A COMPARISON OF CHANGES IN WHOLE-BODY AND SKIN AMINO-ACID-METABOLISM OF SHEEP IN RESPONSE TO 24 H CONTINUOUS INFUSIONS OF VARIANTS OF INSULIN-LIKE-GROWTH-FACTOR-1

Because of the economic significance of wool to many sheep production systems, attempts to partition amino acids towards skin and wool prote in synthesis have included both nutritional and hormonal methods of ma nipulation. A variant of insulin-like growth factor 1 (IGF-1) has prev iously been shown to transiently increase protein synthesis in the ski n of sheep and the current study extended that work by comparing the e ffects of a 24 h, close-arterial, skin infusion of IGF-1, in the form of either recombinant human (rhIGF-1) or an extended variant (LR(3)IGF -1), on both whole body and skin amino acid metabolism adult, castrate d Romney sheep, with three animals allocated to each treatment. There were no differences in food intake between the two treatment groups. T he plasma concentration of immunoreactive IGF-1 of sheep infused with rhIGF-1 increased (P < 0.01) with time of administration, but decrease d (P < 0.05) after 24 h of LR(3)IGF-1 infusion. Administration of both IGF-1 variants caused a substantial and sustained decrease in arteria l insulin to less than 50% (P < 0.05) of pre-infusion values, while ar terial plasma glucose concentrations were only reduced by 7%. Alterati ons in whole body and skin protein metabolism were assessed from conti nuous infusions of mixed [U-C-13] AA, [2,6 ring H-3]phenylalanine and [S-35]cysteine. Within 4 h both AA concentrations and whole body plasm a ILR of essential and non-essential AA were decreased (P < 0.05 for s even AA) by IGF-1 infusions. Both IGF-1 variants caused acute increase s (P < 0.05) in skin blood flow and, for 13 of the 15 AA measured, iso topic transfers (range 50-220%; P < 0.05 for cysteine and tyrosine), w hich probably reflect increased protein synthesis. By 24 h skin blood flow, AA uptake and protein synthesis had returned to pre-infusion val ues. Strategies based on exogenous application, or enhanced endogenous production, of IGF-1 are unlikely, therefore, to produce persistent a nabolic responses.