Metabolism at thermoneutrality and in the cold is reduced in the neonatal preobese Zucker fatty (fa/fa) rat

Minimum and maximum rates of oxygen consumption (V̇O₂) were determined in 2-, 6- and 8-d-old lean (Fa/?) and preobese (fa/fa or 'fatty') Zucker rats. On each study day, three or four naive littermate male pups were placed in individual respiration chambers submerged in a water bath and studied simultaneously over a range of ambient temperatures (24-35°C). The purpose of the cold exposure was to elicit maximal oxygen consumption and to determine the ability of the rat to maintain body temperature during cold stress. Minimum rates of oxygen consumption were observed at 33-34°C in both groups. As ambient temperature was reduced, V̇O₂ increased and rectal temperature decreased. Regression analysis revealed a significantly reduced oxygen consumption in the preobese pups within the thermoneutral zone and during cold exposure. A defect in energy expenditure for heat production was evident in the preobese (fa/fa) pups as indicated by their attenuated cold-induced rate of oxygen consumption and decreased ability to maintain body temperature during cold stress relative to their lean (Fa/?) littermates. Their lower V̇O₂ within the thermoneutral zone implies that the preobese (fa/fa) pups have a decreased metabolic rate even under conditions in which cold-induced thermogenic mechanisms are turned off.