Protein turnover in different types of skeletal muscle during experimental hyperthyroidism in rats

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Abstract
Experimental hyperthyroidism was induced in rats by daily i.p. injection of triiodothyronine (T3; 100 .mu.g/100 g body wt) during 3 or 10 days. Protein synthesis and degradation were measured by incubated soleus and extensor digitorum longus (EDL) muscles by determining rate of tyrosine incorporation into protein and release of tyrosine to the incubation medium, respectively. Protein synthesis was unaffected by T3 administration during 3 or 10 days. Protein breakdown was significantly increased in soleus but unchanged in EDL in the 3-days experiment. Following administration of T3 for 10 days proteolysis was increased in both muscles. Weight of the soleus muscle was reduced after T3 for 3 days. After 10 days weight and protein content were reduced in both muscles. Apparently, reduced muscle protein content followed administration of T3 was the result of increased proteolysis, not decreased protein synthesis. Evidently, slow muscle (soleus) is more sensitive to the effects of thyroid hormone than fast muscle (EDL).