Skeletal Muscle Insulin-Receptor Kinase: Effects of Substrate Inhibition and Diabetes

Insulin receptor tyrosine kinase activity solubilized from hind limb muscle of control and streptozocin-induced diabetic (STZ-D) rats (2–3 wk) was studied with the substrates histone H2B and poly glutamic acid–tyrosine (glu-tyr) (4:1). Basal and insulin-stimulated kinase activities were inhibited when high concentrations of either substrate were added before initiation of phosphorylation with ATP. Under these conditions, insulin-stimulated activities of diabetic- and control-derived receptor kinase toward H2B were similar at 0.008 mg/ml H2B. However, higher concentrations of H2B (0.04–1 mg/ml) progressively reduced the ratios of diabetic-derived to control-derived receptor kinase activities to ∼0.5. When inhibition of receptor kinase activities was prevented by allowing maximal autophosphorylation of insulin receptors before addition of H2B, kinase activity of diabetic- and control-derived receptors was similar at all H2B concentrations. Diabetic-derived insulin-receptor tyrosine kinase activity toward poly glu-tyr (4:1) was not significantly different from that of control rats. Under conditions of substrate inhibition (0.4 mg/ml H2B), insulin receptor H2B kinase activity from muscles of rats with severe diabetes (85 mg/kg STZ, 7 days) was significantly decreased, whereas the same activity from rats with moderate diabetes (50 mg/kg STZ, 7 days) was not significantly different from control rats. Insulin receptor α,β dimers were not detectable in muscle preparations from control or diabetic rats. The data suggest that the impairment of muscle-derived insulin-receptor tyrosine kinase activity associated with insulinopenic diabetes reflects, in part, enhanced inhibition by some substrates. If solubilized insulin receptors and the exogenous substrates studied model in vivo events, impaired signaling of the muscle insulin receptor in insulinopenic diabetes may depend on the type and concentration of intracellular tyrosine kinase substrates and the severity of the metabolic derangements.