Hindlimb vascular responses to sympathetic augmentation during acute anemia

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Abstract
The effect of increased sympathetic activity on skeletal muscle blood flow during acute anemic hypoxia was studied in 16 anesthetized dogs. Sympathetic activity was altered by clamping the carotid arteries bilaterally below the carotid sinus. One group (n = 8) was .beta. blocked by administration of propranolol (1 mg/kg); a 2nd group (n = 8) was untreated. Venous outflow from the left hindlimb was isolated for measurement of blood flow and O2 uptake (.ovrhdot.VO2). After a 20-min control period, both carotid arteries were clamped (CC) for 20 min followed by a 20-min recovery period. The sequence was repeated after hematocrit was lowered to .apprx. 15% by dextran exchange for blood. Prior to anemia, CC did not alter cardiac output or limb blood flow in either group. After induction of anemia, hindlimb resistance was higher with CC in the .beta. block than in the no block group. Limb blood flow and .ovrhdot.VO2 fell in the .beta.-block group with CC during anemia. .beta. Block also prevented the additive increases in whole body .ovrhdot.VO2 seen with CC and induction of anemia. The increased vasoconstrictor tone that was obtained with .beta. block during anemia was evidently successful in redistributing the lower viscosity blood away from resting skeletal muscle, even to the point that muscle .ovrhdot.VO2 was decreased.

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