Increased nonoxidative glycolysis despite continued fatty acid uptake during demand-induced myocardial ischemia

Modulation of NFKB1/p50 by ROS leads to impaired ATP production during MI compared to cardiac hypertrophy

Pathological hypertrophy and myocardial infarction (MI) are two etiologically different cardiac disorders having differential molecular mechanisms of disease manifestation. However, no study has been conducted so far to analyze and compare the differential status of energy metabolism in these two disease forms. It was shown recently by our group that production of ATP is significantly impaired during MI along with inhibition of pyruvate dehydrogenase E1-β (PDHE1 B) by pyruvate dehydrogenase kinase 4 (PDK4). However, the ATP levels showed no significant change during pathological hypertrophy compared to control group. To seek a plausible explanation of this phenomenon, the peroxisome proliferator-activated receptor alpha (PPAR) pathway was studied in all the experimental groups which revealed that PGC1α- ERRα axis remains active in MI while the same remained inactive during pathological hypertrophy possibly by NF-κB that plays a significant role in deactivating this pathway during hypertrophy. At the same time, it was observed that reactive oxygen species (ROS) negatively regulates NF-κB activity during MI by oxidation of cysteine residues of p50- the DNA binding subunit of NF-κB. Thus, this study reports for the first time, a possible mechanism for the differential status of energy metabolism during two etiologically different cardiac pathophysiological conditions involving PGC1α-ERRα axis along with p50 subunit of NF-κB.

Dipropionylcysteine ethyl ester compensates for loss of citric acid cycle intermediates during post ischemia reperfusion in the pig heart

PURPOSE

During reperfusion, following myocardial ischemia, uncompensated loss of citric acid cycle (CAC) intermediates may impair CAC flux and energy transduction. Propionate has an anaplerotic effect when converted to the CAC intermediate succinyl-CoA, and may improve contractile recovery during reperfusion. Antioxidant therapy with N-acetylcysteine decreases reperfusion injury. To synergize the antioxidant effects of cysteine with the anaplerotic effects of propionate, we synthesized a novel bi-functional compound, N,S-dipropionyl cysteine ethyl ester (DPNCE) and tested its anaplerotic and anti-oxidative capacity in anesthetized pigs.

METHODS

Ischemia was induced by a 70% reduction in left anterior descending coronary artery flow for one hour, followed by 1 h of reperfusion. After 30 min of ischemia and throughout reperfusion animals were treated with saline or intravenous DPNCE (1.5 mg x kg(-1) x min(-1), n = 8/group). Arterial concentrations and myocardial propionate, cysteine, free fatty acids, glucose and lactate uptakes, cardiac mechanical functions, myocardial content of CAC intermediates and oxidative stress were assessed.

RESULTS

Ischemia resulted in reduction in myocardial tissue concentration of CAC intermediates. DPNCE treatment elevated arterial propionate and cysteine concentrations and myocardial propionate uptake, and increased myocardial concentrations of citrate, succinate, fumarate, and malate compared to saline treated animals. DPNCE treatment did not affect blood pressure or myocardial contractile function, but increased arterial free fatty acid concentration and myocardial fatty acid uptake. Arterial cysteine concentration was elevated by DPNCE, but there was negligible myocardial cysteine uptake, and no change in markers of oxidative stress.

CONCLUSION

DPNCE elevated arterial cysteine and propionate, and increased myocardial concentration of CAC intermediates, but did not affect mechanical function or oxidative stress.

Effects of exercise training and diet on lipid kinetics during free fatty acid-induced insulin resistance in older obese humans with impaired glucose tolerance

Elevated free fatty acids (FFA) are implicated with insulin resistance at the cellular level. However, the contribution of whole body lipid kinetics to FFA-induced insulin resistance is not well understood, and the effect of exercise and diet on this metabolic defect is not known. We investigated the effect of 12 wk of exercise training with and without caloric restriction on FFA turnover and oxidation (FFA(ox)) during acute FFA-induced insulin resistance. Sixteen obese subjects with impaired glucose tolerance were randomized to either a hypocaloric (n = 8; -598 +/- 125 kcal/day, 66 +/- 1 yr, 32.8 +/- 1.8 kg/m(2)) or a eucaloric (n = 8; 67 +/- 2 yr, 35.3 +/- 2.1 kg/m(2)) diet and aerobic exercise (1 h/day at 65% of maximal oxygen uptake) regimen. Lipid kinetics ([1-(14)C]palmitate) were assessed throughout a 7-h, 40 mU x m(-2) x min(-1) hyperinsulinemic euglycemic clamp, during which insulin resistance was induced in the last 5 h by a sustained elevation in plasma FFA (intralipid/heparin infusion). Despite greater weight loss in the hypocaloric group (-7.7 +/- 0.5 vs. -3.3 +/- 0.7%, P < 0.001), FFA-induced peripheral insulin resistance was improved equally in both groups. However, circulating FFA concentrations (2,123 +/- 261 vs. 1,764 +/- 194 micromol/l, P < 0.05) and FFA turnover (3.20 +/- 0.58 vs. 2.19 +/- 0.58 micromol x kg FFM(-1) x min(-1), P < 0.01) during hyperlipemia were suppressed only in the hypocaloric group. In contrast, whole body FFA(ox) was improved in both groups at rest and during hyperlipemia. These changes were driven by increases in intracellular lipid-derived FFA(ox) (12.3 +/- 7.7 and 14.7 +/- 7.8%, P < 0.05). We conclude that the exercise-induced improvement in FFA-induced insulin resistance is independent of the magnitude of weight loss and FFA turnover, yet it is linked to increased intracellular FFA utilization.

Impact of anaerobic glycolysis and oxidative substrate selection on contractile function and mechanical efficiency during moderate severity ischemia

The role of anaerobic glycolysis and oxidative substrate selection on contractile function and mechanical efficiency during moderate severity myocardial ischemia is unclear. We hypothesize that 1) preventing anaerobic glycolysis worsens contractile function and mechanical efficiency and 2) increasing glycolysis and glucose oxidation while inhibiting free fatty acid oxidation improves contractile function during ischemia. Experiments were performed in anesthetized pigs, with regional ischemia induced by a 60% decrease in left anterior descending coronary artery blood flow for 40 min. Three groups were studied: 1) no treatment, 2) inhibition of glycolysis with iodoacetate (IAA), or 3) hyperinsulinemia and hyperglycemia (HI + HG). Glucose and free fatty acid oxidation were measured using radioisotopes and anaerobic glycolysis from net lactate efflux and myocardial lactate content. Regional contractile power was assessed from left ventricular pressure and segment length in the anterior wall. We found that preventing anaerobic glycolysis with IAA during ischemia in the absence of alterations in free fatty acid and glucose oxidation did not adversely affect contractile function or mechanical efficiency during myocardial ischemia, suggesting that anaerobic glycolysis is not essential for maintaining residual contractile function. Increasing glycolysis and glucose oxidation with HI + HG inhibited free fatty acid oxidation and improved contractile function and mechanical efficiency. In conclusion, these results show a dissociation between myocardial function and anaerobic glycolysis during moderate severity ischemia in vivo, suggesting that metabolic therapies should not be aimed at inhibiting anaerobic glycolysis per se, but rather activating insulin signaling and/or enhancing carbohydrate oxidation and/or decreasing fatty acid oxidation.

Transmural distribution of metabolic abnormalities and glycolytic activity during dobutamine-induced demand ischemia

The heterogeneity across the left ventricular wall is characterized by higher rates of oxygen consumption, systolic thickening fraction, myocardial perfusion, and lower energetic state in the subendocardial layers (ENDO). During dobutamine stimulation-induced demand ischemia, the transmural distribution of energy demand and metabolic markers of ischemia are not known. In this study, hemodynamics, transmural high-energy phosphate (HEP), 2-deoxyglucose-6-phosphate (2-DGP) levels, and myocardial blood flow (MBF) were determined under basal conditions, during dobutamine infusion (DOB: 20 microg x kg(-1) x min(-1) iv), and during coronary stenosis + DOB + 2-deoxyglucose (2-DG) infusion. DOB increased rate pressure products (RPP) and MBF significantly without affecting the subendocardial-to-subepicardial blood flow ratio (ENDO/EPI) or HEP levels. During coronary stenosis + DOB + 2-DG infusion, RPP, ischemic zone (IZ) MBF, and ENDO/EPI decreased significantly. The IZ ratio of creatine phosphate-to-ATP decreased significantly [2.30 +/- 0.14, 2.06 +/- 0.13, and 2.04 +/- 0.11 to 1.77 +/- 0.12, 1.70 +/- 0.11, and 1.72 +/- 0.12 for EPI, midmyocardial (MID), and ENDO, respectively], and 2-DGP accumulated in all layers, as evidenced by the 2-DGP/PCr (0.55 +/- 0.12, 0.52 +/- 0.10, and 0.37 +/- 0.08 for EPI, MID, and ENDO, respectively; P < 0.05, EPI > ENDO). In the IZ the wet weight-to-dry weight ratio was significantly increased compared with the normal zone (5.9 +/- 0.5 vs. 4.4 +/- 0.4; P < 0.05). Thus, in the stenotic perfused bed, during dobutamine-induced high cardiac work state, despite higher blood flow, the subepicardial layers showed the greater metabolic changes characterized by a shift toward higher carbohydrate metabolism, suggesting that a homeostatic response to high-cardiac work state is characterized by more glucose utilization in energy metabolism.

Circadian rhythms in myocardial metabolism and contractile function: influence of workload and oleate

Multiple extracardiac stimuli, such as workload and circulating nutrients (e.g., fatty acids), known to influence myocardial metabolism and contractile function exhibit marked circadian rhythms. The aim of the present study was to investigate whether the rat heart exhibits circadian rhythms in its responsiveness to changes in workload and/or fatty acid (oleate) availability. Thus, hearts were isolated from male Wistar rats (housed during a 12:12-h light-dark cycle: lights on at 9 AM) at 9 AM, 3 PM, 9 PM, and 3 AM and perfused in the working mode ex vivo with 5 mM glucose plus either 0.4 or 0.8 mM oleate. Following 20-min perfusion at normal workload (i.e., 100 cm H(2)O afterload), hearts were challenged with increased workload (140 cm H(2)O afterload plus 1 microM epinephrine). In the presence of 0.4 mM oleate, myocardial metabolism exhibited a marked circadian rhythm, with decreased rates of glucose oxidation, increased rates of lactate release, decreased glycogenolysis capacity, and increased channeling of oleate into nonoxidative pathways during the light phase. Rat hearts also exhibited a modest circadian rhythm in responsiveness to the workload challenge when perfused in the presence of 0.4 mM oleate, with increased myocardial oxygen consumption at the dark-to-light phase transition. However, rat hearts perfused in the presence of 0.8 mM oleate exhibited a markedly blunted contractile function response to the workload challenge during the light phase. In conclusion, these studies expose marked circadian rhythmicities in myocardial oxidative and nonoxidative metabolism as well as responsiveness of the rat heart to changes in workload and fatty acid availability.

Heart failure progression is accelerated following myocardial infarction in type 2 diabetic rats

Clinical studies have shown a greater incidence of myocardial infarction in diabetic patients, and following an infarction, diabetes is associated with an increased risk for the development of left ventricular (LV) dysfunction and heart failure. The goal of this study was to determine if the progression of heart failure following myocardial infarction in type 2 diabetic (T2D) rats is accelerated compared with nondiabetic rats. Male nondiabetic Wistar-Kyoto (WKY) and T2D Goto-Kakizaki (GK) rats underwent coronary artery ligation or sham surgery to induce heart failure. Postligation (8 and 20 wk), two-dimensional echocardiography and LV pressure measurements were made. Heart failure progression, as assessed by enhanced LV remodeling and contractile dysfunction, was accelerated 8 wk postligation in the T2D animals. LV remodeling was evident from increased end-diastolic and end-systolic diameters and areas in the GK compared with the WKY infarcted group. Furthermore, enhanced LV contractile dysfunction was evident from a greater deterioration in fractional shortening and enhanced myocardial performance index (an index of global LV dysfunction) in the GK infarcted group. This accelerated progression was accompanied by greater increases in atrial natriuretic factor and skeletal alpha-actin (gene markers of heart failure and hypertrophy) mRNA levels in GK infarcted hearts. Despite similar decreases in metabolic gene expression (i.e., peroxisome proliferator-activated receptor-alpha-regulated genes associated with fatty acid oxidation) between infarcted WKY and GK rat hearts, myocardial triglyceride levels were elevated in the GK hearts only. These results, demonstrating enhanced remodeling and LV dysfunction 8 wk postligation provide evidence of an accelerated progression of heart failure in T2D rats.

Parallel activation of mitochondrial oxidative metabolism with increased cardiac energy expenditure is not dependent on fatty acid oxidation in pigs

Steady state concentrations of ATP and ADP in vivo are similar at low and high cardiac workloads; however, the mechanisms that regulate the activation of substrate metabolism and oxidative phosphorylation that supports this stability are poorly understood. We tested the hypotheses that (1) there is parallel activation of mitochondrial and cytosolic dehydrogenases in the transition from low to high workload, which increases NADH/NAD+ ratio in both compartments, and (2) this response does not require an increase in fatty acid oxidation (FAO). Anaesthetized pigs were subjected to either sham treatment, or an abrupt increase in cardiac workload for 5 min with dobutamine infusion and aortic constriction. Myocardial oxygen consumption and FAO were increased 3- and 2-fold, respectively, but ATP and ADP concentrations did not change. NADH-generating pathways were rapidly activated in both the cytosol and mitochondria, as seen in a 40% depletion in glycogen stores, a 3.6-fold activation of pyruvate dehydrogenase, and a 50% increase in tissue NADH/NAD+. Simulations from a multicompartmental computational model of cardiac energy metabolism predicted that parallel activation of glycolysis and mitochondrial metabolism results in an increase in the NADH/NAD+ ratio in both cytosol and mitochondria. FAO was blocked by 75% in a third group of pigs, and a similar increase in and the NAHD/NAD+ ratio was observed. In conclusion, in the transition to a high cardiac workload there is rapid parallel activation of substrate oxidation that results in an increase in the NADH/NAD+ ratio.

Malonyl-CoA Decarboxylase Inhibition as a Novel Approach to Treat Ischemic Heart Disease

INTRODUCTION

During and following cardiac ischemia the levels of circulating fatty acids are elevated, resulting in fatty acid oxidation dominating as a source of oxidative metabolism at the expense of pyruvate oxidation. A decrease in the levels of myocardial malonyl-CoA (an endogenous inhibitor of mitochondrial fatty acid uptake) contributes to these high fatty acid oxidation rates. Low pyruvate oxidation rates during and following ischemia results in the accumulation of metabolic byproducts (lactate and protons) that leads to impaired cardiac function, decreased cardiac efficiency, and increased myocardial tissue injury.

METHODOLOGY

One approach to increasing pyruvate oxidation during and following ischemia is to inhibit fatty acid oxidation, which results in an improvement of both cardiac function and cardiac efficiency. A novel approach to decreasing fatty acid oxidation and increasing pyruvate oxidation is to increase myocardial levels of malonyl-CoA. This can be achieved by pharmacologically inhibiting malonyl-CoA decarboxylase (MCD), the principal enzyme involved in the degradation of cardiac malonyl-CoA.

RESULTS

Studies with either genetic deletion of MCD in the mouse or with novel MCD inhibitors show that decreased MCD activity increases cardiac malonyl-CoA, resulting in an inhibition of fatty acid oxidation and a stimulation of pyruvate oxidation.

CONCLUSION

The beneficial effects of MCD inhibition on cardiac function and cardiac efficiency suggest that this approach could be an effective means to treat ischemic heart disease.

Role of Cellular Compartmentation in the Metabolic Response to Stress: Mechanistic Insights from Computational Models

The mechanisms controlling ATP generation in the transition from normal resting conditions to either high work states or ischemia are poorly understood. ATP generation depends upon compartmentation between the mitochondria and cytosol of metabolic pathways and key energy transfer species that cannot be easily assessed experimentally. We developed a multicompartment mathematical model of cardiac metabolism to simulate the metabolic responses to ischemia and increased workload. The model is based on mass balances, transport, and metabolic processes in cardiac tissue, and has three distinct compartments (blood, cytosol, and mitochondria). In addition to distinguishing between cytosol and mitochondria, the model includes a cytosolic subcompartment for glycolytic metabolic channeling. The model simulations predict the rapid activation of glycogenolysis and lactate production at the onset of ischemia, and support the concept of localization of glycolysis to a cytosolic subcompartment. In addition, simulations show that mitochondrial NADH/NAD(+) is primarily determined by oxygen consumption during ischemia, while cytosolic NADH/NAD(+) and lactate production are largely a function of glycolytic flux during the initial phase, and is controlled by mitochondrial NADH/NAD(+) and the malate-aspartate shuttle during the steady state. Finally, the model predicts that metabolic activation with an abrupt increase in workload requires parallel activation of ATP hydrolysis, glycolysis, mitochondrial dehydrogenases, the electron transport chain, and ADP phosphorylation. Taken together, these studies demonstrate the importance of metabolic compartmentation in the regulation of cardiac energetics in response to acute stress, and they highlight the usefulness of computational models in this line of investigation.

Acute lipoprotein lipase deletion in adult mice leads to dyslipidemia and cardiac dysfunction

The most energy-requiring organ in the body, the cardiac muscle, relies primarily on lipoprotein-derived fatty acids. Prenatal loss of cardiac lipoprotein lipase (LPL) leads to hypertriglyceridemia, but no cardiac dysfunction, in young mice. Cardiac specific loss of LPL in 8-wk-old mice was produced by a 2-wk tamoxifen treatment of MerCreMer (MCM)/Lpl(flox/flox) mice. LPL gene deletion was confirmed by PCR analysis, and LPL mRNA expression was reduced by approximately 70%. One week after tamoxifen was completed, triglyceride was increased with LPL deletion, 162 +/- 53 vs. 91 +/- 21 mg/dl, P < 0.01. Tamoxifen treatment of Lpl(flox/flox) mice did not cause a significant increase in triglyceride levels. Four weeks after tamoxifen, MCM/Lpl(flox/flox) mice had triglyceride levels of 190 +/- 27 mg/dl, similar to those of mice with prenatal LPL deletion. One week after the tamoxifen, MCM/Lpl(flox/flox), but not Lpl(flox/flox), mice had decreases in carnitine palmitoyl transferase I mRNA (18%) and pyruvate dehydrogenase kinase 4 mRNA (38%). These changes in gene expression became more robust with time. Acute loss of LPL decreased ejection fraction and increased mRNA levels for atrial natriuretic factor. Our studies show that acute loss of LPL can be produced and leads to rapid alteration in gene expression and cardiac dysfunction.

Differential effects of saturated and unsaturated fatty acid diets on cardiomyocyte apoptosis, adipose distribution, and serum leptin

Fatty acids are the primary fuel for the heart and are ligands for peroxisome proliferator-activated receptors (PPARs), which regulate the expression of genes encoding proteins involved in fatty acid metabolism. Saturated fatty acids, particularly palmitate, can be converted to the proapoptotic lipid intermediate ceramide. This study assessed cardiac function, expression of PPAR-regulated genes, and cardiomyocyte apoptosis in rats after 8 wk on either a low-fat diet [normal chow control (NC); 10% fat calories] or high-fat diets composed mainly of either saturated (Sat) or unsaturated fatty acids (Unsat) (60% fat calories) (n = 10/group). The Sat group had lower plasma insulin and leptin concentrations compared with the NC or Unsat groups. Cardiac function and mass and body mass were not different. Cardiac triglyceride content was increased in the Sat and Unsat groups compared with NC (P < 0.05); however, ceramide content was higher in the Sat group compared with the Unsat group (2.9 +/- 0.2 vs. 1.4 +/- 0.2 nmol/g; P < 0.05), whereas the NC group was intermediate (2.3 +/- 0.3 nmol/g). The number of apoptotic myocytes, assessed by terminal deoxynucleotide transferase-mediated dUTP nick-end labeling staining, was higher in the Sat group compared with the Unsat group (0.28 +/- 0.05 vs. 0.17 +/- 0.04 apoptotic cells/1,000 nuclei; P < 0.04) and was positively correlated to ceramide content (P < 0.02). Both high-fat diets increased the myocardial mRNA expression of the PPAR-regulated genes encoding uncoupling protein-3 and pyruvate dehydrogenase kinase-4, but only the Sat diet upregulated medium-chain acyl-CoA dehydrogenase. In conclusion, dietary fatty acid composition affects cardiac ceramide accumulation, cardiomyocyte apoptosis, and expression of PPAR-regulated genes independent of cardiac mass or function.

High-fat diet prevents cardiac hypertrophy and improves contractile function in the hypertensive dahl salt-sensitive rat

1. The role that dietary lipid and plasma fatty acid concentration play in the development of cardiac hypertrophy in response to hypertension is not clear. 2. In the present study, we treated Dahl salt-sensitive rats with either normal chow (NC), normal chow with salt added (NC + salt) or a diet high in long-chain saturated fatty acids with added salt (HFD + salt). Cardiac function was assessed by echocardiography and left ventricular (LV) catheterization. 3. The HFD + salt group had significantly higher plasma free fatty acid concentrations and myocardial triglyceride content compared with the NC + salt group, but did not upregulate the activity of the fatty acid oxidation enzyme medium chain acyl-coenzyme A dehydrogenase. Systolic blood pressure was elevated to a similar extent in the NC + salt and HFD + salt groups compared with the NC group. Although LV mass was increased in the NC + salt group compared with the NC group, LV mass in the HFD + salt group did not differ from that of the NC group and was significantly lower than that in the NC + salt group. 4. There was no evidence of cardiac dysfunction in the NC + salt group compared with the NC group; however, high fat feeding significantly increased LV contractile performance (e.g. increased cardiac output and peak dP/dt). 5. In conclusion, the HFD + salt diet prevented the hypertrophic response to hypertension and improved the contractile performance of the heart. It remains to be determined whether preventing cardiac hypertrophic adaptations would be deleterious to the heart if the hypertensive stress is maintained long term.

Malonyl-CoA decarboxylase inhibition suppresses fatty acid oxidation and reduces lactate production during demand-induced ischemia

The rate of cardiac fatty acid oxidation is regulated by the activity of carnitine palmitoyltransferase-I (CPT-I), which is inhibited by malonyl-CoA. We tested the hypothesis that the activity of the enzyme responsible for malonyl-CoA degradation, malonyl-CoA decarboxlyase (MCD), regulates myocardial malonyl-CoA content and the rate of fatty acid oxidation during demand-induced ischemia in vivo. The myocardial content of malonyl-CoA was increased in anesthetized pigs using a specific inhibitor of MCD (CBM-301106), which we hypothesized would result in inhibition of CPT-I, reduction in fatty acid oxidation, a reciprocal activation of glucose oxidation, and diminished lactate production during demand-induced ischemia. Under normal-flow conditions, treatment with the MCD inhibitor significantly reduced oxidation of exogenous fatty acids by 82%, shifted the relationship between arterial fatty acids and fatty acid oxidation downward, and increased glucose oxidation by 50%. Ischemia was induced by a 20% flow reduction and β-adrenergic stimulation, which resulted in myocardial lactate production. During ischemia MCD inhibition elevated malonyl-CoA content fourfold, reduced free fatty acid oxidation rate by 87%, and resulted in a 50% decrease in lactate production. Moreover, fatty acid oxidation during ischemia was inversely related to the tissue malonyl-CoA content ( r = −0.63). There were no differences between groups in myocardial ATP content, the activity of pyruvate dehydrogenase, or myocardial contractile function during ischemia. Thus modulation of MCD activity is an effective means of regulating myocardial fatty acid oxidation under normal and ischemic conditions and reducing lactate production during demand-induced ischemia.

Differential effects of heptanoate and hexanoate on myocardial citric acid cycle intermediates following ischemia-reperfusion

In the normal heart, there is loss of citric acid cycle (CAC) intermediates that is matched by the entry of intermediates from outside the cycle, a process termed anaplerosis. Previous in vitro studies suggest that supplementation with anaplerotic substrates improves cardiac function during myocardial ischemia and/or reperfusion. The present investigation assessed whether treatment with the anaplerotic medium-chain fatty acid heptanoate improves contractile function during ischemia and reperfusion. The left anterior descending coronary artery of anesthetized pigs was subjected to 60 min of 60% flow reduction and 30 min of reperfusion. Three treatment groups were studied: saline control, heptanoate (0.4 mM), or hexanoate as a negative control (0.4 mM). Treatment was initiated after 30 min of ischemia and continued through reperfusion. Myocardial CAC intermediate content was not affected by ischemia-reperfusion; however, treatment with heptanoate resulted in a more than twofold increase in fumarate and malate, with no change in citrate and succinate, while treatment with hexanoate did not increase fumarate or malate but increased succinate by 1.8-fold. There were no differences among groups in lactate exchange, glucose oxidation, oxygen consumption, and contractile power. In conclusion, despite a significant increase in the content of carbon-4 CAC intermediates, treatment with heptanoate did not result in improved mechanical function of the heart in this model of reversible ischemia-reperfusion. This suggests that reduced anaplerosis and CAC dysfunction do not play a major role in contractile and metabolic derangements observed with a 60% decrease in coronary flow followed by reperfusion.

Myocardial energy metabolism during ischemia and the mechanisms of metabolic therapies

The primary effect of ischemia is reduced aerobic adenosine triphosphate (ATP) formation in mitochondria. This triggers accelerated glycolysis and reduced cell pH, Ca(2+) accumulation, K(+) efflux, adenosine formation, and the clinical signs of ischemia: chest pain and a shift in the ST segment. Traditional therapies for angina are aimed at either decreasing the need for ATP by suppressing heart rate, blood pressure, and cardiac contractility, or at increasing oxygen delivery to the mitochondria, or both. An additional approach to treating angina is to suppress myocardial fatty acid oxidation, increase pyruvate oxidation, and reduce anaerobic glycolysis. High fatty acid levels result in oxygen wasting and inhibit the oxidation of pyruvate in the mitochondria. In experimental models, the partial inhibition of myocardial fatty acid oxidation with agents such as oxfenicine, ranolazine, and trimetazidine stimulates glucose oxidation and reduces lactate production during ischemia. Clinical studies demonstrate that this approach is as effective as traditional hemodynamic therapies at improving exercise tolerance and reducing the frequency of angina. Moreover, because these agents do not suppress heart rate, blood pressure, or contractility, they are effective as add-on therapy to Ca(2+)-channel and beta-adrenergic receptor antagonists.

Inhibition of very long chain acyl-CoA dehydrogenase during cardiac ischemia

The heart utilizes primarily fatty acids for energy production. During ischemia, however, diminished oxygen supply necessitates a switch from beta-oxidation of fatty acids to glucose utilization and glycolysis. Molecular mechanisms responsible for these alterations in metabolism are not fully understood. Mitochondrial acyl-CoA dehydrogenase catalyzes the first committed step in the beta-oxidation of fatty acids. In the current study, an in vivo rat model of myocardial ischemia was utilized to determine whether specific acyl-CoA dehydrogenases exhibit ischemia-induced alterations in activity, identify mechanisms responsible for changes in enzyme function, and assess the effects on mitochondrial respiration. Very long chain acyl-CoA dehydrogenase (VLCAD) activity declined 34% during 30 min of ischemia. Loss in activity appeared specific to VLCAD as medium chain acyl-CoA dehydrogenase activity remained constant. Loss in VLCAD activity during ischemia was not due to loss in protein content. In addition, activity was restored in the presence of the detergent Triton X-100, suggesting that changes in the interaction between the protein and inner mitochondrial membrane are responsible for ischemia-induced loss in activity. Palmitoyl-carnitine supported ADP-dependent state 3 respiration declined as a result of ischemia. When octanoyl-carnitine was utilized state 3 respiration remained unchanged. State 4 respiration increased during ischemia, an increase that appears specific to fatty acid utilization. Thus, VLCAD represents a likely site for the modulation of substrate utilization during myocardial ischemia. However, the dramatic increase in mitochondrial state 4 respiration would be predicted to accentuate the imbalance between energy production and utilization.

Regulation of cardiac malonyl-CoA content and fatty acid oxidation during increased cardiac power

Myocardial fatty acid oxidation is regulated by carnitine palmitoyltransferase I (CPT I), which is inhibited by malonyl-CoA. Increased cardiac power causes a fall in malonyl-CoA content and accelerated fatty acid oxidation; however, the mechanism for the decrease in malonyl-CoA is unclear. Malonyl-CoA is formed by acetyl-CoA carboxylase (ACC) and degraded by malonyl-CoA decarboxylase (MCD); thus a fall in malonyl-CoA could be due to activation of MCD, inhibition of ACC, or both. This study assessed the effects of increased cardiac power on malonyl-CoA content and ACC and MCD activities. Anesthetized pigs were studied under control conditions and during increased cardiac power in response to dobutamine infusion and aortic constriction alone, under hyperglycemic conditions, or with the CPT I inhibitor oxfenicine. An increase in cardiac power was accompanied by increased myocardial O(2) consumption, decreased malonyl-CoA concentration, and increased fatty acid oxidation. There were no differences among groups in activity of ACC or AMP-activated protein kinase (AMPK), which physiologically inhibits ACC. There also were no differences in V(max) or K(m) of MCD. Previous studies have demonstrated that AMPK can be inhibited by protein kinase B (PKB); however, PKB was activated by dobutamine and the elevated insulin that accompanied hyperglycemia, but there was no effect on AMPK activity. In conclusion, the fall in malonyl-CoA and increase in fatty acid oxidation that occur with increased cardiac work were not due to inhibition of ACC or activation of MCD, suggesting alternative regulatory mechanisms for the work-induced decrease in malonyl-CoA concentration.

Mechanistic model of cardiac energy metabolism predicts localization of glycolysis to cytosolic subdomain during ischemia

A new multidomain mathematical model of cardiac cellular metabolism was developed to simulate metabolic responses to reduced myocardial blood flow. The model is based on mass balances and reaction kinetics that describe transport and metabolic processes of 31 key chemical species in cardiac tissue. The model has three distinct domains (blood, cytosol, and mitochondria) with interdomain transport of chemical species. In addition to distinguishing between cytosol and mitochondria, the model includes a subdomain in the cytosol to account for glycolytic metabolic channeling. Myocardial ischemia was induced by a 60% reduction in coronary blood flow, and model simulations were compared with experimental data from anesthetized pigs. Simulations with a previous model without compartmentation showed a slow activation of glycogen breakdown and delayed lactate production compared with experimental results. The addition of a subdomain for glycolysis resulted in simulations showing faster rates of glycogen breakdown and lactate production that closely matched in vivo experimental data. The dynamics of redox (NADH/NAD+) and phosphorylation (ADP/ATP) states were also simulated. These controllers are coupled to energy transfer reactions and play key regulatory roles in the cytosol and mitochondria. Simulations showed a similar dynamic response of the mitochondrial redox state and the rate of pyruvate oxidation during ischemia. In contrast, the cytosolic redox state displayed a time response similar to that of lactate production. In conclusion, this novel mechanistic model effectively predicted the rapid activation of glycogen breakdown and lactate production at the onset of ischemia and supports the concept of localization of glycolysis to a subdomain of the cytosol.

Regulation of pyruvate dehydrogenase activity and citric acid cycle intermediates during high cardiac power generation

A high rate of cardiac work increases citric acid cycle (CAC) turnover and flux through pyruvate dehydrogenase (PDH); however, the mechanisms for these effects are poorly understood. We tested the hypotheses that an increase in cardiac energy expenditure: (1) activates PDH and reduces the product/substrate ratios ([NADH]/[NAD(+)] and [acetyl-CoA]/[CoA-SH]); and (2) increases the content of CAC intermediates. Measurements were made in anaesthetized pigs under control conditions and during 15 min of a high cardiac workload induced by dobutamine (Dob). A third group was made hyperglycaemic (14 mm) to stimulate flux through PDH during the high work state (Dob + Glu). Glucose and fatty acid oxidation were measured with (14)C-glucose and (3)H-oleate. Compared with control, the high workload groups had a similar increase in myocardial oxygen consumption ( and cardiac power. Dob increased PDH activity and glucose oxidation above control, but did not reduce the [NADH]/[NAD(+)] and [acetyl-CoA]/[CoA-SH] ratios, and there were no differences between the Dob and Dob + Glu groups. An additional group was treated with Dob + Glu and oxfenicine (Oxf) to inhibit fatty acid oxidation: this increased [CoA-SH] and glucose oxidation compared with Dob; however, there was no further activation of PDH or decrease in the [NADH]/[NAD(+)] ratio. Content of the 4-carbon CAC intermediates succinate, fumarate and malate increased 3-fold with Dob, but there was no change in citrate content, and the Dob + Glu and Dob + Glu + Oxf groups were not different from Dob. In conclusion, compared with normal conditions, at high myocardial energy expenditure (1) the increase in flux through PDH is regulated by activation of the enzyme complex and continues to be partially controlled through inhibition by fatty acid oxidation, and (2) there is expansion of the CAC pool size at the level of 4-carbon intermediates that is largely independent of myocardial fatty acid oxidation.

Moderate severity heart failure does not involve a downregulation of myocardial fatty acid oxidation

Recent human and animal studies have demonstrated that in severe end-stage heart failure (HF), the cardiac muscle switches to a more fetal metabolic phenotype, characterized by downregulation of free fatty acid (FFA) oxidation and an enhancement of glucose oxidation. The goal of this study was to examine myocardial substrate metabolism in a model of moderate coronary microembolization-induced HF. We hypothesized that during well-compensated HF, FFA oxidation would predominate as opposed to a more fetal metabolic phenotype of greater glucose oxidation. Cardiac substrate uptake and oxidation were measured in normal dogs ( n = 8) and in dogs with microembolization-induced HF ( n = 18, ejection fraction = 28%) by infusing three isotopic tracers ([9,10-3H]oleate, [U-14C]glucose, and [1-13C]lactate) in anesthetized open-chest animals. There were no differences in myocardial substrate metabolism between the two groups. The total activity of pyruvate dehydrogenase, the key enzyme regulating myocardial pyruvate oxidation (and hence glucose and lactate oxidation) was not affected by HF. We did not observe any difference in the activity of carnitine palmitoyl transferase I (CPT-I) and its sensitivity to inhibition by malonyl-CoA between groups; however, malonyl-CoA content was decreased by 22% with HF, suggesting less in vivo inhibition of CPT-I activity. The differences in malonyl-CoA content cannot be explained by changes in the Michaelis-Menten constant and maximal velocity for malonyl-CoA decarboxylase because neither were affected by HF. These results support the concept that there is no decrease in fatty acid oxidation during compensated HF and that the downregulation of fatty acid oxidation enzymes and the switch to carbohydrate oxidation observed in end-stage HF is only a late-stage phenomemon.

Malonyl coenzyme a decarboxylase inhibition protects the ischemic heart by inhibiting fatty acid oxidation and stimulating glucose oxidation

Abnormally high rates of fatty acid oxidation and low rates of glucose oxidation are important contributors to the severity of ischemic heart disease. Malonyl coenzyme A (CoA) regulates fatty acid oxidation by inhibiting mitochondrial uptake of fatty acids. Malonyl CoA decarboxylase (MCD) is involved in the decarboxylation of malonyl CoA to acetyl CoA. Therefore, inhibition of MCD may decrease fatty acid oxidation and protect the ischemic heart, secondary to increasing malonyl CoA levels. Ex vivo working rat hearts aerobically perfused in the presence of newly developed MCD inhibitors showed an increase in malonyl CoA levels, which was accompanied by both a significant decrease in fatty acid oxidation rates and an increase in glucose oxidation rates compared with controls. Using a model of demand-induced ischemia in pigs, MCD inhibition significantly increased glucose oxidation rates and reduced lactate production compared with vehicle-treated hearts, which was accompanied by a significant increase in cardiac work compared with controls. In a more severe rat heart global ischemia/reperfusion model, glucose oxidation was significantly increased and cardiac function was significantly improved during reperfusion in hearts treated with the MCD inhibitor compared with controls. Together, our data show that MCD inhibitors, which increase myocardial malonyl CoA levels, decrease fatty acid oxidation and accelerate glucose oxidation in both ex vivo rat hearts and in vivo pig hearts. This switch in energy substrate preference improves cardiac function during and after ischemia, suggesting that pharmacological inhibition of MCD may be a novel approach to treating ischemic heart disease.

beta-Hydroxybutyrate inhibits myocardial fatty acid oxidation in vivo independent of changes in malonyl-CoA content

This study tested the hypothesis that an acute infusion of beta-hydroxybutyrate inhibits myocardial fatty acid uptake and oxidation in vivo. Anesthetized pigs were untreated (n = 6) or treated with an intravenous infusion of fat emulsion (n = 7) to elevate plasma free fatty acid levels. A third group received fat emulsion plus an intravenous infusion of beta-hydroxybutyrate (25 micromol.kg-1.min-1; n = 7) for 60 min. All animals received a continuous infusion of [3H]palmitate, and myocardial fatty acid oxidation was measured from the cardiac production of 3H2O. Plasma free fatty acid concentrations were elevated in the fat emulsion group (0.77 +/- 0.11 mM) compared with the untreated group (0.15 +/- 0.03 mM), which resulted in greater myocardial free fatty acid oxidation. In contrast, the group receiving beta-hydroxybutyrate in addition to fat emulsion had elevated beta-hydroxybutyrate concentration (0.87 +/- 0.11 vs. 0.04 +/- 0.01 mM), but suppressed fatty acid oxidation (0.053 +/- 0.013 micromol.g-1.min-1) (P < 0.05) compared with the fat emulsion group (0.116 +/- 0.029 micromol.g-1.min-1). There were no differences among the three groups in the tissue content for malonyl-CoA, acetyl-CoA, or free CoA or the activity of acetyl-CoA carboxylase; thus the inhibition of fatty acid oxidation by elevated beta-hydroxybutyrate did not appear to be due to malonyl-CoA inhibition of carnitine palmitoyl transferase-I or to an increase in the acetyl-CoA-to-free CoA ratio. In conclusion, fatty acid uptake and oxidation is blocked by an infusion of beta-hydroxybutyrate; this effect was not due to elevated myocardial malonyl-CoA content.

Effect of hyperglycemia and fatty acid oxidation inhibition during aerobic conditions and demand-induced ischemia

Metabolic interventions improve performance during demand-induced ischemia by reducing myocardial lactate production and improving regional systolic function. We tested the hypotheses that 1) stimulation of glycolysis would increase lactate production and improve ventricular wall motion, and 2) the addition of fatty acid oxidation inhibition would reduce lactate production and further improve contractile function. Measurements were made in anesthetized open-chest swine hearts. Three groups, hyperglycemia (HG), HG + oxfenicine (HG + Oxf), and control (CTRL), were treated under aerobic conditions and during demand-induced ischemia. During demand-induced ischemia, HG resulted in greater lactate production and tissue lactate content but had no significant effect on glucose oxidation. HG + Oxf significantly lowered lactate production and increased glucose oxidation compared with both the CTRL and HG groups. Myocardial energy efficiency was greater in the HG and HG + Oxf groups under aerobic conditions but did not change during demand-induced ischemia. Thus enhanced glycolysis resulted in increased energy efficiency under aerobic conditions but significantly enhanced lactate production with no further improvement in function during demand-induced ischemia. Partial inhibition of free fatty acid oxidation in the presence of accelerated glycolysis increased energy efficiency under aerobic conditions and significantly reduced lactate production and enhanced glucose oxidation during demand-induced ischemia.

Quantitative assessment of anaplerosis from propionate in pig heart in vivo

Normal cardiac metabolism requires continuous replenishment (anaplerosis) of catalytic intermediates of the citric acid cycle. Little is known about the quantitative aspects of propionate as a substrate of in vivo anaplerosis; therefore, we measured the rate of propionate entry into the citric acid cycle in hearts of anesthetized pigs. [U-(13)C(3)]propionate (0.25 mM) was infused in a coronary artery branch for 1 h via an extracorporeal perfusion circuit, and cardiac biopsies were analyzed for the mass isotopomer distribution of citric acid cycle intermediates. Infusion of propionate did not affect myocardial oxygen consumption, heart rate, or contractile function. In the infused territory, propionate infusion did not affect uptake of glucose and lactate but decreased free fatty acid uptake by one-half (P < 0.05). Propionate extraction and uptake were 57.4 +/- 3.3% and 0.078 +/- 0.009 micromol x min(-1) x g(-1). Anaplerosis from propionate, calculated from the mass isotopomer distribution of succinate, accounted for 8.9 +/- 1.3% of the citric acid cycle flux. Propioylcarnitine release accounted for only 0.033 +/- 0.002% of propionate uptake. Methylcitrate did not accumulate. Thus administration of a low concentration of propionate appears to be a convenient and safe way to boost anaplerosis in the heart.