Endothelial dysfunction, defined as a reduction in the bioavailability of nitric oxide (NO), is a risk factor for the occurrence and progression of various vascular diseases. This study investigates the effect of endothelial dysfunction on age-related changes in aortic extracellular matrix (ECM) microstructure and the relationship between microstructural adaptation and the mechanical response. Here, we used groups of NOS3 knockout (KO), NOS3 heterozygotes (Het), and wild-type (WT) B6 mice (controls) to study changes in hemodynamic parameters, collagen fiber organization, and both active and passive aortic mechanics using biaxial pressure myography over a time course from 1.5 to 12 mo. Our results show that homeostatic levels of passive circumferential stress and stretch were preserved in KO mice by remodeling adventitial collagen fibers toward a more predominantly circumferential direction with age, rather than by increased fibrosis, in response to hypertension induced by endothelial dysfunction. However, passive aortic stiffness in KO mice was significantly increased owing to geometrical changes, including significant increases in wall thickness and decreases in inner diameter, as well as by ECM microstructural reorganization, during this maladaptive vascular remodeling. Furthermore, long-term NO deficiency significantly increased smooth muscle cell (SMC) contractility initially, but this effect was attenuated with age. These findings improve our understanding of microstructural and mechanical changes during the maladaptive vascular remodeling process, demonstrating a role for adventitial collagen fiber re-orientation in the response to hypertension
