FREE INSULIN-LIKE-GROWTH-FACTOR-I (IGF-I) IN HEALTHY-SUBJECTS - RELATIONSHIP WITH IGF-BINDING PROTEINS AND INSULIN SENSITIVITY

The majority of insulin-like growth factor I (IGF-I) circulates in blo od bound to a family of IGF-binding proteins (IGFBPs). Only a small fr action of IGF-I is unbound or free, and one of the postulated roles of the IGFBPs is regulation of this free component, thereby increasing I GF-I bioavailability. Whether free IGF-I plays a physiological role in glucose homeostasis, however, is not clear. In this study, we examine d the effects of acute changes in serum insulin on free IGF-I, total I GF-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 s ensitivity (S-I) was computed by Bergman's minimal model. Serum insuli n reached a first peak after glucose injection and a second, higher pe ak after exogenous insulin administration. Although the total IGF-I le vel remained constant for the duration of the experiment, free IGF-I d ecreased 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% abo ve 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 S-I (r = 0.52; P < 0.005) and inversely correlated wi th glucose (r = -0.51; P < 0.005) and IGFBP-1 (r = -0.65; P < 0.001). In conclusion, free IGF-I is acutely regulated by insulin and correlat es with S-I, suggesting that it may play a physiological role in gluco se homeostasis.