Rapid resetting of the aortic baroreceptors in the rabbit and its implications for short-term and longer term reflex control.

We studied the effects of sustained changes in resting mean arterial pressures (MAP) on arterial baroreceptor properties in anesthetized rabbits and on the baroreceptor-heart rate reflex in conscious animals. The rabbits had balloons implanted round their aorta and vena cava, for producing transient changes in MAP about the resting MAP. Aortic baroreceptor function curves were obtained at different resting MAP by relating balloon-induced changes in MAP to either (1) integrated aortic nerve activity or (2) unit baroreceptor activity. Any sustained change in resting MAP reset the unit function curves in the same direction within 15 minutes by altering their threshold without affecting gain. The effect was reversible and independent of starting pressure. It was the same whether resting MAP was altered by vasoactive drugs (nitroprusside, phenylephrine) or was changed by withdrawing or infusing blood in sympathetically blocked rabbits. We studied baroreflex function in conscious rabbits by deriving MAP-heart period (HP) curves at different resting MAP. Nitroprusside lowered baroreflex threshold for evoking bradycardia, whereas phenylephrine increased threshold. From our analysis, the resting MAP-mediated changes in receptor threshold accounted for the reflex threshold changes. Altered baroreceptor properties did not account for changes in baroreflex HP range produced by both drugs, and in reflex gain; these were probably due to afferent interactions in the CNS. Because of rapid arterial baroreceptor resetting, transient changes (of about 30 seconds or less) in MAP of moderate magnitude evoked normal reflex heart rate responses at each resting MAP. After changes in resting MAP sustained for 15 minutes or longer, reflex changes in resting heart rate were considerably smaller than in the absence of resetting. Therefore the arterial baroreceptors provide the baroreflex with a "floating" rather than a fixed set point, determined by the prevailing MAP.