The influence of angiotensin II-induced increase in aortic wall mass on compliance in rats in vivo

Objective: This study was undertaken to examine the effects of angiotensin II-induced structural changes in the aortic wall on the dynamic mechanical properties of the vessel in the rat. Methods: Wistar rats were infused s.c. with 250 ng/kg/min angiotensin II(Ang II) for 14 days (ANG). Both ANG and control rats (CON) were equipped with an arterial catheter for measurement of arterial blood pressure. Thoracic aorta diameter, compliance coefficient (CC), and distensibility coefficient (DC) were determined non-invasively in pentobarbital-anesthetized animals using a B-mode imager attached to a vessel wall tracking system. After sacrifice, medial cross-sectional area (CSA), and elastin and collagen densities were determined by morphometry on cross-sections. Media thickness (Mt) and wall-to-lumen ratio (W/L) were subsequently calculated. Results: Ang II infusion significantly increased mean arterial blood pressure in conscious rats (122 +/- 3 mmHg CON vs. 157 +/- 4 mmHg ANG). This was normalized when the rats were anesthetized, thus making it possible to determine CC and DC under isobaric conditions where the diastolic diameters were also similar. Two-week infusion of Ang II induced a significant increase in CSA from 0.48 +/- 0.02 mm(2) in CON to 0.61 +/- 0.03 mm(3). Mt and W/L were likewise increased, but collagen and elastin densities remained unchanged. CC and DC were not effected by this increase in aortic wall mass (CC: 0.143 +/- 0.009 CON, 0.147 +/- 0.014 mm(2)/kPa ANG, DC: 0.052 +/- 0.005 CON, 0.051 +/- 0.004 kPa(-1) ANG). Conclusions: An increase in aortic wall mass resulting from chronic infusion of angiotensin II does dot alter the dynamic compliance of the vessel under isobaric conditions.