Effects of acute hyperinsulinemia on insulin signal transduction and glucose transporters in ovine fetal skeletal muscle

Abstract

. 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...

Insulin is required for amino acid stimulation of dual pathways for translational control in skeletal muscle in the late-gestation ovine fetus

“…Previous studies in late-gestation fetal sheep have shown that acute hyperinsulinemia at pharmacological doses (ϳ110 U/ml) activates the insulin signal transduction pathway through PI3-kinase/Akt, leading to substantial upregulation of p70 S6k and 4E-BP1 and assembly of the translation initiation complex eIF4F (2,40). Consistent with these results, we found a sevenfold increase in the phosphorylation of Akt, p70 S6k , and rp S6 with high-dose fetal insulin infusion.…”

Skeletal muscle glucose transporter protein responses to antenatal glucocorticoids in the ovine fetus

“…Similarly, an increase in GLUT 4 concentrations of the extensor digitorum lateralis and biceps femoris muscles was observed. Although the changes in GLUT concentrations that we observed were not large, previous work by some of us has demonstrated that even small changes in GLUT concentrations in the ovine fetus can have a major impact on fetal glucose homeostasis (Anderson et al 2005).…”

Acute supplementation of amino acids increases net protein accretion in IUGR fetal sheep

“…There are several possible mechanisms to account for this observation. First, fetal insulin concentrations as a result of the insulin infusion were much higher than those achieved in previous studies, and, by study design, the infusion period was for a shorter duration to capture acute upregulation of signal transduction in muscle (2). Moreover, the marked increase in fetal insulin concentrations achieved by the insulin clamp, while stimulating protein translation capacity in skeletal muscle, may not promote net protein turnover along the same time frame in the whole animal.…”