PRE-CLINICAL RESEARCH Endothelium-Dependent Regulation of the Mechanical Properties of Aortic Valve Cusps Ismail El-Hamamsy, MD,* Kartik Balachandran, MS,† Magdi H. Yacoub, FRS,* Louis M. Stevens, MD, SM,‡ Padmini Sarathchandra, PHD,* Patricia M. Taylor, PHD,* Ajit P. Yoganathan, PHD,† Adrian H. Chester, PHD* Harefield, United Kingdom; Atlanta, Georgia; and Boston, Massachusetts Objectives The aim of this study was to evaluate the role of valve endothelium in regulating the mechanical properties of aortic valve cusps. Background Mechanical properties of valve cusps are key to their function and durability; however, little is known about the regulation of valve biomechanics. Methods Mechanical properties of porcine aortic valve leaflets were evaluated in response to serotonin (5-hydroxytryptamine [5-HT]), with and without N-nitro-L-arginine-methyl-ester (L-NAME) or endothelial denudation, and endothelin (ET)-1, with and without cytochalasin-B. Results Under physiological loading conditions, 5-HT induced a decrease in the areal stiffness of the cusp (-25.0  4.0%; p  0.01 vs. control), which was reversed by L-NAME or endothelial denudation (+17.5  5.3%, p = 0.07, and +14.7  1.8%, p  0.05 vs. control, respectively). ET-1 caused an increase in stiffness (+34.4  13.8%; p  0.05 vs. control), but not in the presence of cytochalasin-B (p = 0.29 vs. control). Changes in cusp stiffness were accompanied by aortic cusp relaxations to 5-HT (-0.29%  0.1% change in load per 10-fold increase in 5-HT concentration; p = 0.03), which were reversed by endothelial denudation (+0.29  0.06% change in load per 10-fold increase in 5-HT concentration; p = 0.02) and by L-NAME (p  0.05). Valve cusps contracted in re- sponse to ET-1 (+0.29  0.08% change in load per 10-fold increase in ET-1 concentration; p = 0.02), which was inhibited by cytochalasin-B. Conclusions These data highlight the role of the endothelium in regulating the mechanical properties of aortic valve cusps and underline the importance of valve cellular integrity for optimal valve function. (J Am Coll Cardiol 2009;53: 1448–55) © 2009 by the American College of Cardiology Foundation The role of endothelial cells in preventing platelet aggrega- tion, inflammation, and smooth muscle cell contraction and proliferation in the vascular system has been well docu- mented. Endothelial dysfunction is an early occurrence in the cascade of events leading to atherosclerosis. The phys- iological role of heart valve endothelium has not been adequately defined. Recent studies have indicated that heart valve endothelial cells have specific cellular and molecular characteristics that are not shared with endothelium else- where in the vasculature (1,2). While vascular endothelium plays a fundamental role in regulating blood vessel tone, the functional role of endothelium-derived mediators on the mechanical properties of cusp tissue remains unknown. This could be important as the valve is subjected to high mechanical forces at each cardiac cycle, ranging from compression to stretch and shear stress, and is constantly required to adapt to varying hemodynamic conditions. See page 1456 Appropriate adaptations of the mechanical properties of aortic valves are relevant to its function. They can affect stress distribution on the valve cusps (3), improve leaflet coaptation during diastole (3), and optimize the instanta- neous movements of the valve, which could influence flow dynamics, coronary perfusion, and ventricular function (4). Therefore, elucidating the regulation of aortic valve me- From the *Tissue Engineering Department, Harefield Heart Science Centre, Na- tional Heart and Lung Institute, Imperial College London, Harefield, United Kingdom; †Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia; and the ‡School of Public Health, Harvard University, Boston, Massachusetts. Dr. El-Hamamsy is supported by a Research Fellowship Award from the Canadian Institutes of Health Research (CIHR MFE- 83809) and by the Magdi Yacoub Institute. Dr. Balachandran is supported by the National Science Foundation through the ERC program at Georgia Institute of Technology under award number EEC-9731643. Drs. El-Hamamsy and Balachan- dran contributed equally to this work. Manuscript received September 3, 2008; revised manuscript received November 5, 2008, accepted November 6, 2008. Journal of the American College of Cardiology Vol. 53, No. 16, 2009 © 2009 by the American College of Cardiology Foundation ISSN 0735-1097/09/$36.00 Published by Elsevier Inc. doi:10.1016/j.jacc.2008.11.056