Phosphorylation of skeletal muscle contractile proteins in vivo

The phosphorylation of troponin·tropomyosin and the 18,000 dalton light chain of myosin was investigated in white gracilis muscles of rabbits in vivo. Troponin·tropomyosin contained 1 mol phosphate per mol protein in resting muscle. The phosphate content was not altered by the intra-arterial injection of 1 nmol isoproterenol which increased cyclic AMP and phosphorylase a formation. Also tetanic electrical stimulation produced no change in the phosphate content of troponin·tropomyosin. The light chain of myosin contained 0.50 mol phosphate per mol protein in control muscles which increased to 0.90 with tetanic electrical stimulation. These results demonstrate that phosphorylation of myosin, but not troponin can be stimulated in skeletal muscle in vivo in response to contractile activity.