Free Insulin-Like Growth Factor I (IGF-I) in Healthy Subjects: Relationship with IGF-Binding Proteins and Insulin Sensitivity

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Abstract
The majority of insulin-like growth factor I (IGF-I) circulates in blood bound to a family of IGF-binding proteins (IGFBPs). Only a small fraction of IGF-I is unbound or free, and one of the postulated roles of the IGFBPs is regulation of this free component, thereby increasing IGF-I bioavailability. Whether free IGF-I plays a physio- logical role in glucose homeostasis, however, is not clear. In this study, we examined the effects of acute changes in serum insulin on free IGF-I, total IGF-I, IGFBP-1, and IGFBP-3 in 11 healthy subjects. Glucose (0.3 g/kg) and insulin (0.05 U/kg) were injected iv at 0 and 20 min, respectively. Blood samples were drawn at defined intervals for 3 h, and insulin sensitivity (SI) was computed by Bergman's minimal model. Serum insulin reached a first peak after glucose injection and a second, higher peak after exogenous insulin administration. Al- though the total IGF-I level remained constant for the duration of the experiment, free IGF-I decreased by 20% 20 min after the first insulin peak and by 35% 20 min after the second peak. IGFBP-1 first declined to 20% below basal, then rose to 3-fold the basal level. IGFBP-3 increased linearly to 20% above basal by the end of the experiment, and this increase mirrored the decline of free IGF-I. In the fasting state, free IGF-I was positively correlated with SI (r 5 0.52; P , 0.005) and inversely correlated with glucose (r 52 0.51; P , 0.005) and IGFBP-1 (r 52 0.65; P , 0.001). In conclusion, free IGF-I is acutely regulated by insulin and correlates with SI, suggesting that it may play a physiological role in glucose homeostasis. (J Clin Endocrinol Metab 82: 2177-2181, 1997)

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