Calorigenic and cardiovascular responses to norepinephrine in anesthetized and unanesthetized control and cold-acclimated rats.

Cardiac output (Q) and regional blood flow to various tissues were measured by gamma-labeled plastic microspheres (15 +/- 3 micrometer) injected into the left ventricle at rest and during 30-min infusion of norepinephrine (NE infusion, 2 microgram/kg.min) in either anesthetized or unanesthetized control (CT) and cold-acclimated (CA) rats. Besides cardiovascular functions, oxygen consumption (Vo2) and colonic temperature (Tcol) were measured. Under anesthesia (the rat was immersed into warm and initial Tcol was kept at around 37.6 degrees C), NE infusion produced significant increases in V02, Q and blood flow to most of the tissues measured both in CT and CA rats. The increases in V02 and blood flow to the brown adipose tissues (BAT) were significantly higher in CA rats. Both in CT and CA rats, heart rate (HR), blood pressure (BP), Q tissue blood flow, and V02 at rest were significantly higher in unanesthetized conditions (Ta 17 degrees C) than in anesthetized condition. In unanesthetized rats, these resting values were significantly higher in CT than in CA. In CT rats, NE infusion produced decreases in HR, Q and blood flow to most of the tissues measured. The reductions in blood flow to BAT and skeletal muscles were particularly consistent. In CA rats, NE infusion produced significant increases in Q and blood flow to most of the tissues measured particularly in BAT. Blood flow to the ear was decreased. Vo2 and Tco1 increased. The mass of BAT in CA rats was twice as much as that in CT animals. The result indicates that, in contrast to CT rats, CA rats can increase cardiac output and blood flow to BAT with NE in either anesthetized or unanesthetized conditions, which might be responsible for the significant calorigenic response to NE in these rats.