P2 purinoceptor-mediated cyclic AMP accumulation in bovine vascular smooth muscle cells

Adenine nucleotide-induced activation of adenosine A(2B) receptors expressed in Xenopus laevis oocytes: involvement of a rapid and localized adenosine formation by ectonucleotidases

We recently demonstrated that extracellular ATP effectively activates adenosine (Ade) A(2B) receptors indirectly through a localized rapid conversion to Ade by ectonucleotidases on the membrane surface of C6Bu-1 rat glioma cells. These responses were observed even in the presence of adenosine deaminase (ADA). Here, we demonstrate that such responses indeed occur in A(2B) receptor-expressing Xenopus laevis oocytes, which possess endogenous ectonucleotidase activity. In oocytes coexpressing the A(2B) receptor and cystic fibrosis transmembrane conductance regulator (CFTR), Ade induced a concentration-dependent increase in a cyclic AMP-activated CFTR current, a response that was inhibited by the P1 antagonist xanthine-amine congener (XAC). A brief application of ATP and beta,gamma-methylene ATP (beta,gamma-MeATP) also induced the CFTR current in a manner similar to that seen with Ade. Among several nucleotide agonists, ADP, AMP, and adenosine-5'-O-(3-thio)triphosphate induced the CFTR current. Although adenine nucleotide-induced CFTR currents were inhibited by XAC, they were highly resistant to ADA treatment; 5 U/ml ADA was required for inhibition of adenine nucleotide-induced CFTR current, whereas 1 U/ml ADA was sufficient to abolish the Ade-induced response. In addition, the ecto-5'-nucleotidase inhibitor alpha,beta-methylene ADP markedly inhibited the beta,gamma-MeATP-induced response but not the Ade-induced one. These results support our hypothesis that adenine nucleotides are rapidly and locally converted into Ade on the membrane surface, resulting in the activation of A(2B) receptors.

Cyclic adenosine monophosphate-dependent vascular responses to purinergic agonists adenosine triphosphate and uridine triphosphate in the anesthetized mouse

The mechanism by which purinergic agonist adenosine triphosphate (ATP) and uridine triphosphate (UTP) decrease systemic arterial pressure in the anesthetized mouse was investigated. Intravenous injections of adenosine triphosphate (ATP) and uridine triphosphate (UTP) produced dose-dependent decreases in systemic blood pressure in the mouse. The order of potency was ATP > UTP. Vasodilator responses to ATP and UTP were altered by the cyclic adenosine monophosphate (cAMP) phosphodiesterase inhibitor rolipram. The vascular responses to ATP and UTP were not altered by a nitric oxide synthase inhibitor, a cyclooxygenase inhibitor, a cGMP phosphodiesterase inhibitor, or a particular P2 receptor antagonist. These data suggest that ATP and UTP cause a decrease in systemic arterial pressure in the mouse via a cAMP-dependent pathway via a novel P2 receptor linked to adenylate cyclase and that nitric oxide release, prostaglandin synthesis, cGMP, and P2X1, P2Y1, and P2Y4 receptors play little or no role in the vascular effects of these purinergic agonists in the mouse.

Vasodilator responses to ATP and UTP are cAMP dependent in the mesenteric vascular bed of the cat

BACKGROUND

This study was designed to examine the responses to and the mechanism by which purinergic agonists decrease vascular resistance in the mesenteric vascular bed of the cat.

METHODS AND RESULTS

Injections of ATP, UTP, and 2-MethylThioATP (2-MetSATP) into the mesenteric perfusion circuit elicited dose-dependent decreases in perfusion pressure while injections of beta,gamma-MethylATP (beta,gamma-MetATP) produced a biphasic response with an initial vasopressor response followed by a vasodilator response. The order of potency of the vasodilator response was 2-MetSATP > ATP > UTP > beta,gamma-MetATP. The vasodilator responses to ATP, UTP, 2-MetSATP, and beta,gamma-MetATP were increased in duration by the cAMP phosphodiesterase inhibitor, rolipram. However, vasodilator responses were not altered by the adminstration of a nitric oxide synthase inhibitor, a cGMP phosphodiesterase inhibitor, or a cyclooxygenase inhibitor. Treatment with PPADS, a P2X(1), P2Y(1), and P2Y(4) receptor antagonist, did not alter vasodilator responses to the purinergic agonists; however, the vasopressor component of the response to beta,gamma-MetATP was decreased.

CONCLUSIONS

These data suggest that ATP, UTP, 2-MetSATP, and beta,gamma-MetATP dilate the mesentary vascular bed in the cat by a cAMP dependent mechanism, and that nitric oxide or prostaglandin release, cGMP accumulation, or activation of P2X(1), P2Y(1), or P2Y(4) receptors play little or no role in mediating vasodilator responses to the purinergic agonists in this regional vascular bed. In addition, these results suggest that the pressor component of the response to beta,gamma-MetATP is mediated by the activation of P2X(1) receptors.

Beta,gamma-methylene ATP-induced cAMP formation in C6Bu-1 cells: involvement of local metabolism and subsequent stimulation of adenosine A2B receptor

The mechanism underlying beta,gamma-methylene ATP (beta,gamma-MeATP)-induced cAMP elevation was investigated in rat glioma C6Bu-1 cells. Beta,gamma-MeATP increased forskolin-stimulated cAMP formation in a manner sensitive to both the P1 antagonist xanthine amine congener (XAC) and the P2 antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). Adenosine deaminase (ADA; 1 U/mL), which abolished the adenosine-induced response, did not eliminate the beta,gamma-MeATP-induced response. However, combination of ADA with alpha,beta-methylene ADP (alpha,beta-MeADP), an ecto-5'-nucleotidase inhibitor, blocked the beta,gamma-MeATP-induced response. AMP, the substrate for ecto-5'-nucleotidase, also induced cAMP formation in a manner sensitive to XAC and alpha,beta-MeADP inhibition. However, the AMP-induced response was not blocked by PPADS. HPLC analyses revealed that adenosine was generated from beta,gamma-MeATP and AMP. In addition, alpha,beta-MeADP inhibited the conversion of beta,gamma-MeATP and AMP to adenosine, whereas PPADS blocked adenosine formation from beta,gamma-MeATP but not from AMP. [3H]Adenosine generated from [3H]AMP was preserved on the cell surface environment even in the presence of ADA. The mRNAs for ecto-phosphodiesterase/pyrophosphatase 1 (EC 3.1.4.1), ecto-5'-nucleotidase (EC 3.1.3.5) and adenosine A2B receptor were detected by RT-PCR. These results suggest that C6Bu-1 cells possess ecto-enzymes converting beta,gamma-MeATP to adenosine, and the locally accumulated adenosine in this mechanism efficiently stimulates A2B receptors in a manner resistant to exogenous ADA.

ATP induces c-fos expression in C6 glioma cells by activation of P(2Y) receptors

BACKGROUND

Extracellular ATP functions in the enteric nervous system as a neurotransmitter, and recent evidence suggests ATP may regulate development through effects on cellular proliferation.

METHODS

The action of ATP at purinoceptors and the role of second messenger pathways in c-fos mRNA expression in C6 glioma cells were investigated using the techniques of Northern and Western blotting.

RESULTS

Treatment of C6 cells with ATP caused a time- and dose-dependent increase in c-fos expression. The rank order of agonist potency was ATP = ADP > gammasATP > alphabetaATP > betagammaATP > AMP = UTP. The ATP-induced c-fos increment was inhibited by three P(2Y) receptor antagonists-suramin, reactive blue, and DIDS-by 99+/-3, 89+/-7, and 61+/-14%, respectively. The ATP-stimulated c-fos expression was attenuated by phospholipase C inhibitor (U73122), protein kinase C (PKC) down-regulation (4alpha-phorbol 12-myristate 13-acetate and chelerythrine), mitogen-activated protein (MAP) kinase inhibition (apigenin), an inhibitor of MAP kinase kinase (PD98059), down-regulation of adenylate cyclase (SQ22536), and inhibition of type II protein kinase A (8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphorothioate), but was not affected by inhibition of type I protein kinase A (8-bromoadenosine-3',5'-cyclic monophosphorothioate) and inhibitors of calmodulin kinase (KN93 and KN62). Phosphorylated MAP kinase was increased in cells exposed to ATP. This effect was suppressed by chelerythrine.

CONCLUSIONS

These studies demonstrate that ATP-induced c-fos mRNA expression is under multifactorial regulation.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

Salivary gland P2 nucleotide receptors

The effects of ATP on salivary glands have been recognized since 1982. Functional and pharmacological studies of the P2 nucleotide receptors that mediate the effects of ATP and other extracellular nucleotides have been supported by the cloning of receptor cDNAs, by the expression of the receptor proteins, and by the identification in salivary gland cells of multiple P2 receptor subtypes. Currently, there is evidence obtained from pharmacological and molecular biology approaches for the expression in salivary gland of two P2X ligand-gated ion channels, P2Z/P2X7 and P2X4, and two P2Y G protein-coupled receptors, P2Y1 and P2Y2. Activation of each of these receptor subtypes increases intracellular Ca2+, a second messenger with a key role in the regulation of salivary gland secretion. Through Ca2+ regulation and other mechanisms, P2 receptors appear to regulate salivary cell volume, ion and protein secretion, and increased permeability to small molecules that may be involved in cytotoxicity. Some localization of the various salivary P2 receptor subtypes to specific cells and membrane subdomains has been reported, along with evidence for the co-expression of multiple P2 receptor subtypes within specific salivary acinar or duct cells. However, additional studies in vivo and with intact organ preparations are required to define clearly the roles the various P2 receptor subtypes play in salivary gland physiology and pathology. Opportunities for eventual utilization of these receptors as pharmacotherapeutic targets in diseases involving salivary gland dysfunction appear promising.

Effects of P(1) and P2 receptor antagonists on beta, gamma-methyleneATP- and CGS21680-induced cyclic AMP formation in NG108-15 cells

1. We have previously shown that ATP increased cyclic AMP in NG108-15 cells, which was inhibited by P(1) receptor antagonist methylxanthines. In the present study, we examined the effects of P(1) and P2 receptor antagonists on cyclic AMP formation induced by beta,gamma-methyleneATP (beta,gamma-MeATP) and CGS21680, an A(2A) adenosine receptor agonist, in NG108-15 cells. 2. beta,gamma-MeATP and CGS21680 increased intracellular cyclic AMP with EC(50) values of 8. 0+/-0.98 microM (n=4) and 42+/-7.5 nM (n=4), respectively. 3. Several P(1) receptor antagonists inhibited both beta,gamma-MeATP- and CGS21680-induced cyclic AMP increase with a similar rank order of potency; ZM241385>CGS15943>XAC>DPCPX. However, the pK(i) values of these antagonists for beta,gamma-MeATP were larger than those for CGS21680. 4. Alloxazine, a P(1) receptor antagonist, and several P2 receptor antagonists (PPADS, iPPADS, reactive blue-2) inhibited beta, gamma-MeATP-induced response, while these antagonists little affected CGS21680-induced one. Suramin was effective only for beta, gamma-MeATP-induced response at 1 mM. 5. 2-chloroadenosine (2CADO) and 2-chloroATP (2ClATP) increased cyclic AMP with similar potencies. The effects of these agonists were both inhibited by ZM241385, but only 2ClATP-induced response was inhibited by PPADS. 6. ATP- and beta, gamma-MeATP-induced responses were little affected by alpha, beta-methyleneADP, a 5'-nucleotidase inhibitor. 7. These results clearly demonstrate that ATP-stimulated cyclic AMP formation can be distinguished from the A(2A) receptor agonist-induced one by using the several P(1) and P2 receptor antagonists.

Involvement of K+ channel permeability changes in the L-NAME and indomethacin resistant part of adenosine-5'-O-(2-thiodiphosphate)-induced relaxation of pancreatic vascular bed

1. We have previously demonstrated that adenosine-5'-O-(2-thiodiphosphate) (ADPbetaS), a potent P2Y-purinoceptor agonist, relaxed pancreatic vasculature not only through prostacyclin (PGI2) and nitric oxide (NO) release from the endothelium but also through other mechanism(s). In this study, we investigated the effects of an inhibitor of the Na+/K+ pump, of ATP-sensitive K+ (K(ATP)) channels and of small (SK(Ca)) or large (BK(Ca)) conductance Ca2+-activated K+ channels. Experiments were performed at basal tone and during the inhibition of NO synthase and cyclo-oxygenase. 2. In control conditions, ADPbetaS (15 microM) induced an initial transient vasoconstriction followed by a progressive and sustained vasodilatation. In the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME, 200 microM) the transient vasoconstriction was reversed into a one minute vasodilator effect, which was then followed by a progressive and sustained vasodilatation similar to that observed with ADPbetaS alone. The addition of indomethacin (10 microM) did not significantly modify the profile of ADPbetaS-induced vasodilatation. 3. Ouabain (100 microM) decreased basal pancreatic flow rate and did not modify ADPbetaS-induced relaxation. This inhibitor of the Na+/K+ pump increased the pancreatic vasoconstriction induced by L-NAME or by the co-administration of L-NAME and indomethacin. Ouabain did not modify either the L-NAME or the L-NAME/indomethacin resistant part of the ADPbetaS vasodilatation. 4. The K(ATP) inhibitor tolbutamide (185 microM) did not significantly modify basal pancreatic flow rate and ADPbetaS-induced relaxation. This inhibitor which did not change L-NAME-induced vasoconstriction, significantly diminished the L-NAME resistant part of ADPbetaS-induced vasodilatation. Tolbutamide intensified the vasoconstriction induced by the co-administration of L-NAME and indomethacin. In contrast, the L-NAME/indomethacin resistant part of ADPbetaS vasodilatation was not changed by the closure of K(ATP). 5. The SK(Ca) inhibitor apamin (0.1 microM) did not significantly change pancreatic vascular resistance whatever the experimental conditions (in the absence or in presence of L-NAME or L-NAME/indomethacin). In the presence of L-NAME, the closure of SK(Ca) channels changed the one minute vasodilator effect of ADPbetaS into a potent vasoconstriction and thereafter modified only the beginning of the second part of the L-NAME-resistant part of the ADPbetaS-induced vasodilatation. In contrast, the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation remained unchanged in the presence of apamin. 6. Charybdotoxin (0.2 microM), an inhibitor of BK(Ca), increased pancreatic vascular resistance in the presence of L-NAME/indomethacin. In the presence of L-NAME, the closure of BK(Ca) channels reversed the one minute vasodilator effect of ADPbetaS into a potent vasoconstriction and drastically diminished the sustained vasodilatation. In contrast the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation was not modified by the presence of charybdotoxin. Under L-NAME/indomethacin/charybdotoxin/apamin infusions, ADPbetaS evoked a drastic and transient vasoconstriction reaching a maximum at the second minute, which was followed by a sustained increase in the flow rate throughout the ADPbetaS infusion. The maximal vasodilator effect of ADPbetaS observed was not modified by the addition of apamin. 7. The results suggest that the L-NAME-resistant relaxation induced by ADPbetaS in the pancreatic vascular bed involves activation of BK(Ca), K(ATP) and to a lesser extent of SK(Ca) channels, but the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation is insensitive to the closure of K(ATP), SK(Ca) and BK(Ca) channels.

Involvement of protein kinase C in the UTP-mediated potentiation of cyclic AMP accumulation in mouse J774 macrophages

1. We have investigated the effects of nucleotide analogues on cyclic AMP formation in mouse J774 macrophages and the mechanisms involved. 2. UTP, in the concentration range 0.1-100 microM, induced concentration-dependent potentiation of prostaglandin E1 (PGE1)-induced cyclic AMP formation, but had no effect on basal cyclic AMP formation. UDP showed an equal potency, while 2-methylthio ATP, alpha, beta-methylene ATP and beta,gamma-methylene ATP gave either a slight increase or had no effect at concentrations up to 100 microM. ATP, although 100 fold less effective than UTP, also caused cyclic AMP potentiation, but had no effect on agonist-stimulated or basal cyclic AMP levels. 3. The cyclic AMP potentiation effect of UTP correlated with increased [Ca2+]i and inositol phosphate (IP) formation over the same concentration range. 4. Ionomycin, which evokes an increase in [Ca2+]i without affecting IP formation, did not cause an increase in cyclic AMP content, indicating that UTP-induced cyclic AMP regulation is not due to activation of Ca(2+)-sensitive adenylyl cyclase isoforms. 5. Although reduced, UTP potentiation was seen in cells incubated in a Ca(2+)-free and/or BAPTA-containing medium. Under these conditions, the UTP-increased IP accumulation was similarly reduced. 6. Exposure of cells to phorbol 12-myristate 13-acetate (PMA) also increased PGE1 stimulation of cyclic AMP levels, and the UTP-induced potentiation of cyclic AMP formation was inhibited by either staurosporine or Ro 31-8220. Pretreatment of cells with PMA for 4-24 h resulted in marked attenuation of UTP-stimulated cyclic AMP potentiation. 7. Pretreatment with pertussis toxin (24 h, 100 ng ml-1) did not significantly affect UTP-induced cyclic AMP potentiation and IP formation, although it increased the cyclic AMP response to PGE1. 8. Analysis of J774 cells by Western blotting with antibodies specific for different protein kinase C (PKC) isoforms shows the presence of the beta I, beta II, delta, epsilon, eta, mu, lambda and zeta isoforms. Moreover, UTP significantly increased the level of PKC beta I, beta II, delta, epsilon, mu, lambda and zeta immunoreactivity in the membrane fraction and decreased the cytosolic reactivity of PKC beta II, delta, epsilon and zeta. 9. Immunoblot studies also indicate the presence of type II adenylyl cyclase. 10. These results indicate that PKC is required for the potentiation of adenylyl cyclase activity by macrophage pyrimidinoceptors, which exhibit a higher specificity for UTP and UDP than for ATP.

Evidence that most high-affinity ATP binding sites on aortic endothelial cells and membranes do not correspond to P2 receptors

It has recently been demonstrated that two types of ATP receptors, the P2Y and P2U receptors, are coexpressed on bovine aortic endothelial cells. The aim of the present study was to characterize directly P2Y and P2U subtypes on intact bovine aortic endothelial cells and on membranes prepared from these cells using adenosine 5'-0-(3-thio[35S]triphosphate) ([35S]ATP gamma S), [alpha-32P]ATP and [alpha-32P]UTP as radioligands. [35S]ATP gamma S binding to bovine aortic endothelial cell membranes was saturable and apparently involved a single class of high-affinity binding sites (Kd: 14 +/- 11 nM. Bmax 1.6 +/- 0.7 pmol/mg protein; mean +/- S.D.). A similar class of high-affinity binding sites was identified with [alpha-32P]ATP (Kd: 14 +/- 9 nM; Bmax: 1.7 +/- 1.1 pmol/mg protein; mean +/- S.D.). Competition experiments showed that only one third of these sites bound 2-methylthio-ATP (2-MeSATP) with high affinity (Ki: 21 +/- 5 and 14 +/- 10 nM, mean +/- S.D., for [35S]ATP gamma S and [alpha-32P]ATP, respectively) and might therefore represent the P2Y receptors. UTP did not compete with [35S]ATP gamma S or [alpha-32P]ATP for binding at the remaining sites, indicating that they are not the P2U receptors. No high-affinity UTP binding sites could be detected using [alpha-32P]UTP. [35S]ATP gamma S binding to intact bovine aortic endothelial cells was competed by ATP gamma S (Kd: 1.0 +/- 0.5 microM; mean +/- S.D.), but not by 2-MeSATP and UTP, indicating that these binding sites are neither the P2Y nor the P2U receptors.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.

Involvement of multiple receptors in the actions of extracellular ATP: the example of vascular endothelial cells

The role of ATP and ADP as intercellular mediators is now well established. The presence of the nucleotides in extracellular fluids can result from several mechanisms: cell lysis, selective permeabilization of the plasma membrane and exocytosis of secretory vesicles, such as platelet dense bodies. Extracellular adenine nucleotides are rapidly degraded by ectonucleotidases expressed inter alia on the surface of endothelial cells. They act on cells via the family of P2 receptors which encompasses more than 5 subtypes, some of which have been cloned recently. The P2T, P2U and P2Y receptors belong to the superfamily of receptors coupled to G proteins, whereas the P2X receptor is a cation channel and the P2Z receptor a non-selective pore. ATP and ADP stimulate the endothelial production of prostacyclin (PGI2) and nitric oxide (NO), two vasodilators and inhibitors of platelet aggregation, via an increase in cytosolic Ca2+. This action of adenine nucleotides is believed to limit the extent of intravascular platelet aggregation and to help localize thrombus formation to areas of endothelial damage. The endothelial response to nucleotides is mediated by at least two distinct subtypes of P2 receptors, P2Y and P2U, both coupled to phospholipase C.

Mechanisms of ATP-induced Ca2+ signaling in osteoclasts

We investigate the mechanisms underlying the intracellular calcium pulse that occurs in response to extracellular adenosine triphosphate (ATP) in osteoclasts. We find that pre-loading of GDP-beta-S abolishes the response in Ca(2+)-free medium, demonstrating an internal release of Ca2+ via a pathway that involves a G protein. GDP-beta-S does not block in normal Ca(2+)-containing medium, suggesting that ATP also induces a Ca2+ influx across the cell membrane. We confirmed this using the Mn2+ quenching technique, which shows significant opening of Ca2+ channels. We find a smaller response to adenosine diphosphate (ADP) and 2-methylthio-ATP (2-MeSATP), but no response to beta, gamma-methylene-ATP (AMP-PCP), adenosine monophosphate (AMP) or uridine triphosphate (UTP). Prior application of AMP and UTP, but not AMP-PCP, blocks the response to ATP. Our results indicate that the receptor is a P2 subtype that is not characteristic of any previously reported P2 receptor or combination of P2 receptors.

Purine- and nucleotide-mediated relaxation of rabbit thoracic aorta: common and different sites of action

The mechanisms of the relaxant effect of purines and pyrimidines in New Zealand rabbit isolated aorta were investigated at endothelial and smooth muscle cell levels. Endothelium-mediated relaxation by ATP was only partially inhibited by the P2-purinoceptor antagonist suramin (0.1 mM). The pyrimidine UTP produced vasodilation by acting at the endothelial level and relaxation was not antagonized by suramin (0.1 mM). This effect was not mediated by P2 purinoceptors, indicating that UTP, like ATP to a certain extent, produces relaxation via an endothelium nucleotide (N) pyrimidinoceptor. ATP, ADP, AMP, adenosine, 5'-N-ethylcarboxamidoadenosine (NECA) and inosine were all active as relaxants on smooth muscle. The NECA relaxant effect was not antagonized by P1-purinoceptor antagonists 3,7-dimethyl-1-propargylxanthine (50 microM) or 1,3-dipropyl-8-(2-amino-4-chlorophenyl)xanthine (5 microM), excluding a P1-mediated effect. P2-related activity was excluded because adenosine-mediated relaxation was not antagonized by suramin (0.1 mM). UTP was ineffective as a relaxant at smooth muscle level, thus excluding the presence of muscular nucleotide (N) pyrimidinoceptor and suggesting a P3 purinoceptor. The rank order of potency of this muscle purinoceptor was NECA > adenosine > ATP approximately equal to ADP approximately equal to AMP approximately equal to inosine.

Evidence that ATP, ADP and AMP are not ligands of the striatal adenosine A2A receptors

It has been claimed recently that, in several cell types, ATP can induce a stimulation of cAMP production which is sensitive to methylxanthine inhibition and is not mediated by the ATP degradation product, adenosine. One explanation for these results would be direct activation of adenosine A2 receptors by ATP itself. We have therefore investigated whether adenine nucleotides are ligands of adenosine A2A receptors from bovine striatum. We show here that ATP, ADP, AMP and their phosphorothioates analogues (ATP gamma S, ADP beta S and AMP alpha S), at a 100 microM concentration, produced a 83-91% inhibition of the binding of [3H]CGS21680, an adenosine A2A receptor agonist, to striatum membranes. However, this action was inhibited by adenosine deaminase or by adenosine 5'-O-(alpha, beta-methylene)diphosphate (APCP), an inhibitor of 5'-nucleotidase-mediated AMP degradation. The effects of adenosine deaminase and APCP were dependent on their concentration. These results indicate that ATP, ADP and even AMP can exert an effect on the adenosine A2A receptors only through their breakdown into adenosine by ectonucleotidases.

Characterization of P2-purinoceptor mediated cyclic AMP formation in mouse C2C12 myotubes

1. The formation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and inositol(1,4,5)trisphosphate (Ins(1,4,5)P3), induced by ATP and other nucleotides was investigated in mouse C2C12 myotubes. 2. ATP (100 microM) and ATP gamma S (100 microM) caused a sustained increase in cyclic AMP content of the cells, reaching a maximum after 10 min. The cyclic AMP content reached a maximum in the presence of 100 microM ATP, followed by a decline at higher ATP concentrations. ATP-induced cyclic AMP formation was inhibited by the P2-purinoceptor antagonist, suramin. 3. Myotubes hydrolysed ATP to ADP at a rate of 9.7 +/- 1.0 nmol mg-1 protein min-1. However, further hydrolysis of ADP to AMP and adenosine was negligible. 4. The cyclic AMP formation induced by ADP (10 microM-1 mM) showed similar characteristics to that induced by ATP, but a less pronounced decline was observed than with ATP. ADP-induced cyclic AMP formation was blocked by suramin, while cyclic AMP formation elicited by adenosine (10 microM-1 mM) was insensitive to suramin. 5. The ATP analogue, alpha,beta-methylene-ATP also induced a suramin-sensitive cyclic AMP formation, while 2-methylthio-ATP and the pyrimidine, UTP, did not affect cyclic AMP levels. 6. Stimulation of the myotubes with ATP or UTP (10 microM-1 mM) caused a concentration-dependent increase in the Ins(1,4,5)P3 content of the cells. ADP (100 microM-1 mM) was less effective. Adenosine did not affect Ins(1,4,5)P3 levels. 7. Incubation of the cells with UTP (30 microM- 1 mM) inhibited the ATP- and ADP-induced cyclic AMP formation, suggesting that stimulation of the 'nucleotide' type P2-receptor inhibits P2-purinoceptor mediated cyclic AMP formation in C2C12 myotubes. In contrast, UTP (30 microM-I mM) enhanced adenosine-induced cyclic AMP formation.8. Adenosine-sensitive P1-purinoceptors activating cyclic AMP formation were found in C2C12 myotubes.Further, a novel P2-purinoceptor is postulated, sensitive to ATP, ADP and ATPgammaS, which also activates the formation of cyclic AMP in C2C12 myotubes.