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First published online December 14, 2005
Journal of Experimental Biology 209, 115-127 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01883

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Maximum aerobic performance in lines of Mus selected for high wheel-running activity: effects of selection, oxygen availability and the mini-muscle phenotype

Enrico L. Rezende^*, Theodore Garland, Jr, Mark A. Chappell, Jessica L. Malisch and Fernando R. Gomes

Department of Biology, University of California, Riverside, CA 92521, USA

^* Author for correspondence at present address: Integrative Ecology Group, Estación Biológica de Doñana, CSIC, Apdo. 1056, E-41080 Seville, Spain (e-mail: enrico.rezende{at}ebd.csic.es)

Accepted 10 September 2005

We compared maximum aerobic capacity during forced exercise (_O2max) in hypoxia (P_O2=14% O₂), normoxia (21%) and hyperoxia (30%) of lines of house mice selectively bred for high voluntary wheel running (S lines) with their four unselected control (C) lines. We also tested for pleiotropic effects of the `mighty mini-muscle' allele, a Mendelian recessive that causes a 50% reduction in hind limb muscle but a doubling of mass-specific aerobic enzyme activity, among other pleiotropic effects. _O2max of female mice was measured during forced exercise on a motorized treadmill enclosed in a metabolic chamber that allowed altered P_O2. Individual variation in _O2max was highly repeatable within each P_O2, and values were also significantly correlated across P_O2. Analysis of covariance showed that S mice had higher body-mass-adjusted _O2max than C at all P_O2, ranging from +10.7% in hypoxia to +20.8% in hyperoxia. _O2max of S lines increased practically linearly with P_O2, whereas that of C lines plateaued from normoxia to hyperoxia, and respiratory exchange ratio (=CO₂ production/_O2max) was lower for S lines. These results suggest that the physiological underpinnings of _O2max differ between the S and C lines. Apparently, at least in S lines, peripheral tissues may sustain higher rates of oxidative metabolism if central organs provide more O₂. Although the existence of central limitations in S lines cannot be excluded based solely on the present data, we have previously reported that both S and C lines can attain considerably higher _O2max during cold exposure in a He-O₂ atmosphere, suggesting that limitations on _O2max depend on interactions between the central and peripheral organs involved. In addition, mini-muscle individuals had higher _O2max than did those with normal muscles, suggesting that the former might have higher hypoxia tolerance. This would imply that the mini-muscle phenotype could be a good model to test how exercise performance and hypoxia tolerance could evolve in a correlated fashion, as previous researchers have suggested.

Key words: artificial selection, central limitation, exercise, hypoxia, hyperoxia, maximum metabolic rate, oxygen availability, peripheral limitation, respiratory exchange ratio, symmorphosis

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