MUSCLE AND WHOLE-BODY METABOLISM AFTER NOREPINEPHRINE

The effect of an infusion of norepinephrine (0.42 nmol.kg(-1).min(-1)) on energy metabolism in the whole body (using indirect calorimetry) a nd forearm muscle was assessed by using indirect calorimetry and the a rteriovenous forearm catheterization techniques in eight healthy young male adults. The activity of the triglyceride-fatty acid cycle, which mainly operates in nonmuscular tissues, was also assessed by measurin g glycerol turnover using [H-2(5)]glycerol (to indicate lipolysis) and indirect calorimetry (to indicate net fat oxidation). Norepinephrine increased whole body oxygen consumption by almost 10% (P < 0.01), but the estimated oxygen consumption of muscles tended to decrease. Muscle blood flow (measured by Xe-133) and forearm blood flow (measured by s train-gauge plethysmography) were not significantly affected by norepi nephrine, but the rate of uptake of nonesterified fatty acids and beta -hydroxybutyrate increased severalfold (P < 0.05), whereas that of glu cose did not. The activity of the triglyceride-fatty acid cycle increa sed fourfold after norepinephrine administration, having a marginal ef fect on resting energy expenditure (similar to 1.5%) but accounting fo r similar to 15% of the increase in whole body energy expenditure. Thi s study provides no evidence that skeletal muscle is an important site for norepinephrine-induced thermogenesis and suggests that an increas e in the activity of the triglyceride-fatty acid cycle contributes to the norepinephrine-induced increase in energy expenditure of nonmuscul ar tissues.