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 34.996, 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 far. 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.