Skeletal muscle fibre diversity and the underlying mechanisms

Abstract

The review first briefly summarizes how myosin isoforms have been identified as the major determinant of the functional variability among skeletal muscle fibres. The latter feature is a major characteristic of muscle fibres and a major basis of skeletal muscle heterogeneity and plasticity in vivo. Then, evidence is reported, which indicates that the properties of muscle fibres can vary with no change in the myosin isoform they express. Moreover, the physiological and pathological conditions (ageing, disuse, exercise training, muscular dystrophy) in which such myosin isoform independent change in functional properties occurs and the possible underlying mechanisms are considered. Finally, the known molecular bases of the functional differences among slow and fast isoforms are briefly dealt with.

Daily torpor reduces mass and changes stress and power output of soleus and EDL muscles in the Djungarian hamster, Phodopus sungorus

“…Torpor did not change the shape of the work loop power-output-cycle-frequency curves, supporting the findings, from isometric studies, that there was no change in the intrinsic contractile rate of the muscles analysed. The work loop results from the present study are broadly similar (showing no or very limited change in power output) to those found in response to hibernation and aestivation in frogs (West et al, 2006;Symonds et al, 2007), but contrast with the finding that clinical and experimental models of muscle disuse in nonhibernating mammals cause a shift towards faster muscle fibre type (Musacchia et al, 1988;Canepari et al, 2010;James, 2010). Recent studies assessing the relative expression of myosin isoforms in hibernating mammals have demonstrated either a fast to slow muscle fibre type transition or no change in fibre type.…”

“…However, there were no changes in myosin heavy chain expression in the soleus or diaphragm muscle in ground squirrels (Rourke et al, 2004), pectoral muscle in bats (Lee et al, 2008), gastrocnemius in black bears (Rourke et al, 2006) or biceps femoris in captive brown bears (Hershey et al, 2008). Where significant changes in myosin heavy chain expression occur, we would expect concomitant alterations in the contractile rate of skeletal muscle (Harridge, 2007;Canepari et al, 2010). As there were no changes in contractile rate in the muscles in the present study, it seems unlikely that myosin heavy chain expression has changed.…”

“…an increased concentration of myosin. In humans and mice, vastus lateralis and soleus muscle disuse is associated with reductions in absolute muscle force and muscle stress (force per unit area), with the latter caused by a reduction in myosin concentration (Canepari et al, 2010). Previous studies on hibernation have found a significant increase in protein concentration in ground squirrel plantaris muscle (Rourke et al, 2004), significant decreases in skeletal muscle protein concentration in black-tailed prairie dogs (Cotton and Harlow, 2010) and captive brown bears (Hershey et al, 2008), but no significant changes in skeletal muscle protein concentration in other muscles of golden-mantled ground squirrels (Musacchia et al, 1989;Rourke et al, 2004), white-tailed prairie dogs (Cotton and Harlow, 2010) or wild black bears (Lohuis et al, 2007a).…”

“…Skeletal muscle is a highly adaptable tissue that responds rapidly to changes in usage and environment (Harridge, 2007;Canepari et al, 2010). Prolonged periods of muscle disuse in non-hibernating mammals are known to result in large and rapid reductions in skeletal muscle size and mechanical performance (Musacchia et al, 1988;Clark, 2009).…”

Lion (Panthera leo) and caracal (Caracal caracal) type IIx single muscle fibre force and power exceed that of trained humans

“…A quantitave factor associated with effects on primarily specific force production is the myosin concentration within a fibre. A number of studies (reviewed by Canepari et al, 2010) have shown that the concentration of myosin within a fibre is also a determining factor of force production. Given the differences in the MHC IIx isoform protein sizes and the small diameters of the felid fibres compared with those of humans, there is no doubt that these factors do indeed influence the overall power production of the fibre, but would require future studies.…”