Enhanced calorigenesis in brown adipose tissue in physically trained rats

The calorigenesis occurring in brown adipose tissues (BAT) in response to infused norepinephrine [NE: 4 micrograms/(kg . min)] was estimated from the arteriovenous O2 difference across the interscapular brown adipose tissue and the rate of blood flow to BAT in physically trained (TR) and control (CT) rats. TR rats were trained by 2-hr daily swimming in agitated water at 36 degrees C for 18 weeks. The colonic temperature was maintained at approximately 37.5 degrees C throughout the training period. With NE infusion, the O2 content of arterial blood was unchanged, but that from Sulzer's vein significantly decreased from 12.5 to 2.5 vol% in TR and from 15.7 to 4.3 vol% in CT rats. The increase in O2 consumption in BAT with NE was estimated as 1.60 ml/min in TR and 0.83 ml/min in CT rats. The increase in the whole body O2 consumption with NE was 3.19 ml/min in TR and 2.29 ml/min in CT rats. The contribution of calorigenesis in BAT to the whole body calorigenic response to NE was estimated at 50% in TR and 36% in CT rats. These results directly demonstrated that physical training per se causes an enhancement of the calorigenic response of brown adipose tissues to norepinephrine in rats.