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

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Abstract
Cardiac output (.ovrhdot.Q) and regional blood flow to various tissues were measured by .gamma.-labeled plastic microspheres (15 .+-. 3 .mu.m) injected into the left ventricle at rest and during 30-min infusion of norepinephrine (NE infusion, 2 .mu.g/kg .cntdot. min) in either anesthetized or unanesthetized control (CT) and cold-acclimated (CA) rats. Besides cardiovascular functions, O2 consumption (.ovrhdot.VO2) and colonic temperature (Tcol) were measured. Under anesthesia NE infusion produced significant increases in .ovrhdot.VO2, .ovrhdot.Q and blood flow to most of the tissues measured both in CT and CA rats. The increases in .ovrhdot.VO2 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), .ovrhdot.Q, tissue blood flow and .ovrhdot.VO2 at rest were significantly higher under unanesthetized conditions (Ta 17.degree. C) than under 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, .ovrhdot.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 .ovrhdot.Q and blood flow to most of the tissues measured particularly in BAT. Blood flow to the ear was decreased. .ovrhdot.VO2 and Tcol increased. The mass of BAT in CA rats was twice as much as that in CT animals. In contrast to CT rats, CA rats can increase cardiac output and blood flow to BAT with NE in either anesthetized conditions, which might be responsible for the significant calorigenic response to NE in these rats.

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