The effects of insulin, using a 4 d hyperinsulinemic-euglycemic clamp, on plasma concentrations of hormone, metabolites, and binding proteins were evaluated in four Holstein dairy cows during wk 4 and 17 of lactation. Insulin was infused at 1 μg/kg/hr for 96 hr during the clamp period. Compared with the pre-clamp period, plasma insulin concentrations increased 7-fold and 4-fold during the clamp periods in early and mid-lactation, respectively. The total amount of glucose infused was higher (P < 0.05) during the clamp in early lactation. The clamp decreased plasma concentrations of non-esterified fatty acids (P < 0.001) during early lactation while differences in mid-lactation were minor. The clamp also decreased plasma concentration of β-hydroxybutyrate (P < 0.001), plasma urea nitrogen (P < 0.001), and true protein (P < 0.01) although the patterns of decline differed between early and mid-lactation. Growth hormone (GH) concentrations decreased (P < 0.001) and insulin-like growth factor-1 (IGF-1) increased (P < 0.01) during the clamp period suggesting a direct effect of insulin on the un-coupling of the GH/IGF-1 axis. Levels of IGF binding protein-2 (IGFBP-2) decreased (P < 0.01) during the clamp period. The relative proportion of IGFBP-2 decreased (P < 0.001) and that of IGFBP-3 increased (P < 0.001) during the clamp period. There were no interactions between the clamp period and stage of lactation on GH, IGF-1, or IGFBPs. Overall, most plasma variables measured were affected in the same way during the two clamps, but the pattern of change often varied with stage of lactation.
Bibliographical noteFunding Information:
Financial support for this research was provided by the Ministry of Food, Agriculture and Fisheries through the programme “Animal Production considering Product Quality and Environment”, project HUS97-2, the Danish Cattle Industry via the Finance Committee Cattle, and the Department of Animal Health and Welfare, Danish Institute of Agricultural Sciences. The authors wish to thank Novo Nordisk A/S, Bagsvaerd, Denmark for supplying us with the insulin used for infusion in this study, 125 I-IGF-1 tracer for Western ligand blotting and antibodies for the insulin and IGF-1 assays. Likewise, the authors wish to thank Radiometer Medical A/S, Copenhagen, Denmark for supplying us with sterile QS50™ arterial blood sampler (syringes) needed for this study. We furthermore wish to thank for the skillful help during sampling and analysis of samples given by the following laboratory technicians at the Danish Institute of Agricultural Sciences: D. Agnholt, B. Andersen, J.B. Clausen, I.M. Jepsen, L. Niklassen, J. Adamsen, H. Handll and H.M. Purup.