Insulin receptor kinase in human skeletal muscle

Receptor-associated protein kinase activity has been shown in all primary target tissues of insulin action in the rat and a function of insulin receptor phosphorylation in signal transmission was proposed. Insulin receptor phosphorylation so far has not been demonstrated in human target tissues of insulin. We describe here insulin receptor kinase activity in human skeletal muscle. Insulin (10^-8 mol l) stimulates the phosphorylation of a 95-kDa protein from skeletal muscle 2-fold. The phosphoprotein is quantitatively immunoprecipitated with insulin receptor antibody identifying it as the β-subunit of the insulin receptor. The insulin stimulation of phosphorylation is detectable also at physiological insulin concentrations (10^-9 mol l) showing that receptor phosphorylation could be involved in insulin action in human skeletal muscle as well.