Effects of augmented delivery of pyruvate on myocardial high-energy phosphate metabolism at high workstate

Integrated Functions of Cardiac Energetics, Mechanics, and Purine Nucleotide Metabolism

Purine nucleotides play central roles in energy metabolism in the heart. Most fundamentally, the free energy of hydrolysis of the adenine nucleotide adenosine triphosphate (ATP) provides the thermodynamic driving force for numerous cellular processes including the actin-myosin crossbridge cycle. Perturbations to ATP supply and/or demand in the myocardium lead to changes in the homeostatic balance between purine nucleotide synthesis, degradation, and salvage, potentially affecting myocardial energetics and, consequently, myocardial mechanics. Indeed, both acute myocardial ischemia and decompensatory remodeling of the myocardium in heart failure are associated with depletion of myocardial adenine nucleotides and with impaired myocardial mechanical function. Yet there remain gaps in the understanding of mechanistic links between adenine nucleotide degradation and contractile dysfunction in heart disease. The scope of this article is to: (i) review current knowledge of the pathways of purine nucleotide depletion and salvage in acute ischemia and in chronic heart disease; (ii) review hypothesized mechanisms linking myocardial mechanics and energetics with myocardial adenine nucleotide regulation; and (iii) highlight potential targets for treating myocardial metabolic and mechanical dysfunction associated with these pathways. It is hypothesized that an imbalance in the degradation, salvage, and synthesis of adenine nucleotides leads to a net loss of adenine nucleotides in both acute ischemia and under chronic high-demand conditions associated with the development of heart failure. This reduction in adenine nucleotide levels results in reduced myocardial ATP and increased myocardial inorganic phosphate. Both of these changes have the potential to directly impact tension development and mechanical work at the cellular level. © 2024 American Physiological Society. Compr Physiol 14:5345-5369, 2024.

Myocardial Functional Decline During Prolonged Ex Situ Heart Perfusion

BACKGROUND

Myocardial function declines in a time-dependent fashion during ex situ heart perfusion. Cell death and metabolic alterations may contribute to this phenomenon, limiting the safe perfusion period and the potential of ex situ heart perfusion to expand the donor pool. Our aim was to investigate the etiology of myocardial functional decline in ex situ perfused hearts.

METHODS

Cardiac function, apoptosis, effectors and markers of cell death, and metabolic function were assessed in healthy pig hearts perfused for 12 hours. These hearts were perfused in nonworking mode or working mode.

RESULTS

Cardiac function declined during ex situ heart perfusion regardless of perfusion mode but was significantly better preserved in the hearts perfused in working mode (11-hour cardiac index/1-hour cardiac index: working mode, 33%; nonworking mode, 10%; p = 0.025). The rate of apoptosis was higher in the ex situ perfused hearts compared with in vivo samples (apoptotic cells: in vivo, 0.13%; working mode, 0.54%; nonworking mode, 0.88%; p < 0.001), but the absolute values were low and out of proportion to the decline in function in either group. Myocardial dysfunction at the end of the perfusion interval was partially rescued by delivery of a pyruvate bolus.

CONCLUSIONS

A significant decline in myocardial function occurs over time in hearts preserved ex situ that is out of proportion to the magnitude of myocyte cell death present in dysfunctional hearts. Alterations in myocardial substrate utilization during prolonged ex situ heart perfusion may contribute to this phenomenon and represent an avenue to improve donor heart preservation.

Feedback Regulation and Time Hierarchy of Oxidative Phosphorylation in Cardiac Mitochondria

To determine how oxidative ATP synthesis is regulated in the heart, the responses of cardiac mitochondria oxidizing pyruvate to alterations in [ATP], [ADP], and inorganic phosphate ([Pi]) were characterized over a range of steady-state levels of extramitochondrial [ATP], [ADP], and [Pi]. Evolution of the steady states of the measured variables with the flux of respiration shows that: (1) a higher phosphorylation potential is achieved by mitochondria at higher [Pi] for a given flux of respiration; (2) the time hierarchy of oxidative phosphorylation is given by phosphorylation subsystem, electron transport chain, and substrate dehydrogenation subsystems listed in increasing order of their response times; (3) the matrix ATP hydrolysis mass action ratio [ADP] × [Pi]/[ATP] provides feedback to the substrate dehydrogenation flux over the entire range of respiratory flux examined in this study; and finally, (4) contrary to previous models of regulation of oxidative phosphorylation, [Pi] does not modulate the activity of complex III.

Regulation of cardiac cellular bioenergetics: mechanisms and consequences

The regulation of cardiac cellular bioenergetics is critical for maintaining normal cell function, yet the nature of this regulation is not fully understood. Different mechanisms have been proposed to explain how mitochondrial ATP production is regulated to match changing cellular energy demand while metabolite concentrations are maintained. We have developed an integrated mathematical model of cardiac cellular bioenergetics, electrophysiology, and mechanics to test whether stimulation of the dehydrogenase flux by Ca²⁺ or Pi, or stimulation of complex III by Pi can increase the rate of mitochondrial ATP production above that determined by substrate availability (ADP and Pi). Using the model, we show that, under physiological conditions the rate of mitochondrial ATP production can match varying demand through substrate availability alone; that ATP production rate is not limited by the supply of reducing equivalents in the form of NADH, as a result of Ca²⁺ or Pi activation of the dehydrogenases; and that ATP production rate is sensitive to feedback activation of complex III by Pi. We then investigate the mechanistic implications on cytosolic ion homeostasis and force production by simulating the concentrations of cytosolic Ca²⁺, Na⁺ and K⁺, and activity of the key ATPases, SERCA pump, Na⁺/K⁺ pump and actin-myosin ATPase, in response to increasing cellular energy demand. We find that feedback regulation of mitochondrial complex III by Pi improves the coupling between energy demand and mitochondrial ATP production and stabilizes cytosolic ADP and Pi concentrations. This subsequently leads to stabilized cytosolic ionic concentrations and consequentially reduced energetic cost from cellular ATPases.

Effects of ethyl pyruvate on cardiac function recovery and apoptosis reduction after global cold ischemia and reperfusion

The present study used an in vitro model of cold cardioplegia in isolated working rat hearts to evaluate the possible role of ethyl pyruvate (EP) in promoting cardiac function and preventing apoptosis. Two groups of rats were evaluated; the EP (2 mM EP; n=8) and control (n=8) groups. Isolated rat hearts were perfused with Krebs-Henseleit buffer (KHB) for 30 min, arrested with cardioplegic solution and stored for 4 h in B21 solution at 4°C. The hearts were reperfused with KHB for 45 min. EP was added to the cardioplegic and storage solutions and also to KHB for reperfusion. Cardiac parameters of the heart rate, including left ventricular systolic pressure, left ventricular end-diastolic pressure, left ventricular developed pressure and maximal rise rate of the left ventricular pressure, were monitored. In addition, coronary flow, adenosine triphosphate (ATP) levels and malondialdehyde (MDA) content were recorded and apoptotic cell determination was detected. The functional parameters in the EP group were significantly higher compared with those in the control group during the reperfusion period (P<0.05). In addition, ATP levels were higher in the EP group than in the control group and the content of MDA was lower in the EP group than in the control group. A concentration of 2 mM EP significantly reduced the number of apoptotic cells in the EP group compared with that of the control group (P<0.05). Therefore, EP significantly preserved cardiac function, enhanced tissue ATP levels, attenuated myocardial oxidative injury and markedly reduced apoptosis following myocardial ischemia in an in vitro model of 4 h of cold cardioplegia and reperfusion.

Mitochondrial approaches to protect against cardiac ischemia and reperfusion injury

The mitochondrion is a vital component in cellular energy metabolism and intracellular signaling processes. Mitochondria are involved in a myriad of complex signaling cascades regulating cell death vs. survival. Importantly, mitochondrial dysfunction and the resulting oxidative and nitrosative stress are central in the pathogenesis of numerous human maladies including cardiovascular diseases, neurodegenerative diseases, diabetes, and retinal diseases, many of which are related. This review will examine the emerging understanding of the role of mitochondria in the etiology and progression of cardiovascular diseases and will explore potential therapeutic benefits of targeting the organelle in attenuating the disease process. Indeed, recent advances in mitochondrial biology have led to selective targeting of drugs designed to modulate or manipulate mitochondrial function, to the use of light therapy directed to the mitochondrial function, and to modification of the mitochondrial genome for potential therapeutic benefit. The approach to rationally treat mitochondrial dysfunction could lead to more effective interventions in cardiovascular diseases that to date have remained elusive. The central premise of this review is that if mitochondrial abnormalities contribute to the etiology of cardiovascular diseases (e.g., ischemic heart disease), alleviating the mitochondrial dysfunction will contribute to mitigating the severity or progression of the disease. To this end, this review will provide an overview of our current understanding of mitochondria function in cardiovascular diseases as well as the potential role for targeting mitochondria with potential drugs or other interventions that lead to protection against cell injury.

Simulation of cellular biochemical system kinetics

The goal of realistically and reliably simulating the biochemical processes underlying cellular function is achievable through a systematic approach that makes use of the broadest possible amount of in vitro and in vivo data, and is consistent with all applicable physical chemical theories. Progress will be facilitated by establishing: (1) a concrete self-consistent theoretical foundation for systems simulation; (2) extensive and accurate databases of thermodynamic properties of biochemical reactions; (3) parameterized and validated models of enzyme and transporter catalytic mechanisms that are consistent with physical chemical theoretical foundation; and (4) software tools for integrating all these concepts, data, and models into a cohesive representation of cellular biochemical systems. Ongoing initiatives are laying the groundwork for the broad-based community cooperation that will be necessary to pursue these elements of a strategic infrastructure for systems simulation on a large scale.

Potential therapeutic benefits of strategies directed to mitochondria

The mitochondrion is the most important organelle in determining continued cell survival and cell death. Mitochondrial dysfunction leads to many human maladies, including cardiovascular diseases, neurodegenerative disease, and cancer. These mitochondria-related pathologies range from early infancy to senescence. The central premise of this review is that if mitochondrial abnormalities contribute to the pathological state, alleviating the mitochondrial dysfunction would contribute to attenuating the severity or progression of the disease. Therefore, this review will examine the role of mitochondria in the etiology and progression of several diseases and explore potential therapeutic benefits of targeting mitochondria in mitigating the disease processes. Indeed, recent advances in mitochondrial biology have led to selective targeting of drugs designed to modulate and manipulate mitochondrial function and genomics for therapeutic benefit. These approaches to treat mitochondrial dysfunction rationally could lead to selective protection of cells in different tissues and various disease states. However, most of these approaches are in their infancy.

Creatine and phosphate pools are maintained at energetically optimal levels in the heart during hypertrophic remodeling and heart failure

The ability of mitochondria to oxidatively synthesize ATP from ADP and inorganic phosphate is compromised in the failing heart. Specifically, the magnitude of the free energy at which ATP is synthesized in heart failure is diminished compared to control. However the causal mechanisms involved are not clearly understood. Here we used computer simulation to analyze the impact of reduction in three cytoplasmic metabolic pools that is observed with hypertrophic remodeling and heart failure. Our simulations, which are validated based on in vivo data on phosphate metabolites in both the healthy and diseased heart, predict that, given a prescribed reduction in the total adenine nucleotide pool, the pools of total creatine and total exchangeable phosphate are maintained at levels that maintain the ATP hydrolysis potential of the heart at near the normal physiological value.

Experimentally observed phenomena on cardiac energetics in heart failure emerge from simulations of cardiac metabolism

The failing heart is hypothesized to suffer from energy supply inadequate for supporting normal cardiac function. We analyzed data from a canine left ventricular hypertrophy model to determine how the energy state evolves because of changes in key metabolic pools. Our findings--confirmed by in vivo (31)P-magnetic resonance spectroscopy--indicate that the transition between the clinically observed early compensatory phase and heart failure and the critical point at which the transition occurs are emergent properties of cardiac energy metabolism. Specifically, analysis reveals a phenomenon in which low and moderate reductions in metabolite pools have no major negative impact on oxidative capacity, whereas reductions beyond a critical tipping point lead to a severely compromised energy state. The transition point corresponds to reductions in the total adenine nucleotide pool (TAN) of approximately 30%, corresponding to the reduction observed in humans in heart failure [Ingwall JS, Weiss RG (2004) Is the failing heart energy starved? On using chemical energy to support cardiac function. Circ Res 95(2):135-145]. At given values of TAN and the total exchangeable phosphate pool during hypertrophic remodeling, the creatine pool attains a value that is associated with optimal ATP hydrolysis potential. Thus, both increases and decreases to the creatine pool are predicted to result in diminished energetic state unless accompanied by appropriate simultaneous changes in the other pools.

Roles of the creatine kinase system and myoglobin in maintaining energetic state in the working heart

BACKGROUND

The heart is capable of maintaining contractile function despite a transient decrease in blood flow and increase in cardiac ATP demand during systole. This study analyzes a previously developed model of cardiac energetics and oxygen transport to understand the roles of the creatine kinase system and myoglobin in maintaining the ATP hydrolysis potential during beat-to-beat transient changes in blood flow and ATP hydrolysis rate.

RESULTS

The theoretical investigation demonstrates that elimination of myoglobin only slightly increases the predicted range of oscillation of cardiac oxygenation level during beat-to-beat transients in blood flow and ATP utilization. In silico elimination of myoglobin has almost no impact on the cytoplasmic ATP hydrolysis potential (DeltaGATPase). In contrast, disabling the creatine kinase system results in considerable oscillations of cytoplasmic ADP and ATP levels and seriously deteriorates the stability of DeltaGATPase in the beating heart.

CONCLUSION

The CK system stabilizes DeltaGATPase by both buffering ATP and ADP concentrations and enhancing the feedback signal of inorganic phosphate in regulating mitochondrial oxidative phosphorylation.

Superior cardiac function via anaplerotic pyruvate in the immature swine heart after cardiopulmonary bypass and reperfusion

Pyruvate produces inotropic responses in the adult reperfused heart. Pyruvate oxidation and anaplerotic entry into the tricarboxylic acid (TCA) cycle via carboxylation are linked to the stimulation of contractile function. The goals of this study were to determine if these metabolic pathways operate and are maintained in the developing myocardium after reperfusion. Immature male swine (age: 10-18 days) were subjected to cardiopulmonary bypass (CPB). Intracoronary infusion of [2-(13)C]pyruvate (to achieve an estimated final concentration of 8 mM) was given for 35 min, starting either during weaning (group I) and after its discontinuation (group II) or without (control) CPB. Hemodynamic data were collected. 13C NMR spectroscopy was used to determine the fraction of pyruvate entering the TCA cycle via pyruvate carboxylation (PC) to total TCA cycle entry (PC plus decarboxlyation via pyruvate dehydrogenase). Liquid chromatography-mass spectrometry was used to determine total glutamate enrichment. Pyruvate infusion starting during the weaning of mechanical circulatory support improved maximum dP/dt (P<0.05) but waiting to start the infusion until after the discontinuation of CPB did not. Glutamate fractional enrichment was confirmed by liquid chromatography-mass spectroscopy as adequate (>5%) to provide signal to noise in the NMR experiment in all groups. The ratio of pyruvate carboxylase to total pyruvate entry into the TCA cycle did not differ between groups (group I: 20+/-4%, group II: 23+/-7%, and control: 27+/-7%). These data show that robust PC operates in the neonatal pig heart and is maintained during reperfusion under conditions that emulate CPB and reperfusion in human infants.

Phosphate metabolite concentrations and ATP hydrolysis potential in normal and ischaemic hearts

To understand how cardiac ATP and CrP remain stable with changes in work rate - a phenomenon that has eluded mechanistic explanation for decades - data from (31)phosphate-magnetic resonance spectroscopy ((31)P-MRS) are analysed to estimate cytoplasmic and mitochondrial phosphate metabolite concentrations in the normal state, during high cardiac workstates, during acute ischaemia and reactive hyperaemic recovery. Analysis is based on simulating distributed heterogeneous oxygen transport in the myocardium integrated with a detailed model of cardiac energy metabolism. The model predicts that baseline myocardial free inorganic phosphate (P(i)) concentration in the canine myocyte cytoplasm - a variable not accessible to direct non-invasive measurement - is approximately 0.29 mm and increases to 2.3 mm near maximal cardiac oxygen consumption. During acute ischaemia (from ligation of the left anterior descending artery) P(i) increases to approximately 3.1 mm and ATP consumption in the ischaemic tissue is reduced quickly to less than half its baseline value before the creatine phosphate (CrP) pool is 18% depleted. It is determined from these experiments that the maximal rate of oxygen consumption of the heart is an emergent property and is limited not simply by the maximal rate of ATP synthesis, but by the maximal rate at which ATP can be synthesized at a potential at which it can be utilized. The critical free energy of ATP hydrolysis for cardiac contraction that is consistent with these findings is approximately -63.5 kJ mol(-1). Based on theoretical findings, we hypothesize that inorganic phosphate is both the primary feedback signal for stimulating oxidative phosphorylation in vivo and also the most significant product of ATP hydrolysis in limiting the capacity of the heart to hydrolyse ATP in vivo. Due to the lack of precise quantification of P(i) in vivo, these hypotheses and associated model predictions remain to be carefully tested experimentally.

Increased expression of SERCA in the hearts of transgenic mice results in increased oxidation of glucose

While several transgenic mouse models exhibit improved contractile characteristics in the heart, less is known about how these changes influence energy metabolism, specifically the balance between carbohydrate and fatty acid oxidation. In the present study we examine glucose and fatty acid oxidation in transgenic mice, generated to overexpress sarco(endo)plasmic reticulum calcium-ATPase (SERCA), which have an enhanced contractile phenotype. Energy substrate metabolism was measured in isolated working hearts using radiolabeled glucose and palmitate. We also examined oxygen consumption to see whether SERCA overexpression is associated with increased oxygen utilization. Since SERCA is important in calcium handling within the cardiac myocyte, we examined cytosolic calcium transients in isolated myocytes using indo-1, and mitochondrial calcium levels using pericam, an adenovirally expressed, mitochondrially targeted ratiometric calcium indicator. Oxygen consumption did not differ between wild-type and SERCA groups; however, we were able to show an increased utilization of glucose for oxidative metabolism and a corresponding decreased utilization of fatty acids in the SERCA group. Cytosolic calcium transients were increased in myocytes isolated from SERCA mice, and they show a faster rate of decay of the calcium transient. With these observations we noted increased levels of mitochondrial calcium in the SERCA group, which was associated with an increase in the active form of the pyruvate dehydrogenase complex. Since an increase in mitochondrial calcium levels leads to activation of the pyruvate dehydrogenase complex (the rate-limiting step for carbohydrate oxidation), the increased glucose utilization observed in isolated perfused hearts in the SERCA group may reflect a higher level of mitochondrial calcium.

Pyruvate improves cardiac electromechanical and metabolic recovery from cardiopulmonary arrest and resuscitation

Severe depletion of myocardial energy and antioxidant resources during cardiac arrest culminates in electromechanical dysfunction following recovery of spontaneous circulation (ROSC). A metabolic fuel and natural antioxidant, pyruvate augments myocardial energy and antioxidant redox states in parallel with its enhancement of contractile performance of stunned and oxidant-challenged hearts. This study tested whether pyruvate improves post-arrest cardiac function and metabolism. Beagles were subjected to 5 min cardiac arrest and 5 min open-chest cardiac compression (OCCC: 80 compressions min(-1); aortic pressure 60-70 mmHg), then epicardial dc countershocks (5-10 J) were applied to restore sinus rhythm. Pyruvate was infused i.v. throughout OCCC and the first 25 min ROSC to a steady-state arterial concentration of 3.6+/-0.2 mM. Control experiments received NaCl infusions. Phosphocreatine phosphorylation potential (approximately PCr) and glutathione/glutathione disulfide ratio (GSH/GSSG), measured in snap-frozen left ventricle, indexed energy and antioxidant redox states, respectively. In control experiments, left ventricular pressure development, dP/dt and carotid flow initially recovered upon defibrillation, but then fell 40-50% by 3 h ROSC. ST segment displacement in lead II ECG persisted throughout ROSC. Approximately PCr collapsed and GSH/GSSG fell 61% during arrest. Both variables recovered partially during OCCC and completely during ROSC. Pyruvate temporarily increased approximately PCr and GSH/GSSG during OCCC and the first 25 min ROSC and enhanced pressure development, dP/dt and carotid flow at 15-25 min ROSC. Contractile function stabilized and ECG normalized at 2-3 h ROSC, despite post-infusion pyruvate clearance and waning of its metabolic benefits. In conclusion, intravenous pyruvate therapy increases energy reserves and antioxidant defenses of resuscitated myocardium. These temporary metabolic improvements support post-arrest recovery of cardiac electromechanical performance.

Direct action of T3 on phosphorylation potential in the sheep heart in vivo

Thyroid acting through ligand binding to nuclear receptors modifies myocardial respiratory kinetics and oxidative phosphorylation in the heart. Direct nongenomic action of thyroid hormone on high-energy phosphate concentrations and respiratory kinetics has never been proven in vivo but might be responsible for observed changes in oxygen utilization efficiency immediately after triiodothyronine (T3) administration. We tested the hypothesis that T3 directly and rapidly modifies myocardial high-energy phosphate concentrations and phosphorylation potential in vivo. Anesthetized sheep (age 28-40 days) thyroidectomized shortly after birth (Thy) and euthyroid age-matched controls (Con) underwent median sternotomy and received T3 infusion (0.8 microg/kg), followed by epinephrine infusion to increase myocardial oxygen consumption (MVo2). 31P magnetic resonance spectra were monitored via a surface coil over the left ventricle. T3 increased phosphocreatine (PCr)/ATP and decreased ADP in Thy animals without causing a change in MVo2. T3 produced no changes in high-energy phosphates in Con animals. T3 did not modify the PCr/ATP or ADP response to epinephrine and elevation in MVo2 in either group. Cardiac mitochondria isolated from Thy and Con animals showed no change in respiratory rate or ADP/ATP exchange efficiency after T3 incubation. T3 infusion in a hypothyroid state decreases ADP concentration, thereby altering the equilibrium between phosphorylation potential and myocardial respiratory rate. These T3-induced effects are not due to changes in ADP/ATP exchange efficiency through action at the adenine nucleotide translocator but may be due to T3 mediation of substrate utilization, confirmed in other models.

CK flux or direct ATP transfer: versatility of energy transfer pathways evidenced by NMR in the perfused heart

How the myocardium is able to permanently coordinate its intracellular fluxes of ATP synthesis, transfer and utilization is difficult to investigate in the whole organ due to the cellular complexity. The adult myocardium represents a paradigm of an energetically compartmented cell since 50% of total CK activity is bound in the vicinity of other enzymes (myofibrillar sarcolemmal and sarcoplasmic reticulum ATPases as well as mitochondrial adenine nucleotide translocator, ANT). Such vicinity of enzymes is well known in vitro as well as in preparations of skinned fibers to influence the kinetic properties of these enzymes and thus the functioning of the subcellular organelles. Intracellular compartmentation has often been neglected in the NMR analysis of CK kinetics in the whole organ. It is indeed a methodological challenge to reveal subcellular kinetics in a working organ by a global approach such as NMR. To get insight in the energy transfer pathway in the perfused rat heart, we developed a combined analysis of several protocols of magnetization transfer associated with biochemical data and quantitatively evaluated which scheme of energetic exchange best describes the NMR data. This allows to show the kinetic compartmentation of subcellular CKs and to quantify their fluxes. Interestingly, we could show that the energy transfer pathway shifts from the phosphocreatine shuttle in the oxygenated perfused heart to a direct ATP diffusion from mitochondria to cytosol under moderate inhibition of ATP synthesis. Furthermore using NMR measured fluxes and the known kinetic properties of the enzymes, it is possible to model the system, estimate local ADP concentrations and propose hypothesis for the versatility of energy transfer pathway. In the normoxic heart, a 3-fold ADP gradient was found between mitochondrial intermembrane space, cytosol and ADP in the vicinity of ATPases. The shift from PCr to ATP transport observed when ATP synthesis decreases might result from a balance in the activity of two populations of ANT, either coupled or uncoupled to CK. We believe this NMR approach could be a valuable tool to reinvestigate the control of respiration by ADP in the whole heart reconciling the biochemical knowledge of mitochondrial obtained in vitro or in skinned fibers with data on the whole heart as well as to identify the implication of bioenergetics in the pathological heart.

Dobutamine alters carnitine metabolism in the neonatal piglet heart

The use of inotropic agents to support the neonatal heart after sepsis or hypoxia increases cardiac energy demand. Carnitine plays a vital role in energy, fuel metabolism. To test the hypothesis that inotropic agents affect carnitine metabolism, hearts from sow-fed piglets were isolated and perfused with an oxygenated buffer containing glucose and palmitate. Increasing dosages of dobutamine (DOB 2.5–15 µg/Kg body wt per min, 0.007–0.044 µmol/kg per min) or saline vehicle (SAL) were administered. Heart rate (HR), left ventricular systolic (LVSP) and end diastolic pressures (LVEDP) were measured. Left ventricular developed pressure (LVDP = LVSP - LVEDP) and pressure-rate product (LVDP × HR) were calculated. Coronary effluent was collected to measure flow and metabolites. Heart tissue samples were collected for metabolite analysis. Results: DOB increased HR, LVEDP and the pressure-rate product [LVDP × HR]. Mean lactate production increased in DOB, but not in SAL control hearts, and was correlated with heart acylcarnitine, but not with coronary flow. Tissue acylcarnitine levels were higher in the DOB than in the SAL group. Plasma total carnitine was correlated with [LVDP × HR] and LVDP, but not with HR. The findings demonstrate that DOB alters myocardial carnitine metabolism and suggest that carnitine status may affect cardiac response to inotropic agents.Key words: carnitine, dobutamine, neonate, swine, isolated perfused heart.

Ethyl pyruvate preserves cardiac function and attenuates oxidative injury after prolonged myocardial ischemia

OBJECTIVE

Myocardial injury and dysfunction following ischemia are mediated in part by reactive oxygen species. Pyruvate, a key glycolytic intermediary, is an effective free radical scavenger but unfortunately is limited by aqueous instability. The ester derivative, ethyl pyruvate, is stable in solution and should function as an antioxidant and energy precursor. This study sought to evaluate ethyl pyruvate as a myocardial protective agent in a rat model of ischemia-reperfusion injury.

METHODS

Rats underwent 30-minute ischemia and 30-minute reperfusion of the left anterior descending coronary artery territory. Immediately prior to both ischemia and reperfusion, animals received an intravenous bolus of either ethyl pyruvate (n = 26) or vehicle control (n = 26). Myocardial high-energy phosphate levels were determined by adenosine triphosphate assay, oxidative injury was measured by lipid peroxidation assay, infarct size was quantified by triphenyltetrazolium chloride staining, and cardiac function was assessed in vivo.

RESULTS

Ethyl pyruvate administration significantly increased myocardial adenosine triphosphate levels compared with control (87.6 +/- 29.2 nmol/g vs 10.0 +/- 2.4 nmol/g, P =.03). In ischemic myocardium, ethyl pyruvate reduced oxidative injury compared with control (63.8 +/- 3.3 nmol/g vs 89.5 +/- 3.0 nmol/g, P <.001). Ethyl pyruvate diminished infarct size as a percentage of area at risk (25.3% +/- 1.5% vs 33.6% +/- 2.1%, P =.005). Ethyl pyruvate improved myocardial function compared with control (maximum pressure: 86.6 +/- 2.9 mm Hg vs 73.5 +/- 2.5 mm Hg, P <.001; maximum rate of pressure rise: 3518 +/- 243 mm Hg/s vs 2703 +/- 175 mm Hg/s, P =.005; maximal rate of ventricular systolic volume ejection: 3097 +/- 479 microL/s vs 2120 +/- 287 microL/s, P =.04; ejection fraction: 41.9% +/- 3.8% vs 31.4% +/- 4.1%, P =.03; cardiac output: 26.7 +/- 0.9 mL/min vs 22.7 +/- 1.3 mL/min, P =.01; and end-systolic pressure-volume relationship slope: 1.09 +/- 0.22 vs 0.59 +/- 0.2, P =.02).

CONCLUSIONS

In this study of myocardial ischemia-reperfusion injury, ethyl pyruvate enhanced myocardial adenosine triphosphate levels, attenuated myocardial oxidative injury, decreased infarct size, and preserved cardiac function.

The intermediary metabolite pyruvate attenuates stunning and reduces infarct size in in vivo porcine myocardium

The intermediary metabolite pyruvate has been shown to exert significant beneficial effects in in vitro models of myocardial oxidative stress and ischemia-reperfusion injury. However, there have been few reports of the ability of pyruvate to attenuate myocardial stunning or reduce infarct size in vivo. This study tested whether supraphysiological levels of pyruvate protect against reversible and irreversible in vivo myocardial ischemia-reperfusion injury. Anesthetized, open-chest pigs ( n = 7/group) underwent 15 min of left anterior descending coronary artery (LAD) occlusion and 3 h of reperfusion to induce stunning. Load-insensitive contractility measurements of regional preload recruitable stroke work (PRSW) and PRSW area (PRSWA) were generated. Vehicle or pyruvate (100 mg/kg iv bolus + 10 mg·kg–1·min–1 intra-atrial infusion) was administered during ischemia and for the first hour of reperfusion. In infarct studies, pigs ( n = 6/group) underwent 1 h of LAD ischemia and 3 h of reperfusion. Group I pigs received vehicle or pyruvate for 30 min before and throughout ischemia. In group II, the infusion was extended through 1 h of reperfusion. In the stunning protocol, pyruvate significantly improved the recovery of PRSWA at 1 h (50 ± 4% vs. 23 ± 3% in controls) and 3 h (69 ± 5% vs. 39 ± 3% in controls) reperfusion. Control pigs exhibited infarct sizes of 66 ± 1% of the area at risk. The pyruvate I protocol was associated with an infarct size of 49 ± 3% ( P < 0.05), whereas the pyruvate II protocol was associated with an infarct size of 30 ± 2% ( P < 0.05 vs. control and pyruvate I). These findings suggest that pyruvate attenuates stunning and decreases myocardial infarction in vivo in part by reduction of reperfusion injury. Metabolic interventions such as pyruvate should be considered when designing the optimal therapeutic strategies for limiting myocardial ischemia-reperfusion injury.

Limited transfer of cytosolic NADH into mitochondria at high cardiac workload

Glycolysis supplements energy synthesis at high cardiac workloads, producing not only ATP but also cytosolic NADH and pyruvate for oxidative ATP synthesis. Despite adequate Po(2), speculation exists that not all cytosolic NADH is oxidized by the mitochondria, leading to lactate production. In this study, we elucidate the mechanism for limited cytosolic NADH oxidation and increased lactate production at high workload despite adequate myocardial blood flow and oxygenation. Reducing equivalents from glycolysis enter mitochondria via exchange of mitochondrial alpha-ketoglutarate (alpha-KG) for cytosolic malate. This exchange was monitored at baseline and at high workloads by comparing (13)C enrichment between the products of alpha-KG oxidation (succinate) and alpha-KG efflux from mitochondria (glutamate). Under general anesthesia, a left thoracotomy was performed on 14 dogs and [2-(13)C]acetate was infused into the left anterior descending artery for 40 min. The rate-pressure product was 9,035 +/- 1,972 and 21,659 +/- 5,266 mmHg.beats.min(-1) (n = 7) at baseline (n = 7) and with dobutamine, respectively. (13)C enrichment of succinate was 57 +/- 10% at baseline and 45 +/- 13% at elevated workload (not significant), confirming oxidation of [2-(13)C]acetate. However, cytosolic glutamate enrichment, a marker of cytosolic NADH transfer to mitochondria, was dramatically reduced at high cardiac workload (11 +/- 1%) vs. baseline (50 +/- 14%, P < 0.05). This reduced exchange of (13)C from alpha-KG to cytosolic glutamate at high work indicates reduced shuttling of cytosolic reducing equivalents into the mitochondria. Myocardial tissue lactate increased 78%, countering this reduced oxidation of cytosolic NADH. The findings elucidate a contributing mechanism to glycolysis outpacing glucose oxidation in the absence of myocardial ischemia.

Energy dysfunction as a predictor of outcome after moderate or severe head injury: indices of oxygen, glucose, and lactate metabolism

The purpose of this study was to determine if the relationship between abnormalities in glucose, lactate, and oxygen metabolism were predictive of neurologic outcome after moderate or severe head injury, relative to other known prognostic factors. Serial assessments of the cerebral metabolic rates for glucose, lactate, and oxygen were performed using a modified Kety-Schmidt method. In total, 31 normal control subjects were studied once, and 49 TBI patients (mean age 36+/-16 years, median GCS 7) were studied five times median per patient from postinjury days 0 to 9. Univariate and multivariate analyses were performed. Univariate analysis showed that the 6-month postinjury Glasgow Outcome Scale (GOS) was most strongly associated with the mean cerebral metabolic rate of oxygen (CMRO2) (P = 0.0001), mean arterial lactate level (P = 0.0001), mean arterial glucose (P = 0.0008), mean cerebral blood flow (CBF), (P = 0.002), postresuscitation GCS (P = 0.003), and pupillary status (P = 0.004). Brain lactate uptake was observed in 44% of all metabolic studies, and 76% of patients had at least one episode of brain lactate uptake. By dichotomized GOS, patients achieving a favorable outcome (GOS 4-5) were distinguished from those with an unfavorable outcome (GOS1-3) by having a higher CMRO2 (P = 0.003), a higher rate of abnormal brain lactate uptake relative to arterial lactate levels (P = 0.04), and lesser degrees of blood-brain barrier damage based on CT findings (P = 0.03).

CONCLUSIONS

During the first 6 days after moderate or severe TBI, CMRO2 and arterial lactate levels are the strongest predictors of neurologic outcome. However, the frequent occurrence of abnormal brain lactate uptake despite only moderate elevations in arterial lactate levels in the favorable outcome patients suggests the brain's ability to use lactate as a fuel may be another key outcome predictor. Future studies are needed to determine to what degree nonglycolytic energy production from alternative fuels such as lactate occurs after TBI and whether alternative fuel administration is a viable therapy for TBI patients.