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.

Modulation of rabbit carotid baroreflex during positive end-expiratory pressure.

Modification of carotid baroreflex heart and vascular responses during increased lung positive end-expiratory pressure (PEEP) were measured anesthetized aortic-denervated rabbits. Static carotid intrasinus pressure (ISP) was varied in increments of 12.5 mmHg over 25-140 mmHg during lung inflation conditions ranging from spontaneous breathing (SB) to positive-pressure respiration at 0.0-7.5 cmH2O PEEP. To distinguish cardiopulmonary vagally and nonvagally mediated influences, heart rate (HR), mean arterial pressure (MAP), cardiac output (CO), and total peripheral resistance (TPR) were measured and compared before and after sequential vagotomy and beta- and alpha-receptor blockade. When compared with responses of SB animals the HR-ISP relationship was not significantly altered during controlled respiration (CR) with PEEP before or after vagotomy. With CR and then PEEP, MAP-ISP relationship curves and linear slope values (ISP range 62-113 mmHg) were significantly depressed when compared with those of SB rabbits. Before vagotomy slope values were -1.61 (SB), -1.22 (CR + 0.0 cmH2O PEEP), and -0.82 (CR + 7.5 cmH2O PEEP); respective values after vagotomy were -2.26, -0.96, and -0.64. Results of CO and TPR responses during low ISP and PEEP demonstrated components of both vagally and nonvagally mediated influences from inflation sensitive cardiopulmonary receptors.U

Rabbit carotid baroreflexes after carotid sympathectomy, vagotomy, and beta blockade.

Factors that modify carotid vascular baroreflexes were studied in anesthetized rabbits after section of the cervical aortic nerves. We recorded steady-state reflex responses of heart rate (HR) and mean arterial pressure (MAP) to changes in isolated nonpulsatile carotid intrasinus pressures (ISP) over a wide range. These measurements and the procedures were repeated 1) after sympathetic denervation of the carotid sinuses by section of the cervical sympathetic nerve below the superior cervical ganglia; 2) after cervical vagotomy; and 3) after beta-receptor blockade by propranolol administration. The linearity of the MAP-ISP relation curve slope was determined between ISP levels of 63 and 100 mmHg. Comparison of results before and after sympathetic denervation of the sinuses indicated that the denervation significantly lowered the position of the MAP-ISP relation curve over a mid-ISP range. Subsequent vagotomy significantly elevated both HR-ISP relation curve and MAP-ISP relation curve. AFter beta-receptor blockade HR remained constant, whereas the slope of the MAP-ISP relation curve decreased to -1.34 from the slope of -2.26 before blockade. Comparison of results before and after beta blockade in the vagotomized animals suggested a substantial cardiac influence on carotid reflex control of MAP.