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