Postnatal variations of growth hormone bioactivity and of growth hormone-dependent factors

Objective: To evaluate whether the low insulinlike growth factor I (IGF-I) levels that are observed in the neonate depend on the biological inactivity of the molecular forms of growth hormone (GH) or on the immaturity of the hepatic GH receptors during the early postnatal period. Materials aad Methods: Serum samples were collected from 60 normal full-term neonates on day 5 and at 1 and 4 months of age to evaluate the GH concentrations by using both an immunofluorometric assay and Nb2 cell bioassay as well as the GH-binding protein, IGF-II and IGF-binding protein 3 values by radioimmunoassay. Results: Five-day-old neonates showed significantly higher (P<.001) mean+/-SEM GH levels that were measured by using the immunofluorometric assay (27.22+/-1.62 mu g/L) and Nb2 cell bioassay (3.56+/-0.14 U/mL) compared with those levels in 11 prepubertal children who were studied as control subjects (1.26+/-0.28 mu g/L and 0.74+/-0.08 U/mL, respectively). At 1 and 4 months of age, GH values that were measured by using both the immunofluorometric assay (9.15+/-0.89 and 2.58+/-0.32 mu g/L, respectively) and Nb2 cell bioassay (2.52+/-0.11 and 1.71+/-0.15 U/mL, respectively) were decreased significantly (P<.001). In 5-day-old neonates, we observed significantly lower (P<.001) serum GH-binding protein (9.73%+/-0.42%), IGF-I (67.63+/-5.20 ng/mL), and IGF-binding protein 3 (1.46+/-0.17 mg/L) concentrations compared with those in the prepubertal children (30.74%+/-2.01%, 210+/-25 ng/mL, and 3.08+/-0.22 mg/L, respectively). At 1 month of age, serum GH-binding protein (16.00%+/-0.70%) and IGF-binding protein 3 (2.96+/-0.30 mg/L) values were increased significantly (P<.001), while IGF-I levels (72.55+/-7.6 ng/mL, P=.09) were not increased. Serum IGF-I values were increased significantly (P<.005) at 4 months of age (97.94+/-9.68 ng/mL). Conclusion: The interaction of bioactive molecular forms of GH with the increased hepatic GPI receptors induces the rise in postnatal IGF-I levels in early infancy.