Tj. Barstow et al., MUSCLE ENERGETICS AND PULMONARY OXYGEN-UPTAKE KINETICS DURING MODERATE EXERCISE, Journal of applied physiology, 77(4), 1994, pp. 1742-1749
The present study tested whether, during moderate exercise, 1) the dyn
amic responses of ADP and changes in free energy of ATP hydrolysis (De
lta 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 r
eflect the rise in muscle Qo(2) indicated by the calf PCr kinetics. Th
e 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-magnet
ic 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 distor
ted from those of PCr due to dynamic changes in pH. These results are
consistent with mechanisms of respiratory control that feature substra
te 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 const
ants for phase 2 Vo(2) and muscle PCr suggests that phase 2 Vo(2) kine
tics reflect those of muscle Qo(2) in healthy subjects during moderate
exercise.