Mechanisms of aging-induced impairment of endothelium-dependent relaxation: Role of tetrahydrobiopterin

Oxidative stress has been implicated as an important mechanism of vascular endothelial dysfunction induced by aging. Previous studies suggested that tetrahydrobiopterin (BH₄), an essential cofactor of endothelial NO synthase, could be a molecular target for oxidation. We tested the hypothesis that oxidative stress, in particular oxidation of BH₄, may contribute to attenuation of endothelium-dependent relaxation in aged mice. Vasomotor function of isolated carotid arteries was studied using a video dimension analyzer. Vascular levels of BH₄ and its oxidation products were measured via HPLC. In aged mice (age, 95 ± 2 wk), endothelium-dependent relaxation to ACh (10^-5 to 10^-9 M) as well as endothelium-independent relaxation to the NO donor diethylammonium (Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (DEA-NONOate, 10^-5 to 10^-9 M) were significantly reduced compared with relaxation detected in young mice (age, 23 ± 0.5 wk). Incubation of aged mouse carotid arteries with the cell-permeable SOD mimetic Mn(III)tetra(4-benzoic acid)porphyrin chloride normalized relaxation to ACh and DEA-NONOate. Furthermore, production of superoxide anion in aorta and serum levels of amyloid P component, which is the murine analog of C-reactive protein, was increased in old mice. In aorta, neither the concentration of BH₄ nor the ratio of reduced BH₄ to the oxidation products were different between young and aged mice. Our results demonstrate that in mice, aging impairs relaxation mediated by NO most likely by increased formation of superoxide anion. Oxidation of BH₄ does not appear to be an important mechanism underlying vasomotor dysfunction in aged mouse arteries.