Second window of preconditioning normalizes palmitate use for oxidation and improves function during low-flow ischaemia

Metabolic flux in the driver's seat during cardiac health and disease

Cardiac function is a dynamic process that must adjust efficiently to the immediate demands of physical state and activity. So too, the metabolic support of cardiac function is a dynamic process that must respond, in time, to the demands of cardiac function and viability. Flux through metabolic pathways provides chemical energy and generates signaling molecules that regulate activity among intracellular compartments to meet these demands. Thus, flux through metabolic pathways provides a dynamic mode of support of cardiomyocytes during physiological and pathophysiological challenges. Any inability of metabolic flux to keep pace with the demands of the cardiomyocyte results in progressive dysfunction that contributes to cardiac disease. Thus, the priority in maintaining and regulating flux through metabolic pathways in the cardiomyocyte cannot be understated. Great potential exists in current efforts to elucidate metabolic mechanisms as therapeutic targets for the diseased heart. As a consequence, detecting metabolic flux in the functioning myocardium of the heart, under normal and diseased conditions, is essential in elucidating the metabolic basis of contractile dysfunction. As a companion to the 2022 ISHR Research Achievement Award lecture, this review examines the use and applications of stable isotope kinetics to quantify metabolic flux through intermediary pathways and the exchange and transport of intermediates across the mitochondrial membrane and sarcolemma of intact functioning hearts in determining how these intracellular events are coordinated to support cardiac function and health. Finally, this work reviews recently demonstrated metabolic defects in diseased hearts and the potential for metabolic alleviation of heart disease.

Secreted frizzled protein 3 is a novel cardioprotective mechanism unique to the clinically relevant fourth window of ischemic preconditioning

Most studies on ischemic preconditioning (IPC) use one or two ischemic stimuli before examining cardioprotection. To better simulate the clinical situation, we examined, in pigs, the effects of six episodes of 10 min coronary artery occlusion (CAO) 12 h apart, followed by 60 min CAO. We named this model the fourth window of IPC. To determine the novel mechanisms mediating cardioprotection in the fourth window, gene analysis was examined in fourth window IPC cardiac tissue 60 min after the last episode of 10 min CAO. Secreted frizzled-related protein 3 (sFRP3) was the most significantly upregulated gene that was unique to the fourth window, that is, not found in the first, second, or third window IPC. To study the effects of sFRP3 on cardioprotection, sFRP3 was injected in the hearts of wild-type (WT) mice. In the [CAO/coronary artery reperfusion (CAR)] model (30 min CAO followed by 24 h CAR), infarct size was less, P < 0.01, after sFRP3 injection (14% ± 1.7%) compared with vehicle injection (48% ± 1.6%). sFRP3 injection also protected the development of heart failure following permanent CAO for 2 wk. Left ventricular ejection fraction was significantly improved, P < 0.05, at 2 wk after CAO with sFRP3 (53% ± 5%) compared with vehicle (36% ± 2%) and was accompanied by significant, P < 0.01, reductions in myocardial fibrosis (53% ± 4%), myocyte size (17% ± 3%), apoptosis (100%), and mortality (56%). Thus, sFRP3, unique to the clinically relevant fourth window IPC model, is a novel mechanism mediating ischemic cardioprotection.NEW & NOTEWORTHY1) This investigation identifies the novel fourth window of ischemic preconditioning. 2) sFRP3 was identified as the most significantly upregulated gene in the fourth window and was shown to induce cardioprotection when administered to the hearts of wild-type mice.

Ketone Body Infusion With 3-Hydroxybutyrate Reduces Myocardial Glucose Uptake and Increases Blood Flow in Humans: A Positron Emission Tomography Study

BACKGROUND

High levels of ketone bodies are associated with improved survival as observed with regular exercise, caloric restriction, and-most recently-treatment with sodium-glucose linked transporter 2 inhibitor antidiabetic drugs. In heart failure, indices of ketone body metabolism are upregulated, which may improve energy efficiency and increase blood flow in skeletal muscle and the kidneys. Nevertheless, it is uncertain how ketone bodies affect myocardial glucose uptake and blood flow in humans. Our study was therefore designed to test whether ketone body administration in humans reduces myocardial glucose uptake (MGU) and increases myocardial blood flow.

METHODS AND RESULTS

Eight healthy subjects, median aged 60 were randomly studied twice: (1) During 390 minutes infusion of Na-3-hydroxybutyrate (KETONE) or (2) during 390 minutes infusion of saline (SALINE), together with a concomitant low-dose hyperinsulinemic-euglycemic clamp to inhibit endogenous ketogenesis. Myocardial blood flow was measured by ¹⁵O-H₂O positron emission tomography/computed tomography, myocardial fatty acid metabolism by ¹¹C-palmitate positron emission tomography/computed tomography and MGU by ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography. Similar euglycemia, hyperinsulinemia, and suppressed free fatty acids levels were recorded on both study days; Na-3-hydroxybutyrate infusion increased circulating Na-3-hydroxybutyrate levels from zero to 3.8±0.5 mmol/L. MGU was halved by hyperketonemia (MGU [nmol/g per minute]: 304±97 [SALINE] versus 156±62 [KETONE], P<0.01), whereas no effects were observed on palmitate uptake oxidation or esterification. Hyperketonemia increased heart rate by ≈25% and myocardial blood flow by 75%.

CONCLUSIONS

Ketone bodies displace MGU and increase myocardial blood flow in healthy humans; these novel observations suggest that ketone bodies are important cardiac fuels and vasodilators, which may have therapeutic potentials.

Assessing Cardiac Metabolism: A Scientific Statement From the American Heart Association

In a complex system of interrelated reactions, the heart converts chemical energy to mechanical energy. Energy transfer is achieved through coordinated activation of enzymes, ion channels, and contractile elements, as well as structural and membrane proteins. The heart's needs for energy are difficult to overestimate. At a time when the cardiovascular research community is discovering a plethora of new molecular methods to assess cardiac metabolism, the methods remain scattered in the literature. The present statement on "Assessing Cardiac Metabolism" seeks to provide a collective and curated resource on methods and models used to investigate established and emerging aspects of cardiac metabolism. Some of those methods are refinements of classic biochemical tools, whereas most others are recent additions from the powerful tools of molecular biology. The aim of this statement is to be useful to many and to do justice to a dynamic field of great complexity.

A Food and Drug Administration-Approved Antiviral Agent that Inhibits Adenylyl Cyclase Type 5 Protects the Ischemic Heart Even When Administered after Reperfusion

A Food and Drug Administration-approved antiviral agent, known as vidarabine or adenine 9-β-D-arabinofuranoside (AraA), has features of inhibiting adenylyl cyclase type 5 (AC5) and protects against chronic coronary artery occlusion (CAO). The goal of this investigation was to determine whether AraA protects against myocardial ischemia, even when delivered after coronary artery reperfusion (CAR). AraA, delivered after CAR in wild-type mice, reduced infarct size by 55% compared with vehicle-treated controls, whereas an equal dose of adenosine reduced infarct size only when administered before CAR. A 5-fold greater dose of adenosine was required to reduce infarct size when delivered after CAR, which also reduced arterial pressure by 15%, whereas AraA did not affect pressure. The reduction in infarct size with AraA was prevented by a MEK/extracellular signal-regulated kinase blocker, a pathway also involved in the mechanism of protection of the AC5 knockout (KO) model. Infarct size was also reduced in cardiac-specific AC5 KO mice similarly in the presence and absence of AraA, further suggesting that AraA protection involves the AC5 pathway. AraA reduced infarct size in chronically instrumented conscious pigs when delivered after CAR, and in this model, it also reduced post-CAR coronary hyperemia, which could be another mechanism for cardioprotection (i.e., by reducing oxidative stress during CAR). Thus, AraA inhibits AC5 and exhibits unique cardioprotection when delivered after CAR, which is critical for clinical translation.

The mechanism of Intralipid®-mediated cardioprotection complex IV inhibition by the active metabolite, palmitoylcarnitine, generates reactive oxygen species and activates reperfusion injury salvage kinases

BACKGROUND

Intralipid® administration at reperfusion elicits protection against myocardial ischemia-reperfusion injury. However, the underlying mechanisms are not fully understood.

METHODS

Sprague-Dawley rat hearts were exposed to 15 min of ischemia and 30 min of reperfusion in the absence or presence of Intralipid® 1% administered at the onset of reperfusion. In separate experiments, the reactive oxygen species (ROS) scavenger N-(2-mercaptopropionyl)-glycine was added either alone or with Intralipid®. Left ventricular work and activation of Akt, STAT3, and ERK1/2 were used to evaluate cardioprotection. ROS production was assessed by measuring the loss of aconitase activity and the release of hydrogen peroxide using Amplex Red. Electron transport chain complex activities and proton leak were measured by high-resolution respirometry in permeabilized cardiac fibers. Titration experiments using the fatty acid intermediates of Intralipid® palmitoyl-, oleoyl- and linoleoylcarnitine served to determine concentration-dependent inhibition of complex IV activity and mitochondrial ROS release.

RESULTS

Intralipid® enhanced postischemic recovery and activated Akt and Erk1/2, effects that were abolished by the ROS scavenger N-(2-mercaptopropionyl)glycine. Palmitoylcarnitine and linoleoylcarnitine, but not oleoylcarnitine concentration-dependently inhibited complex IV. Only palmitoylcarnitine reached high tissue concentrations during early reperfusion and generated significant ROS by complex IV inhibition. Palmitoylcarnitine (1 µM), administered at reperfusion, also fully mimicked Intralipid®-mediated protection in an N-(2-mercaptopropionyl)-glycine -dependent manner.

CONCLUSIONS

Our data describe a new mechanism of postconditioning cardioprotection by the clinically available fat emulsion, Intralipid®. Protection is elicited by the fatty acid intermediate palmitoylcarnitine, and involves inhibition of complex IV, an increase in ROS production and activation of the RISK pathway.

Metabolomic analysis of two different models of delayed preconditioning

Recently we described an ischemic preconditioning induced by repetitive coronary stenosis, which is induced by 6 episodes of non-lethal ischemia over 3 days, and which also resembles the hibernating myocardium phenotype. When compared with traditional second window of ischemic preconditioning using cDNA microarrays, many genes which differed in the repetitive coronary stenosis appeared targeted to metabolism. Accordingly, the goal of this study was to provide a more in depth analysis of changes in metabolism in the different models of delayed preconditioning, i.e., second window and repetitive coronary stenosis. This was accomplished using a metabolomic approach based on liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) techniques. Myocardial samples from the ischemic section of porcine hearts subjected to both models of late preconditioning were compared against sham controls. Interestingly, although both models involve delayed preconditioning, their metabolic signatures were radically different; of the total number of metabolites that changed in both models (135 metabolites) only 7 changed in both models, and significantly more, p<0.01, were altered in the repetitive coronary stenosis (40%) than in the second window (8.1%). The most significant changes observed were in energy metabolism, e.g., phosphocreatine was increased 4 fold and creatine kinase activity increased by 27.2%, a pattern opposite from heart failure, suggesting that the repetitive coronary stenosis and potentially hibernating myocardium have enhanced stress resistance capabilities. The improved energy metabolism could also be a key mechanism contributing to the cardioprotection observed in the repetitive coronary stenosis and in hibernating myocardium. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Infarct-remodelled hearts with limited oxidative capacity boost fatty acid oxidation after conditioning against ischaemia/reperfusion injury

AIMS

Infarct-remodelled hearts are less amenable to protection against ischaemia/reperfusion. Understanding preservation of energy metabolism in diseased vs. healthy hearts may help to develop anti-ischaemic strategies effective also in jeopardized myocardium.

METHODS AND RESULTS

Isolated infarct-remodelled/sham Sprague-Dawley rat hearts were perfused in the working mode and subjected to 15 min of ischaemia and 30 min of reperfusion. Protection of post-ischaemic ventricular work was achieved by pharmacological conditioning with sevoflurane. Oxidative metabolism was measured by substrate flux in fatty acid and glucose oxidation using [(3)H]palmitate and [(14)C]glucose. Mitochondrial oxygen consumption was measured in saponin-permeabilized left ventricular muscle fibres. Activity assays of citric acid synthase, hydroxyacyl-CoA dehydrogenase, and pyruvate dehydrogenase and mass spectrometry for acylcarnitine profiling were also performed. Six weeks after coronary artery ligation, the hearts exhibited macroscopic and molecular signs of hypertrophy consistent with remodelling and limited respiratory chain and citric acid cycle capacity. Unprotected remodelled hearts showed a marked decline in palmitate oxidation and acetyl-CoA energy production after ischaemia/reperfusion, which normalized in sevoflurane-protected remodelled hearts. Protected remodelled hearts also showed higher β-oxidation flux as determined by increased oxygen consumption with palmitoylcarnitine/malate in isolated fibres and a lower ratio of C16:1+C16OH/C14 carnitine species, indicative of a higher long-chain hydroxyacyl-CoA dehydrogenase activity. Remodelled hearts exhibited higher PPARα-PGC-1α but defective HIF-1α signalling, and conditioning enabled them to mobilize fatty acids from endogenous triglyceride stores, which closely correlated with improved recovery.

CONCLUSIONS

Protected infarct-remodelled hearts secure post-ischaemic energy production by activation of β-oxidation and mobilization of fatty acids from endogenous triglyceride stores.

Acute liver carnitine palmitoyltransferase I overexpression recapitulates reduced palmitate oxidation of cardiac hypertrophy

RATIONALE

Muscle carnitine palmitoyltransferase I is predominant in the heart, but the liver isoform (liver carnitine palmitoyltransferase I [L-CPT1]) is elevated in hearts with low long chain fatty acid oxidation, such as fetal and hypertrophied hearts.

OBJECTIVE

This work examined the effect of acute L-CPT1 expression on the regulation of palmitate oxidation and energy metabolism in intact functioning rat hearts for comparison with findings in hypertrophied hearts.

METHODS AND RESULTS

L-CPT1 was expressed in vivo in rat hearts by coronary perfusion of Adv.cmv.L-CPT1 (L-CPT1, n=15) vs. phosphate-buffered saline (PBS) infusion (PBS, n=7) or empty virus (empty, n=5). L-CPT1 was elevated 5-fold at 72 hours after Adv.cmv.L-CPT1 infusion (P<0.05), but muscle carnitine palmitoyltransferase I was unaffected. Despite similar tricarboxylic acid cycle rates, palmitate oxidation rates were reduced with L-CPT1 (1.12 ± 0.29 μmol/min per gram of dry weight, mean±SE) vs. PBS (1.6 ± 0.34). Acetyl CoA production from palmitate was reduced with L-CPT1 (69 ± 0.02%; P<0.05; PBS=79 ± 0.01%; empty=81 ± 0.02%), similar to what occurs in hypertrophied hearts, and with no difference in malonyl CoA content. Glucose oxidation was elevated with L-CPT1 (by 60%). Surprisingly, L-CPT1 hearts contained elevated atrial natriuretic peptide, indicating induction of hypertrophic signaling.

CONCLUSIONS

The results link L-CPT1 expression to reduced palmitate oxidation in a nondiseased adult heart, recapitulating the phenotype of reduced long chain fatty acid oxidation in cardiac hypertrophy. The implications are that L-CPT1 expression induces metabolic remodeling hypertrophic signaling and that regulatory factors beyond malonyl CoA in the heart regulate long chain fatty acid oxidation via L-CPT1.