SYMPATHETICALLY AND NONSYMPATHETICALLY MEDIATED ONSET OF EXCESS FAT DEPOSITION IN ZUCKER RATS

To evaluate whether a low sympathetic activity is essential for the onset of Zucker rat obesity, we artificially reared fa/fa and Fa/fa littermates from 4 days of age with identical amounts of milk containing norepinephrine (NE). The NE concentration was adjusted to keep the mass-specific metabolic rate (MR) roughly either 100 or 50% above its thermoneutral level. At both doses, no genotype difference in total MR occurred before day 16, but between 16 and 21 days of age, total MR of fa/fa pups was 10% less than that of their Fa/fa littermates. The 21-day-old fa/fa pups had similar to 1 g more body fat than their lean littermates, and this was energetically accounted for to 50-80% by their lower MR and to 10-25% by the reduced growth of their fat-free dry mass. We conclude that factors not dependent on decreased sympathetic outflow trigger the onset of excess fat deposition in the 3rd postnatal wk, even if sympathetic activity was high throughout the previous weeks during which excessive fat deposition is mainly dependent on decreased thermoregulatory thermogenesis.