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High Ratios of Free to Total Insulin-Like Growth Factor-I in Early Infancy

Division of Endocrinology and Metabolism, Tokyo Metropolitan Kiyose Children’s Hospital (Y.H., K.F., H.K., M.A., T.A., S.K.) Tokyo, Japan 204; Yuka Medias, Research Center (M.T.); and Keio University School of Medicine (T.H., Y.T.)

Address all correspondence and requests for reprints to: Yukihiro Hasegawa, Division of Endocrinology and Metabolism, Tokyo Metropolitan Kiyose Children’s Hospital, 1–3-1 Umezono Kiyose, Tokyo, Japan 204.

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.

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