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First published online October 5, 2007
Journal of Experimental Biology 210, 3505-3512 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.000331

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Commentary

The long and winding road: influences of intracellular metabolite diffusion on cellular organization and metabolism in skeletal muscle

Stephen T. Kinsey^1,*, Kristin M. Hardy¹ and Bruce R. Locke²

¹ Department of Biology and Marine Biology, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5915, USA
² Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Florida State University, Tallahassee, FL 32310-6046, USA

^* Author for correspondence (e-mail: kinseys{at}uncw.edu)

Accepted 26 July 2007

A fundamental principle of physiology is that cells are small in order to minimize diffusion distances for O₂ and intracellular metabolites. In skeletal muscle, it has long been recognized that aerobic fibers that are used for steady state locomotion tend to be smaller than anaerobic fibers that are used for burst movements. This tendency reflects the interaction between diffusion distances and aerobic ATP turnover rates, since maximal intracellular diffusion distances are ultimately limited by fiber size. The effect of diffusion distance on O₂ flux in muscle has been the subject of quantitative analyses for a century, but the influence of ATP diffusion from mitochondria to cellular ATPases on aerobic metabolism has received much less attention. The application of reaction–diffusion mathematical models to experimental measurements of aerobic metabolic processes has revealed that the extreme diffusion distances between mitochondria found in some muscle fibers do not necessarily limit the rates of aerobic processes per se, as long as the metabolic process is sufficiently slow. However, skeletal muscle fibers from a variety of animals appear to have intracellular diffusion distances and/or fiber sizes that put them on the brink of diffusion limitation. Thus, intracellular metabolite diffusion likely influences the evolution of muscle design and places limits on muscle function.

Key words: muscle fiber, fiber growth, diffusion, metabolic modeling, reaction-diffusion, exercise, metabolism, scaling, crustacean, fish, phosphagen, arginine phosphate, arginine kinase, creatine phosphate, creatine kinase, mitochondria