Postnatal age and food intake alter insulin-like growth factor-II/mannose-6-phosphate receptors in ovine skeletal muscles.

In female lambs aged from 2 days to 2 years, specific binding of 125I-IGF-II in muscle fibre and connective tissue of M. biceps femoris and M. gastrocnemius was demonstrated using histological autoradiography. The binding site was characterized as the IGF-II/M6P receptor in membrane preparations using competitive displacement assay and SDS-PAGE. In both muscles, 125I-IGF-II binding was more abundant in connective tissue than muscle fibre (P < or = 0.001). Levels changed significantly with age in all cell types studied (P < or = 0.001), while changes as a result of fasting were limited to a decrease in binding to the connective tissue of M. biceps femoris (P < or = 0.01). The overall decline of 125I-IGF-II binding with increasing age is correlated with a slowing of postnatal growth, while the reduction in 125I-IGF-II binding with fasting may be associated with modulating growth and composition of connective tissue, or increasing the bioavailability of IGF-II to specific muscles.