SERUM-FREE IGF-I DURING A HYPERINSULINEMIC CLAMP FOLLOWING 3 DAYS OF ADMINISTRATION OF IGF-I VS SALINE

In a randomized crossover study in eight healthy subjects, we compared the effect of 3 days of continuous subcutaneous administration of ins ulin-like growth factor I (IGF-I; 10 mu g.kg(-1).h(-1)) and saline on fasting serum levels of free IGF-I, total (extractable) IGF-I, and IGF -binding protein (IGFBP)-1 and -3. On the 3rd day a hyperinsulinemic ( euglycemic and hypoglycemic) clamp was performed. When preclamp (basel ine) levels were compared after 3 days, IGF-I administration had incre ased total IGF-I from 225 +/- 21 (means +/- SE) to 1,003 +/- 46 mu g/l (P < 0.0001), free IGF-I from 0.5 +/- 0.2 to 10.4 +/- 1.7 mu g/l (P < 0.001), IGFBP-3 from 2,908 +/- 148 to 3,591 +/- 179 mu g/l (P < 0.01) , and IGFBP-1 from 7.6 +/- 3.8 to 19.6 +/- 2.5 mu g/l (P < 0.01). Duri ng the clamp, levels of free IGF-I increased gradually from baseline t o 1.0 +/- 0.3 mu g/l (saline; P < 0.01) and to 19.6 +/- 4.7 mu g/l (IG F-I; P < 0.005). Concomitantly, levels of IGFBP-1 decreased gradually from baseline to 4.1 +/- 2.3 mu g/l (saline; P < 0.0005) and to 4.6 +/ - 1.8 mu g/l (IGF-I; P < 0.0001). Total IGF-I exhibited minor changes only during the clamp (P < 0.05), and IGFBP-3 was unchanged. In conclu sion, administration of IGF-I increased total IGF-I about fourfold, wh ereas free IGF-I increased 20-fold. Noteworthily, in both situations a further twofold increase in free IGF-I was observed during the hyperi nsulinemic clamp, concomitant with a decrease in IGFBP-1. This support s the hypothesis that IGFBP-1 is important in the short-term regulatio n of free IGF-I in vivo.