Number 27, 2005
Metabolic approach in heart failure

Featured research

Mechanisms of creatine depletion in chronically failing rat heart
Ten Hove M, Chan S, Lygate C, et al. J Mol Cell Cardiol. 2005;38:309–313.

Clinical heart failure trials have uniformly shown that any energy-costly chronic pharmacologic treatment increases mortality, while energy-sparing treatment improves survival. The failing myocardium is characterized by deranged energetics: in addition to decreased concentrations of ATP and reduced creatine kinase activity and flux, a consistent finding has been a substantial reduction in phosphocreatine and creatine contents, resulting in a reduced energy reserve of the failing heart. It has thus been speculated that maintenance of creatine/phosphocreatine concentrations may be a potential mechanism for therapeutic intervention in heart failure. However, for the implementation of such a therapeutic approach, the regulation of creatine content in normal and failing heart needs to be fully understood. The aim of this study was to investigate the mechanisms of creatine regulation in chronically failing myocardium. This was induced in rats by left coronary artery ligation. The results show that depletion of creatine/phosphocreatine content in the failing heart is the result of reduced sarcolemmal uptake of creatine.

Commentary
This study has directly demonstrated that a major mechanism responsible for creatine depletion in the failing myocardium is reduced creatine uptake. Creatine is not synthesized in muscle, and thus the cardiomyocyte content depends on the creatine transporter accumulating creatine intracellularly against a large concentration gradient, opposed by a passive back-leak of creatine out of the cell. Creatine transport is sodium-dependent, with a stoichiometry of either 1 or 2 Na⁺ and not near equilibrium, and is thus a potential site for the control of intracellular creatine content. This study describes that heart failure led to a significant (30%) decrease in intracellular creatine content and to a significant (26%) reduction in creatine uptake.
The signaling pathways involved in downregulation of creatine uptake in heart failure are currently unknown. Among others, factors that regulate the Na⁺ gradient across the cell membrane, thus influencing the thermodynamic driving force of the cell, have been suggested to play a part in the regulation of creatine transporter activity. It has previously been shown that the strategy of chronically providing high dosages of creatine to the failing heart is ineffective in preventing the decrease in creatine content [1]. The results presented here demonstrate that a possible explanation for the inefficiency of oral creatine in chronic heart failure is the fact that the rate of uptake of creatine is reduced in the failing myocardium.
Whether this reduction in the creatine uptake capacity constitutes a pathophysiological or an adaptive mechanism in heart failure remains to be determined. A pathophysiological mechanism would be mediated by a reduction in total creatine content, thereby decreasing the rate and extent of intracellular ATP transfer via the creatine kinase reaction, and limiting energy availability at the myofibrils. In contrast, an adaptive mechanism leading to a reduction in free creatine as a response to decreasing phosphocreatine concentrations would prevent a decrease in the free-energy change of ATP hydrolysis, and maintaining a high free-energy change is essential for maintenance of contractile function.
If reductions in creatine content do have a causal role in heart failure, then understanding the regulation of creatine concentrations and the factors that govern down- and upregulation of the rates of uptake of creatine may be clinically relevant in the search for new treatment strategies specifically targeted at maintaining cardiac energetics.

D. Feuvray

Sildenafil prevents endothelial dysfunction induced by ischemia and reperfusion via opening of adenosine triphosphate-sensitive potassium channels: a human in vivo study
Gori T, Sicuro S, Dragoni S, Donati G, Forconi S, Parker JD. Circulation. 2005;111:742–746.

Animal studies have demonstrated that administration of sildenafil can limit myocardial damage induced by prolonged ischemia, an effect that appears to be mediated by opening of ATP-sensitive potassium (K_ATP) channels. No study has investigated whether sildenafil can also prevent the impairment in endothelium-dependent vasodilatation induced by ischemia-reperfusion in humans. In a double-blind, placebo-controlled, crossover design, 10 healthy male volunteers (25–45 years old) were allocated randomly to groups to receive oral sildenafil (50mg) or placebo. Two hours later, endothelium-dependent, flow-mediated dilatation (FMD) of the radial artery was measured before and after ischemia-reperfusion (15min of ischemia at the level of the brachial artery, followed by 15min of reperfusion). Seven days later, the volunteers received the other treatment (ie, placebo or sildenafil) and underwent the same procedure.
Radial artery diameter and FMD before ischemia-reperfusion, and baseline radial artery diameter after ischemia-reperfusion, were similar between visits (P=NS). After administration of placebo, ischemia-reperfusion significantly blunted FMD (before ischemia-reperfusion: 7.9±1.1%; after ischemia-reperfusion: 1.2±0.7%; P <0.01). Importantly, sildenafil limited this impairment in endothelium-dependent vasodilatation (before ischemia-reperfusion: 7.0±0.9%; after ischemia-reperfusion: 6.2±1.1%; P=NS; P <0.01 compared with placebo). In a separate procedure, this protective effect was completely prevented by previous administration of the sulfonylurea, glibenclamide (5mg), a blocker of K_ATP channels (n=7; FMD before ischemia-reperfusion: 10.3±1.5%; after ischemia-reperfusion: 1.3±1.4%; P <0.05).
We conclude that, in humans, oral sildenafil induces potent protection against ischemia-reperfusion-induced endothelial dysfunction through opening of K_ATP channels. Further studies are needed to test the potential clinical implications of this finding.

Commentary
Sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor, has been shown to improve endothelial function in acute and chronic dosing studies in diabetic and nondiabetic individuals. Tadalafil has also been shown to improve endothelial function in men at increased cardiovascular risk. In mice, sildenafil reversed pre-established ventricular hypertrophy induced by pressure overload and restored chamber function to normal. Similar studies in humans (men and women) are awaited, but in this paper Gori et al have shown, in healthy male volunteers, that sildenafil can induce potent endothelial protection as a result of opening KATP channels.
Animal and human studies suggest that PDE5 inhibitors may have important effects in addition to their use in treating erectile dysfunction. These important vascular and potential myocardial protective actions have already been demonstrated in the treatment of pulmonary hypertension, with minimal adverse effects. With no interaction with metabolic agents, the concept of an additive or synergistic action from combination therapy is one of the many important clinical areas to warrant further study.

G. Jackson

Uncoupling proteins in human heart
Murray AJ, Anderson RE, Watson GC, Radda GK, Clarke K. Lancet. 2004;364:1786–1788.

Abnormal energetic activity in heart failure correlates inversely with plasma free fatty acid concentrations. However, the link between energetic and metabolic abnormalities is unknown. To investigate this association, we obtained blood samples from 39 patients undergoing coronary artery bypass graft surgery. Patients fasted overnight before samples were taken. When plasma free fatty acid concentrations were increased, concentrations of cardiac mitochondrial uncoupling protein (UCP) increased (isoform UCP2, P <0.0001; isoform UCP3, P=0.0036) and those of glucose transporter 4 (GLUT 4) protein decreased (cardiac, P=0.0001; skeletal muscle, P=0.0006). Consequently, energy deficiency in heart failure might result from increased concentrations of mitochondrial UCPs (ie, less efficient ATP synthesis) and depleted GLUT 4 (ie, reduced glucose uptake). New treatments to correct these energy defects would be to achieve a simultaneous decrease in plasma free fatty acids and provision of an alternative energy source.

Commentary
Cardiac energetics and energy reserve can be decreased in patients with heart failure. Alterations in mitochondrial function and ATP production have been implicated in this decreased capacity for energy production in the failing myocardium. Direct assessments of mitochondrial function in the heart have shown that a number of defects can occur in the failing heart, including a decrease in electron transport chain activity. Another metabolic alteration that can occur in patients with heart failure is an increase in plasma fatty acid concentrations. This study by Murray et al shows that, in patients with low ejection fraction undergoing elective cardiac surgery, increased plasma fatty acid concentrations correlated with an increase in mitochondrial UCP2. UCP2 can decrease the proton gradient in the inner mitochondrial matrix, thereby uncoupling electron transport chain activity from ATP production. This study provides a possible alternative mechanism by which energy depletion can occur in heart failure – uncoupling of mitochondrial respiration and a decrease in ATP production. High plasma concentrations of fatty acids seen in patients with heart failure may exacerbate this mitochondrial dysfunction. The authors propose that a reduction in concentrations of circulating fatty acids or provision of an alternative substrate (such as glucose) may reduce the expression of UCP2 and thereby improve the efficiency of mitochondrial energy production. This possibility remains to be explored.

G. Lopaschuk

Cardiovascular risk factors emerge after artificial selection for low aerobic capacity
Wisløff U, Najjar SM, Ellingsen O, et al. Science. 2005;307:418–420.

In humans, the strong statistical association between fitness and survival suggests a link between impaired oxygen metabolism and disease. We hypothesized that artificial selection of rats based on low and high intrinsic exercise capacity would yield models that also contrasted for disease risk. After 11 generations, rats with low aerobic capacity scored high on cardiovascular risk factors that constitute the metabolic syndrome. The decrease in aerobic capacity was associated with decreases in the amounts of transcription factors required for mitochondrial biogenesis and in the amounts of oxidative enzymes in skeletal muscle. Impairment of mitochondrial function may link reduced fitness to cardiovascular and metabolic disease.

Commentary
An individual's aerobic/exercise capacity is probably determined by the interaction of numerous biochemical and physiological characteristics that are both genetically and environmentally influenced. However, studies in twins confirm a substantial genetic component that may determine both aerobic capacity in the untrained state and the additional performance gained by training. There is also a wealth of evidence that shows an individual's aerobic capacity relates to risk of cardiovascular events and to the presence of features of the metabolic syndrome such as insulin resistance and adiposity. The difficulty lies in establishing how these factors interrelate and which is chicken and which is egg.
This fascinating and far-reaching study by Wisløff et al is based on the detailed phenotyping of rats bred for aerobic capacity. The study started with 192 genetically heterogenous rats. Through a structured and regimented training program, the 13 males and females with the greatest exercise capacity were selected and bred. Their offspring were similarly selected and bred; inbreeding was avoided by maintaining at least 13 families throughout the study. Exactly the same graded treadmill exercise test was used to select rats with low aerobic capacity. After 11 generations of selective breeding, offspring of families bred for high exercise performance were capable of running 3.5 times as far as offspring of families bred for low exercise performance. The question is, what other differences existed between the low capacity runners (LCRs) and high capacity runners (HCRs)?
Despite being selected only for traits relating to exercise capacity, the HCR rats were healthier than the LCR rats, even in their sedentary state. For example, their mean daytime blood pressure was 17mm Hg lower (although differences at night were less marked), endothelial function (as assessed by in-vitro responses to acetylcholine) was 48% better, fasting glucose 20% lower, insulin concentrations 131% lower, triglycerides 168% lower, and intra-abdominal fat 63% lower. In their untrained state, the HCRs' greater VO₂max was associated with hearts that were slighter heavier and comprised of myocytes with better contractile performance when isolated. Thus it seems that the genes that determine low exercise performance also determine features of the metabolic syndrome. The authors hypothesized that the two may be interrelated through compromised mitochondrial oxidative function.
Comparing soleus muscle harvested from the LCRs with that obtained from the HCRs, the authors found evidence of markedly reduced mitochondrial biogenesis and function, including reduced amounts of peroxisome proliferative activated receptor gamma (PPAR-?) and PPAR-? coactivator 1a.
The above analyses were performed in rats aged 16–24 weeks, equivalent to young adulthood. At this age, the HCRs were already leaner than the LCRs. It is possible, therefore, that the changes described are secondary to body weight rather than directly genetically determined. To address this, further analyses were performed in 5-week-old HCR and LCR pups that had identical weights and visceral fat. Despite identical adiposity, there were already significant differences in circulating glucose and triglycerides, suggesting that, in this model, genetically determined metabolic derangements may precede, and even contribute to, obesity, rather than vice versa.
This study provides an excellent example of the bidirectional nature of translational research. The notion that impaired regulation of oxidative pathways in mitochondria may be a common factor linking reduced aerobic capacity to cardiovascular and metabolic disease had originally been suggested by epidemiological and clinical studies. The authors have reinforced this interrelationship by showing that the genetic selection of reduced aerobic capacity results in coincident mitochondrial dysfunction and cardiovascular risk. Before using this information to influence choice of partner I, for one, would like to know the answer to the following question: Are the LCRs more intelligent than the HCRs?

M. Marber

Endothelial dysfunction and damage in congestive heart failure: relation of flow-mediated dilation to circulating endothelial cells, plasma indexes of endothelial damage, and brain natriuretic peptide
Chong AY, Blann AD, Patel J, Freestone B, Hughes E, Lip GY. Circulation. 2004;110:1794–1798.

Congestive heart failure (CHF) is associated with endothelial perturbation (as defined by flow-mediated endothelial-dependent vasodilatation [FMD, an index of endothelial dysfunction], circulating endothelial cells [CECs, an index of endothelial damage], or plasma indexes of endothelial damage/dysfunction [eg, von Willebrand factor (vWF) and soluble thrombomodulin (sTM)]) and increased plasma concentrations of brain natriuretic peptide (BNP, a peptide hormone associated with left ventricular systolic dysfunction and prognosis). However, the relationships between these parameters are unclear.
To test the hypothesis that there is a relationship between endothelial perturbation (defined by FMD, CECs, vWF, and sTM) and BNP in CHF, we studied these indexes in 30 patients with CHF who were compared with 20 age-matched control individuals. FMD, CECs, plasma vWF, and BNP concentrations (but not sTM) were all abnormal in patients with CHF. There were significant inverse correlations between FMD and vWF (P=0.001), CECs (P=0.002), and BNP (P=0.006), and a positive correlation between CECs and vWF (P=0.032). In multivariate analysis, BNP (P <0.001) and FMD (P <0.001) were both independently associated with CHF.
We conclude that ample evidence of endothelial cell damage/dysfunction in CHF cannot be fully explained by the variance in plasma BNP per se. Therefore, the routes by which these indexes influence the pathophysiology of CHF and predict adverse outcomes may be independent.

Commentary
A generalized endothelial damage has been suggested to contribute to CHF. Cellular, biochemical, and hemodynamic markers of endothelial dysfunction have been consistently found to be abnormal in patients with heart failure. In the present study, an inverse correlation between FMD of the brachial artery and increased concentrations of vWF is reported. Impaired endothelium-dependent vasodilatation may explain the abnormal vasoconstriction that is a hallmark of CHF. Increased vWF is an established marker of endothelial damage that facilitates platelet adhesion and activation. The presence of increased numbers of circulating endothelial cells in heart failure is proposed as evidence that CECs are desquamated, damaged endothelial cells, raising the possibility that some part of the vascular wall may be denuded. Endothelial denudation exposes the underlying collagen, activating the coagulation system that, coupled with increased vWF, may explain the excess risk of thromboembolism associated with heart failure.
An inverse relationship is also reported between FMD and plasma BNP, neither being apparently related with left ventricular ejection fraction or New York Heart Association class.
Interestingly enough, both increased BNP and endothelial function may be improved in heart failure by the administration of trimetazidine. The mechanism linking the cardiac energy metabolism switch associated with the inhibition of 3-ketoacyl coenzyme A thiolase to the improvement in endothelial function and the reduction in BNP, in addition to reductions in inflammatory markers, is not clear at present. The possibility of a direct protective and anti-inflammatory effect of trimetazidine on the endothelium appears worthy of direct investigation, both into its possible use as a therapeutic alternative in heart failure, and to gain a better understanding of the pathogenetic mechanisms of progressive heart failure.

M. Marzilli

Evaluation of 18F-FDG uptake and arterial wall calcifications using 18F-FDG PET/CT
Ben-Haim S, Kupzov E, Tamir A, Israel O. [Abstract] J Nucl Med. 2004;45:1816–1821.

Glucose metabolic activity expressed as uptake of [¹⁸F]2-fluoro-2-deoxyglucose (18F-FDG) may be increased in active atherosclerotic plaque. Calcium depositions are often increased in mature atherosclerotic plaque. The purpose of the present study was to assess the patterns of vascular wall uptake of 18F-FDG and computed tomography (CT) calcifications, using combined positron emission tomography (PET)/CT.
We evaluated retrospectively 122 consecutive patients older than 50 years (47 women and 75 men; mean age 66±9 years) undergoing whole-body 18F-FDG PET/CT for tumor assessment. PET, CT, and PET/CT slices were generated for review. Abnormal vascular findings in major arteries in the chest and abdomen were categorized as PET positive (PET+), PET negative (PET-), CT positive (CT+), or CT negative (CT-). The topographic relationship between increased vascular wall uptake of 18F-FDG on PET and the presence of calcifications on CT was assessed on PET/CT fused images, with abnormal sites further classified as PET+/CT+, PET+/CT-, or PET-/CT+. The presence of CT calcifications and increased vascular wall uptake of 18F-FDG was correlated with age, sex, presence of cardiovascular risk factors, and cardiovascular disease.
Abnormal findings were identified at 349 sites. CT calcifications (CT+) were observed at 320 sites (92%) in 100 patients (82%), more commonly in men (P<0.03), in older patients (P <0.0001), in patients with hypertension (P <0.003) or hyperlipidemia (P<0.04), and in smokers (P <0.008). Increased vascular wall uptake of 18F-FDG (PET+) was observed at 52 sites (15%) in 38 patients (31%), more commonly in men (P <0.02), in older patients (P <0.0001), and in patients with hypertension (P<0.02), and was borderline in patients with cardiovascular disease (P=0.057). PET+ and CT+ findings correlated in 12 patients, a PET+/CT- pattern was found in 18 patients, and eight patients exhibited increased vascular wall uptake of 18F-FDG in sites with and without calcifications (PET+/CT+, CT-). Twenty-two patients (18%) had a PET-/CT- pattern.
We conclude that hybrid PET/CT can be used to identify and correctly localize vascular wall 18F-FDG activity. Increased vascular wall 18F-FDG activity was found in 15% of sites and CT calcifications were noted in 92% of sites, with congruent findings in 7%. The clinical significance of the relationship between vascular wall uptake of 18F-FDG and CT calcifications needs to be assessed by further prospective studies with long-term follow-up.

Commentary
Electron-beam CT can diagnose atherosclerosis in the arterial wall by detecting the presence of calcium. This is a well established technique and several studies and reviews have indicated its use as a screening tool for atherosclerosis. Moreover, the technique may provide prognostic information. A relatively new technique with which to visualize atherosclerosis is the use of [¹⁸F]2-fluoro-2-deoxyglucose (FDG) in combination with PET. Earlier studies have shown that, in the normal vessel wall, there is no measurable uptake of FDG, whereas in the atherosclerotic plaque FDG is taken up predominantly by macrophages. In the present study by Ben-Haim et al, the authors compared the uptake of FDG and electron-beam CT calcification in patients, using a novel technique of combined PET and CT scanning. Interestingly, the authors found a high prevalence of CT calcifications and a low prevalence of FDG uptake. Moreover, there was a low coincidence of CT-positive and FDG-positive findings.
In an excellent editorial in the same issue of Journal of Nuclear Medicine, Peter Weissberg offers a plausible explanation for these findings. He states that calcification and FDG uptake measure different features of the atherosclerotic plaque. FDG uptake is related to active metabolism of macrophages, and is therefore closely related to inflammation. Inflammation may lead to plaque rupture and coronary events. In contrast, calcification is a long-lasting, cumulative phenomenon, and may be the result of repeated episodes of inflammation. It is therefore a very frequent finding of atherosclerosis, but may have a limited prognostic effect.
Thus combined CT and metabolic imaging of the arterial wall is a new and exciting technique, which may give further insight into the dynamics of the atherosclerotic process.

Frans Visser

REFERENCES