Effects of norepinephrine and alpha-block on O2 uptake and blood flow in dog hindlimb

Norepinephrine (NE) may increase skeletal muscle O2 demand at the same time that it restricts O2 transport by vasoconstriction. We prevented vasoconstriction with 3.0 mg/kg phenoxybenzamine (alpha-Bl) in 10 anesthetized, paralyzed dogs ventilated at constant rate. Hindlimb and whole-body O2 uptake (VO2) and blood flow were measured for a 20-min control period, 20 min of NE infusion at 1 micrograms . kg-1 . min-1 either intravenously (iv) or intra-arterially (ia), and 20 min of recovery. The sequence was repeated for the other route of infusion. A second group of 10 without alpha-Bl was treated the same. Cardiac output increased with both ia and iv infusions in the no-block group and was unchanged in the alpha-Bl group. Limb blood flow increased 25% during the first 5 min of iv but decreased 40% with ia infusion in the no-block group. Whole-body VO2 was significantly increased 9% in both groups by both routes of NE. Limb VO2 was significantly decreased in the no-block group at 5 min of NE infusion iv when limb blood flow was increased. Limb VO2 did not change significantly in the alpha-Bl group with NE by either route. Hindlimb skeletal muscle did not participate in or contribute to the calorigenic effect of NE on the whole body, and that lack of effect was not due to any effect of NE on rate or distribution of blood flow in the limb.