MITOCHONDRIAL OXIDATIVE ENZYME ACTIVITY IN INDIVIDUAL FIBRE TYPES IN HYPO- AND HYPERTHYROID RAT SKELETAL MUSCLES

Abstract

Quantitative cytochemical and biochemical techniques were used in combination to study the response of mitochondrial oxidative enzymes in individual muscle fiber types of hypo- and hyperthyroidism. Hypothyroidism resulted in decreased activity of succinate dehydrogenase (SDH), L-glycerol-3-phosphate dehydrogenase (L-GPDH) and D-3-hydroxybutryate dehydrogenase (D-HBDH) in all fiber types of both slow-twitch soleus and fast-twitch extensor digitorum longus (e.d.l.) muscles. In hyperthyroidism, only L-GPDH activity increased in e.d.l. but more marked increases were seen in soleus muscles, which also showed increased SDH activity. In addition to these alterations in the enzyme activity in individual fiber types, the metabolic profile of the muscle is further modified by the hormone-induced interconversion of slow- to fast-twitch fibers and vice versa.