Evidence of a defect in energy expenditure in 7-day-old Zucker rat (fa/fa).

The aim of the investigation was to see whether a defect in energy expenditure could be found in the Zucker rat at the onset of obesity. Obese (fa/fa) and lean (Fa/fa) 7-day-old pups were studied at three ambient temperatures. At 33 degrees C fa/fa pups showed a reduction in oxygen consumption, respiratory CO2 production, in vivo oxidation of injected [1-14C]palmitic acid, as well as in core temperature. When the pups were kept at 28 degrees C, the difference between genotypes was considerably accentuated, thus indicating a subnormal thermogenic response of the fa/fa pups to a mildly cold environment. At 20 degrees C, however, the metabolic rates dropped to the same low level, and the core temperature equilibrated with ambient temperature in both genotypes. The results demonstrate that the 1-wk-old fa/fa pup has a defect in thermoregulatory thermogenesis. The magnitude of the deficit in energy expenditure was more than adequate to account for the 50% greater fat content of 7-day-old fa/fa pups.