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

OBJECTIVEInsulin-like growth factor-I (IGF-I) levels are lower in older compared with younger subjects. We tested the hypothesis that the reduction in circulating IGF-I would be accompanied by upregulation in tissue IGF-I binding in at least some tissues. We tested erythrocyte IGF-I binding since blood is an accessible tissue in humans, and there is growing evidence to suggest that erythrocyte IGF-I binding is influenced by circulating IGF-I. DESIGN AND PATIENTSWe compared 9 healthy older males (61–68 years old) with 9 healthy younger males (15–19 years old). MEASUREMENTSStandard techniques were used to assay circulating IGF-I and IGF binding proteins 1–5 (IGFBPs 1–5). Erythrocyte IGF-I binding was first measured by studies in which native [¹²⁵I]-IGF-I was displaced with unlabelled native IGF-I. In order to determine a possible role for IGF binding proteins (IGFBP), native [¹²⁵I]-IGF-I was displaced with des-(1-3)IGF-I, which binds with IGF receptors but not IGFBPs. RESULTSAs expected, circulating IGF-I was significantly lower in the older compared with the younger subjects. In addition, IGFBP-3 and 5 were significantly lower, and IGFBP-4 higher, in older compared with younger subjects. When native [¹²⁵I]-IGF-I was displaced with unlabelled native IGF-I, the number of IGF-I binding sites per erythrocyte was higher in the older subjects (43 ± 5 vs. 18 ± 2, older vs. younger, respectively; P < 0.05). In contrast, when native [¹²⁵I]-IGF-I was displaced with des-(1-3), IGF-I binding capacity was not different between the two age groups. CONCLUSIONSErythrocyte IGF binding was increased in older compared with younger subjects. Surprisingly, the mechanism of the increase may not be a simple up regulation of IGF-I receptors in response to reduced circulating IGF-I, but possibly by an increase in the levels of as yet unidentified erythrocyte membrane-associated IGF binding proteins. [ABSTRACT FROM AUTHOR]