CIRCULATING LEVELS OF INSULIN, INSULIN-LIKE GROWTH FACTOR-I (IGF-I), IGF-II, AND IGF-BINDING PROTEINS IN THE SMALL-FOR-GESTATIONAL-AGE FETAL-RAT

Insulin-like growth factors (IGFs) are important regulators of somatic growth in childhood and circulate in association with specific binding proteins (IGFBPs). Insulin is important for the regulation of IGFs and IGFBPs in postnatal life and is required for normal growth in utero. We asked whether alterations in the availability of IGFs and/or their BPs may contribute to impaired fetal growth 24 h after bilateral uterine artery ligation when circulating levels of insulin are low and fetuses are small for gestational ap (SGA). Bilateral uterine arterial ligation or sham surgery was performed on maternal rats on day 19 of gestation (term = 21.5 days). One day after ligation, fetuses were SGA compared to shams (2.9 +/- 0.1 vs. 3.5 +/- 0.1 g/fetus, respectively; P < 0.001), and liver weight was reduced (242 +/-9 vs. 300 +/- 6 mg/liver; P < 0.001). Serum levels of both insulin and IGF-I were reduced approximately 50% in SGA litters compared to those in shams (P < 0.001) and correlated with each other (P < 0.02). IGF-I levels also correlated with fetal body and liver weights (P < 0.005 for each) even after controlling for the effects of insulin. Insulin levels correlated with body and liver weights (P < 0.02 and P < 0.03, respectively), but these relationships were not significant after controlling for the effects of IGF-I. Serum levels of IGF-II were not significantly reduced in SGA and did not correlate with fetal weight or insulin or IGF-I levels. [I-125]IGF-I binding assay demonstrated that the availability of IGFBPs was increased in SGA serum, and Western ligand blotting indicated that circulating levels of 32K IGFBPs were increased in SGA fetuses compared to shams. Densitometric analysis indicated that levels of 32K IGFBPs were 4-fold greater in SGA litters (P < 0.001 vs. shams), and the level of 32K IGFBPs correlated with fetal body and liver weights (P < 0.05 for each) and with levels of both insulin and IGF-I (P < 0.001 for each), but not with levels of IGF-II. Immunoblotting with newly developed antiserum against rat IGFBP-1 confirmed that levels of immunoreactive 32K IGFBP-I are increased in SGA serum, and immunoprecipitation studies confirmed that IGFBP-1 accounted for the rise in 32K IGFBPs in SGA serum. Antiserum against IGFBP-1 also precipitated some 34K BPs in SGA (but not sham) serum, suggesting that levels of both 32K and 34K forms of IGFBP-1 are elevated in SGA fetal rats. In contrast, immunoblotting and immunoprecipitation with antiserum against rat IGFBP-2 showed that levels of 34K IGFBP-2 were not changed in SGA animals. In summary, circulating levels of IGF-I were reduced while levels of immunoreactive IGFBP-1 were increased in the SGA fetal rat. Serum levels of both IGF-I (by immunoassay) and 32K IGFBP-1 (by ligand blotting) correlated with fetal body and liver weights and serum levels of insulin. Alterations in the expression and availability of IGF-I and IGFBP-1 and modification of circulating forms of IGFBP-1 may play an important role in the regulation of fetal growth and provide a mechanism by which insulin may modulate growth in utero.