Wall Stiffness Suppresses Akt/eNOS and Cytoprotection in Pulse-Perfused Endothelium

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Abstract
Increased steady shear stress stimulates nitric oxide synthase (eNOS) in part by Akt-dependent phosphorylation. Arteries in vivo are exposed to pulse perfusion (PP) combining phasic shear with stretch. In compliant vessels, enhancing PP lowers vascular tone by stimulating eNOS; whereas in aged, stiff arteries, flow-mediated dilation declines and PP is a prominent risk factor. Here, we tested the hypothesis that reduced wall distensibility alters PP-induced eNOS/Akt mechano-signaling. Bovine aortic endothelial cells cultured within distensible tubes were exposed to physiological nonreversing steady or PP (7 dynes/cm 2 mean shear, pulse pressure 0 or 90 mm Hg×2 hours) in a custom servo-system. In compliant tubes, PP doubled Akt phosphorylation above nonpulsatile flow levels, whereas P-Akt declined to static levels from PP in stiffer tubes. eNOS phosphorylation (S-1179) similarly increased with PP in compliant tubes but was nearly undetectable with increased PP in stiffer tubes. After PP, brief exposure of cells to ultraviolet irradiation (oxidant stress) and subsequent culture revealed cytoprotection in compliant tubes but diffuse cytotoxicity and cell detachment in stiffer tubes. NOS inhibition by L-NAME converted compliant-tube post-UV behavior to that of stiffer tubes. These data provide novel evidence that wall compliance can directionally mediate endothelial Akt/eNOS phosphorylation and mechano-signaling. This may help explain increased vascular risks resulting from artery stiffening.

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