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

Abstract

SUMMARY.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. {Cue Res 50: 428-439, 1982)

Hemodynamic responses to acute aortic coarctation in conscious sinoaortic denervated rats

“…Therefore, the increased sympathetic drive of SAD rats presumably masked the effect of neurohumoral factors on overall resistance to blood flow as seen in intact animals. On the other hand, the smaller and gradual increase in aortic resistance associated with a sharp bradycardia that leveled off 5 min after aortic constriction in the intact rats might suggest the already known rapid resetting of the baroreceptors (25,26) that was demonstrated to be present 30 min after the onset of the hypertensive response (27).…”

Blockade of Cardiac Nerves by Intrapericardial Local Anaesthetics in the Conscious Rabbit

“…Tbe reflex was elicited by inflating the perivascular cufTs (Biombery and Korner. 1979;Faris et al, 1980), so as to create a fall or rise of mean BP at a rate of 1-2 mmHg/s (Dorward et al, 1982), until HR reached an upper or lower limiting value (Fig. !…”

The effects of acute and chronic dihydropyridine calcium antagonist therapy on baroreflex sensitivity: a re-analysis using the sequence method

“…4 Initial dosing with dihydropyridine calcium antagonists is associated with a reflex tachycardia, 5 attributable to a baroreflex mediated increase in sympathetic activity and reduction in vagal activity in response to the lower level of blood pressure. However, these changes are attenuated by the effects of baroreceptor resetting, whereby there is a shift in the function curve relating afferent baroreceptor activity to blood pressure, in the direction of the prevailing arterial pressure; estimates of the time course of baroreceptor resetting vary from minutes 6 to days or weeks. 7 Thus, as a result of resetting, the deleterious autonomic effects associated with initial doses of dihydropyridines may not persist with chronic therapy.…”