Mechanisms of complete baroreceptor resetting in hypertension.

It is a universally accepted phenomenon that vascular baroreceptors reset to operate at higher pressure levels in hypertension. A rapid or acute resetting can be demonstrated within the first 5 to 15 minutes after arterial pressure has been elevated. However, this resetting is only partial because the increase in pressure threshold for baroreceptor activation represents a fraction (20 to 40%) of the total pressure increase. This acute resetting changes little within the first hours of hypertension. To achieve complete resetting however, that is when the pressure threshold increase equals the total pressure increase, blood pressure needs to be maintained at an elevated level for 48 hours in the rat. Studies of aortic calibre behaviour performed in freely moving rats demonstrated a striking direct relationship between the time taken for the diastolic calibre to reach maximal dilation and the time taken for complete resetting of the aortic baroreceptors. It was also observed that during transient pressure increases the displacement of the diastolic calibre is much greater than the increase in pulsation, indicating that under physiological conditions sustained distension of the diastolic calibre is an important factor in aortic baroreceptor distortion. The data suggest that complete resetting of the baroreceptors in hypertension occurs when the increased stress on the arterial wall is matched by a proportional permanent increase in diastolic calibre.