Cardiac responses to induced lactate oxidation: NMR analysis of metabolic equilibria

Intra-cardiac Side-Firing Light Catheter for Monitoring Cellular Metabolism using Transmural Absorbance Spectroscopy of Perfused Mammalian Hearts

Absorbance spectroscopy of cardiac muscle provides non-destructive assessment of cytosolic and mitochondrial oxygenation via myoglobin and cytochrome absorbance respectively. In addition, numerous aspects of the mitochondrial metabolic status such as membrane potential and substrate entry can also be estimated. To perform cardiac wall transmission optical spectroscopy, a commercially available side-firing optical fiber catheter is placed in the left ventricle of the isolated perfused heart as a light source. Light passing through the heart wall is collected with an external optical fiber to perform optical spectroscopy of the heart in near real- time. The transmission approach avoids numerous surface scattering interference occurring in widely used reflection approaches. Changes in transmural absorbance spectra were deconvolved using a library of chromophore reference spectra, providing quantitative measures of all the known cardiac chromophores simultaneously. This spectral deconvolution approach eliminated intrinsic errors that may result from using common dual wavelength methods applied to overlapping absorbance spectra, as well as provided a quantitative evaluation of the goodness of fit. A custom program was designed for data acquisition and analysis, which permitted the investigator to monitor the metabolic state of the preparation during the experiment. These relatively simple additions to the standard heart perfusion system provide a unique insight into the metabolic state of the heart wall in addition to conventional measures of contraction, perfusion, and substrate/oxygen extraction.

The cycling of acetyl-coenzyme A through acetylcarnitine buffers cardiac substrate supply: a hyperpolarized 13C magnetic resonance study

BACKGROUND

Carnitine acetyltransferase catalyzes the reversible conversion of acetyl-coenzyme A (CoA) into acetylcarnitine. The aim of this study was to use the metabolic tracer hyperpolarized [2-(13)C]pyruvate with magnetic resonance spectroscopy to determine whether carnitine acetyltransferase facilitates carbohydrate oxidation in the heart.

METHODS AND RESULTS

Ex vivo, following hyperpolarized [2-(13)C]pyruvate infusion, the [1-(13)C]acetylcarnitine resonance was saturated with a radiofrequency pulse, and the effect of this saturation on [1-(13)C]citrate and [5-(13)C]glutamate was observed. In vivo, [2-(13)C]pyruvate was infused into 3 groups of fed male Wistar rats: (1) controls, (2) rats in which dichloroacetate enhanced pyruvate dehydrogenase flux, and (3) rats in which dobutamine elevated cardiac workload. In the perfused heart, [1-(13)C]acetylcarnitine saturation reduced the [1-(13)C]citrate and [5-(13)C]glutamate resonances by 63% and 51%, respectively, indicating a rapid exchange between pyruvate-derived acetyl-CoA and the acetylcarnitine pool. In vivo, dichloroacetate increased the rate of [1-(13)C]acetylcarnitine production by 35% and increased the overall acetylcarnitine pool size by 33%. Dobutamine decreased the rate of [1-(13)C]acetylcarnitine production by 37% and decreased the acetylcarnitine pool size by 40%.

CONCLUSIONS

Hyperpolarized (13)C magnetic resonance spectroscopy has revealed that acetylcarnitine provides a route of disposal for excess acetyl-CoA and a means to replenish acetyl-CoA when cardiac workload is increased. Cycling of acetyl-CoA through acetylcarnitine appears key to matching instantaneous acetyl-CoA supply with metabolic demand, thereby helping to balance myocardial substrate supply and contractile function.

Amino acid transamination is crucial for ischaemic cardioprotection in normal and preconditioned isolated rat hearts--focus on L-glutamate

We have found that cardioprotection by l-glutamate mimics protection by classical ischaemic preconditioning (IPC). We investigated whether the effect of IPC involves amino acid transamination and whether IPC modulates myocardial glutamate metabolism. In a glucose-perfused, isolated rat heart model subjected to 40 min global no-flow ischaemia and 120 min reperfusion, the effects of IPC (2 cycles of 5 min ischaemia and 5 min reperfusion) and continuous glutamate (20 mm) administration during reperfusion on infarct size and haemodynamic recovery were studied. The effect of inhibiting amino acid transamination was evaluated by adding the amino acid transaminase inhibitor amino-oxyacetate (AOA; 0.025 mm) during reperfusion. Changes in coronary effluent, interstitial (microdialysis) and intracellular glutamate ([GLUT](i)) concentrations were measured. Ischaemic preconditioning and postischaemic glutamate administration reduced infarct size to the same extent (41 and 40%, respectively; P < 0.05 for both), without showing an additive effect. Amino-oxyacetate abolished infarct reduction by IPC and glutamate, and increased infarct size in both control and IPC hearts in a dose-dependent manner. Ischaemic preconditioning increased [GLUT](i) before ischaemia (P < 0.01) and decreased the release of glutamate during the first 10 min of reperfusion (P = 0.03). A twofold reduction in [GLUT](i) from the preischaemic state to 45 min of reperfusion (P = 0.0001) suggested increased postischaemic glutamate utilization in IPC hearts. While IPC and AOA changed haemodynamics in accordance with infarct size, glutamate decreased haemodynamic recovery despite reduced infarct size. In conclusion, ischaemic cardioprotection of the normal and IPC-protected heart depends on amino acid transamination and activity of the malate-aspartate shuttle during reperfusion. Underlying mechanisms of IPC include myocardial glutamate metabolism.

The use of magnetic resonance methods in translational cardiovascular research

Magnetic resonance methods are widely applicable to research questions posed in translational cardiovascular studies. The main intent of this review was to offer the cardiovascular translational research scientist a "menu" of magnetic resonance (MR) approaches that can be applied to answering research questions posed in a variety of experimental situations including those involving the use of human subjects. Obviously, this menu is not comprehensive and many other topics could have been selected for emphasis. However, we hope that the material presented encompasses a broad enough slice of the field to stimulate thinking about the possible applications of MR methods to specific research questions.

Heart slice NMR

Nuclear magnetic resonance (NMR) spectroscopy of the heart is normally carried out using whole heart preparations under coronary perfusion. In such preparations, either radical changes in ionic composition of the perfusate or applications of numerous drugs would affect coronary microcirculation. This report communicates the first (31)P NMR spectroscopy study using a heart slice preparation (left ventricular slices) superfused with extracellular medium. The ratio of phosphocreatine concentration to ATP concentration was approximately 2.1. Also, intracellular pH and Mg(2+) concentration ([Mg(2+)](i)), estimated from the chemical shifts of inorganic phosphate and ATP, were comparable with those under retrograde perfusion. [Mg(2+)](i) was significantly increased by the removal of extracellular Na(+), supporting the essential role of Na(+)-coupled Mg(2+) transport in Mg(2+) homeostasis of the heart. Heart slice preparation could also be used to evaluate the potency of cardiac drugs, regardless of their possible effects on coronary microcirculation.

A13C isotopomer kinetic analysis of cardiac metabolism: influence of altered cytosolic redox and [Ca2+]o

Rat hearts were perfused with mixtures of [3-(13)C]pyruvate and [3-(13)C]lactate (to alter cytosolic redox) at low (0.5 mM) or high (2.5 mM) Ca(2+) concentrations to alter contractility. Hearts were frozen at various times after exposure to these substrates, were extracted, and were then analyzed by (13)C NMR spectroscopy. The time-dependent multiplets observed in the (13)C NMR resonances of glutamate in all hearts and in malate and aspartate in hearts perfused with high-pyruvate/low-lactate concentrations were analyzed using a kinetic model of the tricarboxylic acid (TCA) cycle. The analysis showed that TCA cycle flux (V(TCA)) and exchange flux (V(X)) that involved cycle intermediates were both sensitive to cell redox and altered Ca(2+) concentration, and the ratio of these fluxes (V(X)/V(TCA)) varied >10-fold.

Myocardial redox state during coronary artery bypass grafting assessed with microdialysis

PURPOSE

Microdialysis allows the biochemical analysis of interstitial fluids of nearly every organ as a bedside procedure. This technique could be useful to reveal data about the myocardial metabolism during cardiopulmonary bypass in human coronary artery bypass graft (CABG) surgery.

METHODS

In 17 patients undergoing CABG a myocardial microdialysis catheter (CMA 70, CMA/Microdialysis AB, Sweden) was inserted in the apical region of the beating heart. Microdialysis measurements were performed at timed intervals before, during, and after cardiopulmonary bypass (CPB). The concentrations of lactate and pyruvate were analyzed semi-continuously.

RESULTS

During CPB the myocardial lactate-pyruvate-ratio (LPR) rose from an initial 11 (8-15) to 33 (29-41) ( P<0.01). After CPB the LPR decreased to 4 (3-7) at the end of observation ( P<0.05). The pyruvate concentration showed an immediate increase from 34 (30-42) microM at the end of CPB to 181 (147-234) microM after removal of the cross-clamp with subsequent increase during reperfusion ( P<0.01). Plasma lactate and pyruvate showed no essential changes during the study.

CONCLUSION

Using the microdialysis technique it was possible to analyze myocardial metabolic changes during CABG. The course of myocardial LPR as a sensitive indicator of the myocardial redox state showed profound changes during and after CPB. We propose the microdialysis technique as an additional monitoring tool in CABG.

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.

Improvement in survival and cardiac metabolism after gene transfer of sarcoplasmic reticulum Ca(2+)-ATPase in a rat model of heart failure

BACKGROUND

In heart failure, sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) activity is decreased, resulting in abnormal calcium handling and contractile dysfunction. We have previously shown that increasing SERCA2a expression by gene transfer improves ventricular function in a rat model of heart failure created by ascending aortic constriction.

METHODS AND RESULTS

In this study, we tested the effects of gene transfer of SERCA2a on survival, left ventricular (LV) volumes, and metabolism. By 26 to 27 weeks after aortic banding, all animals developed heart failure (as documented by >25% decrease in fractional shortening) and were randomized to receive either an adenovirus carrying the SERCA2a gene (Ad.SERCA2a) or control virus (Ad.betagal-GFP) by use of a catheter-based technique. Sham-operated rats, uninfected or infected with either Ad.betagal-GFP or Ad.SERCA2a, served as controls. Four weeks after gene transfer, survival in rats with heart failure treated with Ad.betagal-GFP was 9%, compared with 63% in rats receiving Ad.SERCA2a. LV volumes were significantly increased in heart failure (0.64+/-0.05 versus 0.35+/-0.03 mL, P<0.02). Overexpression of SERCA2a normalized LV volumes (0.46+/-0.07 mL) in the failing hearts. (31)P NMR analysis showed a reduced ratio of phosphocreatine to ATP content in failing+Ad.betagal-GFP compared with sham+Ad.betagal-GFP (0.82+/-0.13 versus 1.38+/-0.14, P<0.01). Overexpression of SERCA2a in failing hearts improved the phosphocreatine/ATP ratio (1.23+/-0.28).

CONCLUSIONS

In this study, we show that unlike inotropic agents that improve contractile function at the expense of increased mortality and worsening metabolism, gene transfer of SERCA2a improves survival and the energy potential in failing hearts.

Substrate-dependent proton load and recovery of stunned hearts during pyruvate dehydrogenase stimulation

Stimulation of pyruvate dehydrogenase (PDH) improves functional recovery of postischemic hearts. This study examined the potential for a mechanism mediated by substrate-dependent proton production and intracellular pH. After 20 min of ischemia, isolated rabbit hearts were reperfused with or without 5 mM dichloroacetate (DCA) in the presence of either 5 mM glucose, 5 mM glucose + 2.5 mM lactate, or 5 mM glucose + 2.5 mM pyruvate. DCA inhibits PDH kinase, increasing the proportion of dephosphorylated, active PDH. Unlike pyruvate or glucose alone, lactate + glucose did not support the effects of DCA on the recovery of rate-pressure product (RPP) (without DCA, RPP = 14,000 +/- 1,200, n = 6; with DCA, RPP = 13,700 +/- 1,800, n = 9). Intracellular pH, from (31)P nuclear magnetic resonance spectra, returned to normal within 2.1 min of reperfusion with all substrates except for lactate + glucose + DCA or lactate + DCA, which delayed pH recovery for up to 12 min (at 2.1 min pH = 6. 00 +/- 0.08, lactate + glucose + DCA; pH = 6.27 +/- 0.34, for lactate + DCA). Hearts were also reperfused after 10 min of ischemia with 0.5 mM palmitate + 5 mM DCA and either 2.5 mM pyruvate or 2.5 mM lactate. Again, intracellular pH recovery was delayed in the presence of lactate. PDH activation in the presence of lactate also decreased coupling of oxidative metabolism to mechanical work. These findings have implications for therapeutic use of stimulated carbohydrate oxidation in stunned hearts.