55.6 Saturday, Jan. 5 Why are muscle fibers so large? Solving diffusion problems to attain maximal cell size KINSEY, S.T.; University of North Carolina Wilmington email@example.com
Muscle fibers are among the largest cell types, but while diffusion appears to limit maximal fiber size, the selective pressures that control minimal size are unclear. During animal growth, muscle fibers generally increase in diameter and this size increase is associated with a number of structural and metabolic changes to the cells. Many of these changes compensate for the increasing diffusion distances associated with hypertrophic fiber growth. Experimental measurement of metabolic rates, diffusion distances and diffusion coefficients, coupled with mathematical reaction-diffusion models have revealed that many fibers grow to sizes that put them on the brink of extreme diffusion limitation in the adult. This suggests that fibers become as large as possible and structural alterations allow fibers to attain larger sizes than would otherwise be possible. These results are consistent with the ‘optimal fiber size hypothesis’ proposed by Ian Johnston and colleagues to explain the very large fibers in cold water fishes. This hypothesis posits that the reduced surface area to volume (SA:V) in larger fibers is favored because it reduces the cost of maintaining the membrane potential. To test this hypothesis, the fiber size dependence of Na+-K+-ATPase cost and activity were measured in white muscle that grows hypertrophically from juveniles and adults of 16 species of crustaceans and fishes that vary dramatically in body mass and fiber size. Changes in Na+-K+-ATPase cost and activity during hypertrophic growth were proportional to changes in SA:V, providing evidence that large fiber size is under positive selection. Ironically, since SA:V is more sensitive to fiber size in smaller fibers, this rule of fiber design may be more relevant to smaller fibers than to the very large fibers for which it was originally proposed.