CHARACTERIZATION OF INSULIN-RECEPTOR KINASE-ACTIVITY AND AUTOPHOSPHORYLATION IN DIFFERENT SKELETAL-MUSCLE TYPES

We have examined insulin binding, autophosphorylation, and tyrosine kinase activity in detergent-solubilized and wheat germ agglutinin-purified insulin receptor preparations from four rat muscles of different fiber composition (i.e., tensor fascia latae, soleus, vastus intermedius, and plantaris). Insulin binding activity was similar in three of the four muscles but lower in tensor fascia latae. No significant differences were noted in the affinity of insulin for its receptor from various muscle types. Insulin receptor tyrosine kinase activity measured in the absence (basal) and presence of insulin (0.3-300 nM) was comparable in all muscle types (normalized to the amount of insulin bound). Insulin sensitivity, measured as the dose of insulin required for half-maximal activation of kinase activity, was also similar in all muscle types. Likewise, incubation of receptor preparations with [gamma-32P]ATP, Mn2+, and insulin (0.25-100 nM) resulted in a dose-dependent auto-phosphorylation of the beta-subunit (relative molecular weight approximately 95 kDa) with similar kinetics in all muscle types. In conclusion, these results show that the functional behavior of the insulin receptor autophosphorylation-kinase system (in vitro) is not changed by alterations in muscle fiber composition, indicating that differences in insulin sensitivity between different skeletal muscle types is probably not due to modulation of the insulin receptor phosphorylation system.