Function and Position determine relative proportions of different fiber types in limb muscles of the lizard Tropidurus psammonastes

PEREIRA ANIELI; ABDALA, V; KOHLSDORF, T.

ZOOLOGY

ELSEVIER GMBH

Año: 2015 vol. 118 p. 27 - 33

0944-2006

Vertebrate limb skeletal muscles can be classified as flexors or extensors according to their function, and as dorsal or ventral according to their position. The latter classification evokes their embryological origin from muscle masses initially divided during limb development, and muscles sharing a given position do not necessarily perform the same function. Variation in relative proportions of fiber types is more frequently interpreted in relation to the function a muscle exerts during contraction regardless of its position. An emerging question is what determines relative proportions of fiber types in different limb muscles ? if position, function, or both. Here we compare the relative proportions of different fiber types among six limb muscles in the lizard Tropidurus psammonastes. We predict that statistical models incorporating both position and function will have a better fit for explaining variation in fiber types among limb muscles. Individual fibers were classified as slow-oxidative (SO), fast-glycolytic (FG) or fast-oxidative-glycolytic (FOG) based on mitochondrial content; muscles were classified according to position and function. Mixed Linear Models considering one or both effects were compared using likelihood-ratio tests. Variation in proportion of FG and FOG fibers is mainly explained by the function the muscle exerts rather than its position, while variation in SO fibers is better explained by position. Flexor muscles have on average lower proportions of FG and higher proportions of FOG fibers. Position explains variation in SO fibers: these are less abundant in ventral muscles than in those developed from a dorsal muscle mass. Our results clarify the roles of position and function in the specification of relative proportions of fibers differing in mitochondrial content, and also provide evidence that these factors may differentially affect distinct fiber-types.