Intrinsic A(1) adenosine receptor activation during ischemia or reperfusion improves recovery in mouse hearts

Dapagliflozin attenuates high glucose-and hypoxia/reoxygenation-induced injury via activating AMPK/mTOR-OPA1-mediated mitochondrial autophagy in H9c2 cardiomyocytes

This study investigated the protective effect of dapagliflozin on H9c2 cardiomyocyte function under high glucose and hypoxia/reoxygenation (HG-H/R) conditions and identified the underlying molecular mechanisms. Dapagliflozin reduced the level of lactate dehydrogenase and reactive oxygen species in cardiomyocytes under HG-H/R conditions and was accompanied by a decrease in caspase-3/9 activity. In addition, Dapagliflozin significantly reduced mitochondrial permeability transition pore opening and increased ATP content, accompanied by upregulation of OPA1 with autophagy-related protein molecules and activation of the AMPK/mTOR signalling pathway in HG-H/R treated cardiomyocytes. OPA1 knockdown or compound C treatment attenuated the protective effects of dapagliflozin on the cardiomyocytes under HG-H/R conditions. Downregulation of OPA1 expression increased mitochondrial intolerance in cardiomyocytes during HG-H/R injury and the AMPK-mTOR-autophagy signalling is a key mechanism for protecting mitochondrial function and reducing cardiomyocyte apoptosis. Collectively, dapagliflozin exerted protective effects on the cardiomyocytes under HG-H/R conditions. Dapagliflozin attenuated myocardial HG-H/R injury by activating AMPK/mTOR-OPA1-mediated mitochondrial autophagy.

Biased agonism at adenosine receptors

Adenosine modulates many aspects of human physiology and pathophysiology through binding to the adenosine family of G protein-coupled receptors, which are comprised of four subtypes, the A₁R, A_2AR, A_2BR and A₃R. Modulation of adenosine receptor function by exogenous agonists, antagonists and allosteric modulators can be beneficial for a number of conditions including cardiovascular disease, Parkinson's disease, and cancer. Unfortunately, many preclinical drug candidates targeting adenosine receptors have failed in clinical trials due to limited efficacy and/or severe on-target undesired effects. To overcome the key barriers typically encountered when transitioning adenosine receptor ligands into the clinic, research efforts have focussed on exploiting the phenomenon of biased agonism. Biased agonism provides the opportunity to develop ligands that favour therapeutic signalling pathways, whilst avoiding signalling associated with on-target undesired effects. Recent studies have begun to define the structure-function relationships that underpin adenosine receptor biased agonism and establish how this phenomenon can be harnessed therapeutically. In this review we describe the recent advancements made towards achieving therapeutically relevant biased agonism at adenosine receptors.

G-Protein-Coupled Receptors in Heart Disease

GPCRs (G-protein [guanine nucleotide-binding protein]-coupled receptors) play a central physiological role in the regulation of cardiac function in both health and disease and thus represent one of the largest class of surface receptors targeted by drugs. Several antagonists of GPCRs, such as βARs (β-adrenergic receptors) and Ang II (angiotensin II) receptors, are now considered standard of therapy for a wide range of cardiovascular disease, such as hypertension, coronary artery disease, and heart failure. Although the mechanism of action for GPCRs was thought to be largely worked out in the 80s and 90s, recent discoveries have brought to the fore new and previously unappreciated mechanisms for GPCR activation and subsequent downstream signaling. In this review, we focus on GPCRs most relevant to the cardiovascular system and discuss traditional components of GPCR signaling and highlight evolving concepts in the field, such as ligand bias, β-arrestin-mediated signaling, and conformational heterogeneity.

An allosteric modulator of the adenosine A1 receptor improves cardiac function following ischaemia in murine isolated hearts

The effect of an allosteric modulator of the adenosine A₁ receptors was investigated using an ischaemia-reperfusion protocol in murine isolated hearts. Isolated hearts were perfused with Kreb-Henseleit solution gassed with carbogen gas (95% O₂ and 5% CO₂) in Langendorff mode and electrically paced at 480 bpm. Following 20 min equilibration and 20 min global normothermic ischaemia, the allosteric modulator VCP333 (1 μM) or the adenosine A₁ receptor partial agonist VCP102 (10 μM) were infused after 5 min of reperfusion for 15 min. Upon termination of the drug treatment, reperfusion continued for a further 40 min. At the end of 60 min reperfusion, treatment with VCP333 or VCP102 improved the recovery of the left ventricular developed pressure when compared to control group responses (p < 0.05). Neither compound affected end diastolic pressure, coronary flow rates or dP/dt_max values when compared to control tissues during reperfusion (p > 0.05). The infusion of VCP102 or VCP333 during reperfusion reduced cardiac troponin I efflux to 6.7% and 25% respectively of control heart efflux (p < 0.05). This data indicates that the allosteric modulator of the adenosine A₁ receptor (VCP333) has similar characteristics to the adenosine receptor partial agonist VCP102 as it improves cardiac function and reduces myocardial cell death following an ischaemic episode.

Extracellular NAADP affords cardioprotection against ischemia and reperfusion injury and involves the P2Y11-like receptor

BACKGROUND AND PURPOSE

Extracellular nucleotides may play important regulatory roles within the cardiovascular system and notably in cardioprotection. We aimed to look for a possible pharmacological preconditioning effect of extracellular NAADP ([NAADP]e) against ischemia/reperfusion injury. [NAADP]e has been recently reported to be a full agonist of the P2Y11 receptor. Therefore, we characterized the involvement of the P2Y11-like receptor in mediating ischemic/reperfusion tolerance induced by [NAADP]e.

EXPERIMENTAL APPROACH

The cardioprotective effects of [NAADP]e were evaluated in a model of ischemia/reperfusion carried out on Langendorff perfused rat hearts. This model was also instrumented with a microdialysis probe. Furthermore, using isolated cardiomyocytes, we assessed cAMP, inositol phosphate accumulation and prosurvival protein kinases activation induced by [NAADP]e pretreatement.

RESULTS

Pretreatment with 1μM [NAADP]e induced cardioprotective effects with regards to functional recovery, necrosis and arrhythmogenesis (p<0.05). These effects were completely suppressed with NF157, an antagonist of the P2Y11 receptor. Moreover, global ischemia induced a time-dependent increase in interstitial concentration of adenosine, NAADP and UTP. In cardiomyocyte cultures, NF157 suppressed cAMP and inositol phosphate accumulation induced by [NAADP]e. [NAADP]e induced phosphorylation of ERK 1/2, AKT and its downstream target GSK-3β (p<0.05). These activations were also suppressed by NF157.

CONCLUSIONS

Evidence suggests that NAADP signalling at the P2Y11-like receptor affords significant cardioprotection against ischemia/reperfusion injury. Besides adenosine and UTP, microdialysis study supports a potential endogenous role of [NAADP]e.

Cardiovascular protection and antioxidant activity of the extracts from the mycelia of Cordyceps sinensis act partially via adenosine receptors

Mycelia of cultured Cordyceps sinensis (CS) is one of the most common substitutes for natural CS and was approved for arrhythmia in China. However, the role of CS in ameliorating injury during ischemia-reperfusion (I/R) is still unclear. We examined effects of extracts from CS on I/R and investigated the possible mechanisms. Post-ischemic coronary perfusion pressure, ventricular function, and coronary flow were measured using the Langendorff mouse heart model. Oxidative stress of cardiac homogenates was performed using an ELISA. Our results indicate that CS affords cardioprotection possibly through enhanced adenosine receptor activation. Cardioprotection was demonstrated by reduced post-ischemic diastolic dysfunction and improved recovery of pressure development and coronary flow. Treatment with CS largely abrogates oxidative stress and damage in glucose- or pyruvate-perfused hearts. Importantly, observed reductions in oxidative stress [glutathione disulfide (GSSG)]/[GSSG + glutathione] and [malondialdehyde (MDA)]/[superoxide dismutase + MDA] ratios as well as the resultant damage upon CS treatment correlate with functional markers of post-ischemic myocardial outcome. These effects of CS were partially blocked by 8-ρ-sulfophenyltheophylline, an adenosine receptor antagonist. Our results demonstrate a suppressive role of CS in ischemic contracture. Meanwhile, the results also suggest pre-ischemic adenosine receptor activation may be involved in reducing contracture in hearts pretreated with CS.

Activation of transient receptor potential canonical 3 (TRPC3)-mediated Ca2+ entry by A1 adenosine receptor in cardiomyocytes disturbs atrioventricular conduction

Although the activation of the A(1)-subtype of the adenosine receptors (A(1)AR) is arrhythmogenic in the developing heart, little is known about the underlying downstream mechanisms. The aim of this study was to determine to what extent the transient receptor potential canonical (TRPC) channel 3, functioning as receptor-operated channel (ROC), contributes to the A(1)AR-induced conduction disturbances. Using embryonic atrial and ventricular myocytes obtained from 4-day-old chick embryos, we found that the specific activation of A(1)AR by CCPA induced sarcolemmal Ca(2+) entry. However, A(1)AR stimulation did not induce Ca(2+) release from the sarcoplasmic reticulum. Specific blockade of TRPC3 activity by Pyr3, by a dominant negative of TRPC3 construct, or inhibition of phospholipase Cs and PKCs strongly inhibited the A(1)AR-enhanced Ca(2+) entry. Ca(2+) entry through TRPC3 was activated by the 1,2-diacylglycerol (DAG) analog OAG via PKC-independent and -dependent mechanisms in atrial and ventricular myocytes, respectively. In parallel, inhibition of the atypical PKCζ by myristoylated PKCζ pseudosubstrate inhibitor significantly decreased the A(1)AR-enhanced Ca(2+) entry in both types of myocytes. Additionally, electrocardiography showed that inhibition of TRPC3 channel suppressed transient A(1)AR-induced conduction disturbances in the embryonic heart. Our data showing that A(1)AR activation subtly mediates a proarrhythmic Ca(2+) entry through TRPC3-encoded ROC by stimulating the phospholipase C/DAG/PKC cascade provide evidence for a novel pathway whereby Ca(2+) entry and cardiac function are altered. Thus, the A(1)AR-TRPC3 axis may represent a potential therapeutic target.

Regulation of foam cells by adenosine

Macrophages rely on reverse cholesterol transport mechanisms to rid themselves of excess cholesterol. By reducing accumulation of cholesterol in the artery wall, reverse cholesterol transport slows or prevents development of atherosclerosis. In stable macrophages, efflux mechanisms balance influx mechanisms, and accumulating lipids do not overwhelm the cell. Under atherogenic conditions, inflow of cholesterol exceeds outflow, and the cell is ultimately transformed into a foam cell, the prototypical cell in the atherosclerotic plaque. Adenosine is an endogenous purine nucleoside released from metabolically active cells by facilitated diffusion and generated extracellularly from adenine nucleotides. Under stress conditions, such as hypoxia, a depressed cellular energy state leads to an acute increase in the extracellular concentration of adenosine. Extracellular adenosine interacts with 1 or more of a family of G protein-coupled receptors (A(1), A(2A), A(2B), and A(3)) to modulate the function of nearly all cells and tissues. Modulation of adenosine signaling participates in regulation of reverse cholesterol transport. Of particular note for the development of atherosclerosis, activation of A(2A) receptors dramatically inhibits inflammation and protects against tissue injury. Potent antiatherosclerotic effects of A(2A) receptor stimulation include inhibition of macrophage foam cell transformation and upregulation of the reverse cholesterol transport proteins cholesterol 27-hydroxylase and ATP binding cassette transporter A1. Thus, A(2A) receptor agonists may correct or prevent the adverse effects of inflammatory processes on cellular cholesterol homeostasis. This review focuses on the importance of extracellular adenosine acting at specific receptors as a regulatory mechanism to control the formation of foam cells under conditions of lipid loading.

Myocardial Stunning with Hemodialysis: Clinical Challenges of the Cardiorenal Patient

We discuss the current state of knowledge related to the pathogenesis of myocardial stunning as well as the potential mechanisms responsible for the clinical presentation of myocardial stunning in hemodialysis patients. We suggest future research areas for this critical and clinically important condition in this high-risk patient population. In consideration of acute and chronic changes secondary to dialysis, especially in patients with risk for coronary artery disease, the prevalence of myocardial stunning and its role in the natural history of these patients' disease progression is considered. We propose a paradigm: that the majority of the pathophysiologic mechanisms by which hemodialysis may induce myocardial stunning falls into two categories with (1) vascular and/or (2) metabolic contributions. In order to prevent eventual myocardial hibernation, myocardial remodeling, scarring, and loss of contractile function with aberrant electrical conductivity that could lead to sudden death, it is imperative to identify the risk factors associated with myocardial stunning during hemodialysis. Further understanding of these mechanisms may lead to novel clinical interventions and pharmacologic therapeutic agents.

Partial adenosine A1 receptor agonists for cardiovascular therapies

Adenosine, a purine nucleoside, is present in all cells in tightly regulated concentrations. It has many different physiological effects in the whole body and in the heart. Adenosine activates four G protein-coupled receptors A1, A2a, A2b, and A3. Activation of myocardial A1 receptors has been shown to inhibit a variety of myocardial pathologies associated with ischemia and reperfusion injury, including stunning, arrhythmogenesis, coronary and ventricular dysfunction, acute myocardial infarction, apoptosis, and chronic heart failure, implying several options for new cardiovascular therapies for diseases, like angina pectoris, control of cardiac rhythm, ischemic injury during an acute coronary syndrome, or heart failure. However, the main issue of using full A1 receptor agonists in such indications is the broad physiologic spectrum of cardiac and extracardiac effects. Desired A1 receptor-mediated protective and regenerative cardiovascular effects might be counter-regulated by unintended side effects when considering full A1 receptor agonists. These effects can be overcome by partial A1 agonists. Partial A1 agonists can be used to trigger only some of the physiological responses of receptor activation depending on endogenous adenosine levels and on receptor reserve in different tissues. CV-Therapeutics reported the identification of a partial A1 receptor agonist CVT-3619, and recently, another partial A1 receptor agonist VCP28 was published. Both compounds are adenosine derivatives. Adenosine-like A1 receptor agonists often have the drawback of a short half-life and low bioavailability, making them not suitable for chronic oral therapy. We identified the first non-adenosine-like partial A1 receptor agonist(s) with pharmacokinetics optimal for oral once daily treatment and characterized the qualities of the partial character of the A1 receptor agonist(s) in preclinical and clinical studies.

The role of muscarinic receptors in the beneficial effects of adenosine against myocardial reperfusion injury in rats

Adenosine, a catabolite of ATP, displays a wide variety of effects in the heart including regulation of cardiac response to myocardial ischemia and reperfusion injury. Nonetheless, the precise mechanism of adenosine-induced cardioprotection is still elusive. Isolated Sprague-Dawley rat hearts underwent 30 min global ischemia and 120 min reperfusion using a Langendorff apparatus. Both adenosine and acetylcholine treatment recovered the post-reperfusion cardiac function associated with adenosine and muscarinic receptors activation. Simultaneous administration of adenosine and acetylcholine failed to exert any additive protective effect, suggesting a shared mechanism between the two. Our data further revealed a cross-talk between the adenosine and acetylcholine receptor signaling in reperfused rat hearts. Interestingly, the selective M(2) muscarinic acetylcholine receptor antagonist methoctramine significantly attenuated the cardioprotective effect of adenosine. In addition, treatment with adenosine upregulated the expression and the maximal binding capacity of muscarinic acetylcholine receptor, which were inhibited by the selective A(1) adenosine receptor antagonist 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX) and the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME). These data suggested a possible functional coupling between the adenosine and muscarinic receptors behind the observed cardioprotection. Furthermore, nitric oxide was found involved in triggering the response to each of the two receptor agonist. In summary, there may be a cross-talk between the adenosine and muscarinic receptors in ischemic/reperfused myocardium with nitric oxide synthase might serve as the distal converging point. In addition, adenosine contributes to the invigorating effect of adenosine on muscarinic receptor thereby prompting to regulation of cardiac function. These findings argue for a potentially novel mechanism behind the adenosine-mediated cardioprotection.

Essential role of EGFR in cardioprotection and signaling responses to A1 adenosine receptors and ischemic preconditioning

Transactivation of epidermal growth factor receptor (EGFR) may contribute to specific protective responses (e.g. mediated by δ-opioid, bradykinin, or muscarinic receptors). No studies have assessed EGFR involvement in cardioprotection mediated by adenosine receptors (ARs), and the role of EGFR in ischemic preconditioning (IPC) is unclear. We tested EGFR, matrix metalloproteinase (MMP), and heparin-binding EGF (HB-EGF) dependencies of functional protection via A1AR agonism or IPC. Pretreatment of mouse hearts with 100 nM of A1AR agonist 2-chloro- N6-cyclopentyladenosine (CCPA) or IPC (3 × 1.5-min ischemia/2-min reperfusion) substantially improved recovery from 25-min ischemia, reducing left ventricular diastolic dysfunction up to 50% and nearly doubling pressure development and positive change in pressure over time (+dP/d t). Benefit with both CCPA and IPC was eliminated by inhibitors of EGFR tyrosine kinase (0.3 μM AG1478), MMP (0.3 μM GM6001), or HB-EGF ligand (0.3 ng/ml CRM197), none of which independently altered postischemic outcome. Phosphorylation of myocardial EGFR, Erk1/2, and Akt increased two- to threefold during A1AR agonism, with responses blocked by AG1478, GM6001, and CRM197. Studies in HL-1 myocytes confirm A1AR-dependent Erk1/2 phosphorylation is negated by AG1478 or GM6001, and reduced with CRM197 (as was Akt activation). These data collectively reveal that A1AR- and IPC-mediated functional protection is entirely EGFR and MMP dependent, potentially involving the HB-EGF ligand. Myocardial survival kinase activation (Erk1/2, Akt) by A1AR agonism is similarly MMP/HB-EGF/EGFR dependent. Thus MMP-mediated EGFR activation appears essential to cardiac protection and signaling via A1ARs and preconditioning.

Cooperative cardioprotection through adenosine A1 and A2A receptor agonism in ischemia-reperfused isolated mouse heart

Recent reports have shown that adenosine A1 receptor-mediated cardioprotection requires concomitant A2 receptor activation, but no study thus far has shown that this phenomenon occurs using A1 agonists at reperfusion. Thus, we compared adenosine A2A receptor knockout (A2AKO) and wild-type mouse hearts (n = 9-11) subjected to global ischemia (30 minutes) and reperfusion (60 minutes) in the presence and absence of the A1 agonist N-cyclopentlyadenosine (CPA). We also determined the effects of selective antagonists at A2A and A2B receptors on CPA-induced protection. In wild-type hearts, CPA (100 nM) significantly (P < 0.05) improved contractility (52.7 ± 6.2% versus 23.9 ± 4.9% of preischemia), left ventricular developed pressure, end diastolic pressure; reduced infarct size (7.9 ± 1.7% versus 23.9 ± 6.6% area at risk); decreased lactate dehydrogenase efflux; and increased ERK1/2 phosphorylation at 60 minutes of reperfusion. Adenosine A2A (ZM241385, 50 nM) and A2B (MRS1754, 100 nM) receptor antagonists abolished CPA-mediated cardioprotection in wild-type groups as did the A1 receptor antagonist DPCPX (P < 0.05). In A2AKO hearts, CPA did not improve functional parameters and protective signaling with the exception of end diastolic pressure. In this model, using a clinically relevant mode of pharmacologic intervention, pERK 1/2-dependent A1-mediated cardioprotection requires a cooperative activation of A2 receptors, presumably through endogenous adenosine.

A novel highly selective adenosine A1 receptor agonist VCP28 reduces ischemia injury in a cardiac cell line and ischemia-reperfusion injury in isolated rat hearts at concentrations that do not affect heart rate

The cardioprotective effects of a novel adenosine A1 receptor agonist N6-(2,2,5,5-tetramethylpyrrolidin-1-yloxyl-3-ylmethyl) adenosine (VCP28) were compared with the selective adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) in a H9c2(2-1) cardiac cell line-simulated ischemia (SI) model (12 hours) and a global ischemia (30 minutes) and reperfusion (60 minutes) model in isolated rat heart model. H9c2(2-1) cells were treated with CPA and VCP28 at the start of ischemia for entire ischemic duration, whereas isolated rat hearts were treated at the onset of reperfusion for 15 minutes. In the H9c2(2-1) cells SI model, CPA and VCP28 (100 nM) significantly (P < 0.05, n = 5-6) reduced the proportion of nonviable cells (30.88% +/- 2.49% and 16.17% +/- 3.77% of SI group, respectively) and lactate dehydrogenase efflux. In isolated rat hearts, CPA and VCP28 significantly (n = 6-8, P < 0.05) improved post-ischemic contractility (dP/dt(max), 81.69% +/- 10.96%, 91.07% +/- 19.87% of baseline, respectively), left ventricular developed pressure, and end diastolic pressure and reduced infarct size. The adenosine A1 receptor antagonist abolished the cardioprotective effects of CPA and VCP28 in SI model and isolated rat hearts. In conclusion, the adenosine A1 receptor agonist VCP28 has equal cardioprotective effects to the prototype A1 agonist CPA at concentrations that have no effect on heart rate.

Insulin-like growth factor-1 overexpression in cardiomyocytes diminishes ex vivo heart functional recovery after acute ischemia

BACKGROUND

Acute insulin-like growth factor-1 administration has been shown to have beneficial effects in cardiac pathological conditions. The aim of the present study was to assess the structural and ex vivo functional impacts of long-term cardiomyocyte-specific insulin-like growth factor-1 overexpression in hearts of transgenic αMHC-IGF-1 Ea mice.

METHODS

Performance of isolated transgenic αMHC-IGF-1 Ea and littermate wild-type control hearts was compared under baseline conditions and in response to 20-min ischemic insult. Cardiac desmin and laminin expression patterns were determined histologically, and myocardial hydroxyproline was measured to assess collagen content.

RESULTS

Overexpression of insulin-like growth factor-1 did not modify expression patterns of desmin or laminin but was associated with a pronounced increase (∼30%) in cardiac collagen content (from ∼3.7 to 4.8 μg/mg). Baseline myocardial contractile function and coronary flow were unaltered by insulin-like growth factor-1 overexpression. In contrast to prior evidence of acute cardiac protection, insulin-like growth factor-1 overexpression was associated with significant impairment of acute functional response to ischemia-reperfusion. Insulin-like growth factor-1 overexpression did not modify ischemic contracture development, but postischemic diastolic dysfunction was aggravated (51±5 vs. 22±6 mmHg in nontransgenic littermates). Compared with wild-type control, recovery of pressure development and relaxation indices relative to baseline performance were significantly reduced in transgenic αMHC-IGF-1 Ea after 60-min reperfusion (34±7% vs. 62±7% recovery of +dP/dt; 35±11% vs. 57±8% recovery of -dP/dt).

CONCLUSIONS

Chronic insulin-like growth factor-1 overexpression is associated with reduced functional recovery after acute ischemic insult. Collagen deposition is elevated in transgenic αMHC-IGF-1 Ea hearts, but there is no change in expression of the myocardial structural proteins desmin and laminin. These findings suggest that sustained cardiac elevation of insulin-like growth factor-1 may not be beneficial in the setting of an acute ischemic insult.

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

Cardioprotection induced by adenosine A1 receptor agonists in a cardiac cell ischemia model involves cooperative activation of adenosine A2A and A2B receptors by endogenous adenosine

Extracellular adenosine concentrations increase within the heart during ischemia, and any exogenous adenosine receptor agonists therefore work in the context of significant local agonist concentrations. We evaluated the interactions between A1, A2A, A2B, and A3 receptors in the presence and absence of adenosine deaminase (ADA, which is used to remove endogenous adenosine) in a cardiac cell ischemia model. Simulated ischemia (SI) was induced by incubating H9c2(2-1) cells in SI medium for 12 hours in 100% N2 gas before assessment of necrosis using propidium iodide (5 microM) or apoptosis using AnnexinV-PE flow cytometry. N6-Cyclopentyladenosine (CPA; 10(-7)M) and N6-(3-iodobenzyl) adenosine-5'-N-methyluronamide (IB-MECA; 10(-7)M) reduced the proportion of nonviable cells to 30.87 +/- 2.49% and 35.18 +/- 10.30%, respectively (% of SI group). In the presence of ADA, the protective effect of CPA was reduced (62.82 +/- 3.52% nonviable), whereas the efficacy of IB-MECA was unchanged (35.81 +/- 3.84% nonviable; P < 0.05, n = 3-5, SI vs. SI + ADA). The protective effects of CPA and IB-MECA were abrogated in the presence of their respective antagonists DPCPX (8-cyclopentyl-1,3-dipropylxanthine) and MRS1191 [3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate], whereas A2A and A2B agonists had no significant effect. CPA-mediated protection was abrogated in the presence of both A2A (ZM241385, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-lamino]ethyl)phenol; 50 nM) and A2B (MRS1754, 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl)xanthine; 200 nM) antagonists (n = 3-5, P < 0.05). In the absence of endogenous adenosine, significant protection was observed with CPA in presence of CGS21680 (4-[2-[[6-amino-9-(N-ethyl-b-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid) or LUF5834 [2-amino-4-(4-hydroxyphenyl)-6-(1H-imidazol-2-ylmethylsulfanyl)pyridine-3,5-dicarbonitrile] (P < 0.05 vs. SI + ADA + CPA). Apoptosis (14.35 +/- 0.15% of cells in SI + ADA group; P < 0.05 vs. control) was not significantly reduced by CPA or IB-MECA. In conclusion, endogenous adenosine makes a significant contribution to A1 agonist-mediated prevention of necrosis in this SI model by cooperative interactions with both A2A and A2B receptors but does not play a role in A3 agonist-mediated protection.

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.

Loss of myocardial ischemic postconditioning in adenosine A1 and bradykinin B2 receptors gene knockout mice

BACKGROUND

Ischemic postconditioning (PostC) is a recently described cardioprotective modality against reperfusion injury, through series of brief reflow interruptions applied at the very onset of reperfusion. It is proposed that PostC can activate a complex cellular signaling cascade, in which cell membrane receptors could serve as the upstream triggers of PostC. However, the exact subtypes of such receptors remain controversial or uninvestigated. To this context, the purpose of present study was to determine the definitive role of adenosine A(1) and bradykinin B(1) and B(2) receptors in PostC.

METHODS AND RESULTS

The hearts isolated from adult male C57BL/6J wild-type mice or the mice lacking adenosine A(1), or bradykinin B(1) or B(2) receptors subjected to zero-flow global ischemia and reperfusion in a Langendorff model. PostC, consisting of 6 cycles of 10 seconds of reperfusion and 10 seconds of ischemia, demonstrated significantly reduced myocardial infarct size (22.8+/-3.1%, mean+/-SEM) as compared with the non-PostC wild-type controls (35.1+/-2.8%, P<0.05). The infarct-limiting protection of PostC was absent in adenosine A(1) receptor knockout mice (34.9+/-2.7%) or bradykinin B(2) receptor knockout mice (33.3+/-1.7%) and was partially attenuated in bradykinin B(1) receptor-deficient mice (25.6+/-2.9%; P>0.05). On the other hand, PostC did not significantly alter postischemic cardiac contractile function and coronary flow.

CONCLUSIONS

With the use of three distinctive strains of gene knockout mice, the current study has provided the first conclusive evidence showing PostC-induced infarct-limiting cardioprotection could be triggered by activation of multiple types of cell membrane receptors, which include adenosine A(1) and bradykinin B(2) receptors.

Adenosinergic cardioprotection: Multiple receptors, multiple pathways

Adenosine, formed primarily via hydrolysis of 5'-AMP, has been historically dubbed a "retaliatory" metabolite due to enhanced local release and beneficial actions during cellular/metabolic stress. From a cardiovascular perspective, evidence indicates the adenosinergic system is essential in mediation of intrinsic protection (e.g., pre- and postconditioning) and determining myocardial resistance to insult. Modulation of adenosine and its receptors thus remains a promising, though as yet not well-realized, approach to amelioration of injury in ischemic-reperfused myocardium. Adenosine exerts effects through A(1), A(2A), A(2B), and A(3) adenosine receptor subtypes (A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR), which are all expressed in myocardial and vascular cells, and couple to G proteins to trigger a range of responses (generally, but not always, beneficial). Adenosine can also enhance tolerance to injurious stimuli via receptor-independent metabolic effects. Given adenosines contribution to preconditioning, it is no surprise that postreceptor signaling typically mimics that associated with preconditioning. This involves activation/translocation of PKC, PI3 kinase, and MAPKs, with ultimate effects at the level of mitochondrial targets-the mitochondrial K(ATP) channel and/or the mitochondrial permeability transition pore (mPTP). Nonetheless, differences in cytoprotective signaling and actions of the different adenosine receptor subtypes have been recently revealed. Our understanding of adenosinergic cytoprotection continues to evolve, with roles for the A(2) subtypes emerging, together with evidence of essential receptor "cross-talk" in mediation of protection. This review focuses on current research into adenosine-mediated cardioprotection, highlighting recent findings which, together with a wealth of prior knowledge, may ultimately facilitate adenosinergic approaches to clinical cardiac protection.

A1 adenosine receptor upregulation accompanies decreasing myocardial adenosine levels in mice with left ventricular dysfunction

BACKGROUND

It is well known that adenosine levels are increased during ischemia and protect the heart during ischemia/reperfusion. However, less is known about the role of adenosine-adenosine receptor (AR) pathways in hearts with left ventricular dilation and dysfunction. Therefore, we assessed adenosine levels and selective AR expression in transgenic mice with left ventricular systolic dysfunction secondary to overexpression of tumor necrosis factor-alpha (TNF 1.6).

METHODS AND RESULTS

Cardiac adenosine levels were reduced by 70% at 3 and 6 weeks of age in TNF 1.6 mice. This change was accompanied by a 4-fold increase in the levels of A1-AR and a 50% reduction in the levels of A2A-AR. That the increase in A1-AR density was of physiological significance was shown by the fact that chronotropic responsiveness to the A1-AR selective agonist 2-chloro-N6-cyclopentanyladenosine was enhanced in the TNF 1.6 mice. Similar changes in adenosine levels were found in 2 other models of heart failure, mice overexpressing calsequestrin and mice after chronic pressure overload, suggesting that the changes in adenosine-AR signaling were secondary to myocardial dysfunction rather than to TNF overexpression.

CONCLUSIONS

Cardiac dysfunction secondary to the overexpression of TNF is associated with marked alterations in myocardial levels of adenosine and ARs. Modulation of the myocardial adenosine system and its signaling pathways may be a novel therapeutic target in patients with heart failure.

Impaired p38 MAPK/HSP27 signaling underlies aging-related failure in opioid-mediated cardioprotection

Cardioprotection and preconditioning mediated via G-protein-coupled receptors may be lost or impaired with advancing age, limiting ischemic tolerance and the ability to pharmacologically protect older hearts from ischemic injury. Our preliminary findings indicated a loss of delta-opioid receptor-mediated protection in aged vs. young mouse hearts, which may involve alterations in protective kinase signaling. In the present study, we tested the hypothesis that aging-related loss of opioid-triggered cardioprotection involves failure to activate p38 MAPK and its distal signaling targets. Langendorff-perfused hearts from young (10-14 weeks) or aged (24-26 months) C57 mice underwent 25-min ischemia and 45-min reperfusion in the presence or absence of 1 micromol/l DPDPE (delta-opioid agonist) or 1 micromol/l anisomycin (activator of p38 MAPK), and functional recovery and protein activation/phosphorylation were assessed. Contractile recovery was similar in untreated young and aged hearts (50+/-2% and 53+/-5%, respectively), and was enhanced by DPDPE in young hearts only (67+/-3%). Immunoblot analysis revealed that DPDPE comparably activated or phosphorylated GRK2, Akt, ERK1/2 and p70S6 kinase in young and aged hearts, whereas aging abrogated the stimulatory effects of DPDPE on p38 MAPK and HSP27. Treatment with anisomycin elicited comparable activation of p38 MAPK and HSP27 in both young and aged hearts, coupled with a pronounced and equivalent cardioprotection in the two groups (73+/-3% and 77+/-2%, respectively), an effect abolished by the p38 MAPK inhibitor, SB203580. These data indicate that aging-related loss of delta-opioid-mediated cardioprotection involves failure to activate p38 MAPK and HSP27. Direct targeting of this pathway elicits comparable protection in both age groups.

Regulated Overexpression of the A 1 -Adenosine Receptor in Mice Results in Adverse but Reversible Changes in Cardiac Morphology and Function

Background— Both the A 1 - and A 3 -adenosine receptors (ARs) have been implicated in mediating the cardioprotective effects of adenosine. Paradoxically, overexpression of both A 1 -AR and A 3 -AR is associated with changes in the cardiac phenotype. To evaluate the temporal relationship between AR signaling and cardiac remodeling, we studied the effects of controlled overexpression of the A 1 -AR using a cardiac-specific and tetracycline-transactivating factor–regulated promoter.

Methods and Results— Constitutive A 1 -AR overexpression caused the development of cardiac dilatation and death within 6 to 12 weeks. These mice developed diminished ventricular function and decreased heart rate. In contrast, when A 1 -AR expression was delayed until 3 weeks of age, mice remained phenotypically normal at 6 weeks, and >90% of the mice survived at 30 weeks. However, late induction of A 1 -AR still caused mild cardiomyopathy at older ages (20 weeks) and accelerated cardiac hypertrophy and the development of dilatation after pressure overload. These changes were accompanied by gene expression changes associated with cardiomyopathy and fibrosis and by decreased Akt phosphorylation. Discontinuation of A 1 -AR induction mitigated cardiac dysfunction and significantly improved survival rate.

Conclusions— These data suggest that robust constitutive myocardial A 1 -AR overexpression induces a dilated cardiomyopathy, whereas delaying A 1 -AR expression until adulthood ameliorated but did not eliminate the development of cardiac pathology. Thus, the inducible A 1 -AR transgenic mouse model provides novel insights into the role of adenosine signaling in heart failure and illustrates the potentially deleterious consequences of selective versus nonselective activation of adenosine-signaling pathways in the heart.

Modulation of ischaemic contracture in mouse hearts: a 'supraphysiological' response to adenosine

While inhibition of ischaemic contracture was one of the first documented cardioprotective actions of exogenously applied adenosine, it is not known whether this is a normal function of endogenous adenosine generated during ischaemic stress. Additionally, the relevance of delayed contracture to postischaemic outcome is unclear. We tested the ability of endogenous versus exogenous adenosine to modify contracture (and postischaemic outcomes) in C57/Bl6 mouse hearts. During ischaemia, untreated hearts developed peak contracture (PC) of 85 +/- 5 mmHg at 8.9 +/- 0.8 min, with time to reach 20 mmHg (time to onset of contracture; TOC) of 4.4 +/- 0.3 min. Adenosine (50 microm) delayed TOC to 6.7 +/- 0.6 min, as did pretreatment with 10 microm 2-chloroadenosine (7.2 +/- 0.5 min) or 50 nm of A(1) adenosine receptor (AR) agonist N(6)-cyclohexyladenosine (CHA) (6.7 +/- 0.3 min), but not A(2A)AR or A(3)AR agonists (20 nm 2-[4-(2-carboxyethyl) phenethylamino]-5' N-methylcarboxamidoadenosine (CGS21680) or 150 nm 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA), respectively). Adenosinergic contracture inhibition was eliminated by A(1)AR gene knockout (KO), mimicked by A(1)AR overexpression, and was associated with preservation of myocardial [ATP]. This adenosine-mediated inhibition of contracture was, however, only evident after prolonged (10 or 15 min) and not brief (3 min) pretreatment. Ischaemic contracture was also insensitive to endogenously generated adenosine, since A(1)AR KO, and non-selective and A(1)AR-selective antagonists (50 microm 8-sulphophenyltheophylline and 150 nm 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), respectively), all failed to alter intrinsic contracture development. Finally, delayed contracture with A(1)AR agonism/overexpression or ischaemic 2,3-butanedione monoxime (BDM; 5 microm to target Ca(2+) cross-bridge formation) was linked to enhanced postischaemic outcomes. In summary, adenosinergic inhibition of contracture is solely A(1)AR mediated; the response is 'supraphysiological', evident only with significant periods of pre-ischaemic AR agonism (or increased A(1)AR density); and ischaemic contracture appears insensitive to locally generated adenosine, potentially owing to the rapidity of contracture development versus the finite time necessary for expression of AR-mediated cardioprotection.

Effects of postconditioning of adenosine and acetylcholine on the ischemic isolated rat ventricular myocytes

In this study, protective effects of adenosine and acetylcholine-induced postconditioning were investigated on the contractile function of the ischemic isolated rat ventricular myocytes. A video-based edge-detection system was used to monitor single ventricular myocytes contraction. Adenosine and acetylcholine were administrated for 6 min before ischemia as preconditioning, or 15 min after ischemia as postconditioning. Adenosine and acetylcholine receptor antagonists and mitoKATP inhibitor were used to analyze pathways underlying the effects on postconditioning.

RESULTS

(1) The peak shortening of ischemic heart cells was improved by both adenosine and acetylcholine during preconditioning (84.72+/-5.34% and 68.61+/-8.10% vs. control: 8.43+/-5.35% of the pre-ischemia value), as well as postconditioning (76.47+/-7.87% and 57.48+/-6.97% vs. control: 8.43+/-5.35% of the pre-ischemia value) and the effects of preconditioning and postconditioning were comparable. More datum in the normal text. (2) Observed effects of adenosine and acetylcholine postconditioning were missing in the presence of adenosine A1 receptor and muscarinic M2 receptor antagonists, respectively. (3) Adenosine and acetylcholine-induced postconditioning was also blocked by mitoKATP antagonist. These results suggest that both adenosine and acetylcholine protect the contractile function of ischemic heart cells to a similar extent during preconditioning and postconditioning. The postconditioning of adenosine and acetylcholine is relative to the adenosine A1 and muscarinic M2 receptors, respectively. MitoKATP is implicated in the postconditioning of both acetylcholine and adenosine.

Effects of targeted deletion of A1 adenosine receptors on postischemic cardiac function and expression of adenosine receptor subtypes

To examine ischemic tolerance in the absence of A(1) adenosine receptors (A(1)ARs), isolated wild-type (WT) and A(1)AR knockout (A(1)KO) murine hearts underwent global ischemia-reperfusion, and injury was measured in terms of functional recovery and efflux of lactate dehydrogenase (LDH). Hearts were analyzed by real-time RT-PCR both at baseline and at intervals during ischemia-reperfusion to determine whether compensatory expression of other adenosine receptor subtypes occurs with either A(1)AR deletion and/or ischemia-reperfusion. A(1)KO hearts had higher baseline coronary flow (CF) and left ventricular developed pressure (LVDP) than WT hearts, whereas heart rate was unchanged by A(1)AR deletion. After 20 min of ischemia, CF was attenuated in A(1)KO compared with WT hearts, and this reduction persisted throughout reperfusion. Final recovery of LVDP was decreased in A(1)KO hearts (54.4 +/- 5.1 vs. WT 81.1 +/- 3.4% preischemic baseline) and correlated with higher diastolic pressure during reperfusion. Postischemic efflux of LDH was greater in A(1)KO compared with WT hearts. Real-time RT-PCR demonstrated the absence of A(1)AR transcript in A(1)KO hearts, and the message for A(2A), A(2B), and A(3) adenosine receptors was similar in uninstrumented A(1)KO and WT hearts. Ischemia-reperfusion increased A(2B) mRNA expression 2.5-fold in both WT and A(1)KO hearts without changing A(1) or A(3) expression. In WT hearts, ischemia transiently doubled A(2A) mRNA, which returned to preischemic level upon reperfusion, a pattern not observed in A(1)KO hearts. Together, these data affirm the cardioprotective role of A(1)ARs and suggest that induced expression of other adenosine receptor subtypes may participate in the response to ischemia-reperfusion in isolated murine hearts.

Growth regulation of the vascular system: an emerging role for adenosine

The importance of metabolic factors in the regulation of angiogenesis is well understood. An increase in metabolic activity leads to a decrease in tissue oxygenation causing tissues to become hypoxic. The hypoxia initiates a variety of signals that stimulate angiogenesis, and the increase in vascularity that follows promotes oxygen delivery to the tissues. When the tissues receive adequate amounts of oxygen, the intermediate effectors return to normal levels, and angiogenesis ceases. An emerging concept is that adenosine released from hypoxic tissues has an important role in driving the angiogenesis. The following feedback control hypothesis is proposed: AMP is dephosphorylated by ecto-5′-nucleotidase, producing adenosine under hypoxic conditions in the extracellular space adjacent to a parenchymal cell (e.g., cardiomyocyte, skeletal muscle fiber, hepatocyte, etc.). Extracellular adenosine activates A2receptors, which stimulates the release of vascular endothelial growth factor (VEGF) from the parenchymal cell. VEGF binds to its receptor (VEGF receptor 2) on endothelial cells, stimulating their proliferation and migration. Adenosine can also stimulate endothelial cell proliferation independently of VEGF, which probably involves modulation of other proangiogenic and antiangiogenic growth factors and perhaps an intracellular mechanism. In addition, hemodynamic factors associated with adenosine-induced vasodilation may have a role in the development and remodeling of the vasculature. Once a new capillary network has been established, and the diffusion/perfusion capabilities of the vasculature are sufficient to supply the parenchymal cells with adequate amounts of oxygen, adenosine and VEGF as well as other proangiogenic and antiangiogenic growth factors return to near-normal levels, thus closing the negative feedback loop. The available data indicate that adenosine might be an essential mediator for up to 50–70% of the hypoxia-induced angiogenesis in some situations; however, additional studies in intact animals will be required to fully understand the quantitative importance of adenosine.

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.

Cardioprotection following adenosine kinase inhibition in rat hearts

Adenosine kinase phosphorylates adenosine to AMP, the primary pathway for adenosine metabolism under basal conditions. Inhibition of adenosine kinase results in a site-specific increase in interstitial adenosine. Using a rat model of myocardial infarction, we examined the protective effects of adenosine kinase inhibition. Male Sprague-Dawley rats underwent 30 min regional occlusion followed by 90 min reperfusion. Infarct size, expressed as a percent of the area-at-risk, IS/AAR(%), was 58.0 +/- 2.1 % in untreated rats. Pretreatment with the adenosine kinase inhibitor, 5-iodotubercidin (1 mg/kg), limited infarct development to 37.5+/-3.7% (P < 0.001). The A(1) adenosine receptor (A(1)AR) antagonist, DPCPX (100 microg/kg), abolished the infarct-sparing effect of 5-iodotubercidin (IS, 62.8 +/- 1.3%). Similarly, the A(3) adenosine receptor (A(3)AR) antagonist, MRS-1523 (2 mg/kg), and the delta-opioid receptor (DOR) antagonist, BNTX, (1 mg/kg) abolished the reduction of IS produced by iodotubercidin. Pretreatment with the ROS scavenger, 2-MPG (20 mg/kg), or the PKC-delta antagonist, rottlerin (0.3 mg/kg) also abolished iodotubercidin-mediated cardioprotection. Furthermore, pretreatment with 5-HD, a mitochondrial K(ATP) (mitoK(ATP)) channel inhibitor, but not the sarcolemmal K(ATP) channel blocker, HMR-1098, abrogated the beneficial effects of adenosine kinase inhibition (IS, 59.5 +/- 3.8%). These data suggest that inhibition of adenosine kinase is effective in reducing infarct development via A(1)AR, A(3)AR and DOR activation. Data also suggest that this protection is mediated via ROS, PKC-delta and mitoK(ATP) channels.

Genetic Deletion of the A 1 Adenosine Receptor Limits Myocardial Ischemic Tolerance

Adenosine receptors may be important determinants of intrinsic ischemic tolerance. Genetically modified mice were used to examine effects of global A 1 adenosine receptor (A 1 AR) knockout (KO) on function and ischemic tolerance in perfused mouse hearts. Baseline contractile function and heart rate were unaltered by A 1 AR KO, which was shown to abolish the negative chronotropic effects of 2-chloroadenosine (A 1 AR-mediated) without altering A 2 adenosine receptor–mediated coronary dilation. Tolerance to 25 minutes global normothermic ischemia (followed by 45 minutes reperfusion) was significantly limited by A 1 AR KO, with impaired contractile recovery (reduced by ≈25%) and enhanced lactate dehydrogenase (LDH) efflux (increased by ≈100%). Functional effects of A 1 AR KO involved worsened systolic pressure development with little to no change in diastolic dysfunction. In contrast, cardiac specific A 1 AR overexpression enhanced ischemic tolerance with a primary action on diastolic dysfunction. Nonselective receptor agonism (10 μmol/L 2-chloroadenosine) protected wild-type and also A 1 AR KO hearts (albeit to a lesser extent), implicating protection via subtypes additional to A 1 ARs. However, A 1 AR KO abrogated effects of 2-chloroadenosine on ischemic contracture and diastolic dysfunction. These data are the first demonstrating global deletion of the A 1 AR limits intrinsic myocardial resistance to ischemia. Data indicate the function of intrinsically activated A 1 ARs appears primarily to be enhancement of postischemic contractility and limitation of cell death.

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

Cells of the cardiovascular system generate and release purine nucleoside adenosine in increasing quantities when constituent cells are "stressed" or subjected to injurious stimuli. This increased adenosine can interact with surface receptors in myocardial, vascular, fibroblast, and inflammatory cells to modulate cellular function and phenotype. Additionally, adenosine is rapidly reincorporated back into 5'-AMP to maintain the adenine nucleotide pool. Via these receptor-dependent and independent (metabolic) paths, adenosine can substantially modify the acute response to ischemic insult, in addition to generating a more sustained ischemia-tolerant phenotype (preconditioning). However, the molecular basis for acute adenosinergic cardioprotection remains incompletely understood and may well differ from more widely studied preconditioning. Here we review current knowledge and some controversies regarding acute cardioprotection via adenosine and adenosine receptor activation.

Ischaemic tolerance in aged mouse myocardium: the role of adenosine and effects of A1 adenosine receptor overexpression

The genesis of the ischaemia intolerant phenotype in aged myocardium is poorly understood. We tested the hypothesis that impaired adenosine-mediated protection contributes to ischaemic intolerance, and examined whether this is countered by A1 adenosine receptor (A1AR) overexpression. Responses to 20 min ischaemia and 45 min reperfusion were assessed in perfused hearts from young (2-4 months) and moderately aged (16-18 months) mice. Post-ischaemic contractility was impaired by ageing with elevated ventricular diastolic (32 +/- 2 vs. 18 +/- 2 mmHg in young) and reduced developed (37 +/- 3 vs. 83 +/- 6 mmHg in young) pressures. Lactate dehydrogenase (LDH) loss was exaggerated (27 +/- 2 vs. 16 +/- 2 IU g-1 in young) whereas the incidence of tachyarrhythmias was similar in young (15 +/- 1 %) and aged hearts (16 +/- 1 %). Functional analysis confirmed equipotent effects of 50 micro M adenosine at A1 and A2 receptors in young and aged hearts. Nonetheless, while 50 micro M adenosine improved diastolic (5 +/- 1 mmHg) and developed pressures (134 +/- 7 mmHg) and LDH loss (6 +/- 2 IU g-1) in young hearts, it did not alter these variables in the aged group. Adenosine did attenuate arrhythmogenesis for both ages (to ~10 %). In contrast to adenosine, 50 micro M diazoxide reduced ischaemic damage and arrhythmogenesis for both ages. Contractile and anti-necrotic effects of adenosine were limited by 100 micro M 5-hydroxydecanoate (5-HD) and 3 micro M chelerythrine. Anti-arrhythmic effects were limited by 5-HD but not chelerythrine. Non-selective (100 micro M 8-sulfophenyltheophylline) and A1-selective (150 nM 8-cyclopentyl-1,3-dipropylxanthine) adenosine receptor antagonism impaired ischaemic tolerance in young but not aged hearts. Quantitative real-time PCR and radioligand analysis indicated that impaired protection is unrelated to changes in A1AR mRNA transcription, or receptor density (~8 fmol mg-1 protein in both age groups). However, A1AR overexpression improved tolerance for both ages, restoring adenosine-mediated protection. These data reveal impaired protection via exogenous and endogenous adenosine contributes to ischaemic intolerance with ageing. This is independent of A1AR expression, and involves ineffective activation of a 5-HD-/diazoxide-sensitive process. The effects of A1AR overexpression indicate that the age-related failure in signalling can be overcome.

Pyruvate-dependent preconditioning and cardioprotection in murine myocardium

1. Whether pyruvate inhibits or can actually initiate myocardial preconditioning is unclear and whether pyruvate provides protection via its action as a 'cosubstrate' with glucose or via alternative mechanisms also remains controversial. We examined effects of a high concentration of pyruvate (10 mmol/L) alone or with 15 mmol/L glucose in mouse hearts subjected to 20 min ischaemia and 30 min reperfusion. 2. Provision of 10 mmol/L pyruvate alone or as a cosubstrate markedly reduced ischaemic contracture and enhanced postischaemic recovery. Time to contracture was increased from approximately 3 min to over 8 min, peak contracture was reduced from 90 mmHg to less than 60 mmHg and postischaemic pressure development was also improved. Effects on contracture were independent of the presence of pyruvate during ischaemia and improved postischaemic recovery was evident with pre-ischaemic pyruvate perfusion. 3. Cardioprotection did not require the presence of pyruvate during ischaemia or reperfusion and effects of pyruvate pretreatment could be mimicked by pretreatment with 1 mmol/L dichloroacetate (DCA), an activator of pyruvate dehydrogenase. 4. Myocardial adenosine efflux and Ca2+ content were elevated (by 215 and 65%, respectively) following pretreatment with pyruvate, potentially triggering a preconditioned state. A role for adenosine A1 receptors is supported by lack of added protection with pyruvate in hearts transgenically overexpressing adenosine A1 receptors. 5. Collectively, these observations demonstrate that pre-ischaemic treatment with pyruvate or DCA provides a beneficial preconditioning-like effect in ischaemic and postischaemic myocardium. The response appears unrelated to glycolytic inhibition, but may be mediated via transient changes in adenosine levels and/or cellular Ca2+.

Receptor and non-receptor-dependent mechanisms of cardioprotection with adenosine

The relative roles of mitochondrial (mito) ATP-sensitive K(+) (mitoK(ATP)) channels, protein kinase C (PKC), and adenosine kinase (AK) in adenosine-mediated protection were assessed in Langendorff-perfused mouse hearts subjected to 20-min ischemia and 45-min reperfusion. Control hearts recovered 72 +/- 3 mmHg of ventricular pressure (50% preischemia) and released 23 +/- 2 IU/g lactate dehydrogenase (LDH). Adenosine (50 microM) during ischemia-reperfusion improved recovery (149 +/- 8 mmHg) and reduced LDH efflux (5 +/- 1 IU/g). Treatment during ischemia alone was less effective. Treatment with 50 microM diazoxide (mitoK(ATP) opener) during ischemia and reperfusion enhanced recovery and was equally effective during ischemia alone. A(3) agonism [100 nM 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide], A(1) agonism (N(6)-cyclohexyladenosine), and AK inhibition (10 microM iodotubercidin) all reduced necrosis to the same extent as adenosine, but less effectively reduced contractile dysfunction. These responses were abolished by 100 microM 5-hydroxydecanoate (5-HD, mitoK(ATP) channel blocker) or 3 microM chelerythrine (PKC inhibitor). However, the protective effects of adenosine during ischemia-reperfusion were resistant to 5-HD and chelerythrine and only abolished when inhibitors were coinfused with iodotubercidin. Data indicate adenosine-mediated protection via A(1)/A(3) adenosine receptors is mitoK(ATP) channel and PKC dependent, with evidence for a downstream location of PKC. Adenosine provides additional and substantial protection via phosphorylation to 5'-AMP, primarily during reperfusion.

Adenosine-mediated cardioprotection in ischemic-reperfused mouse heart

We investigated the roles of A1, A2A, or A3 receptors and purine salvage in cardioprotection with exogenous adenosine, and tested whether A2A -mediated reductions in perfusion pressure modify post-ischemic recovery. Treatment with 10(-5) or 5 x 10(-5) M adenosine improved contractile recovery from 20 min ischemia 45 min reperfusion in isolated mouse hearts. Protection was attenuated by adenosine kinase inhibition (10(-5) M iodotubercidin) and receptor antagonism (5 x 10(-5) M 8-rho-sulfophenyltheophylline, 8-SPT). Enzyme efflux mirrored contractile recoveries. A 3 agonism with 10(-7) M 2-chloro- N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) improved ischemic tolerance whereas A1 agonism with 5 x 10(-8) M N6-cyclopentyladenosine (CPA) and A2A agonism with 10(-9) M 2-[p-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680) or 2 x 10(-8) M methyl-4-(3-[9-[4S,5S,2R,3R)-5-(N-ethylcarbamoyl)-3,4-dihydroxyoxolan-2-yl]-6-aminopurin-2-yl)]prop-2-ynyl) cyclohexane-carboxylate (ATL-146e) were ineffective. Protection via A1 receptor overexpression was enhanced by adenosine, but unaltered by A1 or A2A agonists. Finally, post-ischemic dysfunction in hearts perfused at constant flow was dependent on coronary pressure, with A2A AR-mediated reductions in pressure reducing diastolic contracture, and elevated perfusion pressure worsening contracture. Data indicate that cardioprotection with exogenous adenosine in asanguinous hearts involves purine salvage and activation of A3 but not A1 or A2A receptors.

Functional properties and responses to ischaemia-reperfusion in Langendorff perfused mouse heart

Despite minimal model characterisation Langendorff perfused murine hearts are increasingly employed in cardiovascular research, and particularly in studies of myocardial ischaemia and reperfusion. Reported contractility remains poor and ischaemic recoveries variable. We characterised function in C57/BL6 mouse hearts using a ventricular balloon or apicobasal displacement and assessed responses to 10-30 min global ischaemia. We examined the functional effects of pacing, ventricular balloon design, perfusate filtration, [Ca(2+)] and temperature. Contractility was high in isovolumically functioning mouse hearts (measured as the change in pressure with time (+dP/dt), 6000-7000 mmHg s(-1)) and was optimal at a heart rate of approximately 420 beats min(-1), with the vasculature sub-maximally dilated, and the cellular energy state high. Post-ischaemic recovery (after 40 min reperfusion) was related to the ischaemic duration: developed pressure recovered by 82 +/- 5 %, 73 +/- 4 %, 68 +/- 3 %, 57 +/- 2 % and 41 +/- 5 % after 10, 15, 20, 25 and 30 min ischaemia, respectively. Ventricular compliance and elastance were both reduced post-ischaemia. Post-ischaemic recoveries were lower in the apicobasal model (80 +/- 4 %, 58 +/- 7 %, 40 +/- 3 %, 32 +/- 7 % and 25 +/- 5 %) despite greater reflow and lower metabolic rate (pre-ischaemic myocardial O(2) consumption (V(O2,myo)) 127 +/- 15 vs. 198 +/- 17 microl O(2) min(-1) g(-1)), contracture, enzyme and purine efflux. Electrical pacing slowed recovery in both models, small ventricular balloons (unpressurised volumes < 50-60 microl) artificially depressed ventricular function and recovery from ischaemia, and failure to filter the perfusion fluid to < 0.45 microm depressed pre- and post-ischaemic function. With attention to these various experimental factors, the buffer perfused isovolumically contracting mouse heart is shown to be stable and highly energized, and to possess a high level of contractility. The isovolumic model is more reliable in assessing ischaemic responses than the commonly employed apicobasal model.