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Abstract
The generation of nitric oxide by the vascular endothelium maintains a continuous
vasodilator tone that is essential for the regulation of blood flow and blood pressure.
Nitric oxide also contributes to the control of platelet aggregation and has important
antiatherogenic effects. These properties are mediated by the action of constitutive
nitric oxide synthase and subsequent activation by nitric oxide of soluble guanylate
cyclase. Impaired release of nitric oxide occurs in most animal and human models of
hypertension, contributing to the increased peripheral resistance and most likely
to the development of cardiovascular complications. Antihypertensive medications (angiotensin-converting
enzyme [ACE] inhibitors and calcium channel blockers) appear to prevent the impairment
of nitric oxide-mediated vasodilation in experimental hypertension, though in humans
the data are not as clear. Reduced nitric oxide release appears therefore to be a
consequence rather than a cause of high blood pressure, and the reduction in blood
pressure per se is most important. In hyperlipidaemia, endothelium-dependent relaxations
are reduced probably due to the inhibitory action of oxidized low-density lipoproteins
on endothelium-dependent relaxations. Lipid-lowering strategies and, more recently,
ACE inhibition have been demonstrated to improve nitric oxide dependent coronary vasodilation
in hypercholesterolaemic patients with and without atheromatous coronary disease.
Nitric oxide dependent vasodilation is also impaired in insulin- and non-insulin-dependent
diabetes as well as in healthy aging. Endothelial dysfunction may be improved in non-insulin-dependent
diabetes by administration of the antioxidants, supporting the hypothesis that nitric
oxide inactivation by oxygen-derived free radicals contributes to abnormal vascular
reactivity in diabetes.