Comparison of insulin and insulin-like growth factor I receptors from rat skeletal muscle and L-6 myocytes

Insulin and IGF-I receptors were solubilized from fused L-6 myocytes, a rat skeletal muscle derived cell line, and compared to rat skeletal muscle receptors. In skeletal muscle, 125I-insulin binding was competed by insulin > IGF-I > MSA, whereas in L-6 cells IGF-I > insulin > MSA. 125I-IGF-I binding was competed by IGF-I > insulin = MSA in both tissues. On electrophoresis, differences in Mr were observed between skeletal muscle and L-6 derived receptors both in the α- and β-subunits. Six antibodies directed against the human insulin receptor β-subunit recognized the rat skeletal muscle insulin receptor, while only two reacted strongly with L-6 derived receptors. Skeletal muscle has receptors with relative specificity for insulin and IGF-I respectively; L-6 cells also have two classes of receptors, one is kinetically similar to the IGF-I receptor from skeletal muscle; the other, which binds insulin with relatively high affinity has even greater affinity for IGF-I. This unusual receptor may represent a developmental stage in muscle or the transformed nature of L-6 cells.