SUMMARY We studied the effect of section of both carotid sinus nerves on arterial blood pressure in nine unanesthetized dogs. Four to seven recording sessions were conducted on each dog before and after sinus nerve section (SNS). During each session, histograms of mean arterial pressure during each heart cycle and of heart rate were assembled over 75 minutes. Then, responses to rapid aortic injection of nitroglycerin and phenylephrine were studied. Mean blood pressure was 94.86 ± 11.37 mm Hg with arterial baroreceptors intact. After SNS, mean blood pressure was 92.55 ± 9.82 mm Hg. Mean heart rate was 88.64 ± 11.97 beats/min in the intact dog and 86.20 ± 10.24 beats/min after SNS. Differences in mean blood pressure and heart rate were not significant (P > 0.05). The standard deviation of beat-by-beat mean arterial pressure was slightly, but not significantly, greater following SNS. The standard deviation of heart rate was slightly and significantly smaller (P < 0.02) after SNS. Blood pressure decreases following nitroglycerin were identical before and after SNS. There was a greater rise in blood pressure following phenylephrine after SNS. Heart rate changes following phenylephrine and nitroglycerin were smaller after SNS, but significant (P < 0.05) only for nitroglycerin. The dog with carotid sinus nerves sectioned regulates its mean blood pressure at the same level as the intact dog. Aortic baroreceptors reflexly compensate for both increased and decreased arterial pressure.THE BARORECEPTORS in the carotid sinus and in the aortic arch are usually exposed to the same arterial pressure. The baroreceptor reflex responses from each of these two sites have been compared in the anesthetized dog by changing pressure at one baroreceptor site while maintaining constant or eliminating baroreceptor activity from the other site. Although these responses from the two baroreceptor sites are qualitatively similar, several studies indicate that there are marked quantitative differences. "4 These studies indicate that the most sensitive region (pressure at which the greatest reflex changes are seen), the threshold (lowest pressure at which reflex responses are seen), and the saturation level (pressure at which there is no further response to increased pressure) are higher when pressure changes are applied to the aortic receptors than for pressure changes to the carotid receptors. In most cases, the change in peripheral resistance for a given change in pressure at the baroreceptor site (resistance gain) is much greater for the carotid than for the aortic baroreceptors.2 ' 5 In these studies, the pressure at the baroreceptors was nonpulsatile. However, several investigators concluded that these differences in response from the two baroreceptor sites are also present when the pressure at the baroreceptors is pulsatile. If these differences in reflex response to pressure changes at the carotid sinus and the aortic arch persist in the awake dog, the aortic baroreceptors should be func-

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Studies were conducted on unanesthetized dogs (1) in the control state, (2) after carotid sinus denervation plus section of the cervical aortic nerves (SCAD), and (3) after carotid sinus denervation plus section of intrathoracic vegal branches which innervate arterial and cardiopulmonary baroreceptors, the heart, lungs, and other structures (STD). Mean values and standard deviations of blood pressure and heart rate were measured during many 75-minute recording sessions. The control mean arterial pressure was 94.8 +/- 8.9 mm Hg. The mean pressure after SCAD was 105.5 +/- 9.5 mm Hg, 10.0 mm Hg higher (P less than 0.05) than in the control state, whereas after STD, the mean pressure was 119.5 +/- 16.8 mm Hg, 25.3 mm Hg higher (P less than 0.001) than in the control state. The standard deviation of pressure was increased (P less than 0.01) by either denervation procedure. The mean pressure after STD was higher than after SCAD (P less than 0.05). Ten of 12 animals with SCAD showed residual baroreceptor reflexes (seven from intrathoracic receptors) whereas, after STD, six of 11 animals showed reflexes (one from intrathoracic receptors). SCAD only occasionally produces denervation as complete as that produced by STD. A larger increase in arterial pressure follows a more complete denervation of vagally innervated baroreceptors. We believe that our procedures do not denervate all arterial baroreceptors. Chronic denervation of arterial baroreceptors leads to a widely varying, elevated arterial pressure. The increase in pressure has persisted more than 1 1/2 years.

SUMMARY After cervical aortic nerve section, mean arterial pressure in the unanesthetized dog increased by an average of 7.4 mm Hg. Following a more extensive denervation of aortic arch receptors by section of intrathoracic vagal branches, arterial pressure increased by 16.7 mm Hg. The above changes were seen in the stable state after the effects of surgery had disappeared. In both cases carotid baroreceptors were functional. After administration of nitroglycerin and phenylephrine subsequent to either denervation procedure, blood pressure changes were larger and heart rate responses were smaller than in the control state. The unanesthetized dog regulates mean arterial pressure at a higherthan-normal pressure after aortic baroreceptor denervation. Reflexes from the aortic baroreceptors continuously participate in the normal control of mean arterial pressure. Section of the cervical aortic nerves only partially denervates aortic baroreceptors. Our findings may be relevant to human essential hypertension. Ore Res 45: 26-34, 1979

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