Methodology for measuring in vitro/ex vivo cardiac energy metabolism

Respiratory substrate preferences in mitochondria isolated from different tissues of three fish species

Energy requirements of tissues vary greatly and exhibit different mitochondrial respiratory activities with variable participation of both substrates and oxidative phosphorylation. The present study aimed to (1) compare the substrate preferences of mitochondria from different tissues and fish species with different ecological characteristics, (2) identify an appropriate substrate for comparing metabolism by mitochondria from different tissues and species, and (3) explore the relationship between mitochondrial metabolism mechanisms and ecological energetic strategies. Respiration rates and cytochrome c oxidase (CCO) activities of mitochondria isolated from heart, brain, kidney, and other tissues from Silurus meridionalis, Carassius auratus, and Megalobrama amblycephala were measured using succinate (complex II-linked substrate), pyruvate (complex I-linked), glutamate (complex I-linked), or combinations. Mitochondria from all tissues and species exhibited substrate preferences. Mitochondria exhibited greater coupling efficiencies and lower leakage rates using either complex I-linked substrates, whereas an opposite trend was observed for succinate (complex II-linked). Furthermore, maximum mitochondrial respiration rates were higher with the substrate combinations than with individual substrates; therefore, state III respiration rates measured with substrate combinations could be effective indicators of maximum mitochondrial metabolic capacity. Regardless of fish species, both state III respiration rates and CCO activities were the highest in heart mitochondria, followed by red muscle mitochondria. However, differences in substrate preferences were not associated with species feeding habit. The maximum respiration rates of heart mitochondria with substrate combinations could indicate differences in locomotor performances, with higher metabolic rates being associated with greater capacity for sustained swimming.

Physiological and pharmacological stimulation for in vitro maturation of substrate metabolism in human induced pluripotent stem cell-derived cardiomyocytes

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) enable human cardiac cells to be studied in vitro, although they use glucose as their primary metabolic substrate and do not recapitulate the properties of adult cardiomyocytes. Here, we have explored the interplay between maturation by stimulation of fatty acid oxidation and by culture in 3D. We have investigated substrate metabolism in hiPSC-CMs grown as a monolayer and in 3D, in porous collagen-derived scaffolds and in engineered heart tissue (EHT), by measuring rates of glycolysis and glucose and fatty acid oxidation (FAO), and changes in gene expression and mitochondrial oxygen consumption. FAO was stimulated by activation of peroxisome proliferator-activated receptor alpha (PPARα), using oleate and the agonist WY-14643, which induced an increase in FAO in monolayer hiPSC-CMs. hiPSC-CMs grown in 3D on collagen-derived scaffolds showed reduced glycolysis and increased FAO compared with monolayer cells. Activation of PPARα further increased FAO in cells on collagen/elastin scaffolds but not collagen or collagen/chondroitin-4-sulphate scaffolds. In EHT, FAO was significantly higher than in monolayer cells or those on static scaffolds and could be further increased by culture with oleate and WY-14643. In conclusion, a more mature metabolic phenotype can be induced by culture in 3D and FAO can be incremented by pharmacological stimulation.

Mechanical Postconditioning Promotes Glucose Metabolism and AMPK Activity in Parallel with Improved Post-Ischemic Recovery in an Isolated Rat Heart Model of Donation after Circulatory Death

Donation after circulatory death (DCD) could improve donor heart availability; however, warm ischemia-reperfusion injury raises concerns about graft quality. Mechanical postconditioning (MPC) may limit injury, but mechanisms remain incompletely characterized. Therefore, we investigated the roles of glucose metabolism and key signaling molecules in MPC using an isolated rat heart model of DCD. Hearts underwent 20 min perfusion, 30 min global ischemia, and 60 minu reperfusion with or without MPC (two cycles: 30 s reperfusion—30 s ischemia). Despite identical perfusion conditions, MPC either significantly decreased (low recovery = LoR; 32 ± 5%; p < 0.05), or increased (high recovery = HiR; 59 ± 7%; p < 0.05) the recovery of left ventricular work compared with no MPC (47 ± 9%). Glucose uptake and glycolysis were increased in HiR vs. LoR hearts (p < 0.05), but glucose oxidation was unchanged. Furthermore, in HiR vs. LoR hearts, phosphorylation of raptor, a downstream target of AMPK, increased (p < 0.05), cytochrome c release (p < 0.05) decreased, and TNFα content tended to decrease. Increased glucose uptake and glycolysis, lower mitochondrial damage, and a trend towards decreased pro-inflammatory cytokines occurred specifically in HiR vs. LoR MPC hearts, which may result from greater AMPK activation. Thus, we identify endogenous cellular mechanisms that occur specifically with cardioprotective MPC, which could be elicited in the development of effective reperfusion strategies for DCD cardiac grafts.

Hearts lacking plasma membrane KATP channels display changes in basal aerobic metabolic substrate preference and AMPK activity

Cardiac ATP-sensitive K+ (KATP) channels couple changes in cellular metabolism to membrane excitability and are activated during metabolic stress, although under basal aerobic conditions, KATP channels are thought to be predominately closed. Despite intense research into the roles of KATP channels during metabolic stress, their contribution to aerobic basal cardiac metabolism has not been previously investigated. Hearts from Kir6.2+/+ and Kir6.2−/− mice were perfused in working mode, and rates of glycolysis, fatty acid oxidation, and glucose oxidation were measured. Changes in activation/expression of proteins regulating metabolism were probed by Western blot analysis. Despite cardiac mechanical function and metabolic efficiency being similar in both groups, hearts from Kir6.2−/− mice displayed an approximately twofold increase in fatty acid oxidation and a 0.45-fold reduction in glycolytic rates but similar glucose oxidation rates compared with hearts from Kir6.2+/+ mice. Kir6.2−/− hearts also possessed elevated levels of activated AMP-activated protein kinase (AMPK), higher glycogen content, and reduced mitochondrial density. Moreover, activation of AMPK by isoproterenol or diazoxide was significantly blunted in Kir6.2−/− hearts. These data indicate that KATP channel ablation alters aerobic basal cardiac metabolism. The observed increase in fatty acid oxidation and decreased glycolysis before any metabolic insult may contribute to the poor recovery observed in Kir6.2−/− hearts in response to exercise or ischemia-reperfusion injury. Therefore, KATP channels may play an important role in the regulation of cardiac metabolism through AMPK signaling.

NEW & NOTEWORTHY In this study, we show that genetic ablation of plasma membrane ATP-sensitive K+ channels results in pronounced changes in cardiac metabolic substrate preference and AMP-activated protein kinase activity. These results suggest that ATP-sensitive K+ channels may play a novel role in regulating metabolism in addition to their well-documented effects on ionic homeostasis during periods of stress.

Methods for the Determination of Rates of Glucose and Fatty Acid Oxidation in the Isolated Working Rat Heart

The mammalian heart is a major consumer of ATP and requires a constant supply of energy substrates for contraction. Not surprisingly, alterations of myocardial metabolism have been linked to the development of contractile dysfunction and heart failure. Therefore, unraveling the link between metabolism and contraction should shed light on some of the mechanisms governing cardiac adaptation or maladaptation in disease states. The isolated working rat heart preparation can be used to follow, simultaneously and in real time, cardiac contractile function and flux of energy providing substrates into oxidative metabolic pathways. The present protocol aims to provide a detailed description of the methods used in the preparation and utilization of buffers for the quantitative measurement of the rates of oxidation for glucose and fatty acids, the main energy providing substrates of the heart. The methods used for sample analysis and data interpretation are also discussed. In brief, the technique is based on the supply of ¹⁴C- radiolabeled glucose and a ³H- radiolabeled long-chain fatty acid to an ex vivo beating heart via normothermic crystalloid perfusion. ¹⁴CO2 and ³H2O, end byproducts of the enzymatic reactions involved in the utilization of these energy providing substrates, are then quantitatively recovered from the coronary effluent. With knowledge of the specific activity of the radiolabeled substrates used, it is then possible to individually quantitate the flux of glucose and fatty acid in the oxidation pathways. Contractile function of the isolated heart can be determined in parallel with the appropriate recording equipment and directly correlated to metabolic flux values. The technique is extremely useful to study the metabolism/contraction relationship in response to various stress conditions such as alterations in pre and after load and ischemia, a drug or a circulating factor, or following the alteration in the expression of a gene product.

Assessment of coronary hemodynamics and vascular function

Coronary blood flow closely matches to metabolic demands of heart and myocardial oxygen consumption and is conditioned by function of coronary resistance vessels. The microvascular endothelium of coronary resistance vessels is exposed to a spatially and temporally regulated input from cardiomyocytes and the haemodynamic forces of the cardiac cycle. Functional measurements of coronary pressure and flow are important approaches that provide complementary information on the function of coronary vessel function that could not be assessed by the methods utilized for the anatomic characterization of coronary disease, such as coronary angiography. The goal of this paper is to review the methodologies for assessment of coronary vascular function and haemodynamics which are utilized in research and to discuss their potential applicability in the clinical settings.

The continuing evolution of the Langendorff and ejecting murine heart: new advances in cardiac phenotyping

The isolated retrograde-perfused Langendorff heart and the isolated ejecting heart have, over many decades, resulted in fundamental discoveries that form the underpinnings of our current understanding of the biology and physiology of the heart. These two experimental methodologies have proven invaluable in studying pharmacological effects on myocardial function, metabolism, and vascular reactivity and in the investigation of clinically relevant disease states such as ischemia-reperfusion injury, diabetes, obesity, and heart failure. With the advent of the genomics era, the isolated mouse heart preparation has gained prominence as an ex vivo research tool for investigators studying the impact of gene modification in the intact heart. This review summarizes the historical development of the isolated heart and provides a practical guide for the establishment of the Langendorff and ejecting heart preparations with a particular emphasis on the murine heart. In addition, current applications and novel methods of recording cardiovascular parameters in the isolated heart preparation will be discussed. With continued advances in methodological recordings, the isolated mouse heart preparation will remain physiologically relevant for the foreseeable future, serving as an integral bridge between in vitro assays and in vivo approaches.

Long-term effects of intrauterine growth restriction on cardiac metabolism and susceptibility to ischaemia/reperfusion

AIMS

Adult offspring who are born intrauterine growth restricted (IUGR) are at risk of developing cardiovascular diseases during adulthood. Additionally, several cardiac diseases are associated with changes in myocardial energy metabolism. However, the potential long-term effects of being born IUGR on cardiac energetics are unknown. The aim of this study was to assess the long-term effect of IUGR on cardiac performance and energy metabolism under aerobic conditions and after ischaemia/reperfusion (IR) injury.

METHODS AND RESULTS

To induce IUGR, pregnant Sprague-Dawley rats were randomly assigned to hypoxic (11.5% O(2)) or control (21% O(2)) environments from day 15 to 21 of pregnancy. Cardiac susceptibility to IR was evaluated in male and female offspring at 4 (young-adult) or 12 (ageing) months of age using isolated working hearts. Cardiac production of energy was evaluated using radiolabelled substrates. Both male and female IUGR offspring exhibited an increased susceptibility to IR injury compared with controls (P< 0.05) as well as an increased post-ischaemic production of protons (P< 0.001) secondary to a mismatch between myocardial glycolysis and glucose oxidation rates. Moreover, offspring born IUGR exhibited an increased myocardial production of acetyl-CoA during reperfusion. The mismatch between energy production and cardiac performance indicates that in IUGR offspring, cardiac efficiency during reperfusion was decreased relative to controls.

CONCLUSION

Our results suggest that hypoxia-induced IUGR has long-term effects on cardiac susceptibility to IR injury that are independent of sex and age. Moreover, we identified a mismatch in glucose metabolism, leading to proton accumulation in the post-ischaemic myocardium of offspring born IUGR as a potential mechanism involved.

Novel O-palmitolylated beta-E1 subunit of pyruvate dehydrogenase is phosphorylated during ischemia/reperfusion injury

BACKGROUND

During and following myocardial ischemia, glucose oxidation rates are low and fatty acids dominate as a source of oxidative metabolism. This metabolic phenotype is associated with contractile dysfunction during reperfusion. To determine the mechanism of this reliance on fatty acid oxidation as a source of ATP generation, a functional proteomics approach was utilized.

RESULTS

2-D gel electrophoresis of mitochondria from working rat hearts subjected to 25 minutes of global no flow ischemia followed by 40 minutes of aerobic reperfusion identified 32 changes in protein abundance compared to aerobic controls. Of the five proteins with the greatest change in abundance, two were increased (long chain acyl-coenzyme A dehydrogenase (48 +/- 1 versus 39 +/- 3 arbitrary units, n = 3, P < 0.05) and alpha subunit of ATP synthase (189 +/- 15 versus 113 +/- 23 arbitrary units, n = 3, P < 0.05)), while two were decreased (24 kDa subunit of NADH-ubiquinone oxidoreductase (94 +/- 7 versus 127 +/- 9 arbitrary units, n = 3, P < 0.05) and D subunit of ATP synthase (230 +/- 11 versus 368 +/- 47 arbitrary units, n = 3, P < 05)). Two forms of pyruvate dehydrogenase betaE1 subunit, the rate-limiting enzyme for glucose oxidation, were also identified. The protein level of the more acidic form of pyruvate dehydrogenase was reduced during reperfusion (37 +/- 4 versus 56 +/- 7 arbitrary units, n = 3, P < 05), while the more basic form remained unchanged. The more acidic isoform was found to be O-palmitoylated, while both isoforms exhibited ischemia/reperfusion-induced phosphorylation. In silico analysis identified the putative kinases as the insulin receptor kinase for the more basic form and protein kinase Czeta or protein kinase A for the more acidic form. These modifications of pyruvate dehydrogenase are associated with a 35% decrease in glucose oxidation during reperfusion.

CONCLUSIONS

Cardiac ischemia/reperfusion induces significant changes to a number of metabolic proteins of the mitochondrial proteome. In particular, ischemia/reperfusion induced the post-translational modification of pyruvate dehydrogenase, the rate-limiting step of glucose oxidation, which is associated with a 35% decrease in glucose oxidation during reperfusion. Therefore these post-translational modifications may have important implications in the regulation of myocardial energy metabolism.

The inhibition of pyruvate dehydrogenase kinase improves impaired cardiac function and electrical remodeling in two models of right ventricular hypertrophy: resuscitating the hibernating right ventricle

Right ventricular hypertrophy (RVH) and RV failure contribute to morbidity and mortality in pulmonary arterial hypertension (PAH). The cause of RV dysfunction and the feasibility of therapeutically targeting the RV are uncertain. We hypothesized that RV dysfunction and electrical remodeling in RVH result, in part, from a glycolytic shift in the myocyte, caused by activation of pyruvate dehydrogenase kinase (PDK). We studied two complementary rat models: RVH + PAH (induced by monocrotaline) and RVH + without PAH (induced by pulmonary artery banding (PAB)). Monocrotaline RVH reduced RV O(2)-consumption and enhanced glycolysis. RV 2-fluoro-2-deoxy-glucose uptake, Glut-1 expression, and pyruvate dehydrogenase phosphorylation increased in monocrotaline RVH. The RV monophasic action potential duration and QT(c) interval were prolonged due to decreased expression of repolarizing voltage-gated K(+) channels (Kv1.5, Kv4.2). In the RV working heart model, the PDK inhibitor, dichloroacetate, acutely increased glucose oxidation and cardiac work in monocrotaline RVH. Chronic dichloroacetate therapy improved RV repolarization and RV function in vivo and in the RV Langendorff model. In PAB-induced RVH, a similar reduction in cardiac output and glycolytic shift occurred and it too improved with dichloroacetate. In PAB-RVH, the benefit of dichloroacetate on cardiac output was approximately 1/3 that in monocrotaline RVH. The larger effects in monocrotaline RVH likely reflect dichloroacetate's dual metabolic benefits in that model: regression of vascular disease and direct effects on the RV. Reduction in RV function and electrical remodeling in two models of RVH relevant to human disease (PAH and pulmonic stenosis) result, in part, from a PDK-mediated glycolytic shift in the RV. PDK inhibition partially restores RV function and regresses RVH by restoring RV repolarization and enhancing glucose oxidation. Recognition that a PDK-mediated metabolic shift contributes to contractile and ionic dysfunction in RVH offers insight into the pathophysiology and treatment of RVH.

Suppression of 5'-AMP-activated protein kinase activity does not impair recovery of contractile function during reperfusion of ischemic hearts

Activation of 5'-AMP-activated protein kinase (AMPK) may benefit the heart during ischemia-reperfusion by increasing energy production. While AMPK stimulates glycolysis, mitochondrial oxidative metabolism is the major source of ATP production during reperfusion of ischemic hearts. Stimulating AMPK increases mitochondrial fatty acid oxidation, but this is usually accompanied by a decrease in glucose oxidation, which can impair the functional recovery of ischemic hearts. To examine the relationship between AMPK and cardiac energy substrate metabolism, we subjected isolated working mouse hearts expressing a dominant negative (DN) alpha(2)-subunit of AMPK (AMPK-alpha(2) DN) to 20 min of global no-flow ischemia and 40 min of reperfusion with Krebs-Henseleit solution containing 5 mM [U-(14)C]glucose, 0.4 mM [9, 10-(3)H]palmitate, and 100 microU/ml insulin. AMPK-alpha(2) DN hearts had reduced AMPK activity at the end of reperfusion (82 +/- 9 vs. 141 +/- 7 pmol.mg(-1).min(-1)) with no changes in high-energy phosphates. Despite this, AMPK-alpha(2) DN hearts had improved recovery of function during reperfusion (14.9 +/- 0.8 vs. 9.4 +/- 1.4 beats.min(-1).mmHg.10(-3)). During reperfusion, fatty acid oxidation provided 44.0 +/- 2.8% of total acetyl-CoA in AMPK-alpha(2) DN hearts compared with 55.0 +/- 3.2% in control hearts. Since insulin can inhibit both AMPK activation and fatty acid oxidation, we also examined functional recovery in the absence of insulin. Functional recovery was similar in both groups despite a decrease in AMPK activity and a decreased reliance on fatty acid oxidation during reperfusion (66.4 +/- 9.4% vs. 85.3 +/- 4.3%). These data demonstrate that the suppression of cardiac AMPK activity does not produce an energetically compromised phenotype and does not impair, but may in fact improve, the recovery of function after ischemia.

High rates of residual fatty acid oxidation during mild ischemia decrease cardiac work and efficiency

It is unknown what effects high levels of fatty acids have on energy metabolism and cardiac efficiency during milder forms of ischemia. To address this issue, isolated working rat hearts perfused with Krebs-Henseleit solution (5 mM glucose, 100 muU/mL insulin, and 0.4 (Normal Fat) or 1.2 mM palmitate (High Fat)) were subjected to 30 min of aerobic perfusion followed by 30 min of mild ischemia (39% reduction in coronary flow). Both groups had similar aerobic function and rates of glycolysis, however the High Fat group had elevated rates of palmitate oxidation (150%), and decreased rates of glucose oxidation (51%). Mild ischemia decreased cardiac work (56% versus 40%) and efficiency (29% versus 11%) further in High Fat hearts. Palmitate oxidation contributed a greater percent of acetyl-CoA production during mild ischemia in the High Fat group (81% versus 54%). During mild ischemia glycolysis remained at aerobic levels in the Normal Fat group, but was accelerated in the High Fat group. Triglyceride, glycogen and adenine nucleotide content did not differ at the end of mild ischemia, however glycogen turnover was double in the High Fat group (248%). Addition of the pyruvate dehydrogenase inhibitor dichloroacetate to the High Fat group resulted in a doubling of the rate of glucose oxidation and improved cardiac efficiency during mild ischemia. We demonstrate that fatty acid oxidation dominates as the main source of residual oxidative metabolism during mild ischemia, which is accompanied by suppressed cardiac function and efficiency in the presence of high fat.

Altering and analyzing glucose metabolism in perfused hearts of transgenic mice

Glucose metabolism plays an important role in cardiac bioenergetics that changes under various stress conditions including hypertrophy, diabetic cardiomyopathy, and ischemia-reperfusion injury. To understand the role of glycolysis under these conditions, we have altered several steps of the glycolytic pathway specifically in the heart. In this chapter, we describe methods used to produce cardiac-targeted transgenic mice and procedures for measuring various glucose metabolites including glucose-6-phosphate, fructose-6-phosphate, fructose-1,6-bisphosphate, and glycogen. Also, we describe methods for measuring glucose transport and glycolysis in perfused mouse hearts. Using these methods, we show that mice over-expressing cardiac-specific kinase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (Mykd-PFK-2) show reduced glucose transport and reduced glycolysis when compared with control. The metabolites glucose-6-phosphate, fructose-6-phosphate, and glycogen were elevated, whereas fructose-1,6-bisphosphate was reduced in the transgenic Mykd-PFK-2 mouse hearts.

p38 mitogen-activated protein kinase mediates adenosine-induced alterations in myocardial glucose utilization via 5′-AMP-activated protein kinase

Adenosine-induced acceleration of glycolysis in hearts stressed by transient ischemia is accompanied by suppression of glycogen synthesis and by increases in activity of adenosine 5′-monophosphate-activated protein kinase (AMPK). Because p38 mitogen-activated protein kinase (MAPK) may regulate glucose metabolism and may be activated downstream of AMPK, this study determined the effects of the p38 MAPK inhibitors SB202190 and SB203580 on adenosine-induced alterations in glucose utilization and AMPK activity. Studies were performed in working rat hearts perfused aerobically following stressing by transient ischemia (2 × 10-min ischemia followed by 5-min reperfusion). Phosphorylation of AMPK and p38 MAPK each were increased fourfold by adenosine, and these effects were inhibited by either SB202190 or SB203580. Neither of these inhibitors directly affected AMPK activity. Attenuation of the adenosine-induced increase in AMPK and p38 MAPK phosphorylation by SB202190 and SB203580 occurred independently of any change in tissue ATP-to-AMP ratio and did not alter glucose uptake, but it was accompanied by an increase in glycogen synthesis and glycogen content and by inhibition of glycolysis and proton production. There was a significant inverse correlation between the rate of glycogen synthesis and AMPK activity and between AMPK activity and glycogen content. These data demonstrate that AMPK is likely downstream of p38 MAPK in mediating the effects of adenosine on glucose utilization in hearts stressed by transient ischemia. The ability of p38 MAPK inhibitors to relieve the inhibition of glycogen synthesis and to inhibit glycolysis and proton production suggests that these agents may restore adenosine-induced cardioprotection in stressed hearts.

Impact of lactate in the perfusate on function and metabolic parameters of isolated working rat heart

The goal of this study was to investigate the effect of 1 mM exogenous lactate on cardiac function, and some metabolic parameters, such as glycolysis, glucose oxidation, lactate oxidation, and fatty acid oxidation, in isolated working rat hearts. Hearts from male Sprague-Dawley rats were isolated and perfused with 5 mM glucose, 1.2 mM palmitate, and 100 microU/ml insulin with or without 1 mM lactate. The rates of glycolysis, glucose, lactate, and fatty acid oxidation were determined by supplementing the buffer with radiolabeled substrates. Cardiac function was similar between lactate+ and lactate- hearts. Glycolysis was not affected by 1 mM lactate. The addition of lactate did not alter glucose oxidation rates. Interestingly, palmitate oxidation rates almost doubled when 1 mM lactate was present in the perfusate. This study suggests that subst rate supply to the heart is crucially important when evaluating the data from metabolic studies.

Role of changes in cardiac metabolism in development of diabetic cardiomyopathy

In patients with diabetes, an increased risk of symptomatic heart failure usually develops in the presence of hypertension or ischemic heart disease. However, a predisposition to heart failure might also reflect the effects of underlying abnormalities in diastolic function that can occur in asymptomatic patients with diabetes alone (termed diabetic cardiomyopathy). Evidence of cardiomyopathy has also been demonstrated in animal models of both Type 1 (streptozotocin-induced diabetes) and Type 2 diabetes (Zucker diabetic fatty rats and ob/ob or db/db mice). During insulin resistance or diabetes, the heart rapidly modifies its energy metabolism, resulting in augmented fatty acid and decreased glucose consumption. Accumulating evidence suggests that this alteration of cardiac metabolism plays an important role in the development of cardiomyopathy. Hence, a better understanding of this dysregulation in cardiac substrate utilization during insulin resistance and diabetes could provide information as to potential targets for the treatment of cardiomyopathy. This review is focused on evaluating the acute and chronic regulation and dysregulation of cardiac metabolism in normal and insulin-resistant/diabetic hearts and how these changes could contribute toward the development of cardiomyopathy.

Cardiac Expression of Kinase-deficient 6-Phosphofructo-2-kinase/Fructose-2,6-bisphosphatase Inhibits Glycolysis, Promotes Hypertrophy, Impairs Myocyte Function, and Reduces Insulin Sensitivity

Glycolysis is important to cardiac metabolism and reduced glycolysis may contribute to diabetic cardiomyopathy. To understand its role independent of diabetes or hypoxic injury, we modulated glycolysis by cardiac-specific overexpression of kinase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (kd-PFK-2). PFK-2 controls the level of fructose 2,6-bisphosphate (Fru-2,6-P(2)), an important regulator of glycolysis. Transgenic mice had over 2-fold reduced levels of Fru-2,6-P(2). Heart weight/body weight ratio indicated mild hypertrophy. Sirius red staining for collagen was significantly increased. We observed a 2-fold elevation in glucose 6-phosphate and fructose 6-phosphate levels, whereas fructose 1,6-bisphosphate was reduced 2-fold. Pathways branching off of glycolysis above phosphofructokinase were activated as indicated by over 2-fold elevated UDP-N-acetylglucosamine and glycogen. The kd-PFK-2 transgene significantly inhibited glycolysis in perfused hearts. Insulin stimulation of metabolism and Akt phosphorylation were sharply reduced. In addition, contractility of isolated cardiomyocytes was impaired during basal and hypoxic incubations. The present study shows that cardiac overexpression of kinase-deficient PFK-2 reduces cardiac glycolysis that produced negative consequences to the heart including hypertrophy, fibrosis, and reduced cardiomyocyte function. In addition, metabolic and signaling responses to insulin were significantly decreased.