Effect of diadenosine phosphates on human umbilical vessels: novel platelet-derived vasoconstrictors

Polyphosphate as a donor of high-energy phosphate for the synthesis of ADP and ATP

Here, we studied the potential role of inorganic polyphosphate (polyP) as an energy source for ADP and ATP formation in the extracellular space. In SaOS-2 cells, we show that matrix vesicles are released into the extracellular space after incubation with polyP. These vesicles contain both alkaline phosphatase (ALP) and adenylate kinase (AK) activities (mediated by ALPL and AK1 enzymes). Both enzymes translocate to the cell membrane in response to polyP. To distinguish the process(es) of AMP and ADP formation during ALP hydrolysis from the ATP generated via the AK reaction, inhibition studies with the AK inhibitor A(5')P5(5')A were performed. We found that ADP formation in the extracellular space occurs after enzymatic ATP synthesis. After exposure to polyP, a significant increase of the ADP level was observed, which is likely to be been catalyzed by ALP. This increase is not due to an intensified ATP release via exocytosis. The ATP level in the extracellular space of SaOS-2 cells is strongly increased in response to polyP, very likely mediated by the AK. We propose that the ALP and AK enzymes are involved in the extracellular ADP and ATP synthesis.

Diadenosine polyphosphates are selective vasoconstrictors in human coronary artery bypass grafts

Diadenosine polyphosphates (Ap(n)A) are released by degranulating platelets and high, local concentrations may form at sites of platelet activation. Radial artery grafts, now often used alongside the internal mammary artery in coronary artery bypass surgery, are particularly reactive to several vasoconstrictors but the response to Ap(n)A has not been investigated. This study compared the vasoconstrictor activity of Ap(n)A in human radial artery with other vessels commonly used as bypass grafts. Radial artery demonstrated robust concentration-dependent vasoconstriction to Ap(n)A (n=4-6) at concentrations in the micromolar range. In contrast, average responses in internal mammary artery were negligible. Cross-desensitization revealed that Ap(n)A-mediated vasoconstriction occurred via an alphabetamethyleneATP-sensitive receptor. Responses to both Ap(5)A and alphabetamethyleneATP were inhibited by suramin but were insensitive to the P2X(1) receptor antagonist 8,8'-[Carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino)]bis-1,3,5-naphthalenetrisulfonic acid (NF279). Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) enhanced responses to Ap(5)A. Similar responses were obtained in saphenous vein. In conclusion, diadenosine polyphosphates contract radial artery and saphenous vein by an as yet uncharacterized P2X receptor but have only limited activity in internal mammary artery. The selective activity of diadenosine polyphosphates in radial artery would implicate them as potential mediators of post-operative contraction in this graft.

Isolation and quantification of dinucleoside polyphosphates by using monolithic reversed phase chromatography columns

In former studies, dinucleoside polyphosphates were quantified using ion-pair reversed-phase perfusion chromatography columns, which allows a detection limit in the micromolar range. The aim of this study was both to describe a chromatographic assay with an increased efficiency of the dinucleoside separation, which enables the reduction of analytical run times, and to establish a chromatographic assay using conditions, which allow MALDI-mass spectrometric analysis of the resulting fractions. We compared the performance of conventional silica reversed phase chromatography columns, a perfusion chromatography column and a monolithic reversed-phase C18 chromatography column. The effects of different ion-pair reagents, flow-rates and gradients on the separation of synthetic diadenosine polyphosphates as well as of diadenosine polyphosphates isolated from human platelets were analysed. Sensitivity and resolution of the monolithic reversed-phase chromatography column were both higher than that of the perfusion chromatography and the conventional reversed phase chromatography columns. Using a monolithic reversed-phase C18 chromatography column, diadenosine polyphosphates were separable baseline not only in the presence of tetrabutylammonium hydrogensulfate (TBA) but also in the presence of triethylammonium acetate (TEAA) as ion-pair reagent. The later reagent is useful because, in contrast to TBA, it is compatible with MALDI mass-spectrometric methods. This makes TEAA particularly suitable for identification of unknown nucleoside polyphosphates. Furthermore, because of the lower backpressure of monolithic reversed-phase chromatography columns, we were able to significantly increase the flow rate, decreasing the amount of time for the analysis close to 50%, especially using TBA as ion-pair reagent. In summary, monolithic reversed phase C18 columns markedly increase the sensitivity and resolution of dinucleoside polyphosphate analysis in a time-efficient manner compared to reversed-phase perfusion chromatography columns or conventional reversed-phase columns. Therefore, further dinucleoside polyphosphate analytic assays should be based on monolithic silica C18 columns instead of perfusion chromatography or conventional silica reversed phase chromatography columns. In conclusion, the use of monolithic silica C18 columns will lead to isolation and quantification of up to now unknown dinucleoside polyphosphates. These chromatography columns may facilitate further research on the biological roles of dinucleoside polyphosphates.

Synthetic, nondegradable diadenosine polyphosphates and diinosine polyphosphates: their effects on insulin-secreting cells and cultured vascular smooth muscle cells

Diadenosine polyphosphates show a dissimilarity between their effects in static and perifusion experiments with respect to insulin release that may be due to degradation of the compounds. The aim was to investigate two nondegradable compounds of bisphosphorothioates containing a methylene or chloromethylene group (namely, diadenosine 5',5' "-(P(1),P(4)-dithio-P(2),P(3)-methylene)tetraphosphate and diadenosine 5',5' "-(P(1),P(4)-dithio-P(2),P(3)-chloromethylene)tetraphosphate), as mixtures of three or four diastereomers. Owing to their modified structures, these compounds are resistant to degradation (ectophosphodiesterases, diphosphohydrolases, and phosphorylases). Both compounds tested were minimally degraded (2%) even after 16 h when incubated with insulin-secreting (INS-1) cells. Additionally, diinosine polyphosphates (Ip(5)I and Ip(6)I), putative antagonists of diadenosine polyphosphates, were tested. By use of [(3)H]Ap(4)A, saturable binding sites for both diadenosine polyphosphate analogues were found in INS-1 cells, 3T3 preadipocyte cells, and vascular smooth muscle cells (VSMC) and for both Ip(5)I and Ip(6)I in INS-1 cells. The synthesized diadenosine polyphosphate analogues have the same affinity as Ap(4)A, whereas Ip(5)I and Ip(6)I inhibit binding at higher concentrations (10-100 microM). Insulin release was investigated in static experiments over 90 min in INS-1 cells. Insulin release was inhibited dose-dependently by both of the diadenosine polyphosphate analogues to the same degree as by Ap(4)A. The glucose-induced insulin release curve was not shifted to the right. Both compounds inhibit insulin release only at high (insulin stimulatory) glucose concentrations, e.g., 5.6 mM glucose. Ip(5)I and Ip(6)I antagonized Ap(5)A-mediated inhibition of insulin release. [(3)H]Thymidine incorporation into VSMC was not influenced by either synthetic diadenosine polyphosphate analogue, indicating that Ap(4)A does not act by itself in this case but (active) degradation products mediate the effect. The data indicate the following. (1) Since nondegradable compounds inhibit insulin release as well as Ap(4)A, it is Ap(4)A itself and not any of its degradation products that induces this effect. (2) Diadenosine polyphosphate effects on cell proliferation are mediated via a degradation product in contrast to their effect on insulin release. (3) Ip(5)I and Ip(6)I act like antagonists. Both synthetic analogues and diinosine polyphosphates are valuable tools for diabetes research.

Effects of dinucleoside polyphosphates on regulation of coronary vascular tone

The aim of the present study was to investigate the effects of Xp(5)X and Xp(6)X (X = guanosine (G) or adenosine (A); n = 5 and 6), which have been identified in human platelets, on coronary vascular tone. The activation of purinoceptors in rat coronary vasculature by Xp(5)X and Xp(6)X was evaluated by measuring their effects on perfusion pressure in the Langendorff perfused rat. Ap(5)X and Ap(6)X induced dose-dependent vasodilation that was due to P2Y(1) receptor activation, as evidenced by use of the selective P2Y(1) receptor antagonist 2'-deoxy-N(6)-methyl-adenosine 3',5'-diphosphate diammonium (MRS2179). Vasodilation was induced by NO release, as evidenced by inhibition of nitric oxide synthases (NO synthases) by N(G)-nitro-L-arginine methyl ester (L-NAME). The dose-dependent decrease in coronary perfusion pressure induced by Ap(5)X and Ap(6)X was converted to a dose-dependent increase in perfusion pressure after inhibition of NO synthases by L-NAME. After endothelium removal, the vasodilation elicited by Ap(5)X and Ap(6)X was converted to a vasoconstriction which could be inhibited by P2X receptor blockade. Ap(5)A, Ap(5)G, Ap(6)A and Ap(6)G are vasodilating or vasoconstricting nucleotides that activate P2Y(1) or P2X receptors depending on the status of the coronary vascular endothelium.

In vivo effects of diadenosine polyphosphates on rat renal microcirculation

BACKGROUND

Diadenosine polyphosphates (APXA) are vasoactive nucleotides that elicit effects via purinoceptors. Recent data suggest differential effects of APXA on kidney vasculature.

METHODS

The in vivo effects of AP3A, AP5A, and adenosine on renal microvessels and the role of purinoceptors were investigated by the application of agonists to the hydronephrotic rat kidney and preincubation with respective antagonists.

RESULTS

The addition of the agonists (10-7 mol/L up to 10-4 mol/L) resulted in a concentration-dependent transient vasoconstriction [interlobular artery (ILOB): adenosine 30 +/- 7%, N = 7, AP3A 35 +/- 10%, N = 5; AP5A 66 +/- 19%, N = 5; 10-5 mol/L each] lasting up to one minute, followed by a concentration-dependent vasodilation (ILOB: adenosine 10 +/- 3%, N = 6; AP3A 19 +/- 4%, N = 5; AP5A 12 +/- 5%, N = 6; 10-5 mol/L each). In ILOB and in the afferent arteriole (AFF), the constrictory effects of AP5A were more pronounced than those of AP3A and adenosine. In the efferent arteriole (EFF), vascular tone was only slightly affected by all agonists. The dilatory potency was comparable for all agonists in ILOB and EFF. No significant vasodilation occurred in AFF. The application of the selective A1 receptor antagonist DPCPX (10-5 mol/L) completely abolished the adenosine-induced vasoconstriction, whereas the A2 receptor antagonist DMPX and the P2 purinoceptor antagonists PPADS and A3P5P (all 10-5 mol/L) did not affect adenosine-induced constriction. The AP3A-induced constriction was abolished by DPCPX and was partially inhibited by PPADS. The constriction induced by AP5A was less sensitive to DPCPX but more sensitive to PPADS. In ILOB and EFF, DMPX or A3P5P abolished dilation after the addition of the agonists. The dilation after AP5A was not significantly reduced. In AFF, no significant dilation was observed with these agonists alone, but it was clearly visible in the presence of DPCPX or PPADS.

CONCLUSIONS

APXA evoke transient constrictions in vessels of the hydronephrotic rat kidney, which are mediated by A1 and P2 purinoceptors. The length of the phosphate chain determines the degree of vasoconstriction and the extent to which the substances exert effects on the P2 purinoceptor subtypes. ILOB and AFF are more potently affected by APXA than EFF. Afferent vasodilation is partially overridden by sustained vasoconstriction.

Diadenosine polyphosphates in cultured vascular smooth-muscle cells and endothelium cells--their interaction with specific receptors and their degradation

The role of diadenosine polyphosphates (ApnA, where "A" denotes "adenosine" and "n" denotes the number of phosphate groups "p") as vasoconstrictors of smooth-muscle cells and as blood-pressure regulating and insulin-releasing compounds has been described. It was the aim of this study to investigate whether specific receptors for these compounds, mediating the above mentioned effects, occur in cultured vascular smooth-muscle cells (VSMC) and in endothelium cells, and whether these compounds are degraded during incubation. Saturable binding sites for diadenosine polyphosphate [3H]Ap4A with an extremely quick saturation equilibrium, even at low temperature (4 degrees C), are present in vascular smooth-muscle cells. Diadenosine polyphosphates at micromolar concentrations displaced [3H]Ap4A from binding sites; the ranking order was Ap4A > Ap3A > Ap5A approximately Ap6A. Compounds interacting with purinergic P2X receptors such as suramin, alpha,beta-methylene ATP and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), albeit at high concentrations, displaced [3H]Ap4A from its binding sites. Surprisingly, at low concentrations the compounds tested increased the binding of [3H]Ap4A, which might imply the occurrence of positive receptor cooperativity or inhibition of [3H]Ap4A degradation. By use of thin-layer chromatography it was observed that [3H]Ap4A was quickly degraded (half-life approx. 12 min) in the extracellular medium to (mainly) adenosine and inosine. [3H]Ap4A and its degradation products were quickly taken up by the cells. Degradation can be inhibited by Ap6A, alpha,beta-methylene ATP or PPADS. Rather similar degradation and uptake results were also obtained when endothelium cells were used. These data indicate that specific binding sites for [3H]Ap4A are present in vascular smooth-muscle cells and that diadenosine polyphosphates at physiological concentrations displace binding. The receptors involved might be distinct diadenosine polyphosphate receptors, although the involvement of others, such as P2X receptors, is also possible. Ap4A is quickly degraded in the extracellular space and compounds that inhibit degradation result in an increase in [3H]Ap4A binding. It should be remembered that when diadenosine polyphos-phates are being investigated in physiological and pathophysiological studies of their impact on smooth-muscle cell proliferation and on vasoconstriction (blood-pressure regulation), results obtained from long-term incubations might be critical.

Evidence for two different P2X-receptors mediating vasoconstriction of Ap5A and Ap6A in the isolated perfused rat kidney

The activation of various P2-receptor subtypes in rat renal vasculature by P1, P5-diadenosine pentaphosphate (ApsA) and P1, P6-diadenosine hexaphosphate (Ap6A) were studied by measuring their effects on perfusion pressure during continuous perfusion in a rat isolated perfused kidney. Permanent perfusion with Ap5A and Ap6A elicited both a transient and sustained vasoconstriction with both vasoconstrictions to be different: the transient vasoconstriction can be elicited with concentrations > or = 10 nM, whereas the sustained vasoconstriction is observed with concentrations > or = 1 nM. ApsA and Ap6A act via the same receptors as alpha,beta-methylene ATP (alpha,beta-meATP). The rank order of potency for transient vasconstriction was alpha,beta-meATP = ApsA>Ap6A>B,gamma-meATP, and for sustained vasoconstriction alpha,beta-meATP = Ap5A > beta,gamma-meATP > or = Ap6A. Suramin, a non-selective P2-receptor antagonist, and pyridoxal-phosphate-6-azophenyl-2;4-disulphonic acid (PPADS) a highly selective P2X-receptor antagonist antagonized both the transient and the sustained vasoconstriction. Taken together the results of the agonist profile of Ap5A and Ap6A and comparing its findings to literature it can be demonstrated that the transient but not the sustained vasoconstriction is mediated via the P2X1-receptor which is present in rat renal vasculature. It is demonstrated that the agonist profile of the sustained vasoconstriction induced by ApsA and Ap6A does not fit to any currently known P2X- or P2Y-receptor subtype. We conclude a yet unidentified P2X-receptor or chimeric P2X-receptor may contribute to the effects on rat renal vasculature produced by Ap5A and Ap6A and which may play an important role in glomerular perfusion pressure and blood pressure control.

Acyl-CoA synthetase catalyzes the synthesis of diadenosine hexaphosphate (Ap6A)

The synthesis of diadenosine hexaphosphate (Ap6A), a potent vasoconstrictor, is catalyzed by acyl-CoA synthetase from Pseudomonas fragi. In a first step AMP is transferred from ATP to tetrapolyphosphate (P4) originating adenosine pentaphosphate (p5A) which, subsequently, is the acceptor of another AMP moiety from ATP generating diadenosine hexaphosphate (Ap6A). Diadenosine pentaphosphate (Ap5A) and diadenosine tetraphosphate (Ap4A) were also synthesized in the course of the reaction. In view of the variety of biological effects described for these compounds the potential capacity of synthesis of diadenosine polyphosphates by the mammalian acyl-CoA synthetases may be relevant.

Inotropic effects of diadenosine tetraphosphate in isolated canine cardiac preparations

We studied the effects of diadenosine tetraphosphate (AP4A) on the force of contraction in canine preparations. The force of contraction was measured in isolated electrically driven (1 Hz) atrial and ventricular cardiac trabeculae from adult dogs. AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced force of contraction in atrial preparations by approximately 24%. Moreover, AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced the force of contraction in ventricular preparations by 29 and 29%, respectively. The negative inotropic effects of AP4A were abolished by the A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX). In summary, in canine myocardium, AP4A alone and after prestimulation with a beta-adrenoceptor agonist exerts negative inotropic effects, which are probably mediated via A1-adenosine receptors.

Mediation of the vasoactive properties of diadenosine tetraphosphate via various purinoceptors

OBJECTIVE AND METHODS

The vasoactive properties of P1,P4-diadenosine tetraphosphate (Ap4A) were studied by measuring the effects of perfusion pressure of a rat isolated perfused kidney.

RESULTS

The vasoconstrictive response to Ap4A was mediated to a large extent to a P2X receptor which could be shown by inhibition with pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium. The remaining vasoconstriction of Ap4A could be blocked by a 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A1 receptor antagonist In raised tone preparation Ap4A evoked vasodilation when P2 receptors were blocked by suramin. The dilation was not mediated by a P2Y receptor as the effect could not be blocked by suramin.

CONCLUSION

Ap4A induces vasoconstriction via A1 and P2X receptors and vasodilatation via an unidentified receptor which is not a P2Y receptor. Ap4A may play an important role in kidney perfusion and, thus, in blood-pressure control.

Characterization of the P2 receptors on the human umbilical vein endothelial cell line ECV304

1. To characterize the P2 receptors present on the human umbilical vein endothelial-derived cell line, ECV304, cytosolic Ca2+, ([Ca2+]c), responses were recorded in single cells and in cell suspensions to a series of nucleotides and nucleotide agonists. 2. Concentration response curves were obtained in fura-2-loaded ECV304 cell suspensions, with EC50 values of 4.2 microM for ATP, 2.5 microM for UTP and 14 microM for adenosine-5'-O-(3-thio)triphosphate (ATPgammaS). EC50 values for 2-methylthioATP, ADP, adenosine-5'-O-(2-thio)diphosphate (ADPbetaS) and AMP were 0.5 microM, 3.5 microM, 15 microM and 4.7 microM respectively, but maximal [Ca2+]c responses were less than those produced by a maximal addition of ATP/UTP. ECV304 cells were unresponsive to UDP and beta,gamma,methyleneATP. 3. Cross-desensitization studies on ECV304 cells suggested that ATP and UTP recognized the same receptor. However, ADP recognized a receptor distinct from the UTP-sensitive receptor and AMP recognized a third distinct receptor. 4. ECV304 [Ca2+]c responses to 2-methylthioATP were inhibited in the presence of 30 microM pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), whereas [Ca2+]c responses to UTP were unaffected by this treatment. 5. ECV304 cells responded to the diadenosine polyphosphate Ap3A with rises in [Ca2+]c. Apparent responses to Ap4A, Ap5A and Ap6A, were shown to be due to a minor nucleotide contaminant that could be removed by pre-treatment of the diadenosine samples with either alkaline phosphatase or apyrase. 6. ECV304 cells display a pharmacology consistent with the presence of at least two P2 receptors; a P2Y2 receptor insensitive to the diadenosine polyphosphates and a P2Y1 receptor sensitive to Ap3A. In addition, ECV304 cells respond to AMP with increases in [Ca2+]c via an as yet uncharacterized receptor.

Differential effects of diadenosine phosphates on purinoceptors in the rat isolated perfused kidney

1. The activation of various purinoceptors in rat renal vasculature by P1,P2-diadenosine pyrophosphate (Ap2A), P1,P3-diadenosine triphosphate (Ap3A), P1,P4-diadenosine tetraphosphate (Ap4A), P1,P5-diadenosine pentaphosphate (Ap5A), P1,P6-diadenosine hexaphosphate (Ap6A) was studied by measuring their effects of perfusion pressure of a rat isolated perfused kidney. 2. The vasoconstrictive response to Ap5A was completely due to P2x purinoceptor activation, that to Ap4A and Ap6 was P2x purinoceptor mediated to a large extent, as evidenced by the inhibitory effects of suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium (PPADS). 3. The vasoconstrictive effects of Ap2A and Ap3A were mostly due to stimulation of A1-receptors, as shown by the inhibitory effect of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). 4. The vasoconstrictive response to Ap6A was partially insensitive to A1 and P2x purinoceptor blockers. 5. In raised tone preparations Ap2A and Ap3A evoked vasodilatation, which was blocked by the A2 receptor blocker, 3,7-dimethyl-1-propargylxanthine (DMPX). 6. In raised tone preparations Ap4A evoked vasodilatation when the P2-purinoceptors were blocked by suramin. 7. The activation of different purinoceptor subtypes by diadenosine phosphates critically depends on the number of phosphate groups.

Diadenosine polyphosphates regulate cytosolic calcium in human fibroblast cells by interaction with P2x purinoceptors coupled to phospholipase C

The effects of diadenosine pentaphosphate (AP5A), and diadenosine hexaphosphate (AP6A) on the cytosolic-free Ca2+ concentration ([Ca2+]i) were evaluated in cultured human fibroblast cells (HF cells) using the fluorescent dye technique. AP5A, and AP6A concentration-dependently increased [Ca2+]i in HF cells. The addition of 10 mumol/1 AP5A and AP6A significantly increased [Ca2+]i in HF cells from 71 +/- 3 nmol/1 (n = 184) to 241 +/- 39 nmol/1 (n = 11; P < 0.001 compared to resting value) and to 227 +/- 26 nmol/1 (n = 23; P < 0.001), respectively. The purinoceptor P2 blockers, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), inhibited the diadenosine polyphophate-induced [Ca2+]i increase, whereas the P2y purinoceptor blocker, reactive blue, had no effect. Adenosinetriphosphate (ATP) and the P2x agonist, alpha 1 beta-methylene-ATP also significantly increased [Ca2+]i in HF cells, whereas the P2y agonist methylthio-ATP showed only a small [Ca2+]i response. Diadenosine polyphosphates mainly induced transplasmamembrane Ca2+ influx as was confirmed by experiments in the absence of extracellular Ca2+ or by manganese quenching studies. Organic (verapamil) and inorganic Ca2+ channel blockers (NiCI2) significantly reduced the AP6A induced transplasmamembrane Ca2+ influx. The inhibitor of phosphatidylcholine-specific phospholipase C, D609, significantly reduced the effect of diadenosine polyphosphates on [Ca2+]i in HF cells. It is concluded that diadenosine polyphosphates regulate transplasmamembrane Ca2+ influx after occupation of P2x receptors via activation of phosphatidylcholine-specific phospholipase C and hence of voltage-operated Ca2+ channels.