Aortic caliber changes during development of hypertension in freely moving rats.

The availability of a chronically implanted electrolytic strain gauge has permitted us to monitor, for the first time in freely moving rats, sequential changes of both resting caliber and mechanical properties of the aorta during onset and maintenance of sustained hypertension. Normotensive rats were subjected to subdiaphragmatic aortic constriction under ether anesthesia. There was an immediate and sustained rise of pressure (on average a 50% increase over base-line mean arterial pressure of 101 +/- 4 mmHg), whereas mean aortic caliber (6.552 +/- 0.142 mm) dilated gradually with no significant change at 6 h and had a significant dilation of 3.4 and 6.8% after 24 and 48 h, respectively. On the 2nd day of hypertension, dilation of the aorta was accompanied by a marked increase in both mean distensibility and dynamic distensibility of the aortic wall (7-fold and 1.5-fold increase over basal values of 0.128 +/- 0.015 and 0.056 +/- 0.005 X 10(-6) dyn/cm2, respectively). No further dilation was observed for up to 5 days, but the dynamic distensibility of the aorta returned to normal values on the 3rd day of hypertension. Maximal dilation at 48 h was produced by the increase of both pulsation and diastolic caliber of the aorta with the displacement of diastolic caliber (94.8%) being the major determinant of aortic dilation. Thus the time required for the aorta to achieve the new resting diastolic position during the development of sustained hypertension (48 h) coincides with the time course of complete resetting of aortic baroreceptors as we demonstrated before in the same preparation. These two findings taken together suggest that alterations in the geometry of the aortic wall play an important role in the resetting process.