Relationship between Aortic Diameter and Aortic Baroreceptor Activity in Normal and Hypertensive Rabbits

Abstract Studies of aortic nerve activity (Aars 1968a), and the load/length relationship of strips from the aortic wall (Aars 1968b), have suggested that the hypertensive resetting of baroreceptor activity is due to changes in the aortic wall. In nine normal and four hypertensive rabbits, diameter of the ascending aorta, measured with an ultrasonic gauge, was therefore recorded together with aortic baroreceptor activity. Aortic nerve activity was closely related to aortic diameter in individual rabbits, but wide variations in pressure/diameter relationships made it difficult to compare diameter and activity in a group of rabbits. Assuming that stretching of the receptor area would be almost equal in all rabbits at the inflexion point of their pressure/diameter curves, the diameter corresponding to inflexion point of the curve for dynamic elastic modulus was used as reference diameter. Plotted against per cent of this diameter, the nerve activity showed acceptable uniformity in the group of normal rabbits, and observations in hypertensive rabbits showed less deviation from the normal. The uniformity was further improved by allowing for variations in diameter pulsations. It was therefore concluded that the hypertensive resetting of aortic baroreceptor activity was predominantly due to altered distensibility of the aortic wall.