Effect of Thyroparathyroidectomy and Thyroxin on Rate of Atrophy of Skeletal Muscle

Abstract

Previous investigations have indicated that the thyroid secretion exerts an influence on skeletal muscle. Simpson suggested that the lack of development of skeletal muscle in cretin sheep might be due to inactivity rather than a specific action of thyroid secretion on muscle growth. Gudernatsch showed that the feeding of thyroid material caused an accelerated rate of atrophy of the tadpole's tail during metamorphosis. This effect has usually been attributed to the mechanical influences of increased growth changes rather than a specific effect of thyroid substance on autolysis. This report is concerned with the influence of thyroparathyroidectomy and thyroxin administration on the rate of atrophy of the denervated gastrocnemius muscle of the rat. We have previously reported some effects of denervation on rat's skeletal muscle and described the technique employed. The weight of the denervated muscle has been compared to that of the opposite control muscle in a series of adult animals 7, 14, 21 and 28 days after section of the sciatic nerve. A series of 26 animals were subjected to thyroparathyroidectomy a few weeks prior to the time of nerve section. A group of 36 animals were given subcutaneous injections of dl-thyroxin. The amounts given totaled approximately 1 mg. per rat per week, and the injections were made at intervals of 3 to 4 days. The initial dose was given 3 to 4 days prior to denervation of the muscle. The effectiveness of the thyroxin administration was determined by frequent measurements of the oxygen consumption of the animals. The amount of thyroxin given was adequate to cause an elevation of the metabolic rate but less than that required to cause an appreciable loss of body weight. In order to measure the rate of atrophy it is essential that the control muscle should neither gain nor loose weight subsequent to the time of denervation of the opposite muscle. A group of animals from the same stock served as controls for the above experiments.