Nitroglycerin-mediated S-nitrosylation of proteins: a field comes full cycle.

Nitroglycerin (glyceryl trinitrate) (GTN) has been an important part of the management of patients with angina or heart failure for over 135 years. GTN works through a combined action on the venous circulation and coronary vasculature to reduce preload and improve myocardial blood flow.1 Its attributes include a potent vasodilatory action on diseased coronary vessels as well as antiischemic effects elicited in the microcirculation.1,2 Dilation of conduit vessels by GTN is mediated in large part through nitric oxide (NO) binding to heme within, and activation of, soluble guanylate cyclase (sGC) in vascular smooth muscle, thereby leading to induction of the second messenger, cyclic GMP. The microvascular action of GTN involves additional effects on red blood cells (RBCs) to improve rheology and oxygen delivery.2 GTN is an exceptionally potent vasodilator compared to other organic nitrates (isosorbide dior mononitrates) but loses efficacy over time. Tachyphylaxis to GTN is initially specific to GTN (mechanism-based tolerance), but is ultimately associated with diminished responsiveness to other nitro(so)vasodilators (cross-tolerance) and even other classes of drugs (as a result of fluid retention and perhaps cellular injury).1,3,4 Tolerance and cross-tolerance have generally been thought of in terms of an NO deficiency, resulting in attenuated sGC activity.5,6 Sayed et al had found recently that S-nitrosylation of sGC (the addition of an NO group to a cysteine thiol) by endothelium-derived NO inhibits sGC activity,7 and they now report that exposure to GTN can result in the S-nitrosylation and desensitization of sGC, thereby providing a mechanism for cross-tolerance.8 In other words, they suggest that aberrant or misdirected NO bioactivity, rather than NO deficiency per se, may contribute to crosstolerance. These findings are consistent with an emerging paradigm in NO biology in which NO-based signaling is elicited in substantial part by S-nitrosothiols (SNOs), and, accordingly, dysregulated protein S-nitrosylation contributes to cellular dysfunction and disease.9,10 These new results also help elucidate the long-recognized importance of S-nitrosothiols in GTN biotransformation and metabolism. Classic studies by Murad, Ignarro, and Furchgott originally identified the activity of GTN with that of the endotheliumderived vasodilator, NO.11 Both GTN and NO activated sGC in situ. It is now understood, however, that NO bioactivity cannot be readily differentiated from that of endogenous SNOs, which mediate vasorelaxation and whose role in regulation of vascular resistance has been established by stringent genetic criteria.12,13 SNO-based activity is transduced by sGC/cGMP and by S-nitrosylation of proteins. It is, therefore, of interest that a large part of the acetylcholine-mediated relaxation in the classic Furchgott bioassay (rabbit thoracic aorta) is in fact preserved after inhibition of sGC14,15 and probably attributable to S-nitrosylation of the charybdotoxin-sensitive potassium channel and perhaps of calcium ATPase.12,14 The case for S-nitrosothiols is perhaps even stronger in the microcirculation. Harrison and colleagues noted long ago that coronary microvessels are far more responsive to low mass nitrosothiols such as S-nitrosocysteine than to NO itself.16 S-Nitrosothiols are also impervious to the NO-scavenging chemistry of hemoglobin, which is of particular importance in small vessels where the effective concentration of hemoglobin is highest. Interestingly, vasodilation by GTN is markedly less efficacious in small versus large coronary vessels and is greatly potentiated in microvessels by the addition of cysteine,16–18 which reacts with GTN to produce S-nitrosocysteine.19 Thus, the role of cGMP in the action of GTN in the microcirculation (especially during low flow states), and more generally in the control of microcirculatory blood flow, is poorly understood. In view of this, and of atypical features of the hamster cheek pouch preparation used by Sayed et al8 (which is not representative of vascular beds that contribute principally to the effects of nitro[so]vasodilators), their findings will need to be confirmed in more relevant vascular systems. The observations of Sayed et al8 nonetheless shed new light on shared biochemical and physiological properties of GTN and S-nitrosocysteine with respect to cross-tolerance. These results are reminiscent of early work by Ignarro19 on the participation of S-nitrosothiols, particularly S-nitrosocysteine, in GTN biotransformation, and of work by Needleman and Johnson,20 who suggested that oxidation of protein thiols may constitute a mechanism of GTN tolerance. Recent experiments by Kaul and colleagues2 further suggest that the principal function of these S-nitrosothiols may be in the microcirculation where they subserve RBC-mediated control of blood flow. Notably, GTN augments the S-nitrosylation of hemoglobin in tandem with the increases in oxygen delivery mediated by RBCs.2 S-Nitrosohemoglobin is in equilibrium with low-mass SNOs, which convey NO bioactivity from RBCs,13,21,22 consistent with the accumulating evidence that SNOs play central roles in hypoxic vasodilation, a mechanism that is pivotal in relief from ischemia (Figure). The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Departments of Medicine and Biochemistry, Duke University Medical Center, Durham, NC. Correspondence to Jonathan S. Stamler, MD, Department of Medicine, Box 2612, Duke University Medical Center, Durham, NC 27710. E-mail [email protected] (Circ Res. 2008;103:557-559.) © 2008 American Heart Association, Inc.