Metabolic effects of aldose reductase inhibition during low-flow ischemia and reperfusion

Co-Polarized [1-13C]Pyruvate and [1,3-13C2]Acetoacetate Provide a Simultaneous View of Cytosolic and Mitochondrial Redox in a Single Experiment

Cellular redox is intricately linked to energy production and normal cell function. Although the redox states of mitochondria and cytosol are connected by shuttle mechanisms, the redox state of mitochondria may differ from redox in the cytosol in response to stress. However, detecting these differences in functioning tissues is difficult. Here, we employed ¹³C magnetic resonance spectroscopy (MRS) and co-polarized [1-¹³C]pyruvate and [1,3-¹³C₂]acetoacetate ([1,3-¹³C₂]AcAc) to monitor production of hyperpolarized (HP) lactate and β-hydroxybutyrate as indicators of cytosolic and mitochondrial redox, respectively. Isolated rat hearts were examined under normoxic conditions, during low-flow ischemia, and after pretreatment with either aminooxyacetate (AOA) or rotenone. All interventions were associated with an increase in [P_i]/[ATP] measured by ³¹P NMR. In well-oxygenated untreated hearts, rapid conversion of HP [1-¹³C]pyruvate to [1-¹³C]lactate and [1,3-¹³C₂]AcAc to [1,3-¹³C₂]β-hydroxybutyrate ([1,3-¹³C₂]β-HB) was readily detected. A significant increase in HP [1,3-¹³C₂]β-HB but not [1-¹³C]lactate was observed in rotenone-treated and ischemic hearts, consistent with an increase in mitochondrial NADH but not cytosolic NADH. AOA treatments did not alter the productions of HP [1-¹³C]lactate or [1,3-¹³C₂]β-HB. This study demonstrates that biomarkers of mitochondrial and cytosolic redox may be detected simultaneously in functioning tissues using co-polarized [1-¹³C]pyruvate and [1,3-¹³C₂]AcAc and ¹³C MRS and that changes in mitochondrial redox may precede changes in cytosolic redox.

Aldose Reductase: An Emerging Target for Development of Interventions for Diabetic Cardiovascular Complications

Diabetes is a leading cause of cardiovascular morbidity and mortality. Despite numerous treatments for cardiovascular disease (CVD), for patients with diabetes, these therapies provide less benefit for protection from CVD. These considerations spur the concept that diabetes-specific, disease-modifying therapies are essential to identify especially as the diabetes epidemic continues to expand. In this context, high levels of blood glucose stimulate the flux via aldose reductase (AR) pathway leading to metabolic and signaling changes in cells of the cardiovascular system. In animal models flux via AR in hearts is increased by diabetes and ischemia and its inhibition protects diabetic and non-diabetic hearts from ischemia-reperfusion injury. In mouse models of diabetic atherosclerosis, human AR expression accelerates progression and impairs regression of atherosclerotic plaques. Genetic studies have revealed that single nucleotide polymorphisms (SNPs) of the ALD2 (human AR gene) is associated with diabetic complications, including cardiorenal complications. This Review presents current knowledge regarding the roles for AR in the causes and consequences of diabetic cardiovascular disease and the status of AR inhibitors in clinical trials. Studies from both human subjects and animal models are presented to highlight the breadth of evidence linking AR to the cardiovascular consequences of diabetes.

COVID-19 and the Heart and Vasculature: Novel Approaches to Reduce Virus-Induced Inflammation in Patients With Cardiovascular Disease

The coronavirus disease 2019 (COVID-19) pandemic presents an unprecedented challenge and opportunity for translational investigators to rapidly develop safe and effective therapeutic interventions. Greater risk of severe disease in COVID-19 patients with comorbid diabetes mellitus, obesity, and heart disease may be attributable to synergistic activation of vascular inflammation pathways associated with both COVID-19 and cardiometabolic disease. This mechanistic link provides a scientific framework for translational studies of drugs developed for treatment of cardiometabolic disease as novel therapeutic interventions to mitigate inflammation and improve outcomes in patients with COVID-19.

Cardiospecific Overexpression of ATPGD1 (Carnosine Synthase) Increases Histidine Dipeptide Levels and Prevents Myocardial Ischemia Reperfusion Injury

BACKGROUND Myocardial ischemia reperfusion (I/R) injury is associated with complex pathophysiological changes characterized by pH imbalance, the accumulation of lipid peroxidation products acrolein and 4-hydroxy trans-2-nonenal, and the depletion of ATP levels. Cardioprotective interventions, designed to address individual mediators of I/R injury, have shown limited efficacy. The recently identified enzyme ATPGD1 (Carnosine Synthase), which synthesizes histidyl dipeptides such as carnosine, has the potential to counteract multiple effectors of I/R injury by buffering intracellular pH and quenching lipid peroxidation products and may protect against I/R injury. METHODS AND RESULTS We report here that β-alanine and carnosine feeding enhanced myocardial carnosine levels and protected the heart against I/R injury. Cardiospecific overexpression of ATPGD1 increased myocardial histidyl dipeptides levels and protected the heart from I/R injury. Isolated cardiac myocytes from ATPGD1-transgenic hearts were protected against hypoxia reoxygenation injury. The overexpression of ATPGD1 prevented the accumulation of acrolein and 4-hydroxy trans-2-nonenal-protein adducts in ischemic hearts and delayed acrolein or 4-hydroxy trans-2-nonenal-induced hypercontracture in isolated cardiac myocytes. Changes in the levels of ATP, high-energy phosphates, intracellular pH, and glycolysis during low-flow ischemia in the wild-type mice hearts were attenuated in the ATPGD1-transgenic hearts. Two natural dipeptide analogs (anserine and balenine) that can either quench aldehydes or buffer intracellular pH, but not both, failed to protect against I/R injury. CONCLUSIONS Either exogenous administration or enhanced endogenous formation of histidyl dipeptides prevents I/R injury by attenuating changes in intracellular pH and preventing the accumulation of lipid peroxidation derived aldehydes.

Pyridine Dinucleotides from Molecules to Man

SIGNIFICANCE

Pyridine dinucleotides, nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), were discovered more than 100 years ago as necessary cofactors for fermentation in yeast extracts. Since that time, these molecules have been recognized as fundamental players in a variety of cellular processes, including energy metabolism, redox homeostasis, cellular signaling, and gene transcription, among many others. Given their critical role as mediators of cellular responses to metabolic perturbations, it is unsurprising that dysregulation of NAD and NADP metabolism has been associated with the pathobiology of many chronic human diseases. Recent Advances: A biochemistry renaissance in biomedical research, with its increasing focus on the metabolic pathobiology of human disease, has reignited interest in pyridine dinucleotides, which has led to new insights into the cell biology of NAD(P) metabolism, including its cellular pharmacokinetics, biosynthesis, subcellular localization, and regulation. This review highlights these advances to illustrate the importance of NAD(P) metabolism in the molecular pathogenesis of disease.

CRITICAL ISSUES

Perturbations of NAD(H) and NADP(H) are a prominent feature of human disease; however, fundamental questions regarding the regulation of the absolute levels of these cofactors and the key determinants of their redox ratios remain. Moreover, an integrated topological model of NAD(P) biology that combines the metabolic and other roles remains elusive.

FUTURE DIRECTIONS

As the complex regulatory network of NAD(P) metabolism becomes illuminated, sophisticated new approaches to manipulating these pathways in specific organs, cells, or organelles will be developed to target the underlying pathogenic mechanisms of disease, opening doors for the next generation of redox-based, metabolism-targeted therapies. Antioxid. Redox Signal. 28, 180-212.

Protective role of fructokinase blockade in the pathogenesis of acute kidney injury in mice

Acute kidney injury is associated with high mortality, especially in intensive care unit patients. The polyol pathway is a metabolic route able to convert glucose into fructose. Here we show the detrimental role of endogenous fructose production by the polyol pathway and its metabolism through fructokinase in the pathogenesis of ischaemic acute kidney injury (iAKI). Consistent with elevated urinary fructose in AKI patients, mice undergoing iAKI show significant polyol pathway activation in the kidney cortex characterized by high levels of aldose reductase, sorbitol and endogenous fructose. Wild type but not fructokinase knockout animals demonstrate severe kidney injury associated with ATP depletion, elevated uric acid, oxidative stress and inflammation. Interestingly, both the renal injury and dysfunction in wild-type mice undergoing iAKI is significantly ameliorated when exposed to luteolin, a recently discovered fructokinase inhibitor. This study demonstrates a role for fructokinase and endogenous fructose as mediators of acute renal disease.

The polyol pathway, which converts glucose into sorbitol and fructose, is active in chronic conditions like hepatic steatosis and chronic kidney disease. Here, Andres-Hernando et al. show that fructose production promotes renal injury and fructokinase inhibition protects against kidney damage during ischaemic acute kidney disease.

Aldose reductase (AKR1B) deficiency promotes phagocytosis in bone marrow derived mouse macrophages

Macrophages are critical drivers of the immune response during infection and inflammation. The pathogenesis of several inflammatory conditions, such as diabetes, cancer and sepsis has been linked with aldose reductase (AR), a member of the aldo-keto reductase (AKR) superfamily. However, the role of AR in the early stages of innate immunity such as phagocytosis remains unclear. In this study, we examined the role of AR in regulating the growth and the phagocytic activity of bone marrow-derived mouse macrophages (BMMs) from AR-null and wild-type (WT) mice. We found that macrophages derived from AR-null mice were larger in size and had a slower growth rate than those derived from WT mice. The AR-null macrophages also displayed higher basal, and lipopolysaccharide (LPS) stimulated phagocytic activity than WT macrophages. Moreover, absence of AR led to a marked increase in cellular levels of both ATP and NADPH. These data suggest that metabolic pathways involving AR suppress macrophage energy production, and that inhibition of AR could induce a favorable metabolic state that promotes macrophage phagocytosis. Hence, modulation of macrophage metabolism by inhibition of AR might represent a novel strategy to modulate host defense responses and to modify metabolism to promote macrophage hypertrophy and phagocytosis under inflammatory conditions.

Damaging effects of hyperglycemia on cardiovascular function: spotlight on glucose metabolic pathways

The incidence of cardiovascular complications associated with hyperglycemia is a growing global health problem. This review discusses the link between hyperglycemia and cardiovascular diseases onset, focusing on the role of recently emerging downstream mediators, namely, oxidative stress and glucose metabolic pathway perturbations. The role of hyperglycemia-mediated activation of nonoxidative glucose pathways (NOGPs) [i.e., the polyol pathway, hexosamine biosynthetic pathway, advanced glycation end products (AGEs), and protein kinase C] in this process is extensively reviewed. The proposal is made that there is a unique interplay between NOGPs and a downstream convergence of detrimental effects that especially affect cardiac endothelial cells, thereby contributing to contractile dysfunction. In this process the AGE pathway emerges as a crucial mediator of hyperglycemia-mediated detrimental effects. In addition, a vicious metabolic cycle is established whereby hyperglycemia-induced NOGPs further fuel their own activation by generating even more oxidative stress, thereby exacerbating damaging effects on cardiac function. Thus NOGP inhibition, and particularly that of the AGE pathway, emerges as a novel therapeutic intervention for the treatment of cardiovascular complications such as acute myocardial infarction in the presence hyperglycemia.

Detrimental effects of acute hyperglycaemia on the rat heart

AIM

Hyperglycaemia is an important risk factor for acute myocardial infarction. It can lead to increased induction of non-oxidative glucose pathways (NOGPs) - polyol and hexosamine biosynthetic pathways, advanced glycation end products and protein kinase C - that may contribute to cardiovascular diseases onset. However, the precise underlying mechanisms remain poorly understood. Here we hypothesized that acute hyperglycaemia increases myocardial oxidative stress and NOGP activation resulting in cardiac dysfunction during ischaemia-reperfusion and that inhibition of, and/or shunting flux away from NOGPs [by benfotiamine (BFT) treatment], leads to cardioprotection.

METHODS

We employed several experimental systems: (i) Isolated rat hearts were perfused ex vivo with Krebs-Henseleit buffer containing 33 mm glucose vs. controls (11 mm glucose) ± global ischaemia and reperfusion ± BFT (first 20 min of reperfusion); (ii) Infarct size determination as per the ischaemic protocol, but with regional ischaemia and reperfusion ± BFT treatment; in separate experiments, NOGP inhibitors were also employed for (i) and (ii); and (iii) In vivo coronary ligations performed on streptozotocin-treated rats ± BFT treatment (early reperfusion).

RESULTS

Acute hyperglycaemia generated myocardial oxidative stress, NOGP activation and apoptosis, but caused no impairment of cardiac function during pre-ischaemia, thereby priming hearts for later damage. Following ischaemia-reperfusion (under hyperglycaemic conditions), such effects were exacerbated together with cardiac contractile dysfunction. Moreover, inhibition of respective NOGPs and shunting away by BFT treatment (in part) improved cardiac function during ischaemia-reperfusion.

CONCLUSION

Coordinate NOGP activation in response to acute hyperglycaemia results in contractile dysfunction during ischaemia-reperfusion, allowing for the development of novel cardioprotective agents.

Role of aldose reductase in the metabolism and detoxification of carnosine-acrolein conjugates

Oxidation of unsaturated lipids generates reactive aldehydes that accumulate in tissues during inflammation, ischemia, or aging. These aldehydes form covalent adducts with histidine-containing dipeptides such as carnosine and anserine, which are present in high concentration in skeletal muscle, heart, and brain. The metabolic pathways involved in the detoxification and elimination of these conjugates are, however, poorly defined, and their significance in regulating oxidative stress is unclear. Here we report that conjugates of carnosine with aldehydes such as acrolein are produced during normal metabolism and excreted in the urine of mice and adult human non-smokers as carnosine-propanols. Our studies show that the reduction of carnosine-propanals is catalyzed by the enzyme aldose reductase (AR). Carnosine-propanals were converted to carnosine-propanols in the lysates of heart, skeletal muscle, and brain tissue from wild-type (WT) but not AR-null mice. In comparison with WT mice, the urinary excretion of carnosine-propanols was decreased in AR-null mice. Carnosine-propanals formed covalent adducts with nucleophilic amino acids leading to the generation of carnosinylated proteins. Deletion of AR increased the abundance of proteins bound to carnosine in skeletal muscle, brain, and heart of aged mice and promoted the accumulation of carnosinylated proteins in hearts subjected to global ischemia ex vivo. Perfusion with carnosine promoted post-ischemic functional recovery in WT but not in AR-null mouse hearts. Collectively, these findings reveal a previously unknown metabolic pathway for the removal of carnosine-propanal conjugates and suggest a new role of AR as a critical regulator of protein carnosinylation and carnosine-mediated tissue protection.

NAD+ metabolism and oxidative stress: the golden nucleotide on a crown of thorns

In the twentieth century, NAD+ research generated multiple discoveries. Identification of the important role of NAD+ as a cofactor in cellular respiration and energy production was followed by discoveries of numerous NAD+ biosynthesis pathways. In recent years, NAD+ has been shown to play a unique role in DNA repair and protein deacetylation. As discussed in this review, there are close interactions between oxidative stress and immune activation, energy metabolism, and cell viability in neurodegenerative disorders and ageing. Profound interactions with regard to oxidative stress and NAD+ have been highlighted in the present work. This review emphasizes the pivotal role of NAD+ in the regulation of DNA repair, stress resistance, and cell death, suggesting that NAD+ synthesis through the kynurenine pathway and/or salvage pathway is an attractive target for therapeutic intervention in age-associated degenerative disorders. NAD+ precursors have been shown to slow down ageing and extend lifespan in yeasts, and protect severed axons from degeneration in animal models neurodegenerative diseases.

Aldose reductase modulates cardiac glycogen synthase kinase-3β phosphorylation during ischemia-reperfusion

Earlier studies have demonstrated that aldose reductase (AR) plays a key role in mediating ischemia-reperfusion (I/R) injury. Our objective was to investigate if AR mediates I/R injury by influencing phosphorylation of glycogen synthase kinase-3β (p-GSK3β). To investigate this issue, we used three separate models to study the effects of stress injury on the heart. Hearts isolated from wild-type (WT), human expressing AR transgenic (ARTg), and AR knockout (ARKO) mice were perfused with/without GSK3β inhibitors (SB-216763 and LiCl) and subjected to I/R. Ad-human AR (Ad-hAR)-expressing HL-1 cardiac cells were exposed to hypoxia (0.5% O(2)) and reoxygenation (20.9% O(2)) conditions. I/R in a murine model of transient occlusion and reperfusion of the left anterior descending coronary artery (LAD) was used to study if p-GSK3β was affected through increased AR flux. Lactate dehydrogenase (LDH) release and left ventricular developed pressure (LVDP) were measured. LVDP was decreased in hearts from ARTg mice compared with WT and ARKO after I/R, whereas LDH release and apoptotic markers were increased (P < 0.05). p-GSK3β was decreased in ARTg hearts compared with WT and ARKO (P < 0.05). In ARKO, p-GSK3β and apoptotic markers were decreased compared with WT (P < 0.05). WT and ARTg hearts perfused with GSK3β inhibitors improved p-GSK3β expression and LVDP and exhibited decreased LDH release, apoptosis, and mitochondrial pore opening (P < 0.05). Ad-hAR-expressing HL-1 cardiac cells, exposed to hypoxia (0.5% O(2)) and reoxygenation (20.9% O(2)), had greater LDH release compared with control HL-1 cells (P < 0.05). p-GSK3β was decreased and correlated with increased apoptotic markers in Ad-hAR HL-1 cells (P < 0.05). Treatment with phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) inhibitor increased injury demonstrated by increased LDH release in ARTg, WT, and ARKO hearts and in Ad-hAR-expressing HL-1 cells. Cells treated with protein kinase C (PKC) α/β inhibitor displayed significant increases in p-Akt and p-GSK3β expression, and resulted in decreased LDH release. In summary, AR mediates changes in p-GSK3β, in part, via PKCα/β and Akt during I/R.

Amelioration of acute kidney injury in lipopolysaccharide-induced systemic inflammatory response syndrome by an aldose reductase inhibitor, fidarestat

Background

Systemic inflammatory response syndrome is a fatal disease because of multiple organ failure. Acute kidney injury is a serious complication of systemic inflammatory response syndrome and its genesis is still unclear posing a difficulty for an effective treatment. Aldose reductase (AR) inhibitor is recently found to suppress lipopolysaccharide (LPS)-induced cardiac failure and its lethality. We studied the effects of AR inhibitor on LPS-induced acute kidney injury and its mechanism.

Methods

Mice were injected with LPS and the effects of AR inhibitor (Fidarestat 32 mg/kg) before or after LPS injection were examined for the mortality, severity of renal failure and kidney pathology. Serum concentrations of cytokines (interleukin-1β, interleukin-6, monocyte chemotactic protein-1 and tumor necrosis factor-α) and their mRNA expressions in the lung, liver, spleen and kidney were measured. We also evaluated polyol metabolites in the kidney.

Results

Mortality rate within 72 hours was significantly less in LPS-injected mice treated with AR inhibitor both before (29%) and after LPS injection (40%) than untreated mice (90%). LPS-injected mice showed marked increases in blood urea nitrogen, creatinine and cytokines, and AR inhibitor treatment suppressed the changes. LPS-induced acute kidney injury was associated with vacuolar degeneration and apoptosis of renal tubular cells as well as infiltration of neutrophils and macrophages. With improvement of such pathological findings, AR inhibitor treatment suppressed the elevation of cytokine mRNA levels in multiple organs and renal sorbitol accumulation.

Conclusion

AR inhibitor treatment ameliorated LPS-induced acute kidney injury, resulting in the lowered mortality.

The redox basis of epigenetic modifications: from mechanisms to functional consequences

Epigenetic modifications represent mechanisms by which cells may effectively translate multiple signaling inputs into phenotypic outputs. Recent research is revealing that redox metabolism is an increasingly important determinant of epigenetic control that may have significant ramifications in both human health and disease. Numerous characterized epigenetic marks, including histone methylation, acetylation, and ADP-ribosylation, as well as DNA methylation, have direct linkages to central metabolism through critical redox intermediates such as NAD(+), S-adenosyl methionine, and 2-oxoglutarate. Fluctuations in these intermediates caused by both normal and pathologic stimuli may thus have direct effects on epigenetic signaling that lead to measurable changes in gene expression. In this comprehensive review, we present surveys of both metabolism-sensitive epigenetic enzymes and the metabolic processes that may play a role in their regulation. To close, we provide a series of clinically relevant illustrations of the communication between metabolism and epigenetics in the pathogenesis of cardiovascular disease, Alzheimer disease, cancer, and environmental toxicity. We anticipate that the regulatory mechanisms described herein will play an increasingly large role in our understanding of human health and disease as epigenetics research progresses.

Aldose reductase pathway contributes to vulnerability of aging myocardium to ischemic injury

Aging men and women display both increased incidence of cardiovascular disease and complications of myocardial infarction and heart failure. We hypothesized that altered glucose metabolism, in particular, flux of glucose via the polyol pathway (PP) may be responsible, in part, for the enhanced vulnerability of aging myocardium to ischemic injury, even in the absence of superimposed disease processes linked to PP flux, such as diabetes. To test our hypothesis, we determined the expression and products of PP enzymes aldose reductase (AR) and sorbitol dehydrogenase (SDH) in hearts from Fischer 344 aged (26 months) and young (4 months) rats subjected to global ischemia followed by reperfusion in the presence or absence of blockers of PP and the measures of ischemic injury and functional recovery were determined. Expression and activities of AR and SDH were significantly higher in aged vs. young hearts, and induction of ischemia further increased AR and SDH activity in the aged hearts. Myocardial ischemic injury was significantly greater in aged vs. young hearts, and blockade of AR reduced ischemic injury and improved cardiac functional recovery on reperfusion in aged hearts. These data indicate that innate increases in activity of the PP enzymes augment myocardial vulnerability to I/R injury in aging, and that blockers of PP protect the vulnerable aging hearts.

The anti-necrosis role of hypoxic preconditioning after acute anoxia is mediated by aldose reductase and sorbitol pathway in PC12 cells

It has been demonstrated that hypoxic preconditioning (HP) enhances the survival ability of the organism against the subsequent acute anoxia (AA). However, it is not yet clear whether necrosis induced by AA can be prevented by HP, and what are the underlying mechanisms. In this study, we examined the effect of HP (10% O(2), 48 h) on necrosis induced by AA (0% O(2), 24 h) in PC12 cells. We found that HP delayed the regulatory volume decrease and reduced cell swelling after 24 h of exposure to AA. Since aldose reductase (AR) is involved in cell volume regulation, we detected AR mRNA expression with reverse transcription-polymerase chain reaction (RT-PCR) techniques. The AR mRNA level was dramatically elevated by HP. Furthermore, an HP-induced decrease in cell injury was reversed by berberine chloride (BB), the inhibitor of AR. In addition, sorbitol synthesized from glucose catalyzed by AR is directly related to cell volume regulation. Subsequently, we tested sorbitol content in the cytoplasm. HP clearly elevated sorbitol content, while BB inhibited the elevation induced by HP. Further study showed that a strong inhibitor of sorbitol permease, quinidine, completely reversed the protection induced by HP after AA. These data provide evidence that HP prevents necrosis induced by AA and is mediated by AR and sorbitol pathway.

Aldose reductase and cardiovascular diseases, creating human-like diabetic complications in an experimental model

Hyperglycemia and reduced insulin actions affect many biological processes. One theory is that aberrant metabolism of glucose via several pathways including the polyol pathway causes cellular toxicity. Aldose reductase (AR) is a multifunctional enzyme that reduces aldehydes. Under diabetic conditions AR converts glucose into sorbitol, which is then converted to fructose. This article reviews the biology and pathobiology of AR actions. AR expression varies considerably among species. In humans and rats, the higher level of AR expression is associated with toxicity. Flux via AR is increased by ischemia and its inhibition during ischemia reperfusion reduces injury. However, similar pharmacological effects are not observed in mice unless they express a human AR transgene. This is because mice have much lower levels of AR expression, probably insufficient to generate toxic byproducts. Human AR expression in LDL receptor knockout mice exacerbates vascular disease, but only under diabetic conditions. In contrast, a recent report suggests that genetic ablation of AR increased atherosclerosis and increased hydroxynonenal in arteries. It was hypothesized that AR knockout prevented reduction of toxic aldehydes. Like many in vivo effects found in genetically manipulated animals, interpretation requires the reproduction of human-like physiology. For AR, this will require tissue specific expression of AR in sites and at levels that approximate those in humans.

The role of the polyol pathway in acute kidney injury caused by hindlimb ischaemia in mice

The polyol pathway, a collateral glycolytic process, previously considered to be active in high glucose milieu, has recently been proposed to play a crucial role in ischaemia/reperfusion tissue injury. In this study, we explored the role of the polyol pathway in acute kidney injury (AKI), a life-threatening condition, caused by hindlimb ischaemia, and determined if inhibition of the polyol pathway by aldose reductase (AR) inhibitor is beneficial for this serious disorder. Mice 8 weeks of age rendered hindlimb ischaemic for 3 h by the clipping of major supporting arteries revealed marked muscle necrosis with accumulation of sorbitol and fructose in ischaemic muscles. Serum concentrations of blood urea nitrogen (BUN), creatinine phosphokinase (CPK), creatinine, tumour necrosis factor (TNF)-alpha as well as interleukin (IL)-6 were all elevated in these mice. Treatment with AR inhibitor (ARI) effectively suppressed muscle necrosis and accompanying inflammatory reactions and prevented renal failure. Similar to ARI-treated mice, AR-deficient mice were protected from severe ischaemic limb injury and renal failure, showing only modest muscle necrosis and significant suppression of serum markers of renal failure and inflammation. Thus, these findings suggest that the polyol pathway is implicated in AKI caused by ischaemic limb injury and that AR may be a potential therapeutic target for this condition.

Polyol pathway impairs the function of SERCA and RyR in ischemic-reperfused rat hearts by increasing oxidative modifications of these proteins

A number of studies have shown that the polyol pathway, consisting of aldose reductase (AR) and sorbitol dehydrogenase (SDH), contributes to ischemia-reperfusion (I/R)-induced myocardial infarction due to depletion of ATP. In this report we show that the polyol pathway in I/R heart also contributes to the impairment of sacro/endoplasmic reticulum Ca(2+)-ATPase (SERCA) and ryanodine receptor (RyR), two key players in Ca(2+) signaling that regulate cardiac contraction. Rat hearts were isolated and retrogradely perfused with either Krebs' buffer containing 1 microM AR inhibitor, zopolrestat, or 200 nM SDH inhibitor, CP-170,711, and challenged by 30 min of regional ischemia and 45 min of reperfusion. We found that post-ischemic contractile function of the isolated perfused hearts was improved by pharmacological inhibition of the polyol pathway. I/R-induced contractile dysfunction is most likely due to impairment in Ca(2+) signaling and the activities of SERCA and RyR. All these abnormalities were significantly ameliorated by treatment with ARI or SDI. We showed that the polyol pathway activities increase the level of peroxynitrite, which enhances the tyrosine nitration of SERCA and irreversibly modifies it to form SERCAC674-SO(3)H. This leads to reduced level of S-glutathiolated SERCA, contributing to its inactivation. The polyol pathway activities also deplete the level of GSH, leading to decreased active RyR, the S-glutathiolated RyR. Thus, in I/R heart, inhibition of polyol pathway improved the function of SERCA and RyR by protecting them from irreversible oxidation.

Aldose reductase mediates myocardial ischemia-reperfusion injury in part by opening mitochondrial permeability transition pore

Aldose reductase (AR), a member of the aldo-keto reductase family, has been demonstrated to play a central role in mediating myocardial ischemia-reperfusion (I/R) injury. Recently, using transgenic mice broadly overexpressing human AR (ARTg), we demonstrated that AR is an important component of myocardial I/R injury and that inhibition of this enzyme protects heart from I/R injury (20-22, 48, 49, 56). To rigorously delineate mechanisms by which AR pathway influences myocardial ischemic injury, we investigated the role played by reactive oxygen species (ROS), antioxidant enzymes, and mitochondrial permeability transition (MPT) pore opening in hearts from ARTg or littermates [wild type (WT)] subjected to I/R. MPT pore opening after I/R was determined using mitochondrial uptake of 2-deoxyglucose ratio, while H2O2 was measured as a key indicator of ROS. Myocardial 2-deoxyglucose uptake ratio and calcium-induced swelling were significantly greater in mitochondria from ARTg mice than in WT mice. Blockade of MPT pore with cyclosphorin A during I/R reduced ischemic injury significantly in ARTg mice hearts. H2O2 measurements indicated mitochondrial ROS generation after I/R was significantly greater in ARTg mitochondria than in WT mice hearts. Furthermore, the levels of antioxidant GSH were significantly reduced in ARTg mitochondria than in WT. Resveratrol treatment or pharmacological blockade of AR significantly reduced ROS generation and MPT pore opening in mitochondria of ARTg mice hearts exposed to I/R stress. This study demonstrates that MPT pore opening is a key event by which AR pathway mediates myocardial I/R injury, and that the MPT pore opening after I/R is triggered, in part, by increases in ROS generation in ARTg mice hearts. Therefore, inhibition of AR pathway protects mitochondria and hence may be a useful adjunct for salvaging ischemic myocardium.

Single sample extraction protocol for the quantification of NAD and NADH redox states in Saccharomyces cerevisiae

A robust redox extraction protocol for quantitative and reproducible metabolite isolation and recovery has been developed for simultaneous measurement of nicotinamide adenine dinucleotide (NAD) and its reduced form, NADH, from Saccharomyces cerevisiae. Following culture in liquid media, yeast cells were harvested by centrifugation and then lysed under nonoxidizing conditions by bead blasting in ice-cold, nitrogen-saturated 50 mM ammonium acetate. To enable protein denaturation, ice cold nitrogen-saturated CH(3)CN/50 mM ammonium acetate (3:1 v/v) was added to the cell lysates. Chloroform extractions were performed on supernatants to remove organic solvent. Samples were lyophilized and resuspended in 50 mM ammonium acetate. NAD and NADH were separated by HPLC and quantified using UV-Vis absorbance detection. NAD and NADH levels were evaluated in yeast grown under normal (2% glucose) and calorie restricted (0.5% glucose) conditions. Results demonstrate that it is possible to perform a single preparation to reliably and robustly quantitate both NAD and NADH contents in the same sample. Robustness of the protocol suggests it will be (i) applicable to quantification of these metabolites in other cell cultures; and (ii) amenable to isotope labeling strategies to determine the relative contribution of specific metabolic pathways to total NAD and NADH levels in cell cultures.

Polyol pathway and modulation of ischemia-reperfusion injury in Type 2 diabetic BBZ rat hearts

We investigated the role of polyol pathway enzymes aldose reductase (AR) and sorbitol dehydrogenase (SDH) in mediating injury due to ischemia-reperfusion (IR) in Type 2 diabetic BBZ rat hearts. Specifically, we investigated, (a) changes in glucose flux via cardiac AR and SDH as a function of diabetes duration, (b) ischemic injury and function after IR, (c) the effect of inhibition of AR or SDH on ischemic injury and function. Hearts isolated from BBZ rats, after 12 weeks or 48 weeks diabetes duration, and their non-diabetic littermates, were subjected to IR protocol. Myocardial function, substrate flux via AR and SDH, and tissue lactate:pyruvate (L/P) ratio (a measure of cytosolic NADH/NAD+), and lactate dehydrogenase (LDH) release (a marker of IR injury) were measured. Zopolrestat, and CP-470,711 were used to inhibit AR and SDH, respectively. Myocardial sorbitol and fructose content, and associated changes in L/P ratios were significantly higher in BBZ rats compared to non-diabetics, and increased with disease duration. Induction of IR resulted in increased ischemic injury, reduced ATP levels, increases in L/P ratio, and poor cardiac function in BBZ rat hearts, while inhibition of AR or SDH attenuated these changes and protected hearts from IR injury. These data indicate that AR and SDH are key modulators of myocardial IR injury in BBZ rat hearts and that inhibition of polyol pathway could in principle be used as a therapeutic adjunct for protection of ischemic myocardium in Type 2 diabetic patients.

Role of nitric oxide in regulating aldose reductase activation in the ischemic heart

Aldose reductase (AR) catalyzes the reduction of several aldehydes ranging from lipid peroxidation products to glucose. The activity of AR is increased in the ischemic heart due to oxidation of its cysteine residues, but the underlying mechanisms remain unclear. To examine signaling mechanisms regulating AR activation, we studied the role of nitric oxide (NO). Treatment with the NO synthase (NOS) inhibitor, N-nitro-l-arginine methyl ester prevented ischemia-induced AR activation and myocardial sorbitol accumulation in rat hearts subjected to global ischemia ex vivo or coronary ligation in situ, whereas inhibition of inducible NOS and neuronal NOS had no effect. Activation of AR in the ischemic heart was abolished by pretreatment with peroxynitrite scavengers hesperetin or 5, 10, 15, 20-tetrakis-[4-sulfonatophenyl]-porphyrinato-iron [III]. Site-directed mutagenesis and electrospray ionization mass spectrometry analyses showed that Cys-298 of AR was readily oxidized to sulfenic acid by peroxynitrite. Treatment with bradykinin and insulin led to a phosphatidylinositol 3-kinase (PI3K)-dependent increase in the phosphorylation of endothelial NOS at Ser-1177 and, even in the absence of ischemia, was sufficient in activating AR. Activation of AR by bradykinin and insulin was reversed upon reduction with dithiothreitol or by inhibiting NOS or PI3K. Treatment with AR inhibitors sorbinil or tolrestat reduced post-ischemic recovery in the rat hearts subjected to global ischemia and increased the infarct size when given before ischemia or upon reperfusion. These results suggest that AR is a cardioprotective protein and that its activation in the ischemic heart is due to peroxynitrite-mediated oxidation of Cys-298 to sulfenic acid via the PI3K/Akt/endothelial NOS pathway.

WY 14,643 inhibits human aldose reductase activity

Aldose reductase (AR) is implicated to play a critical role in diabetes and cardiovascular complications because of the reaction it catalyzes. Our data reveal that peroxisome proliferator WY 14,643, follows a pure non-competitive inhibition pattern in the aldehyde reduction activity as well as in the alcohol oxidation activity of AR. This finding communicates for the first time a novel feature of WY 14,643 in regulating AR activity. In addition, this observation indicates that AR, AR-like proteins and aldo-keto reductase (AKR) members may be involved in the WY 14,643 mechanism of action when it is administered as PPAR agonist.

Aldose reductase inhibitors improve myocardial reperfusion injury in mice by a dual mechanism

Aldose reductase (AR) has been implicated in the pathogenesis of diabetic complications, although the clinical efficacy of AR inhibitors has not been clearly proven. To clarify the pathophysiological role of AR in the heart, we investigated effects of AR inhibitors applied either during the pre-ischemic phase, or during the post-ischemic reperfusion phase on ischemia-reperfusion injury in isolated heart from transgenic mice overexpressing human AR. On reperfusion following global ischemia, transgenic mouse hearts exhibited lower left developed pressure, increased release of creatine kinase, and lower ATP content compared with their littermates. When inhibitors of AR were applied during the pre-ischemic phase, they significantly improved deranged cardiac function, creatine kinase release, and ATP content. On the other hand, inhibition of AR during the post-ischemic reperfusion phase did not affect cardiac performance and ATP content, but it significantly attenuated creatine kinase release and the level of thiobarbiturate-reactive substances in transgenic mouse hearts. These results suggest a dual role of AR in ischemia-reperfusion injury. Inhibition of AR during ischemia preserved generation of ATP via glycolysis, whereas inhibition during the reperfusion phase reduced myocardial injury by attenuating oxidative stress elicited by ischemic insult and reoxygenation.

Advanced glycation end products and RAGE: a common thread in aging, diabetes, neurodegeneration, and inflammation

The products of nonenzymatic glycation and oxidation of proteins and lipids, the advanced glycation end products (AGEs), accumulate in a wide variety of environments. AGEs may be generated rapidly or over long times stimulated by a range of distinct triggering mechanisms, thereby accounting for their roles in multiple settings and disease states. A critical property of AGEs is their ability to activate receptor for advanced glycation end products (RAGE), a signal transduction receptor of the immunoglobulin superfamily. It is our hypothesis that due to such interaction, AGEs impart a potent impact in tissues, stimulating processes linked to inflammation and its consequences. We hypothesize that AGEs cause perturbation in a diverse group of diseases, such as diabetes, inflammation, neurodegeneration, and aging. Thus, we propose that targeting this pathway may represent a logical step in the prevention/treatment of the sequelae of these disorders.

Aldose reductase pathway mediates JAK‐STAT signaling: a novel axis in myocardial ischemic injury

The aldose reductase pathway has been demonstrated to be a key component of myocardial ischemia reperfusion injury. Previously, we demonstrated that increased lactate/pyruvate ratio, a measure of cytosolic NADH/NAD+, is an important change that drives the metabolic cascade mediating ischemic injury. This study investigated signaling mechanisms by which the aldose reductase pathway mediates myocardial ischemic injury. Specifically, the influence of the aldose reductase pathway flux on JAK-STAT signaling was examined in perfused hearts. Induction of global ischemia in rats resulted in JAK2 activation followed by STAT5 activation. Pharmacological inhibition of aldose reductase or sorbitol dehydrogenase blocked JAK2 and STAT5 activation and was associated with lower lactate/pyruvate ratio and lower protein kinase C activity. Niacin, known to lower cytosolic NADH/NAD+ ratio independent of the aldose reductase pathway inhibition, also blocked JAK2 and STAT5 activation. Inhibition of protein kinase C also blocked JAK2 and STAT5 activation. Transgenic mice overexpressing human aldose reductase exhibited increased JAK2 and STAT5 activation. Pharmacological inhibition of JAK2 reduced ischemic injury and improved functional recovery similar to that observed in aldose reductase pathway inhibited mice hearts. These data, for the first time, demonstrate JAK-STAT signaling by the aldose reductase pathway in ischemic hearts and is, in part, due to changes in cytosolic redox state.

Central role for aldose reductase pathway in myocardial ischemic injury

Aldose reductase (AR), a member of the aldo-keto reductase family, has been implicated in the development of vascular and neurological complications of diabetes. Recently, we demonstrated that aldose reductase is a component of myocardial ischemic injury and that inhibitors of this enzyme protect rat hearts from ischemia-reperfusion injury. To rigorously test the effect of aldose reductase on myocardial ischemia-reperfusion injury, we used transgenic mice broadly overexpressing human aldose reductase (ARTg) driven by the major histocompatibility complex I promoter. Hearts from these ARTg or littermate mice (WT) (n=6 in each group) were isolated, perfused under normoxic conditions, then subjected to 50 min of severe low flow ischemia followed by 60 min of reperfusion. Creatine kinase (CK) release (a marker of ischemic injury) was measured during reperfusion; left ventricular developed pressure (LVDP), end diastolic pressure (EDP), and ATP were measured throughout the protocol. CK release was significantly greater in ARTg mice compared with the WT mice. LVDP recovery was significantly reduced in ARTg mice compared with the WT mice. Furthermore, ATP content was higher in WT mice compared with ARTg mice during ischemia and reperfusion. Infarct size measured by staining techniques and myocardial damage evaluated histologically were also significantly worse in ARTg mice hearts than in controls. Pharmacological inhibition of aldose reductase significantly reduced ischemic injury and improved functional recovery in ARTg mice. These data strongly support key roles for AR in ischemic injury and impairment of functional and metabolic recovery after ischemia. We propose that interventions targeting AR may provide a novel adjunctive approach to protect ischemic myocardium.

Sorbitol dehydrogenase: a novel target for adjunctive protection of ischemic myocardium

Sorbitol dehydrogenase (SDH) is a polyol pathway enzyme that catalyzes conversion of sorbitol to fructose. Recent studies have demonstrated that activation of aldose reductase, the first enzyme of the polyol pathway, is a key response to ischemia and that inhibition of aldose reductase reduces myocardial ischemic injury. In our efforts to understand the role of pathway in affecting metabolism under normoxic and ischemic conditions, as well as in ischemic injury in myocardium, we investigated the importance of SDH by use of a specific inhibitor (SDI), CP-470,711. SDH inhibition increased glucose oxidation, whereas palmitate oxidation remained unaffected. Global ischemia increased myocardial SDH activity by approximately 1.5 fold. The tissue lactate/pyruvate ratio, a measure of cytosolic NADH/NAD+, was reduced by SDH inhibition under both normoxic and ischemic conditions. ATP was higher in SDI hearts during ischemia and reperfusion. Creatine kinase release during reperfusion, a marker of myocardial ischemic injury, was markedly attenuated in SDH-inhibited hearts. These data indicate that myocardial SDH activation is a component of ischemic response and that interventions that inhibit SDH protect ischemic myocardium. Furthermore, these data identify SDH as a novel target for adjunctive cardioprotective interventions.

Aldose reductase activation is a key component of myocardial response to ischemia

Aldose reductase, a member of the aldo-keto reductase family, has been implicated in the development of vascular and neurological complications in diabetes. Despite recent studies from our laboratory demonstrating protection of ischemic hearts by an aldose reductase inhibitor, the presence and influence of aldose reductase in cardiac tissue remain unknown. Our goal in this study was to isolate and characterize the kinetic properties of cardiac aldose reductase, as well as to study the impact of flux via this enzyme on glucose metabolism and contractile function in hearts subjected to ischemia-reperfusion. Results demonstrate that ischemia increases myocardial aldose reductase activity and that these increases are, in part, due to activation by nitric oxide. The kinetic parameter of cardiac aldose reductase (Kcat) was significantly higher in ischemic tissues. Aldose reductase inhibition increased glycolysis and glucose oxidation. Aldose reductase inhibited hearts, when subjected to ischemia/reperfusion, exhibited less ischemic injury and was associated with lower lactate/pyruvate ratios (a measure of cytosolic NADH/NAD+), greater tissue content of adenosine triphosphate, and improved cardiac function. These findings indicate that aldose reductase is a component of ischemic injury and that pharmacological inhibitors of aldose reductase present a novel adjunctive approach for protecting ischemic hearts.

Evidence of separate pathways for lactate uptake and release by the perfused rat heart

The simultaneous release and uptake of lactate by the heart has been observed both in vivo and ex vivo; however, the pathways underlying these observations have not been satisfactorily explained. Consequently, the purpose of this study was to test the hypothesis that hearts release lactate from glycolysis while simultaneously taking up exogenous lactate. Therefore, we determined the effects of fatty acids and diabetes on the regulation of lactate uptake and release. Hearts from control and 1-wk diabetic animals were perfused with 5 mM glucose, 0.5 mM [3-(13)C]lactate, and 0, 0.1, 0.32, or 1.0 mM palmitate. Parameters measured include perfusate lactate concentrations, fractional enrichment, and coronary flow rates, which enabled the simultaneous, but independent, measurements of the rates of 1) uptake of exogenous [(13)C]lactate and 2) efflux of unlabeled lactate from metabolism of glucose. Although the rates of lactate uptake and efflux were both similarly inhibited by the addition of palmitate, (i.e., the ratio of lactate uptake to efflux remained constant), the ratio of lactate uptake to efflux was significantly higher in the controls compared with the diabetic group (1.00 +/- 0.14 vs. 0.50 +/- 0.07, P < 0.002). These data, combined with heterogeneous (13)C enrichment of tissue lactate, pyruvate, and alanine, suggest that glycolytically derived lactate production and oxidation of exogenous lactate operate as functionally separate metabolic pathways. These results are consistent with the concept of an intracellular lactate shuttle.

Protection of ischemic myocardium in diabetics by inhibition of electroneutral Na+-K+-2Cl- cotransporter

Diabetes increases both the incidence of cardiovascular disease and complications of myocardial infarction and heart failure. Studies using diabetic animals have shown that changes in myocardial sodium transporters result in alterations in intracellular sodium (Na(i)) homeostasis. Because the changes in sodium homeostasis can be due to increased entry of Na+ via the electroneutral Na+-K+-2Cl- cotransporter (NKCC), we conducted experiments in acute diabetic hearts to determine if 1) net inward cation flux via NKCC is increased, 2) this cotransporter contributes to a greater increase in Na(i) during ischemia, and 3) inhibition of NKCC limits injury and improves function after ischemia-reperfusion. These issues were investigated in perfused type I diabetic and nondiabetic rat hearts subjected to ischemia and 60 min of reperfusion. A group of diabetic and nondiabetic hearts was perfused with 5 microM of bumetanide, an inhibitor of NKCC. Flux via NKCC, Na(i), and ATP was measured in each group with the use of radiotracer 86Rb, 23Na, and 31P nuclear magnetic resonance spectroscopy, respectively, whereas ischemic injury was assessed by measuring creatine kinase release on reperfusion. Cation flux via NKCC, as measured by 86Rb uptake, was significantly increased in diabetic hearts. Inhibition of NKCC significantly reduced ischemic injury in diabetic hearts, improved functional recovery on reperfusion, attenuated the ischemic rise in Na(i), and conserved ATP during ischemia-reperfusion. Parallel studies in nondiabetic hearts showed that NKCC inhibition was not cardioprotective. These findings demonstrate that flux via NKCC is increased in type I diabetic hearts and that inhibition with bumetanide attenuates changes in Na(i) and ATP during ischemia and protects against ischemic injury. The data suggest a therapeutic role for pharmacological agents that inhibit flux via NKCC in diabetic patients with myocardial ischemia.

Protection of ischemic hearts by high glucose is mediated, in part, by GLUT-4

Metabolic interventions that promote glucose use during ischemia have been shown to protect ischemic myocardium and improve functional recovery on reperfusion. We evaluated whether the cardioprotection afforded by high glucose during low-flow ischemia is associated with changes in the sarcolemmal content of glucose transporters, specifically GLUT-4. Isolated rat hearts were paced at 300 beats/min and perfused under normal glucose (5 mM) or high glucose (10 mM) conditions in buffer containing 0.4 mM albumin, 0.4 mM palmitate, and 70 mU/l insulin and subjected to 50 min of low-flow ischemia and 60 min of reperfusion. To determine the importance of insulin-sensitive glucose transporters in mediating cardioprotection, a separate group of hearts were perfused in the presence of cytochalasin B (10 microM), a preferential inhibitor of insulin-sensitive glucose transporters. Ischemic contracture during low-flow ischemia and creatine kinase release on reperfusion was decreased, and the percent recovery of left ventricular function with reperfusion was enhanced in hearts perfused with high glucose (P < 0.03). Hearts perfused with high glucose exhibited increased GLUT-4 protein expression in the sarcolemmal membrane compared with control hearts under baseline conditions, and these changes were additive with low-flow ischemia. In addition, high glucose did not affect the baseline distribution of sarcolemmal GLUT-1 and blunted any changes with low-flow ischemia. These salutary effects were abolished when glucose transporters are blocked with cytochalasin B. These data demonstrate that protection of ischemic myocardium by high glucose is associated with increased sarcolemmal content of the insulin-sensitive GLUT-4 and suggest a target for the protection of jeopardized myocardium.

Aldose reductase inhibition alone or combined with an adenosine A(3) agonist reduces ischemic myocardial injury

This study investigated whether aldose reductase (AR) inhibition with zopolrestat, either alone or in combination with an adenosine A(3)-receptor agonist (CB-MECA), reduced myocardial ischemic injury in rabbit hearts subjected to 30 min of regional ischemia and 120 min of reperfusion. Zopolrestat reduced infarct size by up to 61%, both in vitro (2 nM to 1 microM; EC(50) = 24 nM) and in vivo (50 mg/kg). Zopolrestat reduced myocardial sorbitol concentration (index of AR activity) by >50% (control, 15.0 +/- 2.2 nmol/g; 200 nM zopolrestat, 6.7 +/- 1.3 nmol/g). A modestly cardioprotective concentration of CB-MECA (0.2 nM) allowed a 50-fold reduction in zopolrestat concentration while providing a similar reduction in infarct size (infarct area/area at risk: control, 62 +/- 2%; 1 microM zopolrestat, 24 +/- 5%; 20 nM zopolrestat plus 0.2 nM CB-MECA, 20 +/- 4%). In conclusion, AR inhibition is cardioprotective both in vitro and in vivo. Furthermore, combining zopolrestat with an A(3) agonist allows a reduction in the zopolrestat concentration while maintaining an equivalent degree of cardioprotection.

Niacin protects the isolated heart from ischemia-reperfusion injury

Nicotinic acid (niacin) has been shown to decrease myocyte injury. Because interventions that lower the cytosolic NADH/NAD(+) ratio improve glycolysis and limit infarct size, we hypothesized that 1) niacin, as a precursor of NAD(+), would lower the NADH/NAD(+) ratio, increase glycolysis, and limit ischemic injury and 2) these cardioprotective benefits of niacin would be limited in conditions that block lactate removal. Isolated rat hearts were perfused without (Ctl) or with 1 microM niacin (Nia) and subjected to 30 min of low-flow ischemia (10% of baseline flow, LF) and reperfusion. To examine the effects of limiting lactate efflux, experiments were performed with 1) Ctl and Nia groups subjected to zero-flow ischemia and 2) the Nia group treated with the lactate-H(+) cotransport inhibitor alpha-cyano-4-hydroxycinnamate under LF conditions. Measured variables included ATP, pH, cardiac function, tissue lactate-to-pyruvate ratio (reflecting NADH/NAD(+)), lactate efflux rate, and creatine kinase release. The lactate-to-pyruvate ratio was reduced by more than twofold in Nia-LF hearts during baseline and ischemic conditions (P < 0.001 and P < 0.01, respectively), with concurrent lower creatine kinase release than Ctl hearts (P < 0.05). Nia-LF hearts had significantly greater lactate release during ischemia (P < 0.05 vs. Ctl hearts) as well as higher functional recovery and a relative preservation of high-energy phosphates. Inhibiting lactate efflux with alpha-cyano-4-hydroxycinnamate and blocking lactate washout with zero flow negated some of the beneficial effects of niacin. During LF, niacin lowered the cytosolic redox state and increased lactate efflux, consistent with redox regulation of glycolysis. Niacin significantly improved functional and metabolic parameters under these conditions, providing additional rationale for use of niacin as a therapeutic agent in patients with ischemic heart disease.

Fructose-2,6-bisphosphate, a potent stimulator of phosphofructokinase, is increased by high exogenous glucose perfusion

BACKGROUND

We have previously demonstrated that perfusion of isolated hearts with high concentrations of glucose results in increased glycolysis during ischemia, diminished ischemic injury, and improved functional recovery with reperfusion.

OBJECTIVE

To evaluate a possible mechanism by which glucose conferred this protection. We examined the hypothesis that increased exogenous glucose concentrations results in increased concentrations of fructose-2,6-bisphosphate, a potent activator of phosphofructokinase-1, and thus increases glycolysis.

METHODS

Perfused rabbit hearts were subjected to 60 min of low-flow ischemia. Control hearts were perfused with buffer containing 0.4 mmol/l palmitate, 5 mmol/l glucose, and 70 mU/l insulin, and treated hearts were perfused with buffer containing 0.4 mmol/l palmitate, 15 mmol/l glucose and 210 mU/l insulin.

RESULTS

Ischemic contracture was attenuated by perfusion of high concentrations of glucose (high glucose) (P < 0.05 compared with control). Glucose uptake and lactate production were greater in hearts perfused with high glucose, as was the ATP concentration at the end of ischemia (P < 0.05 compared with controls). Exogenous glucose uptake and lactate production correlated well with fructose-2,6-bisphosphate content (P = 0.007).

CONCLUSIONS

Enhancement of glycolysis in hearts perfused with high glucose may be the result of stimulation of phosphofructokinase-1 by fructose-2,6-bisphosphate. Accordingly, this may serve as an important mechanism by which cardioprotection may be achieved.