Effect of rhIGF-I infusion on whole fetal and fetal skeletal muscle protein metabolism in sheep

Insulin-like growth factor I (IGF-I) has been shown to have significant ana bolic effects in the regulation of fetal protein metabolism. To investigate the tissue-specific effects of IGF-I on fetal skeletal muscle metabolism, we infused recombinant human (rh) IGF-I directly into the hindlimb of nine chronically catheterized, late-gestation fetal sheep. Substrate balance and amino acid kinetics were measured across the hindlimb and were compared wi th the effects at the whole body level before and during a 3-h infusion of rhIGF-I into the external iliac artery at 150 mu g/h. Infusion of rhIGF-I r esulted in increases in IGF-I concentrations by 2- to 5.75-fold in the ipsi lateral iliac vein and by nearly 3-fold in the abdominal aorta. In the stud y limb, IGF-I had no effect on protein synthesis (phenylalanine rate of dis posal 0.88 +/- 0.13 before vs. 0.73 +/- 0.19 mu mol/min during IGF-I) or br eakdown (phenylalanine rate of appearance 0.67 +/- 0.13 before vs. 0.60 +/- 0.17 mu mol/min during IGF-I) and did not alter net phenylalanine balance. IGF-I also did not affect hindlimb oxygen or glucose uptake. In contrast, at the whole body level, the rate of appearance of leucine, indicative of f etal protein breakdown, decreased during IGF-I infusion (rate of appearance of leucine 41.1 +/- 3.3 to 37.6 +/- 2.7 mu mol/min) as did fetal leucine o xidation (8.4 +/- 0.8 to 6.8 +/- 0.6 mu mol/min). There was no change in th e umbilical uptake of leucine, and although not statistically significant, fetal leucine accretion increased 2.4-fold. These results provide further e vidence that IGF-I promotes fetal protein accretion; however, its site of a ction is in tissues other than skeletal muscle.