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First published online August 4, 2005
Journal of Experimental Biology 208, 3159-3167 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01703

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Cost of transport is increased after cold exposure in Monodelphis domestica: training for inefficiency

Paul J. Schaeffer^1,2,*, Jason J. Villarin^1,, David J. Pierotti¹, Daniel P. Kelly^2,3 and Stan L. Lindstedt¹

¹ Department of Biological Sciences, Physiology and Functional Morphology Group, Northern Arizona University, Flagstaff, AZ 86011, USA
² Center for Cardiovascular Research and Department of Medicine, Washington University School of Medicine, St Louis, MO 63110, USA
³ Department of Molecular Biology and Pharmacology, and Department of Pediatrics, Washington University School of Medicine, St Louis, MO 63110, USA

^* Author for correspondence (e-mail: pschaeff{at}im.wustl.edu)

Accepted 17 May 2005

Monodelphis domestica (Didelphidae: Marsupialia) lacks brown adipose tissue and thus relies on skeletal muscle as its primary thermogenic organ. Following cold exposure, the aerobic capacity of skeletal muscle in these animals is greatly increased. We investigated the effects of this plastic response to thermogenesis on locomotion and muscle mechanics. In cold-exposed animals, cost of transport was 15% higher than in controls but was unaffected by exercise training. Twitch kinetics in isolated semitendinosus muscles of cold-exposed animals were characteristic of slow-oxidative fiber types. Both time-to-peak tension and half-relaxation time were longer and maximal shortening velocity was slower following cold exposure compared to either thermoneutral controls or exercise-trained animals. Further, muscles from the cold-exposed animals had greater fatigue resistance than either control or exercise-trained animals, indicating greater oxidative capacity. Finally, we identified an uncoupling protein 3 homologue, whose gene expression was upregulated in skeletal muscle of cold-exposed Monodelphis domestica. Cold exposure provided a potent stimulus for muscle plasticity, driving a fast-to-slow transition more effectively than exercise training. However, linked to the dramatic shift in muscle properties is an equally dramatic increase in whole animal muscle energetics during locomotion, suggesting an uncoupled state, or `training for inefficiency'.

Key words: oxygen consumption, locomotion, thermogenesis, uncoupling protein 3, muscle mechanics, marsupial, Monodelphis domestica

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