Role of adenosine and glycogen in ischemic preconditioning of rat hearts

Myocardial autophagic energy stress responses--macroautophagy, mitophagy, and glycophagy

An understanding of the role of autophagic processes in the management of cardiac metabolic stress responses is advancing rapidly and progressing beyond a conceptualization of the autophagosome as a simple cell recycling depot. The importance of autophagy dysregulation in diabetic cardiomyopathy and in ischemic heart disease - both conditions comprising the majority of cardiac disease burden - has now become apparent. New findings have revealed that specific autophagic processes may operate in the cardiomyocyte, specialized for selective recognition and management of mitochondria and glycogen particles in addition to protein macromolecular structures. Thus mitophagy, glycophagy, and macroautophagy regulatory pathways have become the focus of intensive experimental effort, and delineating the signaling pathways involved in these processes offers potential for targeted therapeutic intervention. Chronically elevated macroautophagic activity in the diabetic myocardium is generally observed in association with structural and functional cardiomyopathy; yet there are also numerous reports of detrimental effect of autophagy suppression in diabetes. Autophagy induction has been identified as a key component of protective mechanisms that can be recruited to support the ischemic heart, but in this setting benefit may be mitigated by adverse downstream autophagic consequences. Recent report of glycophagy upregulation in diabetic cardiomyopathy opens up a novel area of investigation. Similarly, a role for glycogen management in ischemia protection through glycophagy initiation is an exciting prospect under investigation.

A hypoxic episode during cardiogenesis downregulates the adenosinergic system and alters the myocardial anoxic tolerance

To what extent hypoxia alters the adenosine (ADO) system and impacts on cardiac function during embryogenesis is not known. Ectonucleoside triphosphate diphosphohydrolase (CD39), ecto-5'-nucleotidase (CD73), adenosine kinase (AdK), adenosine deaminase (ADA), equilibrative (ENT1,3,4), and concentrative (CNT3) transporters and ADO receptors A1, A2A, A2B, and A3 constitute the adenosinergic system. During the first 4 days of development chick embryos were exposed in ovo to normoxia followed or not followed by 6 h hypoxia. ADO and glycogen content and mRNA expression of the genes were determined in the atria, ventricle, and outflow tract of the normoxic (N) and hypoxic (H) hearts. Electrocardiogram and ventricular shortening of the N and H hearts were recorded ex vivo throughout anoxia/reoxygenation ± ADO. Under basal conditions, CD39, CD73, ADK, ADA, ENT1,3,4, CNT3, and ADO receptors were differentially expressed in the atria, ventricle, and outflow tract. In H hearts ADO level doubled, glycogen decreased, and mRNA expression of all the investigated genes was downregulated by hypoxia, except for A2A and A3 receptors. The most rapid and marked downregulation was found for ADA in atria. H hearts were arrhythmic and more vulnerable to anoxia-reoxygenation than N hearts. Despite downregulation of the genes, exposure of isolated hearts to ADO 1) preserved glycogen through activation of A1 receptor and Akt-GSK3β-GS pathway, 2) prolonged activity and improved conduction under anoxia, and 3) restored QT interval in H hearts. Thus hypoxia-induced downregulation of the adenosinergic system can be regarded as a coping response, limiting the detrimental accumulation of ADO without interfering with ADO signaling.

Involvement of adenosine and standardization of aqueous extract of garlic (Allium sativum Linn.) on cardioprotective and cardiodepressant properties in ischemic preconditioning and myocardial ischemia-reperfusion induced cardiac injury

The present study investigated the effect of garlic (Allium sativum Linn.) aqueous extracts on ischemic preconditioning and ischemia-reperfusion induced cardiac injury, as well as adenosine involvement in ischemic preconditioning and garlic extract induced cardioprotection. A model of ischemia-reperfusion injury was established using Langendorff apparatus. Aqueous extract of garlic dose was standardized (0.5%, 0.4%, 0.3%, 0.2%, 0.1%, 0.07%, 0.05%, 0.03%, 0.01%), and the 0.05% dose was found to be the most effective. Higher doses (more than 0.05%) were highly toxic, causing arrhythmia and cardiodepression, whereas the lower doses were ineffective. Garlic exaggerated the cardioprotective effect of ischemic preconditioning. The cardioprotective effect of ischemic preconditioning and garlic cardioprotection was significantly attenuated by theophylline (1,000 µmol/L) and 8-SPT (10 mg/kg, i.p.) and expressed by increased myocardial infarct size, increased LDH level, and reduced nitrite and adenosine levels. These findings suggest that adenosine is involved in the pharmacological and molecular mechanism of garlic induced cardioprotection and mediated by the modulation of nitric oxide.

Cladribine inhibits a diltiazem-induced increase in red blood cell purine nucleotide concentrations in a zebrafish model

Minimizing drug interactions is paramount to improving the efficacy and tolerability of cancer therapy. The zebrafish represents an innovative cancer model due to highly conserved genetics and inherent capacity for high-throughput chemical screening. This pilot study extends the utility of the zebrafish to a preclinical model for pharmacodynamics by examining the interaction of the nucleoside analogue, cladribine with the calcium channel blocker, diltiazem. Cladribine (0.7-3.5 mM) and/or diltiazem (2.4 mM), was injected intraperitoneally into adult zebrafish and red blood cell (RBC) lysates were assayed by HPLC for levels of purine nucleotides (e.g. ATP), potential biomarkers of cardiovascular health. Diltiazem increased RBC ATP concentrations, which were inhibited by co-injection of cladribine. These results suggest a novel drug interaction and highlight the feasibility of the zebrafish as an in vivo model for pharmacodynamic studies.

Adenosine receptors and reperfusion injury of the heart

Adenosine, a catabolite of ATP, exerts numerous effects in the heart, including modulation of the cardiac response to stress, such as that which occurs during myocardial ischemia and reperfusion. Over the past 20 years, substantial evidence has accumulated that adenosine, administered either prior to ischemia or during reperfusion, reduces both reversible and irreversible myocardial injury. The latter effect results in a reduction of both necrosis or myocardial infarction (MI) and apoptosis. These effects appear to be mediated via the activation of one or more G-protein-coupled receptors (GPCRs), referred to as A(1), A(2A), A(2B) and A(3) adenosine receptor (AR) subtypes. Experimental studies in different species and models suggest that activation of the A(1) or A(3)ARs prior to ischemia is cardioprotective. Further experimental studies reveal that the administration of A(2A)AR agonists during reperfusion can also reduce MI, and recent reports suggest that A(2B)ARs may also play an important role in modulating myocardial reperfusion injury. Despite convincing experimental evidence for AR-mediated cardioprotection, there have been only a limited number of clinical trials examining the beneficial effects of adenosine or adenosine-based therapeutics in humans, and the results of these studies have been equivocal. This review summarizes our current knowledge of AR-mediated cardioprotection, and the roles of the four known ARs in experimental models of ischemia-reperfusion. The chapter concludes with an examination of the clinical trials to date assessing the safety and efficacy of adenosine as a cardioprotective agent during coronary thrombolysis in humans.

Evaluating the protective role of ischaemic preconditioning in rat hearts using a stationary small-animal SPECT imager and 99mTc-glucarate

OBJECTIVE

To examine the protective role of ischaemic preconditioning (IPC) in rat hearts using Tc-glucarate (GLA) and a stationary SPECT imager, FastSPECT.

METHODS

Twenty-four rats with 30 min myocardial ischaemia and 150 min reperfusion (IR) were studied as follows. The IPC group (n=6) underwent IPC (five cycles of 4 min ligation of the left coronary artery and reflow) before IR. The control group (n=7) was treated by IR without IPC. The SPT group (n=6) was subjected to IPC and an adenosine antagonist, 8-(p-sulfophenyl)-theophylline (SPT). The vehicle group (n=5) received IPC and SPT carrier vehicle. GLA was delivered intravenously 30 min post-reperfusion, and 2-h dynamic cardiac images were acquired by FastSPECT.

RESULTS

GLA showed 'hot-spot' accumulation in the ischaemic area-at-risk (IAR) and exhibited lower retention (% 5 min peak) in the IPC and vehicle groups (33.8+/-2.6 vs. 35.7+/-9.2, P>0.05) than in the control and SPT groups (63.1+/-5.3 vs. 54.8+/-4.8, P>0.05). The infarct size (% IAR) was larger in the control and SPT groups (48.2+/-6.3 vs. 41.7+/-6.3, P>0.05) than that in the IPC and vehicle groups (21.0+/-1.9 vs. 19.1+/-4.6, P>0.05). In terms of the ex-vivo IAR-to-normal radioactivity ratio, there was a statistical difference between the control and IPC groups (7.4+/-0.9 vs. 3.0+/-0.4), as well as the SPT and vehicle groups (7.4+/-1.0 vs. 3.4+/-0.5).

CONCLUSION

IPC offers cardioprotection and relates to the activation of adenosine receptors in rat hearts. FastSPECT GLA imaging is not only useful in detecting early ischaemia-reperfusion injury, but also valuable in evaluating cardioprotection.

Metabolic and genetic regulation of cardiac energy substrate preference

Proper heart function relies on high efficiency of energy conversion. Mitochondrial oxygen-dependent processes transfer most of the chemical energy from metabolic substrates into ATP. Healthy myocardium uses mainly fatty acids as its major energy source, with little contribution of glucose. However, lactate, ketone bodies, amino acids or even acetate can be oxidized under certain circumstances. A complex interplay exists between various substrates responding to energy needs and substrate availability. The relative substrate concentration is the prime factor defining preference and utilization rate. Allosteric enzyme regulation and protein phosphorylation cascades, partially controlled by hormones such as insulin, modulate the concentration effect; together they provide short-term adjustments of cardiac energy metabolism. The expression of metabolic machinery genes is also dynamically regulated in response to developmental and (patho)physiological conditions, leading to long-term adjustments. Specific nuclear receptor transcription factors and co-activators regulate the expression of these genes. These include peroxisome proliferator-activated receptors and their nuclear receptor co-activator, estrogen-related receptor and hypoxia-inducible transcription factor 1. Increasing glucose and reducing fatty acid oxidation by metabolic regulation is already a target for effective drugs used in ischemic heart disease and heart failure. Interaction with genetic factors that control energy metabolism could provide even more powerful pharmacological tools.

Anti-infarct effect of magnesium is not mediated by adenosine A1 receptors in rat globally ischaemic isolated hearts

1. The aim of present study was to investigate the effects of magnesium (Mg) on cardiac function and infarct size and to compare it effects with those of adenosine. The mechanism of Mg-mediated cardioprotection was explored by combined use of Mg and a selective adenosine A(1) receptor antagonist. 2. Rat isolated hearts were used for Langendorff perfusion. Hearts were either non-preconditioned or preconditioned with Mg (6 mmol/L) or adenosine (1 mmol/L) before 30 min sustained ischaemia followed by 120 min reperfusion. Within each of these protocols, hearts were divided into two groups; one group was exposed to the A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 200 nmol/L). Infarct size was measured by the triphenyltetrazolium chloride method. Left ventricular function was assessed by left ventricular developed pressure (LVDP), the product of heart rate x LVDP and coronary flow (CF). 3. The administration of Mg had an anti-infarct effect independent of its effect on postischaemic functional recovery in rats. Both Mg and adenosine equipotently reduced infarct size, but this effect of Mg was not blocked by the simultaneous administration of DPCPX. Cardiac function was improved by both adenosine and Mg and blockade of adenosine A(1) receptors attenuated these effects for both agents. 4. In conclusion, the results of the present study indicate that stimulation of adenosine A(1) receptors is not responsible for the anti-infarct effect of Mg in ischaemic myocardium in rats, but that the Mg-mediated protection of postischaemic functional recovery in rats is mediated by these receptors.

Myocardial adenosine does not correlate with the protection mediated by ischaemic or pharmacological preconditioning in rat heart

1. We tested the hypothesis that ischaemic preconditioning of the rat heart activates cardiovascular adenosine formation to provide enhanced cardioprotection. 2. Rat isolated perfused hearts were either non-preconditioned, preconditioned with 5 min ischaemia or treated for 5 min with the alpha1-adrenoceptor agonist phenylephrine (50 micro mol/L) before being subjected to 30 min sustained ischaemia followed by 30 min reperfusion. Isolated cardiomyocytes were either non-preconditioned, subjected to 10 min simulated ischaemia or treated for 10 min with phenylephrine (50 micro mol/L) before being subjected to 30 min simulated ischaemia. Functional recovery of hearts and cell viability were used as indices of the effects of ischaemia. 3. Myocardial adenosine, as well as intracellular pH, was determined at the end of the preconditioning period and at 10, 20 and 30 min of sustained ischaemia. Intracellular pH was also determined during the reperfusion. 4. Ischaemic or pharmacological preconditioning with phenylephrine correlated with an improved functional recovery of perfused hearts during reperfusion and increased cell viability during ischaemia. 5. In perfused hearts, ischaemic preconditioning resulted in increased adenosine production in the myocardium during the following sustained ischaemia. However, in isolated cardiomyocytes, adenosine levels during sustained ischaemia were lower in ischaemically preconditioned cells compared with the respective non-preconditioned cardiomyocytes. 6. The increase in adenosine production was not observed in hearts preconditioned with phenylephrine instead of transient ischaemia. Similarly, pharmacological preconditioning resulted in decreased adenosine levels during sustained ischaemia in isolated cardiomyocytes. 7. Intracellular pH was preserved during ischaemia to the same extent in both ischaemically or pharmacologically preconditioned hearts and cardiomyocytes, indicating that less acidosis during ischaemia is related to protection. 8. Taken together, the results suggest that cardioprotection does not necessarily correlate with increased adenosine production. Thus, adenosine concentration is not crucial to the beneficial effects of preconditioning in rat heart.

Resveratrol provides late-phase cardioprotection by means of a nitric oxide- and adenosine-mediated mechanism

We used two experimental models to prove that resveratrol (trans-3,4',5-trihydroxystilbene) reduces cardiac ischemic-reperfusion injury by means of a nitric oxide- and adenosine-dependent mechanism. (1). ACUTE EX VIVO: resveratrol (10 microM, 10 min) infusion in Langendorff-perfused normoxic rat hearts significantly increased adenosine release and coronary flow compared with baseline. After 30-min low-flow ischemia, vasodilation, still present at reperfusion, was completely abolished by resveratrol plus adenosine antagonist 8-(p-sulfophenyl)theophylline (SPT, 50 microM) administration. (2). CHRONIC IN VIVO: rats received tap water containing 25 mg/l resveratrol for 15 days or normal water. Twenty-four hours after, their hearts were Langendorff-perfused and submitted to 60-min low-flow ischemia and reperfusion. The resveratrol-treated hearts showed better functional recovery at reperfusion and significant vasodilation, but no variation in high-energy phosphates (31P Nuclear Magnetic Resonance). N(G)-nitro-L-arginine methyl ester (L-NAME, 30 microM), a nonselective nitric oxide synthase inhibitor, or SPT (50 microM) administered for 10 min prior to the low-flow ischemia cancelled the effects. This suggests that long-term moderate resveratrol consumption could play an important role in late cardioprotective effects.

Preconditioning of rat hearts by adenosine A1 or A3 receptor activation

Our study in rat hearts examined whether activation of adenosine A(1) or A(3) receptors improved functional recovery and reduced apoptosis resulting from low-flow ischemia. Prior to 30 min low-flow ischemia (0.6 ml/min; 6% of baseline flow), Langendorff rat hearts were preconditioned with two 5-min cycles of (a) ischemia (PC; n=7), (b) infusion of 250 nM adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA; n=6), or (c) infusion of 50 nM adenosine A(3) receptor agonist N(6)-(3-iodobenzyl)-adenosine-5'-N-methyl-uronamide (IB-MECA; n=8). Recovery of function was improved in PC (71+/-3%), CCPA (68+/-6%) and IB-MECA (68+/-4%) groups compared to control hearts (46+/-5%; P<0.05). Cumulative release of total purines during ischemia-reperfusion was approx. 50% lower in PC, CCPA and IB-MECA groups compared to controls (P<0.05) and was significantly correlated to the percentage functional recovery (R(2)=0.55; P<0.05). The number of cytosolic histone-associated-DNA fragments, a hallmark of apoptosis and measured by Enzyme Linked ImmunoSorbent Assay (ELISA), was small and not different between groups after 30 min reperfusion. However, CCPA (0.6+/-0.1 absorbance units) and MECA (0.7+/-0.1 units; P<0.05 vs. PC) decreased apoptosis after 150 min reperfusion compared to PC (1.4+/-0.3 units) and control (1.2+/-0.1 units) hearts. This study shows that adenosine triggers protection of function in preconditioned rat hearts via both the adenosine A(1) and A(3) receptor. In clinical practice, pharmacological stimulation of adenosine A(3) receptors may be advantageous over adenosine A(1) receptor activation due to a lack of contractile side-effects. In contrast to ischemic preconditioning, pharmacological stimulation of adenosine A(1) or A(3) receptors reduced apoptosis. Furthermore, total purine release may serve as a marker of the degree of functional protection.