Mitochondrial pyruvate transport in working guinea-pig heart. Work-related vs. carrier-mediated control of pyruvate oxidation

Nutritional modulation of heart failure in mitochondrial pyruvate carrier-deficient mice

The myocardium is metabolically flexible; however, impaired flexibility is associated with cardiac dysfunction in conditions including diabetes and heart failure. The mitochondrial pyruvate carrier (MPC) complex, composed of MPC1 and MPC2, is required for pyruvate import into the mitochondria. Here we show that MPC1 and MPC2 expression is downregulated in failing human and mouse hearts. Mice with cardiac-specific deletion of Mpc2 (CS-MPC2-/-) exhibited normal cardiac size and function at 6 weeks old, but progressively developed cardiac dilation and contractile dysfunction, which was completely reversed by a high-fat, low-carbohydrate ketogenic diet. Diets with higher fat content, but enough carbohydrate to limit ketosis, also improved heart failure, while direct ketone body provisioning provided only minor improvements in cardiac remodelling in CS-MPC2-/- mice. An acute fast also improved cardiac remodelling. Together, our results reveal a critical role for mitochondrial pyruvate use in cardiac function, and highlight the potential of dietary interventions to enhance cardiac fat metabolism to prevent or reverse cardiac dysfunction and remodelling in the setting of MPC deficiency.

Cardioprotective effect of N-methylnicotinamide salt of pyruvate in experimental model of cardiac hypoxia

BACKGROUND

Pyruvate improves contractility of normal, hypoxic, and post-ischemic myocardium. However, sodium overload is a major problem with its therapeutic application if sodium pyruvate is used. Development of alternative forms such as N-1-methylnicotinamide (MNA) pyruvate may help to overcome this problem. The aim of the study was to investigate the effect of MNA pyruvate in a murine model of cardiac ischemia.

METHODS

Seven month old male ApoE^-/-LDLr^-/- mice that develop myocardial infarction when exposed to hypoxic stress, were used in this study. Hypoxia (8% O2 in inspired air) was maintained for 8min and was followed by reoxygenation (21% O2 in inspired air). Four groups of mice were treated 10min before the hypoxic event by intravenous injection of MNA, MNA pyruvate, sodium pyruvate, and saline as control. The myocardial ischemia and damage was recorded by ECG. Four hours following the hypoxic episode serum troponin T and creatine kinase activity were measured.

RESULTS

Significant hypernatremia was found in the sodium pyruvate group. During hypoxia, control and MNA group developed profound STU depressions on ECG while no changes were observed in MNA pyruvate and sodium pyruvate group. Creatine kinase activity and troponin T content in the mice plasma were significantly higher in the control and MNA group as compared to the MNA pyruvate and sodium pyruvate group.

CONCLUSIONS

This study demonstrated that administration of MNA pyruvate prior to a hypoxia-induced cardiac event was cardioprotective. This intervention did not cause hypernatremia in contrast to sodium pyruvate.

Pyruvate enhancement of cardiac performance: Cellular mechanisms and clinical application

Cardiac contractile function is adenosine-5'-triphosphate (ATP)-intensive, and the myocardium's high demand for oxygen and energy substrates leaves it acutely vulnerable to interruptions in its blood supply. The myriad cardioprotective properties of the natural intermediary metabolite pyruvate make it a potentially powerful intervention against the complex injury cascade ignited by myocardial ischemia-reperfusion. A readily oxidized metabolic substrate, pyruvate augments myocardial free energy of ATP hydrolysis to a greater extent than the physiological fuels glucose, lactate and fatty acids, particularly when it is provided at supra-physiological plasma concentrations. Pyruvate also exerts antioxidant effects by detoxifying reactive oxygen and nitrogen intermediates, and by increasing nicotinamide adenine dinucleotide phosphate reduced form (NADPH) production to maintain glutathione redox state. These enhancements of free energy and antioxidant defenses combine to augment sarcoplasmic reticular Ca²⁺ release and re-uptake central to cardiac mechanical performance and to restore β-adrenergic signaling of ischemically stunned myocardium. By minimizing Ca²⁺ mismanagement and oxidative stress, pyruvate suppresses inflammation in post-ischemic myocardium. Thus, pyruvate administration stabilized cardiac performance, augmented free energy of ATP hydrolysis and glutathione redox systems, and/or quelled inflammation in a porcine model of cardiopulmonary bypass, a canine model of cardiac arrest-resuscitation, and a caprine model of hypovolemia and hindlimb ischemia-reperfusion. Pyruvate's myriad benefits in preclinical models provide the mechanistic framework for its clinical application as metabolic support for myocardium at risk. Phase one trials have demonstrated pyruvate's safety and efficacy for intravenous resuscitation for septic shock, intracoronary infusion for heart failure and as a component of cardioplegia for cardiopulmonary bypass. The favorable outcomes of these trials, which argue for expanded, phase three investigations of pyruvate therapy, mirror findings in isolated, perfused hearts, underscoring the pivotal role of preclinical research in identifying clinical interventions for cardiovascular diseases. Impact statement This article reviews pyruvate's cardioprotective properties as an energy-yielding metabolic fuel, antioxidant and anti-inflammatory agent in mammalian myocardium. Preclinical research has shown these properties make pyruvate a powerful intervention to curb the complex injury cascade ignited by ischemia and reperfusion. In ischemically stunned isolated hearts and in large mammal models of cardiopulmonary bypass, cardiac arrest-resuscitation and hypovolemia, intracoronary pyruvate supports recovery of myocardial contractile function, intracellular Ca²⁺ homeostasis and free energy of ATP hydrolysis, and its antioxidant actions restore β-adrenergic signaling and suppress inflammation. The first clinical trials of pyruvate for cardiopulmonary bypass, fluid resuscitation and intracoronary intervention for congestive heart failure have been reported. Receiver operating characteristic analyses show remarkable concordance between pyruvate's beneficial functional and metabolic effects in isolated, perfused hearts and in patients recovering from cardiopulmonary bypass in which they received pyruvate- vs. L-lactate-fortified cardioplegia. This research exemplifies the translation of mechanism-oriented preclinical studies to clinical application and outcomes.

Functional response of the isolated, perfused normoxic heart to pyruvate dehydrogenase activation by dichloroacetate and pyruvate

Dichloroacetate (DCA) and pyruvate activate pyruvate dehydrogenase (PDH), a key enzyme that modulates glucose oxidation and mitochondrial NADH production. Both compounds improve recovery after ischemia in isolated hearts. However, the action of DCA and pyruvate in normoxic myocardium is incompletely understood. We measured the effect of DCA and pyruvate on contraction, mitochondrial redox state, and intracellular calcium cycling in isolated rat hearts during normoxic perfusion. Normalized epicardial NADH fluorescence (nNADH) and left ventricular developed pressure (LVDP) were measured before and after administering DCA (5 mM) or pyruvate (5 mM). Optical mapping of Rhod-2AM was used to measure cytosolic calcium kinetics. DCA maximally activated PDH, increasing the ratio of active to total PDH from 0.48 ± 0.03 to 1.03 ± 0.03. Pyruvate sub-maximally activated PDH to a ratio of 0.75 ± 0.02. DCA and pyruvate increased LVDP. When glucose was the only exogenous fuel, pyruvate increased nNADH by 21.4 ± 2.9 % while DCA reduced nNADH by 21.4 ± 6.1 % and elevated the incidence of premature ventricular contractions (PVCs). When lactate, pyruvate, and glucose were provided together as exogenous fuels, nNADH increased with DCA, indicating that PDH activation with glucose as the only exogenous fuel depletes PDH substrate. Calcium transient time-to-peak was shortened by DCA and pyruvate and SR calcium re-uptake was 30 % longer. DCA and pyruvate increased SR calcium load in myocyte monolayers. Overall, during normoxia when glucose is the only exogenous fuel, DCA elevates SR calcium, increases LVDP and contractility, and diminishes mitochondrial NADH. Administering DCA with plasma levels of lactate and pyruvate mitigates the drop in mitochondrial NADH and prevents PVCs.

Pyruvate stabilizes electrocardiographic and hemodynamic function in pigs recovering from cardiac arrest

Cardiac electromechanical dysfunction may compromise recovery of patients who are initially resuscitated from cardiac arrest, and effective treatments remain elusive. Pyruvate, a natural intermediary metabolite, energy substrate, and antioxidant, has been found to protect the heart from ischemia-reperfusion injury. This study tested the hypothesis that pyruvate-enriched resuscitation restores hemodynamic, metabolic, and electrolyte homeostasis following cardiac arrest. Forty-two Yorkshire swine underwent pacing-induced ventricular fibrillation and, after 6 min pre-intervention arrest, 4 min precordial compressions followed by transthoracic countershocks. After defibrillation and recovery of spontaneous circulation, the pigs were monitored for another 4 h. Sodium pyruvate or NaCl were infused i.v. (0.1 mmol·kg(-1)·min(-1)) throughout precordial compressions and the first 60 min recovery. In 8 of the 24 NaCl-infused swine, the first countershock converted ventricular fibrillation to pulseless electrical activity unresponsive to subsequent countershocks, but only 1 of 18 pyruvate-treated swine developed pulseless electrical activity (relative risk 0.17; 95% confidence interval 0.13-0.22). Pyruvate treatment also lowered the dosage of vasoconstrictor phenylephrine required to maintain systemic arterial pressure at 15-60 min recovery, hastened clearance of excess glucose, elevated arterial bicarbonate, and raised arterial pH; these statistically significant effects persisted up to 3 h after sodium pyruvate infusion, while infusion-induced hypernatremia subsided. These results demonstrate that pyruvate-enriched resuscitation achieves electrocardiographic and hemodynamic stability in swine during the initial recovery from cardiac arrest. Such metabolically based treatment may offer an effective strategy to support cardiac electromechanical recovery immediately after cardiac arrest.

Mitochondrial pyruvate transport: a historical perspective and future research directions

Pyruvate is the end-product of glycolysis, a major substrate for oxidative metabolism, and a branching point for glucose, lactate, fatty acid and amino acid synthesis. The mitochondrial enzymes that metabolize pyruvate are physically separated from cytosolic pyruvate pools and rely on a membrane transport system to shuttle pyruvate across the impermeable inner mitochondrial membrane (IMM). Despite long-standing acceptance that transport of pyruvate into the mitochondrial matrix by a carrier-mediated process is required for the bulk of its metabolism, it has taken almost 40 years to determine the molecular identity of an IMM pyruvate carrier. Our current understanding is that two proteins, mitochondrial pyruvate carriers MPC1 and MPC2, form a hetero-oligomeric complex in the IMM to facilitate pyruvate transport. This step is required for mitochondrial pyruvate oxidation and carboxylation-critical reactions in intermediary metabolism that are dysregulated in several common diseases. The identification of these transporter constituents opens the door to the identification of novel compounds that modulate MPC activity, with potential utility for treating diabetes, cardiovascular disease, cancer, neurodegenerative diseases, and other common causes of morbidity and mortality. The purpose of the present review is to detail the historical, current and future research investigations concerning mitochondrial pyruvate transport, and discuss the possible consequences of altered pyruvate transport in various metabolic tissues.

Acetate transiently inhibits myocardial contraction by increasing mitochondrial calcium uptake

Background

There is a close relationship between cardiovascular disease and cardiac energy metabolism, and we have previously demonstrated that palmitate inhibits myocyte contraction by increasing K_v channel activity and decreasing the action potential duration. Glucose and long chain fatty acids are the major fuel sources supporting cardiac function; however, cardiac myocytes can utilize a variety of substrates for energy generation, and previous studies demonstrate the acetate is rapidly taken up and oxidized by the heart. In this study, we tested the effects of acetate on contractile function of isolated mouse ventricular myocytes.

Results

Acute exposure of myocytes to 10 mM sodium acetate caused a marked, but transient, decrease in systolic sarcomere shortening (1.49 ± 0.20% vs. 5.58 ± 0.49% in control), accompanied by a significant increase in diastolic sarcomere length (1.81 ± 0.01 μm vs. 1.77 ± 0.01 μm in control), with a near linear dose response in the 1–10 mM range. Unlike palmitate, acetate caused no change in action potential duration; however, acetate markedly increased mitochondrial Ca²⁺ uptake. Moreover, pretreatment of cells with the mitochondrial Ca²⁺ uptake blocker, Ru-360 (10 μM), markedly suppressed the effect of acetate on contraction.

Conclusions

Lehninger and others have previously demonstrated that the anions of weak aliphatic acids such as acetate stimulate Ca²⁺ uptake in isolated mitochondria. Here we show that this effect of acetate appears to extend to isolated cardiac myocytes where it transiently modulates cell contraction.

Cardiovascular applications of hyperpolarized contrast media and metabolic tracers

Modern hyperpolarization technology enhances the recordable magnetic resonance signal four to five orders of magnitude, making in vivo assessments of tracer pathways and metabolic compartments feasible. Existing hyperpolarization instrumentation and previous tracer studies using hydroxyethylpropionate (HEP) as an extracellular marker and 14-carbon label pyruvate as examples are described and reviewed as applicable to the working heart. Future metabolic imaging based on the use of hyperpolarized pyruvate needs to consider extra- and intra-cellular label dilution due to glycolysis, lactate oxidation and protein degradation. This dilution can substantially decrease the recordable signals from PDH flux (oxidative decarboxylation of pyruvate) and other pyruvate pathways. The review of previous literature and data suggests that the (13)C-alanine signal is a better index of mitochondrially oxidized pyruvate than L-lactate. These facts and considerations will help in the interpretation of the in vivo recorded hyperpolarization signals of metabolic tracers and contrast media.

Age-related compensatory activation of pyruvate dehydrogenase complex in rat heart

Mitochondrial uptake and beta-oxidation of long-chain fatty acids are markedly impaired in the aging rat heart. While these alterations would be expected to adversely affect overall pyridine nucleotides, NADH levels do not change significantly with age. This conundrum suggests that specific compensatory mechanisms occur in the aging heart. The comparison of cardiac pyruvate dehydrogenase complex (PDC) kinetics in 4- and 24- to 28-month-old F344 rats revealed a 60% significant increase in V(max) with no change in PDC expression, and a 1.6-fold decrease in the Michaelis constant (K(m)) in old compared to young rats. The observed kinetic adjustments were selective to PDC, as neither the V(max) nor K(m) of citrate synthase changed with age. PDC kinase-4 mRNA levels decreased by 57% in old vs young rat hearts and correlated with a 45% decrease in PDC phosphorylation. We conclude that PDC from old rat hearts catabolizes pyruvate more efficiently due to an adaptive change in phosphorylation.

Reconstruction and Functional Characterization of the Human Mitochondrial Metabolic Network Based on Proteomic and Biochemical Data

Diverse datasets including genomic, proteomic, isotopomer, and DNA sequence variation are becoming available for human mitochondria. Thus there is a need to integrate these data within an in silico modeling framework where mitochondrial biology and related disorders can be studied and analyzed. This paper reports a reconstruction and characterization of the human mitochondrial metabolic network based on proteomic and biochemical data. The 189 reactions included in this reconstruction are both elementally and charge-balanced and are assigned to their respective cellular compartments (mitochondrial, cytosol, or extracellular). The capabilities of the reconstructed network to fulfill three metabolic functions (ATP production, heme synthesis, and mixed phospholipid synthesis) were determined. Network-based analysis of the mitochondrial energy conversion process showed that the overall ATP yield per glucose is 31.5. Network flexibility, characterized by allowable variation in reaction fluxes, was evaluated using flux variability analysis and analysis of all of the possible optimal flux distributions. Results showed that the network has high flexibility for the biosynthesis of heme and phospholipids but modest flexibility for maximal ATP production. A subset of all of the optimal network flux distributions, computed with respect to the three metabolic functions individually, was found to be highly correlated, suggesting that this set may contain physiological meaningful fluxes. Examinations of optimal flux distributions also identified correlated reaction sets that form functional modules in the network.

Pyruvate-enhanced cardioprotection during surgery with cardiopulmonary bypass

OBJECTIVES

To determine whether pyruvate-fortified cardioplegia solution provides cardioprotection superior to lactate-based cardioplegia solutions in patients undergoing elective coronary revascularization, with specific attention to post-surgical recovery of left ventricular performance as well as biochemical markers of ischemic injury.

DESIGN

Prospective, randomized, semi-blinded human trial.

SETTING

Community-based academic medical center.

PARTICIPANTS

Thirty adult patients undergoing elective coronary artery bypass graft surgery.

INTERVENTIONS

Patients were randomized to two 4:1 blood cardioplegia solutions, one pyruvate enhanced and the other lactate based. Hemodynamic and laboratory variables were measured in all patients at pre-cross-clamp, post-cross-clamp, and 4, 6, 8, and 12 hours after bypass.

MEASUREMENTS AND MAIN RESULTS

Relative to lactate-based cardioplegia, pyruvate-fortified cardioplegia sharply increased left ventricular stroke work at 4 to 12 hours after bypass (p < 0.001), lowered coronary sinus troponin I and creatine phosphokinase-MB activities 67% (p < 0.001) and 53% (p < 0.01), respectively, and increased coronary sinus hemoglobin O(2) saturation 18% (p < 0.001). Ten patients treated with lactate cardioplegia required beta-adrenergic inotropic support postbypass, but only 4 pyruvate-treated patients required beta-adrenergic support (p = 0.067). Pyruvate cardioplegia shortened postsurgery hospitalization from 6.3 +/- 0.3 to 5.2 +/- 0.1 days (p < 0.002).

CONCLUSIONS

Pyruvate-fortified cardioplegia mitigated myocardial injury during coronary artery bypass surgery and facilitated postsurgical recovery of cardiac performance. Thus, pyruvate-enhanced cardioplegia may provide cardioprotection superior to lactate-based solutions during surgical cardiac arrest.

Pyruvate modulates cardiac sarcoplasmic reticulum Ca2+ release in rats via mitochondria-dependent and -independent mechanisms

The glycolytic product pyruvate has beneficial effects on cardiac contractile function. The postulated cellular mechanisms underlying the positive inotropic effect of pyruvate, however, are contradictory or have remained elusive. Therefore, we studied the effects of pyruvate on cardiac Ca2+ regulation, intracellular pH (pHi) and flavoprotein oxidation using fluorescence confocal microscopy in intact and permeabilized rat ventricular myocytes and single channel recordings from rat cardiac ryanodine receptors (RyRs) incorporated into planar lipid bilayers. In intact cells extracellular pyruvate (10 mM) elevated diastolic [Ca2+]i, which was due, at least in part, to a concomitant acidification of the cytosol. Furthermore, pyruvate increased the amplitude and slowed the kinetics of the electrically evoked [Ca2+]i transient, and augmented sarcoplasmic reticulum (SR) Ca2+ content. Recording of flavoprotein (FAD) fluorescence indicated that pyruvate caused a reduction of mitochondrial redox potential, which is proportional to an increase of the rate of ATP synthesis. Inhibitors of mitochondrial monocarboxylate transport (alpha-cyano-4-hydroxycinnamate, 0.5 mM), adenine nucleotide translocation (atractyloside, 0.3 mM) and the electron transport chain (cyanide, 4 mM) abolished or attenuated the pyruvate-mediated increase of the amplitude of the [Ca2+]i transient, but did not change the effect of pyruvate on diastolic [Ca2+]i. Results from experiments with permeabilized myocytes indicated a direct correlation between ATP/ADP ratio and SR Ca2+ content. Furthermore, pyruvate (4 mM) reduced the frequency of spontaneous Ca2+ sparks by approximately 50%. Single RyR channel recordings revealed a approximately 60% reduction of the open probability of the channel by pyruvate (1 mM), but no change in conductance. This effect of pyruvate on RyR channel activity was neither Ca2+ nor ATP dependent. Taken together, these findings suggest that, in cardiac tissue, pyruvate has a dual effect on SR Ca2+ release consisting of a direct inhibition of RyR channel activity and elevation of SR Ca2+ content. The latter effect was most probably mediated by an enhanced SR Ca2+ uptake due to an augmentation of mitochondria-dependent ATP synthesis.

Insulin-like effects of a physiologic concentration of carnitine on cardiac metabolism

Pharmacologic (millimolar) levels of carnitine have been reported to increase myocardial glucose oxidation, but whether physiologically relevant concentrations of carnitine affect cardiac metabolism is not known. We employed the isolated, perfused rat heart to compare the effects of physiologic levels of carnitine (50 microM) and insulin (75 mU/l [0.5 nM]) on the following metabolic processes: (1) glycolysis (release of 3H2O from 5-3H-glucose); (2) oxidation of glucose and pyruvate (production of 14CO2 from U-14C-glucose, 1-14C-glucose, 3,4-14C-glucose, 1-14C-pyruvate, and 2-14C-pyruvate); and (3) oxidation of palmitate (release of 3H2O from 9,10-3H-palmitate). We found that addition of carnitine (50 microM) to a perfusate containing both glucose (10 mM) and palmitate (0.5 mM) stimulated glycolytic flux by 20%, nearly doubled the rate of glucose oxidation, and inhibited palmitate oxidation by 20%. These actions of carnitine were uniformly similar to those of insulin. When carnitine and insulin were administered together, their effects on the oxidation of glucose and palmitate, but not on glycolysis, were additive. When pyruvate (1 mM) was substituted for glucose, neither carnitine nor insulin influenced the rate of oxidation of pyruvate or palmitate. In combination, however, carnitine and insulin sharply suppressed pyruvate oxidation (75%) and doubled the rate of palmitate oxidation. None of the responses to carnitine or insulin was affected by varying the isotopic labeling of glucose or pyruvate. The results show that carnitine, at normal blood levels, exerts insulin-like effects on myocardial fuel utilization. They also suggest that plasma carnitine in vivo may interact with insulin both additively and permissively on the metabolism of carbohydrates and fatty acids.

Citrin and aralar1 are Ca(2+)-stimulated aspartate/glutamate transporters in mitochondria

The mitochondrial aspartate/glutamate carrier catalyzes an important step in both the urea cycle and the aspartate/malate NADH shuttle. Citrin and aralar1 are homologous proteins belonging to the mitochondrial carrier family with EF-hand Ca(2+)-binding motifs in their N-terminal domains. Both proteins and their C-terminal domains were overexpressed in Escherichia coli, reconstituted into liposomes and shown to catalyze the electrogenic exchange of aspartate for glutamate and a H(+). Overexpression of the carriers in transfected human cells increased the activity of the malate/aspartate NADH shuttle. These results demonstrate that citrin and aralar1 are isoforms of the hitherto unidentified aspartate/glutamate carrier and explain why mutations in citrin cause type II citrullinemia in humans. The activity of citrin and aralar1 as aspartate/glutamate exchangers was stimulated by Ca(2+) on the external side of the inner mitochondrial membrane, where the Ca(2+)-binding domains of these proteins are localized. These results show that the aspartate/glutamate carrier is regulated by Ca(2+) through a mechanism independent of Ca(2+) entry into mitochondria, and suggest a novel mechanism of Ca(2+) regulation of the aspartate/malate shuttle.

Intramitochondrial pyruvate attenuates hydrogen peroxide-induced apoptosis in bovine pulmonary artery endothelium

In the hydrogen peroxide (H2O2) apoptosis model of the murine thymocyte, redox reactant and antioxidant pyruvate prevents programmed cell death. We tested the hypothesis that such protection was mediated, at least in part, via pyruvate handling by mitochondrial metabolism. Cultured bovine pulmonary artery endothelial cells were incubated for 30 min with 0.5 mM H2O2 in the absence and presence of 0.5 mM alpha-cyano-3-hydroxycinnamate, as a selective inhibitor of the mitochondrial pyruvate transporter. In controls H2O2 decreased cell viability by 30% within 24 h; this was associated with apoptosis-like bodies, nuclear condensation, and biochemical DNA damage consistent with programmed cell death. Pyruvate (0.1-20 mM) enhanced cell viability in a dose-dependent manner, with > or = 85% viable cells at > or = 3 mM and no DNA laddering, no positive nick-end labeling (TUNEL), and no detectable Annexin V or propidium iodide staining. In contrast, using > or = 5 mM L-lactate as a cytosolic reductant or acetate as a redox-neutral substrate, cell death increased to approximately 40%, which was associated with intense DNA laddering, positive TUNEL and Hoechst 33258 assays. Alpha-cyano-3-hydroxycinnamate alone did not significantly decrease endothelial viability but reduced viability from 85+/-3 to 71+/-4% (p = 0.023) in presence of 3 mM pyruvate plus H2O2; pathological cell morphology and DNA laddering under the same conditions suggested loss of pyruvate protection against apoptosis. Since alpha-cyano-3-hydroxycinnamate re-distributed medium pyruvate and L-lactate consistent with selective blockade of pyruvate uptake into the mitochondria, the findings support the hypothesis that pyruvate protection against H2O2 apoptosis is mediated in part via the mitochondrial matrix compartment. Possible mediators include anti-apoptotic bcl-2 and/or products of mitochondrial pyruvate metabolism such as citrate that affect metabolic regulation and anti-oxidant status in the cytoplasm.

Antioxidant pyruvate inhibits cardiac formation of reactive oxygen species through changes in redox state

Myocardial ischemia-reperfusion is associated with bursts of reactive oxygen species (ROS) such as superoxide radicals (O(2)(-).). Membrane-associated NADH oxidase (NADHox) activity is a hypothetical source of O(2)(-)., implying the NADH concentration-to-NAD(+) concentration ratio ([NADH]/[NAD(+)]) as a determinant of ROS. To test this hypothesis, cardiac NADHox and ROS formation were measured as influenced by pyruvate or L-lactate. Pre- and postischemic Langendorff guinea pig hearts were perfused at different pyruvate/L-lactate concentrations to alter cytosolic [NADH]/[NAD(+)]. NADHox and ROS were measured with the use of lucigenin chemiluminescence and electron spin resonance, respectively. In myocardial homogenates, pyruvate (0.05, 0.5 mM) and the NADHox blocker hydralazine markedly inhibited NADHox (16 +/- 2%, 58 +/- 9%). In postischemic hearts, pyruvate (0.1-5.0 mM) dose dependently inhibited ROS up to 80%. However, L-lactate (1.0-15.0 mM) stimulated both basal and postischemic ROS severalfold. Furthermore, L-lactate-induced basal ROS was dose dependently inhibited by pyruvate (0.1-5.0 mM) and not the xanthine oxidase inhibitor oxypurinol. Pyruvate did not inhibit ROS from xanthine oxidase. The data suggest a substantial influence of cytosolic NADH on cardiac O(2)(-). formation that can be inhibited by submillimolar pyruvate. Thus cytotoxicities due to cardiac ischemia-reperfusion ROS may be alleviated by redox reactants such as pyruvate.

Intra- and extra-cellular lactate shuttles

The "lactate shuttle hypothesis" holds that lactate plays a key role in the distribution of carbohydrate potential energy that occurs among various tissue and cellular compartments such as between: cytosol and mitochondria, muscle and blood, blood and muscle, active and inactive muscles, white and red muscles, blood and heart, arterial blood and liver, liver and other tissues such as exercising muscle, intestine and portal blood, portal blood and liver, zones of the liver, and skin and blood. Studies on resting and exercising humans indicate that most lactate (75-80%) is disposed of through oxidation, with much of the remainder converted to glucose and glycogen. Lactate transport across cellular membranes occurs by means of facilitated exchange along pH and concentration gradients involving a family of lactate transport proteins, now called monocarboxylate transporters (MCTs). Current evidence is that muscle and other cell membrane lactate transporters are abundant with characteristics of high Km and Vmax. There appears to be long-term plasticity in the number of cell membrane transporters, but short-term regulation by allosteric modulation or phosphorylation is not known. In addition to cell membranes, mitochondria also contain monocarboxylate transporters (mMCT) and lactic dehydrogenase (mLDH). Therefore, mitochondrial monocarboxylate uptake and oxidation, rather than translocation of transporters to the cell surfaces, probably regulate lactate flux in vivo. Accordingly, the "lactate shuttle" hypothesis has been modified to include a new, intracellular component involving cytosolic to mitochondrial exchange. The intracellular lactate shuttle emphasizes the role of mitochondrial redox in the oxidation and disposal of lactate during exercise and other conditions.

Pyruvate: metabolic protector of cardiac performance

Pyruvate, a metabolic product of glycolysis and an oxidizable fuel in myocardium, increases cardiac mechanical performance and energy reserves, especially when supplied at supraphysiological concentrations. The inotropic effects of pyruvate are most impressive in hearts that have been reversibly injured (stunned) by ischemia/reperfusion stress. Glucose appears to be an essential co-substrate for pyruvate's salutary effects in stunned hearts, but other fuels including lactate, acetate, fatty acids, and ketone bodies produce little or no improvement in postischemic function over glucose alone. In contrast to pharmacological inotropism by catecholamines, metabolic inotropism by pyruvate increases cardiac energy reserves and bolsters the endogenous glutathione antioxidant system. Pyruvate enhancement of cardiac function may result from one or more of the following mechanisms: increased cytosolic ATP phosphorylation potential and Gibbs free energy of ATP hydrolysis, enhanced sarcoplasmic reticular calcium ion uptake and release, decreased cytosolic inorganic phosphate concentration, oxyradical scavenging via direct neutralization of peroxides and/or enhancement of the intracellular glutathione/NADPH antioxidant system, and/or closure of mitochondrial permeability transition pores. This review aims to summarize evidence for each of these mechanisms and to consider the potential utility of pyruvate as a therapeutic intervention for clinical management of cardiac insufficiency.

Pyruvate prevents hydrogen peroxide-induced apoptosis

Studies were carried out to investigate the protective effects of pyruvate, a key glycolytic intermediate and alpha-keto-monocarboxylate, against oxidative stress-induced apoptosis. Oxidative stress was induced by treating mouse thymocytes with 25 microM hydrogen peroxide for 15 min at 37 degrees C under 5% CO2 in air. Pre- and post-treatment of cells with 10 mM pyruvate inhibited morphological changes, internucleosomal DNA fragmentation, and translocation of phosphatidylserine to the plasma membrane surface, which are characteristic features of apoptosis. L-lactate (10 mM) and acetate (10 mM) were ineffective in inhibiting apoptosis and appeared to be toxic to the cells under similar conditions. The results suggest that pyruvate has therapeutic potential for use in the treatment of oxidative stress-induced disorders associated with increased apoptosis.

Effects of CO2-induced acidification on the fatigue resistance of single mouse muscle fibers at 28 degrees C

The role of reduced muscle pH in the development of skeletal muscle fatigue is unclear. This study investigated the effects of lowering skeletal muscle intracellular pH by exposure to 30% CO2 on the number of isometric tetani needed to induce significant fatigue. Isolated single mouse muscle fibers were stimulated repetitively at intervals of 4-2.5 s by using 80-Hz, 400-ms tetani at 28 degrees C in Tyrode solution bubbled with either 5 or 30% CO2. Stimulation continued until tetanic force had fallen to 40% of the initial value. Exposure to 30% CO2 caused a significant fall in intracellular pH of approximately 0.3 pH unit but did not cause any significant changes in initial peak tetanic force. During the course of repetitive stimulation, intracellular pH fell by approximately 0.3 pH unit in both normal and acidified fibers. The number of tetani needed to reduce force to 40% of the initial value was not significantly different in 5 and 30% CO2 Tyrode. The sole effect of acidosis was to reduce the rate of relaxation of force, especially in fatigued fibers. It is concluded that, at 28 degrees C, acidosis per se does not accelerate the development of fatigue during repeated tetanic stimulation of isolated mouse skeletal muscle fibers.

Regulation of skeletal muscle carbohydrate oxidation during steady-state contraction

Pyruvate dehydrogenase complex (PDC) activation status has been described as being central in the regulation of tissue substrate oxidation as outlined by the glucose fatty-acid cycle. In the present study we examined the effects of reduced lipolysis, with use of nicotinate, and increased PDC activation, with use of dichloroacetate (DCA), on substrate utilization during 20 min of submaximal steady-state contraction (approximately 80% of maximal O2 uptake) in canine gracilis skeletal muscle. At rest, PDC activation was unchanged by nicotinate but was approximately 2.5-fold higher in the DCA group than in the control group (P < 0.05). During contraction, PDC activation status increased to 3.5 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the control group, remained at 4.5 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the DCA group, but only increased to 2.2 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the nicotinate group (P < 0.05). However, the estimated amount of carbohydrate oxidized during the 20-min contraction was similar across groups and did not follow the degree of PDC activation (81.2 +/- 22.9, 95.9 +/- 11.7, and 89.3 +/- 18.9 mmol glucosyl units/kg dry muscle for control, nicotinate, and DCA, respectively). Thus it would appear that, during steady-state contraction, PDC activation status does not determine the rate of carbohydrate oxidation in skeletal muscle.

Pyruvate augments calcium transients and cell shortening in rat ventricular myocytes

Pyruvate has been shown to be a metabolic inotrope in the myocardium. In millimolar concentrations, it has been shown to increase both myocardial phosphorylation potential and the cytosolic [NAD+]-to-[NADH] ratio. To determine if changes in these parameters can alter intracellular Ca2+ concentration ([Ca2+]i) and hence contractile function, Ca2+ transients and cell shortening (CS) were measured in isolated rat ventricular myocytes superfused with a physiological N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer (11 mmol/l glucose) with and without additional pyruvate, L-lactate, acetate, or isoproterenol. The addition of 5 mmol/l pyruvate resulted in a 33% increase in CS and a 39% increase in systolic [Ca2+]i. These pyruvate effects were 70% of those observed with 100 nmol/l isoproterenol. The mitochondrial monocarboxylate transport inhibitor alpha-cyano-4-hydroxycinnamate (250 mumol/l) strongly inhibited pyruvate inotropy, suggesting a substantial obligatory coupling between pyruvate inotropism and its oxidation by the mitochondria. A possible role of the cytosolic [NAD+]-to-[NADH] ratio was assessed by comparing the effects of 20 mmol/l L-lactate to those of equimolar pyruvate. In contrast to 20 mmol/l pyruvate, excess L-lactate failed to appreciably increase CS or systolic [Ca2+]i. The findings imply that, at levels substantially above 5 mmol/l, a portion of pyruvate inotropism might be due to extreme cytosolic [NAD+]-to-[NADH] ratios. This study is the first evidence that augmented [Ca2+]i transients are most likely the mechanism of cardiac pyruvate inotropism.

Escherichia coli heat-stable enterotoxin receptors. A novel marker for colorectal tumors

PURPOSE

Receptors for Escherichia coli heat-stable toxin (ST) are selectively expressed in membranes of intestinal mucosa cells and colon carcinoma cells in vitro, suggesting their use as a marker for colorectal tumors in vivo. The present studies examined the expression and function of ST receptors in normal human tissues and primary and metastatic colorectal tumors obtained from patients at surgery.

METHODS

Surgical specimens were obtained as follows: from normal colon; from primary adenocarcinomas from all anatomic divisions of the colon and rectum; from gallbladder, kidney, liver, lung, lymph node, ovary, peritoneum, stomach; and from colon carcinomas metastatic to liver, lung, lymph node, ovary, and peritoneum. Membranes prepared from these specimens were assessed for the presence and functional characteristics of ST receptors.

RESULTS

ST bound specifically to membranes from each division of normal colon and rectum and all primary and metastatic colorectal tumors examined. The affinity and density of ST receptors were similar in tumors of different grades and from various metastatic sites. ST-receptor interaction was coupled to activation of guanylyl cyclase in all normal samples of colon and rectum and all primary and metastatic colorectal tumors examined. In contrast, neither ST binding nor ST activation of guanylyl cyclase was detected in any extraintestinal tissues examined.

CONCLUSIONS

Functional ST receptors are expressed in normal colonic tissue and primary and metastatic colorectal tumors but not by extraintestinal tissues in humans. Expression of ST receptors does not vary as a function of the metastatic site or grade of these tumors. Receptors expressed by colorectal tumors retain their characteristic function, with binding of ST coupled to activation of guanylyl cyclase. These studies support the suggestion that ST receptors represent a specific marker for human colorectal tumors that may have use as a target for directing diagnostics and therapeutics to these tumors in vivo.

Continuous measurement of 13C16O2 production from [13C]pyruvate by intact liver mitochondria: effect of HCO3-

We have measured continuously the production of mass 45 CO2(13C16O2) from 13C-labeled pyruvate in a guinea pig liver mitochondrial suspension and simultaneously the O2 consumption at 37 degrees C and pH 7.4. The reactions took place in a closed 3-ml volume, stirred, thermoregulated chamber separated from the ion source of a mass spectrometer by a gas-permeable membrane that permitted recording the mass peaks of any gas dissolved in the reaction mixture with a response time as fast as 3 s. If the pyruvate was labeled on C-2, no 13C16O2 was formed, even after 1 h, indicating that C-2 and C-3 were not metabolized in the citric acid cycle. We found that production of 13C16O2 was five times greater in the presence of 25 mM HCO3- than in its absence. A probable mechanism of this CO2/HCO3- effect is carboxylation of pyruvate to oxaloacetate, which would react with acetyl CoA to form citrate and with NADH to form malate, thus removing two major inhibitors of pyruvate dehydrogenase. We conclude that CO2/HCO3- has a potent and hitherto unappreciated regulatory effect on liver pyruvate dehydrogenase.

Hypothyroid state and membrane fatty acid composition influence cardiac mitochondrial pyruvate oxidation

Pyruvate oxidation was measured in cardiac mitochondria from euthyroid and hypothyroid rats fed diets enriched with either omega-6 or omega-3 fatty acids. Both State 4 and State 3 rates of pyruvate-dependent respiration were markedly reduced in hypothyroid mitochondria, regardless of diet consumed, compared to euthyroid controls. Respiratory control ratios and ADP/O ratios were the same under all treatments. While there was no significant effect of diet on respiration in euthyroid mitochondria, pyruvate oxidation was 28% higher in hypothyroid mitochondria from animals fed the omega-3 diet compared to those fed the omega-6 diet. Depressed respiration in the hypothyroid state was correlated with 18% more phosphatidylcholine in the inner mitochondrial membrane whereas phosphatidylethanolamine was 17% lower and cardiolipin 32% lower compared to controls. The total phospholipid fatty acid composition was not affected by the hypothyroid state. However, enhanced respiration in hypothyroid animals fed the omega-3 diet was associated with a 3-fold increase in monounsaturated fatty acids in the cardiolipin fraction, and a 12-fold increase in omega-3 fatty acids, primarily 22:5(omega-3) and 22:6(omega-3). The data suggest that membrane levels of cardiolipin and its omega-3 fatty acid content modulate pyruvate transport in hypothyroid mitochondria.