Characterization of Insulin-Like Growth Factor I Receptor on Human Erythrocytes*

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Abstract
[125I]Insulin-like growth factor I (IGF-I) specifically bound to erythrocytes; the binding was saturable, and time and temperature dependent. Steady state binding was reached at 16 h at 4 C, and specific binding averaged 14.3 ± 0.7% (±SEM) at a concentration of 3.6 × 109 cells/ml in seven normal subjects. [125I]IGF-I binding to the cells was displaced by unlabeled IGF-I in a dose-dependent manner. Scatchard analysis indicated a linear plot, and Ka and number of binding sites/cell were 1.43 ± 0.07 × 109 M-1 and 20.7 ± 2.2, respectively. Compared to IGF-I, the relative potencies of multiplication-stimulating activity and insulin for displacing [125I]IGF-I binding were 20% and 1%, respectively. [125I] IGF-I binding to erythrocytes from patients with acromegaly was lower than binding to cells from pituitary drawfs. An inverse correlation between plasma IGF-I level and the number of IGF-I-binding sites per cell was found (r = −0.75; P < 0.005). These results demonstrate that [126I] IGF-I binding to erythrocytes can be used for clinical measurement of the IGF-I receptor.(J Clin Endocrinol Metab61: 1066, 1985)

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