The influence of the thyroid state on energy turnover during tetanic stimulation in the fast-twitch (mixed type) muscle of rats

The effects of thyroid hormones on energy metabolism and force development during tetanic stimulation of fast-twitch skeletal muscle (mixed type) were studied. Hypothyroid, euthyroid, and hyperthyroid (ten days of daily administration of 15 μg T 3 100 gm body weight ) rats showed the same tension-time integral (force × time) under tetanic stimulation. The ATP turnover rate was significantly lower in the hypothyroid group than in the euthyroid and hyperthyroid animals. The ratio between ATP turnover and force development was lower in hypothyroidism. With the experimental set-up used, this difference in the energy/force ratio in the T x group cannot be ascribed to differences in internal work. Lactate production was diminished in the hypothyroid group; this was probably not the result of a block in glycogenolysis but rather was a reflection of more economic energy expenditure during contraction. After tetanic stimulation, the phosphorylation potential ( [ATP] [ADP] f [Pi] ) was decreased most strongly in the hyperthyroid group, mainly because of the higher accumulation of Pi compared with the other groups, which suggests that during the recovery phase the stimulation of oxidative phosphorylation is greater in this group.