Gas Exchange, MRS and NIRS Assessment of Metabolic Transients in Skeletal Muscle

Abstract

The study of the kinetics of O-2 consumption (Vo(2)) at the onset and offset of constant-load submaximal exercise (Vo(2) 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 Vo(2) on- and off-kinetics were determined: a) Directly, by dynamic solution of the Fick equation throughout the transients; attempts were also trade to obtain similar informations by near-infrared spectroscopy. b) Indirectly, froth the kinetics of phosphocreatine hydrolysis and resynthesis, by chemical methods or by P-31 magnetic resonance spectroscopy. At the whole body level, Vo(2) on- and off- kinetics are determined from breath-by-breath measurements of pulmonary gas exchange. The Vo(2) = f(t) function is a complex one, particularly during the on-transient. The so-called "phase 2" of the Vo(2) on-response, as well as the Vo(2) off-response, yield relevant metabolic informations. In muscle the Vo(2) on- and off-kinetics are characterized by half-times (t1/2) of 15- 20 sec. At the whole-body level, t1/2 of the Vo(2) on-kinetics show a wider variability, related to the experimental protocol and to other factors. The Vo(2) off-phase is more constant, and its kinetic parameters appear closer to those obtained at the tissue level. The study of the Voz kinetics is valuable for a functional evaluation of skeletal muscle oxidative metabolism. In ordinary conditions muscle Vo(2) kinetics appears mainly imposed by intrinsic (metabolic) rather than extrinsic (O-2 delivery) factors