Effects of Carotid Sinus Nerve Stimulation on the Coronary Circulation of the Conscious Dog

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Abstract
Control of the coronary circulation by the carotid sinus was studied in intact, unanesthetized dogs instrumented with Doppler ultrasonic flow probes on the left circumflex coronary artery, miniature pressure gauges in the aorta, and stimulating electrodes on the carotid sinus nerves. A radiofrequency pacemaker was used to stimulate the nerves in dogs at rest, during sleep, exercise, and after autonomic blockade. Thirty-second periods of stimulation in the resting conscious dog resulted in an average decrease in aortic pressure of 28%, an average decrease in mean coronary flow of 7%, while heart rate decreased by 13% at the beginning of stimulation and then returned to control levels. Mean and late diastolic coronary resistances decreased by an average of 22% from control. Similar results occurred with carotid sinus nerve stimulation during sleep and during treadmill exercise. Combined beta-receptor blockade with propranolol and atropine prevented the changes in heart rate with carotid sinus nerve stimulation but not the decrease in arterial pressure or the coronary dilatation. After alpha-receptor blockade with phenoxybenzamine or sympathetic blockade with guanethidine, coronary dilatation was not observed with carotid sinus nerve stimulation. Thus sympathetic constrictor tone is present in the resting conscious dog and the coronary dilatation observed with electrical stimulation of the carotid sinus nerves is due to a reduction in resting sympathetic constrictor tone.