RECEPTORS FOR INSULIN-LIKE GROWTH FACTOR-II IN THE GROWING TIP OF THEDEER ANTLER

Insulin-like growth factor-II (IGF-II) binding in the growing tip of t he deer antler was examined using autoradiographical studies, radiorec eptor assays and affinity cross-linking studies. Antler tips from red deer stags were removed 60 days after the commencement of growth, and cryogenically cut into sections. Sections were incubated with radiolab elled IGF-II, with or without an excess of competing unlabelled IGF-II and analysed autoradiographically. Radiolabelled IGF-II showed high s pecific binding in the reserve mesenchyme and perichondrium zones, whi ch are tissues undergoing rapid differentiation and cell division in t he antler. Binding to all other structural zones was low and significa ntly (P<0.001) less than binding to the reserve mesenchyme/perichondri um zones. Radioreceptor assays on antler microsomal membrane preparati ons revealed that the IGF-II binding was to a relatively homogeneous r eceptor population (K(d) = 1.3 x 10(-10) mol/l) with characteristics t hat were not entirely consistent with those normally attributed to the type 2 IGF receptor. Tracer binding was partly displaceable by IGF-I and insulin at concentrations above 10 nmol/l. However, affinity cross -linking studies revealed a single band migrating at 220 kDa under non -reducing conditions, indicative of the type 2 IGF receptor. These res ults indicate that, in antler tip tissues, IGF-II binds to sites which have different binding patterns and properties from receptors binding IGF-1. This may have functional significance as it appears that, whil st IGF-I has a role in matrix development of cartilage, IGF-II may hav e a role in the most rapidly differentiating and proliferating tissues of the antler.