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.


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-I, 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 micrograms/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 micrograms/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 micrograms/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).


CONCLUSIONS
The interaction of bioactive molecular forms of GH with the increased hepatic GH receptors induces the rise in postnatal IGF-I levels in early infancy.
Bozzola
M
M
Department of Pediatrics, University of Pavia, Italy.
Tettoni
K
K
Locatelli
F
F
Radetti
G
G
Belloni
C
C
Autelli
M
M
Zecca
M
M
Valentini
R
R
Severi
F
F
Tatò
L
L
D000367:Age Factors; D002648:Child; D002675:Child, Preschool; D005314:Embryonic and Fetal Development / Q000502:physiology; D005260:Female; D019382:Human Growth Hormone / Q000097:blood*; D006801:Humans; D007223:Infant; D007231:Infant, Newborn / Q000097:blood*; D018969:Insulin-Like Growth Factor Binding Proteins / Q000097:blood*; D008099:Liver / Q000502:physiology; D008297:Male; D011863:Radioimmunoassay; D011986:Receptors, Somatotropin / Q000502:physiology
D018969:Insulin-Like Growth Factor Binding Proteins; D011986:Receptors, Somatotropin; D019382:Human Growth Hormone