Evidence supporting a direct suppressive effect of growth hormone on serum IGFBP-1 levels. Experimental studies in normal, obese and GH-deficient adults

It has occasionally been suggested that GH directly suppresses circulating IGFBP-1 levels, although it is generally believed that such an effect is secondary to a GH-induced increase in insulin levels. We present data from several experiments in which the effects of GH on IGFBP-1 could be studied more extensively. In normal subjects (n=36), an i.v. GH bolus caused a small but significant decrease in plasma IGFBP-1 concentrations without changes in insulin [IGFBP-1 (μg/l): 2.6 ± 0.3 (GH) vs 3.2 ± 0.4 (placebo), P<0.05]. Conversely, a 28-h somatostatin infusion with and without GH administration during fasting in normal subjects yielded higher IGFBP-1 levels in the non-GFI substituted study [50.5 ± 5.3 (GH-suppression) vs 22.6 ± 5.6 (GH-substitution), P<0.01], comparable with an increased concentration of IGFBP-1 during fasting in GH-deficient patients without usual GH substitution [23.4 ± 7.6 (GH pause) vs 14.1 ± 4.9 (GH substitution), P<0.01]. In both fasting studies insulin levels remained stable. During a hypocaloric diet, long-term GH treatment in obesity lead to a significant decline in IGFBP-1 level (2.3 ± 0.6 vs 1.2 ± 0.2, P<0.01), while no changes were found in the placebo group. Again, insulin levels remained equally low in both studies. Finally, a significant rebound increase in IGFBP-1 level in response to insulin induced hypoglycemia was only observed among GH-deficient patients, but not in control subjects, the latter of whom responded to hypoglycemia with a significant increase in serum GH levels [23.2 ± 7.2 (GHDA) vs 2.5 ± 0.3 (controls), P<0.01]. In conclusion, a suppressive effect of GH on IGFBP-1 appears to be unmasked in the presence of low or suppressed insulin levels, making GH a potential regulator of IGF-I bioactivity in a hitherto unrecognized way.