Cardiovascular responses to unilateral and bilateral stimulation of rabbit aortic nerves

Reflex heart and mean arterial pressure (MAP) responses during unilateral and bilateral electrical stimulation of the central end of the cut aortic nerves were studied in 14 anesthetized closed-thorax rabbits. During control of carotid intrasinus pressure (ISP), with ISP = MAP, heart rate was 248 +/- 12 beats/min and fell -79 +/- 14, -61 +/- 16, and -117 +/- 16 beats/min during left (LAN), right (RAN), and bilateral (BAN) nerve stimulation. MAP was 79 +/- 5 mmHg and fell -57 +/- 4 (LAN), -46 +/- 6 (RAN), and -65 +/- 4 mmHg (BAN). Responses were also determined following blockade of cardiac vagal efferents (atropine) and then vagotomy (n = 4) or vagotomy alone (n = 10). Results indicated that cardiac parasympathetic effects of LAN and RAN stimulation were additive, whereas the respective summation of cardiac and arterial vascular sympathetic effects were mutually inhibitory. BAN stimulation at low (25 mmHg) and high (greater than or equal to 125 mmHg) ISP levels resulted in different magnitudes of MAP and heart rate responses before and after vagotomy and beta-receptor blockade. These results indicated that summation was mutually inhibitory for cardiac and vasomotor sympathetics when maximal stimulation of opposite influence was applied to aortic and carotid afferents. However, arterial baroreceptor afferents may summate differently at more normal blood pressure conditions.