Ventilatory behavior and metabolism in two strains of obese rats

This study examined differences in metabolism and ventilatory responsiveness in the Zucker (Z) and Koletsky (K) rat which each carry a different recessive mutation of the leptin receptor gene. The Null hypothesis was that the obese (homozygous) rats from the strains would not differ among the variables assessed. Male and female rats of obese and lean phenotypes were studied, with 5-6 animals in each group. Animals of the same age were assessed for ventilation and metabolism by whole-body plethysmography and the open circuit method. During quiet wakefulness, each animal was exposed to 5 min presentations of: room air; 10% O-2/bal N-2; 100%, O-2; room air, and 7%, O-2/93'%, O-2. Differences in metabolism, independent of phenotype included: K <: Z for RQ in both lean and obese rats; oxygen consumption in obese K > Z females and Z > K for males; CO2 production in obese K < Z. Some differences in breathing were independent of obese phenotype, with K < Z in frequency (f) for air (P<0.02), and persisting with each chemical challenge: for hypoxia (P < 0.01); hyperoxia (P < 0.002); and hypercapnia (P < 0.001). Factors dependent on obesity were K K often for every challenge (P < 0.001). A higher f and VE in Z compared with K rats was present in both genders, and persisted with each challenge. In conclusion, obese rats from these two strains do not breathe the same, even when age, weight, body mass index, and diet are alike. We conclude that that factors other than fat accumulation contribute to the expression of respiratory control and ventilation in obesity in the rat. (C) 2001 Elsevier Science B.V. All rights reserved.