Inferences from pulmonary O2 uptake with respect to intramuscular [phosphocreatine] kinetics during moderate exercise in humans

Abstract

1 In the non-steady state of moderate intensity exercise, pulmonary O₂ uptake (V̇_p,O2) is temporally dissociated from muscle O₂ consumption (V̇_m,O2) due to the influence of the intervening venous blood volume and the contribution of body O₂ stores to ATP synthesis. A monoexponential model of V̇_p,O2 without a delay term, therefore, implies an obligatory slowing of V̇_p,O2 kinetics in comparison to V̇_m,O2. 2 During moderate exercise, an association of V̇_m,O2 and [phosphocreatine] ([PCr]) kinetics is a necessary consequence of the control of muscular oxidative phosphorylation mediated by some function of [PCr]. It has also been suggested that the kinetics of V̇_p,O2 will be expressed with a time constant within 10 % of that of V̇_m,O2. 3 V̇_p,O2 and intramuscular [PCr] kinetics were investigated simultaneously during moderate exercise of a large muscle mass in a whole-body NMR spectrometer. Six healthy males performed prone constant-load quadriceps exercise. A transmit-receive coil under the right quadriceps allowed determination of intramuscular [PCr]; V̇_p,O2 was measured breath-by-breath, in concert with [PCr], using a turbine and a mass spectrometer system. 4 Intramuscular [PCr] decreased monoexponentially with no delay in response to exercise. The mean of the time constants (τ_PCr) was 35 s (range, 20–64 s) for the six subjects. 5 Two temporal phases were evident in the V̇_p,O2 response. When the entire V̇_p,O2 response was modelled to be exponential with no delay, its time constant (τ′_V̇p,O2) was longer in all subjects (group mean = 62 s; range, 52–92 s) than that of [PCr], reflecting the energy contribution of the O₂ stores. 6 Restricting the V̇_p,O2 model fit to phase II resulted in matching kinetics for V̇_p,O2 (group mean τ_V̇p,O2= 36 s; range, 20–68 s) and [PCr], for all subjects. 7 We conclude that during moderate intensity exercise the phase II τ_V̇p,O2 provides a good estimate of τ_PCr and by implication that of V̇_m,O2 (τ_V̇m,O2).