Background: Aortic valve stenosis is the most frequent valvular heart disease in the elderly in the western world. Pathological changes in blood flow across the aortic valve affect endothelial function through changes in vascular shear stress and in mechanical and functional properties of red blood cell. While previous studies revealed endothelial dysfunction in patients with aortic valve stenosis, underlying mechanisms are still not fully understood. Therefore, we aim to investigate the mechanisms involved in endothelial dysfunction in a murine model of aortic valve stenosis.

 

Hypothesis: We hypothesized that aortic valve stenosis facilitates free hemoglobin release from red blood cells, which deteriorates endothelial function by altered NO bioavailability.

 

Methods: 12-week-old, male wild-type mice (C57Bl/6) were subjected to wire- injury of the aortic valve to induce aortic valve stenosis. Peak flow velocities and pressure gradients across the aortic valve were assessed by echocardiography after 4 and 12 weeks. Furthermore, endothelial function was analyzed by flow-mediated dilation (FMD) of the femoral artery. For in vitro endothelial function studies, segments of femoral arteries were mounted on a wire-myograph to examine endothelium- dependent and independent relaxation in the presence of vasoactive drugs. Additionally, an analysis of free hemoglobin levels was carried out by enzyme-linked immunosorbent assay. To determine nitric oxide (NO) levels, plasma nitrite and nitrate levels were measured by chemiluminescence detection and HPLC based approach, respectively. 

 

Results: We observed an endothelial dysfunctional phenotype in mice with aortic valve stenosis. Flow-mediated dilation showed a decrease in vasodilation function four weeks after induction of aortic valve stenosis with a further decline of vasodilation response after 12 weeks. Wire-myograph studies showed no alterations in vasodilator and vasoconstrictor function in femoral artery segments compared to sham mice. However, vasodilation in the presence of eNOS inhibitor L-NAME revealed a shift towards endothelium-dependent relaxation in the femoral artery 12 weeks compared to four weeks after wire-injury and sham surgery. Furthermore, free hemoglobin levels were significantly increased in mice with aortic valve stenosis. Nitric oxide metabolite nitrate showed a significant increase while there was no change in nitrite levels.

 

Conclusion: The results demonstrate that aortic valve stenosis is accompanied by endothelial dysfunction, which is not attributable to a loss of eNOS activity than rather a decrease in NO bioavailability due to scavenging by cell- free hemoglobin.