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Metabolism at Thermoneutrality and in the Cold is Reduced in the Neonatal Preobese Zucker Fatty (fa/fa) Rat¹

Departments of Animal Physiology and Nutrition and Food Intake Laboratory, University of California, Davis, California 95616

Minimum and maximum rates of oxygen consumption (Vo₂) 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, Vo₂ 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 Vo₂ 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.

KEY WORDS: • obesity • energy expenditure • Zucker rat • neonate • brown adipose tissue

¹ This work was supported in part by National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases research grants AM-32907 and AM 18899, training grant T32 AM-HL-07355 and a Jastro-Shields grant from the University of California, Davis, to P. J. B.

² To whom requests for reprints should be addressed.

Manuscript received 4 September 1985. Revision accepted 28 July 1986.