Erythrocyte insulin-like growth factor-i binding in younger and older males

Insulin-like growth factor-I (IGF-I) levels are naturally lower in older compared with younger subjects, and we used this phenomenon to assess the relationship between circulating IGF-I and erythrocyte IGF-I binding. Serum levels of IGF-I were measured in 9 healthy older males (61 to 68 years old) and in 9 healthy younger males (15 to 19 years old). Erythrocyte IGF-I binding was first measured by studies in which native I25I-IGF-1 was displaced with unlabelled native IGF-I. As expected, serum levels of IGF-I were significantly lower in older males (173 ±13 ug/L) compared with the younger males (355 ±24 ug/L, pO.OOOl). In contrast, binding capacity was significantly higher in the older compared with the younger males (p<0.05), and this was reflected by the increased number of IGF-I binding sites per erythrocyte (43 ±5 vs. 18 ±2, older vs. younger, respectively; pO.05). In order to determine a possible role for IGF binding proteins, we performed experiments in which native I2'I-IGF-I was displaced with des-(l-3)IGF-I which binds with IGF receptors but not IGFBPs. In these studies, binding capacity was not different between the two age groups. The data suggest that the lowered circulating IGF-I levels normally observed in older subjects is accompanied by an increase in erythrocyte IGF-I binding. However, the mechanism of this increase may not be a simple upregulation of IGF-I receptors, but rather an increase in the levels of erythrocyte membraneassociated IGF binding proteins.