Biochemical properties of a P2Y-purinergic receptor

ATP-evoked inositol phosphates formation through activation of P2U purinergic receptors in cultured astrocytes: regulation by PKC subtypes alpha, delta, and theta

ATP-induced phosphoinositide (PI) hydrolysis was studied in cultured astrocytes. To characterize the P2 purinergic receptor-mediated effects of ATP, the subtype-specific agonists 2-methylthio ATP (2-MeSATP), UTP, and alpha, beta-methylene ATP were compared. ATP, UTP, or 2-MeSATP induced a dose-dependent increase of inositol phosphates (IP) accumulation; alpha, beta-methylene ATP and adenosine had no effect. The order of potency was ATP > or = UTP >> 2-MeSATP. Cross-desensitization experiments indicated that ATP interacted with both P2U and P2Y receptors. P2U was the predominant P2 receptor in mediating PI hydrolysis in astrocytes. The effect of ATP, UTP, or 2-MeSATP was markedly inhibited by pretreatment of cells with pertussis toxin (PTX), indicating that both P2U and P2Y receptors coupled to phospholipase C through PTX-sensitive G protein. Short-term (10 min) treatment of cells with 1 microM TPA attenuated ATP, UTP, and 2-MeSATP-induced PI breakdown; however, long-term (24 h) pretreatment resulted in marked potentiation of both ATP and UTP, and restoration of 2-MeSATP responses. In a further analysis of the effect of TPA, 10 min and 1.5 h pretreatment attenuated ATP-and UTP-induced PI breakdown, but this inhibitory action was lost after 3 h of treatment. Both 6 and 24 h pretreatments resulted in a potentiation. Western blot analysis showed translocation of protein kinase C (PKC) alpha, -delta, and -theta from the cytosol to the membrane following 10 min and 1.5 h treatments, and restoration to basal levels in the membrane fraction was seen after 3 h of treatment. On the other hand, partial and complete down-regulation of these three isoforms was seen after 6 and 24 h of treatment, respectively. PKC eta was translocated but not down-regulated by TPA. These results suggested that PKC alpha, -delta, and -theta, not -eta may exert tonic inhibition on P2U receptor-mediated PI turnover in unstimulated astrocytes.

A novel P2-purinoceptor expressed by a subpopulation of astrocytes from the dorsal spinal cord of the rat

1. Astrocytes from the dorsal spinal cord express P2-purinoceptors which, when stimulated, produce a rise in the intracellular level of free Ca2+ ([Ca2+]i). Previously we have found that the P2Y class of receptor is expressed by nearly all astrocytes from the dorsal horn. To determine whether other metabotropic P2-purinoceptor classes are also present, in this study we investigated the effects of UTP. 2. Application of UTP (1-500 microM, 5-20 s) produced a transient rise in [Ca2+]i in a subpopulation of astrocytes. The magnitude of the peak increase in [Ca2+]i was dependent upon UTP concentration and the EC50 was found to be 5.2 +/- 0.2 microM. Ca2+ responses were maximum at 100 microM UTP. 3. The rise in [Ca2+]i in response to UTP was not affected by removal of extracellular Ca2+. On the other hand, application of the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, abolished responses to UTP. These findings indicate that UTP stimulates the release of Ca2+ from a thapsigargin-sensitive intracellular pool. 4. The Ca2+ response to UTP was unaffected by treatment with pertussis toxin, suggesting that UTP responses may be mediated via a pertussis toxin-insensitive G protein. 5. While all cells tested (n = 52) responded to the P2Y-purinoceptor agonist, 2-methylthio-ATP, only a subpopulation of astrocytes (n = 67/93) was responsive to UTP. The presence of UTP-sensitive and UTP-insensitive cells requires the existence of two discrete types of receptor. One receptor, expressed by UTP-insensitive cells, appears to be activated selectively by 2-methylthio-ATP. 6. To investigate whether UTP and 2-methylthio-ATP activate a common type of receptor in UTP-responsive cells, a cross-desensitization strategy was used. Desensitization with prolonged exposure to a high concentration of 2-methylthio-ATP failed to affect responses to UTP and vice versa, indicating that receptors activated by UTP are distinct from those activated by 2-methylthio-ATP. 7. The P2-purinoceptor antagonist, suramin (100 microM), blocked Ca2+ responses to UTP and to 2-methylthio-ATP. 8. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), has been reported to block responses mediated by P2X- and P2Y-purinoceptors in other systems and therefore we investigated its effects on responses to 2-methylthio-ATP and to UTP. PPADS was found to block Ca2+ responses to 2-methylthio-ATP in a concentration-dependent manner with an IC50 of 0.92 +/- 0.1 microM. PPADS also blocked UTP-evoked responses and the IC50 was 7.2 +/- 1.9 microM. At a concentration of 10 microM, PPADS produced a rightward shift in the dose-response curve for UTP and did not affect the maximum response. 9. Calcium responses evoked by the muscarinic agonist, carbachol, were unaffected either by suramin (100 microM) or by PPADS (50 microM). 10. The present results indicate the presence of a novel class of metabotropic P2U-purinoceptor in dorsal spinal astrocytes. In contrast to P2Y-purinoceptors, the P2U-purinoceptor is expressed only by a subpopulation of astrocytes and its sensitivity to suramin and PPADS distinguish this receptor from P2U-purinoceptors found in other tissues.

Extracellular ATP activates different signalling pathways in rat Sertoli cells

1. The present study describes effects of extracellular ATP (ATPe) on plasma membrane potential and cytoplasmic Ca2+ concentrations ([Ca2+]i) in rat Sertoli cells. Sertoli cells in suspension were stimulated with ATPe and other nucleotides and ionic changes were monitored utilizing the fluorescent dyes bis-oxonol and fura-2/AM. ATPe induced a prompt plasma membrane depolarization which was dependent on Na+ influx from the extracellular medium, since it was abolished by omission of extracellular Na+. Depolarization was independent of [Ca2+]i rise as it also occurred in the absence of extracellular Ca2+ and after intracellular Ca2+ stores were discharged with thapsigargin. ATPe also stimulated a rapid and biphasic increase in [Ca2+]i: a prompt spike was followed by a prolonged sustained plateau. The initial spike was dependent on Ca2+ release from intracellular stores since it was also present when cells were incubated in EGTA-supplemented Ca(2+)-free medium and was abolished by pretreatment with ionomycin and thapsigargin, agents that discharge intracellular Ca2+ stores. The sustained phase was dependent on Ca2+ influx from the extracellular medium as it was abolished when cells were incubated in EGTA-supplemented Ca(2+)-free medium. Ca2+ influx was due to activation of voltage-operated calcium channels (VOCCs) since it was abolished by the VOCC inhibitors verapamil and nifedipine or incubation in sucrose medium, an experimental condition which precludes plasma membrane depolarization by ATPe. 2. ATPe-induced rises in intracellular Ca2+ concentration and plasma membrane depolarization were reduced by pretreatment with pertussis toxin, suggesting that ATPe-activated transduction mechanisms are in part under the control of pertussis toxin-sensitive G-proteins. These data show that Sertoli cells possess P2-purinergic receptor subtypes coupled to influx of Na+ and release of Ca2+ from intracellular stores and provide evidence for an activation of different pathways by extracellular ATPe. Activation of these receptors induces Na+ influx that causes a rapid plasma membrane depolarization. Furthermore, ATPe also triggers Ca2+ release from intracellular stores and Ca2+ influx from extracellular space via dihydropyridine-sensitive VOCCs.

A novel receptor for diadenosine polyphosphates coupled to calcium increase in rat midbrain synaptosomes

1. Diadenosine polyphosphates, Ap4A and Ap5A, as well as ATP, alpha,beta-MeATP and ADP-beta-S, were able to elicit variable intrasynaptosomal Ca2+ increases in rat midbrain synaptic terminals. The origin of the Ca2+ increment was the extra synaptosomal space since the elimination of extracellular Ca2+ abolished the effect of all the agonists. 2. The P2-purinoceptor antagonist, suramin, did not affect the Ca(2+)-increase evoked by diadenosine polyphosphates but dramatically blocked the Ca2+ entry induced by ATP and its synthetic analogues. 3. The actions of Ap5A and ATP on the intrasynaptosomal Ca2+ increase did not cross-desensitize. 4. Concentration-response studies for diadenosine polyphosphates showed pD2 values of 54.5 +/- 4.2 microM and 55.6 +/- 3.8 microM for Ap4A and Ap5A, respectively. 5. The entry of calcium induced by diadenosine polyphosphates could be separated into two components. The first represented a selective voltage-independent Ca2+ entry; the second, a sustained phase which was voltage-dependent. 6. Studies on the voltage-dependent Ca(2+)-channels involved in the effects of the diadenosine polyphosphates, demonstrated that omega-conotoxin G-VI-A inhibited the sustained Ca(2+)-entry, suggesting the participation of an N-type Ca(2+)-channel. This toxin was unable to abolish the initial cation entry induced by Ap4A or Ap5A. omega-Agatoxin IV-A, tetrodotoxin, or nifedipine did not inhibit the effects of the diadenosine polyphosphates. 7. The effect of ATP on Ca(2+)-entry was abolished by nifedipine and omega-conotoxin G-VI-A, suggesting the participation of L- and N-type Ca(2+)-channels in the response to ATP. 8. These data suggest that Ap4A, Ap5A and ATP activate the same intracellular Ca2+ signal through different receptors and different mechanisms. Ap4A and Ap5A induce a more selective Ca2+-entry in a voltage-independent process. This is the first time that a selective action of diadenosine polyphosphate through receptors other than P1 and P2-purinoceptors has been described.

The phospholipase C activating P2U purinoceptor also inhibits cyclicAMP formation in DDT1 MF-2 smooth muscle cells

The P2U purinoceptor mediated effect on cellular cAMP was investigated in DDT1 MF-2 smooth muscle cells. Stimulation of these receptors by ATP or UTP caused a pronounced decrease of about 50% in cellular cAMP levels in forskolin or isoprenaline pretreated cells. This action of the nucleotides was concentration dependent with an IC50 of 9.4 +/- 0.2 microM and 29.0 +/- 0.5 microM for UTP and ATP, respectively and was inhibited by the P2-purinoceptor antagonist suramin. The cAMP level appeared to be modified by intracellular Ca2+, represented by an initial decline in cAMP. Neither inactivation of protein kinase C by staurosporine nor elevated cytoplasmic Ca2+ concentrations interfered with the sustained decrease in cAMP levels induced by ATP or UTP, showing that this effect is not mediated via the phospholipase C pathway known to be activated after P2U purinoceptor stimulation in DDT1 MF-2 cells. Pertussis toxin inhibited the action of these nucleotides on the cellular cAMP level. It can be concluded that the P2U purinoceptor in DDT1 MF-2 cells is coupled to different G-proteins, activating phospholipase C and inhibiting adenylyl cyclase activity.

Calcium-dependent release of arachidonic acid in response to purinergic receptor activation in airway epithelium

The effect of purinergic receptor agonists on arachidonic acid release was investigated in [3H]arachidonic acid-prelabeled human airway epithelial cells. Exposure of bronchial epithelial BEAS39 cells to extracellular ATP resulted in a marked release of unesterified [3H]arachidonic acid with maximal effect observed within 60-90 s. [3H]diacylglycerol and [3H]phosphatidic acid accumulated in parallel with [3H]arachidonic acid. ATP-stimulated [3H]arachidonic acid release with a K0.5 of 9 +/- 2 microM and UTP was equipotent; no effect was observed with P2Y- or P2X-purinergic receptor agonists or with adenosine. Similar results were obtained with primary cultures of normal human nasal epithelium, CF/T43 and HBE1 airway epithelial cell lines derived from a cystic fibrosis patient and from a normal donor, respectively, and HT-29 human colon carcinoma cells. ATP stimulated inositol phosphate formation in BEAS39 cells with a concentration dependence identical to that for [3H]arachidonic acid release. The effect of ATP on both [3H]arachidonic acid release and inositol phosphate formation was equally inhibited by pertussis toxin. The Ca2+ ionophore A-23187 mimicked the effects of ATP or UTP on arachidonic acid release, and a marked inhibitory effect was observed with thapsigargin. The protein kinase C inhibitor staurosporine partially inhibited ATP-stimulated [3H]arachidonic acid release. These data are consistent with the hypothesis that phospholipase A2 activation is secondary to P2U-purinergic receptor stimulation of D-myoinositol 1,4,5-trisphosphate production and calcium mobilization from intracellular stores.

Distribution and characterisation of [3H]alpha,beta-methylene ATP binding sites in the rat

Radioligand binding studies have been performed to study the distribution of the binding sites for the P2x purinoceptor selective agonist radioligand, [3H]alpha,beta-methylene ATP ([3H]alpha beta-meATP), in membranes prepared from various peripheral organs and several brain regions of the rat. In agreement with previous studies in the rat vas deferens, [3H]alpha beta-meATP labelled two populations of sites. One site exhibited high affinity for the ligand (Kd = 0.7 nM; Bmax = 1012 fmol.mg-1 protein) while the other site exhibited lower affinity (Kd = 70.8 nM) and higher capacity (Bmax) = 7470 fmol.mg-1 protein). In competition studies, using a low concentration of radioligand (1 nM), the high affinity alpha beta-meATP binding sites in vas deferens membranes could be preferentially labelled (84-91%). Under these conditions, the P2x purinoceptor agonists, alpha beta-meATP and beta, gamma-methylene ATP, had the highest affinity with pIC50 values of 8.3 and 7.3 respectively. The P2y purinoceptor agonist, 2-methyl-thio-ATP (2-me-S-ATP), had lower affinity (pIC50 = 6.7), while uridine triphosphate, adenosine diphosphate and adenosine, agonists at the P2u, P2t and P1 purinoceptors, respectively, possessed low affinity (pIC50 values < 5.6). In addition, the P2 purinoceptor antagonists, cibacron blue and suramin, inhibited binding over the same concentration range at which they behave as functional antagonists at the P2x purinoceptor. High and low affinity binding sites for [3H]alpha beta-meATP were also identified in a range of other peripheral tissues (spleen, heart and liver) and in several brain regions (striatum, cerebral cortex, hippocampus). In the spleen, heart, cerebral cortex and liver the Kd values at both the high affinity binding sites (Kd = 1-1.2 nM) and the low affinity binding sites (Kd = 98-158 nM) were similar to the respective Kd values at the high and low affinity binding sites in the vas deferens. In competition studies performed using a low concentration of radioligand (1 nM) these sites exhibited a similar pharmacological profile to that seen in the vas deferens. Detailed analysis of competition curves to several of the ATP analogues in each of the tissues revealed that the binding profile of the radioligand was complex since several compounds, and in particular ATP and 2-me-S-ATP, identified a lower proportion of sites with high affinity than did alpha beta-meATP.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of L-type Ca2+ current by extracellular ATP in ferret isolated right ventricular myocytes

1. The effects of extracellular adenosine triphosphate (ATP) on the basal L-type Ca2+ current (ICa) were investigated in ferret isolated right ventricular myocytes using the gigaohm seal voltage clamp in the whole-cell and cell-attached configurations. 2. Micromolar levels of extracellular ATP reversibly inhibited ICa in a concentration-dependent manner, without any significant changes in the voltage dependence of either the peak ICa I-V relationship or steady-state activation curve. 3. In contrast, micromolar levels of extracellular ATP did significantly alter the inactivation characteristics of ICa. Ten micromolar ATP: (i) increased the degree of steady-state inactivation of ICa; (ii) altered the time constants of ICa inactivation at 0 mV; and (iii) decreased the time constant of ICa recovery from inactivation at -70 mV. 4. The inhibitory effect of ATP on ICa was not blocked by atropine, a muscarinic cholinergic receptor antagonist, or CPDPX (8-cyclopentyl-3,4-dipropylxanthine), an A1 adenosine receptor antagonist. In contrast, the inhibitory effect of 10 microM ATP could be nearly completely antagonized by 100 microM suramin, a purinergic P2 receptor antagonist. 5. The potency order of ATP analogues in inhibiting ICa was 2-methyl-thio-ATP > ATP > alpha,beta-methylene-ATP, indicating involvement of a P2Y-type ATP receptor. 6. Pretreatment of cells with pertussis toxin (PTX) did not prevent the ATP-induced decrease in ICa. However, (i) ATP produced an irreversible decrease of ICa in the presence of intracellular GTP gamma S, and (ii) the inhibitory effect was significantly attenuated in the presence of intracellular GDP beta S, indicating the involvement of a PTX-insensitive G protein in the P2Y receptor-coupling process. 7. Neither (i) replacing extracellular Ca2+ with 1 mM Ba2+, nor (ii) intracellular perfusion of 10 mM BAPTA for at least 30 min attenuated the inhibitory effect of ATP on the current through Ca2+ channels, suggesting that the inhibitory effect was not obligatorily dependent upon influx of Ca2+ or changes in [Ca2+]i. 8. Ensemble-average current behaviour constructed from cell-attached patch recordings of single L-type Ca2+ channels (110 mM BaCl2) demonstrated that when 10 microM ATP was added to the superfusate on the outside of the patch electrode the inhibition of ICa was still observed, providing evidence for the involvement of intracellular diffusible second messenger(s).(ABSTRACT TRUNCATED AT 400 WORDS)

Aggregation of human and canine platelets: modulation by purine nucleotides

This study compared the responses of canine and human platelets to various aggregating agonists in the presence or absence of extracellular ATP and ATP analogues. Canine and human platelets were approximately equally reactive with ADP or collagen while the canine platelets were about 10 fold more sensitive to thrombin. Canine platelets were insensitive to the thromboxane mimetic U46619 but were synergistically aggregated by a mixture of ADP and U46619. Human platelets were very sensitive to U46619. Aggregations of human platelets with all of the above agonists were inhibited by extracellular ATP; beta, gamma methylene ATP (beta gamma ATP) and benzoyl ATP (BzATP) with a rank order suggestive of an interaction with P2x-like purinoceptors which support our previous findings. The comparable aggregations of canine platelets were likewise inhibited by ATP and its analogues but with a rank order suggestive of an interaction with P2y-like purinoceptors. ATP inhibited U46619- and ADP-induced aggregation of human platelets and ADP-induced aggregation of canine platelets, presumably, in part, due to competition for the ADP P2T receptor. However, when U46619 was added to either ATP or ATP analogue-inhibited ADP-treated canine platelets, the inhibition was nullified. Furthermore, we demonstrated, for the first time, that the canine thromboxane receptor becomes reactive to U46619 alone after incubation at room temperature for 3.5-5 hrs while human platelets become inactive under similar conditions. The implication of these studies is that there are significant differences in the canine and human platelet thromboxane and purine receptors. The future characterization of these differences and the mechanism by which they function should further our understanding of the impact of extracellular ATP on hemostasis and thrombosis.

Osteoblast-like cells have a variable mixed population of purino/nucleotide receptors

Osteoblast-like UMR 106.06 cells respond to extracellular application of nucleotides with a fast intracellular calcium pulse (latency of about 20 s, half-width of about 10 s), as measured with fluo-3 on a confocal laser scanning system. Cross-inhibition experiments at 50 microM show that, on a cell population basis, adenosine triphosphate (ATP) strongly inhibits the effect of uridine triphosphate (UTP) or 2-methylthio-ATP (2-MeSATP) applied within 2 min after the end of the ATP-induced pulse, while prior application of UTP or 2-MeSATP only weakly inhibits the ATP effect, and UTP and 2-MeSATP weakly inhibit each other. Furthermore, there are clear differences in cross-inhibition between individual cells. Our measurements provide strong evidence that these cells have at least two types of purino/nucleotide receptors, probably P2y and P2u, with a proportion that varies between individual cells.

Occupancy of P2 purinoceptors with unique properties modulates the function of human platelets

This report demonstrates that platelets possess P2 purinoceptors with unique properties that distinguish them from the ADP (P2T) receptor. Extracellular ATP, and its poorly hydrolyzable analogues, inhibit collagen- and U46619 (a thromboxane mimetic)-induced platelet aggregations. Adenosine deaminase was without effect on ATP action while reversing the inhibitory effect of adenosine. A unique aspect of the P2 receptor is the sensitivity to UTP and CTP and insensitivity to GTP. The rank order of inhibition by beta gamma-methylene ATP, alpha beta-methylene ATP > ATP indicates that a P2x-like receptor is present on the platelet membrane. This conclusion is further supported by the nearly complete desensitization to ATP by pre-exposure of platelets to alpha beta-methylene-ATP. However, unlike previously described P2x purinoceptors, the inhibition of platelet aggregation by extracellular ATP appears to result, at least in part, from the ATP-induced increase of intracellular cyclic AMP levels apparently coupled through a Gs protein. The combined addition of iloprost (0.14 to 1.39 nM) and ATP (18 microM) or ATP (20-40 microM) and the phosphodiesterase inhibitor theophylline (0.5 to 1 mM) synergistically inhibited platelet aggregation implying a common interactive site with adenylate cyclase. This is further substantiated by the ability of the adenylate cyclase inhibitor, 2',5'-dideoxyadenosine, to abrogate the inhibitory effects of ATP. The protein kinase A (PKA) inhibitor H1004 blocks ATP inhibition of platelet aggregation while the protein kinase C inhibitor H7 did not. This implies that the generation of cyclic AMP, with the subsequent activation of PKA and phosphorylation of selected proteins is required, in part, for the action of ATP.

P2Y purinoceptors in normal NIH 3T3 and in NIH 3T3 overexpressing c-ras

The ability of purinergic agonists to induce Ca2+ responses has been tested in two lines of murine fibroblasts: normal NIH 3T3 fibroblasts and NIH 115.14, a clone expressing high levels [1] of the c-ras protooncogene. Both kinds of cells are responsive to ATP in the range 1 microM-1 mM; ADP and ATP gamma S are almost as potent as ATP, while AMP is unable to elicit a response. Ca2+ measurements performed in single cells by image analysis show great variability among cells but in each individual responding cell the Ca2+ rise occurs in an all-or-none fashion. The transient Ca2+ response does not depend on influx from the extracellular medium. Electrophysiological experiments reveal the activation of an outward current (at -50 mV) by ATP, probably due to Ca(2+)-activated K+ channels, confirming the absence of a substantial Ca2+ influx. Finally, stimulation by ATP produces a small but significant increase in the production of inositol phosphates. These results indicate that these cell lines possess purinergic receptors which are not integral membrane channels and which are coupled to InsP3 formation and may be therefore classified as P2Y.