THE EFFECTS OF HYPOTHYROIDISM AND HYPERTHYROIDISM ON FIBER COMPOSITION AND MITOCHONDRIAL ENZYME-ACTIVITIES IN RAT SKELETAL-MUSCLE

Hypo- and hyperthyroidism have been associated with changes in the activities of mitochondrial enzymes in homogenates of skeletal muscles, but it is unclear whether such changes were due to changes in single fibre enzyme activities or to previously documented changes in relative numbers of fibres. In this study the activities of the mitochondrial enzymes-alpha-glycerol phosphate dehydrogenase (m-alpha-GPDH) and succinate dehydrogenase (SDH) were measured in single fibres of the soleus and gastrocnemius muscles of the rat by cytochemical assays. In the soleus muscles of hypothyroid animals there was a decrease in the mean percentage (+/- S.D.) Of type II fibres from 8.0+/-6.0 to 0.8+/-1.9% (P<0.05) and decreases in SDH activities in all fibre types (P<0.005). In the gastrocnemius muscles of these animals there were no changes in fibre composition but type IIB fibres had reduced (P<0.05) m-alpha-GPDH activities. In the hyperthyroid animals, in which body weight had increased relative to the euthyroid animals, there were increases in the percentages of type IC and type II fibres in the soleus from 4.3+/-1.7 to 13.1+/-9.0% (P<0.05) and from 9.6+/-7.2 to 33.4+/-9.6% (P<0.005) respectively and an increase in the percentage of type IIA fibres in the gastrocnemius from 92.9+/-2.3 to 97.0+/-2.9% (P<0.05). However, there were no increases in single fibre mitochondrial enzyme activities. It is therefore suggested that the administration of moderate, growth-promoting doses of thyroid hormones to euthyroid animals can cause changes in muscle fibre composition without stimulating the activities of mitochondrial enzymes.