Insulin-Like Growth Factor I (IGF-I) and Insulin Binding to Erythrocytes of Normal Prepubertal Children and Adults

Summary

Erythrocyte insulin-like growth factor I (IGF-I) and insulin receptors were characterized in 10 normal prepubertal children (5 girls and 5 boys) aged 4-11 yrs and 10 normal adults (4 women and 6 men) aged 32-47 yrs. erythrocytes were purified from 5 ml of blood by Ficoll-Paque gradient centrifugation. Reticulocytes count in the erythrocyte suspensions were lower than 1%. Insulin and IGF-I binding assays were performed simultaneously.

Maximal percent binding of [¹²⁵I] labelled IGF-I was significantly higher in prepubertal children than in adults (8.7 ± 0.7% versus 6.2 ± 0.5% at a concentration of 5 × 10⁹ erythrocytes/ml). Scatchard analysis revealed the high affinity constant was better in prepubertal children (Ka = 4.6 ± 1.3 nM^-1 versus 1.8 ± 0.2 nM^-1) whereas the binding capacity was similar (5.8 ± 1.1 versus 7.7 ± 0.8 high affinity binding sites/cell). In both groups, unlabelled IGF-I inhibited tracer-binding half maximally at about 1 nM. Insulin was 100-fold less potent. In adults, specific binding of [¹²⁵I] labelled IGF-I was higher in women (7.6 ± 0.7%) than in men (5.3 ± 0.4%). No significant difference was observed in maximal specific binding of [¹²⁵I] labelled insulin between prepubertal children (8.2 ± 0.5%) and adults (7.2 ± 0.7%). In both groups, competition by unlabelled insulin for [¹²⁵I] labelled insulin binding gave 50% displacement for approximately 0.25 nM and IGF-I was about 80-fold less potent. Both IGF-I and insulin binding parameters were not significantly correlated with plasma hormone levels. In prepubertal children, the high-affinity IGF-I receptors number decreased with increasing high-affinity insulin receptors number. It was concluded that erythrocyte IGF-I receptors exhibit age-related changes in humans whereas insulin ones do not. The data suggests that measurement of IGF-I binding on erythrocytes might be useful in clinical or nutritional investigations.

Key words

IGF-I Receptors - Insulin Receptors - Erythrocytes - Human - Children