Blood flow to brown adipose tissue and norepinephrine- induced calorigenesis in physically trained rats

Cardiac output (CO) and regional blood flow to various tissue were measured by means of gamma-labeled plastic microspheres (15 +/- 3 micrometers) injected into the left ventricle during 30-min infusion of l-norepinephrine (NE:4 micrograms(kg.min)) or the same amount of saline (11 microliters/min per animal) in rats physically trained by 2-hr swimming in water of 32 (SW-32), 36 (SW-36), and 38 degree C (SW-38) for 10-11 weeks and in control (CT) rats. Body weight and skinfold thickness were significantly less in SW-36 and SW-38 than CT rats. The weights of heart and adrenal glands significantly increased in the trained rats in comparison with CT rats. The average weight of brown adipose tissue (BAT) was significantly greater in SW-32 and significantly less in SW-36 and SW-38 as compared with CT rats. Infusion of NE induced significant increases in oxygen consumption (VO2), colonic temperature, heart rate, CO, blood flow to BAT, and mean blood pressure in anesthetized rats. The increases in VO2 and CO were significantly greater in SW-32 and SW-36 than in CT rats. The increases in blood flow to BAT were significantly greater in SW-32, SW-36, and SW-38 than in CT rats. These results indicate that physical training per se enhances calorigenic response to NE supported by an increased cardiac output and blood flow to the dominant calorigenic tissues such as BAT in rats.