Nitrate therapy: new aspects concerning molecular action and tolerance.

Although the short-term vasodilatory properties of organic nitrates are potent and well known, a number of vascular and extravascular changes have been shown to compromise their hemodynamic effects on long-term administration. Among these changes, systemic phenomena such as neurohormonal activation and intravascular volume expansion1 as well as specific vascular changes such as increased vascular superoxide (O2 · ) production,2 increased sensitivity to vasoconstrictors,3 and decreased responsiveness to nitric oxide (NO) donors4,5 have long been identified as playing a role. Several hypotheses have been proposed to explain these abnormalities, and over the last 15 years, our groups have focused on the concept that an inappropriate production of reactive oxygen species (ROS), an impairment in the scavenging of these mediators (Figure 1), or both might have a crucial mechanistic importance in all these modifications.6,7 Independently of the role of ROS in tolerance, the possibility that nitrate-induced oxidative stress might affect patients’ prognosis has important implications, and the observation that long-term therapy with most of the drugs in this class causes endothelial dysfunction, the prognostic significance of which is well accepted in patients with coronary artery disease, hypertension, and heart failure,8 should not be taken as an academic curiosity. Beyond these as-yet insufficiently investigated prognostic implications, recognition of the role of ROS suggests that a number of interventions thought to interfere with the vascular redox balance might also retard or prevent the development of nitrate tolerance and nitrate-induced side effects. Evidence that therapy with angiotensin-converting enzyme (ACE) inhibitors, angiotensin-1 receptor blockers, certain -blockers, statins, and vitamins such as folic acid or vitamin C beneficially influence nitrate tolerance and glyceryl trinitrate (GTN)–induced endothelial dysfunction suggests that nitrate therapy might have profoundly different implications since these therapies have become diffusely available. In addition, the recognition of important differences in the mechanisms triggered by different nitrates should also be attributed clinical relevance, and evidence suggests that, rather than the generic nitrate tolerance, more specific expressions (GTN tolerance, isosorbide mononitrate [ISMN] tolerance, etc) should be used.9 This review summarizes the current concepts underlying tolerance and endothelial dysfunction in response to longterm therapy with different nitrates and addresses the question of whether the use of these drugs remains indicated despite these side effects.