Thyroidectomy significantly alters carbonic anhydrase III concentration and fiber distribution in rat muscle

Thyroidectomy has a dramatic effect on rat muscle, greatly increasing the number of Type I fibers and the concentrations of carbonic anhydrase III (CAIII) in the muscle. Carbonic anhydrase III is not confined to the Type I fibers, as was previously believed, but also occurs in fibers that exhibit a level of ATPase staining less than that of 2A fibers but greater than 2B. These fibers are rare in normal muscle but become numerous after thyroidectomy, when they stain heavily for CAIII.