Effects of Thyroxin on Skeletal Muscle Function

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Abstract
Simultaneous measurements of isometric tension exerted by the gastrocnemius m. and rate of O2 consumption of the entire animal were made in control hyper- and hypothyroid rats. When the muscle was stimulated at 40/second frequency, tension was lowest and increment in rate of O2 consumption was highest in hyperthyroid rats, the converse being true for the hypothyroids; mechanical fusion of contractions was complete for hypothyroids and markedly incomplete for hyperthyroids, controls yielding moderate clonus. At a stimulus frequency of 120/second (above fusion frequency for all rats) tension output was highest for controls, but O2 consumption increment per unit tension-time exertion was the same for all rats. It is concluded that thyroid hormone can alter the degree of muscle fusion, and thereby the economy, at low stimulus frequencies.

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