IGF-I and IGF-binding protein-3 in plasma of GH-deficient rats

The majority of IGF-I circulates in a large (150 kDa) ternary complex with IGF-binding protein-3 (IGFBP-3) and a non-IGF-binding acid-labile subunit. The secretion of ternary complex into the circulation from liver has been considered to be GH-dependent; however, recent data indicate that GH does not directly regulate hepatic IGFBP-3 synthesis. To examine the role of insulin in regulating plasma IGFBP-3 levels, postpubertal male GH-deficient (dw/dw) rats were treated every 8 h with injections (s.c.) of 0.9% saline, 20 micrograms insulin/day, 200 micrograms hIGF-I/day, or 20 micrograms insulin/day plus 200 micrograms hIGF-I/day, for 10 days with the animals being killed 2-3 h after the final injection. Hypoglycaemia was not observed in any of the treatment groups. hIGF-I treatment increased longitudinal growth and weight gain (P < 0.05), while insulin treatment had no effect. Plasma IGF-I levels were increased in groups treated with hIGF-I (P < 0.05), while insulin treatment resulted in a reduction (P < 0.05): saline = 267.1 +/- 15.6 (ng/ml +/- S.E.M.), insulin = 219.3 +/- 17.5, hIGF-I = 391.7 +/- 17.6, insulin plus hIGF-I = 357.5 +/- 31.8. Hepatic IGF-I mRNA expression was increased in insulin-treated dw/dw rats in comparison with hIGF-I-treated animals (P < 0.05) but not in comparison with saline control or the combined treatment groups. Plasma levels of intact IGFBP-3, measured by ligand blot analysis, were increased in all treatment groups compared with saline (P < 0.05): saline = 100.0 +/- 9.4% (% of saline +/- S.E.M.), insulin = 149.9 +/- 17.5%, hIGF-I = 191.4 +/- 17.3%, insulin plus hIGF-I = 205.4 +/- 15.3%. The levels of the 28/32 kDa IGFBPs and IGFBP-4 in plasma were increased by hIGF-I treatment (P < 0.05) but not by insulin treatment. Hepatic specific 125I-bovine GH binding was not significantly different in any of the treatment groups. This study provides the first evidence in nondiabetic animals that insulin regulates hepatic IGF-I mRNA expression, plasma IGF-I and plasma IGFBP-3 levels in the GH-deficient state without changes in hepatic GH receptors. The divergent response of plasma IGF-I and IGFBP-3 levels to insulin treatment in the present study may indicate an effect of insulin on the clearance of IGF-I from the circulation.