Action of long(R3)-insulin-like growth factor-1 on protein metabolism in beef heifers

Insulin-like growth factor-1 (IGF-1) is perhaps the most important endogenous 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 i.v. infusion of the IGF-1 analog Long(R3)-IGF-1 on protein metabolism in beef heifers that were slowly losing liveweight because of restricted feeding. There was a tendency for both whole-body protein and skeletal muscle protein to be conserved in Long(R3)-IGF-1-treated heifers. Long(R3)-IGF-1 administration markedly reduced the plasma concentrations of all amino acids measured and glucose. There was a significant change in the profile differences of endogenous plasma IGF-1 concentrations 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), whereas the test group decreased dramatically (by about 140 ng/ml). Increased plasma concentrations of a 31-32-kDa IGF-binding protein (possibly IGF-binding protein-1) in the treated group was detected by radioligand blot. We found that Long(R3)-IGF-1 infusion tended to preserve whole-body and muscle protein in beef heifers on a low-quality diet, and suggest that further investigation of this treatment may provide an alternative approach to reducing weight loss during the dry season.