. Effects of acute hyperinsulinemia on insulin signal transduction and glucose transporters in ovine fetal skeletal muscle. Am J Physiol Regul Integr Comp Physiol 288: R473-R481, 2005. First published November 11, 2004; doi:10.1152/ajpregu.00405.2004.-To test the effects of acute fetal hyperinsulinemia on the pattern and time course of insulin signaling in ovine fetal skeletal muscle, we measured selected signal transduction proteins in the mitogenic, protein synthetic, and metabolic pathways in the skeletal muscle of normally growing fetal sheep in utero. In experiment 1, 4-h hyperinsulinemiceuglycemic clamps were conducted in anesthetized twin fetuses to produce selective fetal hyperinsulinemia-euglycemia in one twin and euinsulinemia-euglycemia in the other. Serial skeletal muscle biopsies were taken from each fetus during the clamp and assayed by Western blot for selected insulin signal transduction proteins. Tyrosine phosphorylation of the insulin receptor, insulin receptor substrate-1, and the p85 subunit of phosphatidylinositol 3-kinase doubled at 30 min and gradually returned to control values by 240 min. Phosphorylation of extracellular signal-regulated kinase 1,2 was increased fivefold through 120 min of insulin infusion and decreased to control concentration by 240 min. Protein kinase B phosphorylation doubled at 30 min and remained elevated throughout the study. Phosphorylation of p70 S6K increased fourfold at 30, 60, and 120 min. In the second experiment, a separate group of nonanesthetized singleton fetuses was clamped to intermediate and high hyperinsulinemic-euglycemic conditions for 1 h. GLUT4 increased fourfold in the plasma membrane at 1 h, and hindlimb glucose uptake increased significantly at the higher insulin concentration. These data demonstrate that an acute increase in fetal plasma insulin concentration stimulates a unique pattern of insulin signal transduction proteins in intact skeletal muscle, thereby increasing pathways for mRNA translation, glucose transport, and cell growth. insulin signaling; fetal growth; glucose transporter 4 IN ADULT ANIMALS and humans, insulin has primarily a metabolic role to regulate skeletal muscle glucose uptake with less effect on skeletal muscle protein synthesis than in rapidly growing tissues. Although increased serum insulin concentration in the fetal sheep has been shown to increase fetal skeletal muscle glucose uptake, especially at pharmacological doses (17,18,20,21), insulin in the rapidly growing fetus seems to act largely as an anabolic hormone, promoting skeletal muscle protein synthesis and growth. In human fetuses, abnormal insulin concentrations contribute to variations in fetal growth; excessive insulin promotes fetal overgrowth, whereas insufficient insulin leads to fetal growth restriction. Animal studies have produced experimental deficiencies in insulin secretion in fetal lambs, resulting in fetal growth restriction and limited carcass protein accretion (27). Also, it has been shown in the neonatal pig that insulin action to promote s...