Free rather than total circulating insulin-like growth factor-I determines the feedback on growth hormone release in normal subjects

Pituitary GH secretion is feedback regulated by circulating IGF-I. However, it remains to be determined whether the feedback control is mediated through circulating free or total IGF-I. To study this, we compared the temporal changes in circulating levels of GH vs. free and total IGF-I during fasting. Seventeen healthy normal-weight subjects (body mass index 23.4 +/- 0.6 kg/m(2)) were studied during 80 h of fasting. Serum was assayed for GH every 3 h; total, free, and bioactive IGF-I, IGF binding protein (IGFBP)-1, -2, and -3 as well as IGFBP-1 bound IGF-I were assayed every morning. During fasting, mean 24-h GH levels increased from 1.41 +/- 0.20 to 3.01 +/- 0.46 and 2.09 +/- 0.30 mug/liter (d 1 vs. d 2 and 3; P < 0.03). After 24 h of fasting, free and bioactive IGF-I had decreased by 40 +/- 5 and 17 +/- 5%, respectively (P < 0.02), and both concentrations remained suppressed for the rest of the study. In contrast, total IGF-I remained unchanged until the end of d 3, at which levels were slightly reduced (P < 0.007). IGFBP-1 increased from 38 +/- 2 to137 +/- 24, 212 +/- 32, and 214 +/- 22 mu g/liter (d 1 vs. d 2, d 3, and end of d 3; P < 0.0001), and these changes closely paralleled those of IGFBP-1- bound IGF-I (P < 0.0001). IGFBP-2 increased only transiently at d 2 (P < 0.05), and IGFBP-3 remained unchanged. The increase in mean 24-h GH levels from d 1 to d 2 correlated inversely with the relative reduction in free IGF-I from d 1 to d 2 (r = -0.51; P = 0.04), i.e. the larger the reduction in free IGF-I, the larger the increase in GH. None of the other IGF-related parameters correlated with GH. In conclusion, the temporal relationship between the increase in GH and the reduction in free IGF-I supports the hypothesis that circulating free IGF-I mediates the feedback regulation of GH secretion.