Plasma leptin determination in ruminants: effect of nutritional status andbody fatness on plasma leptin concentration assessed by a specific RIA in sheep

A specific leptin RIA was developed to assess concentrations of leptin in o vine plasma, and was shown to be efficient with bovine and caprine plasma. A specific, high-affinity antibody was generated against recombinant ovine leptin which, when used in a competitive leptin RIA, provided valid estimat es of linearity (r=+0.989-0.998), recovery (102%), repeatability (13%) and limit of sensitivity (0.83 ng/ml for 100 mu l sample size). Serial dilution s of five ovine, bovine or caprine plasma samples showed good linearity and parallelism with the recombinant ovine leptin standard curve. A comparison of this RIA was made with a commercial 'multi-species' RIA kit using 56 ov ine plasma samples. Major differences were found in assay sensitivity. Non- lactating, non-pregnant, ovariectomized ewes were fed a ration for 65 days which provided 90 +/- 9% (control; n=12) or 39 +/- 2% of maintenance energy requirements (underfed; n=16) in order to analyse the respective effects o f body fatness (estimated by either an in vivo dilution technique or body c ondition scoring) and of nutritional status on plasma leptin concentration. There was a significant positive correlation between body fatness or body condition score and plasma leptin levels (r=+0.68, P<0.001 or r=+0.72, P<0. 001 respectively). When concentrations of leptin were assessed over time, u nderfed ewes exhibited a dramatic reduction in plasma leptin values (- 56%, P<0.001). These data provide strong evidence that, in sheep, the variation s in plasma concentrations of leptin are related to variations in body fatn ess (35%) and, to a lesser extent, in nutritional status (17%).