ABSTRACT
We tested the hypothesis that a force reduction in soleus muscles from hyperthyroid rats would be associated with oxidative modification of myofibrillar proteins. Daily injection of thyroid hormone [3, 5, 3’-triiodo-<sub>L</sub>-thyronine (T<sub>3</sub>)] for 21 days depressed isometric forces in whole soleus muscle across a range of stimulus frequencies (1, 10, 20, 40, 75 and 100 Hz) (<i>P</i><0.05). In fiber bundles, hyperthyroidism also led to pronounced reductions (<i>P</i><0.05) in both K<sup>+</sup>- and 4-chloro-<i>m</i>-cresol-induced contracture forces. The degrees of the reductions were similar between these two contractures. These reductions in force production were accompanied by a remarkable increment (103% ; <i>P</i><0.05) in carbonyl groups comprised in myofibrillar proteins. In additional experiments, we have also tested the efficacy of carvedilol, a non-selective β<sub>1</sub>-β<sub>2</sub>-blocker that possesses anti-oxidative properties. Treatment with carvedilol prevented T<sub>3</sub>-induced oxidation of myofibrillar proteins. However, carvedilol did not improve the hyperthyroid-induced reductions in force production. These data suggest that oxidative modification of myofibrillar proteins may not account for the reductions in force production of hyperthyroid rat soleus muscle.