Diadenosine tetraphosphate (AP4A) mimics cardioprotective effect of ischemic preconditioning in the rat heart: contribution of KATP channel and PKC

Modulation of P2Y11-related purinergic signaling in inflammation and cardio-metabolic diseases

Chronic inflammation is the hallmark of cardiovascular pathologies with a major role in both disease progression and occurrence of long-term complications. The massive release of ATP during the inflammatory process activates various purinergic receptors, including P2Y11. This receptor is less studied but ubiquitously expressed in all cells relevant for cardiovascular pathology: cardiomyocytes, fibroblasts, endothelial and immune cells. While several studies suggested a potential pro-inflammatory role for P2Y11 receptors, recent literature data are supportive of an anti-inflammatory profile characterized by the immunosuppression of dendritic cells, inhibition of fibroblast proliferation and of cytokines and ATP secretion. Moreover, modulation of its activity appears to mediate the positive inotropic effect of ATP and mitigate endothelial dysfunction, thus rendering this receptor a promising therapeutic target in the cardiovascular disease armamentarium. The aim of the present review is to summarize the current available knowledge on P2Y11-related purinergic signaling in the setting of inflammation and cardio-metabolic diseases.

Current knowledge on the role of P2Y receptors in cardioprotection against ischemia-reperfusion

During ischemia, numerous effective endogenous extracellular mediators have been identified, particularly, nucleosides such as adenosine as well as purinergic and pyrimidinergic nucleotides. They may play important regulatory roles within the cardiovascular system and notably as cardio-protectants. Indeed, the distribution of the P2Y receptors in mammalian heart includes several cellular constituents relevant for the pathophysiology of myocardial ischemia. Beside the well-known cardioprotective effect of adenosine, the additional protective role of P2Y receptors has emerged. However, interpretation of experimental results may be sometimes perplexing. This is due to the variability of: the experimental models, the endpoints criteria, the chemical structure of agonist and antagonist ligands and their concentrations, the sequences of drug administration with respect to the model used (before and/or during and/or after ischemia). The net effect may be in the opposite direction after a transient or a prolonged stimulation. Nevertheless, the overall reading of published data highlights the beneficial role of the P2Y_2/4 receptor stimulation, the useful and synergistic role of P2Y_6/11 receptor activation and even of the P2Y₁₁ receptor alone in cardioprotection. More, the P2Y₁₁ receptor could be involved in counter-regulation of profibrotic processes. Paradoxically, transient P2X₇ receptor stimulation could contribute to the net cardioprotective effect of ATP. Recently, experimental data have shown that blocking the P2Y₁₂ receptor after ischemia confers cardioprotection independently of platelet antiaggregatory effect. This suggests for P2Y receptors an important role in primary prevention and as a therapeutic target in myocardial protection during ischemia and reperfusion.

Quantification of diadenosine polyphosphates in blood plasma using a tandem boronate affinity-ion exchange chromatography system

Endogenous diadenosine polyphosphates (Ap(n)As) have been associated with a variety of biological effects but quantifying their concentration in blood is difficult. We report on the development of a tandem affinity-ion exchange high-performance liquid chromatography (HPLC) system that employs boronate affinity upstream of ion exchange chromatography for automated rapid (45-min) resolution and extraction of Ap(n)As from human plasma. This system obviates previous requirements for multiple column separations and handling steps, so it is ideally set up for time- and cost-efficient screening of blood samples for Ap(n)A pharmacokinetic and biodistribution studies.

Diadenosine tetraphosphate stimulates atrial ANP release via A(1) receptor: involvement of K(ATP) channel and PKC

Diadenosine polyphosphates (APnAs) are endogenous compounds and exert diverse cardiovascular functions. However, the effects of APnAs on atrial ANP release and contractility have not been studied. In this study, the effects of diadenosine tetraphosphate (AP4A) on atrial ANP release and contractility, and their mechanisms were studied using isolated perfused rat atria. Treatment of atria with AP4A resulted in decreases in atrial contractility and extracellular fluid (ECF) translocation whereas ANP secretion and cAMP levels in perfusate were increased in a dose-dependent manner. These effects of AP4A were attenuated by A(1) receptor antagonist but not by A(2A) or A(3) receptor antagonist. Other purinoceptor antagonists also did not show any effects on AP4A-induced ANF release and contractility. The increment of ANP release and negative inotropy induced by AP4A was similar to those induced by AP3A, AP5A, and AP6A. Protein kinase A inhibitors accentuated AP4A-induced ANP secretion. In contrast, an inhibitor of phospholipase C, protein kinase C or sarcolemma K(ATP) channel completely blocked AP4A-induced ANP secretion. However, an inhibitor of adenylyl cyclase or mitochondria K(ATP) channel had no significant modification of AP4A effects. These results suggest that AP4A regulates atrial inotropy and ANP release mainly through A(1) receptor signaling involving phospholipase C-protein kinase C and sarcolemmal K(ATP) channel and that protein kinase A negatively modulates the effects of AP4A.

Diadenosine polyphosphate analog controls postsynaptic excitation in CA3-CA1 synapses via a nitric oxide-dependent mechanism

Previously, we have described the modulatory effect of diadenosine polyphosphates Ap4A and Ap5A on synaptic transmission in the rat hippocampal slices mediated by presynaptic receptors (Klishin et al., 1994). In contrast, we now describe how nonhydrolyzable Ap4A analog diadenosine-5',5'''-P1,P4-[beta,beta'-methylene]tetraphosphate (AppCH2ppA) at low micromolar concentrations exerts strong nondesensitizing inhibition of orthodromically evoked field potentials (OFPs) without affecting the amplitude of excitatory postsynaptic currents and antidromically evoked field potentials, as recorded in hippocampal CA1 zone. The effects of AppCH2ppA on OFPs are eliminated by a P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) but not mimicked by purinoceptor agonists alpha,beta-methylene-ATP and adenosine 5'-O-(3-thio)-triphosphate, indicating that a P2-like receptor is involved but not one belonging to the conventional P2X/P2Y receptor classes. Diadenosine polyphosphate receptor (P4) antagonist Ip4I (diinosine tetraphosphate) was unable to modulate AppCH2ppA effects. Thus, the PPADS-sensitive P2-like receptor for AppCH2ppA seems to control selectively dendritic excitation of the CA1 neurons. The specific nitric oxide (NO)-scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide is shown to significantly attenuate AppCH2ppA-mediated inhibitory effects, indicating that NO is involved in the cascade of events initiated by AppCH2ppA. Further downstream mediation by adenosine A1 receptors is also demonstrated. Hence, AppCH2ppA-mediated effects involve PPADS-sensitive P2-like receptor activation leading to the production of NO that stimulates intracellular synthesis of adenosine, causing in turn postsynaptic A1 receptor activation and subsequent postsynaptic CA1 dendritic inhibition. Such spatially selective postsynaptic dendritic inhibition may influence dendritic electrogenesis in pyramidal neurons and consequently mediate control of neuronal network activity.

The K(Ca) channel as a trigger for the cardioprotection induced by kappa-opioid receptor stimulation -- its relationship with protein kinase C

We first determined whether the cardioprotection resulting from kappa opioid receptor (kappa-OR) stimulation was blocked by the K(Ca) channel inhibitor, paxilline (Pax), administered before or during ischaemic insults in vitro. In isolated rat hearts, 30 min of ischaemia and 120 min of reperfusion induced infarction and increased lactate dehydrogenase (LDH) release. In isolated ventricular myocytes subjected to 5 min of metabolic inhibition and anoxia followed by 10 min of reperfusion, the percentage of live cells and the amplitude of the electrically induced intracellular Ca(2+) ([Ca(2+)](i)) transient decreased, while diastolic [Ca(2+)](i) increased. Pretreatment with 10 microM U50,488H, a kappa-OR agonist, attenuated the undesirable effects of ischaemic insults in both preparations. The beneficial effects of kappa-OR stimulation, that were abolished by 5 microM nor-BNI, a kappa-OR antagonist, were also abolished by 1 microM Pax administered before ischaemic insults or 20 microM atractyloside, an opener of the mitochondrial permeability transition pore. Activation of protein kinase C (PKC) with 0.1 microM phorbol 12-myristate 13-acetate decreased the infarct size and LDH release in isolated rat hearts subjected to ischaemia/reperfusion, and these effects were abolished by blockade of PKC with its inhibitors, 10 microM GF109203X or 5 microM chelerythrine, and more importantly by 1 microM Pax. On the other hand, the cardioprotective effects of opening the K(Ca) channel with 10 microM NS1619 were not altered by either PKC inhibitor. In conclusion, the high-conductance K(Ca) channel triggers cardioprotection induced by kappa-OR stimulation that involves inhibition of MPTP opening. The K(Ca) channel is located downstream of PKC.

Positive inotropic and sustained anti-beta-adrenergic effect of diadenosine pentaphosphate in human and guinea pig hearts. Role of dinucleotide receptors and adenosine receptors

AIM

Diadenosine polyphosphates are present intracellularly and in extracellular fluid due to release from secretory vesicles in platelets, chromaffin cells and other cells. This study investigates effects of diadenosine pentaphosphate (AP5A) on heart muscle function.

METHODS

Contractile force amplitude and action potential duration at 90% repolarization (APD90) were measured after challenge with AP5A 50 microm or isoproterenol 50-70 nM in guinea pig papillary muscles. Isoproterenol was given immediately after AP5A-exposure or after 45 min washout. AP5A was combined with antagonists to the purinergic P2 receptor (suramin 100 microm), the dinucleotide receptor [diinosine pentaphosphate 30 microm (IP5I)] or adenosine receptors [8-(P-sulfophenyl) theophylline 50 microm (8-SPT)].

RESULTS

Results are %-change (mean +/- SEM) from value before exposure. AP5A increased contractile force by 22 +/- 3%* (*P <0.05), and IP5I abolished this. AP5A prolonged APD90 by 7 +/- 2%*. AP5A significantly reduced response to isoproterenol acutely from 31 +/- 4* (controls) to 9 +/- 4% and after 45 min washout from 61 +/- 14* (controls) to 16 +/- 5%. 8-SPT abolished the sustained effect. Increase in contractile force by AP5A was confirmed in human atria trabecula preparations.

CONCLUSION

AP5A increased contractile force and prolonged APD90. Contractile force increased by stimulation of the dinucleotide receptor in guinea pig myocardium. The sustained anti-beta-adrenergic effect of AP5A was due to adenosine receptor stimulation.

Endogenous diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate in human myocardial tissue

Diadenosine polyphosphates have been characterized as extracellular mediators controlling numerous physiological effects. In this study, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate were isolated and identified in human myocardial tissue. Human myocardial tissue was homogenized and fractionated by affinity chromatography, displacement chromatography, anion-exchange chromatography, and reversed-phase chromatography. In fractions purified to homogeneity, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate were revealed by matrix-assisted laser desorption/ionization mass spectrometry and ultraviolet spectroscopy. These diadenosine polyphosphates were further identified by enzymatic analysis, which demonstrated an interconnection of the phosphate groups with the adenosines in the 5' positions of the riboses. Furthermore, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate were found in human cardiac-specific granules, and the amount of diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate was estimated in the range of approximately 500 micromol/L. In conclusion, the experiments show that the diadenosine polyphosphates with 2 and 3 phosphate groups occur in human myocardial tissue, and so do the diadenosine polyphosphates with 4 to 6 phosphate groups. After being released by cholinergic stimulation, which is known to affect diadenosine polyphosphate release from secretory granules, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate activate P2X purinoceptors in vascular smooth muscle; hence, they can act as vasoconstrictors. It may be inferred that the differential action of both predominantly vasodilator and vasoconstrictor diadenosine polyphosphates allow a fine-tuning of myocardial blood flow by locally released diadenosine polyphosphates.

Cardioprotection of Interleukin-2 Is Mediated via κ-Opioid Receptors

We examined whether interleukin-2 (IL-2) protects the myocardium against injury induced by ischemia and reperfusion via the kappa-opioid receptor (OR). The cardioprotective effect of IL-2 was evaluated by measuring infarct size and lactate dehydrogenase (LDH) release in response to ischemia and reperfusion in the isolated rat heart. IL-2 at an optimal dose of 50 U/ml mimicked the effect of ischemic preconditioning by reducing infarct size and LDH release. The infarct and LDH-reducing effects of IL-2 were blocked by nor-binaltorphimine (5 microM), a kappa-OR antagonist, but not naltrindole (5 microM), a delta-OR antagonist known to block the action of its stimulation. Moreover, blockade of the mitochondrial ATP-sensitive potassium (mito-K(ATP)) channel with a selective antagonist, 5-hydroxydecanoate (100 microM), or a nonselective antagonist of K(ATP) channels, glybenclamide (100 microM), or blockade of protein kinase C (PKC) with its inhibitors chelerythrine (5 microM) or GF 109203X (10 microM) [3-[1-[3-(dimethylaminopropyl]-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione monohydrochloride] abolished the protective effect of IL-2. Administration of free radical scavengers N-acetylcysteine (4 mM) or N-(2-mercaptopropionyl)-glycine (1 mM) also abolished the protective effects of IL-2 and U50,488H [(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide], a selective kappa-OR agonist. This study provides the first evidence that IL-2 confers cardioprotection against injury induced by ischemia/reperfusion. The effect of IL-2 is mediated via kappa-OR as evidenced by kappa-OR antagonism and similar signaling mechanisms, mito-K(ATP), PKC, and reactive oxygen species involved in the cardioprotective effects of both IL-2 and kappa-OR stimulation.

Diadenosine tetraphosphate protects against injuries induced by ischemia and 6-hydroxydopamine in rat brain

Diadenosine tetraphosphate (AP4A), an endogenous diadenosine polyphosphate, reduces ischemic injury in the heart. In this study, we report the potent and protective effects of AP4A in rodent models of stroke and Parkinson's disease. AP4A, given intracerebroventricularly before middle cerebral artery (MCA) ligation, reduced cerebral infarction size and enhanced locomotor activity in adult rats. The intravenous administration of AP4A also induced protection when given early after MCA ligation. AP4A suppressed terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) induced by hypoxia/reperfusion in primary cortical cultures, and reduced both ischemia-induced translocation of mitochondrial cytochrome c and the increase in cytoplasmic caspase-3 activity in vivo. The purinergic P2/P4 antagonist di-inosine pentaphosphate or P1-receptor antagonist sulfonylphenyl theophylline, but not the P2-receptor antagonist suramin, antagonized the effect of AP4A, suggesting that the observed protection is mediated through an anti-apoptotic mechanism and the activation of P1- and P4-purinergic receptors. AP4A also afforded protection from toxicity induced by unilateral medial forebrain bundle injection of 6-hydroxydopamine (6-OHDA). One month after lesioning, vehicle-treated rats exhibited amphetamine-induced rotation. Minimal tyrosine hydroxylase immunoreactivity was detected in the lesioned nigra or striatum. No KCl-induced dopamine release was found in the lesioned striatum. All of these indices of dopaminergic degeneration were attenuated by pretreatment with AP4A. In addition, AP4A reduced TUNEL in the lesioned nigra 2 d after 6-OHDA administration. Collectively, our data suggest that AP4A is protective against neuronal injuries induced by ischemia or 6-OHDA through the inhibition of apoptosis. We propose that AP4A may be a potentially useful target molecule in the therapy of stroke and Parkinson's disease.

Changes in extracellular pH and myocardial ischaemia alter the cardiac effects of diadenosine tetraphosphate and pentaphosphate

1. The structural conformation of diadenosine tetraphosphate (Ap(4)A) and pentaphosphate (Ap(5)A) has been reported to alter as pH is reduced. As such, it is possible that the cardiac effects of Ap(4)A and Ap(5)A vary during acidosis and myocardial ischaemia due to changes in ligand structure, receptor proteins or intracellular signalling. 2. We investigated whether the cardiac electrophysiological and coronary vasomotor effects of Ap(4)A and Ap(5)A are preserved under conditions of extracellular acidosis (pH 6.5) and alkalosis (pH 8.5) and whether Ap(4)A has any electrophysiological or antiarrhythmic effects during ischaemia. 3. Transmembrane right ventricular action potentials, refractory periods and coronary perfusion pressure were recorded from isolated, Langendorff-perfused guinea-pig hearts under constant flow conditions. The effects of 1 nM and 1 microM Ap(4)A and Ap(5)A were studied at pH 7.4, 6.5 and 8.5. The effects of 1 microM Ap(4)A were studied during global low-flow ischaemia and reperfusion. 4. At pH 7.4, Ap(4)A and Ap(5)A increased action potential duration (APD(95)) and refractory period (RP) and reduced coronary perfusion pressure. The electrophysiological effects were absent at pH 6.5 while the reductions in perfusion pressure were attenuated. At pH 8.5, Ap(4)A increased RP but the effects of Ap(4)A and Ap(5)A on perfusion pressure were attenuated. During ischaemia, Ap(4)A had no antiarrhythmic or electrophysiological effects. 5. These data demonstrate the importance of extracellular pH in influencing the effects of Ap(4)A and Ap(5)A on the heart and indicate that any potentially cardioprotective effects of these compounds during normal perfusion at physiological pH are absent during ischaemia.

Myocardial protection using diadenosine tetraphosphate with pharmacological preconditioning

BACKGROUND

We have reported a similar cardioprotective effect and mechanism of diadenosine tetraphosphate (AP4A) and ischemic preconditioning in rat hearts. In this study, the applicability of AP4A administration to cardiac surgery was tested by using a canine cardiopulmonary bypass model.

METHODS

Hearts underwent 60 minutes of cardioplegic arrest (34 degrees C) by a single dose of cardioplegia. Cardioplegia contained either AP4A (40 micromol/L; n = 6) or saline (n = 6). Beagles were weaned from cardiopulmonary bypass 30 minutes after reperfusion, and left ventricular function was evaluated after another 30 minutes by using the cardiac loop analysis system.

RESULTS

Administration of AP4A significantly improved the postischemic recovery of cardiac function and reduced the leakage of serum creatine kinase compared with saline. Systemic vascular resistance, mean aortic blood pressure, and the electrocardiographic indices were not significantly altered by AP4A administration.

CONCLUSIONS

Administration of AP4A was cardioprotective without apparent adverse effects. Because the cardioprotective mechanism may be similar to that of ischemic preconditioning, the addition of AP4A into cardioplegia may be a novel safe method for clinical application of preconditioning cardioprotection.