INSULIN-LIKE GROWTH FACTOR-II INCREASES PLASMA OSTEOCALCIN CONCENTRATION IN NEWBORN LAMBS

The influence of GH, IGF-I and IGF-II on plasma 1,25-(OH)2D and osteocalcin (OC) concentrations was studied in four groups of five newborn lambs each. They received a single i.v. injection of either purified bovine GH (I IU/mg 600-mu-g/kg body wt), synthetic human IGF-I (10 or 200-mu-g/kg body wt) or synthetic human IGF-II (10-mu-g/kg body wt). Five controls were injected the same volume (I ml) of solvent. IGF-II, but not GH or IGF-I, induced a rapid (1.5 h) and sustained (up to 18 h) increase of serum osteocalcin. This effect of IGF-II on plasma OC concentration was not mediated by 1,25-(OH)2D, which increased in plasma only 18 h after IGF-II injection. These data suggest that IGF-II stimulates, the in vivo secretion of OC by osteoblasts through a mechanism independent of 1,25-(OH)2D. Further studies are necessary to elucidate the mechanism of IGF-II action on osteocalcin synthesis and to explain the late increase of plasma 1,25-(OH)2 D after IGF-II injection.