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First published online March 12, 2009
Journal of Experimental Biology 212, 977-985 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.026625

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Changes in efficiency and myosin expression in the small-muscle phenotype of mice selectively bred for high voluntary running activity

David G. McGillivray¹, Theodore Garland, Jr², Elizabeth M. Dlugosz², Mark A. Chappell² and Douglas A. Syme^1,*

¹ Department of Biological Sciences, University of Calgary, Calgary, Alberta T2N 1N4, Canada
² Department of Biology, University of California, Riverside, CA 92521, USA

^* Author for correspondence (e-mail: syme{at}ucalgary.ca)

Accepted 17 January 2009

Mice from lines selectively bred for high levels of voluntary wheel running express a high incidence of a small muscle phenotype (`mini-muscles') that may confer an adaptive advantage with respect to endurance-running capacity. Plantar flexors in the mini-muscle phenotype exhibit a high capacity for aerobic activity, including altered enzyme activities, loss of expression of type II<sub>b</sub> myosin heavy chain (MHC), increased expression of type I, II<sub>x</sub> and II<sub>a</sub> MHC, and mechanical performance consistent with slower, more fatigue-resistant muscles. We hypothesized that these changes may accompany enhanced efficiency of contraction, perhaps in support of the enhanced capacity for endurance running. To assess efficiency, we measured work and associated oxygen consumption from isolated soleus and medial gastrocnemius muscles from mice with mini-muscle and normal phenotypes. We also measured the MHC expression of the plantar flexor muscles to better understand the physiological basis of any differences in efficiency. The proportion of the various MHC isoforms in the soleus was shifted toward a slightly faster phenotype in the mini-muscle mice, whereas in the gastrocnemius and plantaris it was shifted toward a markedly slower phenotype, with large reductions in type II<sub>b</sub> MHC and large increases in type I, II<sub>a</sub>, and II<sub>x</sub> MHC. Soleus muscles from normal and mini-muscle mice showed no statistical differences in efficiency, but medial gastrocnemius from mini-muscle mice were significantly less efficient than those from normal mice, despite the distinctly slower MHC phenotype in mini-muscle mice. Thus, based on measures of efficiency from isolated muscles under conditions near optimal for power output, the shift toward a slower phenotype in `mini' gastrocnemius muscles does not appear to confer advantages directly through increased efficiency. Rather, the slower phenotype may reduce energy used by the muscles and be permissive to enhanced running ability, perhaps by reducing reliance on anaerobic metabolism.

Key words: artificial selection, efficiency, energetics, experimental evolution, multiple solutions, muscle, myosin heavy chain, oxygen, work

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This article has been cited by other articles:

				E. M. Dlugosz, M. A. Chappell, D. G. McGillivray, D. A. Syme, and T. Garland Jr Locomotor trade-offs in mice selectively bred for high voluntary wheel running J. Exp. Biol., August 15, 2009; 212(16): 2612 - 2618. [Abstract] [Full Text] [PDF]