Insuline-Like Growth Factor-I (IGF-I) is of involved in the endocrine control of fetal and postnatal growth. However newborns growth is rapid despite low serum IGF-I concentrations which are even lower in Small for Gestational Age (SGA).

Hypothesis: The serum formation of binary complexes between IGF-I and either of the six IGF-Binding Protein (IGFBP-1 to -6) or of long-lived ternary complexes between IGF-I, IGFBP-3 and acid-labile subunit is thought to regulate IGF-I bioavailability by increasing its half life.

Methods: We studied the serum IGFBPs in 29 term and pretem SGA vs 50 eutrophic (AGA) newborns of the same gestational age (GA). Serum samples were obtained between 2 and 5 days of life. We performed analysis of IGFBPs by Western Ligand Blotting and of IGFBP-3 by Western Immuno Blotting using a specific polyclonal antibodies, and by Radio Immuno Assay (RIA). We assessed for proteolytic activity capable of degrading radiolabeled IGFBP-3.

Results: By Western Ligand Blot analysis, the mean value of IGFBP-1 or -2 were higher in SGA than in AGA newborns. RIA measurement of IGFBP-3 showed a lower level in SGA (0,67 ng/ml vs 0,98 ng/ml for AGA newborns, p<0.05). Western immuno blot revealed two bands migrating at 38 and 40 kDa whose intensity increased with GA, and a third band at 29 kDa. This 29 kDa band was increased in SGA compared to AGA newborns at 35 and 36 weeks GA. To explain the increase of low molecular weight form of IGFBP-3, we looked for an IGFBP-3 protease activity in serum but it was not increased in either group.

Conclusion: This study suggests that IGF-I-IGFBPs ratios may be different in SGA and eutrophic newborns. We can hypothesize that the increase of IGFBP-1 and -2, the decrease of IGFBP-3, and the modifications of the 29 kDa IGFBP-3 might decrease the ternary complexes formation in serum therefore increasing the availability of IGF-I in SGA newborns.