Impaired myocardial autophagy linked to energy metabolism disorders

Autophagy represents an evolutionarily conserved catabolic mechanism that promotes cell survival by releasing energy substrates via degradation of cellular constituents and by eliminating defective organelles under conditions of stress, such as starvation and hypoxia. The link between enhanced autophagy and nutrient deprivation has been well established. For example, chronic myocardial ischemia, a condition of insufficient oxygen and nutrition, activates autophagy to degrade and recycle damaged cellular structures, thereby ameliorating cardiomyocyte injury.     

Impaired myocardial autophagy linked to energy metabolism disorders

Autophagy represents an evolutionarily conserved catabolic mechanism that promotes cell survival by releasing energy substrates via degradation of cellular constituents and by eliminating defective organelles under conditions of stress, such as starvation and hypoxia. The link between enhanced autophagy and nutrient deprivation has been well established. For example, chronic myocardial ischemia, a condition of insufficient oxygen and nutrition, activates autophagy to degrade and recycle damaged cellular structures, thereby ameliorating cardiomyocyte injury.     

Re-balancing cellular energy substrate metabolism to mend the failing heart

Fatty acids and glucose are the main substrates for myocardial energy provision. Under physiologic conditions, there is a distinct and finely tuned balance between the utilization of these substrates. Using the non-ischemic heart as an example, we discuss that upon stress this substrate balance is upset resulting in an over-reliance on either fatty acids or glucose, and that chronic fuel shifts towards a single type of substrate appear to be linked with cardiac dysfunction. These observations suggest that interventions aimed at re-balancing a tilted substrate preference towards an appropriate mix of substrates may result in restoration of cardiac contractile performance. Examples of manipulating cellular substrate uptake as a means to re-balance fuel supply, being associated with mended cardiac function underscore this concept. We also address the molecular mechanisms underlying the apparent need for a fatty acid–glucose fuel balance. We propose that re-balancing cellular fuel supply, in particular with respect to fatty acids and glucose, may be an effective strategy to treat the failing heart.     

Effect of cardioplegia on nitrogen and energy metabolism of the human heart

The interrelation between the energy and nitrogenous metabolism of the myocardium during cardioplegia has been studied in patients with congenital valvular heart disease (tetralogy of Fallot--12 patients, ventricular septal defect--5 patients). Whole body hypothermia with repeated heart reperfusion with cold cardioplegic blood perfusate was used for the protection of the myocardium. However, ATP level of the myocardium of some patients decreased by 20% and more of the baseline. This loss was accompanied by a reduction in glutamate and aspartate levels and a rise in ammonium and alanine levels in the myocardium (by 17.7 +/- 3.8; 17.6 +/- 5.9; 61.4 +/- 12.5 and 92.4 +/- 26.3% of the baseline, respectively).     

Impairment of glucose metabolism and energy transfer in the hypertriglyceridemic rat heart

The metabolic pathways involved in ATP production in hypertriglyceridemic rat hearts were evaluated. Hearts from male Wistar rats with sugar-induced hypertriglyceridemia were perfused in an isolated organ system. Mechanical performance, oxygen uptake and beat rate were evaluated under perfusion with different oxidizable substrates. Age- and weight-matched animals were used as control. The hypertriglyceridemic (HTG) hearts showed a decrease in the mechanical work and slight diminution in the oxygen uptake when perfused with glucose, pyruvate or lactate. No differences were found when perfused with palmitate, octanoate or -hydroxybutyrate. The glycolytic flux in HTG hearts was 2.4 times lower than in control hearts. Phosphofructokinase-I (PFK-I) was 16% decreased in HTG hearts, whereas pyruvate kinase activity did not change. The increased levels of glucose-6hyphen;phosphate in HTG heart, suggested a flux limitation by the PFK-I. Pyruvate dehydrogenase in its active form (PDHa) diminished as well. The PDHa level in the HTG hearts was restored to control values by dichloroacetate; however, this addition did not significantly improve the mechanical performance. Levels of ATP and phosphocreatine as well as total creatine kinase activity and the MB fraction were significant lower in the HTG hearts perfused with glucose. The data suggested that supply of ATP by glucose oxidation did not suffice to support cardiac work in the HTG hearts; this impairment was exacerbated by the diminution of the creatine kinase system output.     

Impaired mitophagy at the heart of injury

Recent publications link mitophagy mediated by PINK1 and Parkin with cardioprotection and attenuation of inflammation and cell death. The field is in need of methods to monitor mitochondrial turnover in vivo to support the development of new therapies targeting mitochondrial turnover.     

Impaired mitophagy at the heart of injury

Recent publications link mitophagy mediated by PINK1 and Parkin with cardioprotection and attenuation of inflammation and cell death. The field is in need of methods to monitor mitochondrial turnover in vivo to support the development of new therapies targeting mitochondrial turnover.     

Comparative energy metabolism in cultured heart muscle and hela cells

The yields of energy from oxidation of fatty acids, glucose, and glutamine were compared in cultures of chick embryo heart muscle (heart) and HeLa cells. Aerobic energy production, as measured by oxygen utilization, was comparable in the two cell types. In media containing dialyzed sera, the rates of incorporation of fatty acids directly into lipids were similar in both cells and accounted for > 97% of fatty acid metabolism in HeLa cells. However, in heart cells only 45% ended in lipid, 42% in protein, and 13% was released as CO₂; the latter two products probably reflect the oxidation of fatty acids to acetyl-coenzyme A (-CoA) and its subsequent metabolism in the citrate cycle. Increased serum concentration in the medium did not affect fatty acid metabolism in HeLa cultures, but resulted in greater oxidation by heart cells (> 100 times that by HeLa cells). The metabolisms of both glucose and glutamine were similar in heart and HeLa cells with ? 60% of glucose carbon ending as medium lactate and only 3–5% converted to acetyl-CoA. About 25% of glutamine carbon ended as CO₂ and increased utilizations with increasing serum concentrations was accountable in both cells by increased lactate from glucose and glutamate from glutamine. CO₂ production (and energy) from glutamine was independent of glutamine concentration within a tenfold range of physiological concentrations. The yields of energy have been calculated. In 10% dialyzed calf serum, oxidation of glutamine carbon provided about half of the total energy in heart cells, glucose about 35–45%, with most coming from glycolysis; oxidation of fatty acid carbon provided only 5–10%. That > 90% of the aerobic energy comes from glutamine in both cells can account for the comparable rates of oxygen utilization. HeLa cells derived little or no energy from fatty acids.     

The effect of K+ concentration on the energy metabolism in perfused rat heart

From experiments at various perfusion pressures in hemoglobin-free perfused rat hearts, oxygen consumption and redox shift of pyridine nucleotide were found to vary linearly with cardiac work. This relation was used for analysis of the energy metabolism associated with ion pumps. Mechanical activities such as left ventricular pressure and heart rate varied with the extracellular K+ concentration. Ion-pump dependent changes in oxygen consumption and redox state of pyridine nucleotide, estimated as the difference of the values at normal (4.7 mM) and various other extracellular K+ concentrations with corrections for the change due to mechanical work, were found to vary linearly with the K+ concentration. The slope for oxygen consumption was about 0.1 mumol/min/g X wet wt per mM K+. Lactate release changed markedly but transiently, about 1 min after changing the extracellular K+ concentration, and its amount varied linearly with the K+ concentration. In the steady state, however, lactate release was almost independent of the extracellular K+ concentration, although oxidized pyridine nucleotide increased with increasing K+ concentration. Coronary flow increased with the extracellular K+ concentration. Heart rate changed little between 1 and 12 mM K+, but decreased sharply above 12 mM K+. At 20 mM K+, heart beat was arrested and approximately 40% of myoglobin was deoxygenated. The intracellular oxygen concentration was estimated to be about 10 microM even during aerobic perfusion. Similarly, Ca2+-free arrested heart was found to be in a hypoxic state. The results showed that oxygen entry into cardiac tissue is facilitated by the cardiac cycle.     

In silico studies of energy metabolism of normal and diseased heart

Biotechnology research is developing into genomic analyses that involve the simultaneous monitoring of thousands of genes. The development of various bioinformatics resources that provide efficient access to information is necessary. We have used single-pass sequencing of randomly selected cDNA clones to generate expressed sequence tags (ESTs). These ESTs data has been widely used to study gene expression in a variety of heart libraries [1, 21]. Data annotation on our recent finding allows us to construct the profiles of genes in the energy metabolizing pathways (glycolysis and glycogen metabolism) that are expressed in heart cDNA libraries. In silico studies of genes of energy metabolism yields data that are consistent with results derived from conventional metabolic experiments. The change in gene profiles describing the metabolism of diseased hearts is also presented here.     

Substrate-induced alterations of high energy phosphate metabolism and contractile function in the perfused heart

The bioenergetic basis by which the Krebs cycle substrate pyruvate increased cardiac contractile function over that observed with the Embden-Meyerhof substrate glucose was investigated in the isovolumic guinea pig heart. Alterations in the content of the high energy phosphate metabolites and the rate of high energy phosphate turnover were measured by 31P NMR. These were correlated to the changes in contractile function and rates of myocardial oxygen consumption. Maximum left ventricular developed pressure (LVDP) and high energy phosphates were observed with 16 mM glucose or 10 mM pyruvate. In hearts perfused with 16 mM glucose, the intracellular phosphocreatine (PCr) concentration was 15.2 +/- 0.6 mM with a PCr/Pi ratio of 10.3 +/- 0.9. The O2 consumption was 5.35 mumol/g wet weight/min, and these hearts exhibited a LVDP of 97 +/- 3.7 mm Hg at a constant paced rate of 200 beats/min. In contrast, when hearts were switched to 10 mM pyruvate, the PCr concentration was 18.3 +/- 0.4 mM, the PCr/Pi ratio was 30.4 +/- 2.2, the O2 consumption was 6.67 mumol/g wet weight/min, and the LDVP increased to 125 +/- 3.3 mm Hg. From NMR saturation transfer experiments, the steady-state flux of ATP synthesis from PCr was 4.9 mumol/s/g of cell water during glucose perfusion and 6.67 mumol/s/g of cell water during pyruvate perfusion. The flux of ATP synthesis from ADP was measured to be 0.99 mumol/s/g of cell water with glucose and calculated to be 1.33 mumol/s/g of cell water with pyruvate. These results suggest that pyruvate quite favorably alters myocardial metabolism in concert with the increased contractile performance. Thus, as a mechanism to augment myocardial performance, pyruvate appears to be unique.     

Impaired renal function and aluminum metabolism

The consequences of renal functional impairment on aluminum (Al) excretion are not clear inasmuch as little is known about its glomerular filtration, tubular reabsorption, or secretion. The association of Al and the etiology of the dialysis encephalopathy syndrome and osteomalacia among patients with uremia suggests that renal functional impairment is a prerequisite for increased body Al stores. However, considerable evidence argues against the concept that tissue Al accumulation occurs as a simple consequence of renal failure. Many dialysis patients have high parathyroid hormone (PTH) concentrations that have been associated with neurologic abnormalities, bone disease, and anemia. The toxicity of PTH could be either direct or indirect by influencing the metabolism of potentially toxic substances such as Al. Our studies in normal rats suggest that gastrointestinal Al absorption and specific tissue burdens are enhanced by PTH, but not irreversibly, because the withdrawal of PTH resulted in Al egress. Dialysis patients are often treated with vitamin D analogs to prevent or control consequences of hyperparathyroidism and impaired 1,25-dihydroxycholecalciferol synthesis. Although some reports suggest that high bone Al in osteomalacia may be responsible for vitamin D resistance, our studies with normal rats suggest that its metabolites may also increase tissue Al burdens independent of PTH action. Thus, several factors operative in uremia other than impaired renal function may contribute to altered Al metabolism and, consequently, to its toxicity.     

Impaired glucose metabolism in hypertensive patients

We have studied glucose tolerance under carefully controlled conditions in 79 patients with arterial hypertension. The results show that, in patients with arterial hypertension but without clinical diabetes mellitus, the glucose tolerance was abnormal in 77.3% and normal in 22.3%. The corresponding figure in the control group of normotensive subjects was 0%. In each test the responses to glucose administration were analyzed by plotting the logarithm of the blood glucose concentration against time. For the points between 60 and 120 min, corresponding to the periods following glucose administration, a linear relationship was obtained and showed a decline at an exponential rate, as noted by other observers. An estimate of the volume of distribution of glucose was obtained as follows. Values observed in hypertensives with a pathological percent fall in blood glucose per minute (Kg) were 29.8 +/- 12.0 (mean +/- SD) liters and those in normal subjects with normal Kg values had a mean of 14.35 +/- 2.98, the difference being highly significant (p less than 0.0001). The results of the theoretical glucose concentration are also presented. Those obtained from subjects with normal Kg values (359.0 +/- 58.4 mg/dl) are significantly higher than in subjects with pathological Kg values (257.6 +/- 51.3 mg/dl; p less than 0.0001). All patients with either pathological or normal Kg values had normal glucose concentration levels, fasting blood sugar and no glucose in the urine specimen. The difference between pathological Kg values (107.0 +/- 25.8 mg/dl) and normal Kg values (90.6 +/- 13.0 mg/dl) was not found to be statistically different (p greater than 0.05). The distribution and means of glucose half time in controls with normal Kg values and hypertensives with pathological Kg values were: 63.5 +/- 11.5 and 137.8 +/- 48.1 min, respectively. The difference between normal and pathological Kg values being statistically significant at a confidence level above 99.5%. We also studied the free glucose pool at zero time. A significantly higher level was found in hypertensives with pathological Kg values, again indicating an impairment in glucose metabolism in this group: 90.6 +/- 26.5 vs. 65.0 +/- 5.4 g (p less than 0.0001). Another study showed an estimate of the mean cellular glucose uptake (MCUg) per minute and per kilogram body weight. The MCUg following glucose loading decreased considerably in hypertensives with pathological Kg values. The percentage reduction ranged between 50 and 55% hypertensives with pathological Kg values 4.1 +/- 0.8, and normotensives with normal Kg values, 8.0 +/- 0.6 (p less than 0.0001).(ABSTRACT TRUNCATED AT 400 WORDS)     

Cardiac mitochondrial energy metabolism in heart failure: Role of cardiolipin and sirtuins

Mitochondrial oxidation of fatty acids accounts for the majority of cardiac ATP production in the heart. Fatty acid utilization by cardiac mitochondria is controlled at the level of fatty acid uptake, lipid synthesis, mobilization and mitochondrial import and oxidation. Consequently defective mitochondrial function appears to be central to the development of heart failure. Cardiolipin is a key mitochondrial phospholipid required for the activity of the electron transport chain. In heart failure, loss of cardiolipin and tetralinoleoylcardiolipin helps to fuel the generation of excessive reactive oxygen species that are a by-product of inefficient mitochondrial electron transport chain complexes I and III. In this vicious cycle, reactive oxygen species generate lipid peroxides and may, in turn, cause oxidation of cardiolipin catalyzed by cytochrome c leading to cardiomyocyte apoptosis. Hence, preservation of cardiolipin and mitochondrial function may be keys to the prevention of heart failure development. In this review, we summarize cardiac energy metabolism and the important role that fatty acid uptake and metabolism play in this process and how defects in these result in heart failure. We highlight the key role that cardiolipin and sirtuins play in cardiac mitochondrial β-oxidation. In addition, we review the potential of pharmacological modulation of cardiolipin through the polyphenolic molecule resveratrol as a sirtuin-activator in attenuating mitochondrial dysfunction. Finally, we provide novel experimental evidence that resveratrol treatment increases cardiolipin in isolated H9c2 cardiac myocytes and tetralinoleoylcardiolipin in the heart of the spontaneously hypertensive rat and hypothesize that this leads to improvement in mitochondrial function. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.