Pregnancy is associated with rapid growth of maternal and fetal tissues. Insulin-like growth factors (IGF-I and -II) have roles in mediating both fetal and placental growth. In this study serum IGFs and IGF-binding proteins (IGFBPs) were characterized, IGFBP protease activity was quantified, and hepatic IGFBP-1, -2, -3, and -4 mRNA were investigated throughout rat pregnancy. IGF-I in maternal serum was elevated (P < 0.001) on days 5 and 10 (d5 and d10) of gestation compared to levels in nonpregnant controls (NP), but was significantly decreased below NP levels (P < 0.001) after d10 of pregnancy. Serum IGF-II levels were unaffected by pregnancy. Using Western ligand blotting (WLB), six IGFBP bands were visualized in NP, d5, and d10 pregnancy rat sera. At 15 and 20 days gestation, the IGFBP-3 bands were no longer detectable by WLB. Using an in vitro IGFBP protease assay, sera from rats at 15 and 20 days gestation proteolyzed 63 ± 4% and 81 ± 5% of recombinant human IGFBP-3, respectively. Regression analyses demonstrated that serum IGF-I was positively correlated with serum IGFBP-3 (r2= 0.73; P = 0.001), whereas serum IGFBP-3 (r2= -0.85; P = 0.001) and serum IGF-I (r2= -0.78; P = 0.001) were negatively correlated with serum protease activity. In addition, no change was observed in liver IGFBP-3 mRNA during pregnancy, further suggesting that protease activity is primarily responsible for the decrease in serum IGFBP-3. However, IGFBP-1 and -4 mRNA levels were increased 3-to 11-fold after d5 of gestation. The hormonal and/or metabolic regulators of hepatic IGFBP-1 and -4 expression during rat pregnancy remain to be determined.
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