To assess the relationship between insulin receptor (IR) kinase activity and insulin action in vivo in humans, we measured glucose disposal rates (GDR) during a series of euglycemic clamp studies. Simultaneously, we measured IR kinase activity in IRs extracted from skeletal muscle obtained by needle biopsy at the end of each clamp. By preserving the phosphorylation state of the receptors as it existed in vivo at the time of biopsy, we could correlate GDR and IR kinase in skeletal muscle. Eight nondiabetic, nonobese male subjects underwent studies at insulin infusion rates of 0, 40, 120, and 1,200 mU/m2 per min. Kinase activity, determined with receptors immobilized on insulin agarose beads, was measured at 0.5 microM ATP, with 1 mg/ml histone, followed by SDS-PAGE. Insulin increased GDR approximately sevenfold with a half-maximal effect at approximately 100 microU/ml insulin and a maximal effect by approximately 400 microU/ml. Insulin also increased IR kinase activity; the half-maximal effect occurred at approximately 500-600 microU/ml insulin with a maximal 10-fold stimulation over basal. Within the physiologic range of insulin concentrations, GDR increased linearly with kinase activation (P less than 0.0006); at supraphysiologic insulin levels, this relationship became curvilinear. Half-maximal and maximal insulin-stimulated GDR occurred at approximately 20 and approximately 50% maximal kinase activation, respectively. These results are consistent with a role of the kinase in insulin action in vivo. Furthermore, they demonstrate the presence of a large amount of "spare kinase" for glucose disposal.
G R Freidenberg, S L Suter, R R Henry, D Reichart, J M Olefsky