Protein Synthesis during Work-induced Growth of Skeletal Muscle

The capacity of skeletal and cardiac muscle to increase in size in response to increased physiological demand is well known. However, little is known of either the biochemical mechanisms involved in this compensatory hypertrophy or the cellular and physiological changes resulting from this growth process. Studies of protein synthesis during work-induced hypertrophy are relevant to such questions, since over 80% of the dry weight of muscles is protein (1). The present in vivo experiments compare the incorporation of amino acids into proteins by hypertrophying and control muscles, and they attempt to define the types of proteins made during compensatory growth. In order to study work-induced hypertrophy as an isolated process, we carried out these experiments with hypophysectomized animals. Although removal of the pituitary leads to cessation of normal developmental growth, compensatory hypertrophy of muscle occurs normally in such animals (2). Rapid compensatory growth of the soleus and plantaris muscles of one hind limb of the rat can be induced by severing the connections of the gastrocnemius to the Achilles tendon on that side (2, 3). Loss of this muscle imposes a greater work load on the remaining extensors of the ankle, and within 5 days the soleus of that limb increases in mass by 40%'7 and the plantaris by about 20%. In the present experiments, rats received repeated injections of leucine4 C after tenotomy of the gastrocnemius, and the extent of labeling of proteins in the hypertrophied and contralateral muscles was measured at the termination of the compensatory growth.