Cyclic Pressure and Angiotensin II Influence the Biomechanical Properties of Aortic Valves

Hypertension is a known risk factor for aortic stenosis. The elevated blood pressure increases the transvalvular load and can elicit inflammation and extracellular matrix (ECM) remodeling. Elevated cyclic pressure and the vasoactive agent angiotensin II (Ang II) both promote collagen synthesis, an early hallmark of aortic sclerosis. In the current study, it was hypothesized that elevated cyclic pressure and/or angiotensin II decreases extensibility of aortic valve leaflets due to an increase in collagen content and/or interstitial cell stiffness. Porcine aortic valve leaflets were exposed to pressure conditions of increasing magnitude (static atmospheric pressure, 80, and 120 mmHg) with and without 10⁻⁶ M Ang II. Biaxial mechanical testing was performed to determine extensibility in the circumferential and radial directions and collagen content was determined using a quantitative dye-binding method at 24 and 48 h. Isolated aortic valve interstitial cells exposed to the same experimental conditions were subjected to atomic force microscopy to assess cellular stiffness at 24 h. Leaflet tissue incubated with Ang II decreased tissue extensibility in the radial direction, but not in the circumferential direction. Elevated cyclic pressure decreased extensibility in both the radial and circumferential directions. Ang II and elevated cyclic pressure both increased the collagen content in leaflet tissue. Interstitial cells incubated with Ang II were stiffer than those incubated without Ang II while elevated cyclic pressure caused a decrease in cell stiffness. The results of the current study demonstrated that both pressure and Ang II play a role in altering the biomechanical properties of aortic valve leaflets. Ang II and elevated cyclic pressure decreased the extensibility of aortic valve leaflet tissue. Ang II induced direction specific changes in extensibility, demonstrating different response mechanisms. These findings help to provide a better understanding of the responses of aortic valves to mechanical and biochemical changes that occur under hypertensive conditions.