Biochemical adaptations in cardiac hypertrophy

Cardiac hypertrophy is the adaptive response of the heart to chronic overload. The metabolic adaptations that occur during hypertrophy are initially beneficial, but can ultimately deteriorate into heart failure. The mechanisms underlying this are unknown. Evidence of impaired energy reserve, which may be caused by changes in the profile of substrate use, has been implicated in the transition of compensatory hypertrophy to heart failure. The work of this thesis characterises the alterations in substrate utilisation that occur in the heart, secondary to pressure-overload induced cardiac hypertrophy, the their implications on heart function.Pressure-overload hypertrophy was induced surgically in male Sprague- Dawley rats by inter-renal ligation. 13C-NMR spectroscopy was performed on extracts from hypertrophied and control hearts perfused with 13C-labelled substrate mixtures to determine the profile of substrate utilisation. Nine weeks pressure- overload achieved a moderate hypertrophy, evidenced by a 10-15% increase in heart mass to tibia length. The hypertrophied hearts showed an increased reliance on glucose and endogenous substrate contribution to TCA cycle oxidation for the production of ATP (15.0% versus 11.0%) compared to control hearts.Prolonged fifteen weeks pressure-overload resulted in further metabolic changes including impaired long-chain fatty acid oxidation and the accumulation of long-chain acylcarnitines. Alteration in substrate utilisation preceded any change in heart function and is strong evidence to suggest that impaired substrate delivery at the level of the mitochondria in cardiac hypertrophy plays an important role in the development of heart failure and is not a secondary phenomenon. At high workloads both hypertrophied and control hearts, showed similar profiles of substrate use, with glucose being the predominant substrate utilised for TCA cycle oxidation. At high workloads, hypertrophied hearts initially exhibited significantly higher mechanical function, but was not sustained, suggesting that physiological changes were becoming detrimental. This study highlights that sequential metabolic adaptations occur during the development of hypertrophy and precede any functional abnormality, providing potential prognostic markers

Fuel availability and fate in cardiac metabolism: A tale of two substrates

The heart’s extraordinary metabolic flexibility allows it to adapt to normal changes in physiology in order to preserve its function. Alterations in the metabolic profile of the heart have also been attributed to pathological conditions such as ischemia and hypertrophy; however, research during the past decade has established that cardiac metabolic adaptations can precede the onset of pathologies. It is therefore critical to understand how changes in cardiac substrate availability and use trigger events that ultimately result in heart dysfunction. This review examines the mechanisms by which the heart obtains fuels from the circulation or from mobilization of intracellular stores. We next describe experimental models that exhibit either an increase in glucose use or a decrease in FA oxidation, and how these aberrant conditions affect cardiac metabolism and function. Finally, we highlight the importance of alternative, and relatively under investigated, strategies for the treatment of heart failure

Metabolic therapy: cardioprotective effects of orotic acid and its derivatives

Metabolic therapy involves the administration of a substance normally found in the human body to enhance cellular reactions involved in the pathogenesis of disease. Myocardial ischaemia/reperfusion injury represents a leading cause of morbidity and mortality, also in cardiovascular disease. Therapeutic strategies aimed at limiting cardiomyocyte death during the postischaemic reperfusion and in the perioperative settings are nowadays extensively studied. Conceived originally as a dietary constituent (known as vitamin B13) only, it is now apparent that most orotic acid is synthesized in the human body where it arises as an intermediate in the biosynthetic pathway of pyrimidine nucleotides. Previous investigations in the heart suggest that orotate and its derivatives could be of significant clinical benefit in the treatment of heart disease. The present brief review is concerned with the current knowledge of the major effects of these compounds in both experimental and clinical cardiology. The potential mechanisms and biochemical pathways responsible for cardioprotection are highlighted.Biomedical Reviews 2010; 21: 47-55

Pathophysiology of Heart Failure

Heart failure occurs when the heart is unable to maintain a cardiac output sufficient to satisfy the oxygen requirements of the body despite adequate blood volume and hemoglobin content. Regardless of the initial cause of heart failure and in spite of compensatory mechanisms, patients often follow a course of worsening heart failure that is characterized by a low cardiac output, high filling pressures, and increased peripheral vascular resistance. In addition to persistence of the initiating event, cardiac deterioration may be caused or aggravated by a variety of factors including depletion of cardiac norepinephrine stores, down-regulation of myocardial beta-adrenergic receptors, microvascular spasm with resultant further cellular necrosis, and subendocardial ischemia perpetuating myocardial failure

Myocardial substrate metabolism in the normal and failing heart

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest that impaired substrate metabolism contributes to contractile dysfunction and to the progressive left ventricular remodeling that are characteristic of the heart failure state. The general concept that has recently emerged is that myocardial substrate selection is relatively normal during the early stages of heart failure; however, in the advanced stages there is a downregulation in fatty acid oxidation, increased glycolysis and glucose oxidation, reduced respiratory chain activity, and an impaired reserve for mitochondrial oxidative flux. This review discusses 1) the metabolic changes that occur in chronic heart failure, with emphasis on the mechanisms that regulate the changes in the expression of metabolic genes and the function of metabolic pathways; 2) the consequences of these metabolic changes on cardiac function; 3) the role of changes in myocardial substrate metabolism on ventricular remodeling and disease progression; and 4) the therapeutic potential of acute and long-term manipulation of cardiac substrate metabolism in heart failure

Probing cardiac metabolism in uraemic cardiomyopathy

Cardiovascular complications are the leading cause of death in patients with chronic kidney disease (CKD). Uraemic cardiomyopathy (UCM) is characterised by structural and cellular remodelling including left ventricular hypertrophy (LVH), metabolic remodelling and mitochondrial dysfunction. Although ex vivo studies have highlighted evidence of enhanced glucose utilisation in the hypertrophied heart, cardiac glucose metabolism in uraemia has yet to be established in vivo. In addition, little is known about mitochondrial morphology or the impact of iron therapy on cardiac mitochondrial function in CKD. The aims of this study were to (I) investigate cardiac glucose metabolism in vivo using ¹⁸F-flurodeoxyglucose positron emission tomography (¹⁸F-FDG PET) during the development of UCM and (II) characterise mitochondrial morphology and the impact of iron therapy on cardiac mitochondrial function in uraemia.Experimental uraemia was induced surgically in male Sprague-Dawley rats via a subtotal nephrectomy. Dynamic PET/CT scans were acquired at 5, 9 and 13 weeks post-surgery using ¹⁸F-FDG PET. The rate and distribution of ¹⁸F-FDG uptake were determined using Patlak and polar map analysis. In a separate series of experiments the iron complex, ferumoxytol, was administered 6 weeks post-surgery and mitochondrial respiratory rates and enzyme activities determined following sacrifice 6 weeks later. Cardiac mitochondrial morphology was characterised by probing the expression of key mitochondrial fusion and fission proteins and evaluating mitochondrial size and structure in left ventricular tissue and isolated mitochondria.Renal dysfunction was prominent in uraemic animals by 12 weeks as evidenced by elevated serum creatinine, urea and the presence of anaemia. LVH was associated with moderately increased ¹⁸F-FDG uptake in the uraemic heart at 5, 9 and 13 weeks. This was paralleled at the cellular level by altered mitochondrial morphology, characterised by a more sparsely packed cristae, and increased mitochondrial state 4 respiration, indicative of reduced efficiency. However, ferumoxytol treatment did not impact on cardiac mitochondrial function at this stage of uraemia. Collectively these data suggest there is evidence of enhanced glucose utilisation in the uraemic heart in vivo and these changes are associated with altered mitochondrial structure and bioenergetics

Changing strategies in the management of heart failure

AbstractForty years ago therapy for congestive heart failure was limited largely to the mercurial diuretics and a variety of cardiac glycoside preparations; these were often ineffective, and the common practice of “pushing” digitalis caused serious, sometimes lethal side effects. Today, a more complete understanding of the regulation of cardiac work and pathophysiology of heart failure is having a profound impact on therapeutic strategy for this common condition. Despite more powerful means to augment myocardial contractility and much more effective diuretics, therapy that relies only on inotropic stimulation and diuresis is no longer optimal for the majority of patients with heart failure. Thus, strategies for the therapy of heart failure must take into account new understanding of mechanisms that initiate, perpetuate and exacerbate the hemodynamic and myocardial abnormalities in these patients.Recognition of the detrimental effects of excessive afterload and the importance of relaxation (lusitropic) as well as contraction (inotropic) abnormalities has led to widespread acceptance of vasodilator therapy, which has dramatically improved our ability to alleviate the symptoms of heart failure. Changes that result from altered gene expression in the hypertrophied myocardium of patients with congestive heart failure can give rise to a cardiomyopathy of overload that, although initially compensatory, may hasten death. These and other advances in our understanding of the pathophysiology, biochemistry and molecular biology of heart failure provide a basis for new therapeutic strategies that can slow the progressive myocardial damage that causes many of these patients to die, while at the same time improving well-being in patients with congestive heart failure

Myocardial energy depletion and dynamic systolic dysfunction in hypertrophic cardiomyopathy

Evidence indicates that anatomical and physiological phenotypes of hypertrophic cardiomyopathy (HCM) stem from genetically mediated, inefficient cardiomyocyte energy utilization, and subsequent cellular energy depletion. However, HCM often presents clinically with normal left ventricular (LV) systolic function or hyperkinesia. If energy inefficiency is a feature of HCM, why is it not manifest as resting LV systolic dysfunction? In this Perspectives article, we focus on an idiosyncratic form of reversible systolic dysfunction provoked by LV obstruction that we have previously termed the 'lobster claw abnormality' — a mid-systolic drop in LV Doppler ejection velocities. In obstructive HCM, this drop explains the mid-systolic closure of the aortic valve, the bifid aortic pressure trace, and why patients cannot increase stroke volume with exercise. This phenomenon is characteristic of a broader phenomenon in HCM that we have termed dynamic systolic dysfunction. It underlies the development of apical aneurysms, and rare occurrence of cardiogenic shock after obstruction. We posit that dynamic systolic dysfunction is a manifestation of inefficient cardiomyocyte energy utilization. Systolic dysfunction is clinically inapparent at rest; however, it becomes overt through the mechanism of afterload mismatch when LV outflow obstruction is imposed. Energetic insufficiency is also present in nonobstructive HCM. This paradigm might suggest novel therapies. Other pathways that might be central to HCM, such as myofilament Ca2+ hypersensitivity, and enhanced late Na+ current, are discussed

Myocardial metabolism evaluation and ketones utilization in the failing human heart

Background: Under normal circumstances, free fatty acids (FFAs) are the predominant energetic substrate of the heart. In experimental and end-stage models of heart failure (HF) a quantitative switch from a predominance of FFAs utilization to the more energy favourable ketone bodies has been demonstrated. The aim of this study was to identify and quantify the heart substrates utilization in mild to moderate human HF. Methods: β-hydroxybutyrate, lactate, triacyclglycerols, glucose and FFAs concentrations in arterial, coronary sinus (CS), and central venous beds were measured after an overnight fast to derive myocardial substrates utilization in HF patients and controls scheduled for cardiac device implanting procedures. Results: A total of 15 HF patients and 11 controls were enrolled. Arterial and CS metabolites concentration were similar between the groups. A significant reduction in the myocardial FFAs extraction was showed in HF patients compared to controls (HF 0.07 +- 0.23 mmol/L vs non-HF 0.25 +- 0.16 mmol/L, p=0.03), together with an inverse association between FFAs and neurohormonal and echocardiographic HF hallmarks. Opposite, β-hydroxybutyrate, lactate, triacyclglycerols and glucose extractions were relatively unchanged between groups. The net cardiac extraction of β-hydroxybutyrate was directly associated to HF duration. When diabetic and non-diabetic patients were compared among HF population, the results were substantially similar, with a slight trend in reduction of FFAs net extraction (HF 0.03 +- 0.30 mmol/L vs non-HF 0.26 +- 0.12 mmol/L, p=0.09). Conclusions: In our study ketone bodies utilization was unchanged between mild to moderate HF patients and controls. We showed a reduced myocardial FFAs extraction consistent with a downregulation of beta-oxidation in the failing heart in HF population. Future studies are needed to clarify mechanisms that regulate the metabolic switch and its timing

A study of mitochondrial redox state in cardiac muscle

This thesis describes the use of intrinsic fluorescence measurements as a means for examining mitochondrial function in different cardiac preparations and phenotypes. Cardiac myocytes are intrinsically fluorescent and spectroscopic analysis of rabbit ventricular myocytes indicated that the majority of this fluorescence arises from the metabolic coenzymes nicotinamide adenine dinucleotide in the reduced state (NADH) and flavin adenine dinucleotide (FAD) in the oxidised state. Calibration of the NADH and FAD fluorescence signal with the mitochondrial inhibitors sodium cyanide (NaCN) and carbonyl cyanide p- (trifluoromethoxy) phenylhydrazone (FCCP) enabled calculation of mitochondrial redox states. Redox measurements reflect the balance between reduced and oxidised forms of the NAD and FAD pools and provide an index for assessing mitochondrial function in cells and tissue. The major advantage of this technique is that the intrinsically fluorescent nature of these metabolites obviates the need for exogenous indicators of mitochondrial function, which can themselves influence mitochondrial behaviour. Mitochondrial redox state was established using a variety of fluorescence techniques. Values for NADstate represent the proportion of the NADH/NAD+ redox couple in the reduced state. Calculation of NADstate using single photon, two photon and wide-field epifluorescence microscopy revealed very similar values ranging from 0.57±0.18 to 0.59±0.17 (mean±SD). FAD fluorescence measurements were used to establish FADstate (the proportion of the FADH2/FAD redox couple in the oxidised state). However, FAD fluorescence could only be detected by epifluorescence and single photon excitation fluorescence microscopy. Once again, comparable values of 0.17±0.10 and 0.18±0.07 respectively were obtained, thus demonstrating the reproducibility of the technique. Attempts were made to perform these measurements in intact cardiac tissue preparations. However, difficulties encountered with the delivery of mitochondrial inhibitor to specific areas of tissue and problems with inner filter effects complicating the interpretation of fluorescence recordings meant that this was not possible. Measurements of intrinsic fluorescence were utilised in order to assess the mitochondrial redox response of cardiac cells to increased energy demand. Isolated rabbit ventricular myocytes were field stimulated and fractional shortening was simultaneously recorded with epifluorescence measurements of NADH and FAD. Cells were paced at 0.5Hz and the stimulation frequency step increased to 1Hz, 2Hz and 3Hz in order to increase work intensity and energy demand. Step increasing stimulation frequency resulted in a decrease in NADH fluorescence and an increase in FAD fluorescence before reaching an essentially steady state. This indicated oxidation of the cell environment, suggesting a transient mismatch between metabolite supply and demand. The magnitude of this response was related to stimulation frequency, with the biggest responses taking place at the highest work intensity. Reducing work intensity back to 0.5Hz pacing resulted in immediate recovery of metabolite fluorescence. Investigation into the redox response to increased work intensity in the stroke prone spontaneously hypertensive rat (SHRSP) model of cardiac hypertrophy found that energy supply and demand matching was in fact improved in these cardiomyocytes compared to Wistar Kyoto (WKY) control myocytes. Work intensity was increased from 1Hz to 2, 4 and 6Hz pacing and the oxidative response to increased workload was found to be significantly less in SHRSP cardiomyocytes compared to WKY myocytes (p<0.01). This was despite similar levels of contractile work being performed by the two groups and may be related to the young age of the animals (16 weeks). At this age, hypertrophy of the SHRSP hearts is likely to still be in the compensated state and mitochondrial function may indeed be improved rather than detrimentally affected at this stage