Effect of alterations in the thyroid state on the intrinsic contractile properties of isolated rat skeletal muscle

Contractile properties of soleus muscles isolated from 31 euthyroid (EU), 20 hyperthyroid (HT), and 18 myxedematous (MY) rats were studied in a myograph. At 100 stimuli/sec maximum isometric tension was essentially identical in EU (17.2 ±0.5 g/mm²) and HT (17.7 ±0.5 g/mm²) muscles, but was significantly depressed in MY muscles (11.5 ±0.7 g/mm²). The rate of tension development was increased in HT (103 ±4.5 g/sec per mm²) as compared to both EU (86.2 ±4.6 g/sec per mm²) and MY (38.4 ±2.2 g/sec per mm²) muscles, while the duration of the active state was shortened in HT (77.1 ±2.3 msec) as compared to EU (105.1 ±1.1 msec) muscles and was prolonged in MY muscles (153.3 ±6.0 msec). The mean rate of isometric relaxation was 26.5 ±4.9 g/mm² per sec in EU muscles, more rapid in HT muscles (33.1 ±1.3 g/sec per mm²), and slower in MY muscles (16.0 ± g/mm² per sec). The fusion frequency was greater in HT muscles, averaging 68.5 ±3.6 stimuli/sec compared to EU muscles (38.1 ±1.2 stimuli/sec) and to MY muscles (33.3 ±4.0 stimuli/sec). At 40 stimuli/sec tension averaged 16.4 ±0.8 g/mm² in EU muscles while at the same frequency tension was reduced in HT muscle, averaging 14.2 ±0.5 g/mm². All differences were significant (P < 0.01). In conclusion, HT and MY result in profound alterations in the intrinsic contractile properties of skeletal muscle. While tension in HT muscles is maintained in vitro at a stimulus frequency of 100 stimuli/sec, the reduction in duration of active state may lower tension in vivo by preventing complete fusion of contractile events. In MY tension is reduced as a consequence of the lowered intensity of the active state. These changes explain, at least in part, the weakness of muscle activity in both HT and MY.

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