Effect of the autonomic nervous system on arteriolar tone in the experimental animal.

Evidence obtained from experiments involving perfusion of vascular beds and cardiac output studies with controlled filling pressure of the right heart before and during ganglionic blockade support the view that the heart of the dog is tonically stimulated by its sympathetic in-nervation. The blockade of this tonic stimulation to the heart permits a reduction in cardiac output. The fall in cardiac output results in a decrease in passive stretch of the arterial system which permits a passive rise in peripheral resistance thus compensating for the loss of arteriolar tone so that the peripheral resistance may be unchanged during ganglionic blockade despite a reduction in neurogenic arteriolar tone. UPON administration of ganglionic blocking agents to anesthetized animals, the blood pressure and the cardiac output fall considerably."w 2 Calculation of the total peripheral resistance discloses that the resistance remains essentially unchanged. Similar results have been found in the hyperten-sive patient.3 The failure of the peripheral resistance to fall after the administration of ganglionic blocking agents has raised the following questions: 1. Is significant arteriolar tone of sympathetic origin tonically present? 2. If present, do the ganglionie blocking agents fail to block such activity? 3. Is the fall in cardiac output accomplished by a decrease in venomotor tone which causes peripheral venous pooling resulting in a reduction in central venous pressure? 4. Is there another mechanism whereby the ganglionic blocking agents exert their activity? Because the blood vessel is essentially an elastic tissue, the fall in blood pressure produced by ganglionic blocking agents should cause a rise in resistance when the stretch on the blood vessel is reduced as a result of the passive narrowing of the vessels. The effect of changes in perfusion pressure on vessel dis-tensibility was tested by perfusing vascular and Welfare. 798 beds in the dog before and during ganglionic blockade. The perfusion experiments explained why the peripheral resistance failed to decrease after ganglionic blockade, but did not elucidate the mechanism whereby the reduction in cardiac output was accomplished. A second set of experiments was devised to investigate whether venous pooling was responsible for the decrease in cardiac output. Perfusion Experiments In the perfusion experiments a special pump* with tubing of suitable size to permit variation in blood flow over a wide range was employed. Perfusion was carried out either on the vascular bed supplied by the femoral artery or on the vascular bed below the kidney supplied by …