Constitutive nitric oxide synthase activation is a significant route for nitroglycerin-mediated vasodilation

Abstract

The physiological effects of nitroglycerin as a potent vasodilator have long been documented. However, the molecular mechanisms by which nitroglycerin exerts its biological functions are still a matter of intense debate. Enzymatic pathways converting nitroglycerin to vasoactive compounds have been identified, but none of them seems to fully account for the reported clinical observations. Here, we demonstrate that nitroglycerin triggers constitutive nitric oxide synthase (NOS) activation, which is a major source of NO responsible for low-dose (1-10 nM) nitroglycerin-induced vasorelaxation. Our studies in cell cultures, isolated vessels, and whole animals identified endothelial NOS activation as a fundamental requirement for nitroglycerin action at pharmacologically relevant concentrations in WT animals.endothelial NOS ͉ neuronal NOS ͉ blood pressure I t has been Ͼ150 years since the effect of nitroglycerin as a potent vasodilator was first reported (1). Since then, intense research has led to extensive and well documented literature regarding nitroglycerin's physiological effects (2-4) and the identity of its derived metabolites in cells, tissues, laboratory animals, and humans (5, 6). Although much of nitroglycerin's pharmacology is known, the mechanisms through which nitroglycerin acts on the endothelium and the heart as well as the enzymatic pathways leading to its bioactivation are still controversial and under intense investigation. A number of hypotheses for nitroglycerin bioconversion in vivo have been formulated, implicating a multitude of enzymes such as GST (6, 7), oxidoreductases (8), and mitochondrial aldehyde dehydrogenase (9, 10) in the bioconversion of nitroglycerin to NO and/or other vasoactive compounds. For example, GST has been shown to catalyze the transnitration of lower thiols in the presence of nitroglycerin (6, 7). Xanthine oxidase and mitochondrial aldehyde dehydrogenase (which are closely related oxidoreductases) have been found to mediate nitroglycerin reduction to nitrite (11) and NO itself (8, 10). Several intermediate compounds, such as partially nitrated glycerin, nitrite (11), and nitrosothiols (6, 7), have been indicated as precursors of nitroglycerin-derived NO, which is ultimately responsible for the observed effects on the vasculature. Collectively, these studies contributed to establishing nitroglycerin as a metabolismdependent NO donor. Although some pathways have received more attention than others, none of the above-mentioned mechanisms seems to satisfactorily delineate nitroglycerin's peculiar kinetic and pharmacological behavior, which is distinct from that of other well characterized NO donors such as sodium nitroprusside (12). For instance, the nitrate groups of nitroglycerin are chemically resistant to rapid reduction because they are esters of nitrate. Also, minute doses of nitroglycerin [maximum plasma concentration Ϸ6 nM for 0.5 mg of nitroglycerin administered sublingually (13)], which are comparable to the basal levels of free NO [Ϸ5 nM as free NO (14)], result in...

Doubt about an essential role for constitutive nitric oxide synthase in nitroglycerin-mediated vasodilation

“…In fact, many articles have shown that endogenously produced NO antagonizes the effect of nitroglycerin. As an example, mechanical removal of the endothelium, which rids the vessel of NO production, has repeatedly been shown to enhance nitroglycerin vasodilatation, in contrast to the finding by Bonini et al (1). Inhibition of NOS with agents such as L-NAME has also been shown to enhance nitroglycerin-induced vasodilatation, again contradicting the findings of Bonini et al (1).…”

“…As an example, mechanical removal of the endothelium, which rids the vessel of NO production, has repeatedly been shown to enhance nitroglycerin vasodilatation, in contrast to the finding by Bonini et al (1). Inhibition of NOS with agents such as L-NAME has also been shown to enhance nitroglycerin-induced vasodilatation, again contradicting the findings of Bonini et al (1). Finally, mice lacking the endothelial isoform of NOS exhibit augmented vasodilatation to nitroglycerin (2, 3), and overexpression of this enzyme in transgenic mice paradoxically reduces nitroglycerin-and sodium nitroprusside-induced vasodilatation (4).…”

“…1 To whom correspondence should be addressed at: Klinikum der Johannes GutenbergUniversitä t Mainz, II. Medizinische Klinik, Labor fü r Molekulare Kardiologie, Verfü gungsgebä ude fü r Forschung und Entwicklung, Raum 00349, Obere Zahlbacher Strasse 63, 55101 Mainz, Germany.…”

Transdermal Glyceryl Trinitrate as an Effective Adjunctive Treatment with Artemether for Late-Stage Experimental Cerebral Malaria

“…This means that a large fraction of the NO derived from the glyceryl trinitrate patches is diverted to react and be scavenged by cell-free hemoglobin instead of playing its expected effect on blood vessels. Third, it has been shown that the vasodilatory action of low-dose glyceryl trinitrate is partly due to NOS-dependent mechanisms (65). NOS (both eNOS and nNOS isoforms) are dysfunctional in mice with ECM (37); therefore, it is conceivable that the action of glyceryl trinitrate through this mechanism is impaired in mice with ECM, also explaining the need for higher doses.…”

Reply to Daiber et al. : “Doubt about an essential role for constitutive nitric oxide synthase in nitroglycerin-mediated vasodilation”

“…In a recent PNAS article (1) we demonstrated that endothelial NOS (eNOS) is phosphorylated at Ser-1177, an index of eNOS activation, in HUVEC and blood vessels of rodents challenged with nitroglycerin. In addition, we showed that the time scale in which this phosphorylation happens correlates with the drop in the blood pressure.…”