Selective and competitive antagonism by suramin of ATP-stimulated catecholamine-secretion from PC12 phaeochromocytoma cells

The effects of Suramin on Ca2+ activated force and sarcoplasmic reticulum Ca2+ release in skinned fast-twitch skeletal muscle fibers of the rat

Suramin has long been used in the treatment of various human diseases. Intravenous infusions of Suramin are commonly administered to patients over extended periods of time but there are a number of significant contraindications with peripheral muscle weakness being one of the most frequently reported. Previous work has shown that even after a single infusion (300 mg kg⁻¹) Suramin remains in skeletal muscle in effective concentrations (11.6 μg mL⁻¹; 84 days) for prolonged periods. These observations provide a strong rationale for investigation of the specific effects of Suramin on skeletal muscle function. Single mechanically skinned fibers were directly exposed to Suramin (10, 100 or 500 μmol L⁻¹) for defined durations (2–10 min) in controlled physiological solutions that mimic the intracellular ionic environment of a fiber. Suramin treatment (10–500 μmol L⁻¹) directly affected the contractile apparatus in a dose‐dependent manner causing a decrease in Ca²⁺‐sensitivity (pCa50 = −log (Ca²⁺) concentration, where 50% of maximum Ca²⁺‐ activated force is produced) by 0.14 to 0.42 pCa units and reduction in maximum Ca²⁺‐activated force by 14 to 62%. Suramin treatment (100 μmol L⁻¹ for 10 min and 500 μmol L⁻¹ for 2 min) also caused development of a Ca²⁺‐independent force corresponding to 2.89 ± 4.33 and 16.77 ± 7.50% of pretreatment maximum Ca²⁺‐activated force, respectively. Suramin treatment (100 μmol L⁻¹, 2 min) also increased the rate of sarcoplasmic reticulum (SR) Ca²⁺ release without significant changes in SR Ca²⁺ uptake. We report new functional effects for Suramin related to alterations in both the contractile apparatus and SR Ca²⁺‐handling of skeletal muscle that may contribute to the peripheral muscle weakness noted in human pharmacological treatments.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Study of the reversal effect of NF449 on neuromuscular blockade induced by d-tubocurarine

AIMS

The aim of this study was to investigate the mechanism for the reversal effect of NF449 (a suramin analogue) on the neuromuscular block induced by d-tubocurarine (d-TC).

MAIN METHODS

Nerve-stimulated muscle contractions and end-plate potentials were performed in mouse phrenic nerve-diaphragm preparations. Acetylcholine (ACh)-induced muscle contractions were performed in the chick biventer cervicis preparations. Presynaptic nerve terminal waveform recordings were performed in mouse triangularis sterni preparations.

KEY FINDINGS

Amongst the suramin analogues in this study, only the NF449 and suramin were able to reverse the blockade effect produced by d-TC on nerve-stimulated muscle contractions. Each of these suramin analogues (NF007, NF023, NF279 and NF449) alone has no significant effect on the amplitude of nerve-stimulated muscle contractions. NF449 and suramin also showed the antagonising effects on the inhibition of end-plate potentials induced by d-TC. Furthermore, pre-treatment with NF449 can antagonise the inhibition of d-TC in ACh-induced contractions of chick biventer cervicis muscle. NF449 produced a greater rightward shift of the dose-response inhibition curve for d-TC than did suramin. Because other purinergic 2X (P2X) receptor antagonists, NF023 and NF279, do not have the reverse effects on the neuromuscular blockade of d-TC, the effect of NF449 seems irrelevant to inhibition of P2X receptors.

SIGNIFICANCE

These data suggest that NF449 was able to compete with the binding of d-TC on the nicotinic ACh receptors, and the effect of NF449 was more potent than suramin in reducing the inhibition of d-TC. The structure of NF449 may provide useful information for designing potent antidotes against neuromuscular toxins.

Retinoic acids increase P2X2 receptor expression through the 5'-flanking region of P2rx2 gene in rat phaeochromocytoma PC-12 cells

The P2X2 receptor is a subtype of ionotropic ATP receptor and plays a significant role in regulating fast synaptic transmission in the nervous system. Because the expression level of the P2X2 receptor is known to determine its channel properties and functional interactions with other neurotransmitter channels, elucidating the mechanisms underlying the regulation of P2X2 receptor expression in neuronal cells is important. Here, we identified three motifs that correspond to the retinoic acid response element in the 5'-flanking region of the rat P2X2 gene. In rat pheochromocytoma PC-12 cells, treatment with 9-cis-retinoic acid as well as all-trans-retinoic acid significantly increased the mRNA and protein level of P2X2 receptor. In addition, in PC-12 cells transiently transfected with a luciferase reporter gene driven by the promoter region of the rat P2X2 gene, both 9-cis-retinoic acid and all-trans-retinoic acid increased the luciferase activity, whereas their effects were diminished by truncation of the retinoic acid response elements in the promoter. Furthermore, 9-cis-retinoic acid enhanced the ATP-evoked whole cell currents and intracellular Ca2+- and ATP-evoked dopamine release, indicating the up-regulation of functional P2X2 receptors on the plasma membrane. These results provide the molecular mechanism underlying the transcriptional regulation of P2X2 receptors and suggest that retinoid is an important factor in regulating P2X2 receptors in the nervous system.

Inhibition of adenylyl cyclase by neuronal P2Y receptors

P2Y receptors inhibiting adenylyl cyclase have been found in blood platelets, glioma cells, and endothelial cells. In platelets and glioma cells, these receptors were identified as P2Y(12). Here, we have used PC12 cells to search for adenylyl cyclase inhibiting P2Y receptors in a neuronal cellular environment. ADP and ATP (0.1 - 100 microM) left basal cyclic AMP accumulation unaltered, but reduced cyclic AMP synthesis stimulated by activation of endogenous A(2A) or recombinant beta(2) receptors. Forskolin-dependent cyclic AMP production was reduced by <or=1 microM and enhanced by 10 - 100 microM ADP; this latter effect was turned into an inhibition when A(2A) receptors were blocked. The nucleotide inhibition of cyclic AMP synthesis was not altered when P2X receptors were blocked, but abolished by pertussis toxin. The rank order of agonist potencies for the reduction of cyclic AMP was (IC(50) values): 2-methylthio-ADP (0.12 nM)=2-methylthio-ATP (0.13 nM)>ADPbetaS (71 nM)>ATP (164 nM)=ADP (244 nM). The inhibition by ADP was not antagonized by suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid, or adenosine-3'-phosphate-5'-phosphate, but attenuated by reactive blue 2, ATP(alpha)S, and 2-methylthio-AMP. RT - PCR demonstrated the expression of P2Y(2), P2Y(4), P2Y(6), and P2Y(12), but not P2Y(1), receptors in PC12 cells. In Northern blots, only P2Y(2) and P2Y(12) were detectable. Differentiation with NGF did not alter these hybridization signals and left the nucleotide inhibition of adenylyl cyclase unchanged. We conclude that P2Y(12) receptors are expressed in neuronal cells and inhibit adenylyl cyclase activity.

Aggravation of chemically-induced injury in perfused rat liver by extracellular ATP

The effects of purinergic receptor agonists on acute liver damage and hemodynamics were studied using chemically-induced liver injury. Rat livers were perfused in situ 24 h after treatment with D-galactosamine (800 mg/kg, i.p.). In these livers, infusion of ATP (50 microM) into the portal vein caused a rapid increase in the leakage of LDH and AST from perfused liver in a dose dependent manner, accompanied with flow reduction. The similar but less effective responses were also observed by the infusion of ADP. Infusion of adenosine, a P1-receptor agonist, induced only minimal changes of liver damage and flow rate. The ATP-induced changes were almost completely suppressed by P2-receptor antagonist, suramin, but not affected by P1-receptor antagonist, 8-phenyltheophylline. Pretreatment of rats with gadolinium chloride, which depletes Kupffer cells, did not inhibit the potentiation of liver damage caused by ATP, whereas hemodynamic effects of ATP were significantly attenuated by gadolinium. These results indicate that extracellular ATP aggravates acute liver injury mediated by P2-type purinergic receptors.

Impaired arterial baroreflex regulation of heart rate after blockade of P2-purinoceptors in the nucleus tractus solitarius

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of ATP mimics baroreflex responses (bradycardia, hypotension); however, the physiological role of these receptors in cardiovascular control remains unclear. We tested whether blockade of these receptors attenuates arterial baroreflex control of heart rate (HR). Baroreflex-induced changes in HR (via graded i.v. infusion of phenylephrine and nitroprusside) were observed in seven alpha-chloralose/urethane anesthetized male Sprague-Dawley rats before and after microinjection of the purinergic P2 receptor antagonist suramin (0.5 nmol in 50 nL) into the subpostremal NTS. Before suramin, typical baroreflex changes in HR were observed (maximum gain, Gmax = 2.94 +/- 0.54 bpm/mmHg). Suramin markedly impaired baroreflex-induced changes in HR (gain = 0.02 +/- 0.08 and 0.18 +/- 0.09 bpm/mmHg for increases and decreases in mean arterial blood pressure, respectively); however, after 90-130 min, HR and baroreflex reactivity returned to control levels. Microinjections of vehicle into the same area did not alter baroreflex function. In addition, suramin did not alter the depressor responses to microinjections of glutamate into the same site of the NTS. We conclude that normal P2x-purinoceptor function in subpostremal NTS may be necessary for baroreflex regulation of HR.

ATP-stimulated activation of the mitogen-activated protein kinases through ionotrophic P2X2 purinoreceptors in PC12 cells. Difference in purinoreceptor sensitivity in two PC12 cell lines

Extracellular purine nucleotides elicit a diverse range of biological responses through binding to specific cell surface receptors. The ionotrophic P2X subclass of purinoreceptors respond to ATP by stimulation of calcium ion permeability; however, it is unknown how P2X purinoreceptor activation is linked to intracellular signaling pathways. We report that stimulation of PC12 cells with ATP results in the activation of the mitogen-activated protein (MAP) kinases ERK1 and ERK2 and was wholly dependent upon extracellular calcium ions. Treatment of the cells with adenosine, AMP, ADP, UTP, or alpha,beta-methylene ATP was without effect; however, MAP kinase activation was abolished by pretreatment with suramin and reactive blue 2. The calcium-activated tyrosine kinase, Pyk2, acts as an upstream regulator of the MAP kinases and became tyrosine phosphorylated following treatment of the cells with ATP. We have ruled out the involvement of depolarization-mediated calcium influx because specific blockers of voltage-gated calcium channels did not affect MAP kinase activation. These data provide direct evidence that calcium influx through P2X2 receptors results in the activation of the MAP kinase cascade. Finally, we demonstrate that a different line of PC12 cells respond to ATP through P2Y2 purinoreceptors, providing an explanation for the conflicting findings of purine nucleotide responsiveness in PC12 cells.

P2 purinoceptor localization along rat nephron and evidence suggesting existence of subtypes P2Y1 and P2Y2

Effects of extracellular ATP on intracellular free calcium concentration([Ca2+]i) were examined in rat single nephron segments using the fura 2-AM. ATP (10 ¿M) induced a significant transient increase in [Ca2+]i in the glomerulus, the early proximal convoluted tubule (S1), the cortical collecting tubule (CCT), and the outer medullary collecting tubule (OMCT). The magnitude of the response was the greatest in the OMCT among four segments. ATP induced an increase in the [Ca2+]i in a dose-dependent manner in S1 and OMCT. In the OMCT, ATP caused a biphasic increase in [Ca2+]i consisting of an initial rapid rise and a sustained phase. Removal of calcium from the medium resulted in an attenuation of the sustained phase of [Ca2+]i and an approximately 30% reduction in the height of the initial [Ca2+]i peak in response to 10 ¿M ATP. Effects of ATP, its analogs, and its metabolites were tested in the S1 and OMCT. ATP, 2-methylthio-ATP (2-MeS-ATP), ADP, and UTP increased [Ca2+]i dose dependently. AMP and adenosine did not affect [Ca2+]i in the S1 and OMCT. The ATP- or 2-MeS-ATP-induced [Ca2+]i increase was inhibited by the pretreatment of the S1 and OMCT with suramin or reactive blue 2. Neomycin, a phospholipase C inhibitor, attenuated the ATP-induced [Ca2+]i increase. To investigate the hormonelike action of ATP in OMCT, a heterologous cross desensitization was performed. The pretreatment of OMCT with ATP inhibited increases in vasopressin-, ANG II-, endothelin-1-, or bradykinin-induced[Ca2+]i increase. These findings suggest that ATP might affect the above peptidyl agonist-activated calcium mobilizations.

P2 receptor-mediated inhibition of adenylyl cyclase in PC12 cells

PC12 pheochromocytoma cells have P2 receptors which are coupled to Ca2+ influx and catecholamine release. Previously we reported that ATP stimulated cyclic AMP accumulation at low concentrations up to 100 microM but showed inhibitory effects above this concentration [Yakushi, Y., Watanabe. A.. Murayama, T., Nomura, Y., 1996. Eur. J. Pharmacol. (314) 243-248]. In this study we investigated the characteristics of the inhibitory effects of ATP analogs. In the presence of 10 microM forskolin, an activator of adenylyl cyclase, ATP, adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), 2',3'-O-(4-benzoyl) benzoyl ATP, 2-methylthio ATP and adenosine 5'-O-(2-thiodiphosphate) inhibited cyclic AMP accumulation in a dose-dependent manner from 100 microM. UTP, alphabeta and betagamma-methylene ATP had no or very limited effects. The relative order of ATP analogs suggests that the ATP receptor appears to be P2Y-like. However, suramin, an antagonist of P2X and P2Y receptors, and reactive blue-2, which inhibited betagamma-methylene ATP-induced cyclic AMP accumulation, did not modify the inhibitory effect of ATPgammaS. Treatment with pertussis toxin, which completely abolished the effect of carbachol, had no effect on the action of ATP over 300 microM. The existence of a new type of ATP receptor-mediated inhibition of adenylyl cyclase is proposed in PC12 cells.

An aspartic acid residue near the second transmembrane segment of ATP receptor/channel regulates agonist sensitivity

Charged or polarized amino acid residues near or within the second transmembrane (M2) segment of neuronal ATP receptor/channels (P2X2 receptors) were neutralized by site-directed mutagenesis, and the properties of the mutants were electrophysiologically characterized using Xenopus oocytes. When Asp315 was substituted with Val (D315V), the sensitivity to ATP was reduced by about 60-fold. The sensitivity to ATP was not affected by the neutralization of Lys324, which is involved in a Walker type A ATP-binding sequence, Lys366, Tyr330, or Asn333. With D315V channels, the sensitivities to other agonists (ADP, ATP gamma S, and 2-methylthio ATP) were also reduced. The sensitivities to antagonists (suramin and Cibacron Blue F3GA) were, however, not affected by this neutralization. The results suggest that Asp315, which is assumed to be present in the extracellular region near the M2 segment of P2X2 receptor/channels, serves to maintain agonist sensitivity.

ATP stimulates release of excitatory amino acids from cultured Schwann cells

The release of excitatory amino acids from Schwann cell cultures in the rat was monitored using high-performance liquid chromatography. The basal concentration of glutamate and aspartate was 33 +/- 4 nM (mean +/- S.E.M., n = 12) and 8 +/- 1 nM (mean +/- S.E.M., n = 12), respectively. ATP (100 microM) caused a receptor-mediated increase in release of glutamate and aspartate from Schwann cell cultures. Bath application of adenosine (100 microM) was without effect on release of excitatory amino acids suggesting involvement of P2 receptors. Suramin, a competitive antagonist at P2 receptors, prevented the response to ATP. The release of excitatory amino acids evoked by ATP was not abolished in calcium-depleted saline. Pretreatment of the Schwann cultures with 50 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetracetic acid-acetoxymethyl ester (BAPTA-AM) abolished the effect of ATP. ATP-evoked release of glutamate from cultured Schwann cells was significantly reduced by thapsigargin (1 microM), an inhibitor of Ca(2+)-ATPase of the Ca2+ pump of internal stores. U73122, a selective inhibitor of receptor-coupled phospholipase C-dependent processes, abolished stimulatory effect of ATP suggesting that ATP's action is mediated through an inositol 1,4,5-triphosphate-sensitive calcium store. The action of ATP was not blocked by L-trans-pyrrolidine-2,4-dicarboxylate, an inhibitor of the electrogenic glutamate transporter, nor was it blocked in Na(+)-free medium, and glutamate release was not stimulated by a depolarizing stimulus, suggesting that ATP-evoked release of glutamate from Schwann cells is not due to the reversal of the glutamate uptake. An anion transport blocker, furosemide, reduced ATP-induced glutamate release. These results suggest that ATP-stimulated glutamate and aspartate release from Schwann cells may be through a calcium-dependent furosemide-sensitive mechanism.

Purinoceptors in neuromuscular transmission

At the neuromuscular junction, P2-purinoceptors mediate the actions of the co-transmitter ATP and P1-purinoceptors, those of its degradation product adenosine. The classification of the subtypes of P1- and P2-purinoceptors and their signal transduction routes is presented. Purinoceptor-mediated effects on the prejunctional release of acetylcholine and the postjunctional desensitization and expression of nicotinic receptors are discussed in depth. An additional section on the reversal action of the P2-purinoceptor antagonist suramin on neuromuscular block underscores the importance of testing purinoceptor-targeted drugs once they will be marketed, to avoid adverse effects in patients.

Activation of P2x-purinoceptors in the nucleus tractus solitarius elicits differential inhibition of lumbar and renal sympathetic nerve activity

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of alpha,beta-methylene ATP (alpha,beta-MeATP) elicits large dose-dependent decreases in mean arterial pressure (MAP) and heart rate (HR) and preferential dilation of the iliac vascular bed in comparison to renal and mesenteric vascular beds. We investigated whether sympathoinhibition contributes to the depressor responses and whether differential changes in regional sympathetic output occur. In 43 chloralose/urethane anesthetized male Sprague-Dawley rats, MAP, HR, renal (RSNA) and lumbar sympathetic nerve activity (LSNA) were recorded. Data were analyzed as both the maximum decrease and the integral of the decrease over the duration of the depressor response. Microinjection of alpha,beta-MeATP (25 and 100 pmol in 50 nl volume) into the subpostremal NTS caused significant and dose-dependent decreases in MAP, HR, RSNA and LSNA. However, the changes in RSNA were significantly greater than those observed in LSNA for both doses and both methods of analysis of data (maximum responses in delta %: 84 +/- 3 vs 62 +/- 4, and 93 +/- 3 vs 74 +/- 4 for low and high dose of alpha,beta-MeATP, respectively; integral responses in delta % x min: 32 +/- 4 vs 18 +/- 3 and 179 +/- 7 vs 134 +/- 14 for low and high dose of alpha,beta-MeATP, respectively). Blockade of P2-purinoceptors in the NTS by the specific P2-receptor antagonist suramin abolished responses to 100 pmol alpha,beta-MeATP and microinjections of vehicle did not alter neural nor hemodynamic parameters. We conclude that activation of P2x-purinoceptors in the NTS inhibits sympathetic nerve activity and evokes differential regional sympathetic responses. However, differential sympathoinhibition does not explain differential vascular responses to the activation of P2x-purinoceptors in the NTS.

P2 purinoceptor-mediated stimulation of adenylyl cyclase in PC12 cells

PC12 pheochromocytoma cells have P2 purinoceptors which are activated by ATP and coupled to Ca2+ influx and catecholamine release. Also PC12 cells have adenosine receptors coupled positively to adenylyl cyclase, and cyclic AMP regulates cell functions such as catecholamine release. The effects of ATP and ATP analogs on cyclic AMP accumulation in PC12 cells were investigated in this study. ATP and adenosine 5'-0-(3-thiotriphosphate) stimulated cyclic AMP accumulation at low concentrations up to 300 microM but showed inhibitory effects above this concentration. 2',3'-O-(4-Benzoyl)benzoyl ATP and 2-methylthio ATP showed similar effects, although the responses were very limited. Addition of adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) or beta, gamma-methylene ATP, but not alpha, beta-methylene ATP, stimulated cyclic AMP accumulation markedly without causing an inhibitory phase. The effects of ATP, ADP beta S and beta, gamma-methylene ATP were not inhibited by adenosine deaminase or specific antagonists to A1 and A2 adenosine receptors. Neither ADp beta S nor beta, gamma-methylene ATP showed any effect on Ca2+ influx or noradrenaline release. Suramin, a P2 receptors antagonists, had no inhibitory effect against ATP analog-stimulated cyclic AMP accumulation, although reactive blue 2 inhibited the beta, gamma-methylene ATP-stimulated reaction but not that up-regulated by ADP beta S. These findings suggest that the pharmacological characteristics of these ATP receptors coupled to adenylyl cyclase are clearly different from those of ligand-gated ion channels defined by P2X purinoceptors, which have been cloned and shown to be coupled to Ca2+ influx and catecholamine release in PC12 cells. The existence of a new type of P2 purinoceptor-mediating stimulation of adenylyl cyclase is proposed in PC12 cells.

The prejunctional inhibitory effect of suramin on neuromuscular transmission in vitro

The P2 purinoceptor antagonist suramin reverses skeletal muscle paralysis evoked by non-depolarizing neuromuscular blocking agents in vitro and in vivo. To further study the action of suramin on neuromuscular transmission, (miniature) endplate potentials ((m.)e.p.ps), motor nerve terminal currents and the release of radiolabeled acetylcholine was measured in isolated nerve-muscle preparations. In preparations paralysed by low Ca2+/high Mg2+ conditions, suramin (10 microM-1 mM) induced a concentration-dependent decrease in quantal content of the e.p.ps without affecting m.e.p.ps. Suramin reversed neuromuscular block by d-tubocurarine in these preparations. In erabutoxin paralysed preparations, suramin (40 microM-1 mM) inhibited the motor nerve terminal currents related to Ca2+ influx concentration-dependently, but did not affect Na+ currents. Suramin-induced inhibition of Ca2+ currents was not antagonized by ATP gamma S. Suramin (300 microM) reduced [14C]acetylcholine outflow in non-paralysed rat phrenic nerve-hemidiaphragm preparations by 32%. As suramin did not chelate Ca2+, these results indicate that suramin inhibits neuromuscular transmission by blocking prejunctional Ca2+ channels, thereby decreasing acetylcholine release upon nerve stimulation.

The mechanism of action of hepatic sympathetic nerves on ketone-body output from perfused rat liver. The effect of the interaction of noradrenaline with ATP on the release of beta-hydroxybutyrate

The regulatory mechanism of ketone-body output by the hepatic sympathetic nerves was studied in rat liver perfused in situ. Enrichment of the perfusion medium with 1 mM octanoate increased the basal ketone-body output from the liver up to 1.5 mumol.min-1.g liver-1. Under these conditions, electrical stimulation of the hepatic nerves (20 V, 20 Hz, 2 ms) decreased the output of both acetoacetate and beta-hydroxybutyrate, and was accompanied by an accumulation of beta-hydroxybutyrate in the liver. The effects of nerve stimulation were inhibited by the alpha 1-antagonist bunazosin (10 microM). However, noradrenaline, a typical sympathetic neurotransmitter, at a concentration of 1 microM decreased the output of acetoacetate but did not affect beta-hydroxybutyrate output. Prostaglandin F2 alpha at a concentration of 10 microM produced an effect similar to treatment with noradrenaline, without a decrease in beta-hydroxybutyrate output. ATP at 50 microM, however, decreased the output of both acetoacetate and beta-hydroxybutyrate and increased the tissue concentration of beta-hydroxybutyrate, mimicking the effect of nerve stimulation. Moreover, in the presence of 0.2 microM ATP, a concentration that produced neither metabolic nor hemodynamic changes, noradrenaline (1 microM) was shown to decrease the beta-hydroxybutyrate output. These results indicate the possible involvement of ATP in the action of hepatic sympathetic nerves on beta-hydroxybutyrate output from the liver, presumably through its interaction with noradrenaline.

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

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

ATP receptor-mediated increase of Ca ionophore-stimulated arachidonic acid release from PC12 pheochromocytoma cells

Phospholipase A2 has recently been proposed as the effector enzyme involved in the receptor-mediated release of arachidonic acid (AA). Released AA and its metabolites have been demonstrated to play an important role in the regulation of cell functions. [3H]AA release from prelabeled PC12 cells was stimulated by a Ca ionophore such as ionomycin or A23187. Although ATP and its effective analog, adenosine 5'-O-(3-thiotrisphosphate) (ATP gamma S), 2-methylthio ATP and 3'-O-(4-benzoyl)benzoyl ATP, did not stimulate [3H]AA release on their own, they did enhance Ca ionophore-stimulated [3H]AA release. The effect of ATP analogs was dose-dependent. ADP, UTP, GTP, ITP, alpha beta-methylene ATP, beta gamma-methylene ATP and 8-bromo ATP showed no effect or very limited effect. The effect of ATP gamma S was antagonized by suramin, a putative P2Y receptor antagonist. The effective ATP analogs also increased [Ca2+]i (cytosolic free Ca2+ concentration) via Ca2+ influx. However, the addition of 50 mM KCl or 10 microM bradykinin, which are well-known to increase [Ca2+]i by different pathways, did not stimulate [3H]AA release, either with or without the Ca ionophore. The addition of phorbol 12-myristate 13-acetate, an activator of protein kinase C, showed no effect on [3H]AA release, either with or without the Ca ionophore. These data suggest that 1) ATP increased Ca ionophore-stimulated AA release via a P2Y-like ATP receptor, and that 2) the elevation of [Ca2+]i by ATP does not quantitatively explain the ATP-stimulated AA release in PC12 cells.

Purine and pyrimidine nucleotides activate distinct signalling pathways in PC12 cells

The role of extracellular nucleotides in intracellular signalling and neurosecretion was assessed in PC12 cells. Activation of phospholipase C and increased [Ca2+]i were mediated by purinoceptors with an agonist potency profile, ATP approximately UTP > 2-methylthioadenosine triphosphate (2-MeSATP), typical of P2U. ATP also evoked a rapid acidification followed by a more gradual alkalinization (measured with 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF)), while UTP induced only a gradual alkalinization. The amiloride analogue 5-(N-ethyl-N-isopropyl)amiloride (EIPA) attenuated the alkalinization phase suggesting activation of the Na+/H+ exchanger by ATP and UTP. Using bisoxonol and [3H]tetraphenylphosphonium ([3H]TPP+) as potential-sensitive probes, we showed that while ATP rapidly depolarized PC12 cells in an Na(+)-dependent manner, UTP evoked a much reduced and delayed response. The potency profile (ATP approximately 2-MeSATP approximately adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) >> UTP, alpha, beta-methyleneATP) suggested involvement of a receptor subtype distinct from P2U. Secretion of endogenous dopamine was also assessed. Those nucleotides that induced depolarization (ATP, 2-MeSATP, ATP gamma S) were also the most potent secretagogues. UTP was ineffective. Our results suggest that ATP stimulates distinct purinoceptor subtypes and induces neurosecretion through the activation of multiple signalling pathways.

Enhancement by zinc of ATP-evoked dopamine release from rat pheochromocytoma PC12 cells

The effects of zinc (Zn2+) on ATP-evoked dopamine release was investigated in rat pheochromocytoma PC12 cells. Zn2+ potentiated the dopamine release evoked by 30 microM ATP in a concentration-dependent manner over a concentration range from 3 to 300 microM. High concentration of Zn2+ (> 1 mM) inhibited the release. Zn2+ (10 microM) shifted the concentration-response curve of the ATP-evoked dopamine release to the left without affecting the maximal response. The dopamine release evoked by 40 mM KCl was not affected by Zn2+ (1-100 microM), whereas high concentration of Zn2+ ( > 300 microM) attenuated the response. The dopamine release evoked by 30 microM ATP in the presence of 10 microM Zn2+ were suppressed by suramin (30 microM), an antagonist to P2-purinoceptors, to an extent similar to that in the absence of Zn2+. Zn2+ (1-100 microM) enhanced the ATP-evoked increase in intracellular Ca2+ concentration ([Ca]i) in the cells. The Ca2+ responses to ATP in the presence and absence of Zn2+ were abolished by external Ca(2+)-depletion. Under whole-cell voltage-clamp, Zn2+ (10 microM) augmented by two-fold the peak amplitude of an inward current evoked by 30 microM ATP. Taken together, it is suggested that Zn2+ enhances the ATP-evoked dopamine release by increasing sensitivity to ATP. The enhancement may be due to the augmentation of ATP-gated Ca(2+)-influx, but not due to modulation of cellular machinery downstream to [Ca]i rise. The enhancement of the ATP-mediated responses may underlie modulation by Zn2+ of physiological functions in various types of neuronal cells.

Inhibition by suramin and reactive blue 2 of GABA and glutamate receptor channels in rat hippocampal neurons

Effects of suramin and reactive blue 2 (RB2), compounds known as antagonists at P2-purinoceptors, on ionic currents mediated through GABA and glutamate receptor channels were investigated in rat hippocampal neurons. Under whole-cell voltage-clamp, suramin (30 to 300 microM) and RB2 (10 to 100 microM) inhibited a current activated by 10 microM GABA in a concentration-dependent manner. Suramin (100 and 300 microM) and RB2 (10 and 30 microM) also inhibited an inward current activated by kainic acid (100 microM), an agonist at non-NMDA type glutamate receptor channels, and an inward current activated by N-methyl-D-aspartate (NMDA; 100 microM), an agonist at NMDA type glutamate receptor channels. The inhibition by suramin or RB2 did not exhibit voltage-dependence between -30 and -90 mV in the case of the GABA- or the kainate-evoked current. In contrast, the inhibition by these compounds of the NMDA-evoked current exhibited voltage-dependence and was enhanced by hyperpolarization. When the concentration of agonists was increased by 5- or 10-fold, the magnitude of the inhibition by suramin of the kainate-evoked current and the magnitude of the inhibition by RB2 of the NMDA-evoked current were attenuated. alpha,beta-Methylene ATP (100 microM) did not affect the GABA-, kainate- or NMDA-activated current. The results suggest that suramin and RB2 inhibit GABA receptor channels and glutamate receptor channels. The blockade of these channels must be taken into account when these compounds are used as pharmacological tools to examine an involvement of P2-purinoceptors, especially in preparations where GABAergic or glutamatergic neurotransmission is expected.

Effects of ATP and UTP in pheochromocytoma PC12 cells: evidence for the presence of three P2 receptors, only one of which subserves stimulation of norepinephrine release

1. In pheochromocytoma PC12 cells ATP and, to a lesser extent, 2-methylthioATP stimulate phosphoinositide breakdown, release of intracellular calcium, and influx of external calcium, leading to stimulation of norepinephrine release. In contrast, although UTP also stimulates phosphoinositide breakdown, release of intracellular calcium, and influx of external calcium, there is no stimulation of norepinephrine release. 2. 2-MethylthioATP, presumably acting at P2y receptors, and UTP, presumably acting at P2u receptors, in combination elicit a phosphoinositide breakdown greater than that elicited by either alone. Intracellular levels of calcium measured with Fura-2 increase to greater levels with ATP than with UTP and are sustained, while the UTP intracellular levels of calcium rapidly return to basal values. Both ATP and UTP cause a similar influx of 45 Ca2+ presumably by stimulation of a P2 receptor directly linked to a cation channel. 3. It is proposed that PC12 cells contain two distinct G protein-coupled P2 receptors that activate phospholipase C and a P2 receptor linked to a cation channel. The P2y receptor sensitive to ATP (and to 2-methylthioATP) causes the depletion of a pool of intracellular calcium, sufficient to activate so-called "receptor-operated calcium entry". The sustained elevation of intracellular calcium after ATP treatment is proposed to result in stimulation of norepinephrine release and activation of calcium-dependent potassium channels and sodium-calcium exchange pathways. 4. The P2u receptor sensitive to UTP (and to ATP) causes only a transient elevation in levels of intracellular calcium, perhaps from a different pool, insufficient to activate so-called receptor-operated calcium entry. Further sequelae do not ensue, and the functional role of the UTP-sensitive P2u receptor is unknown.

Suramin and reactive blue 2 are antagonists for a newly identified purinoceptor on rat megakaryocyte

1. The effects of purinoceptor antagonists on ATP-induced oscillatory K(+)-currents in rat isolated megakaryocytes were investigated. 2. Both reactive blue-2 (RB-2), a selective antagonist of the P2Y purinoceptor, purinoceptor, at concentrations of 0.3-10 microM and suramin, a non-selective P2 purinoceptor antagonist, at 1-30 microM blocked the ATP-induced oscillation in a concentration-dependent manner. 3. RB-2 and suramin also blocked the ADP-induced K(+)-current oscillation at the same concentration range as in the case of ATP. However, both suramin and RB-2 had no effect on thrombin- and inositol 1,4,5-trisphosphate (IP3)-induced K+ current oscillation, indicating that they act as specific purinoceptor antagonists. 4. Thus, the purinoceptors on megakaryocytes show the properties of the P2 subtype according to their blockade by antagonists.

Prejunctional modulation of noradrenaline release in mouse and rat vas deferens: contribution of P1- and P2-purinoceptors

1. Prejunctional purinoceptors modulating the release of noradrenaline were compared in mouse and rat vas deferens. Tissue slices were preincubated with [3H]-noradrenaline and then superfused and stimulated electrically, in most experiments by trains of 60 pulses, 1 Hz. 2. In mouse vas deferens, 2-chloroadenosine (IC50 0.24 microM), beta,gamma-methylene-ATP (IC50 3.8 microM), alpha,beta-methylene-ATP (IC50 2.9 microM) and 2-methylthio-ATP (only 30 microM tested) reduced the evoked overflow of tritium. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX), 10 nM, antagonized the effect of 2-chloro-adenosine (apparent pKB 10.2) as well as of beta,gamma-methylene-ATP (apparent pKB 9.6) and alpha,beta-methylene-ATP. Suramin, 300 microM, attenuated the effect of 2-chloroadenosine at best very slightly, antagonized the effect of beta,gamma-methylene-ATP (apparent pKB 4.5) and, when combined with DPCPX 10 nM, caused a further marked shift to the right of the concentration-response curve of beta,gamma-methylene-ATP beyond the shift produced by DPCPX alone. 3. In rat vas deferens, 2-chloroadenosine (IC50 0.20 microM), beta,gamma-methylene-ATP (IC50 4.8 microM), alpha,beta-methylene-ATP (IC50 3.0 microM) and 2-methylthio-ATP (only 30 microM tested) also reduced the evoked overflow of tritium. DPCPX, 10 nM, antagonized the effect of 2-chloroadenosine (apparent pKB 9.7) as well as of beta,gamma-methylene-ATP (apparent pKB 9.6) and alpha,beta-methylene-ATP. Suramin, 300 microM, did not change the effect of 2-chloroadenosine, attenuated the effect of beta,gamma-methylene-ATP at best very slightly and, when combined with DPCPX, caused at best a very small shift to the right of the concentration-response curve of beta,gamma-methylene-ATP beyond the shift produced by DPCPX alone.4. It is concluded that prejunctional purinoceptor mechanisms in mouse and rat vas deferens are similar. In either species, both nucleosides such as adenosine and nucleotides such as beta,gamma-methylene-ATP activate a common release-inhibiting receptor which is a Pl- or, more specifically, A1-purinoceptor.There seems to be no need to postulate the existence of a novel prejunctional P3-purinoceptor.Moreover, the sympathetic terminal axons possess an additional P2-purinoceptor in both species which is activated by some nucleotides such as beta,gamma-methylene-ATP and 2-methylthio-ATP, although the activation of the P2-purinoceptor by beta,gamma-methylene-ATP is difficult to demonstrate in the rat.

Suramin selectively inhibits the non-adrenergic non-cholinergic inhibitory junction potential in the guinea-pig stomach

In smooth muscle cells of the guinea-pig stomach fundus, transmural nerve stimulation evoked a cholinergic excitatory junction potential (e.j.p.) and, in the presence of atropine, a non-adrenergic non-cholinergic (NANC) inhibitory junction potential (i.j.p.). Suramin (> 10(-5) M), a putative inhibitor of the P2 purinoceptor, enhanced the e.j.p. amplitude and inhibited the i.j.p., with no significant effect on the membrane potential. Thus, a possible involvement of ATP in the generation of the NANC i.j.p. has to be considered.

Characterization of ATP receptor which mediates norepinephrine release in PC12 cells

PC12 cells, a rat pheochromocytoma cell line, has been reported to release norepinephrine in response to extracellular ATP in the presence of extracellular Ca2+. The potency order of ATP analogues was adenosine 5'-O-(3-thiotriphosphate) greater than ATP greater than adenosine 5'-O-(1-thiotriphosphate) = 2-methylthioadenosine 5'-triphosphate (MeSATP) greater than 2'- and 3'-O-(4-benzoyl-benzoyl)ATP (BzATP) greater than ADP greater than 5-adenylylimidodiphosphate. Adenosine 5'-O-(2-thiodiphosphate), beta, gamma-methyleneadenosine 5'-triphosphate, AMP and adenosine were inactive. The ATP action in the absence of extracellular Ca2+, suggests a small but appreciable contribution of intracellular Ca2+ mobilization, for norepinephrine release. However, for some ATP derivatives, like BzATP, almost no contribution of the phospholipase C-Ca2+ pathway is suggested, based on their low activity in inositol phosphates production. To identify the ATP-receptor protein, PC12 cell membranes were photoaffinity-labeled with [32P]BzATP. SDS-PAGE analysis showed that a 53-kDa protein labeling was inhibited by ATP and its derivatives, as well as by P2-antagonists, suramin and reactive blue 2, which inhibit the nucleotide-induced norepinephrine release. The inhibitory activity of the nucleotides was, in parallel with their potency, to induce norepinephrine release. Despite their inability to release norepinephrine, GTP and GTP gamma S inhibited the BzATP labeling, suggesting the participation of a putative G protein in the ATP-receptor-mediated actions. We suggest that the 53-kDa protein on the PC12 cell surface is an ATP receptor, which mediates the norepinephrine release, depending, mainly, on extracellular Ca2+ gating.

Suramin reverses non-depolarizing neuromuscular blockade in rat diaphragm

Unexpectedly, it was observed that the P2-purinoceptor antagonist, suramin (10 microM to 1 mM), reversed the muscle paralysis caused by structurally unrelated non-depolarizing relaxants. Suramin competitively reversed the blocking action of pancuronium. Both the pre- and postsynaptic blockade of nicotinic receptors by pancuronium was counteracted, as shown by the action of suramin, using train-of-four stimulation. Suramin did not affect the paralysis caused by the depolarizing relaxant, succinylcholine. The reversal action of suramin was not due to an increase in the acetylcholine concentration in the synaptic cleft, since neither the contraction of preparations partially paralysed by diminished acetylcholine release in the presence of low Ca2+ or high Mg2+ nor acetylcholinesterase activity were affected. Suramin did not affect the reduction in twitch tension caused by adenosine and potentiated the ATP-induced reduction in twitch, indicating that ATP-sensitive receptors are not involved in the reversal action of suramin. Consequently, these results suggest that the action of suramin is due to binding with a site on the acetylcholine receptor also occupied by non-depolarizing relaxants, but different from the site occupied by succinylcholine.

Dopamine receptor agonists and antagonists enhance ATP-activated currents

The effects of dopamine and related compounds on ATP-activated channels were investigated in pheochromocytoma PC12 cells. Dopamine (10 microM) enhanced an inward current activated by 100 microM ATP. A similar enhancement of the ATP-activated current was observed with apomorphine (10 microM), a non-selective dopamine receptor agonist, with (+)-SKF-38393 (10 microM), a selective dopamine D1 receptor agonist, and with (-)-quinpirole (10 microM), a selective dopamine D2 receptor agonist. Moreover, (+)-SCH-23390 (30 microM), a dopamine D1 receptor antagonist, and (-)-sulpiride (30 microM), a dopamine D2 receptor antagonist, also enhanced the ATP-activated current. The results suggest that ATP-activated channels are modulated by dopaminergic mechanisms, and that this modulation cannot be attributed to any single class of dopamine receptors.

Effects of the P2-purinoceptor antagonist, suramin, on human platelet aggregation induced by adenosine 5'-diphosphate

1. The effects of suramin, a trypanocidal drug which has been reported to be a P2-purinoceptor antagonist on smooth muscle, were investigated in human platelets, where adenosine 5'-diphosphate (ADP) induces aggregation by acting on a subtype of purinoceptors which has been called P2T. 2. Suramin (100 microM) had no inhibitory effect on ADP-induced platelet aggregation in plasma, even after 40 min incubation in the presence of bacitracin, a peptidase inhibitor, and did not affect the ability of adenosine 5'-triphosphate (ATP) (40 microM) to inhibit competitively ADP-induced aggregation. This lack of effect of suramin on platelets in plasma is probably due to its extensive binding to plasma proteins. 3. In washed platelets, suramin (50-400 microM) acted as an apparently competitive antagonist, causing parallel shifts to the right of the log concentration-response curve to ADP. No depression of the maximal response to ADP was observed at concentrations of suramin (50-150 microM) for which full log concentration-response curves to ADP could be obtained, but the slope of the Schild plot was around 2, indicating that this antagonism was not simply competitive. The apparent pA2 value for suramin, taken from this Schild plot, was 4.6. 4. Suramin (200-400 microM) also noncompetitively inhibited aggregation induced by U46619 (a thromboxane receptor agonist) or by 5-hydroxytryptamine in the presence of adrenaline (100 microM), and caused a depression of the maximal response to these agonists. This nonspecific effect of suramin may explain the high Schild plot slope obtained against ADP.5. These results provide evidence that the ADP receptor on human platelets is indeed similar to the P2-purinoceptors responding to adenine nucleotides on smooth muscle and other tissues, and show that suramin cannot distinguish between the proposed subtypes of the P2-purinoceptors.

Extracellular adenosine 5′-triphosphate-evoked glutamate release in cultured hippocampal neurons

Characteristics of extracellular ATP-evoked electrical responses in rat hippocampal neurons were investigated. Extracellular ATP (100 microM) induced a rapid depolarization followed by repetitive firings of spikes in these cells under whole-cell current-clamp. In whole-cell voltage-clamp experiments, ATP activated 2 types of inward currents that were inhibited by P2-purinoceptor blocker suramin (300 microM). One is a small (about -20 pA) sustained current which is insensitive to tetrodotoxin (TTX), and the other is a large (-100 to -300 pA) transient current which abolished in the presence of 3 microM TTX. The ATP-induced transient current was blocked by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 30 microM), a non-N-methyl-D-aspartate (non-NMDA) receptor antagonist. ATP failed to induce the transient current in the cell which showed the desensitization to quisqualic acid (QA; 10 microM), a non-NMDA receptor agonist. These findings suggest that ATP directly activates small sustained currents, and indirectly induces the transient currents by evoking glutamate release.

Antagonism by reactive blue 2 but not by brilliant blue G of extracellular ATP-evoked responses in PC12 phaeochromocytoma cells

1. The effects of reactive blue 2 and brilliant blue G, which have been shown to block extracellular ATP-evoked responses, were investigated to discover whether these compounds act as P2-purinoceptor antagonists in PC12 phaeochromocytoma cells. 2. Reactive blue 2 (10 to 100 microM) suppressed the ATP-stimulated dopamine secretion from PC12 cells in a dose-dependent manner. The concentration-response curve for ATP was shifted to the right and the maximal response was decreased by reactive blue (30 and 100 microM). Brilliant blue G (up to 100 microM) did not significantly affect the secretion. 3. Reactive blue 2 (10 to 100 microM) suppressed the ATP-activated inward current recorded from the voltage-clamped cells in a concentration-dependent manner. Brilliant blue G (up to 100 microM) did not affect the current. 4. The results suggest that reactive blue 2 but not brilliant blue G is a P2-purinoceptor antagonist in PC12 cells. The purinoceptors in these cells may be the same type as those involved in ATP-evoked smooth muscle relaxation, judging from the antagonism by reactive blue 2.

Effects of ATP antagonists on purinoceptor-operated inward currents in rat phaeochromocytoma cells

The effects of suramin, reactive blue 2 (RB2) and d-tubocurarine (d-TC) were investigated electrophysiologically to elucidate the mechanisms underlying their antagonism of P2 purinoceptor-mediated responses. All three compounds inhibited an adenosine triphosphate (ATP)-activated inward current in rat phaeochromocytoma PC12 cells in a concentration-dependent manner. The order of potency was RB2 greater than suramin greater than d-TC. The inhibition induced by suramin or RB2 was reversible, whereas that induced by d-TC was not reversed after a 5-min rinse. The inactivation of the ATP-activated current was accelerated by d-TC but not by suramin or RB2. RB2 administered simultaneously with ATP exerted much weaker inhibition compared to that induced by prior administration, suggesting that RB2 is a slowly acting antagonist. This was not observed for suramin or d-TC. Suramin and RB2 caused a parallel shift in the concentration/response curve for the ATP-activated current. With d-TC the maximal response of ATP was decreased but the concentration producing half-maximal response was unchanged. The voltage dependency of the ATP-activated current showed less inward rectification in the presence of d-TC. Suramin or RB2 did not affect the voltage dependency. These results suggest that suramin and RB2 reversibly block binding of ATP to receptors, whereas d-TC blocks ion permeability through the ATP-activated channel.