Persistence of motor unit and muscle fiber types in the presence of inactivity - PubMed (original) (raw)

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

Roland R Roy et al. J Exp Biol. 2008 Apr.

Abstract

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 alpha-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.

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Figures

Fig. 1

The relationship between normalized succinate dehydrogenase (SDH) and normalized myofibrillar adenosine triphosphatase (mATPase) activity for the motor unit fibers in the tibialis anterior (TA) of all control (A) and all spinal cord isolated (SI, B) cats. Note the clusters for each fiber type, particularly for the type I and type IIx fibers.

Fig. 2

The relationship between normalized α-glycerophosphate dehydrogenase (GPD) and normalized mATPase activity for the motor unit fibers in the TA of all control (A) and all SI (B) cats. Again, note the clusters for each fiber type, particularly for the type I and type IIx fibers. Abbreviations as in Fig. 1.

Fig. 3

The relationship between normalized SDH and mATPase (A,C) and normalized mATPase and GPD (B,D) activity for type I motor unit fibers (glycogen depleted) and type I non-motor unit fibers (not glycogen depleted, located within the territory of the motor unit fibers) in a slow TA motor unit from a control (A,B) and a SI (C,D) cat is shown. In addition, data for all type I fibers across all cats are included for comparative purposes. Note that the ranges in the enzyme activities of motor unit fibers generally overlap those of non-motor unit fibers in both the control and SI cats. Abbreviations as in Figs 1 and 2.

Fig. 4

The relationship between normalized SDH and mATPase (A,C) and normalized mATPase and GPD (B,D) activity for type IIa motor unit fibers (glycogen depleted) and type IIa non-motor unit fibers (not glycogen depleted, located within the territory of the motor unit fibers) in a fast TA motor unit from a control (A,B) and a SI (C,D) cat is shown. In addition, data for all type IIa fibers across all cats are included for comparative purposes. Note that the ranges in the enzyme activities of motor unit fibers generally overlap those of non-motor unit fibers in both the control and SI cats. Abbreviations as in Figs 1 and 2.

Fig. 5

The relationship between normalized SDH and mATPase (A,C) and normalized mATPase and GPD (B,D) activity for type IIx motor unit fibers (glycogen depleted) and type IIx non-motor unit fibers (not glycogen depleted, located within the territory of the motor unit fibers) in a fast TA motor unit from a control (A,B) and a SI (C,D) cat is shown. In addition, data for all type IIx fibers across all cats are included for comparative purposes. Note that the ranges in the enzyme activities of motor unit fibers generally overlap those of non-motor unit fibers in both the control and SI cats. Abbreviations as in Figs 1 and 2.

Fig. 6

A 3-D representation of the relationship among SDH, GPD and mATPase activities in the fibers of the motor units from all control (A) and all SI (B) cats. Note the presence of three clusters of fiber types for both the control and the SI cats. Abbreviations as in Figs 1 and 2.

Fig. 7

Location of centroids of control (C) and SI (E) fibers in each of the three fiber types (type I, 1; type IIa, 2; and type IIx, 3). Metric re-scaling (see text in Results section) produced a new coordinate system (dimension 1 and dimension 2) in which the six centroids had maximum separation.

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Persistence of motor unit and muscle fiber types in the presence of inactivity - PubMed (original) (raw)