[Role of the thyroid gland in skeletal muscle adaptation to increased motor activity]

Experiments in 16-week old male Wistar strain rats showed that deficiency of thyroid hormones caused a decrease of cytochrome aa3, RNA, myofibrillar and sarcoplasmic protein contents in m. quadriceps femoris, Mg2+ activated actomyosin ATPase activity and physical working capacity of untrained and trained animals also reduced. Physical activity the intensity of which corresponded to metabolic power output of 190%. Vo2max stimulated thyroid function, elicited a significant elevation of blood thyroxine level, oxidative potential and intensity of protein synthesis in m. quadriceps femoris, the character of these changes depending on the type of muscle fibers. The stimulation of thyroid function caused an elevation of muscle content of cytochrome aa3 and content of RNA, especially in the glycolytic fibers of hypothyroid rats.