Effects of a warm environment on energy balance in obese (ob/ob) mice.

Low rates of thermoregulatory heat production associated with low metabolic activity of brown adipose tissue, the primary site of thermoregulatory heat production, contribute substantially to the high efficiency of energy retention in obese (ob/ob) mice housed at 20 degrees C to 28 degrees C. To eliminate the need for thermoregulatory heat production lean and ob/ob mice were housed at 34.5 degrees C. At this temperature ob/ob mice still retained energy with a greater efficiency than lean littermates. Next, we investigated the possibility that the high efficiency of energy retention in ob/ob mice housed at 34.5 degrees C was related to depressed dietary-induced thermogenesis associated with low metabolic activity of brown adipose tissue. The sympathetic nervous system is a primary regulator of brown adipose tissue metabolism. Therefore, rates of norepinephrine (NE) turnover in brown adipose tissue, as an indicator of sympathetic nervous system stimulation of the tissue, were measured. Lean and ob/ob mice housed at 34.5 degrees C had equally low rates of NE turnover in their brown adipose tissue. Thus, the high efficiency of energy retention in ob/ob mice maintained at 34.5 degrees C is caused by factors other than low sympathetic stimulation of brown adipose tissue.