Muscular abnormalities in experimental hypothyroidism of rats with special reference to the mounding phenomenon.

The mounding phenomenon was examined in vitro on the excised extensor digitorum longus or soleus muscle of hypothyroid rats. The mounding phenomenon was more prominent and lasted longer in hypothyroid muscles than in controls. In single skinned muscle fibers of both type 1 and type 2 from hypothyroid animals, the maximum tension produced by Ca ion was reduced and activity of Ca uptake by the sarcoplasmic reticulum was also decreased, judging from the response to caffeine. Decreased Ca uptake might retard the relaxation of contracture, thereby rendering the mounding more prominent. Morphological studies of both extensor digitorum longus and soleus muscles showed an increase of type 1 muscle fiber population as well as mild changes in arrangement of the internal membrane system. However, there was no muscle necrosis or other derangements. These results supported the idea that functional abnormality of the sarcoplasmic reticulum could be the basis of the exaggeration of the mounding phenomenon in hypothyroidism.