CATECHOLAMINES STIMULATE THE SYNTHESIS AND RELEASE OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-1 (IGFBP-1) BY FETAL SHEEP LIVER IN-VIVO

In fetal sheep, prolonged hypoxia (for 24 h) induced by a reduction in maternal uterine artery blood flow, increases insulin-like growth fac tor binding protein-1 (IGFBP-1) levels and decreases IGFBP-2 levels in the plasma, with corresponding changes in messenger RNA (mRNA) levels in the liver. Since IGFBP-1 synthesis in liver cells in vitro is stim ulated by compounds that increase intracellular cAMP concentrations, w e hypothesized that the increased IGFBP-1 synthesis during prolonged h ypoxemia may be induced by circulating catecholamines, that are releas ed during hypoxia, and that elevate fetal liver cAMP levels. Our aim w as to determine the effect of 24-h catecholamine infusions on the synt hesis and release of IGFBP-1 and IGFBP-2 in fetal sheep. Vascular cath eters were implanted into fetuses at 110-115 days gestation in 14 preg nant ewes. After a 5-day recovery period, fetuses received a 24-h infu sion of either norepinephrine (1 mu g/kg.min, n = 5), epinephrine (0.2 5 mu g/kg min, n = 5), or vehicle (normal saline, n = 4). Fetal caroti d arterial samples were collected at specified intervals throughout th e infusion for the determination of blood glucose concentrations, plas ma catecholamine concentrations by HPLC, insulin, and glucagon concent rations by RIA, and IGFBP levels by Western ligand blotting. After 24 h, the ewe and fetus were killed and selected fetal tissues (liver and kidney) were collected, and analyzed for IGFBP mRNA levels by norther n blotting followed by laser densitometric quantification. Plasma cate cholamine concentrations were increased in treated fetuses to levels t hat may be expected in fetuses subjected to prolonged hypoxia. In epin ephrine and norepinephrine infused fetuses, blood glucose and plasma g lucagon concentrations were increased significantly, whereas plasma in sulin concentrations were decreased significantly. Norepinephrine and epinephrine infusions increased IGFBP-1 levels significantly (2- to 5- fold) in fetal plasma within 8-12 h, and the time course pattern of el evation of plasma IGFBP-1 levels was similar to that observed in prolo nged hypoxia. After 24 h of either norepinephrine or epinephrine infus ion, IGFBP-1 mRNA levels in the liver of fetuses were increased signif icantly (5- to 7-fold) compared to those of vehicle infused fetuses. I GFBP-2, -3, and -4 levels in fetal plasma were not affected by either infusion, nor were IGFBP-2 mRNA levels in fetal liver and kidney. Our results indicate for the first time that the infusion of cAMP generati ng compounds (catecholamines) induce the expression of IGFBP-1 gene in the fetal liver in vivo, and that the increase in synthesis and relea se of IGFBP-1 during prolonged fetal hypoxia may result from a sustain ed elevation in circulating catecholamine concentrations.