Malte Kelm

Control of coronary vascular tone by nitric oxide.

A specific difference-spectrophotometric method was used to measure nitric oxide (NO) release into the coronary effluent perfusate of isolated, constant-flow-perfused guinea pig hearts. Authentic NO applied into the coronary circulation decreased vascular resistance dose dependently and enhanced coronary release of cyclic GMP (cGMP) fivefold. Increasing oxygen tension in aqueous solutions from 150 to 700 mm Hg decreased NO half-life (5.6 seconds) by 32%. During single passage through the intact coronary system, 86% of the infused NO was converted to nitrite ions. Oxidation of NO was more than 30 times faster within the heart than in aqueous solution. Endogenously formed NO was constantly released into the coronary effluent perfusate at a rate of 161 +/- 11 pmol/min. The NO scavenger oxyhemoglobin and methylene blue increased coronary resistance and decreased cGMP release (basal release, 342 +/- 4 fmol/min), whereas superoxide dismutase reduced coronary resistance. L-Arginine (10(-5) M) slightly decreased coronary perfusion pressure and enhanced release of cGMP. NG-Monomethyl L-arginine (10(-4) M) reduced basal release of NO and cGMP by 26% and 31%, respectively, paralleled by a coronary vasoconstriction. Bradykinin in the physiological range from 5 x 10(-11) M to 10(-7) M dilated coronary resistance vessels, which was paralleled by the release of NO and cGMP. Onset of NO release preceded onset of coronary vasodilation in all cases. Upon stimulation with bradykinin, amounts of endogenously formed NO were within the same range as the dose-response curves for exogenously applied NO both for changes in coronary resistance and cGMP release. Acetylcholine (10(-5) M), ATP (10(-5) M), and serotonin (10(-8) M) increased the rate of NO and cGMP release, resulting in coronary vasodilation. Our data suggest the following: 1) NO, the most rapidly acting vasodilator presently known, is metabolized within the heart mainly to nitrite and exhibits a half-life of only 0.1 second; 2) in the unstimulated heart, basal formation of NO may play an important role in setting the resting tone of coronary resistance vessels; 3) the kinetics and quantities of NO formation suggest that NO is causally involved in the bradykinin-induced coronary vasodilation; and 4) amounts of NO formed within the heart stimulated with ATP, acetylcholine, and serotonin are effective for vasodilation.

Guidelines for pre-operative cardiac risk assessment and perioperative cardiac management in non-cardiac surgery

The American College of Cardiology, American Heart Association, and the European Society of Cardiology are all in the process of completing updated versions of our Guidelines for Perioperative Care. Our respective writing committees are undertaking a careful analysis of all relevant validated studies and always incorporate appropriate new trials and meta-analyses into our evidence review. In the interim, our current joint position is that the initiation of beta blockers in patients who will undergo non-cardiac surgery should not be considered routine, but should be considered carefully by each patients treating physician on a case-by-case basis. Please see the expression of concern which is free to view in Eur Heart J (2013) 34 (44): 3460; doi: 10.1093/eurheartj/eht431. AAA : abdominal aortic aneurysm ACC : American College of Cardiology ACE : angiotensin-converting enzyme ACS : acute coronary syndrome AHA : American Heart Association AR : aortic regurgitation ARB : angiotensin receptor blocker AS : aortic stenosis AF : atrial fibrillation BBSA : β-blocker in spinal anaesthesia BNP : brain natriuretic peptide CABG : coronary artery bypass grafting CARP : coronary artery revascularization prophylaxis CASS : coronary artery surgery study CI : confidence interval COX-2 : cyclooxygenase-2 COPD : chronic obstructive pulmonary disease CPET : cardiopulmonary exercise testing CPG : Committee for Practice Guidelines CRP : C-reactive protein CT : computed tomography cTnI : cardiac troponin I cTnT : cardiac troponin T CVD : cardiovascular disease DECREASE : Dutch Echocardiographic Cardiac Risk Evaluating Applying Stress Echo DES : drug-eluting stent DIPOM : Diabetes Postoperative Mortality and Morbidity DSE : dobutamine stress echocardiography ECG : electrocardiography ESC : European Society of Cardiology FEV1 : forced expiratory volume in 1 s FRISC : fast revascularization in instability in coronary disease HR : hazard ratio ICU : intensive care unit IHD : ischaemic heart disease INR : international normalized ratio LMWH : low molecular weight heparin LQTS : long QT syndrome LR : likelihood ratio LV : left ventricular MaVS : metoprolol after surgery MET : metabolic equivalent MI : myocardial infarction MR : mitral regurgitation MRI : magnetic resonance imaging MS : mitral stenosis NICE-SUGAR : normoglycaemia in intensive care evaluation and survival using glucose algorithm regulation NSTEMI : non-ST-segment elevation myocardial infarction NT-proBNP : N-terminal pro-brain natriuretic peptide NYHA : New York Heart Association OPUS : orbofiban in patients with unstable coronary syndromes OR : odds ratio PaCO2 : mixed expired volume of alveolar and dead space gas PAH : pulmonary arterial hypertension PETCO2 : end-tidal expiratory CO2 pressure PCI : percutaneous coronary intervention PDA : personal digital assistant POISE : PeriOperative ISchaemic Evaluation trial QUO-VADIS : QUinapril On Vascular ACE and Determinants of ISchemia ROC : receiver operating characteristic SD : standard deviation SMVT : sustained monomorphic ventricular tachycardia SPECT : single photon emission computed tomography SPVT : sustained polymorphic ventricular tachycardia STEMI : ST-segment elevation myocardial infarction SVT : supraventricular tachycardia SYNTAX : synergy between percutaneous coronary intervention with taxus and cardiac surgery TACTICS : treat angina with aggrastat and determine cost of therapy with an invasive or conservative strategy TIA : transient ischaemic attack TIMI : thrombolysis in myocardial infarction TOE : transoesophageal echocardiography UFH : unfractionated heparin VCO2 : carbon dioxide production VE : minute ventilation VHD : valvular heart disease VKA : vitamin K antagonist VO2 : oxygen consumption VPB : ventricular premature beat VT : ventricular tachycardia Guidelines and Expert Consensus Documents aim to present management and recommendations based on the relevant evidence on a particular subject in order to help physicians to select the best possible management strategies for the individual patient suffering from a specific condition, taking into account not only the impact on outcome, but also the risk–benefit ratio of particular diagnostic or therapeutic means. Guidelines are no substitutes for textbooks. The legal implications of medical guidelines have been discussed previously.1 A great number of Guidelines and Expert Consensus Documents have been issued in recent years by the European Society of Cardiology (ESC) and also by other organizations or related societies. Because of the impact on clinical practice, quality criteria for development of guidelines have been established in order to make all decisions transparent to the user. The recommendations for formulating and issuing ESC guidelines and Expert Consensus Documents can be found on the ESC website in the guidelines section (www.escardio.org). In brief, experts in the field are selected and undertake a comprehensive review of the published evidence for management and/or prevention of a given condition. …

Plasma nitrite reflects constitutive nitric oxide synthase activity in mammals

Changes in plasma nitrite concentration in the human forearm circulation have recently been shown to reflect acute changes in endothelial nitric oxide synthase (eNOS)-activity. Whether basal plasma nitrite is a general marker of constitutive NOS-activity in vivo is yet unclear. Due to the rapid metabolism of nitrite in blood and the difficulties in its analytical determination literature data on levels of nitrite in mammals are largely inconsistent. We hypothesized that constitutive NOS-activity in the circulatory system is relatively uniform throughout the mammalian kingdom. If true, this should result in comparable systemic plasma nitrite levels in different species. Using three different analytical approaches we determined plasma nitrite concentration to be in a nanomolar range in a variety of species: humans (305 +/- 23 nmol/l), monkeys (367 +/- 62 nmol/l), minipigs (319 +/- 24 nmol/l), dogs (305 +/- 50 nmol/l), rabbits (502 +/- 21 nmol/l), guinea pigs (412 +/- 44 nmol/l), rats (191 +/- 43 nmol/l), and mice (457 +/- 51 nmol/l). Application of different NOS-inhibitors in humans, minipigs, and dogs decreased NOS-activity and thereby increased vascular resistance. This was accompanied by a significant, up to 80%, decrease in plasma nitrite concentration. A comparison of plasma nitrite concentrations between eNOS(-/-) and NOS-inhibited wild-type mice revealed that 70 +/- 5% of plasma nitrite is derived from eNOS. These results provide evidence for a uniform constitutive vascular NOS-activity across mammalian species.

Plasma nitrite rather than nitrate reflects regional endothelial nitric oxide synthase activity but lacks intrinsic vasodilator action

The plasma level of NOx, i.e., the sum of NO2− and NO3−, is frequently used to assess NO bioavailability in vivo. However, little is known about the kinetics of NO conversion to these metabolites under physiological conditions. Moreover, plasma nitrite recently has been proposed to represent a delivery source for intravascular NO. We therefore sought to investigate in humans whether changes in NOx concentration are a reliable marker for endothelial NO production and whether physiological concentrations of nitrite are vasoactive. NO2− and NO3− concentrations were measured in blood sampled from the antecubital vein and brachial artery of 24 healthy volunteers. No significant arterial-venous gradient was observed for either NO2− or NO3−. Endothelial NO synthase (eNOS) stimulation with acetylcholine (1–10 μg/min) dose-dependently augmented venous NO2− levels by maximally 71%. This effect was paralleled by an almost 4-fold increase in forearm blood flow (FBF), whereas an equieffective dose of papaverine produced no change in venous NO2−. Intraarterial infusion of NO2− had no effect on FBF. NOS inhibition (NG-monomethyl-l-arginine; 4–12 μmol/min) dose-dependently reduced basal NO2− and FBF and blunted acetylcholine-induced vasodilation and NO release by more than 80% and 90%, respectively. In contrast, venous NO3− and total NOx remained unchanged as did systemic arterial NO2− and NO3− levels during all these interventions. FBF and NO release showed a positive association (r = 0.85; P < 0.001). These results contradict the current paradigm that plasma NO3− and/or total NOx are generally useful markers of endogenous NO production and demonstrate that only NO2− reflects acute changes in regional eNOS activity. Our results further demonstrate that physiological levels of nitrite are vasodilator-inactive.

Nitric oxide metabolism and breakdown

The steady-state concentration and thus the biological effects of NO are critically determined not only by its rate of formation, but also by its rate of decomposition. Bioreactivity of NO at physiological concentrations may differ substantially from that suggested by in vitro experiments. The charge neutrality and its high diffusion capacity are hallmarks that characterize NO bioactivity. Reactive oxygen derived species are major determinants of NO breakdown. Biotransformation of NO and its related N-oxides occurs via different metabolic routes within the body. S-Nitrosothiols formed upon reaction of NO with redox-activated thiols represent an active storage pool for NO. The major oxidative metabolites represent nitrite and nitrate, the ratio of both is determined by the microenvironmental redox conditions. In humans, circulating nitrite represents an attractive estimate of regional endothelial NO formation, whereas nitrate, with some caution, appears useful in estimating overall nitrogen/NO turnover. Within the near future, more specific biochemical tools for diagnosis of reduced NO bioactivity will become available. Increasing knowledge on the complex metabolism of NO in vivo will lead to the development of new therapeutic strategies to enhance bioactivity of NO via modulation of its metabolism.

Guidelines for pre-operative cardiac risk assessment and perioperative cardiac management in non-cardiac surgery: The task force for preoperative cardiac risk assessment and perioperative cardiac management in non-cardiac surgery of the European society of Cardiology (ESC) and endorsed by the European society of anaesthesiology (ESA)

ESC Committee for Practice Guidelines (CPG): Alec Vahanian (Chairperson) (France), Angelo Auricchio (Switzerland), Jeroen J. Bax (The Netherlands), Claudio Ceconi (Italy), Veronica Dean (France), Gerasimos Filippatos (Greece), Christian Funck-Brentano (France), Richard Hobbs (UK), Peter Kearney (Ire

Nitrate and nitrite in biology, nutrition and therapeutics

Inorganic nitrate and nitrite from endogenous or dietary sources are metabolized in vivo to nitric oxide (NO) and other bioactive nitrogen oxides. The nitrate-nitrite-NO pathway is emerging as an important mediator of blood flow regulation, cell signaling, energetics and tissue responses to hypoxia. The latest advances in our understanding of the biochemistry, physiology and therapeutics of nitrate, nitrite and NO were discussed during a recent 2-day meeting at the Nobel Forum, Karolinska Institutet in Stockholm.

Quantitative and kinetic characterization of nitric oxide and EDRF released from cultured endothelial cells.

Endothelial cells (EC) contribute to the control of local vascular diameter by formation of an endothelium derived relaxant factor (EDRF) (1). Whether nitric oxide (NO) is identical with (EDRF) or might represent only one species of several EDRFs has not been decided as yet (2-5). Therefore, we have directly compared in cultured EC the kinetics of NO formation determined in a photometric assay with the vasodilatory effect of EDRF and NO in a bioassay. Basal release of NO was 16, 4 pmol/min/ml packed EC column. After stimulation with bradykinin (BK) and ATP onset of endothelial NO release and maximal response preceded the EDRF-mediated relaxation. Concentrations of NO formed by stimulated EC were quantitatively sufficient to fully explain the smooth muscle relaxation determined in the bioassay. Our data provide convincing evidence that under basal, BK and ATP-stimulated conditions 1. endothelial cells release nitric oxide as free radical, 2. nitric oxide is solely responsible for the vasodilatory properties of EDRF.

Nitrite reductase activity of myoglobin regulates respiration and cellular viability in myocardial ischemia-reperfusion injury

The nitrite anion is reduced to nitric oxide (NO•) as oxygen tension decreases. Whereas this pathway modulates hypoxic NO• signaling and mitochondrial respiration and limits myocardial infarction in mammalian species, the pathways to nitrite bioactivation remain uncertain. Studies suggest that hemoglobin and myoglobin may subserve a fundamental physiological function as hypoxia dependent nitrite reductases. Using myoglobin wild-type (+/+) and knockout (−/−) mice, we here test the central role of myoglobin as a functional nitrite reductase that regulates hypoxic NO• generation, controls cellular respiration, and therefore confirms a cytoprotective response to cardiac ischemia-reperfusion (I/R) injury. We find that myoglobin is responsible for nitrite-dependent NO• generation and cardiomyocyte protein iron-nitrosylation. Nitrite reduction to NO• by myoglobin dynamically inhibits cellular respiration and limits reactive oxygen species generation and mitochondrial enzyme oxidative inactivation after I/R injury. In isolated myoglobin+/+ but not in myoglobin−/− hearts, nitrite treatment resulted in an improved recovery of postischemic left ventricular developed pressure of 29%. In vivo administration of nitrite reduced myocardial infarction by 61% in myoglobin+/+ mice, whereas in myoglobin−/− mice nitrite had no protective effects. These data support an emerging paradigm that myoglobin and the heme globin family subserve a critical function as an intrinsic nitrite reductase that regulates responses to cellular hypoxia and reoxygenation. myoglobin knockout mice

Nitrite as Regulator of Hypoxic Signaling in Mammalian Physiology

In this review we consider the effects of endogenous and pharmacological levels of nitrite under conditions of hypoxia. In humans, the nitrite anion has long been considered as metastable intermediate in the oxidation of nitric oxide radicals to the stable metabolite nitrate. This oxidation cascade was thought to be irreversible under physiological conditions. However, a growing body of experimental observations attests that the presence of endogenous nitrite regulates a number of signaling events along the physiological and pathophysiological oxygen gradient. Hypoxic signaling events include vasodilation, modulation of mitochondrial respiration, and cytoprotection following ischemic insult. These phenomena are attributed to the reduction of nitrite anions to nitric oxide if local oxygen levels in tissues decrease. Recent research identified a growing list of enzymatic and nonenzymatic pathways for this endogenous reduction of nitrite. Additional direct signaling events not involving free nitric oxide are proposed. We here discuss the mechanisms and properties of these various pathways and the role played by the local concentration of free oxygen in the affected tissue.