Blood flow and oxygen consumption in skeletal muscle during sympathetic stimulation

We studied the effects of physiological rates (0-4 Hz) of sympathetic stimulation on blood flow and oxygen consumption of extensor digitorum longus muscles isolated from anesthetized dogs. Observations were made with the preparation at rest and during isometric twitch exercise at 1 and 4 Hz produced by somatic nerve stimulation. Graded increases in sympathetic stimulation rate resulted in graded and sustained reductions in muscle blood flow in all cases. A given rate of sympathetic stimulation reduced muscle blood flow by nearly the same absolute amount regardless of muscle exercise rate. Consequently the curve relating muscle blood flow to muscle oxygen consumption was progressively shifted downward in a parallel fashion with graded increases in sympathetic stimulation rate. Sympathetic stimulation at 0.5 and 1 Hz reduced muscle oxygen consumption during 4-Hz exercise but failed to do so during rest or 1-Hz exercise. Thus even during heavy exercise, local metabolic mechanisms do not override sympathetic vasoconstriction sufficiently to prevent the latter from limiting muscle oxygen consumption. In addition, the functional consequences of sympathetic activation appear to be greater for heavily exercising muscle than resting muscle.