The cardiocardiac sympathetic reflex during coronary occlusion in anesthetized dogs

Cardiac sympathetic afferent fibers activated during coronary occlusion exert an excitatory influence on sympathetic discharge to the heart in cats after spinal cord section. The significance of this cardiocardiac sympathetic reflex response during myocardial ischemia in animals with an intact neuraxis is unknown. We studied the responses of efferent cardiac sympathetic nerve activity (CSNA), arterial pressure, and heart rate to coronary occlusion in two groups of dogs with cardiac sympathetic reflexes intact and with other reflex inputs affecting CSNA sectioned or controlled. In group I (n = 10), the vagi were sectioned, the spinal cord remained intact, and the carotid sinuses were isolated and perfused to maintain baroreceptor input constant. Coronary occlusion was performed at moderate and low basal levels of CSNA by setting carotid sinus pressure at 125-150 and 200 mm Hg, respectively. Under these conditions, CSNA was not altered by occlusion of either the anterior descending or the circumflex coronary artery. In group II (n = 4), the vagi were sectioned and the spinal cord was interrupted. In these dogs, CSNA increased significantly (61 +/- 19%) during coronary occlusion. These results show that an excitatory cardiocardiac sympathetic reflex can be demonstrated in dogs with spinal cords sectioned but not with spinal cords intact. This finding is consistent with the view that inhibitory bulbospinal pathways minimize the influence of the spinal cardiocardiac sympathetic reflex during myocardial ischemia in anesthetized dogs.