498
Insulin sensitivity of the lipid metabolism of precalving dairy cows across a range of BCS
Insulin plays a central role during the transition period of dairy cows by influencing glucose, lipid and protein metabolism. At the adipose tissue, insulin stimulates lipogenesis and inhibits lipolysis thereby regulating the circulating NEFA concentration. Overconditioning is known to induce insulin resistance of the glucose metabolism in dairy cows. In the present study, we identified if factors related to adiposity (BCS, BFT, NEFA concentration during the dry period) influenced insulin sensitivity of the lipid metabolism in 8 healthy dairy cows at the end of pregnancy across a range of BCS (2.75-5).
Hyperinsulinemic euglycemic clamp tests were performed consisting of 4 insulin infusions: 0.1; 0.5; 2; 5 mU/kg/min. At regular time intervals during the infusions, blood glucose concentration was determined using a glucometer and the speed of a concomitant glucose infusion was adapted to keep blood glucose concentration constant. At the end of each infusion, a steady state (SS) was maintained for 30 minutes. During the SS, minor changes of the glucose infusion were necessary to maintain normal blood glucose level. During the SS, blood samples were taken at 10 minutes interval to determine SS insulin (SSIC) and NEFA (SSNEFA) concentrations. The SSIC was 8.77±3.04; 52.38±16.11; 339.04±122.01; 1411.5±500.08 µU/ml and the SSNEFA was 0.62±0.20; 0.26±0.08; 0.14±0.09; 0.11±0.08 mmol/l for the insulin infusions of 0.1; 0.5; 2; 5mU/kg/min respectively. The SSNEFA is the resultant of both the inhibitory effect of insulin on lipolysis and the stimulatory effect of insulin on lipogenesis. To correct for different basal NEFA levels, the NEFA lowering effect of insulin was calculated as % compared to basal values. Dose response curves were created using PROC NLIN in SAS to determine maximal effect and insulin dose needed to elicit half-maximal effect (logED50). Maximal effect and logED50 were respectively 0.90±0.07% and 1.24±0.29µU/ml. Effects on both parameters were analyzed using PROC MIXED in SAS with parity as random factor and BCS, BFT and NEFA concentration during the dry period as independent variables.
Maximal effect of insulin was negatively influenced by NEFA concentrations during the dry period (β=-0.3065; P<0.05) while the effect of BFT and BCS was not significant. None of the independent variables had a significant influence on logED50. It can be concluded that elevated NEFA concentrations during the dry period decrease the maximal effect of insulin on lipolysis and lipogenesis.
Keywords:
Insulin sensitivity, lipid metabolism, dairy cow