HEMODYNAMIC-EFFECTS OF ACUTE STRESSORS IN THE CONSCIOUS RABBIT

Chronically instrumented, conscious rabbits were used to test the hypo thesis that sensory stimulation with an airjet or oscillation produces differential hemodynamic changes that may be appropriate for an activ e or a passive behavioral response, respectively. Both stressors incre ased arterial pressure, central venous pressure, and hindquarters bloo d flow and produced visceral vasoconstriction. Neither stimulus altere d hindquarters conductance. Although air jet increased heart rate and cardiac output, oscillation did not. The two stressors affected arteri al baroreflex control of heart rate differently. Oscillation reset art erial pressure to a higher level with no change in heart rate maximum or minimum, whereas air jet reset both heart rate and arterial pressur e to higher levels. Neither stressor affected baroreflex sensitivity W e conclude that the conscious rabbit shows at least two distinct cardi ovascular responses when exposed to acute stressors. Airjet produces a cardiovascular response including tachycardia, which resembles the de fense reaction and appears appropriate for active defense or flight. T he response to oscillation, on the other hand, appears better suited f or a passive response such as ''freezing'' behavior. During exposure t o either stressor, the baroreflex is altered to allow simultaneous inc reases in heart rate and arterial blood pressure, but the sensitivity is maintained, allowing normal moment to moment control of heart rate.