Plasma Leptin Turnover Rates in Lean and Obese Zucker RatsThis work was supported by Laboratoris SALVAT, SA, as well as by grants from the CIRIT of the Government of Catalonia and the Plan Nacional de Ciencia de los Alimentos (ALI96–1094) of the Government of Spain. Work was carried out within the frame of the EC Network on Metabolic Integration and Energy Control ERBCHRX-CT94–0490

Abstract Conscious female adult lean and obese Zucker rats were injected through the jugular vein with radioactive iodine-labeled murine leptin; in the ensuing 8 min, four blood samples were sequentially extracted from the carotid artery. The samples were used in a modified RIA for leptin, in which paired tubes received the same amount of either labeled or unlabeled leptin, thus allowing us to estimate both leptin levels and specific radioactivity. The data were used to determine the decay curve parameters from which the half-life of leptin (5.46 ± 0.23 min for lean rats and 6.99 ± 0.75 min for obese rats) as well as the size of its circulating pool (32 pmol/kg for lean rats and 267 pmol/kg for obese rats) and the overall degradation rate (96 fkat/kg for lean rats and 645 fkat/kg for obese rats) were estimated. These values are consistent with the hormonal role of leptin and the need for speedy changes in its levels in response to metabolic challenge.