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.
In order to evaluate the relative importance of groups of tissues containing lipoprotein lipase (LPL) in the removal of serum triacylglycerols during development, LPL activity was determined in cardiac and skeletal muscle, brown (BAT) and white adipose tissue (WAT), lung and kidney of rats aged 0-60 days, either fed or fasted for 6 h. On fasting, LPL activity did not change in lung and kidney, decreased in WAT except at 14 days and to a lesser extent in BAT, whereas muscle LPL decreased at 0, 3, and 7 days and increased from 14 days onward. The fasting-induced changes in serum triacylglycerol concentrations in suckling pups could be attributed to corresponding changes in muscle LPL. In the adult, the main contributor to total LPL activity was WAT in fed rats and muscle in fasted rats, as expected. In suckling pups, however, muscle LPL contributed 63-85% of the total and WAT less than 25%, whatever the nutritional state. The results strongly suggest that clearing of circulating chylomicrons during suckling largely depends on muscle LPL.
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