Action of Long(R-3)-insulin-like growth factor-1 on protein metabolism in beef heifers

Insulin-like growth factor-1 (IGF-1) is perhaps the most important endogeno us factor controlling growth. Most studies to date in livestock have shown that IGF-1 has greatest efficacy when animals are in a catabolic state. We have determined the effects of an iv infusion of the IGF-1 analog Long(R-3) -IGF-1 on protein metabolism in beef heifers that were slowly losing livewe ight because of restricted feeding. There was a tendency for both whole-bod y protein and skeletal muscle protein to be conserved in Long(R-3)-IGF-1-tr eated heifers. Long(R-3)-IGF-1 administration markedly reduced the plasma c oncentrations of all amino acids measured and glucose. There was a signific ant change in the profile differences of endogenous plasma IGF-1 concentrat ions during the 8-hr infusion period, with plasma IGF-1 decreasing sharply in the test group. There was a significant difference in mean profiles for plasma IGF-2 between the test and control groups. Overall, plasma IGF-2 for the control group decreased only slightly over time (about 40 ng/ml), wher eas the test group decreased dramatically (by about 140 ng/ml). Increased p lasma concentrations of a 31-32-kDa IGF-binding protein (possibly IGF-bindi ng protein-1) in the treated group was detected by radioligand blot. We fou nd that Long(R-3)-IGF-1 infusion tended to preserve whole-body and muscle p rotein in beef heifers on a low-quality diet, and suggest that further inve stigation of this treatment may provide an alternative approach to reducing weight loss during the dry season. (C) Elsevier Science Inc. 1999.