The administration of glucocorticoids has been shown to induce atrophy and weakness in whole skeletal muscle. These effects frequently appear to be more pronounced in pale muscle than in red muscle. The present study was undertaken to examine the effects of steroid treatment on the contractile, electrical, and fatigue properties of single meter units in pale and red cat hindlimb muscles. Single motor units in the medial gastrocnemius (MG) and soleus muscles were isolated using techniques developed by McPhedran, et al., (1965). The properties of a large number of motor units were examined in normal animals and were compared to those of single units from the muscles of steroid-treated (3-4 mg. of triamcinolone acetonide per kg. per day for 10 -16 days) animals. Motor units examined in both control and experimental animals were classified by type according to criteria developed by Burke, et al., (1973,1974). The results show that steroids produce alterations in the strength- and speed-related properties of motor units which are more pronounced in fast-twitch than in slow-twitch units. The mean maximum tetanic tension (Pmax) for types FF (fast-twitch, readily fatiguable) and FR (fast-twitch, fatigue-resistant) units in MG were reduced by 63% and 71% respectively; The Pmax for type S units in MG was reduced by 25% as a result of steroid treatment but was unchanged for type S units in soleus. The mean maximum rate of rise of tetanic tension (dP/dt) for the three classes of motor units in MG was reduced in a pattern similar to that for tetanic tension. The dP/dt for types FF, FR, and S units in MG were reduced by 79%, 54%, and 25% respectively. The dP/dt for soleus units was increased by 73% as compared to control soleus units. Steroid treatment lengthened the mean twitch contraction time (CF) of FF units and shortened the CT for type S units in both MG and soleus. The integrated EMG signals elicited from FF and FR units were unchanged following steroid treatment but were significantly increased for type S units in both pale and red muscles. Steroid administration did not markedly alter the susceptibility to fatigue of the three classes of units in MG or in soleus units. There were however fewer MG units with intermediate fatigue indeces in steroid-treated animals. These finding suggest that those units most frequently activated in muscular contractions are the least susceptible to steroid-induced changes in their contractile properties and vice versa. Since the overall frequency of activation of motor ufiits is determined by cell size, these results imply that the Size Principle (Hennemann, et al., 1965) can be extended to not only explain the recruitment order of motor units in muscular contractions but also to account for the patterns of motor unit involvement in steroid-induced myopathy.
| Identifer | oai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-6029 |
| Date | 01 January 1981 |
| Creators | Robinson, Andrew James |
| Publisher | VCU Scholars Compass |
| Source Sets | Virginia Commonwealth University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | © The Author |
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