Abstract
Precise mechanism responsible for the regulation of morphological, functional, and/or metabolic properties of skeletal muscles remains unclear. Thus, the role(s) of metabolic stimulation, not necessarily relying on the intensive mechanical and neural activity, on the properties of skeletal muscles were reviewed. Stimulation of oxidative metabolism was induced by chronic reduction of the contents of high-energy phosphates (HEPs) in resting skeletal muscles by oral supplementation of creatine-analogue, β-guanidinopropionic acid (β-GPA), exposure to cold and/or hypoxia in rodents and frogs. Increases of mitochondrial enzyme activities and shift of muscle fiber phenotype and contractile properties toward the oxidative slow type were noted following the chronic decrease of resting HEPs. Contractile property and whole-body work capacity at a submaximal intensity were increased following β-GPA feeding and exposure to hypoxia. Beneficial responses of the glucose tolerance were also seen following creatine depletion. Further, effects of oxygen transport and/or utilization capacity on the work tolerance in human were also discussed. The cumulative results suggested that active contractility is not necessarily essential, if the mild passive tension development is maintained keeping the fibers at an optimum length. It is also suggested that metabolic stimulation of muscles, associated with lowered HEPs, plays the beneficial role for regulation of functional properties.
