Insulin resistance in denervated skeletal muscle. Inability of insulin to stimulate dephosphorylation of glycogen synthase in denervated rat epitrochlearis muscles

We have investigated the effects of insulin and motor denervation on the phosphorylation of glycogen synthase in skeletal muscle. Rat epitrochlearis muscles were denervated in vivo 3 days before the contralateral and denervated muscles were incubated in vitro with 32Pi to label sites in glycogen synthase. The 32P-labeled synthase was rapidly immunoprecipitated from extracts under conditions which prevented changes in the phosphorylation state of the enzyme. When 32P-labeled synthase from contralateral muscles was cleaved with CNBr, essentially all of the 32P was recovered in two fragments, denoted CB-1 and CB-2. Incubating these muscles with insulin decreased the 32P content of each fragment by approximately 25%, indicating that the hormone stimulated dephosphorylation of at least two sites. Peptide mapping by reverse phase high performance liquid chromatography was performed to resolve phosphorylation sites more completely. The results suggest that the enzyme was phosphorylated in sites 1a, 1b, 2, 3(a+b+c), and 5. Insulin stimulated dephosphorylation of sites in peptides presumed to contain sites 1b, 2, and 3(a+b+c). Synthase from denervated muscles appeared to contain the same amount of phosphate as enzyme from contralateral muscles, and denervation did not detectably affect the distribution of 32P within the subunit. However, denervation abolished the effect of insulin on decreasing the 32P content of synthase. The results indicate that the insulin resistance induced by denervation involves a loss in the ability of insulin to stimulate dephosphorylation of glycogen synthase.