High ratios of free to total insulin-like growth factor-I in early infancy.
Insulin-like growth factor-I (IGF-I) is a major effector of somatic growth and metabolism. In normal, nonpregnant plasma, most of the IGF-I is complexed to specific IGF-binding proteins (IGFBPs), particularly IGFBP-3; only a minor fraction of plasma IGF-I exists in a free form. Recently, we have reported that free IGF-I levels, as measured using a new immunoradiometric assay, are relatively high in maternal plasma during pregnancy because of increased IGFBP-3 proteolytic activity. These high free IGF-I levels are physiologically important for the growth of maternal tissues such as uterus and placenta, which are related to the fetal growth. Growth during early infancy may be a continuation of fetal growth. In the present study, we have analyzed free and total plasma IGF-I and IGFBP-3 proteolytic activity in early infancy. Although the levels of free and total IGF-I were not significantly different in early infancy as compared with prepubertal periods, the ratio of free to total IGF-I (mean = 2.04%) was relatively increased and was similar to the ratio in pregnancy plasma (1.86%). However, unlike in maternal plasma, the high ratios were not totally caused by increased IGFBP-3 proteolytic activity. Our results suggest that there may be an increased conversion of plasma IGF-I to a free form in early infancy. The resultant increase in IGF-I bioavailability could contribute to the rapid somatic growth in early infancy.
Hasegawa
Y
Y
Division of Endocrinology and Metabolism, Tokyo Metropolitan Kiyose Children's Hospital, Japan.
Hasegawa
T
T
Fujii
K
K
Konii
H
H
Anzo
M
M
Aso
T
T
Koto
S
S
Takada
M
M
Tsuchiya
Y
Y
D000328:Adult; D000375:Aging / Q000097:blood; D015153:Blotting, Western; D002648:Child; D002675:Child, Preschool; D010450:Endopeptidases / Q000097:blood; D005260:Female; D006128:Growth; D006801:Humans; D007223:Infant; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000097:blood*; D007334:Insulin-Like Growth Factor I / Q000032:analysis* / Q000378:metabolism; D008297:Male; D011247:Pregnancy; D012016:Reference Values
D018972:Insulin-Like Growth Factor Binding Protein 3; D007334:Insulin-Like Growth Factor I; D010450:Endopeptidases