Arginine and mixed amino acids increase protein accretion in the growth-restricted and normal ovine fetus by different mechanisms

Protein metabolism may be perturbed in intrauterine growth restriction (IUGR). Arginine is indispensable for growth and nitrogen balance in young mammals. Fetuses with IUGR therefore may benefit from arginine supplementation. The purpose of this study was to determine 1) the effects of IUGR on protein metabolism in the ovine fetus and 2) the effects of arginine or mixed amino acid (AA) infusion on protein metabolism in these fetuses. Pregnant ewes and their fetuses were catheterized at 110 d gestation and randomly assigned to control or IUGR groups. IUGR was induced by repetitive placental embolization. Parameters of fetal protein metabolism were determined from [ring-²H ₅]phenylalanine kinetics at baseline and in response to a 4-h infusion of either arginine or an isonitrogenous AA mixture. There were no differences in protein metabolism between control and IUGR groups either at baseline or in response to arginine or AA treatment. Both arginine and AA infusion increased fetal protein accretion in both groups. Arginine did this by decreasing protein turnover, synthesis, and breakdown. AAs increased protein turnover and synthesis while decreasing protein breakdown. AA infusion resulted in a significantly higher increase in protein accretion than arginine infusion. Thus, in the ovine fetus, placental embolization has no clear effect on protein metabolism. Arginine and AAs both, stimulate protein accretion but do so in distinctly different ways. Mixed AA infusion has a greater effect on protein accretion than arginine alone and therefore may be a better strategy for stimulating fetal growth.