in contrast to the finding by Bonini et al. (1). Inhibition of LETTER NOS with agents such as L-NAME has also been shown to enhance nitroglycerin-induced vasodilatation, again contra- Doubt about an essential role for dicting the findings of Bonini et al. (1). Finally, mice lacking the endothelial isoform of NOS exhibit augmented vasodilata- constitutive nitric oxide synthase in tion to nitroglycerin (2, 3), and overexpression of this enzyme nitroglycerin-mediated vasodilation in transgenic mice paradoxically reduces nitroglycerin- and sodium nitroprusside-induced vasodilatation (4). We have In their recent PNAS article (1) Bonini et al. show that inhi- made similar observations and found nitroglycerin potencies Ϯ Ϯ bition of endothelial NOS (eNOS) or removal of the endothe- (pD2 values) of 6.85 0.05 and 7.4 0.1 in aorta from wild- lium impairs the nitroglycerin-mediated vasodilatation (see type and eNOS-deficient mice, respectively (Fig. 1). Addition- figure 2 in ref. 1). Although some data in the article are in- ally, there is an appreciable discrepancy between nitroglycer- teresting, we believe that several points deserve further atten- in-mediated vasodilation and NO formation (reviewed in ref. 5). Although the hemodynamic data presented by Bonini et tion and discussion. The major issue is that the article contra- al. (1) are interesting, they completely contradict previous dicts a large body of previous literature and makes no effort results on pharmacological, mechanical, and knockout-in- to reconcile these differences. In fact, many articles have duced eNOS inhibition and nitroglycerin potency. shown that endogenously produced NO antagonizes the effect of nitroglycerin. As an example, mechanical removal of the Andreas Daibera,1, David G. Harrisonb, and Thomas Mu¨nzela a endothelium, which rids the vessel of NO production, has re- II. Medizinische Klinik, Labor fu¨r Molekulare Kardiologie, Jo- hannes-Gutenberg-Universita¨t, 55101 Mainz, Germany; and bDi- peatedly been shown to enhance nitroglycerin vasodilatation, vision of Cardiology, Department of Medicine, Emory University School of Medicine, Atlanta, GA 30322

1. Bonini MG, et al. (2008) Constitutive nitric oxide synthase activation is a significant route for nitroglycerin-mediated vasodilation. Proc Natl Acad Sci USA 105:8569–8574. 2. Wang EQ, Lee WI, Fung HL (2002) Lack of critical involvement of endothelial nitric oxide synthase in vascular nitrate tolerance in mice. Br J Pharmacol 135:299–302. 3. Kojda G, et al. (1999) Protein expression, vascular reactivity and soluble guanylate cyclase activity in mice lacking the endothelial cell nitric oxide synthase: Contributions of NOS isoforms to blood pressure and heart rate control. Cardiovasc Res 42:206–213. 4. Ohashi Y, et al. (1998) Hypotension and reduced nitric oxide-elicited vasorelaxation in transgenic mice overexpressing endothelial nitric oxide synthase. J Clin Invest 102:2061–2071. 5. Munzel T, Daiber A, Mulsch A (2005) Explaining the phenomenon of nitrate tolerance. Circ Res 97:618–628.

Fig. 1. Concentration–relaxation curves for acetylcholine (ACh, 10Ϫ9 to Author contributions: A.D., D.G.H., and T.M. wrote the paper. 10Ϫ5.5 M, Left) and nitroglycerin (GTN, 10Ϫ9 to 10Ϫ4.5 M, Right) were obtained by isometric tension recordings in aortic segments from wild-type and The authors declare no conflict of interest. eNOSϪ/Ϫ (eNOS ko) mice. The endothelium (ACh)-dependent relaxation was 1To whom correspondence should be addressed at: Klinikum der Johannes Gutenberg- almost completely blunted in aortas from eNOSϪ/Ϫ mice, whereas relaxation Universita¨t Mainz, II. Medizinische Klinik, Labor fu¨r Molekulare Kardiologie, Verfu¨gungs- by the endothelium-independent nitrovasodilator nitroglycerin (GTN) was geba¨ude fu¨r Forschung und Entwicklung, Raum 00349, Obere Zahlbacher Strasse 63, improved in eNOS-deficient vessels. Data are mean Ϯ SEM of 8 independent 55101 Mainz, Germany. E-mail: [email protected]. experiments with tissue from 4 animals per group. *, P Ͻ 0.05 vs. wild type. © 2008 by The National Academy of Sciences of the USA

E92 ͉ PNAS ͉ November 25, 2008 ͉ vol. 105 ͉ no. 47 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0807190105 Downloaded by guest on September 23, 2021