Inferences from pulmonary O-2 uptake with respect to intramuscular [phosphocreatine] kinetics during moderate exercise in humans

1. In the non-steady state of moderate intensity exercise, pulmonary O-2 up take (V-p,V-o2) is temporally dissociated from muscle O-2 consumption (V-m, V-o2) due to the influence of the intervening venous blood volume and the c ontribution of body O, stores to ATP synthesis. A monoexponential model of V-p,V-o2 without a delay term, therefore, implies an obligatory slowing of V-p,V-o2 kinetics in comparison to V-m,V-o2 2. During moderate exercise, an association of V-m,V-o2 and [phosphocreatin e] ([PCr]) kinetics is a necessary consequence of the control of muscular o xidative phosphorylation mediated by some function of [PCr]. It has also be en suggested that the kinetics of V-p,V-o2, will be expressed with a time c onstant within 10 % of that of V-m,V-o2. 3. V-p,V-o2 and intramuscular [PCr] kinetics were investigated simultaneous ly during moderate exercise of a large muscle mass in a whole-body NMR spec trometer. Six healthy males performed prone constant-load quadriceps exerci se. A transmit-receive coil under the right quadriceps allowed determinatio n of intramuscular [PCr]; V-p,V-o2 was measured breath-by-breath, in concer t with [PCr], using a turbine and a mass spectrometer system. 4. Intramuscular [PCr] decreased monoexponentially with no delay in respons e to exercise. The mean of the time constants (tau(PCr)) was 35 s (range, 2 0-64 s) for the six subjects. 5. Two temporal phases were evident in the V-p,V-o2 response. When the enti re V-p,V-o2, response was modelled to be exponential with no delay, its tim e constant (tau'(Vp),(o2)) was longer in all subjects (group mean = 62 s; r ange, 52-92 s) than that of [PCr], reflecting the energy contribution of th e O-2 stores. 6. Restricting the V-p,V-o2 model fit to phase II resulted in matching kine tics fur V-p,V-o2,, (group mean tau(Vp,O2) = 36 s; range, 20-68 s) and [PCr ], for all subjects. 7. We conclude that during moderate intensity exercise the phase II tau(Vp, o2) provides a good estimate of tau(PCr) and by implication that of V-m,V-o 2(tau(Vp,o2)).