [24.2 +/- 14.2 (SE) s] were not significantly different from those for PCr (26.3 +/- 17.3 s) and Pi (30.7 +/- 22.5 s) (P > 0.05). The time constants for phase 2 VO2 (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 GATP. However, these results are not consistent with substrate control by ADP in a simple fashion. Furthermore, the similarity of time constants for phase 2 VO2 and muscle PCr suggests that phase 2 VO2 kinetics reflect those of muscle QO2 in healthy subjects during moderate exercise.

Gas exchange response to high-intensity exercise differs in children and adults. We hypothesized that these findings are related to a lower anaerobic ATP supply in children. We predicted therefore a maturation of muscle high-energy phosphate metabolism during exercise. To test this hypothesis, we measured calf muscle Pi, phosphocreatine (PCr), and pH with 31P-nuclear magnetic resonance spectroscopy during rest and progressive exercise in 10 children and 8 adults. No differences were found at rest in pH and Pi/PCr between children and adults. Exercise resulted in a greater increase in Pi/PCr (P < 0.001) and decrease in pH (P < 0.0001) in adults than in children. Six adults and five children exhibited a transition from a slow to a faster rate of Pi/PCr increase and pH decrease during exercise. No significant differences were found between the two groups in the initial slow-phase slopes of Pi/PCr and pH as a function of work rate. In contrast, during the fast phase, Pi/PCr increased (slope: adults 23.6 +/- 9.8, children 10.7 +/- 2.5; P < 0.05) and pH decreased (slope: adults -6.0 +/- 1.9, children -3.7 +/- 1.2; P < 0.05) more rapidly in adults than in children. In conclusion, high-intensity exercise results in different kinetics of Pi/PCr and pH between children and adults. These results suggest that children are less able than adults to affect ATP rephosphorylation by anaerobic metabolic pathways during high-intensity exercise.

The purpose of this study was to evaluate the consequences of non-linear changes in phosphocreatine (PCr) and pH during incremental calf exercise on estimates of ADP and cytosolic free energy of ATP hydrolysis (delta GATP). Six subjects performed incremental plantar flexion exercise on a treadle ergometer while muscle P(i) metabolism (PCr, P(i), ATP) and pH were followed using 31P-nuclear magnetic resonance spectroscopy. Changes in ADP and delta GATP were estimated with the assumption that there was equilibrium of the creatine kinase reaction and homogeneous tissue metabolite pools. All six subjects showed a threshold for onset of cellular acidosis that occurred on average at 47.3 +/- 12.7% of peak work rate (PWR). In five of the six subjects, PCr and P(i) showed accelerated rates of change above the threshold for onset of cellular acidosis. In all six subjects, ADP, when correctly calculated considering changes in pH, rose in a curvilinear fashion that was well described by a Michaelis-Menten hyperbola through 60-100% of PWR, with a mean apparent Michaelis-Menten constant of 43.1 +/- 17.1 microM ADP and a predicted maximal oxidative rate at PCr = 0, which was 241 +/- 94% of PWR. delta GATP rose linearly with work rate from -62.9 +/- 1.8 kJ/mol during unloaded treadling to -55.0 +/- 1.8 kJ/mol at PWR. If we assume a linear O2 uptake-to-work rate relationship, these results are most consistent with control of respiration being exerted through delta GATP under these conditions (incremental exercise by human calf muscle).(ABSTRACT TRUNCATED AT 250 WORDS)