The effects of hypo- and hyperthyroidism on fibre 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 less than 0.05) and decreases in SDH activities in all fibre types (P less than 0.005). In the gastrocnemius muscles of these animals there were no changes in fibre composition but type IIB fibres had reduced (P less than 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 less than 0.05) and from 9.6 +/- 7.2 to 33.4 +/- 9.6% (P less than 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 less than 0.05). However, there were no increases in single fibre mitochondrial enzyme activities.(ABSTRACT TRUNCATED AT 250 WORDS)