Pharmacological analysis of ecto-ATPase inhibition: evidence for combined enzyme inhibition and receptor antagonism in P2X-purinoceptor ligands

The Concise Guide to PHARMACOLOGY 2013/14: ligand-gated ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full.

Ligand-gated ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets.

It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

P2X4 subunits are part of P2X native channels in murine myenteric neurons

The aim of the present study was to investigate if P2X4 receptors are expressed in murine myenteric neurons and if these receptors contribute to form functional channels in the neuronal membrane by using molecular and electrophysiological techniques. The whole-cell recording technique was used to measure membrane currents induced by ATP (I(ATP)) in myenteric neurons. Compared with recombinant P2X4 receptor-channels (reported by others in a previous study), native myenteric P2X receptors have a relative lower sensitivity for ATP (EC₅₀=102 µM) and α,β methylene ATP (not effect at 30 or 100 µM). BzATP was a weak agonist for native P2X receptors. KN-62 had no effect on myenteric P2X channels whereas PPADS (IC₅₀=0.54 µM) or suramin (IC₅₀=134 µM) were more potent antagonists than on P2X4 homomeric channels. I(ATP) were potentiated by ivermectin (effect that is specific on P2X4 receptors) and zinc. Western blotting shows the presence of P2X4 protein and RT-PCR the corresponding mRNA transcript in the small intestine. Immunoreactivity for P2X4 receptors was found in most myenteric neurons in culture. Single-cell RT-PCR shows the presence of P2X4 mRNA in 90% of myenteric neurons. Our results indicate that P2X4 receptors are expressed in the majority of myenteric neurons, contribute to the membrane currents activated by ATP, and because most properties of I(ATP) does not correspond to P2X4 homomeric channels it is proposed that P2X4 are forming heteromeric channels in these neurons. P2X4 subunits have a widespread distribution within the myenteric plexus and would be expected to play an important role in cell signaling.

Inhibition of ecto-ATPase activity by curcumin in hepatocellular carcinoma HepG2 cells

Effects of curcumin, a major constituent of turmeric, on ecto-nucleotidases have not been clarified. Here, we investigated whether curcumin affects ecto-nucleotidase activities in human hepatocellular carcinoma HepG2 cells. In the cells, high levels of Mg(2+)-dependent activity of ecto-nucleotidases were observed in the presence of 1 mM adenosine triphosphate (ATP). The activity was inhibited by ecto-ATPase inhibitors such as suramin, ZnCl(2) and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. On the other hand, the activity was significantly decreased at alkaline pH (pH 9) and was not inhibited by levamisole, an inhibitor of alkaline phosphatase. In the presence of ATP, curcumin inhibited the activity in a concentration-dependent manner (IC(50) = 6.2 μM). In contrast, curcumin had no effects on ecto-nucleotidase activity in the presence of ADP (1 mM) or AMP (1 mM). The K (m) value for ATP hydrolysis of curcumin-sensitive ecto-ATPase was similar to the value of NTPDase2, an isoform of ecto-nucleoside triphosphate diphosphohydrolase. These results suggest that curcumin is a potent inhibitor of ecto-ATPase and may affect extracellular ATP-dependent responses.

Structure-activity relationships of anthraquinone derivatives derived from bromaminic acid as inhibitors of ectonucleoside triphosphate diphosphohydrolases (E-NTPDases)

Reactive blue 2 (RB-2) had been characterized as a relatively potent ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) inhibitor with some selectivity for NTPDase3. In search for the pharmacophore and to analyze structure-activity relationships we synthesized a series of truncated derivatives and analogs of RB-2, including 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinones, 1-amino-2-methyl-4-arylaminoanthraquinones, 1-amino-4-bromoanthraquinone 2-sulfonic acid esters and sulfonamides, and bis-(1-amino-4-bromoanthraquinone) sulfonamides, and investigated them in preparations of rat NTPDase1, 2, and 3 using a capillary electrophoresis assay. Several 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinone derivatives inhibited E-NTPDases in a concentration-dependent manner. The 2-sulfonate group was found to be required for inhibitory activity, since 2-methyl-substituted derivatives were inactive. 1-Amino-2-sulfo-4-p-chloroanilinoanthraquinone (18) was identified as a nonselective competitive blocker of NTPDases1, 2, and 3 (K_i 16–18 μM), while 1-amino-2-sulfo-4-(2-naphthylamino)anthraquinone (21) was a potent inhibitor with preference for NTPDase1 (K_i 0.328 μM) and NTPDase3 (K_i 2.22 μM). Its isomer, 1-amino-2-sulfo-4-(1-naphthylamino)anthraquinone (20), was a potent and selective inhibitor of rat NTPDase3 (K_i 1.5 μM).

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.

Transduction mechanisms of P2Z purinoceptors

The ability of extracellular ATP to increase the cation permeability of a variety of fresh and cultured cells has been known for decades, but evidence of a separate class of P2 purinoceptor, termed P2Z, which mediates this effect has only recently been obtained. Several features of the P2Z purinoceptor clearly distinguish it from other P2 purinoceptors and show that it is a ligand-gated ion channel. P2Z purinoceptors are highly selective for the ATP4- species and addition of Mg2+ in excess over ATP closes the channel. The most potent agonist is 3'-O-(4-benzoyl)benzoyl ATP which has a 10-fold lower EC50 than ATP. Ca2+ is the preferred permeant for the P2Z ion channel although it will pass ions up to the size of ethidium(+) (314 Da) in lymphocytes or fura-2 (813 Da) in macrophages. The inhibitors of the P2Z purinoceptor or its associated ion channel include suramin, amiloride analogues, high extracellular Na+ concentrations and 2',3'-dialdehyde ATP (oxidized ATP), which blocks irreversibly. Occupancy of P2Z purinoceptors stimulates a phospholipase D activity, which may be involved in membrane remodelling. Moreover, extracellular ATP causes loss of the glycosylated adhesion molecule L-selection from the surfaces of human lymphocytes by enzymic cleavage, suggesting a possible role for P2Z purinoceptors in intercellular interactions.

Basolateral P2X4-like receptors regulate the extracellular ATP-stimulated epithelial Na+ channel activity in renal epithelia

Extracellular ATP initiates potent effects on sodium transport across renal epithelia through membrane-associated purinergic receptors. Dependent on the location of these receptors, ATP either inhibits or stimulates sodium reabsorption. Using A6 cells, transepithelial electrical resistance measurements, and scanning ion conductance microscopy, we have identified the purinergic receptors involved in the stimulatory action on the epithelial cell basolateral plasma membrane. Addition of the potent P2X(4) receptor agonist 2-methylthio-ATP (2MeSATP) to the basolateral side of the A6 monolayer stimulated amiloride-sensitive sodium conductance and produced similar cell morphological changes to those found with ATPgammaS, aldosterone, or hypotonic stress. The agonist potency order determined by sodium conductance changes of the monolayer was: 2MeSATP >or= ATPgammaS > CTP, a similar agonist potency profile to that of cloned P2X(4) receptors but with higher sensitivity for beta, gamma-methylene-ATP and alpha,beta-methylene-ATP. We further demonstrated that the ATP effect on sodium transport was potentiated by ivermectin, not blocked by suramin and PPADS, enhanced by Zn(2+) but not by Cu(2+), and significantly reduced but not totally inhibited by brilliant blue G. These results led us to conclude that basolateral P2X(4)-like receptors were involved. We suggest that there is a reciprocal purinergic system acting both at a basolateral and apical location for control of Na(+) transport. This requires a mechanism within the cell that leads to either basolateral or apical ATP release to regulate renal tubular function.

A capillary electrophoresis method for the characterization of ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) and the analysis of inhibitors by in-capillary enzymatic microreaction

A capillary electrophoresis (CE) method for the characterization of recombinant NTPDases 1, 2, and 3, and for assaying NTPDase inhibitors has been developed performing the enzymatic reaction within the capillary. After hydrodynamic injection of plugs of substrate solution with or without inhibitor in reaction buffer, followed by a suspension of an enzyme-containing membrane preparation, and subsequent injection of another plug of substrate solution with or without inhibitor, the reaction took place close to the capillary inlet. After 5 min, the electrophoretic separation of the reaction products was initiated by applying a constant current of -60 muA. The method employing a polyacrylamide-coated capillary and reverse polarity mode provided baseline resolution of substrates and products within a short separation time of less than 7 min. A 50 mM phosphate buffer (pH 6.5) was used for the separations and the products were detected by their UV absorbance at 210 nm. The Michaelis-Menten constants (K (m)) for the recombinant rat NTPDases 1, 2, and 3 obtained with this method were consistent with previously reported data. The inhibition studies revealed pronounced differences in the potency of reactive blue 2, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suramin, and N (6)-diethyl-beta,gamma-dibromomethylene-ATP (ARL67156) towards the NTPDase isoforms. Notably, ARL67156 does not inhibit all NTPDases, having only a minor inhibitory effect on NTPDase2. Dipyridamole is not an inhibitor of the NTPDase isoforms investigated. The new method is fast and accurate, it requires only tiny amounts of material (nanoliter scale), no sample pretreatment and can be fully automated; thus it is clearly superior to the current standard methods.

Altered membrane NTPase activity in Lesch-Nyhan disease fibroblasts: comparison with HPRT knockout mice and HPRT-deficient cell lines

Lesch-Nyhan disease (LND) is a rare disorder caused by a defect of an enzyme in the purine salvage pathway, hypoxanthine phosphoribosyl transferase (HPRT). It is still unknown how the metabolic defect translates into the complex neuropsychiatric phenotype characterized by self-injurious behavior, dystonia and mental retardation. There are abnormalities in purine and pyrimidine nucleotide content in HPRT-deficient cells. We hypothesized that altered nucleotide concentrations in HPRT deficiency change G-protein-mediated signal transduction. Therefore, our original study aim was to examine the high-affinity GTPase activity of G-proteins in membranes from primary human skin and immortalized mouse skin fibroblasts, rat B103 neuroblastoma cells and mouse Neuro-2a neuroblastoma cells. Unexpectedly, in membranes from human fibroblasts, B103- and Neuro-2a cells, V(max) of low-affinity nucleoside 5'-triphosphatase (NTPase) activities was decreased up to 7-fold in HPRT deficiency. In contrast, in membranes from mouse fibroblasts, HPRT deficiency increased NTPase activity up to 4-fold. The various systems analyzed differed from each other in terms of K(m) values for NTPs, absolute V(max) values and K(i) values for nucleoside 5'-[beta,gamma-imido]triphosphates. Our data show that altered membrane NTPase activity is a biochemical hallmark of HPRT deficiency, but species and cell-type differences have to be considered. Thus, future studies on biochemical changes in LND should be conducted in parallel in several HPRT-deficient systems.

Purification and characterization of NTPDase1 (ecto-apyrase) and NTPDase2 (ecto-ATPase) from porcine brain cortex synaptosomes

We purified to homogeneity and characterized NTPDase1 and NTPDase2 from porcine brain cortex synaptosomes. SDS/PAGE and immunoblotting with antibodies specific to these enzymes revealed a molecular mass estimated at 72 kDa for NTPDase1 and 66 for NTPDase2. Both enzymes exhibited kinetic properties typical for all members of the NTPDase family, e.g. low substrate specificity for tri- and diphosphonucleosides, divalent cations dependency and insensitivity towards ATPase inhibitors. The calculated Km value for NTPDase1 in respect to ATP as a substrate (97 microm) was three times lower in comparison to analogous values for NTPDase2 (270 microm). Additionally, NTPDase1 had a three times higher Kcat/Km coefficient than NTPDase2 (860 and 833 micromol product.s(-1), respectively). We have also demonstrated that in spite of differences in the affinity of ATP for both hydrolases, these enzymes have similar molecular activity. Taken together, these results indicate that NTPDase1 would terminate P2 receptor-mediated signal transmission whereas activity of NTPDase2 may contribute to decreasing high (toxic) concentrations of ATP and/or to production of another signal molecule, ADP.

Soluble purine-converting enzymes circulate in human blood and regulate extracellular ATP level via counteracting pyrophosphatase and phosphotransfer reactions

Extracellular ATP and other purines play a crucial role in the vasculature, and their turnover is selectively governed by a network of ectoenzymes expressed both on endothelial and hematopoietic cells. By studying the whole pattern of purine metabolism in human serum, we revealed the existence of soluble enzymes capable of both inactivating and transphosphorylating circulating purines. Evidence for this was obtained by using independent assays, including chromatographic analyses with 3H-labeled and unlabeled nucleotides and adenosine, direct transfer of gamma-terminal phosphate from [gamma-32P]ATP to NDP/AMP, and bioluminescent measurement of ATP metabolism. Based on substrate-specificity and competitive studies, we identified three purine-inactivating enzymes in human serum, nucleotide pyrophosphatase (EC 3.6.1.9), 5'-nucleotidase (EC 3.1.3.5), and adenosine deaminase (EC 3.5.4.4), whereas an opposite ATP-generating pathway is represented by adenylate kinase (EC 2.7.4.3) and NDP kinase (EC 2.7.4.6). Comparative kinetic analysis revealed that the Vmax values for soluble nucleotide kinases significantly exceed those of counteracting nucleotidases, whereas the apparent Km values for serum enzymes were fairly comparable and varied within a range of 40-70 micro mol/l. Identification of soluble enzymes contributing, along with membrane-bound ectoenzymes, to the active cycling between circulating ATP and other purines provides a novel insight into the regulatory mechanisms of purine homeostasis in the blood.

Purines and pyrimidines are not involved in NANC relaxant responses in the rabbit vaginal wall

1. Non-adrenergic non-cholinergic (NANC) relaxant responses were elicited by electrical field stimulation (EFS) in rabbit vaginal wall strips after treatment with guanethidine and scopolamine and raising smooth muscle tone with phenylephrine. Under these conditions treatment with NOS inhibitors revealed a non-nitrergic NANC relaxant response. The possible role of purines and pyrimidines in these non-nitrergic NANC responses was investigated. 2. Exogenous application of ATP, ADP, adenosine, UTP, or UDP (all at 0.03-10 mM) induced concentration-dependent relaxant responses. 3. Responses to exogenous application of ATP were reduced by the general P2 antagonist cibacron blue (500 micro M), but not by suramin (100 micro M) and were unaffected by L-NAME (500 micro M), omega-conotoxin GVIA (omega-CTX, 500 nM) or tetrodotoxin (TTX, 1 micro M). 4. Responses to exogenous application of adenosine were reduced by the A(2A) antagonist ZM-241385 (30 micro M). 5. ATP- and ADP-induced responses were unaffected by the G-protein inhibitor pertussis toxin (100 ng ml(-1)), whilst ADP- but not ATP-induced responses were reduced by GDPbetaS (100 micro M), which stabilizes G-proteins in their inactive state. 6. EFS-induced non-nitrergic NANC relaxant responses were unaffected by suramin, cibacron blue, ZM-241385, pertussis toxin or GDPbetaS, but were completely inhibited by TTX. 7. Exogenous application of ATP (10 mM) and adenosine (10 mM) increased intracellular cyclic adenosine-3', 5'-monophosphate (cAMP). However, non-nitrergic NANC responses were not associated with increased cAMP. Neither non-nitrergic NANC responses nor responses to ATP or adenosine were associated with increased intracellular cyclic guanosine-3', 5'-monophosphate (cGMP) concentrations. 8. These results suggest that adenosine A(2A) receptors and P2 receptors are present in the rabbit vaginal wall, but that they are not involved in non-nitrergic NANC relaxant responses.

Evidence for the involvement of purinergic signalling in the control of respiration

The ventrolateral medulla has a critical role in the generation and patterning of respiration via an extensive network of respiratory neurones. We have investigated the effects of activating purinergic P2 receptors within the ventrolateral medulla of the anaesthetised rat on the overall pattern of respiratory activity. In addition, using immunohistochemical techniques, we have identified the subtypes of P2X receptors in the ventrolateral medulla. Unilateral microinjection of ATP into the ventrolateral medulla reduced in a dose-dependent manner, or abolished, resting phrenic nerve discharge recorded as an indication of central inspiratory drive. ATP also elicited increases in blood pressure and variable changes in heart rate. These effects were mimicked by microinjection of the P2X receptor agonist alpha,beta-methylene ATP into the ventrolateral medulla. Whilst microinjection of suramin, a P2 receptor antagonist, had no effect on resting cardiorespiratory variables it blocked the respiratory and cardiovascular effects of ATP microinjected into the ventrolateral medulla. Immunohistochemical staining using IgG antibodies showed that P2X1, P2X2, P2X5 and P2X6, but not P2X3, P2X4 or receptor subunits were localised in the rostral ventrolateral medulla.Our results indicate that several P2X receptor subtypes are localised within areas of the ventrolateral medulla that are important for cardiorespiratory control (including the pre-Bötzinger and Bötzinger complexes), and that activation of these receptors can have profound effects on both the cardiovascular and the respiratory networks. Our pharmacological data suggest that different P2X subunits in this region may co-assemble to form hetero-oligomeric assemblies as well as homomultimers within this region.

Apoptosis induced by extracellular ATP in the mouse neuroblastoma cell line N1E-115: studies on involvement of P2 receptors and adenosine

Adenosine triphosphate (ATP) can be released in large amounts from (damaged) cells, leading to locally high concentrations. In this study, we investigated the effect of such high concentrations of ATP on neuroblastoma cells. ATP (>or=30 microM) induced apoptosis in the mouse neuroblastoma cell line N1E-115. Activation of the ATP receptor P2X(7) is one of the routes via which ATP has been shown to induce apoptosis. Although the P2X(7) receptor was present in N1E-115 cells, both at the protein and mRNA level, studies with the P2X(7) receptor agonist benzoyl-benzoyl ATP showed that this receptor was not involved in ATP-induced apoptosis. It has been shown previously that adenosine induces apoptosis in N1E-115 cells after transport inside the cell. In this study, both dipyridamole, a nucleoside transport protein blocker, and uridine, a substrate for this transporter, were able to block ATP-induced apoptosis. This indicated that ATP had to be broken down to adenosine to induce apoptosis. The ecto-nucleotidase inhibitors 6-N,N-diethyl-beta-dibromomethylene-D-adenosine-5'-triphosphate (ARL67156) and alpha,beta-methylene adenosine 5'-diphosphate (AOPCP) commonly used to slow breakdown of ATP did not inhibit ATP breakdown appreciably, while the ATP antagonist PPADS inhibited the breakdown of AMP to adenosine; PPADS was also the only compound capable of inhibiting ATP-induced apoptosis. We conclude that the main route of ATP-induced apoptosis in N1E-115 cells was via breakdown to adenosine.

Nucleotide-evoked relaxation of rat vas deferens: possible mechanisms

ATP causes relaxation of the K(+)-contracted rat vas deferens. Possible sites of action were investigated. ATP and adenosine relaxed the vas deferens precontracted with 80 mM K(+); EC(50) values and maximal relaxations averaged, respectively, 760 microM and 56% for ATP and 74 microM and 30% for adenosine. The adenosine P1 receptor antagonist 8-(para-sulfophenyl)theophylline (8-SPT) reduced relaxations caused by adenosine and low concentrations of ATP, as did the Rp-diastereomer of adenosine 3',5'-cyclic phosphorothioate (Rp-cAMPS), an inhibitor of protein kinase A. The phosphodiesterase inhibitor 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724) augmented responses to adenosine and low concentrations of ATP. alpha,beta-Methylene ADP, an inhibitor of 5'-nucleotidase, reduced relaxations caused by ATP to a similar extent as did 8-SPT. In the presence of an almost saturating concentration of adenosine, ATP caused further relaxation. Conversely, in the presence of ATP, adenosine had little effect. Like ATP, UTP and other nucleoside triphosphates relaxed the vas deferens. The P2 receptor antagonists reactive blue 2, acid blue 25 and 4,4'-diisothiocyanotostilbene-2,2'-disulphonate (DIDS) attenuated the relaxation caused by ATP; suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulphonate (PPADS), Evans blue, trypan blue, reactive red 2 and brilliant blue G had no effect. Three non-selective inhibitors of protein kinases, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), staurosporine and (8R*,9S*,11S*)-(-)-9-hydroxy-9-carboxy-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo[a,g]cycloocta[cde]trinden-1-one (K-252b), markedly reduced the relaxation caused by ATP. The results indicate that adenosine, derived from enzymatic dephosphorylation, contributes to the relaxant effect of ATP, presumably by activation of a smooth muscle adenosine receptor linked to the accumulation of cAMP and activation of protein kinase A. Yet, the main part of the response to ATP is mediated by a site distinct from the adenosine receptor. The pharmacological properties of this site differ from known P2 receptor subtypes. Possibly, the nucleotide-evoked relaxation is due to a phosphoryl transfer catalyzed by an ecto-protein kinase.

Regulation of chicken gizzard ecto-ATPase activity by modulators that affect its oligomerization status

The major ectonucleoside triphosphate phosphohydrolase in the chicken gizzard smooth muscle membranes is an ecto-ATPase, an integral membrane glycoprotein belonging to the E-ATPase (or E-NTPDase) family. The gizzard ecto-ATPase is distinguished by its unusual kinetic properties, temperature dependence, and response to a variety of modulators. Compounds that promote oligomerization of the enzyme protein, i.e., concanavalin A, chemical cross-linking agent, and eosin iodoacetamide, increase its activity. Compounds that inhibit some ion-motive ATPases, e.g., sulfhydryl reagents, xanthene derivatives, NBD-halides, and suramin, also inhibit the gizzard ecto-ATPase, but not another E-ATPase, the chicken liver ecto-ATP-diphosphohydrolase, which contains the same conserved regions as the ecto-ATPase. Furthermore, inhibition of the gizzard ecto-ATPase by these compounds as well as detergents is not prevented by preincubation of the membranes with the substrate, ATP, indicating that their interaction with the enzyme occurs at a locus other than the catalytic site. On the other hand, the inhibitory effect of these compounds, except suramin, is abolished or reduced if the membranes are preincubated with concanavalin A. It is concluded that these structurally unrelated modulators exert their effect by interfering with the oligomerization of the ecto-ATPase protein. Our findings suggest that, under physiological conditions, the gizzard smooth muscle ecto-ATPase may exhibit a range of activities determined by membrane events that affect the status of oligomerization of the enzyme.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Peritubular fluid viscosity modulates H+ flux in proximal tubules through NO release

We evaluated the effects of increasing the viscosity (eta) in peritubular capillary perfusates (PCP; 20 mM HNaPO4--Ringer, pH 7.4) on proximal convoluted tubule (PCT) acidification. Micropuncture experiments were performed with simultaneous luminal and peritubular perfusion. Changes in pH of a 20 mM HNaPO4--Ringer (pH 7.4 at t = 0) droplet placed in PCT lumen were measured with H+-sensitive microelectrodes. By adding neutral dextran (molecular wt 300,000-400,000) to the PCP, eta was increased. The effect of 10(-5) M ATP added to normal-eta PCP was evaluated. High eta increased H+ flux (85 and 97% when eta was increased 20 and 30%, respectively, above the control value). This increase was abolished by adding the nitric oxide antagonist N(omega)-nitro-L-arginine (L-NNA; 10(-4) M) or the purinoreceptor antagonists suramin (10(-4) M) and reactive blue 2 (3 x 10(-5) M). Addition of 5 x 10(-3) M L-arginine to the peritubular perfusate overcame the inhibitory effect of L-NNA on high-eta-induced increase in H+ flux. ATP increased H+ flux (80%), and this effect was blocked by L-NNA. These results suggest that changes in eta can modulate proximal H+ flux, at least in part, through ATP-dependent nitric oxide release from the endothelial cells of the peritubular capillaries.

Purine and pyrimidine nucleotide-sensitive phospholipase A(2) in ampulla from frog semicircular canal

This study was attempted to characterize pharmacologically the P2Y receptors triggering phospholipase A(2) (PLA(2)) activation in ampulla from frog semicircular canal. A microassay was developed to screen the abilities of UTP analogs to stimulate [(3)H]arachidonic acid release by labeled ampullas. At 26 degrees C UTP induced a dose-dependent and saturable increase of PLA(2) activity (apparent activation constant 1.3 +/- 0.4 microM, Hill coefficient 0.9 +/- 0.2, maximal stimulating factor 2.0 +/- 0.1). The rank order of potency of agonists for PLA(2) activation was UTP > or = UDP > adenosine 5'-O-(2-thiodiphosphate) = adenosine 5'-O-(3-thiotriphosphate) > or = ATP = 2-methylthio-ATP > or = ADP = diadenosine tetraphosphate > or = alpha,beta-methylene-ATP = CTP > 2' and 3'-O-(4-benzoylbenzoyl)-ATP > or = AMP = UMP >> uridine and adenosine. UTP- and 2-methylthio-ATP-induced PLA(2) activations were inhibited by U-73122, GF-109203X, and methyl arachidonyl fluorophosphate. Basal activity was stimulated by phorbol ester and epinephrine and reduced by vasotocin, isoproterenol, prostaglandin E(2), cAMP, and forskolin. H-89 restored the cAMP- and forskolin-inhibited PLA(2) activities. Results indicate that P2Y receptor-mediated PLA(2) stimulation requires phopholipase C and protein kinase C activations and basal activity is inhibited by agonist-stimulated cAMP-dependent mechanisms.

UTP binding and phosphoinositidase C activation in ampulla from frog semicircular canal

Pyrimidine nucleotide-sensitive phosphoinositidase C activity (PLC), previously identified in frog semicircular canal ampulla, was pharmacologically characterized. Binding of [(3)H]UTP and abilities of unlabeled nucleotide analogs to inhibit binding and to stimulate PLC in myo-[(3)H]inositol-loaded ampullas were determined. Specific [(3)H]UTP binding was competitively inhibited by UTP [apparent dissociation binding constant = 0.8 microM; Hill coefficient = 0.7]. Scatchard analysis revealed a minor class of high-affinity binding sites [45 fmol UTP bound/microgram protein; dissociation constant (K(D1)) = 0.4 microM] and a major class of moderate-affinity binding sites (365 fmol UTP bound/microgram protein; K(D2) = 10 microM). The stereospecificity pattern for UTP analog recognition was UMP > UDP >/= ADP = UTP = dTTP > adenosine 5'-O-(3-thiotriphosphate) = ATP = CTP = 2'-and 3'-O-4-(benzoylbenzoyl)-ATP (Bz-ATP) >/= AMP >/= 2-methylthio-ATP = alpha,beta-methylene-ATP > uridine = diadenosine tetraphosphate (Ap(4)A); cAMP and adenosine were inactive. Antagonist recognition pattern was DIDS = pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) = reactive blue 2 > suramin. The rank order of potencies for agonist-induced PLC activation was UDP >/= UTP >/= Ap(4)A >/= UMP = Bz-ATP; uridine was inactive. UTP-stimulated PLC activity was inhibited by DIDS = reactive blue 2 = PPADS > suramin. These results suggest that the population of [(3)H]UTP-labeled binding sites is heterogeneous, with a low number of high-affinity UTP receptors whose function(s) need to be determined and a large number of moderate-affinity receptors triggering PLC activation.

Adventures in the pharmacological analysis of P2 receptors

The pharmacological classification of P2 receptors owes its origin to the pioneering efforts of Geoff Burnstock and those who followed him, research that was conducted primarily in physiological experimental systems. Over recent years, the techniques of molecular biology have been increasingly applied in the study of P2 receptors while, at the same time, advances in their pharmacological analysis have been limited by a lack of potent and selective agonist or antagonist ligands. This has resulted in a classification scheme which is largely structural in nature, with relatively little contribution from pharmacology. Our endeavours in this area have been directed towards the discovery of ligands with which the pharmacological analysis and definition of P2 receptors could be advanced, the ultimate goal being the design of therapeutic agents. This article will describe some of our experiences in this challenging but rewarding area.

Inhibitory effects of some purinergic agents on ecto-ATPase activity and pattern of stepwise ATP hydrolysis in rat liver plasma membranes

Inhibitory effects of various purinergic compounds on the Mg(2+)-dependent enzymatic hydrolysis of [(3)H]ATP in rat liver plasma membranes were evaluated. Rat liver enzyme ecto-ATPase has a broad nucleotide-hydrolyzing activity, displays Michaelis-Menten kinetics with K(m) for ATP of 368+/-56 microM and is not sensitive to classical inhibitors of the ion-exchange and intracellular ATPases. P2-antagonists and diadenosine tetraphosphate (Ap(4)A) progressively and non-competitively inhibited ecto-ATPase activity with the following rank order of inhibitory potency: suramin (pIC(50), 4.570)>Reactive blue 2 (4.297)&z.Gt;Ap(4)A (3. 268)>pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (2. 930). Slowly hydrolyzable P2 agonists ATPgammaS, ADPbetaS, alpha, beta-methylene ATP and beta,gamma-methylene ATP as well as the diadenosine polyphosphates Ap(3)A and Ap(5)A did not exert any inhibitory effects on the enzyme activity at concentration ranges of 10(-4)-10(-3) M. Thin-layer chromatography analysis of the formation of [(3)H]ATP metabolites indicated the presence of other enzyme activities on liver surface (ecto-ADPase and 5'-nucleotidase), participating in concert with ecto-ATPase in the nucleotide hydrolysis through the stepwise reactions ATP-->ADP-->AMP-->adenosine. A similar pattern of sequential [(3)H]ATP dephosphorylation still occurs in the presence of ecto-ATPase inhibitors suramin, Ap(4)A and PPADS, but the appearance of the ultimate reaction product, adenosine, was significantly delayed. In contrast, hydrolysis of [(3)H]ATP in the presence of Reactive blue 2 only followed the pattern ATP-->ADP, with formation of the subsequent metabolites AMP and adenosine being virtually eliminated. These data suggest that although nucleotide-binding sites of ecto-ATPase are distinct from those of P2 receptors, some purinergic agonists and antagonists can potentiate cellular responses to extracellular ATP through non-specific inhibition of the ensuing pathways of purine catabolism.

Ecto-nucleotide pyrophosphatase modulates the purinoceptor-mediated signal transduction and is inhibited by purinoceptor antagonists

1. The effect of ecto-nucleotide pyrophosphatase (ecto-NPPase; EC 3.6.1. 9) on the ATP- and ADP-mediated receptor activation was studied in rat C6 glioma cells. The P2-purinoceptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and reactive blue (RB2) are potent inhibitors (IC(50)=12+/-3 microM) of the latter enzyme. 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid (DIDS), 5'-phosphoadenosine 3'-phosphate (PAP) and suramin were less potent inhibitors with an IC(50) of 22+/-4, 36+/-7 and 72+/-11 microM respectively. 2. P1-purinoceptor antagonists CGS 15943, cyclo-pentyl theophylline (CTP) and theophylline did not affect the activity of the ecto-NPPase. 3. ATP- and ADP-mediated P2Y(1)-like receptor activation inhibited the (-)-isoproterenol-induced increase of intracellular cyclic AMP concentration. PPADS, an ineffective P2Y-antagonist in C6, potentiated the ATP and ADP effect approximately 3 fold due to inhibition of nucleotide hydrolysis by the ecto-NPPase. 4. We conclude that ecto-NPPase has a modulator effect on purinoceptor-mediated signalling in C6 glioma cell cultures.

Differential effects of drugs interacting with autonomic transmitters on responses of rat vas deferens to field stimulation

1. Frequency-response curves (0.1-30 Hz) were obtained in the epididymal portion of rat vas deferens. At low frequencies (0.1-1 Hz), the parameters evaluated were the first twitch and the fourth twitch at each frequency. The responses to trains of stimuli at intermediate (2-5 Hz) and high (10-30 Hz) frequencies were biphasic consisting of phase I (the first rapid phase of tetanus) and of phase II (the secondary slowly developing one). 2. Prazosin inhibited the first and the fourth twitch but not when the frequency was < 1 Hz. Suramin inhibited the first twitch while substantially depressing the fourth one. The combination of prazosin and suramin almost completely abolished all the twitches evoked by a train of stimuli at low frequencies. Nifedipine left almost unaltered the first twitch while markedly depressing the fourth one, especially at relatively high frequency (1 Hz). Verapamil was devoid of any inhibitory action. Papaverine depressed the first twitch while only at the highest concentration used (1 x 10(-4) M) markedly inhibited the fourth one. Chloroethylclonidine (CEC) depressed the first twitch and increased the fourth. 3. When intermediate (2-5 Hz) and high (10-30 Hz) frequencies are considered, prazosin and suramin partially inhibited both phase I and phase II, while in combination they almost completely abolished both phases. Nifedipine and verapamil selectively suppressed phase II, leaving phase I unaffected. Papaverine completely abolished both phase I and phase II. CEC was able to completely abolish phase I but increased phase II. 4. These results suggest that the response to the first twitch of a train at low frequency is prevailingly noradrenergic, prazosin-sensitive, while when the twitches are close enough (i.e. at 1 Hz) a summation of stimuli takes place and a predominant purinergic component, both suramin- and nifedipine-sensitive, becomes evident. 5. At high frequencies, both phases are due to the concomitant release of noradrenaline and adenosine triphosphate (ATP). The noradrenergic component of phase I is nifedipine-insensitive and CEC-sensitive, resembling the pharmacological profile of the endogenously released noradrenaline by single pulse, while that of phase II, nifedipine-sensitive and CEC-insensitive, is similar to that produced by exogenously applied noradrenaline.

Inhibition of rat parotid ecto-ATPase activity

The inhibitory profile of several known and suspected ecto-ATPase inhibitors was compared on ecto-ATPase activity in rat parotid plasma membranes. Those chemicals with high IC50 (above 130 microM) were the nucleotides alpha,beta-methylene ATP, beta,gamma-methylene ATP, 2-methylthio ATP, inosine triphosphate, 5'-p-fluorosulphonylbenzoyladenosine, the sulphonates, 1-amino-2-naphthol-4-sulphonic acid, Coomassie brilliant blue G, and the stilbene disulphonates, DIDS and SITS. Those agents with low IC50 were: Coomassie brilliant blue R (114 microM), ATPgammaS (49 microM), suramin (72 microM) and Reactive blue 2 (28 microM). The last three inhibitors have similar potencies as inhibitors of ATP hydrolysis by whole parotid acinar cells. ARL67156, a selective inhibitor of ecto-ATPase, had an IC50 of approx. 120 microM. Suramin displayed non-competitive inhibition of ecto-ATPase whereas the inhibitory effects of ATPgammaS and Reactive blue 2 were curvilinear on Dixon plots. These results define the effects of various agents on ecto-ATPase in an exocrine tissue that has been shown to respond to extracellular ATP.

Effects of suramin on hippocampal apyrase activity and inhibitory avoidance learning of rats

The action of suramin on apyrase activity in hippocampal synaptosomes and its effects on retention of inhibitory avoidance learning were evaluated. Suramin, a P2-purinoceptor antagonist, significantly inhibited in a noncompetitive manner the ATP and ADP hydrolysis promoted by apyrase in hippocampal synaptosomes of adult rats. The Ki values obtained were 72.8 and 109 microM for ATP and ADP hydrolysis, respectively. Intrahippocampal infusion of suramin (0.01, 0.1, 1, and 10 microg) immediately posttraining, in a dose-dependent effect, significantly reduced the response latency during the retention test applied 24 h after the rats received step-down inhibitory avoidance training. The amnesic effects promoted by suramin probably occur by its antagonist action on hippocampal P2-purinoceptors and NMDA receptors. In view of the fact that ATP-metabolizing enzymes and P2-purinoceptors have similar binding domains, these results suggest that suramin can either alter ATP degradation and/or block purinergic neurotransmission.

Are there functional P2X receptors on cell bodies in intact dorsal root ganglia of rats?

P2X purinoceptors have been suggested to participate in transduction of painful stimuli in nociceptive neurons. In the current experiments, ATP (1-10 mM), alpha,beta-methylene-ATP (10-30 microM) and capsaicin (10 nM-1 microM) were applied to neurons impaled with high resistance microelectrodes in rat dorsal root ganglia (L4 and L5) isolated in vitro together with the sciatic nerve and dorsal roots. The agonists were either bath applied or focally applied using a picospritzer. GABA (100 microM) and 40-80 mM K+ solutions gave brisk responses when applied by either technique. Only three of 22 neurons with slowly conducting axons (C cells) showed evidence of P2X-purinoceptor-mediated responses. Only two of 13 cells which responded to capsaicin (putative nociceptors), and none of 29 cells with rapidly conducting axons (A cells), responded to the purinergic agonists. When acutely dissociated dorsal root ganglion cells were studied using patch-clamp techniques, all but four of 30 cells of all sizes responded with an inward current to either ATP or alpha,beta-methylene-ATP (both 100 microM). Our data suggest that few sensory cell bodies in intact dorsal root ganglia express functional purinoceptors.

Modification of the ATP-Induced Increase in

BACKGROUND: It is generally accepted that the plasma membrane of mammalian ventricular myocytes regulates the cytosolic concentration of Ca(2+). In this study we investigated the effects of some P2-purinoceptor antagonists and metals such as copper and zinc on the adenosine triphosphate (ATP)-induced increase in intracellular concentration of free Ca(2+) ([Ca(2+)](i)). METHODS AND RESULTS: Cardiomyocytes were isolated from adult male Sprague-Dawley rats loaded with Fura-2, and fluorescence measurements were performed by employing stirred cell suspensions at room temperature. ATP (50 µM) increased [Ca(2+)](i) over the basal value, and 10 µM cibacron blue or verapamil virtually abolished it. The ATP-induced increase in [Ca(2+)](i) was not observed in Ca(2+)- or Mg(2+)-free buffers. Incubation of cells with ZnCl(2) produced a significant depression of the ATP-induced increase in [Ca(2+)](i); 25 µM Zn(2+) decreased the peak response to approximately 50% of the control value. The ATP-induced increase in [Ca(2+)](i), was inhibited by low concentrations (1-5 µM) of Cu(2+) but was markedly augmented by high concentrations (25 µM) of Cu(2+). The increase in the [Ca(2+)](i) response to cron blue, and Zn(2+), but not by ryanodine or caffeine pretreatment. CONCLUSIONS: The ATP-induced increase in [Ca(2+)](i) is dependent on the extracellular concentrations of Ca(2+) as well as Mg(2+) and is antagonized by cibacron blue and Zn(2+). On the other hand, Cu(2+) produced a biphasic response to the ATP-induced increase in [Ca(2+)](i) in cardiomyocytes.

P2X receptor-mediated excitation of nociceptive afferents in the normal and arthritic rat knee joint

1. We tested the hypothesis that functional P2X receptors are present on peripheral terminals of primary afferent articular nociceptors in the rat knee joint. Neural activity was recorded extracellularly from the medial articular nerve innervating the knee joint in rats anaesthetized with pentobarbitone. 2. The selective P2X receptor agonist, alphabeta methylene ATP (alphabetameATP), and the endogenous ligand, ATP, caused a rapid short-lasting excitation of a sub-population of C and Adelta nociceptive afferent nerves innervating normal knee joints when injected intra-arterially or intra-articularly, and this effect was antagonized by the non-selective P2 receptor antagonist PPADS. 3. Induction of a chronic (14-21 days) unilateral inflammatory arthritis of the knee joint using locally injected Freund's adjuvant neither increased or decreased responsiveness of joint nociceptors to alphabetameATP or ATP. 4. Our results support the hypothesis that alphabetameATP-sensitive P2X receptors are expressed on peripheral nociceptive afferents in the rat knee joint suggesting that they may be involved in the initiation of nociception and pain.

Purinoceptors mediate renal vasodilation by nitric oxide dependent and independent mechanisms

BACKGROUND

Adenosine triphosphate (ATP) and its metabolites including adenosine modulate renal vascular tone under physiological and pathophysiological conditions. Their effects are brought about by activation of membrane bound P1- and P2-purinoceptors located on smooth muscle and endothelial cells. In this study we analyzed the purinoceptor mediated dilation of rabbit and human renal arteries, and evaluated the possible involvement of endothelium-derived relaxing factors.

METHODS

Segments of rabbit and human renal arteries were incubated and perfused with medium containing indomethacin. After preconstriction, drug induced changes in the vessel diameters were measured by a photoelectric device.

RESULTS

ATP (EC50 = 1 mumol/liter), added intraluminally, caused maximal vasodilation of 80 to 100% of the preconstriction response in both species. This effect was inhibited by the P1-purinoceptor antagonist 8-p-(sulphophenyl)theophylline (100 mumol/liter), suggesting that it was in part due to breakdown of ATP to adenosine. The nature of purinoceptor mediated renal vasodilation was studied further in rabbit renal arteries. Adenosine (EC50 = 1 mumol/liter) as well as the P2Y-receptor agonists ADP beta S (EC50 = 0.4 mumol/liter) and 2-MeSATP (EC50 = 0.2 mumol/liter) dilated the arteries by 80 to 100%. The effects of 2-MeSATP, which were to a much lesser extent that of ADP beta S but not that of adenosine, were attenuated by the P2Y-antagonist reactive blue 2 (3 mumol/liter). Removal of the endothelium almost abolished the vasodilation induced by adenosine and ATP. In contrast, these dilator response were only slightly attenuated by the nitric oxide synthase blockers NG-nitro-L-arginine methyl ester and NG-nitro-L-arginine (300 mumol/liter each), whereas acetylcholine and 2-MeSATP induced dilation was markedly reduced by NG-nitro-L-arginine methyl ester.

CONCLUSIONS

P1-purinoceptors activated by adenosine dilate rabbit renal arteries by an endothelium-derived relaxing factor that appears to be distinct from nitric oxide. In contrast, P2Y-purinoceptor induced renal dilation is mediated by nitric oxide. ATP, the physiological activator of P2Y-purinoceptors, is rapidly broken down to adenosine in rabbit and human renal arteries. Therefore, in rabbit and human renal arteries the vasodilatory effect of exogenous ATP mainly results from P1-purinoceptor activation probably through its breakdown product, adenosine.

Pharmacological characterization of ATP receptors in ampulla from frog semicircular canal

Phosphoinositidase C activities sensitive to purine and pyrimidine nucleotides have been identified earlier in ampulla from Rana ridibunda semicircular canal. The aim of this study was to characterize the pharmacological properties of other P2 receptors borne by this structure. A microassay was developed to measure the binding of [35S]adenosine 5'-O-(2-thiodiphosphate) ([35S]ADPbetaS) to a few ampullas microdissected from frog semicircular canals. When determined at 4 degrees C in the absence of divalent cations, [35S]ADPbetaS binding was saturable with incubation time and reversible after elimination of free radioligand. The dissociation kinetics were biphasic and comprised a major component that was rapidly reversible and a minor component that dissociated slowly. [35S]ADPbetaS binding was competitively inhibited by unlabeled ADPbetaS with an apparent dissociation constant of 0.48 +/- 0.09 microM and a Hill coefficient of 0.70 +/- 0.06, and Scatchard analysis revealed a minor class of high-affinity binding sites (RT1 = 52 +/- 11 fmol [35S]ADPbetaS bound/ampulla and Kd1 = 0.15 +/- 0.04 microM) and a major class of low-affinity binding sites (RT2 = 436 +/- 79 fmol [35S]ADPbetaS bound/ampulla and Kd2 = 2.0 +/- 0.8 microM). The pattern of stereospecificity for recognition of unlabeled structural ATP analogs was ADPbetaS >/= alpha, beta-methyleneadenosine 5'-triphosphate = ADP = adenosine 5'-O-(3-thiotriphosphate) > ATP = diadenosine tetraphosphate = AMP > 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate >/= 2-methylthioadenosine 5'-triphosphate > 2-desoxythymidine 5'-triphosphate = guanosine 5'-triphosphate = inosine-5'-triphosphate = xanthosine 5'-triphosphate = cytosine 5'-triphosphate = uridine 5'-triphosphate = uridine-5'-diphosphate, whereas cAMP and adenosine were devoid of activity. For antagonists, suramin revealed competitive inhibitor potencies, whereas reactive blue 2 and DIDS acted as pure noncompetitive inhibitors. Results suggest that the population of labeled receptors is heterogeneous and contains a low number of P2Y-like receptors and a large number of P2X-like receptors whose molecular subtypes and functions in endolymph homeostasis remain to be defined.

Neurogenic and non-neurogenic responses in the urinary bladder of hibernating hamster

1. Purinergic and cholinergic components of parasympathetic neurotransmission and contractile responses to exogenous alpha,beta-methylene ATP, acetylcholine, substance K, substance P, calcitonin gene-related peptide, vasoactive intestinal polypeptide and capsaicin have been investigated in the urinary bladder of hibernating hamsters (4 weeks), cold exposed (4 weeks) and age-matched controls. 2. Electrical field stimulation (EFS) evoked increased frequency-dependent contractions in the detrusor strips from hibernating hamsters compared with those obtained from cold-exposed and age-matched animals. Tetrodotoxin (10(-6) M) completely blocked the frequency-dependent contractions in all groups. 3. The purinergic component of the parasympathetic neurotransmission was not affected in hibernating and cold-exposed animals while the cholinergic component was increased with respect to age-matched animals. The neurogenic response to EFS, still present after incubation with atropine (10(-6) M) and suramin (10(-4) M), was attenuated by indomethacin (10(-6) M) and blocked by tetrodotoxin (10(-6) M). 4. Exogenous administration of alpha,beta-methylene ATP elicited a significantly reduced contraction in strips from hibernating and cold-exposed hamsters relative to age-matched animals. The contractile response to exogenous acetylcholine was greater in the detrusors from hibernating hamsters than in cold-exposed and age-matched animals. Substance K elicited reduced contractions in preparations from hibernating animals compared with cold-exposed and control animals. Calcitonin gene-related peptide, vasoactive intestinal polypeptide, substance P and capsaicin did not elicit any relaxant or contractile response either at resting tone or in carbachol (5 x 10(-7) M)-precontracted tissues. 5. In summary, our findings indicate that 4 weeks of hibernation can significantly increase neurogenic responses in the hamster urinary bladder. This appears to be due to an increase in postjunctional responses to acetylcholine. In contrast, there was a decrease of the postjunctional responses to the parasympathetic cotransmitter ATP and also to the sensory-motor neurotransmitter substance K.

Coexpression of ligand-gated P2X and G protein-coupled P2Y receptors in smooth muscle. Preferential activation of P2Y receptors coupled to phospholipase C (PLC)-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3

P2 receptor subtypes and their signaling mechanisms were characterized in dispersed smooth muscle cells. UTP and ATP stimulated inositol 1,4,5-triphosphate formation, Ca2+ release, and contraction that were abolished by U-73122 and guanosine 5'-O-(3-thio)diphosphate, and partly inhibited (50-60%) by pertussis toxin (PTX). ATP analogs (adenosine 5'-(alpha, beta-methylene)triphosphate, adenosine 5'-(beta, gamma-methylene)triphosphate, and 2-methylthio-ATP) stimulated Ca2+ influx and contraction that were abolished by nifedipine and in Ca2+-free medium. Micromolar concentrations of ATP stimulated both Ca2+ influx and Ca2+ release. ATP and UTP activated Gq/11 and Gi3 in gastric and aortic smooth muscle and heart membranes, Gq/11 and Gi1 and/or Gi2 in liver membranes, and Go and Gi1-3 in brain membranes. Phosphoinositide hydrolysis stimulated by ATP and UTP was mediated concurrently by Galphaq/11-dependent activation of phospholipase (PL) C-beta1 and Gbetagammai3-dependent activation of PLC-beta3. Phosphoinositide hydrolysis was partially inhibited by PTX or by antibodies to Galphaq/11, Gbeta, PLC-beta1, or PLC-beta3, and completely inhibited by the following combinations (PLC-beta1 and PLC-beta3 antibodies; Galphaq/11 and Gbeta antibodies; PLC-beta1 and Gbeta antibodies; PTX with either PLC-beta1 or Galphaq/11 antibody). The pattern of responses implied that P2Y2 receptors in visceral, and probably vascular, smooth muscle are coupled to PLC-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3. These receptors co-exist with ligand-gated P2X1 receptors activated by ATP analogs and high levels of ATP.

Effect of cromakalim on the purinergic and cholinergic transmission in the rat detrusor muscle

Contraction of the rat detrusor muscle is mediated by cholinergic and purinergic mechanisms. The present study was carried out to look at the influence of cromakalim, compared with atropine, suramin and nifedipine on the contractile response evoked by single shock and exogenous agonists (carbachol and ATP) in rat urinary bladder. Cromakalim was able to inhibit only partially the response to carbachol and profoundly affected the response to exogenous ATP. Atropine suppressed the response to carbachol and was inactive versus ATP. Suramin was inactive versus carbachol and was able to antagonize the response to ATP. Nifedipine proved to be a non-competitive antagonist versus carbachol (pD2 = 7.66 +/- 0.05) and deeply affected the response to ATP. Cromakalim inhibited only partially the first, purinergic, phase of the electrically evoked response but was able to inhibit in a concentration-dependent manner the second, cholinergic, phase (logIC50 = 6.87 +/- 0.05). Nifedipine blocked both the phases. Atropine blocked partially only the second phase. Suramin inhibited the first phase but, at least partially, also the second one. The combination of atropine and suramin enhanced the inhibition of the second phase. The antagonistic effect of suramin on the second phase could indicate an overlap of the purinergic and cholinergic components. The comparison between pre- and postjunctional effects indicates that cromakalim acts on purinergic transmission predominantly postjunctionally. On the contrary, the action on cholinergic transmission seems to occur mainly at prejunctional level. This conclusion can be relevant in view of the claimed importance of K+ channel openers in the treatment of urinary disorders.

Properties of ATP receptor-mediated synaptic transmission in the rat medial habenula

The properties of central ATP-mediated synaptic currents were studied using whole-cell patch-clamp recording in rat medial habenula slices. Release was shown to be calcium dependent with a Hill coefficient of approximately 2. The voltage dependence of synaptic current amplitudes was approximately linear. Some reduction of the synaptic current amplitudes was observed at 10 mM extracellular calcium, suggesting calcium block/permeability of the channels. This was confirmed by observation of current-voltage reversal potentials in different calcium concentrations. We estimate that the channels underlying half the synapses showed a negligible calcium permeability. In the other four out of eight synapses the results suggest a very high calcium permeability with an estimated PCa/PCs of > 10. Thus, at -70 mV, in 1 mM calcium, more than 15% of the ATP-mediated synaptic current is estimated to be carried by calcium, but only at synapses with calcium-permeable channels. Net current through these synaptic channels is also controlled by the voltage dependence of synaptic current decay time constants (increasing e-fold for 158 mV depolarization) and by a strong dependence of transmitter release on the frequency of stimulation of the presynaptic neurone, with failure rates increasing 3-fold as stimulation rates were increased from 1 to 10 Hz.

P2-receptor-mediated inhibition of serotonin release in the rat brain cortex

The possibility of a P2-receptor-mediated modulation of the release of serotonin in the rat brain cortex was investigated in occipito-parietal slices preincubated with [3H]serotonin and then superfused and stimulated electrically (10 pulses, 1 Hz). Adenosine receptor agonists decreased the stimulation-evoked overflow of tritium at best slightly; the selective A1 agonist N6-cyclopentyl-adenosine caused no change. Several nucleotides had more marked effects: ATP (3-1000 microM), adenosine-5'-O-(3-thiotriphosphate) (3-300 microM) and P1,P5-di(adenosine-5')-pentaphosphate (3-300 microM) decreased the evoked overflow by up to ca 35%. AMP, alpha,beta-methylene-ATP and UTP produced smaller decreases and 2-methylthio-ATP and UMP caused no change. The inhibition by ATP was attenuated both by the P1-receptor antagonist 8-(p-sulphophenyl)-theophylline (100 microM) and by the P2-receptor antagonist suramin (300 microM) but was not changed by indomethacin (10 microM) and NG-nitro-L-arginine (10 microM). We conclude that the release of serotonin in the rat brain cortex is inhibited through presynaptic P1-receptors (which are not A1) as well as P2-receptors. Inhibition of release via P2-receptors has been previously shown for noradrenaline (brain cortex and hippocampus) and dopamine (neostriatum) and, hence, may be widespread. Differences between transmitter systems exist, however, in the degree of their sensitivity to presynaptic P2-receptor-mediated modulation.

Competition of adenine nucleotides for a 1,3-[3H]-dipropyl-8-cyclopentylxanthine binding site in rat vas deferens

1. The binding of 1,3-[3H]-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), a specific adenosine A1 receptor antagonist, was examined in rat vas deferens membrane preparations using radioligand binding techniques. 2. 1,3-[3H]-Dipropyl-8-cyclopentylxanthine bound to these preparations with a KD of 1.07 +/- 0.14 nmol/L (n = 6). The density of [3H]-DPCPX binding sites was 133.38 +/- 5.57 fmol/mg protein. 3. Computer analysis indicated that nucleosides competed for [3H]-DPCPX binding at two distinct sites. The rank order of potency at the higher affinity site corresponded to R-phenylisopropyladenosine (R-PIA) > or = 2-chloroadenosine (2-CIADO) > or = cyclopentyladenosine (CPA) > or = N-ethylcarboxamidoadenosine (NECA) > s-phenylisopropyladenosine (s-PIA). Ki values were in the low nmol/L range. The rank order of nucleoside potency at the lower affinity site corresponded to R-PIA > or = CPA > or = NECA > or = 2-CIADO > S-PIA. Ki values were in the low mumol/L range. 4. Nucleotides competed for [3H]-DPCPX binding at a single site only. The rank order of potency at this site corresponded to alpha, beta-methylene ATP > or = beta, gamma-methylene ATP > or = ATP. Ki values were in the high mumol/L range. The site seemed to correspond with one of the two binding sites predicted by nucleoside competition binding. 5. The ATP-regenerating compound myokinase did not significantly change the competition curve for ATP, indicating that the competition for [3H]-DPCPX binding observed in the presence of ATP was due to an effect of ATP per se and not to an action of a degradation product. 6. The results demonstrate that in rat vasa deferentia there exist two distinct binding sites for [3H]-DPCPX. One of these sites binds only nucleosides and may represent an adenosine A1 receptor, as usually defined. The other site binds both nucleosides and nucleotides and may represent an atypical adenosine A1 receptor, an atypical P2 or a P3 purinoceptor.

Characterization of P2 receptors for purine and pyrimidine nucleotides in human placental cotyledons

1. The aim of this study was to characterize P2 receptors in the arterial vascular bed of human perfused placental cotyledons. Vasoconstrictor responses to bolus injections of purine and pyrimidine nucleotides were tested at basal tone, and vasodilator responses in preparations with tone raised by perfusion with prostaglandin F2alpha (PGF2alpha; 10-50 nM). 2. At basal tone, bolus injections of the P2X-selective agonist alpha,beta-methylene ATP (alpha,beta-meATP; 0.5-500 nmol) elicited dose-dependent vasoconstriction. ATP (0.005-5 micromol) also elicited dose-dependent vasoconstriction, but was less potent than alpha,beta-meATP. Vasoconstriction was also elicited by other nucleotides, but only at the highest dose tested (5 micromol): UTP > CTP = ITP (n = 6). GTP and TTP did not cause vasoconstriction. 3. Constrictor responses to bolus injections of alpha,beta-meATP were resistant to desensitization and were not significantly affected when carried out in the presence of 1 microM alpha,beta-meATP added to the perfusate. However, responses to bolus injections of alpha,beta-meATP were partially blocked by perfusion with 10 microM alpha,beta-meATP. In contrast, responses to ATP and UTP were unaffected by 10 microM alpha,beta-meATP. The P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 10 and 100 microM) had no significant effect on vasoconstriction mediated by alpha,beta-meATP and ATP. 4. Removal of the endothelium had no significant effect on constrictor responses to alpha,beta-meATP, ATP and UTP. Inhibition of nitric oxide (NO) synthesis with N(G)-nitro-L-arginine methyl ester (L-NAME; 100 microM) had no significant effect on vasoconstriction to ATP and alpha,beta-meATP. 5. In preparations with tone raised with PGF2alpha (10-50 nM) vasodilatation was elicited by nucleotides with the following order of potency: 2MeSATP = ADP >> ATP > UTP > CTP = GTP = ITP = TTP. pD2 values were: 2MeSATP, 10.03+/-0.26 (n=7); ADP, 9.97+/-0.40 (n=5); ATP, 8.89+/-0.18 (n=7); UTP, 7.79+/-0.35 (n=7). Maximal responses to 2MeSATP and ADP were similar and were approximately 40% greater than maximal responses to ATP and UTP. 6. Vasodilator responses to nucleotides were abolished by L-NAME (100 microM) and by removal of the endothelium. 7. In conclusion, contractile responses mediated by alpha,beta-meATP and ATP in human placental smooth muscle are resistant to desensitization and insensitive to PPADS and, thus, show a dissimilar pharmacological profile to the classic smooth muscle P2X1 receptor. There may be two subtypes of smooth muscle P2 receptor based on differential antagonism of alpha,beta-meATP and ATP with alpha,beta-meATP. A smooth muscle P2 receptor mediates vasoconstriction to UTP, and may indicate a further subtype. Endothelium-dependent, NO-dependent, vasodilatation to 2MeSATP and ADP may be mediated by P2Y1 receptors, while endothelial P2Y2 receptors are likely to mediate NO-dependent relaxation to ATP and UTP.

Coexistence of purino- and pyrimidinoceptors on activated rat microglial cells

1. Nucleotide-induced currents in untreated (proliferating) and lipopolysaccharide (LPS; 100 ng ml(-1)) treated (non-proliferating) rat microglial cells were recorded by the whole-cell patch-clamp technique. Most experiments were carried out on non-proliferating microglial cells. ATP (100 nM-1 mM), ADP (10 nM-10 mM) and UTP (1 microM-100 mM), but not uridine (100 microM-10 mM) produced a slow outward current at a holding potential of 0 mV. The effect of UTP (1 mM) did not depend on the presence of extracellular Mg2+ (1 mM). The outward current response to UTP (1 mM) was similar in non-proliferating and proliferating microglia. 2. In non-proliferating microglial cells, the ATP (10 microM)-induced outward current was antagonized by suramin (300 microM) or reactive blue 2 (50 microM), whereas 8-(p-sulphophenyl)-theophylline (8-SPT; 100 microM) was inactive. By contrast, the current induced by UTP (1 mM) was increased by suramin (300 microM) and was not altered by reactive blue 2 (50 microM) or 8-SPT (100 microM). 3. The current response to UTP (1 mM) disappeared when K+ was replaced in the pipette solution by an equimolar concentration of Cs+ (150 mM). However, the effect of UTP (1 mM) did not change when most Cl- was replaced with an equimolar concentration of gluconate (145 mM). The application of 4-aminopyridine (1 mM) or Cs+ (1 mM) to the bath solution failed to alter the UTP (1 mM)-induced current. UTP (1 mM) had almost no effect in a nominally Ca2+-free bath medium, or in the presence of charybdotoxin (0.1 microM); the inclusion of U-73122 (5 microM) or heparin (5 mg ml(-1)) into the pipette solution also blocked the responses to UTP (1 mM). By contrast, the effect of ATP (10 microM) persisted under these conditions. 4. I-V relations were determined by delivering fast voltage ramps before and during the application of UTP (1 mM). In the presence of extracellular Cs+ (1 mM) and 4-aminopyridine (1 mM) the UTP-evoked current crossed the zero current level near -75 mV. Omission of Ca2+ from the Cs+ (1 mM)- and 4-aminopyridine (1 mM)-containing bath medium or replacement of K+ by Cs+ (150 mM) in the pipette solution abolished the UTP current. 5. Replacement of GTP (200 microM) by GDP-beta-S (200 microM) in the pipette solution abolished the current evoked by UTP (1 mM). 6. When the pipette solution contained Cs+ (150 mM) instead of K+ and in addition inositol 1,4,5,-trisphosphate (InsP3; 10 microM), an inward current absolutely dependent on extracellular Ca2+ was activated after the establishment of whole-cell recording conditions. This current had a typical delay, a rather slow time course and did not reverse its amplitude up to 100 mV, as measured by fast voltage ramps. 7. A rise of the internal free Ca2+ concentration from 0.01 to 0.5 microM on excised inside-out membrane patches produced single channel activity with a reversal potential of 0 mV in a symmetrical K+ solution. The reversal potential was shifted to negative values, when the extracellular K+ concentration was decreased from 144 to 32 mM. By contrast, a decrease of the extracellular Cl- concentration from 164 to 38 mM did not change the reversal potential. 8. Purine and pyrimidine nucleotides act at separate receptors in rat microglial cells. Pyrimidinoceptors activate via a G protein the enzyme phospholipase C with the subsequent release of InsP3. The depletion of the intracellular Ca2+ pool appears to initiate a capacitative entry of Ca+ from the extracellular space. This Ca2+ then activates a Ca2+-dependent K+ current.

Inhibition of ecto-ATPase by the P2 purinoceptor agonists, ATPgammaS, alpha,beta-methylene-ATP, and AMP-PNP, in endothelial cells

Ecto-ATPase is a plasma membrane-bound enzyme that sequentially dephosphorylates extracellular nucleotides such as ATP. This breakdown of ATP and other nucleotides makes it difficult to characterize and classify P2 purinoceptors. We have previously shown that the P2 purinergic antagonists, PPADS, suramin and reactive blue, act as ecto-ATPase inhibitors in various cell lines. Here, we show that the P2 purinergic agonists, ATPgammaS, alpha,beta-methylene ATP (alpha,beta-MeATP) and AMP-PNP, inhibit the ecto-ATPase of bovine pulmonary artery endothelial cells (CPAE), with pIC50 values of 5.2, 4.5 and 4.0, respectively. In CPAE, FPL67156, a selective ecto-ATPase inhibitor, also inhibits ecto-ATPase activity, with a pIC50 value of 4.0. In addition, alpha,beta-MeATP (3-100 microM), which itself does not induce phosphoinositide (PI) turnover, left-shifted the agonist-concentration effect (E/[A]) curves for ATP, 2MeS-ATP and UTP by approximate 100-300 fold, while those for ATPgammaS and AMP-PNP were only shifted approximately 2-3 fold. Moreover, in the presence of alpha,beta-MeATP, not only was the potentiation effect of PPADS on the UTP response lost, but a slight inhibition of the UTP response by PPADS was also seen. Thus, we conclude that the action of ATPgammaS, alpha,beta-MeATP and AMP-PNP as ecto-ATPase inhibitors account for their high agonist potency, and also provide information for the development of ecto-ATPase inhibitors of high selectivity and potency.

Nonadrenergic contractile response of guinea pig portal vein to electrical field stimulation mimics response to UTP but not to ATP

Transmural electrical field stimulation (EFS, 4-32 Hz) produced a biphasic contractile response consisting of a rapid and transient contraction (first phase) followed by a slow contraction (second phase) in ring preparations of guinea pig portal veins. Both contractions were enhanced by the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 30 microM). In the presence of L-NAME, tetrodotoxin (1 microM) and guanethidine (3 microM) inhibited both contractions and phentolamine (10 microM), and reserpine treatment abolished the first-phase contraction without affecting the second-phase contraction. These results suggest that the first-phase contraction is caused by norepinephrine released from the perivascular nerves. In the presence of phentolamine and L-NAME, the second-phase contraction was inhibited by the nonselective P2-purinoceptor antagonist suramin (30-300 microM) and the P(2Y)-purinoceptor antagonist reactive blue 2 (RB2; 10-100 microM). alpha,beta-Methylene-adenosine triphosphate (alpha,beta-mATP; 3-30 microM), which desensitizes P(2X)-purinoceptors, and the P(2X)-purinoceptor antagonist 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS; 1-10 microM) had a little effect. Exogenous ATP (0.1-3 mM) and UTP (0.1-3 mM) in the presence of L-NAME produced contractions in a concentration-dependent manner. The ATP-induced contraction was enhanced by suramin, RB2, and DIDS but unaltered by alpha,beta-mATP. The UTP-induced contraction was inhibited by suramin and RB2 but unaltered by alpha,beta-mATP and DIDS. These results indicate that in the guinea pig portal vein, the classic P(2X)-purinoceptors do not contribute to the nonadrenergic component of sympathetic neurotransmission. Furthermore, the pharmacology of the nonadrenergic component of neurotransmission resembles that of vasoconstrictor responses to exogenous UTP rather than to ATP.

Vasoconstrictor responses via P2X-receptors are selectively antagonized by NF023 in rabbit isolated aorta and saphenous artery

1. The effects of NF023, the symmetrical 3'-urea of 8-(benzamido)naphthalene-1,3,5-trisulphonic acid), and its parent compound suramin were investigated on vasoconstrictor responses to alpha, beta-methylene ATP in rabbit isolated saphenous artery and vasodilator responses to ATP in noradrenaline-precontracted rabbit isolated thoracic aorta. 2. In rabbit isolated saphenous artery, alpha, beta-methylene ATP-induced vasoconstrictor responses via P2X-receptors were concentration-dependently and competitively antagonised by NF023 (30-300 microM; pA2 = 5.69 +/- 0.04). Suramin (100-1000 microM) also competitively blocked vasoconstrictor responses to alpha, beta-methylene ATP, albeit with lower potency (pA2 = 4.79 +/- 0.05). In contrast, NF023 (100 microM) did not significantly affect contractile responses to noradrenaline or histamine in the saphenous artery. 3. In noradrenaline-precontracted rabbit isolated thoracic aorta preparations, ATP (3-3000 microM) concentration-dependently induced relaxations via endothelium-dependent or smooth muscle P2Y-receptor subtypes. NF023 (30-300 microM) failed to block relaxant responses to ATP at endothelium-dependent P2Y-receptors, whereas suramin (100-1000 microM) did antagonise endothelium-dependent vasodilator responses to ATP. Neither NF023 (100 microM) nor suramin (300 microM) influenced vasorelaxant responses to ATP via endothelium-independent P2Y-receptors. 4. In conclusion, this study outlines the selectivity of NF023 as an effective P2X-receptor antagonist in rabbit isolated blood vessels without affecting endothelium-dependent or endothelium-independent P2Y-receptor subtypes, adrenoceptors or histamine receptors.

Prominent sympathetic purinergic vasoconstriction in the rabbit splenic artery: potentiation by 2,2'-pyridylisatogen tosylate

1. Vasoconstrictions induced by transmural electrical field stimulation were frequency-dependent from 2 to 32 Hz in the rabbit isolated splenic artery. All contractions were abolished in the presence of tetrodotoxin 1 microM or guanethidine 100 microM. Stimulation at a frequency of more than 32 Hz induced both neurogenic and myogenic responses. 2. Prazosin (1 microM) did not significantly affect vascular contractions to electrical stimulation. Desensitization of P2X-purinoceptors with alpha, beta-methylene ATP (alpha, beta-meATP, 3 microM) abolished the contractions to stimulation at 2-8 Hz and inhibited more than 80% of the vascular response at 16 Hz, but it did not significantly change the responses at 32 Hz. Contractile responses at 32 Hz were inhibited by a combination of prazosin and alpha, beta-meATP. Effects of pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid tetrasodium salt (a selective P2X-purinoceptor antagonist) and suramin (a competitive P2-purinoceptor antagonist) on the neurogenic responses were investigated in this study. 3. 2,2'-Pyridylisatogen tosylate (PIT, 0.3-3 microM) significantly potentiated the vasoconstrictions to electrical stimulation at 2-32 Hz in a concentration-dependent manner. Potentiated responses were restored to the control level 30 min after washing. Concentration-dependent response curves for noradrenaline (NA) or alpha, beta-meATP were not significantly changed by 3 microM PIT, and vasoconstriction by adenosine 5'-triphosphate (ATP, 300 microM) was unaffected by PIT. Coomassie brilliant blue-G (1 microM), which shares the potentiating effect on a recombinant P2Y-purinoceptor with PIT (King et al., 1996), did not inhibit or potentiate the purinergically-mediated component of the response to sympathetic nerve stimulation. The selective alpha 2-adrenoceptor antagonist yohimbine (1 microM) also potentiated the vascular responses to electrical stimulation. 4. The present results indicate that ATP evokes postjunctional contractile responses at low and high frequency electrical stimulation of sympathetic nerves supplying the rabbit splenic artery. PIT potentiates the responses to sympathetic (purinergic) nerve stimulation; this appears to be mainly via prejunctional rather than postjunctional actions.