Normal mammalian skeletal muscle and its phenotypic plasticity

Since muscle mass makes up such a high proportion of total body mass, there must have been considerable selective pressure to minimize the cost of maintenance and to maximize the functionality of muscle tissue for all species. Phenotypic plasticity of muscle tissue allows the species blueprint of muscle tissue to be modified to accommodate specific demands experienced by animals over their lifetime. In this review, we report the scaling of muscle structural compartments in a set of mammals spanning five orders of magnitude (17 g woodmice to 450 kg horses and steers). Muscle mass, muscle myofibrillar volume and sarcoplasmic space were found to represent similar relative quantities in all species studies (scaling factor close to unity). Mitochondrial volumes were found to be systematically smaller in larger animals (scaling factor 0.91) and closely related to the scaling of (V) over dot o(2max) (0.92) and were tracked by the scaling of total capillary length (0.95). In this set of species, we therefore found that maximal metabolic rate and supporting structures did not scale to the 0.75 power of body mass as generally suggested. Muscle phenotypic plasticity is reasonably well characterized on a structural and functional basis, but we still know little about the signals that cause the changes in gene expression necessary for phenotypic changes in muscle. The molecular responses of human m. vastus lateralis to endurance exercise indicate that a single bout of exercise causes specific transient transcriptional adaptations that may gradually accumulate after their translation into the (structural) modifications seen with phenotypic plasticity. Metabolic and mechanical factors are recognized candidate factors for the control of exercise-induced gene transcription in muscle. Distinct protein kinases and transcription factors emerge as possible interfaces that integrate the mechanical (MAPKs and jun/fos) and metabolic (AMPK, HIF-1alpha and PPARalpha) stimuli into enhanced gene transcription in skeletal muscle.