Force development and metabolism in perfused skeletal muscle of euthyroid and hyperthyroid rats

The effect of experimental hyperthyroidism on skeletal muscle metabolism during force development and subsequent recovery was studied in a perfused preparation of the gastrocnemius and plantaris muscle of rats. After equilibration, the muscles were stimulated for 15 minutes at 1 Hz, and were allowed to recover during the following 15 minutes. The basal oxygen consumption of the skeletal muscle preparation was increased by 40% in the hyperthyroid rats as compared with euthyroid rats. The results show that: 1. Active force at 1 Hz was significantly reduced in the hyperthyroid rats compared with euthyroid rats, but the decay of force was similar in the two groups. 2. The increase in oxygen and glucose consumption and in lactate production during stimulation of the muscles was equal in hyperthyroid and euthyroid rats, but during recovery, oxygen consumption was significantly higher in the hyperthyroid than in euthyroid rats. 3. The increase in ATP turnover during force development and recovery (calculated from changes in O2 uptake, lactate production, and the breakdown of creatine-phosphate) in relation to the sum of force was significantly increased in the hyperthyroid state.