Enhancement of calorigenic response to cold and to norepinephrine in physically trained rats

Calorigenic response to cold and to norepinephrine (NE: 4 microgram . kg-1 . min-1) in physically fit (SW) rats was compared with control (CT) and cold-acclimated (CA) rats. Physical fitness was obtained by daily 3-hr swimming in water at 36 degree C for 6-8 weeks. Resting heart rate (HR) was significantly less in SW rats. Cold immersion (in water at 18 degree C up to the neck) increased oxygen consumption (VO2) significantly in CA and SW compared to CT rats (p less than 0.01). Cold immersion decreased colonic temperature (Tco1) in all groups. Despite the increased calorigenic response, SW rats cooled as quickly as CT rats in cold water. Body weight and skinfold thickness were significantly less in SW than in CT rats (p less than 0.01). Compared with CT rats, average weight of the interscapular BAT was significantly less in SW rats (p less than 0.05). Infusion of NE significantly increased VO2, Tco1, HR and mean aortic pressure in anesthetized rats. The increase in VO2 was significantly greater in CA (p less than 0.01) and SW (p less than 0.05) than in CT rats. Tco1 increased significantly in CA (p less than 0.05) and slightly in SW compared to CT animals.