Cardiovascular Research

Cardiovascular Research 1999 42(1):15-26; doi:10.1016/S0008-6363(99)00004-8
© 1999 by European Society of Cardiology

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Copyright © 1999, European Society of Cardiology

The effects of diadenosine polyphosphates on the cardiovascular system

Nicholas A. Flores^*, Brigitte M. Stavrou and Desmond J. Sheridan

Academic Cardiology Unit, National Heart and Lung Institute, Imperial College School of Medicine, London W2 1NY, UK

n.flores{at}ic.ac.uk

^* Corresponding author. Tel.: +44-171-886-6129/6267; fax: +44-171-886-6732

Received 7 April 1998; accepted 16 December 1998

	Abstract

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
Diadenosine polyphosphates are members of a group of dinucleoside polyphosphates that are ubiquitous, naturally occurring molecules. They form a recently identified class of compounds derived from ATP and consist of two adenosine molecules bridged by up to six phosphate groups. These compounds are stored in high concentrations in platelet dense granules and are released when platelets become activated. Some of the compounds promote platelet aggregation, while others are inhibitory. Possible roles as neurotransmitters, extracellular signalling molecules or ‘alarmones’ secreted by cells in response to physiologically stressful stimuli have been postulated. Recent studies suggest a role for these compounds in atrial and synaptic neurotransmission. Studies using isolated mesenteric arteries indicate an important role of phosphate chain length in determining whether diadenosine polyphosphates produce vasodilatation or vasoconstriction, but in the coronary circulation, diadenosine polyphosphates generally produce vasodilatation via mechanisms thought to involve release of NO or prostacyclin (PGI₂). They produce cardiac electrophysiological effects by altering ventricular refractoriness at submicromolar concentrations and reduce heart rate. Mechanisms involving K_ATP channels have been proposed in addition to the involvement of P1- and P2-purinergic receptors and the specific diadenosine polyphosphate receptor identified on isolated cardiac myocytes. Clinical evidence suggests a role for diadenosine polyphosphates in hypertensive patients and those with the Chédiak–Higashi syndrome. This review outlines the effects of these compounds on the cardiovascular system and considers their potential involvement in mediating the pathophysiological effects associated with platelet activation during myocardial ischaemia.

KEYWORDS Vasoconstriction/dilation; Bradycardia; K-ATP channel; Platelets; Haemodynamics

	1. Introduction – the physiological role of diadenosine polyphosphates

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
Diadenosine polyphosphates are members of a group of dinucleoside polyphosphates that are ubiquitous, naturally occurring molecules [1]. They form a recently identified class of compounds derived from ATP and consist of two adenosine molecules bridged by two to six phosphate groups [1], see Fig. 1. Diadenosine triphosphate (AP₃A), diadenosine tetraphosphate (AP₄A), diadenosine pentaphosphate (AP₅A) and diadenosine hexaphosphate (AP₆A) occur naturally, while the synthetic compound diadenosine pyrophosphate (AP₂A) completes the sequence. The naturally occurring compounds are stored in high concentrations in platelet dense granules [1–3] and are released when platelets become activated. They are also stored in chromaffin granules of the adrenal medulla, cholinergic synaptic vesicles, midbrain synaptosomes, the caudate putamen and neostriatum of rats (reviewed in Ref. [4]) and act as second messengers mediating the glucose-induced blockade of the ATP-regulated K⁺ channel in the pancreatic β-cell [5]. Possible roles as neurotransmitters, extracellular signalling molecules or ‘alarmones’ secreted by cells in response to physiologically stressful stimuli (metabolic or oxidative stress, or heat shock) have been postulated [1–3,6–8]. Recent studies suggest a role for these compounds in neurotransmission [9,10] and this has attracted much interest. AP₂A has been studied by some investigators, but the majority have focussed on the naturally occurring compounds. Somewhat less attention has been given to the effects of diadenosine polyphosphates on the cardiovascular system even though their ability to alter coronary vascular resistance and cardiac electrophysiology suggests a potentially important role in the heart [11,12]. The aim of this review is to outline the effects of diadenosine polyphosphates on the cardiovascular system and discuss their physiological and pathophysiological involvement in mediating the effects associated with platelet activation during myocardial ischaemia.

View larger version (16K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 1 Diagrammatic representation of the structure of diadenosine polyphosphates (diadenosine tetraphosphate, AP4A as an example, upper panel) and their relation to adenosine, AMP, ADP and ATP (lower panel). Hydrolysis of ATP at the points indicated by z. scis yields ADP and orthophosphate or AMP and pyrophosphate, etc. It is thus easy to see how hydrolysis of diadenosine polyphosphates yields AMP and ADP or ATP depending on their structure (see Fig. 2). These metabolites may be partly responsible for the cardiovascular effects of diadenosine polyphosphates although they are known to be more resistant to hydrolysis than the nucleotides.

 

	2. Biochemical synthesis and metabolism of diadenosine polyphosphates

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
The biochemical pathways involved in the synthesis and degradation of diadenosine polyphosphates have been outlined in [1,13]. Briefly, the synthetic pathways of diadenosine polyphosphates involve the actions of aminoacyl-tRNA synthetases [1,7,14,15], see Fig. 2. Synthesis of diadenosine polyphosphates occurs in megakaryocytes, the progeny of which is platelets [16].

View larger version (30K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 2 Schematic representation of the synthesis (panel 1) and metabolism (panels 2 and 3) of diadenosine polyphosphates. Synthesis of diadenosine tetraphosphate (AP4A) involves the actions of aminoacyl-tRNA synthetase and occurs in megakaryocytes [1,7,14–16]. Diadenosine polyphosphate hydrolases (a), present in plasma, convert diadenosine polyphosphates (APnA) to AMP and the corresponding adenosine polyphosphate (Apn–1) [19–21]. Adenosine pentaphosphate (Ap5) and adenosine tetraphosphate (Ap4) are only slowly hydrolyzed (dashed line) [20] and it is not firmly established to what extent they are metabolized further [20] (dashed line leading to a question mark). Ap4 can inhibit the hydrolysis of AP4A and the actions of the ATPase (b) [20] (dashed lines leading to a X to indicate enzyme inhibition). ATP and ADP also inhibit the diadenosine polyphosphate hydrolase (not indicated) [24]. ATPases which degrade ATP to ADP (b) are present on erythrocytes, leukocytes and platelets (illustrated diagrammatically) [24]. ADP can be metabolized further to AMP via adenylate kinase (c) and AMP can be converted to adenosine (ado) by 5'-nucleotidase (d). Adenosine can be converted to inosine by the actions of adenosine deaminase (e) and inosine can be metabolized further to hypoxanthine via the actions of purine nucleoside phosphorylases (not indicated). Adenosine and inosine can then be taken up (solid line) via the nucleoside transporter in cardiac myocytes (panel 3) and rephosphorylated back to AMP via adenosine kinase (f), to ADP and to ATP [via the nucleoside diphosphate kinase (g)]. Diadenosine polyphosphates themselves may penetrate intact cell membranes via undetermined mechanisms [26–29] (dashed line labelled with a question mark traversing panels 2 and 3).

 
From the perspective of the cardiovascular system, knowledge of their metabolism has come mainly from haematological studies which have shown that they are hydrolyzed asymmetrically, producing AMP and ADP or ATP, depending on their structure [17], see Figs. 1 and 2. (Hydrolysis can be symmetrical in prokaryotes [1].) Lüthje et al. [18] reported that a AP₃A hydrolyzing enzyme (termed AP₃A hydrolase), present in plasma, converts AP₃A to ADP and AMP while the same enzyme is capable of hydrolyzing AP₄A to produce ATP and AMP [19]. The hydrolase enzyme requires divalent metal cations for catalytic activity [19] and Zn²⁺ is thought to be the normal physiological ion while fluoride is an inhibitor [20].

The hydrolase is regarded as a 5'-nucleotide phosphodiesterase and three AP₃Aase/AP₄Aase isoenzymes have been identified in human serum [21]. A Mg²⁺-dependent enzyme which is specific for AP₄A and does not hydrolyze AP₃A (termed diadenosine tetraphosphatase) has been isolated from a human leukaemic cell line [22].

2.1 Stability
AP₃A and AP₄A are more stable than ATP and ADP [18,23] and studies by Lüthje and Ogilvie [24] have shown that ATP and ADP can affect the rate of hydrolysis of AP₃A and AP₄A in blood by competitively inhibiting the AP_nA hydrolase. The importance of this is related to the relative concentrations of ATP and AP_nA released from platelet dense granules (a ratio of between 20 and 40 to 1) [16] and demonstrates that AP₃A and AP₄A are long-lived molecules which diffuse away from the site of thrombus formation without being destroyed, enabling them to act as long-range signalling molecules [24]. The relative stability of AP_nA is also due to the fact that red blood cells, platelets and leukocytes lack ectoenzymes capable of metabolizing AP₃A and AP₄A and that the enzymes responsible are only present in plasma. (The half-life of AP₃A and AP₄A is longer in whole blood than plasma [24] while the opposite is true for ATP since the ectoenzymes which degrade ATP are present on erythrocytes, leukocytes and platelets [24].) Furthermore, in plasma, AP₄A is cleaved to ATP and AMP which are then metabolized further to adenosine and inosine, but in whole blood, ATP is the main product [24]. Adenosine and inosine can then be taken up via the nucleoside transporter (present in endothelial cells, erythrocytes and cardiac myocytes [25]) and rephosphorylated and converted back to ATP intracellularly [24]. This is illustrated schematically in Fig. 2.

2.2 Cellular uptake and intracellular metabolism
AP₃A and AP₄A themselves do not normally penetrate intact cell membranes [26] but uptake of AP₄A has been reported in tumour cells [27]. Delaney et al. [28] have postulated that the decrease in adenosine levels in the brain induced by AP₄A may be due to actions on adenosine transporters mediated through P2 purinergic receptors but this has not been established. A recent report proposes that diadenosine polyphosphates can be internalized in guinea-pig atrial myocytes by an undetermined mechanism [29]. Previous studies using isolated mouse myocytes demonstrated that diadenosine polyphosphates were not taken up by cardiac myocytes [30] (discussed below), and an explanation for this apparent discrepancy requires further investigation.

Human erythrocytes, leukocytes and platelets have recently been shown to contain an intracellular AP₄A hydrolase which can also hydrolyze AP₅A and AP₆A [31]. The reasons for the presence of the hydrolase in erythrocytes is not clear since they do not synthesize AP₄A and may be related to their previous existence as reticulocytes.

2.3 Endothelial involvement
Ectonucleotidases are also present on the vascular endothelium and they hydrolyze diadenosine polyphosphates [26,32], see Fig. 2. Goldman et al. [26] have described hydrolysis of AP₃A by aortic endothelial cells and how the presence of ATP inhibits its hydrolysis. This prolongs the life of AP₃A in the circulation and contributes to its ability to function as a long-range signalling molecule. Eventually, after ATP and ADP released from platelets have been metabolized by ectoenzymes, the residual, intact AP₃A can then be hydrolyzed, acting as an additional source of ADP to prolong or amplify an initial stimulus to platelet aggregation [26]. In addition to biochemical studies, other functional, physiological studies have commented on the greater stability of diadenosine polyphosphates relative to ATP and this undoubtedly contributes to their physiological role [11,17,33–35].

	3. Diadenosine polyphosphates as neurotransmitters

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
A recent report by Rubino and Burnstock [9] indicates that diadenosine polyphosphates influence neurotransmission in isolated guinea-pig atria. Sensory–motor neurotransmission was evaluated as an increase in contractile tension observed following electrical stimulation and AP₂A, AP₃A, AP₄A, AP₅A and AP₆A at physiological concentrations (0.1 to 30 µM) all reduced the cardiac responses to electrical stimulation and mimicked the negative inotropism of adenosine. The effects of diadenosine polyphosphates on the nervous system have been reviewed recently [1,4,36].

	4. Extracellular and intracellular concentrations of diadenosine polyphosphates

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
AP₃A and AP₄A are released from platelet dense granules as part of the release reaction [16,23,37] such that the concentrations of AP₃A and AP₄A in the circulating blood of an adult following platelet stimulation have been estimated to be of the order of 1 µM [23]. Platelet aggregation results in the release of almost all of the intraplatelet store of diadenosine polyphosphates from an individual platelet, but under basal conditions when platelets are not activated the free concentration of diadenosine polyphosphates in the plasma is low, like that of ADP or ATP [23]. Concentrations within platelet granules are of the order of 1 pmol/10⁶ platelets while the concentrations of ADP and ATP are 20–40-fold higher [16,38]. The intraplatelet concentrations of diadenosine polyphosphates have been reported to vary with platelet size [38] and since larger platelets are considered more haemostatically reactive (reviewed by Brown and Martin [39]) this may have implications under pathological conditions when locally elevated concentrations of diadenosine polyphosphates result. These have not been measured directly, but the suggestion made by Ogilvie [40] that, since platelets aggregate in a limited place, local concentrations could be much higher than 1 µM and that in the microenvironment of a platelet thrombus adherent to a damaged vessel wall concentrations of diadenosine polyphosphates could be 100 µM or higher initially. It is important to remember that there is no experimental evidence to confirm this and that experimental studies employing excessive extracellular concentrations of diadenosine polyphosphates are consequently of questionable relevance, even pathologically.

In normally growing cells the intracellular concentration of AP₄A is between 0.03 and 1.2 µM [15]. In organisms such as Salmonella typhimurium, Saccharomyces cerevisiae and Escherichia coli, intracellular concentrations of diadenosine polyphosphates as high as 100 µM have been reported under conditions of heat shock [41]. Concentrations this high are not found in mammalian cells.

When considering the effects of diadenosine polyphosphates it is therefore important to differentiate between potential systemic and local effects and in view of the above, local effects can be considered as occurring at concentrations up to 100 µM and systemic effects as occurring at concentrations from 0.1 nM up to 1 µM.

	5. Effects of diadenosine polyphosphates on platelet function

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
AP₃A promotes platelet aggregation at concentrations greater than 0.1 µM which is partly due to its slow hydrolysis to ADP (which is proaggregatory) and at 5 µM it synergizes with other agonists such as platelet activating factor [18,37]. AP₄A has inhibitory actions at 5 µM [18,37] which is partly due to its slow hydrolysis to ATP which is antiaggregatory, or to a competitive antagonistic effect at platelet ADP (P2T) receptors [18]. AP₅A and AP₆A have recently been identified in platelet dense granules [42]. AP₅A at 100 µM is a strong inhibitor of platelet aggregation to ADP producing greater inhibition than AP₄A and AP₆A [43]. Analogues of AP₄A have been synthesized which are resistant to hydrolysis and are approximately ten-times more potent than AP₄A with regard to inhibition of platelet aggregation [14,44,45].

	6. Effects of diadenosine polyphosphates

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
6.1 Effects on smooth muscle
AP₅A and AP₆A at 10 µM increase cytosolic free Ca²⁺ concentrations in aortic smooth muscle [46] indicating the ability of these compounds to alter the contractile properties of muscle [42]. The effects of diadenosine polyphosphates on other types of smooth muscle have been reviewed in Ref. [4].

6.2 Effects on the mesenteric circulation
In isolated rabbit mesenteric arteries, Busse et al. [17] reported how removal of the endothelium was capable of converting the vasodilatation seen in response to AP₄A (1–10 µM) to vasoconstriction, while endothelial removal had no effect on the vasodilator response to AP₃A. In vessels with intact endothelium the vasodilator responses to AP₃A were approximately 50% greater than responses to AP₄A suggesting the involvement of different mechanisms in producing them. Other studies using the same vascular bed have described how the presence or absence of endothelium influences the direction of the response to diadenosine polyphosphates [47]. A similar suggestion of different mechanisms controlling vasodilatation in response to AP₃A and AP₄A has been made for the coronary circulation [11] (discussed below).

Studies using the isolated perfused rat mesenteric arterial bed have reported an important role of phosphate chain length in determining whether diadenosine polyphosphates in the physiologically relevant nanomolar to micromolar range have vasodilator or vasoconstrictor actions [47] mediated by P2Y- or P2X-receptors respectively (Fig. 3) due to the total negative charge presented by each molecule. The classification of purinoceptors is reviewed below, but briefly, P2X-receptors, found in vascular smooth muscle cells and which mediate vasoconstriction, are coupled to receptor-operated ion channels with charge being important in mediating activation of these receptors, while P2Y-receptors, found in vascular smooth muscle and endothelial cells [48] are most often coupled to G-proteins [49]. Vasodilator responses, dependent on endothelial P2Y-receptors, can be converted to vasoconstriction if the endothelium is removed [48,50], but vasoconstrictor responses are endothelium-independent [48].

View larger version (21K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 3 Diagrammatic representation of the interaction of diadenosine polyphosphates (APnA) with purinergic receptors. Known interactions with defined receptors are indicated by bold arrows and bold type. Postulated interactions for which there is some experimental evidence are indicated by dashed arrows but these may be due to hydrolysis to ATP, ADP and or adenosine, indicated by the normal arrows. Proposed mechanisms for the cardiovascular effects of diadenosine polyphosphates include those involving ATP-dependent potassium channels (KATP), and release of nitric oxide (NO) or prostacyclin (PGI2) from the vascular endothelium, independently of actions at purinergic receptors, indicated by the dotted arrow.

 
6.3 Effects on the renal and hepatic circulations
The renal circulation is particularly sensitive to vasoconstrictor effects of diadenosine polyphosphates. In the isolated buffer perfused rat kidney [42,51] AP₄A, AP₅A and AP₆A (at physiologically relevant concentrations in the nanomolar range) produced vasoconstriction by P2X-receptor dependent mechanisms, while the vasoconstrictive effects of AP₂A and AP₃A (also in the nanomolar to micromolar range) were ascribed to P1-receptor (A₁) dependent mechanisms [51], see Fig. 3. In raised tone preparations, AP₂A and AP₃A induced vasodilatation by P1-receptor (A₂) mechanisms (Fig. 3), but AP₄A induced vasodilatation by P2-receptor mechanisms [51]. Diadenosine polyphosphates appear to activate different receptors in the renal circulation depending on the number of phosphate groups, but the mechanisms seem to differ from those involved in the mesenteric bed [47,51].

In the isolated perfused rat liver, Busshardt et al. [52] described how AP₃A and AP₄A at 10 µM increased portal pressure, stimulated glucose output and induced a redistribution of Ca²⁺ and K⁺ ions. Both AP₃A and AP₄A were hydrolyzed much more slowly than ATP. In contrast to these observations of vasoconstriction, the response of the coronary circulation to diadenosine polyphosphates is a reduction in coronary vascular resistance.

	7. Cardiac effects of diadenosine polyphosphates

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
7.1 Effects on coronary flow
Studies which we and others [11,12,53] have undertaken have described reductions in coronary vascular resistance in response to both AP₃A and AP₄A in isolated hearts. Perfusion of isolated rabbit hearts with physiological concentrations (0.1 to 1 µM) of AP₃A and AP₄A produces vasodilatation via mechanisms thought to involve direct effects on the cells of the vascular wall [11]. Release of nitric oxide (NO) accounts for part of the response to AP₄A [11] and release of prostacyclin (PGI₂) is involved in the response to AP₃A [11], suggesting the involvement of different mechanisms in the vasodilator response of the heart to these two compounds. Studies using isolated guinea-pig hearts perfused under constant flow conditions [12] indicate that AP₃A and AP₄A at concentrations of 1 nM produce a transient vasodilatation which gradually recovers (Fig. 4), but that maintained reductions in coronary perfusion pressure occur at a concentration of 1 µM. These effects occur at concentrations within the physiological range. Infusion of AP₄A into anaesthetized dogs and pigs decreases mean arterial pressure from 20% to 80% of control depending on the concentration infused (72.5 µg kg^–1 min^–1 to 675.7 µg kg^–1 min^–1) [54]. Cardiac output increased with lower concentrations of AP₄A and decreased with higher concentrations [54]. Systemic vascular resistance was markedly decreased by AP₄A with an apparent vasodilator action on arterial resistance vessels [54]. The conclusions derived from these experiments was that AP₄A reduced heart rate through an undetermined mechanism and that the hypotension associated with increasing doses of AP₄A was not associated with a reflex tachycardia [54]. Taken together, these studies indicate that release of diadenosine polyphosphates produces initial, transient changes in coronary perfusion, but as the extent of platelet activation increases, sustained decreases in blood pressure occur and impair myocardial perfusion.

View larger version (20K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 4 Effects of diadenosine triphosphate (AP3A), diadenosine tetraphosphate (AP4A), diadenosine pentaphosphate (AP5A) and diadenosine hexaphosphate (AP6A) on ventricular refractoriness (1 nM, upper panel) and perfusion pressure (1 nM and 1 µM added at the points indicated by the arrows, lower panel) recorded from isolated, constant flow-perfused, electrically paced guinea-pig hearts [12]. Perfusion of the hearts with all compounds increased refractory period and transiently reduced perfusion pressure at a concentration of 1 nM but during perfusion with 1 µM a sustained reduction in perfusion pressure was observed. The mechanisms responsible for these changes are currently under investigation. Data from [12]. *P<0.05 vs. corresponding control (upper panel).

 
In isolated rat hearts AP₅A (at 5 µM) increased coronary flow and left ventricular developed pressure [55], and at 1 to 1000 µg kg^–1 in anaesthetized rats, produced a transient increase in mean arterial pressure, followed by a longer lasting decrease in pressure [56]. Apart from these observations, relatively little is known about the effects of AP₅A and AP₆A on the heart. In isolated guinea-pig hearts, AP₅A and AP₆A produce coronary vasodilatation (transiently with 1 nM but maintained with 1 µM [12]), see Fig. 4. These effects occur at concentrations within the physiological range. With AP₆A at 1 nM the vasodilatation tends to be followed by a sustained vasoconstriction (Fig. 4) suggesting multiphasic behaviour in this model, the mechanism for which requires investigation. The involvement of NO and PGI₂ in mediating the vasodilatation is unknown at present but since NO and PGI₂ are involved to varying extents in the vasodilator response to AP₃A and AP₄A [11], a similar involvement in response to AP₅A and AP₆A is likely but requires confirmation.

7.2 Electrophysiological effects
Studies in electrically paced guinea-pig hearts [12] indicate that AP₃A and AP₄A produce a slowly developing increase in refractory period and action potential duration at concentrations as low as 1 nM, see Fig. 4. Electrophysiological effects of AP₃A have not been reported previously, but infusion of AP₄A into dogs and pigs reduces heart rate only at high concentrations (163.4 µg kg^–1 min^–1) [54,57].

AP₅A (5 µM and 100 µg kg^–1) decreases heart rate in the rat [55,56] and AP₆A (at concentrations of 3 to 10 µM) has negative chronotropic and inotropic effects in human and guinea-pig cardiac tissues [58]. AP₅A and AP₆A increase action potential duration and refractory period at a physiologically relevant concentration of 1 nM in isolated perfused guinea-pig hearts [12], see Fig. 4.

Unpublished observations of Hilderman and Pivorun (cited by Hilderman et al. [59]) report that extracellular addition of AP₄A (at a concentration not stated) to isolated hearts decreases heart rate by a factor of two by a mechanism which has not yet been identified but has been postulated to involve interaction with the AP₄A receptor and transduction of the signal mediated by an ion channel or by activation of the phospholipase-inositol pathway or other second messenger [1].

The involvement of ATP-dependent K⁺ (K_ATP) channels in mediating the electrophysiological and vasodilatory effects of AP₄A in pigs has been proposed [57] since cromakalim (a K_ATP activator) enhanced the reductions in coronary vascular resistance observed with AP₄A, while glibenclamide (a K_ATP inhibitor) inhibited the vasodilator response. At the isolated myocyte level, diadenosine polyphosphates inhibit K_ATP channel activity when applied intracellularly to isolated ventricular myocytes or when cell membranes are studied in the inside-out configuration [60]. Although the concentration required for this effect is unnaturally high (50 µM) for mammalian cells as discussed above, the rationale for these experiments was that since diadenosine polyphosphates are synthesized intracellularly and are structurally related to ATP which gates K_ATP channels, the compounds could conceivably alter channel function which they have been shown to do.

	8. Receptor-mediated effects

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
The pharmacology of cardiovascular purinoceptors has been reviewed extensively by Olsson and Pearson [61] and there is a movement towards revision of the nomenclature [62,63] to accommodate experimental observations of receptors which are responsive to pyrimidine nucleotides and to diadenosine polyphosphates [64]. Briefly, receptors can be classified according to whether they respond to nucleosides such as adenosine (termed P1 and subdivided into A₁, A_{2a and b} and A₃) or nucleotides such as ATP, ADP or uridine 5'-triphosphate (termed P2 and subdivided into P2X, P2Z, P2Y, P2U, P2D, P2I and P2T) [63,65], see Fig. 3. P2X and P2Y (principally involving ligand-gated ion channel receptor mechanisms and G protein-coupled receptor mechanisms respectively) have been subdivided further [63,65], see Fig. 3. AP₄A and AP₅A can interact with P2D receptors [65] or P₄ receptors [10,66], see Fig. 3. The name P₄ receptor is now considered best avoided [62] and the adoption of the term P2Y_Ap4A receptor has been proposed provisionally [67] to describe the receptor for diadenosine polyphosphates, but this is considered by some to be inappropriate [10].

8.1 Evidence for a diadenosine polyphosphate receptor
The effects of AP₄A have been studied in the greatest detail compared to those of the other diadenosine polyphosphates and a specific receptor to AP₄A identified on the surface of isolated cardiac myocytes dissociated from mouse hearts [30,59]. ATP, ADP, AMP and adenosine at concentrations up to 7.5 µM do not bind to this receptor (distinguishing it from P1- and P2-receptors), but AP₅A, in addition to AP₄A, is a competitive agonist (K_D values 0.14 µM and 0.074 µM respectively) [30]. AP₆A can also bind to the receptor but the affinity of the receptor is preferential for AP₄A such that receptor activation enhances only AP₄A binding [68]. In the experiments of Walker et al. [30] AP₄A did not enter the myocytes by either endocytosis or pinocytosis and it was concluded that localization of the receptor to the cell surface supports the suggestion that AP₄A is a modulator of cellular function. Although Walker et al. [30] reported an extracellular mechanism of action they stressed that their data do not eliminate the possibility of an internal receptor in cardiac myocytes and an intracellular effect on K_ATP channels has been demonstrated [29,60]. AP₄A binding is specific, saturable and reversible and can be antagonized by a monoclonal antibody raised against the receptor [30,69,70]. Binding is temperature-dependent, optimal at 20°C and 40% less at 37°C [30]. Baxi et al. [71] reported that diadenosine polyphosphates with >3 phosphate groups bind to the receptor indicating that the number of phosphates between the two adenosines is important in determining the biological effects of these compounds. In endothelial cells, binding of AP₄A to its receptor induces activation of phospholipase C through the inositol triphosphate pathway and is associated with the opening of Ca²⁺ channels [1]. Hoyle [3] proposed that diadenosine polyphosphates interact with receptors distinct from P1- and P2-receptors. Furthermore, receptors activated by diadenosine polyphosphates in neurological tissue are reported to be distinct from those activated by ATP [10].

A recent report of the synthesis of diinosine polyphosphates, which act as antagonists for the diadenosine polyphosphate receptor, indicates a potentially valuable class of compounds which can be used as pharmacological tools with which to investigate the effects of diadenosine polyphosphates in more detail [72].

8.2 Evidence to suggest purinoceptor involvement in the cardiovascular effects of diadenosine polyphosphates
Despite the identification of a diadenosine polyphosphate receptor on isolated cardiac myocytes, studies using intact preparations suggest that some of the cardiovascular effects of diadenosine polyphosphates are mediated by purinoceptors. Hoyle et al. [34] reported that the negative inotropic responses of guinea-pig left atria to diadenosine polyphosphates (10 to 100 µM) are mediated by P1- and P2-receptors. In the rat, the responses to AP₅A were shown to involve P1-receptors [56], while the responses of isolated human and guinea-pig ventricular preparations to AP₆A appear to involve P1-receptors [58]. Studies in other tissues (mesenteric arteries and isolated perfused kidneys) suggest the involvement of P1- and P2Y-receptors [47,51], in addition to the diadenosine polyphosphate receptor [51]. The current status of knowledge of the involvement of receptors and other mechanisms in mediating the effects of diadenosine polyphosphates on the cardiovascular system are summarized in Table 1.

View this table: [in this window] [in a new window]   Table 1 The effects of diadenosine polyphosphates on the cardiovascular system

 

	9. Similarities to the effects of ATP

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
Diadenosine polyphosphates are structurally related to ATP and there are strong parallels between their cardiac electrophysiological and haemodynamic effects where known. Detailed discussion has been made elsewhere [73].

	10. Platelet activation during myocardial ischaemia

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
It is now well established that platelet activation occurs during myocardial ischaemia [74] and there is substantial evidence that platelet activation is a contributory arrhythmogenic mechanism [75]. While there is ample evidence for the potent effects mediated by platelet-derived mediators such as eicosanoids, platelet activating factor, 5-hydroxytryptamine, etc. (reviewed in Ref. [75]) on the cardiovascular system, the pathophysiological role of platelets during myocardial ischaemia is complex and incompletely understood [74]. Release of diadenosine polyphosphates from activated platelets may be involved: AP₃A promotes platelet aggregation [18,37], while AP₄A, AP₅A and AP₆A are inhibitory [18,37,43] as discussed above. What is not clear is how these compounds interact since when platelets become activated all diadenosine polyphosphates stored within an individual platelet are released [16,23], achieving concentrations in the micromolar range [23], and the net effects need to be considered. The effects of AP₅A and AP₆A may be less important relative to those of AP₃A and AP₄A due to the different concentrations required for an effect.

10.1 Electrophysiology and arrhythmogenesis
The effects of diadenosine polyphosphates on cardiac electrophysiology are relevant physiologically as they occur at submicromolar concentrations [12]. Similarities to ATP and the rapid metabolism of ATP to adenosine are noteworthy since the negative chronotropic and dromotropic actions of adenosine have important clinical implications in the pathogenesis of cardiovascular disease and treatment of arrhythmias [61]. Adenosine has cardioprotective effects during myocardial ischaemia (reductions in infarct size, contractile dysfunction, microvascular injury and other markers of reperfusion injury) by virtue of its actions on myocardial conduction, cardiac electrophysiology and coronary blood flow [25,76–78].

Impaired conduction and alterations in ventricular refractoriness are important substrates for arrhythmogenesis [79] and the ability of diadenosine polyphosphates to increase ventricular refractoriness at nanomolar concentrations [12] is important. The increases in refractoriness and action potential duration with nanomolar concentrations are slowly developing (over several minutes) but are sustained unlike the transient changes in perfusion pressure observed with the same concentrations [12]. In addition since the conformation of diadenosine polyphosphates changes depending on pH, their binding ability and their effects are likely to change during ischaemia when pH and P_O2 are reduced [47]. Experimental studies to compare their effects during acidosis, alkalosis and alterations in oxygen tensions are needed to investigate this in more detail.

10.2 Therapeutic potential of diadenosine polyphosphate analogues and antagonists
Development of compounds to mimic the beneficial effects or to block the deleterious effects of diadenosine polyphosphates on the cardiovascular system is potentially useful and in progress [14,44,45]. As part of this process, it will be important to understand the mechanisms of action of diadenosine polyphosphates on cardiovascular physiology and pathophysiology as reviewed here.

The development of specific antagonists of the effects of diadenosine polyphosphates will be difficult because of the multiple role that these compounds have (Table 1). Additionally, the relative involvement of P1- and P2-receptors and their subclasses in mediating the effects require further elaboration and this is complicated further by the involvement of metabolites in mediating some effects. The most logical and fruitful avenue to explore is the development of stable, nonhydrolyzable analogues of diadenosine polyphosphates with antiaggregatory effects [14,44,45] or to achieve a similar end by the development of compounds which selectively prevent the actions of diadenosine polyphosphate hydrolases. The former is under investigation and is concentrating on stable analogues of AP₄A and AP₅A (antiaggregatory) rather than AP₃A (proaggregatory), see Table 1. The potential of stable analogues of AP₆A has yet to be explored. Nonhydrolyzable analogues have the advantage of a maintained duration of action and could be used as adjunctive therapies. The development of selective inhibitors of diadenosine polyphosphate hydrolases has not yet been addressed.

	11. Clinical relevance of diadenosine polyphosphates

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
Platelet fractions isolated from hypertensive patients have a greater vasopressor activity compared to fractions from normotensive subjects and this is related to the activity of AP₅A and AP₆A [42,80]. Platelets from patients with the Chédiak–Higashi syndrome (a rare disorder characterized by albinism, susceptibility to infection and a bleeding tendency associated with a functional defect of the platelets [81]) contain 99% less AP₄A compared to normal subjects and 53% less ATP [82].

	12. Conclusions

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 
Diadenosine polyphosphates are a newly described class of compounds whose physiological role is being established. They alter coronary flow and increase ventricular refractory period and action potential duration. The limited information available suggests the involvement of endothelium-derived substances in mediating part of the coronary vasodilatation seen in response to some diadenosine polyphosphates. More work is required to determine the mechanism for their effects on cellular electrophysiology. Their relative importance as mediators of some of the deleterious effects associated with platelet activation during myocardial ischaemia also requires further elaboration. These data will be important in defining the need for specific receptor antagonists which might have therapeutic value in man, but this will only be possible once the relative involvement of P1- and P2-receptors and the P2Y_Ap4A or "dinucleotide" receptor in mediating the cardiovascular responses to diadenosine polyphosphates have been established more clearly.

Time for primary review 35 days.

	Acknowledgements

 
The authors gratefully acknowledge the support of British Heart Foundation project grants PG/96074 and PG/98012.

	References

Top Abstract 1. Introduction - the... 2. Biochemical synthesis and... 3. Diadenosine polyphosphates as... 4. Extracellular and... 5. Effects of diadenosine... 6. Effects of diadenosine... 7. Cardiac effects of... 8. Receptor-mediated effects 9. Similarities to the... 10. Platelet activation during... 11. Clinical relevance of... 12. Conclusions References

 

1. Baxi M.D., Vishwanatha J.K. Diadenosine polyphosphates: their biological and pharmacological significance. J Pharmacol Toxicol Methods (1995) 33:121–128.[CrossRef][Web of Science][Medline]
2. Bo X., Fischer B., Maillard M., Jacobson K.A., Burnstock G. Comparative studies on the affinities of ATP derivatives for P_2X-purinoceptors in rat urinary bladder. Br J Pharmacol (1994) 112:1151–1159.[Web of Science][Medline]
3. Hoyle C.H.V. Pharmacological activity of adenine dinucleotides in the periphery: possible receptor classes and transmitter function. Gen Pharmacol (1990) 21:827–831.[Web of Science][Medline]
4. Schlüter H., Tepel M., Zidek W. Vascular actions of diadenosine phosphates. J Auton Pharmacol (1996) 16:357–362.[Web of Science][Medline]
5. Ripoll C., Martin F., Rovira J.M., et al. Diadenosine polyphosphates. A novel class of glucose-induced intracellular messengers in the pancreatic β-cell. Diabetes (1996) 45:1431–1434.[Abstract]
6. Bochner B.R., Lee P.C., Wilson S.W., Cutler C.W., Ames B.N. AppppA and related adenylylated nucleotides are synthesized as a consequence of oxidative stress. Cell (1984) 37:225–232.[CrossRef][Web of Science][Medline]
7. Varshavsky A. Diadenosine 5',5'''-P¹,P⁴-tetraphosphate: A pleiotropically acting alarmone? Cell (1983) 34:711–712.[CrossRef][Web of Science][Medline]
8. Lee P.C., Bochner B.R., Ames B.N. AppppA, heat shock stress, and cell oxidation. Proc Nat Acad Sci USA (1983) 80:7496–7500.[Abstract/Free Full Text]
9. Rubino A., Burnstock G. Possible role of diadenosine polyphosphates as modulators of cardiac sensory–motor neurotransmission in guinea-pigs. J Physiol (1996) 495:515–523.[Abstract/Free Full Text]
10. Pintor J., Puche J.A., Gualix J., Hoyle C.H.V., Miras-Portugal M.T. Diadenosine polyphosphates evoke Ca²⁺ transients in guinea-pig brain via receptors distinct from those for ATP. J Physiol (1997) 504:327–335.[Abstract/Free Full Text]
11. Pohl U., Ogilvie A., Lamontagne D., Busse R. Potent effects of AP₃A and AP₄A on coronary resistance and autacoid release of intact rabbit hearts. Am J Physiol (1991) 260:H1692–H1697.[Web of Science][Medline]
12. Stavrou B.M., Sheridan D.J., Flores N.A. Cardiac electrophysiological and haemodynamic effects of diadenosine polyphosphates in the isolated perfused guinea-pig heart (abstract). J Physiol (1998) 509P:150P–151P.
13. Kisselev L.L., Justesen J., Wolfson A.D., Frolova L.Y. Diadenosine oligophosphates (Ap_nA), a novel class of signalling molecules? FEBS Lett (1998) 427:157–163.[CrossRef][Web of Science][Medline]
14. Zamecknik P.C., Kim B., Gao M.J., Taylor G., Blackburn G.M. Analogues of diadenosine 5',5'''-P¹,P⁴-tetraphosphate (Ap₄A) as potential anti-platelet-aggregation agents. Proc Natl Acad Sci USA (1992) 89:2370–2373.[Abstract/Free Full Text]
15. Rapaport E., Zamecnik P.C. Presence of diadenosine 5',5'''-P¹,P⁴-tetraphosphate (Ap₄A) in mammalian cells in levels varying widely with proliferative activity of the tissue: A possible positive "pleiotypic activator". Proc Natl Acad Sci USA (1976) 73:3984–3988.[Abstract/Free Full Text]
16. Lüthje J., Ogilvie A. The presence of diadenosine 5',5'''-P¹,P³-triphosphate (Ap₃A) in human platelets. Biochem Biophys Res Commun (1983) 115:253–260.[CrossRef][Web of Science][Medline]
17. Busse R., Ogilvie A., Pohl U. Vasomotor activity of diadenosine triphosphate and diadenosine tetraphosphate in isolated arteries. Am J Physiol (1988) 254:H828–H832.[Web of Science][Medline]
18. Lüthje J., Baringer J., Ogilvie A. Effects of diadenosine triphosphate (Ap₃A) and diadenosine tetraphosphate (Ap₄A) on platelet aggregation in unfractionated human blood. Blut (1985) 51:405–413.[CrossRef][Web of Science][Medline]
19. Lüthje J., Ogilvie A. Catabolism of Ap₃A and Ap₄A in human plasma. Purification and characterization of a glycoprotein complex with 5'-nucleotide phosphodiesterase activity. Eur J Biochem (1985) 149:119–127.[Web of Science][Medline]
20. Mateo J., Rotllan P., Marti E., et al. Diadenosine polyphosphate hydrolase from presynaptic plasma membranes of Torpedo electric organ. Biochem J (1997) 323:677–684.[Web of Science][Medline]
21. Lüthje J., Ogilvie A. Catabolism of Ap₄A and Ap₃A in human serum. Identification of isoenzymes and their partial characterization. Eur J Biochem (1987) 169:385–388.[Web of Science][Medline]
22. Ogilvie A., Antl W. Diadenosine tetraphosphatase from human leukemia cells. Purification to homogeneity and partial characterization. J Biol Chem (1983) 258:4105–4109.[Abstract/Free Full Text]
23. Flodgaard H., Klenow H. Abundant amounts of diadenosine 5',5'''-P¹,P⁴-tetraphosphate are present and releasable, but metabolically inactive, in human platelets. Biochem J (1982) 208:737–742.[Web of Science][Medline]
24. Lüthje J., Ogilvie A. Catabolism of Ap₄A and Ap₃A in whole blood. The dinucleotides are long-lived signal molecules in the blood ending up as intracellular ATP in the erythrocytes. Eur J Biochem (1988) 173:241–245.[Web of Science][Medline]
25. Lerman B.B., Belardinelli L. Cardiac electrophysiology of adenosine. Basic and clinical concepts. Circulation (1991) 83:1499–1509.[Free Full Text]
26. Goldman S.J., Gordon E.L., Slakey L.L. Hydrolysis of diadenosine 5',5''-P',P''-triphosphate (Ap₃A) by porcine aortic endothelial cells. Circ Res (1986) 59:362–366.[Abstract/Free Full Text]
27. Elmaleh D.R., Zamecnik P.C., Castronovo F.P. Jr., Straus H.W., Rapaport E. ^99mTc-labeled nucleotides as tumor-seeking radiodiagnostic agents. Proc. Natl. Acad. Sci. USA (1984) 81:918–921.[Abstract/Free Full Text]
28. Delaney S.M., Blackburn G.M., Geiger J.D. Diadenosine polyphosphates inhibit adenosine kinase activity but decrease levels of endogenous adenosine in rat brain. Eur J Pharmacol (1997) 332:35–42.[CrossRef][Web of Science][Medline]
29. Brandts B., Brandts A., Wellner-Kienitz M.-C., et al. Non-receptor-mediated activation of I_K(ATP) and inhibition of I_K(Ach) by diadenosine polyphosphates in guinea-pig atrial myocytes. J Physiol (1998) 512:407–420.[Abstract/Free Full Text]
30. Walker J., Bossman P., Lackey B.R., et al. The adenosine 5',5''',P₁,P₄-tetraphosphate receptor is at the cell surface of heart cells. Biochemistry (1993) 32:14009–14014.[CrossRef][Web of Science][Medline]
31. Hankin S., Matthew N., Thorne H., McLennan A.G. Diadenosine 5',5'''-P¹,P⁴-tetraphosphate hydrolase is present in human erythrocytes, leukocytes and platelets. Int Biochem Cell Biol (1995) 27:201–206.[CrossRef]
32. Rodriguez-Pascal F., Torres M., Rotllán P., Miras-Portugal M.T. Extracellular hydrolysis of diadenosine polyphosphates, AP_nA, by chromaffin cells in culture. Arch Biochem Biophys (1992) 297:176–183.[CrossRef][Web of Science][Medline]
33. Pintor J., King B.F., Ziganshin A.U., Miras-Portugal M.T., Burnstock G. Diadenosine polyphosphate-activated inward and outward currents in follicular oocytes of Xenopus laevis. Life Sci (1996) 59:PL179, PL184.
34. Hoyle C.H.V., Ziganshin A.U., Pintor J., Burnstock G. The activation of P₁- and P₂-purinoceptors in the guinea-pig left atrium by diadenosine polyphosphates. Br J Pharmacol (1996) 118:1294–1300.[Web of Science][Medline]
35. Edgecombe M., McLennan A.G., Fisher M.J. Characterization of the binding of diadenosine 5',5'''-P¹,P⁴-tetraphosphate (Ap₄A) to rat liver cell membranes. Biochem J (1996) 314:687–693.[Web of Science][Medline]
36. Pintor J., Miras-Portugal M.T. P₂ purinergic receptors for diadenosine polyphosphates in the nervous system. Gen Pharmacol (1995) 26:229–235.[CrossRef][Web of Science][Medline]
37. Lüthje J., Ogilvie A. Diadenosine triphosphate (Ap₃A) mediates human platelet aggregation by liberation of ADP. Biochem Biophys Res Commun (1984) 118:704–709.[CrossRef][Web of Science][Medline]
38. Lüthje J., Miller D., Ogilvie A. Unproportionally high concentrations of diadenosine triphosphate (Ap₃A) and diadenosine tetraphosphate (Ap₄A) in heavy platelets. Consequences for in vitro studies with human platelets. Blut (1987) 54:193–200.[CrossRef][Web of Science][Medline]
39. Brown A.S., Martin J.F. The megakaryocyte platelet system and vascular disease. Eur J Clin Invest (1994) 24(Suppl. 1):9–15.[Web of Science][Medline]
40. Ogilvie A. Ap₄A and Other Dinucleoside Polyphosphates. McLennan A.G., ed. (1992) Boca Raton, FL: CRC Press. 229–273.
41. Garrison P.N., Barnes L.D. Ap₄A and Other Dinucleoside Polyphosphates. McLennan A.G., ed. (1992) Boca Raton, FL: CRC Press. 29–61.
42. Schlüter H., Offers E., Brüggemann G., et al. Diadenosine phosphates and the physiological control of blood pressure. Nature (1994) 367:186–188.[CrossRef][Medline]
43. Harrison M.J., Brossmer R., Goody R.S. Inhibition of platelet aggregation and the platelet release reaction by , diadenosine polyphosphates. FEBS Lett (1975) 54:57–60.[CrossRef][Web of Science][Medline]
44. Kim B.K., Zamecknik P., Taylor G., Guo M.J., Blackburn G.M. Antithrombotic effect of β,β'-monochloromethylene diadenosine 5',5'''-P¹,P⁴-tetraphosphate. Proc Natl Acad Sci USA (1992) 89:11056–11058.[Abstract/Free Full Text]
45. Chan S.W., Gallo S.J., Kim B.K., et al. P¹,P⁴-dithio-P²,P³-monochloromethylene diadenosine 5',5'''-P¹,P⁴-tetraphosphate: A novel antiplatelet agent. Proc Natl Acad Sci USA (1997) 94:4034–4039.[Abstract/Free Full Text]
46. Tepel M., Bachmann J., Schlüter H., Zidek W. Diadenosine polyphosphate-induced increase in cytosolic free calcium in vascular smooth muscle cells. J Hypertens (1995) 13:1686–1688.[Web of Science][Medline]
47. Ralevic V., Hoyle C.H.V., Burnstock G. Pivotal role of phosphate chain length in vasoconstrictor versus vasodilator actions of adenine dinucleotides in rat mesenteric arteries. J Physiol (1995) 483:703–713.[Abstract/Free Full Text]
48. Ralevic V., Burnstock G. Role of P₂-purinoceptors in the cardiovascular system. Circulation (1991) 84:1–14.[Abstract/Free Full Text]
49. Chen Z.-P., Levy A., Lightman S.L. Nucleotides as extracellular signalling molecules. J Neuroendocrinol (1995) 7:83–96.[CrossRef][Web of Science][Medline]
50. Gordon J.L. Extracellular ATP: effects, sources and fate. Biochem J (1986) 233:309–319.[Web of Science][Medline]
51. van der Giet M., Khattab M., Börgel J., Schlüter H., Zidek W. Differential effects of diadenosine phosphates on purinoceptors in the rat isolated perfused kidney. Br J Pharmacol (1997) 120:1453–1460.[CrossRef][Web of Science][Medline]
52. Busshardt E., Gerok W., Häussinger D. Regulation of hepatic parenchymal and non-parenchymal cell function by the diadenine nucleotides Ap₃A and Ap₄A. Biochim Biophys Acta (1989) 1010:151–159.[Medline]
53. Nees S. Coronary flow increases induced by adenosine and adenine nucleotides are mediated by the coronary endothelium: a new principle of the regulation of coronary flow. Eur Heart J (1989) 10(Suppl. F):2835.
54. Kikuta Y., Sekine A., Tezuka S., et al. Intravenous diadenosine tetraphosphate in dogs. Cardiovascular effects and influence on blood gases. Acta Anaesthesiol Scand (1994) 38:284–288.[Web of Science][Medline]
55. Humphrey S.M., Holliss D.G., Cartner L.A. Influence of inhibitors of ATP catabolism on myocardial recovery after ischaemia. J Surg Res (1987) 43:187–195.[CrossRef][Web of Science][Medline]
56. Kengatharan M., Thiemermann C., Vane J.R. Analysis of the cardiovascular responses to diadenosine pentaphosphate in the anaesthetised rat (abstract). Br J Pharmacol (1994) 113:62P.
57. Nakae I., Takahashi M., Takaoka A., et al. Coronary effects of diadenosine tetraphosphate resemble those of adenosine in anesthetized pigs: involvement of ATP-sensitive potassium channels. J Cardiovasc Pharmacol (1996) 28:124–133.[CrossRef][Web of Science][Medline]
58. Vahlensieck U., Boknék P., Knapp J., et al. Negative chronotropic and inotropic effects exerted by diadenosine hexaphosphate (AP₆A) via A₁-adenosine receptors. Br J Pharmacol (1996) 119:835–844.[Web of Science][Medline]
59. Hilderman R.H., Martin M., Zimmerman J.K., Pivorun E.B. Identification of a unique membrane receptor for adenosine 5',5'''-P₁,P₄-tetraphosphate. J Biol Chem (1991) 266:6915–6918.[Abstract/Free Full Text]
60. Jovanovic A., Alekseev A.E., Terzic A. Intracellular diadenosine polyphosphates. A novel family of inhibitory ligands of the ATP-sensitive K⁺ channel. Biochem Pharmacol (1997) 54:219–225.[CrossRef][Web of Science][Medline]
61. Olsson R.A., Pearson J.D. Cardiovascular purinoceptors. Physiol Rev (1990) 70:761–845.[Free Full Text]
62. Fredholm B.B., Abbracchio M.P., Burnstock G., et al. Towards a revised nomenclature for P1 and P2 receptors. Trends Pharmacol Sci (1997) 18:79–82.[Medline]
63. Jacobsen K.A. van Rhee A.M. Moro S. Pharmacology of purinergic receptors. http://mgddk1.niddk.nih.gov.8000/pharmacol.html 1996. Accessed 24th October 1997 and 16th September 1998.
64. Communi D., Boeynaems J.-M. Receptors responsive to extracellular pyrimidine nucleotides. Trends Pharmacol Sci (1997) 18:83–86.[CrossRef][Medline]
65. Fredholm B.B., Abbracchio M.P., Burnstock G., et al. Nomenclature and classification of purinoceptors. Pharmacol Rev (1994) 46:143–156.[Web of Science][Medline]
66. Miras-Portugal M.T., Castro E., Mateo J., Pintor J. The diadenosine polyphosphate receptors: P2D purinoceptors. Ciba Found Symp (1996) 198:35–47.[Medline]
67. Alexander SPH, Peters JA, editors. Receptor and Ion Channel Nomenclature Supplement. 8th ed. Trends Pharmacol Sci 1997.
68. Hilderman R.H., Lilien J.E., Zimmerman J.K., et al. Adenylated dinucleotide binding to the adenosine 5',5'''-P₁,P₄-tetraphosphate mouse heart receptor. Biochem Biophys Res Commun (1994) 200:749–755.[CrossRef][Web of Science][Medline]
69. Walker J., Hilderman R.H. Identification of a serine protease which activates the mouse heart adenosine 5',5''',P₁,P₄-tetraphosphate receptor. Biochemistry (1993) 32:3119–3123.[CrossRef][Web of Science][Medline]
70. Walker J., Lewis T.E., Pivorun E.P., Hilderman R.H. Activation of the mouse heart adenosine 5',5'''-P₁-P₄-tetraphosphate receptor. Biochemistry (1993) 32:1264–1269.[CrossRef][Web of Science][Medline]
71. Baxi M.D., McLennan A.G., Vishwanatha J.K. Characterization of the HeLa cell DNA polymerase -associated Ap₄A binding protein by photoaffinity labeling. Biochemistry (1994) 33:14601–14607.[CrossRef][Web of Science][Medline]
72. Pintor J., Gualix J., Miras-Portugal M.T. Diinosine polyphosphates, a group of dinucleotides with antagonistic effects on diadenosine polyphosphate receptor. Mol Pharmacol (1997) 51:277–284.[Abstract/Free Full Text]
73. Flores N.A., Botchway A.N.S., Stavrou B.M., Sheridan D.J. Cardiac electrophysiological effects of platelet-derived substances. Exp Physiol (1999) 84:253–274.[CrossRef][Web of Science][Medline]
74. Flores N.A., Sheridan D.J. The pathophysiological role of platelets during myocardial ischaemia. Cardiovasc Res (1994) 28:295–302.[Free Full Text]
75. Flores N.A. Platelet activation during myocardial ischaemia: a contributory arrhythmogenic mechanism. Pharmacol Ther (1996) 72:83–108.[CrossRef][Web of Science][Medline]
76. Pantely G.A., Bristow J.D. Adenosine. Renewed interest in an old drug. Circulation (1990) 82:1854–1856.[Free Full Text]
77. Hori M., Kitakaze M. Adenosine, the heart, and coronary circulation. Hypertension (1991) 18:565–574.[Abstract/Free Full Text]
78. Ely S.W., Berne R.M. Protective effects of adenosine in myocardial ischaemia. Circulation (1992) 85:893–904.[Abstract/Free Full Text]
79. Janse M.J., Wit A.L. Electrophysiological mechanisms of ventricular arrhythmias resulting from myocardial ischemia and infarction. Physiol Rev (1989) 69:1049–1169.[Free Full Text]
80. Agha A., Schlüter H., Konig S., et al. A novel platelet-derived renal vasoconstrictor agent in normotensives and essential hypertensives. J Vasc Res (1992) 29:281–289.[Web of Science][Medline]
81. Apitz-Castro R., Cruz M.R., Ledezma E., et al. The storage pool deficiency in platelets from humans with the Chédiak–Higashi syndrome: study of six patients. Br J Haematol (1985) 59:471–483.[Web of Science][Medline]
82. Kim B.K., Chao F.C., Leavitt R., et al. Diadenosine 5',5'''-P¹,P⁴-tetraphosphate deficiency in blood platelets of the Chédiak–Higashi syndrome. Blood (1985) 66:735–737.[Abstract/Free Full Text]

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