© 2001 by The Society for Integrative and Comparative Biology
Gas Exchange, MRS and NIRS Assessment of Metabolic Transients in Skeletal Muscle1
1 I.T.B.A.—National Research Council, Via F.lli Cervi 93, I-20090 Segrate (MI), Italy
The study of the kinetics of O2 consumption (
o2) at the onset and offset of constant-load submaximal exercise (o2 on- and off-kinetics) is useful from a practical point of view (a faster adjustment of oxidative metabolism following an increased metabolic demand reduces the need for substrate level phosphorylation, with implications on exercise tolerance and muscle fatigue) and can give valuable insights into the regulation of oxidative metabolism in skeletal muscle. Measurements have been carried out both in man and in animals, at the tissue and at the whole body level. At the tissue level, the o2 on- and off-kinetics were determined: a) Directly, by dynamic solution of the Fick equation throughout the transients; attempts were also made to obtain similar informations by near-infrared spectroscopy. b) Indirectly, from the kinetics of phosphocreatine hydrolysis and resynthesis, by chemical methods or by 31P magnetic resonance spectroscopy. At the whole body level, o2 on- and off-kinetics are determined from breath-by-breath measurements of pulmonary gas exchange. The o2 = f(t) function is a complex one, particularly during the on-transient. The so-called "phase 2" of the o2 on-response, as well as the o2 off-response, yield relevant metabolic informations. In muscle the o2 on- and off-kinetics are characterized by half-times (t
) of 15–20 sec. At the whole-body level, t of the o2 on-kinetics show a wider variability, related to the experimental protocol and to other factors. The o2 off-phase is more constant, and its kinetic parameters appear closer to those obtained at the tissue level. The study of the o2 kinetics is valuable for a functional evaluation of skeletal muscle oxidative metabolism. In ordinary conditions muscle o2 kinetics appears mainly imposed by intrinsic (metabolic) rather than extrinsic (O2 delivery) factors.