Relationship between serum leptin immunoreactivity and body fat mass as estimated by use of a novel gas-phase Fourier transform infrared spectroscopy deuterium dilution method in cats

Objective - To validate a recently developed commercially available leptin radioimmunoassay (RIA) for use with feline serum and evaluate the relationship between serum leptin concentrations and body fat mass in domestic cats. Animals - 19 sexually intact male specific-pathogen-free domestic cats that weighed 3.8 to 7.1 kg and were 1.1 to 3.5 years old. Procedure - Specificity for feline leptin was evaluated by use of gel filtration chromatography and reverse-phase high-performance liquid chromatography fractionation of serum. Body fat mass was determined by use of the deuterium oxide (D₂O) dilution method. Serum water D₂O enrichment was measured by use of gas-phase Fourier transform infrared spectroscopy. Results - Body fat mass and percentage body fat ranged from 0.3 to 2.3 kg and 7.5 to 34.9%, respectively. Serum leptin concentrations were lower in the unfed versus the fed state and ranged between 1.6 and 4.9 ng/ml human equivalent (HE); mean ± SD value was 2.9 ± 0.2 ng/ml HE. Leptin concentrations increased with increasing body fat mass and percentage of body fat. Conclusions-Leptin is in the serum of domestic cats in free (> 78%) and apparently bound forms. The relationship between body fat and serum leptin concentration was similar to that observed in humans and rodents and indicative of a lipostatic role for leptin in cats. Cats that have an overabundance of body fat appear to be less sensitive to the weight-normalizing action of leptin than cats of ideal body condition. (Am J Vet Res 2000;61:796-801) From the Department of Molecular Biosciences, School of Veterinary Medicine (Backus, Rogers) and the Department of Nutrition, College of Agricultural and Environmental Sciences (Havel), University of California, Davis, CA 95616; and Linco Research, St Louis, MO 63301 (Gingerich).