368 Heart 2001;86:368–372 REVIEW Heart: first published as 10.1136/heart.86.4.368 on 1 October 2001. Downloaded from Nitric oxide: an emerging role in cardioprotection? R D Rakhit, M S Marber Over a decade of research has shown nitric products of NO.5–7 The isoform specific oxide (NO) to be a ubiquitous modulator of amount of NO generated may account, in part, biological phenomena from cell signal to eVec- for physiological versus pathological eVects of tor and from physiology to pathophysiology. NO in biological systems; low concentrations The involvement of NO in cardiovascular biol- are associated with cytostasis and high concen- ogy has contributed significantly to our under- trations are associated with cytotoxicity.8 standing of complex disease states including A further explanation for the dichotomous atherosclerosis, systemic and pulmonary eVects of NO may lie in its complex interaction hypertension, endotoxic shock, pre-eclampsia,1 with reactive oxygen species, which is particu- cardiomyopathy,2 and cardiac allograft rejec- larly pertinent in the context of ischaemia– tion.3 However, the emerging role of NO in the maintenance of cell physiology from immu- reperfusion. NO can interact in direct equimo- nomodulation to calcium signalling has high- lar concentrations with superoxide to form the lighted the importance of this fascinating mol- potent oxidant peroxynitrite, which is toxic to 9 ecule in cytostasis. This dichotomy of eVector cardiac myocytes. The amount of peroxyni- function is the “double edged sword” of NO in trite production therefore depends on the ratio biological systems. However, the balance be- of superoxide to NO. The greater availability of tween cytostatic and cytotoxic eVects of NO superoxide may therefore favour peroxynitrite may lie in the tissue concentration of NO pro- production and toxicity. Thus, superoxide may duced, the particular NO synthase (NOS) iso- be an important rate limiting factor determin- form activated (that is, “high output” or “low ing the protective versus toxic eVects of NO.10 output”), and the complex interaction with Although the interaction of NO with reactive other free radicals such as superoxide. How- oxygen species is very complex, this simple http://heart.bmj.com/ ever, a much greater understanding of the hypothesis may explain why, even though the molecular and cellular actions of NO as a majority of animal studies have shown cytopro- physiological regulator has resulted in a body of tective eVects11–14 of NO against ischaemia– recent research increasing our understanding reperfusion injury, others have shown cytotox- of NO, and thus NO releasing agents, in cyto- icity.15 16 protection. Current evidence is outlined below. Emerging evidence suggests that a funda- mental explanation for the dichotomous roles The NO dichotomy: physiology versus of NO may lie at a subcellular level. NO has on October 3, 2021 by guest. Protected copyright. pathology? been shown to modulate mitochondrial func- NO is produced by the catalytic action of NOS tion through reversible and irreversible interac- on the substrate L-arginine. The reaction tions with respiratory chain complexes.17 Physi- involves the oxidation of one of the guanidi- ological concentrations of NO inhibit nonitrogens of arginine and the process cytochrome oxidase (complex IV) in a revers- involves the oxidation of NADPH (nicotina- ible manner, in competition with oxygen. mide adenine dinucleotide phosphate, reduced However, long term exposure can irreversibly form) and the reduction of molecular oxygen. inhibit complex I by S-nitrosation of critical Three NOS enzymes have been characterised: thiols in the enzyme complex. The reversible type I (ncNOS), type II (iNOS), and type III interaction may play an important part in the Waller Department of (ecNOS). ncNOS and ecNOS are calcium Cardiology, St Marys dependent and low output enzymes associated physiological regulation of mitochondrial func- Hospital, Praed Street, tion by reducing oxygen consumption without London W2 1NY, UK with NO production in the picomolar range, R D Rakhit whereas iNOS is a calcium independent and causing adenosine triphosphate (ATP) deple- high output enzyme associated with NO tion. This may be beneficial during ischaemia. Department of production in the nanomolar (nmol) range. All Cardiology, St three NOS isoforms have been shown to be Thomas’s Hospital, 4 London, UK present in human myocardium and may be M S Marber activated in response to hypoxia or ischaemia. Novel cellular mechanisms of protection Laboratory studies of experimental myocardial ANTIAPOPTOSIS Correspondence to: infarction have shown an increased induction Apart from promoting apoptosis NO can also Dr Rakhit of iNOS, ecNOS, and NO concentrations in paradoxically inhibit apoptosis. Proposed [email protected] the heart, together with increased plasma con- mechanisms include the suppression of caspase Accepted 16 May 2001 centrations of nitrate and nitrite, the oxidation 1 and 3 activity by NO induced S-nitrosation; www.heartjnl.com Nitric oxide in cardioprotection 369 cyclic guanosine monophosphate (GMP) me- oxygen consumption28 and that reduced oxy- diated suppression of calcium mediated apop- gen consumption by this mechanism may con- totic cell death; and induction of the cytopro- tribute to cardioprotection during precondi- Heart: first published as 10.1136/heart.86.4.368 on 1 October 2001. Downloaded from tective stress proteins heat shock protein 70 tioning.29 Mitochondrial dysfunction is a and heat shock protein 32 (haemoxygenase).18 critical component of ischaemia–reperfusion injury, which is characterised by dissipation of the membrane potential, ATP depletion, in- ANTIOXIDANT EFFECTS NO has been shown to induce the expression of duction of the mitochondrial permeability transient, and mitochondrial calcium over- haemoxygenase in vascular and smooth muscle 30 cells. Haemoxygenase and carbon monoxide, load. We have data to suggest that NO the product of the breakdown of haeme by induced depolarisation of the mitochondrial haemoxygenase, can bind the haeme moieties membrane potential protects cardiomyocytes by reducing mitochondrial calcium overload of NOS and soluble guanylate cyclase and 31 thereby inhibit NO production. This may during hypoxia–reoxygenation injury. This therefore provide a novel adaptive defence may be a novel explanation linking the NO mechanism against the oxidative stress associ- mediated modulation of mitochondrial ener- ated with sustained production of NO.19 getics to cytoprotection. INHIBITION OF MYOCARDIAL DYSTROPHIN ANTI-INFLAMMATORY EFFECTS PROTEOLYSIS NO has for a long time been linked to the Recent exciting data have shown that NO inac- modulation of the immune response and tivates coxsackieviral protease 2A, which eVects on cell mediated immunity may have a cleaves human and mouse dystrophin in a dose role in cytoprotection. High doses of NO have dependent manner.32 This provides a further been shown to modulate the production of novel mechanism for protection by NO against interleukin 12 negatively, thus reducing the T viral myocarditis and may explain why NO helper cell 1 immune response.20 In addition concentrations are significantly raised in idio- NO appears to reduce polymorphonuclear leu- pathic dilated cardiomyopathy. cocyte mediated endothelial dysfunction in myocardial ischaemia–reperfusion, probably NO and clinical cardioprotection through the specific interaction with adhesion The known physiological eVects of NO in molecules.21 reducing left or right ventricular filling pres- sure, augmenting collateral coronary flow, and 33 CYCLIC GMP inhibiting platelet aggregation provide a pow- This important second messenger is produced erful theoretical basis for the routine use of NO by the action of NO on soluble guanylate in the treatment of acute coronary syndromes, cyclase, which catalyses the conversion of gua- myocardial infarction, and heart failure. De- nasine triphosphate (GTP) to cyclic GMP. spite this the routine use of NO donors, such as http://heart.bmj.com/ Cyclic GMP may exert protective eVects by nitrates, for clinical cardioprotection remains a reducing the influx of calcium through L type controversial area. The clinical evidence for the calcium channels and by stimulating a cyclic use of NO in cardiovascular disease is re- GMP sensitive phosphodiesterase with a re- viewed. sultant reduction in concentrations of cyclic AMP. This, together with the known eVect of MYOCARDIAL INFARCTION NO in reducing myocyte contractility,22 would In a small number of myocardial infarction trials predating the thrombolytic era intra- serve to reduce oxygen consumption and on October 3, 2021 by guest. Protected copyright. energy demand. venous nitrates were thought to have beneficial eVects on infarct size and mortality. Subse- quent meta-analysis of these trials indicated up PRECONDITIONING to a 49% reduction in mortality with the use of Ischaemic preconditioning is a powerful adap- prolonged intravenous glyceryl trinitrate.34 tive phenomenon whereby exposure to peri- However, the later and larger megatrials such od(s) of sublethal ischaemia protects against as ISIS-4 (fourth international study of infarct subsequent lethal ischaemia. NO may trigger 35 23 survival) and GISSI-3 (Gruppo Italiano per an early preconditioned state, and an exten- lo studio della sopravvivenza nell’infarto mio- sive body of evidence has implicated NO as a 36 24 cardico) could not confirm a beneficial eVect trigger and mediator of late preconditioning.