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First published online March 14, 2008
Journal of Experimental Biology 211, 1041-1049 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.013722

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Persistence of motor unit and muscle fiber types in the presence of inactivity

Roland R. Roy^1,*, David J. Pierotti², Alan Garfinkel³, Hui Zhong¹, Kenneth M. Baldwin⁴ and V. Reggie Edgerton^1,3

¹ Brain Research Institute, University of California, Los Angeles, CA 90024-1761, USA
² Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86001-5640, USA
³ Department of Physiological Science, University of California, Los Angeles, CA 90024-1761, USA
⁴ Department of Physiology and Biophysics, University of California, Irvine, CA 92697, USA

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

Accepted 4 February 2008

The clarity of categorizing skeletal muscle fibers in individual motor units into phenotypes based on quantitative single fiber enzyme activities and as a function of neuromuscular activity level was examined. Neuromuscular activity was eliminated in adult cat hindlimb muscles by spinal cord isolation (SI), i.e. complete spinal cord transection at a low thoracic and a high sacral level with bilateral dorsal rhizotomy between the transection sites. One motor unit was isolated via ventral root teasing procedures from the tibialis anterior (TA) muscle of each hindlimb in control and SI cats, and physiologically tested and glycogen depleted through repetitive stimulation; fibers comprising each motor unit were visualized through glycogen staining. Each motor unit was composed of fibers of the same myosin immunohistochemical type. Myofibrillar adenosine triphosphatase, succinate dehydrogenase and -glycerophosphate dehydrogenase activities were determined for a sample of motor unit and non-motor unit fibers, providing a measure of three enzyme activities often used to characterize fiber phenotype within a single unit. Although normal enzyme activities were altered after 6 months of inactivity, the relationships among the three enzymes were largely maintained. These data demonstrate that it is not the diversity in any single enzyme property but the profile of several metabolic pathways that underlies the significance of fiber phenotypes. These profiles must reflect a high level of coordination of expression of selected combinations of genes. Although neuromuscular activity level influences fiber phenotype, the present results demonstrate that activity-independent mechanisms remain important sources of the control of phenotype establishment in the near absence of activity.

Key words: 3-D representations, motor unit types, succinate dehydrogenase, myofibrillar adenosine triphosphatase, -glycerophosphate dehydrogenase

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