Circulatory changes during spontaneous motor activity: role of arterial baroreflexes

Systemic arterial pressure (SAP) and heart rate (HR) were continuously measured in rabbits during spontaneous motor activity such as postural change, exploration, grooming, and eating, and during interposed periods of inactivity within which SAP and HR were defined as basal. Six rabbits were observed for 2 h under each of 3 conditions of arterial baroreceptor afferents: all intact (B4); one carotid sinus intact (B1); all interrupted (B0). In B4, SAP and HR were above basal levels 82% of the time; in B1, 64% and 76% of the time, respectively; in B0, only 21% and 28% of the time, respectively. Supplementary experiments in another six rabbits established that the activity-associated falls of SAP in condition B0 were independent of HR, were not due to engagement of cardiac receptor reflexes, and were not abolished by autonomic ganglion blockade. We conclude that the transient rises of SAP and HR that occur during everyday activity are associated with partial or complete suppression of the reflex effects of arterial baroreceptor input, and we suggest that this is due to transient upward resetting of the arterial baroreflex.