Suramin antagonizes responses to P2-purinoceptor agonists and purinergic nerve stimulation in the guinea-pig urinary bladder and taenia coli

Nitric oxide synthase in the enteric nervous system of the guinea-pig: a quantitative description

The distribution and abundance of nitric oxide synthase (NOS)-containing neurons and their terminals in the gastrointestinal tract of the guinea-pig were examined in detail using NADPH diaphorase histochemistry and NOS immunohistochemistry. NOS-containing cell bodies were found in the myenteric plexus throughout the gastrointestinal tract and in the submucous plexus of the stomach, colon and rectum. NOS-containing neurons comprised between 12% (in the duodenum) and 54% (in the esophagus) of total myenteric neurons. In the ileum, NOS neurons represented 19% of total myenteric neurons. Most of the NOS neurons throughout the gastrointestinal tract possessed lamellar dendrites and a single axon. NOS-containing terminals were abundant in the circular muscle, including that of the sphincters, but were rare in the longitudinal muscle, except for the taeniae of the caecum. The muscularis mucosae of the esophagus, stomach, colon and rectum received a medium to dense innervation by NOS terminals. Within myenteric ganglia, NOS-containing terminals were extremely sparse in the esophagus, stomach and duodenum, common in the ileum and distal colon and extremely dense in the proximal colon and rectum. The submucous plexus in the ileum and large intestine contained a sparse plexus of NOS-containing terminals. NOS terminals were not observed in the mucosa of any region. We conclude that throughout the gastrointestinal tract of the guinea-pig, NOS neurons are inhibitory motor neurons to the circular muscle; in the ileum and large intestine, NOS neurons may also function as interneurons.

Novel competitive antagonists for P2 purinoceptors

Binding of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) (ADP beta 35S) to P2 gamma purinoceptors on turkey erythrocyte membranes was used to determine the affinity of suramin and various suramin congeners belonging to different structure classes (large urea, small urea, dibenzamides and benzamides) for these receptors. Suramin was shown to be a competitive antagonist with a Ki value of 7.3 +/- 2.2 microM. The simple benzamide compound XAMR0721 (8-(3,5-dinitrophenylene carbonylimino)-1,3,5-naphthalene trisulfonate, trisodium salt) displays a high affinity for the P2 gamma purinoceptor (Ki value of 19 +/- 6 microM). Similar to suramin, compound XAMR0721 is a competitive antagonist at P2 gamma purinoceptors. In contrast to suramin, which is a potent inhibitor of the ecto-nucleotidase activity in human blood cells (44 +/- 2% residual activity at 100 microM), compound XAMR0721 is hardly active in this assay (93 +/- 1% residual activity at 100 microM). So XAMR0721, the first competitive antagonist for P2 purinoceptors that is able to discriminate between P2 purinoceptor affinity and ecto-nucleotidase activity, is an interesting pharmacological tool for the characterization of P2 purinoceptor mediated effects.

Local neural control of intestinal motility: nerve circuits deduced for the guinea-pig small intestine

1. Propulsion of digesta along the intestine appears to occur by the action of a series of local reflexes which cause contraction oral to the digesta and relaxation of circular muscle on the anal side. 2. There is now substantial evidence available about the identities of the enteric neurons that mediate these reflexes. 3. The motor neurons and interneurons of the reflex pathways lie within the myenteric plexus. These neurons can be classified electrophysiologically as S-neurons and have distinctive projections and neurochemistries. 4. The sensory neurons may lie in the myenteric plexus, but there is some evidence for sensory neurons in the submucous plexus. A contribution from extrinsic sensory neurons to local motility reflexes cannot be ruled out. Intrinsic sensory neurons are probably AH-neurons and are large multi-axonal cells.

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

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

Differential effects of suramin on P2-purinoceptors mediating contraction of the guinea-pig vas deferens and urinary bladder

1. The effect of the P2-purinoceptor antagonist, suramin, was investigated on contractions of the guinea-pig vas deferens and urinary bladder induced by adenosine 5'-triphosphate (ATP) and by the other naturally occurring nucleoside triphosphates. 2. ATP, guanosine 5'-triphosphate (GTP), cytidine 5'-triphosphate (CTP), inosine 5'-triphosphate (ITP) and uridine 5'-triphosphate (UTP) (0.1-500 microM) each contracted both the guinea-pig bladder and the guinea-pig vas deferens. In the vas deferens the order of potency of the nucleotides was ATP >> CTP > GTP > or = UTP = ITP, and in the bladder it was ATP >> CTP = GTP > UTP = ITP, although maximal responses to these agonists were not achieved in either tissue. 3. Suramin (30 microM-1 mM) dose-dependently inhibited ATP-induced contractions of the bladder in an apparently non-competitive manner, causing a reduction in the slope of the concentration-response curve to ATP. In contrast, suramin (5 microM-1 mM) had little inhibitory effect on ATP-induced contractions of the vas deferens, and indeed at concentrations of 100 microM and above markedly potentiated high concentrations of ATP (100-500 microM). The contractions induced by CTP, GTP, UTP and ITP (1-500 microM) were, however, abolished by suramin (1 mM) in each tissue. 4. Desensitization of the P2X purinoceptors in the guinea-pig vas deferens with adenosine 5'-alpha,beta-methylenetriphosphonate (AMPCPP) (300 microM) abolished contractions induced by ATP (1 microM-1 mM) in the absence of suramin. However, the contractions induced in the presence of suramin were unaffected by prior desensitization, indicating that they were not mediated by P2X-purinoceptors.5. ATP (100 MicroM) was dephosphorylated by both isolated tissue preparations under the conditions of these experiments, breakdown products being detectable after 2 min, with the major breakdown product in the bladder being inosine whereas that in the vas deferens was adenosine. Approximately 35% of the ATP remained intact after incubation for 30 min with the bladder, and approximately 45% remained after incubation for 30 min with the vas deferens. In each tissue this degradation was inhibited by suramin (1 mM), so that after incubation of ATP (100 MicroM) in the presence of suramin for 30 min,approximately 50% remained in the case of the bladder and approximately 65% remained in the vas deferens. However, inhibition of the production of the inhibitory agonist, adenosine by suramin did not appear to be responsible for the potentiation observed in the vas deferens, as the PI-purinoceptor antagonist 8-sulphophenyltheophylline (100 MicroM) did not reduce this potentiation.6. Chelation of divalent cations did not appear to account for the enhancement by suramin of ATP-induced contractions of the vas deferens, as the enhancement was still observed when Mg2+ was omitted from the buffer or when its concentration (normally 1.2 mM) was increased ten fold to 12 mM,or when the concentration of Ca2+ (normally 2.5 mM) was reduced to 0.83 mM. Even in the absence of Mg2+ and with the Ca2+ concentration reduced to 0.83 mM, no inhibition by suramin (1 mM) of ATP-induced contractions was observed.7. The most likely explanation for the potentiation by suramin of the ATP-induced contractions of the vas deferens is the co-existence of inhibitory P2Y-purinoceptors. However, no consistent relaxations to ATP (1-100 MicroM) or to the more potent P2Y-purinoceptor agonist 2-methylthioadenosine 5'-triphosphate(2-MeSATP) (0.01-100 MicroM) could be detected in the vas deferens precontracted with KCl (35 mM), even after desensitization of P2x-purinoceptors with AMPCPP (300 MicroM). Similarly, ATP (1-100 MicroM) or 2-MeSATP (0.01-1100 MicroM) added before KCI (35 mM), carbachol (10 JM) or noradrenaline (10 MicroM) did not reduce subsequent contractions to these agents.8. The differential effect of suramin on the contractions induced by ATP in the bladder and the vas deferens was unexpected, and shows that the receptor populations by which ATP acts in these tissues may not be identical. The failure of suramin to inhibit responses to ATP in the vas deferens suggests that this tissue, in addition to possessing P2x-purinoceptors may also possess a suramin-insensitive contractile ATP receptor revealed in the presence of suramin.

The use of antagonists to characterize the receptors mediating depolarization of the rat isolated vagus nerve by alpha, beta-methylene adenosine 5'-triphosphate

1. We have previously found that the P2x-purinoceptor agonist, alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-methylene ATP), depolarizes the rat cervical vagus nerve, measured with a 'grease-gap' extracellular recording technique. This effect was attenuated by the P2 purinoceptor antagonist, suramin. In the present study we have investigated in more detail the antagonism produced by suramin and have also investigated the actions of two other putative P2 purinoceptor antagonists, cibacron blue and pyridoxal-phosphate-6-azophenyl-2', 5'-disulphonic acid (iso-PPADS). Furthermore, we have studied the interactions between suramin and cibacron blue or iso-PPADS in an attempt to determine whether these antagonists act at a common receptor site. 2. Suramin (1 x 10(-5)-1 x 10(-4) M) produced reversible, concentration-related rightward displacements of the concentration-effect curve to alpha, beta-methylene ATP. Schild analysis of this antagonism yielded a pA2 value of 5.90 with a slope value of 0.47. 3. Cibacron blue (3 x 10(-5)-1 x 10(-4) M) also antagonized depolarizations induced by alpha, beta-methylene ATP. The antagonistic effects of cibacron blue were slow to reach equilibrium but could be readily reversed on washout. At low concentrations for antagonism, cibacron blue (1 x 10(-5) M and 3 x 10(-5) M) produced enhancement of the maximal response to alpha, beta-methylene ATP. At the highest concentration tested (1 x 10(-4) M) the concentration-effect curve to alpha, beta-methylene ATP was shifted to the right in a parallel manner, yielding a pKB estimate of 4.96. 4. Iso-PPADS (1 X 10-6 1 X 10-5- M) produced a concentration-related depression in the maxima ofthe concentration-effect curves to alpha,beta-methylene ATP. Analysis of these data by a double reciprocal plot yielded a pKB estimate of 6.02. This profile of insurmountable antagonism could not be attributed to irreversible binding of iso-PPADS to the receptor since the effect of iso-PPADS could be reversed on washing, albeit slowly.5. In the presence of suramin (1 x 10-4 M), cibacron blue (1 x 10-4 M) produced no further rightward displacement of the alpha,beta-methylene ATP concentration-effect curve. The mean agonist concentration ratios in the presence of suramin or cibacron blue alone (11.7 and 10.3, respectively) were not significantly different from the mean concentration-ratio in the presence of both antagonists (11.8). This finding suggests that high concentrations of alpha,beta-methylene ATP activate a receptor population which is resistant to blockade by either antagonist.6. The antagonistic effect of iso-PPADS (1 x 10-5 M) was partially attenuated by suramin (1I x 10-4 M).It is possible that this interaction reflects a slow dissociation of iso-PPADS from the receptor with which suramin and alpha,beta-methylene ATP interact.7. Suramin, cibacron blue or iso-PPADS had no marked effect on depolarization produced by 5-hydroxytryptamine (5-HT, 1 x 10-7-3 x 10-5 M), indicating their specificity in antagonizing responses to alpha, beta-methylene ATP.8. Responses to alpha,beta-methylene ATP were not antagonized by 8-para-sulphophenyltheophylline (3 x 10-5M), ondansetron (1 x 10-7 M), bicuculline (1 x I0-5 M), phentolamine (1 X 10-6 M) or hexamethonium(1 X 10-4 M), which are antagonists at P1-purinoceptors, 5-HT3 receptors, GABAA receptors, a-adrenoceptors and nicotinic cholinoceptors, respectively, thereby excluding the involvement of these receptors.Indomethacin (3 X 10-6 M) had no effect on responses to alpha,beta-methylene ATP.9. The results obtained with three purinoceptor antagonists confirm and extend our original supposition that alpha,beta-methylene ATP-induced depolarization of the rat vagus nerve is mediated predominantly via P2 purinoceptors, thought to be of the P2,X subtype. The finding that responses induced by high concentrations of agonist were resistant to blockade by suramin and cibacron blue, but could be attenuated by iso-PPADS, adds further weight to our speculation that the purinoceptor population in the rat vagus nerve is heterogeneous.

Characterization of 5-HT receptors mediating contraction and relaxation of the longitudinal muscle of guinea-pig distal colon in vitro

A range of agonists and antagonists were used to characterize the receptors through which 5-hydroxytryptamine (5-HT) contracts and relaxes the longitudinal muscle of segments of guinea-pig distal colon, in vitro. 5-HT contracted the longitudinal muscle over the concentration range 10(-9) to 10(-4) mol/l. The 5-HT3 receptor agonist, 2-methyl-5-HT, produced concentration dependent contractions over the range 10(-6) to 10(-4) mol/l. 5-methoxytryptamine, an agonist at 5-HT4 receptors, caused contractions over a concentration range of 10(-8) to 10(-4) mol/l. The 5-HT4 antagonist, SDZ 205-557 (5 x 10(-7) mol/l) substantially suppressed the responses to low concentrations of 5-HT and to 5-methoxytryptamine, but had no effect on the responses to higher concentrations of 5-HT. In contrast, the 5-HT3 antagonist, granisetron (10(-6) mol/l), blocked the effect of 2-methyl-5-HT and substantially depressed responses to high concentrations of 5-HT, but had no effect on lower concentrations of 5-HT. Granisetron produced a small reduction in the response to 5-methoxytryptamine. Tetrodotoxin (TTX) (3 x 10(-7) mol/l) almost abolished the response to 5-methoxytryptamine and markedly suppressed the response to 2-methyl-5-HT, but the responses to 5-HT were only partially reduced. The 5-HT1 antagonist, methiothepin (10(-6) mol/l) depressed the response to 5-HT (10(-7) to 10(-4) mol/l) and blocked its TTX insensitive component. The 5-HT2 antagonist, ketanserin, in concentrations up to 10(-5) mol/l, had no effect on the contractions evoked by 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of specific ATP receptors induces a rapid increase in intracellular calcium ions in rat hypothalamic neurons

We have used real-time dynamic video imaging of Fura-2 fluorescence to study the acute effects of external ATP on [Ca2+]i in cultured rat hypothalamic neurons. The addition of ATP at microM concentrations, but not adenosine, AMP, ADP or GTP, produced a rapid, dose-dependent increase in cytosolic Ca2+. The hydrolysis-resistant ATP analogues 3-thio-ATP and beta,gamma-imido-ATP produced a similar response but alpha,beta-methylene ATP had much lower efficacy. The ATP response was inhibited by 10 microM nifedipine, abolished by 50 microM cadmium and by the absence of extracellular Ca2+, but was unaffected by ryanodine or omega-conotoxin GVIA. The P2-purinoceptor antagonist suramin reversibly and selectively inhibited the ATP response but had no effect on other neurotransmitter-induced Cai2+ responses. Antagonists to muscarinic, nicotinic, NMDA, non-NMDA, GABA, 5-HT and adenosine receptors had no effect on the ATP response. Thus the Ca2+ response of hypothalamic neurons to ATP is mediated by specific suramin-sensitive ATP-receptors, activation of which is independent of ATP hydrolysis and results in an influx of extracellular Ca2+ largely through high voltage-gated Ca2+ channels. These findings support the assertion that ATP acts in the CNS as an excitatory neurotransmitter.

The effects of the purinergic system on digitalis-induced epileptiform activity

It has been suggested that endogenous chemical substances, such as adenosine, released during a seizure attack, may act as anticonvulsants in vivo. We have investigated electrophysiologically the effects of purinoceptor agonists and antagonists on the epileptiform activity induced by intracortical digitalis in anesthetized rats. Intracortical injections of 1, 2, or 4 micrograms digitalis (desacetyl lanatocid C) caused an epileptiform electrocorticogram (ECoG). The application of adenosine (25 or 100 microM) or adenosine triphosphate (ATP) (3 mM) after desacetyl lanatocid C blocked the epileptiform activity. beta, gamma-Methylene ATP (0.1-0.8 mM), a stable analog of ATP, produced inhibition and then death. The epileptogenic effect of desacetyl lanatocid C was enhanced by theophylline (1 mM); however, suramin (1 mM) changed the pattern of epilepsy. These results indicate that the purinergic system may be involved in the mechanism of action of digitalis glycosides.

Dual effect of ATP and UTP on rat atria: which types of receptors are involved?

The effects of adenine compounds and UTP were examined in electrically driven rat left atria. ATP, ADP, AMP, adenosine and UTP caused a dual inotropic effect: first a rapid decrease in contractility, and second an increase in contractile tension. alpha,beta-Methylene ATP caused an increase in contractile tension only, whereas 2-methylthio-ATP only induced a negative inotropic effect, 1,3-Dipropyl-8-cyclopentylxanthine inhibited the negative effects of ATP and adenosine, whereas 3,7-dimethyl-1-propargylxanthine did not influence the effects of ATP. Suramin but not reactive blue 2 antagonized the positive inotropism induced by ATP and alpha,beta-methylene ATP. Suramin also abolished the positive inotropic effect induced by UTP. These results demonstrate that ATP may induce negative inotropism directly by an action on A1-adenosine receptors and positive inotropism by an action on P2x-purinoceptors. UTP induces a positive inotropic effect mediated by suramin-sensitive receptors.

Effects of suramin on increases in cytosolic calcium and on inhibition of adenylate cyclase induced by adenosine 5'-diphosphate in human platelets

The effects of the P2-purinoceptor antagonist, suramin, on ADP-induced increases in human platelet cytosolic calcium concentration ([Ca2+]i) and inhibition of prostaglandin E1 (PGE1)-stimulated adenylate cyclase activity were investigated. Suramin (50-200 microM) acted as an antagonist of ADP-induced increases in [Ca2+]i, causing parallel, rightward shifts of the log concentration-response curve to ADP with no apparent depression of the maximal response. However, the slope of the Schild plot was 2.3 +/- 0.3, similar to that obtained in previous studies on aggregation, indicating that the antagonism was not simply competitive. The apparent pA2 for suramin, taken from the Schild plot, was 4.63, similar to that for suramin's inhibition of aggregation, which suggests that these two effects are closely related. Suramin was not specific for the ADP receptor, however, as it was also able to inhibit, non-competitively, increases in [Ca2+]i induced by 5-hydroxytryptamine. Suramin (50-400 microM) also inhibited the effect of ADP on PGE1-stimulated accumulation of cyclic AMP, causing parallel shifts of the log concentration-response curve to ADP, with a Schild plot slope of 1.00 +/- 0.10, suggesting competitive antagonism, and a pA2 value of 5.09. Suramin (400 microM) did not reduce the inhibition of cyclic AMP accumulation by adrenaline, although it was able to inhibit the accumulation of cyclic AMP caused by PGE1, again showing that suramin has some non-specific effects. These data suggest that suramin is an antagonist at the platelet ADP receptor mediating increases in [Ca2+]i and inhibition of adenylate cyclase, but that it also shows non-specific effects and can depress platelet responses to other agonists. In addition, the similar pA2 value of suramin for the two effects of ADP does not support suggestion that they are mediated by two different receptors on human platelets.

Adenosine 5'-triphosphate (ATP), the neurotransmitter in the prostatic portion of the longitudinal muscle layer of the rat vas deferens

Suramin (1-100 microM) and alpha, beta-methylene adenosine 5'-triphosphate (AMPCPP, 39 microM), antagonized the motor activity induced by exogenous adenosine 5'-triphosphate (ATP) but not exogenous noradrenaline (NA) in the longitudinal musculature of prostatic (P) and epididymal (E) segments of the rat vas deferens. Likewise, application of these drugs reduced the fast component of the nerve-stimulated contraction in response to a single transmural electrical pulse in E and P. Suramin also blocked in a concentration-dependent fashion, the contractile responses to trains of 1.5, 5, 15 or 30 Hz transmural electrical pulses in P, while it did not affect those in E. AMPCPP obliterated responses to trains of 1.5, 5, and 15 Hz in P, while reducing these responses in E to a significantly lesser extent. Present results strongly support that ATP is the motor transmitter in P, while in E, ATP and NA are likely the co-transmitters responsible for the motor tone.

Structure Activity Relationships for Derivatives of Adenosine-5'-Triphosphate as Agonists at P(2) Purinoceptors: Heterogeneity Within P(2X) and P(2Y) Subtypes

The structure-activity relationships for a variety of adenine nucleotide analogues at P(2x)- and P(2Y)-purinoceptors were investigated. Compounds formed by structural modifications of the ATP molecule including substitutions of the purine ring (C2, C8, N1, and N(6)-substituents, and a uridine base instead of adenine), the ribose moiety (2' and 3'-positions), and the triphosphate group (lower phosphates, bridging oxygen substitution, and cyclization) were prepared. Pharmacological activity at P(2Y)-purinoceptors was assayed in the guinea pig taenia coli, endothelial cells of the rabbit aorta, smooth muscle of the rabbit mesenteric artery, and turkey erythrocyte membranes. Activity at P(2X)-purinoceptors was assayed in the rabbit saphenous artery and the guinea-pig vas deferens and urinary bladder. Some of the analogues displayed selectivity, or even specificity, for either the P(2X)- or the P(2Y)-purinoceptors. Certain analogues displayed selectivity or specificity within the P(2X)- or P(2Y)-purinoceptor superfamilies, giving hints about possible subclasses. For example, 8-(6-aminohexylamino)ATP and 2',3'-isopropylidene-AMP were selective for endothelial Pzypurinoceptors over P(2Y)-purinoceptors in the guinea pig taenia coli, rabbit aorta, and turkey erythrocytes. These compounds were both inactive at P(2X)-purinoceptors. The potent agonist N(6)-methyl ATP and the somewhat less potent agonist 2'-deoxy-ATP were selective for P(2Y)-purinoceptors in the guinea pig taenia coli, but were inactive at P(2X)-purinoceptors and the vascular P(2Y)-purinoceptors. 3'-Benzylamino-3'-deoxyATP was very potent at the P(2X)-purinoceptors in the guinea pig vas deferens and bladder, but not in the rabbit saphenous artery and was inactive at P(2Y) receptors. These data suggest that specific compounds can be developed that can be utilized to activate putative subtypes of the P(2X)- and P(2Y)-purinoceptor classes.

Potentiation by endogenously released ATP of spontaneous transmitter secretion at developing neuromuscular synapses in Xenopus cell cultures

1. Previously we have shown that extracellular application of ATP, a substance co-stored and co-released with acetylcholine (ACh) in the peripheral nervous system, markedly potentiated the frequency of spontaneous synaptic currents (SSCs) produced by ACh. In the present study, we have further characterized the purinoceptor which mediates the potentiation effect of ATP and the role of endogenously released ATP. 2. Pretreatment with a P2-purinoceptor antagonist, suramin (0.3 mM), but not a P1-purinoceptor antagonist, 8-phenyltheophylline (0.1 mM), prevented the potentiating effect of ATP. 3. We studied the role of endogenously released ATP by examining the effect of a specific P2-purinoceptor antagonist on the frequency of spontaneous synaptic events at high-activity synapses (> or = 3 Hz) and found that suramin, but not 8-phenyltheophylline markedly reduced the frequency of SSCs at these high-activity synapses. In addition, desensitizing the P2-purinoceptor with alpha,beta-methylene ATP also produced similar effects to suramin. 4. Extracellular application of the L-type Ca2+ channel blockers, verapamil, nifedipine or diltiazem (10 microM each) reduced SSC frequency of high-activity synapses, while the N-type Ca2+ channel blocker, omega-conotoxin had no appreciable effect. The potentiating effect of ATP was further prevented by pretreatment with the L-type Ca2+ channel blockers. On the other hand, Bay K 8644, which is a depolarization-dependent L-type Ca2+ channel agonist, potentiated SSC frequency at these high-activity synapses. 5. These results suggest that endogenous release of ATP at developing neuromuscular synapses is responsible for the maintenance of high levels of spontaneous ACh release, which is known to play a crucial role in regulating postsynaptic differentiation.

Selective antagonism by PPADS at P2X-purinoceptors in rabbit isolated blood vessels

1. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a P2-purinoceptor antagonist, was investigated for its ability to antagonize: (1) P2X-purinoceptor-mediated contractions of the rabbit central ear artery and saphenous artery evoked by either alpha,beta-methylene ATP (alpha,beta-MeATP) or electrical field stimulation (EFS); (2) P2Y-purinoceptor-mediated relaxations of the rabbit mesenteric artery; (3) endothelium-dependent and endothelium-independent, P2Y-purinoceptor-mediated relaxations of the rabbit aorta. 2. alpha,beta-MeATP (0.1-100 microM) caused concentration-dependent contractions of the rabbit ear and saphenous arteries. The negative log[alpha,beta-MeATP] that produced a contraction equivalent to the EC25 for noradrenaline (ear artery) or histamine (saphenous artery) in the absence of PPADS was 6.60 +/- 0.18 (9) and 6.18 +/- 0.17 (9) in the ear artery and saphenous artery, respectively. These effects of exogenous alpha,beta-MeATP were concentration-dependently inhibited by PPADS (1-30 microM). In the ear artery, the negative log[alpha,beta-MeATP] producing a contractile response equivalent to the EC25 of noradrenaline, in the presence of PPADS at 1, 3 and 10 microM was 6.16 +/- 0.18 (8), 5.90 +/- 0.18 (8) and 4.72 +/- 0.36 (8), respectively (P < 0.01). In the saphenous artery, the negative log[alpha,beta-MeATP] values equivalent to the EC25 for histamine in the presence of PPADS at concentrations of 1, 3, 10 and 30 microM were 5.90 +/- 0.19 (8), 5.73 +/- 0.16 (8), 4.99 +/- 0.14 (8) and 4.51 +/- 0.13 (8), respectively (P < 0.01). 3. PPADS at a concentration of 1 microM had no effect on contractions of the ear artery evoked by EFS (4-64 Hz; 1 microM phentolamine present). At higher concentrations (3-30 MicroM) it caused concentration dependent inhibition of neurogenic contractions. In the saphenous artery, PPADS (1-30 MicroM) concentration-dependently inhibited contractions evoked by EFS at frequencies of 4, 8 and 16 Hz. Contractions evoked by EFS at frequencies of 32 and 64 Hz were significantly inhibited by PPADS only at concentrations of 10 and 30 MicroM.4. PPADS (30 MicroM) had no effect on relaxations to 2-methylthio ATP (3 nM-3 MicroM) in rabbit mesenteric artery and to ATP (1 MicroM-I mM) in rabbit aorta (with endothelium intact or removed). In addition,PPADS (30 MicroM) had no significant influence on the contractile potency of noradrenaline and histamine in rabbit ear and saphenous artery, respectively.5. In conclusion, these results support the evidence that PPADS is a selective antagonist of P2X-purinoceptor-mediated responses.

Accommodation mediated by enteric inhibitory reflexes in the isolated guinea-pig small intestine

1. The aim of the present study was to investigate whether the guinea-pig small intestine shows accommodation to infused fluid, similarly to other regions of the gastrointestinal tract. Tetrodotoxin, papaverine and transmitter antagonists were used to establish the existence of reflex pathways and the nature of the neurotransmitters involved. 2. Compliance, measured as the change in volume of infused fluid divided by the intraluminal pressure change, was reduced by tetrodotoxin (0.6 microM), indicating that there is an overall neurally mediated relaxation of the circular muscle in response to low rates of distension. Papaverine (10 microM) did not have any significant effect on compliance at the low rates of distension, suggesting that the circular muscle is fully relaxed. 3. At each rate of distension, 400 microM N omega-nitro-L-arginine methyl ester (L-NAME, a nitric oxide synthase inhibitor) significantly decreased the compliance of the intestinal wall, indicating that the circular muscle was relaxed by a nitric oxide-mediated mechanism. Apamin (0.5 microM), which blocks a component of inhibitory transmission, did not have a significant effect. 4. In control preparations, the intestinal wall was less compliant when distended by fluid at a fast rate, compared with the lower rates of distension. This was not due to changes in passive components of the intestinal wall or a myogenic response to rapid stretch. 5. When the intestine was distended rapidly, 1 microM hyoscine and 100 microM hexamethonium increased intestinal compliance. However, they had no detectable effect on compliance with low rates of distension.(ABSTRACT TRUNCATED AT 250 WORDS)

P2-purinoceptor-mediated autoinhibition of sympathetic transmitter release in mouse and rat vas deferens

Effects of drugs acting at P2-purinoceptors on the release of newly taken up [3H]-noradrenaline were studied in slices of mouse and rat vas deferens. The slices were superfused and stimulated electrically, in most experiments by trains of 60 pulses/8 Hz. In mouse vas deferens, the P2-purinoceptor antagonists reactive blue 2 (1.8-100 microM) and brilliant blue G (10-300 microM) increased the stimulation-evoked overflow of tritium in a concentration-dependent manner as shown previously for suramin. Reactive blue 2, which preferentially blocks the P2Y-subtype, was the most potent compound and the compound with highest maximal effect, an increase by 104%. Pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), in contrast, caused a small increase only at a single concentration (30 microM). The effects of reactive blue 2, brilliant blue G and suramin were not additive. The P2 agonist adenosine 5'-O-(3-thio)-triphosphate (ATP gamma S) reduced the evoked overflow of tritium. As shown previously for suramin, reactive blue 2 30 microM and brilliant blue G 100 microM antagonized the effect of ATP gamma S. From the shift of the ATP gamma S concentration-response curve to the right, an apparent pKB value of 5.3 was estimated for reactive blue 2 and an apparent pKB of 4.5 for brilliant blue G. In rat vas deferens, reactive blue 2 (3-30 microM), brilliant blue G (10 microM) and suramin (30-300 microM) also increased the evoked overflow of tritium. As in the mouse, reactive blue 2 was the most potent compound and the compound with highest maximal effect, an increase by 90%.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of suramin on vasodilator responses to ATP and 2-methylthio-ATP in the Sprague-Dawley rat coronary vasculature

The effect of suramin, a P2 purinoceptor antagonist, on the vasodilator response to adenosine 5'-triphosphate (ATP), 2-methylthio-ATP (2-meSATP) and adenosine were examined in the Sprague-Dawley rat coronary vasculature using the Langendorff heart preparation. Relaxation induced by 2-meSATP was significantly inhibited by suramin. Only responses to low doses of adenosine and ATP were inhibited by suramin. 8-(p-Sulphophenyl)theophylline (8-PSPT) did not affect the relaxant response to ATP and 2-meSATP at a concentration that significantly inhibited the response to adenosine. It is concluded that 2-meSATP acts via P2Y purinoceptors while ATP appears to be acting largely through a different mechanism. It is not acting via a P1 purinoceptor because ATP was not inhibited by the P1 purinoceptor antagonist 8-PSPT.

Purinergic modulation of the evoked release of [3H]acetylcholine from the hippocampus and cerebral cortex of the rat: role of the ectonucleotidases

Modulation by exogenous and endogenous adenine nucleotides and adenosine of [3H]acetylcholine release evoked by veratridine (10 microM) was compared in synaptosomal fractions from the hippocampus and the cerebral cortex of the rat. In both brain areas, exogenously added ATP or adenosine (10-100 microM) inhibited the evoked tritium release. In the hippocampus, ATP gamma S, an ATP analogue more resistant to catabolism than ATP, was virtually devoid of effect on tritium release, and the effect of ATP was prevented by the ecto-5'-nucleotidase inhibitor alpha,beta-methylene ADP (100 microM), by adenosine deaminase (2 U/ml) and by the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 20 nM). In contrast, in the cerebral cortex, the effect of ATP on tritium release was not prevented by either alpha,beta-methylene ADP (100 microM) or adenosine deaminase (2 U/ml), and several ATP analogues (30 microM) inhibited release. The order of intensity of the inhibitory effects of the ATP analogues was: ATP gamma S > ATP > beta,gamma-imido ATP > beta,gamma-methylene ATP >> 2-methyl-S-ATP, alpha,beta-methylene ATP. The effect of ATP gamma S in the cerebral cortex was prevented by DPCPX (20 nM) and was not affected by the P2 purinoceptor antagonist suramin (100 microM). In the hippocampus, alpha,beta-methylene ADP (100 microM) increased the evoked release of tritium, and adenosine deaminase (2 U/ml) produced an even greater increase; when adenosine deaminase was added in the presence of alpha,beta-methylene ADP, adenosine deaminase still increased the evoked release of tritium. In the cerebral cortex, DPCPX (20 nM) and adenosine deaminase (2 U/ml) increased the evoked tritium release by a similar magnitude, but the effect of adenosine deaminase was smaller than in the hippocampus. It is concluded that in the cerebral cortex ATP as such presynaptically inhibits acetylcholine release, whereas in the hippocampus the role of adenine nucleotides is as a source of endogenous extracellular adenosine that tonically inhibits acetylcholine release. The results also show that besides formation of adenosine from adenine nucleotides, released adenosine as such contributes in nearly equal amounts to the pool of endogenous adenosine that presynaptically inhibits acetylcholine release in the hippocampus.

Some effects of purines on neurones of guinea-pig superior cervical ganglia

1. The actions of adenosine 5'-triphosphate (ATP) and related purine analogues on neurones of isolated superior cervical ganglia in guinea-pig were studied using intracellular recording techniques. 2. Application of ATP (0.1-1 mM) by superfusion evoked a response in half (26 out of 52) of the neurones studied. The response consisted of a slow depolarization (in 23 of the cells) or a hyperpolarization (in 3 cells). The amplitude of depolarization was concentration-dependent and was accompanied by a decrease in input resistance (in 12 of 23 responses). 3. Superfusion of the P2 chi-purinoceptor selective agonist alpha, beta-methylene ATP (1-100 microM), evoked a response in the majority (42 of 58) of neurones studied. The response consisted of a depolarization (in 39 cells) or a hyperpolarization (in 3 cells). The depolarization was also concentration-dependent and was accompanied by a decrease in input resistance (36 of 39 responses). Superfusion of another P2 chi-purinoceptor agonist, beta, gamma-methylene ATP (10 microns), also produced a small depolarization (5 of 8 cells) which was accompanied by a decrease in input resistance. 4. Application of the P2 gamma-purinoceptor selective agonist, 2-methylthio ATP (1-100 microM) evoked a response in approximately half (28 of 55) of the neurones studied. The response consisted of a depolarization (in 23 cells) or a hyperpolarization (in 5 cells). The amplitude of the depolarization was concentration-dependent and accompanied by a decrease in input resistance (13 of 21 responses). 5. Superfusion of the P1-purinoceptor selective agonist 2-chloroadenosine (100 microM) evoked a response in only a few (3 of 21) of the neurones tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Gastrointestinal neurotransmitters

The enteric nervous system contains neurones that are intrinsic to the gastrointestinal tract and the axons of extrinsic neurones. More than 30 functional types of neurone are present and about 25 different possible neurotransmitters have been identified in enteric neurones. Most neurones utilize several transmitters; amongst the transmitters of an individual neurone, one is usually a primary transmitter and other substances are subsidiary transmitters or neuromodulators. The primary transmitter is the substance that has the major role in acutely changing the excitability of the innervated cell. Current evidence indicates that primary transmitters are strongly conserved; that is, the same substance will be the neurotransmitter in functionally equivalent neurones in different regions of the gastrointestinal tract and in different species. In contrast, subsidiary transmitters and neuromodulators of equivalent neurones in different regions are not necessarily the same. Only about seven of the approximately 25 enteric neurotransmitters are known to be primary transmitters. Acetylcholine is the primary transmitter of vagal and pelvic preganglionic neurones, of enteric interneurones, of one class of secretomotor neurone in the intestine and of motor neurones controlling gastric acid secretion. Acetylcholine and tachykinins are co-primary transmitters of muscle motor neurones, with acetylcholine appearing to have the greater role. Tachykinins are probably primary transmitters of enteric sensory neurones at neuroneuronal synapses. Serotonin may also be a transmitter to neurones in the enteric ganglia. Nitric oxide appears to be the usual primary transmitter of enteric inhibitory motor neurones to the muscle. ATP and vasoactive intestinal peptide are subsidiary transmitters of these neurones, although in some regions they may have a primary transmitter role. Vasoactive intestinal peptide is the primary transmitter of non-cholinergic secretomotor neurones. Gastrin releasing peptide is the primary transmitter of motor neurones to gastrin cells. Noradrenaline is the primary transmitter of sympathetic neurones that supply the intestine.

P2-purinoceptor antagonists discriminate three contraction-mediating receptors for ATP in rat vas deferens

The sites of action at which ATP elicits contraction of the rat vas deferens were studied by means of the P2-purinoceptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (iso-PPADS), suramin and reactive blue 2. Increasing concentrations of iso-PPADS (up to 1 mM), suramin (up to 1 mM) and reactive blue 2 (up to 320 microM) reduced and eventually abolished contractions elicited by the P2x-purinoceptor-selective agonist alpha,beta-methylene ATP 3 microM with IC50 values of 2.1, 10.1 and 27.0 microM, respectively. In contrast, iso-PPADS and suramin caused only a partial inhibition of contractions elicited by ATP 1 mM, maximal reduction by about 40%, IC50 values 1.3 and 5.0 microM, respectively; reactive blue 2 did not change ATP-induced contractions. In tissues exposed to iso-PPADS 320 microM throughout, increasing concentrations of reactive blue 2 or suramin decreased contractions elicited by ATP 1 mM, IC50 values 2.6 and 14.5 microM, respectively. In tissues exposed to suramin 320 microM throughout, increasing concentrations of iso-PPADS decreased contractions elicited by ATP 1 mM, IC50 37.9 microM, whereas reactive blue 2 slightly enhanced these contractions. In tissues exposed to reactive blue 2 100 microM throughout, increasing concentrations of iso-PPADS reduced contractions elicited by ATP 1 mM, IC50 26.6 microM, whereas suramin caused no change. Pre-exposure to alpha,beta-methylene ATP 1 microM to desensitize P2x-purinoceptors reduced the response to ATP 1 mM by 91% in otherwise untreated tissues, but did not reduce the response to ATP 1 mM in tissues exposed throughout to iso-PPADS 320 microM, suramin 320 microM or reactive blue 2 100 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Purinoceptors: are there families of P2X and P2Y purinoceptors?

There has been an exponential growth in interest in purinoceptors since the potent effects of purines were first reported in 1929 and purinoceptors defined in 1978. A distinction between P1 (adenosine) and P2 (ATP/ADP) purinoceptors was recognized at that time and later, A1 and A2, as well as P2x and P2y subclasses of P1 and P2 purinoceptors were also defined. However, in recent years, many new subclasses have been claimed, particularly for the receptors to nucleotides, including P2t, P2z, P2u(n) and P2D, and there is some confusion now about how to incorporate additional discoveries concerning the responses of different tissues to purines. The studies beginning to appear defining the molecular structure of P2-purinoceptor subtypes are clearly going to be important in resolving this problem, as well as the introduction of new compounds that can discriminate pharmacologically between subtypes. Thus, in this review, on the basis of this new data and after a detailed analysis of the literature, we propose that: (1) P2X(ligand-gated) and P2Y(G-protein-coupled) purinoceptor families are established; (2) four subclasses of P2X-purinoceptor can be identified (P2X1-P2X4) to date; (3) the variously named P2-purinoceptors that are G-protein-coupled should be incorporated into numbered subclasses of the P2Y family. Thus: P2Y1 represents the recently cloned P2Y receptor (clone 803) from chick brain; P2Y2 represents the recently cloned P2u (or P2n) receptor from neuroblastoma, human epithelial and rat heart cells; P2Y3 represents the recently cloned P2Y receptor (clone 103) from chick brain that resembles the former P2t receptor; P2Y4-P2Y6 represent subclasses based on agonist potencies of newly synthesised analogues; P2Y7 represents the former P2D receptor for dinucleotides. This new framework for P2 purinoceptors would be fully consistent with what is emerging for the receptors to other major transmitters, such as acetylcholine, gamma-aminobutyric acid, glutamate and serotonin, where two main receptor families have been recognised, one mediating fast receptor responses directly linked to an ion channel, the other mediating slower responses through G-proteins. We fully expect discussion on the numbering of the different receptor subtypes within the P2X and P2Y families, but believe that this new way of defining receptors for nucleotides, based on agonist potency order, transduction mechanisms and molecular structure, will give a more ordered and logical approach to accommodating new findings. Moreover, based on the extensive literature analysis that led to this proposal, we suggest that the development of selective antagonists for the different P2-purinoceptor subtypes is now highly desirable, particularly for therapeutic purposes.

Pharmacological actions of the coenzymes NAD(H) and NADP(H) on the rat anococcygeus muscle

1. The pharmacological actions of the oxidized and reduced forms of nicotinamide-adenosine dinucleotide (NAD, NADH) and nicotinamide-adenosine dinucleotide phosphate (NADP, NADPH) were studied on rat isolated anococcygeus muscles. 2. The actions of the two nucleotides were different, but there were no apparent qualitative differences between the oxidized and reduced forms of each. 3. In fully relaxed anococcygeus muscles, NADP(H) produced transient contractions that were subject to desensitization, but NAD(H) had no effect. 4. NADP(H) slightly enhanced contractions elicited by noradrenergic nerve stimulation. In contrast, noradrenergic contractions were inhibited by NAD(H). NADH reduced the stimulation-induced release of noradrenaline, but enhanced contractions elicited by exogenous noradrenaline. 5. In anococcygeus muscles partly contracted with guanethidine, NAD(H) produced a further sustained increase in tone; in contrast, NADP(H) mainly produced transient relaxations to which there was immediate desensitization. 6. Relaxations of anococcygeus muscle elicited by nitrergic nerve stimulation were not affected by NAD. In contrast, NADP(H) reduced them. 7. The actions of NAD(H) were generally the same as those of adenosine and can be attributed to activation of P1-purinoceptors since they were blocked by the selective antagonist 8-sulphophenyltheophylline. 8. The actions of NADP resembled those of the P2-purinoceptor agonist ATP to some extent, but there were some differences. As suggested by others, NADP may act on a unique receptor.

New transmitters and new targets in the autonomic nervous system

Several recent findings have made research into the autonomic nervous system even more exciting, such as the revelation that nitric oxide is a major neurotransmitter, the delineation of the physiological roles for purines and vasoactive intestinal peptide, and the discovery that the interstitial cells of Cajal are major target cells for enteric innervation. Nitric oxide is probably the major neurotransmitter evoking inhibitory junction potentials in smooth muscle. ATP is a mediator of non-adrenergic non-cholinergic enteric innervation, as well as being a fast neurotransmitter in peripheral and autonomic neuro-neuronal synapses. The interactions between enteric nerves and both immune cells and interstitial cells of Cajal (as pacemaker cells of gut smooth muscle) are forcing a rethink of many aspects of gut physiology.

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.

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

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

Intercellular calcium signaling induced by extracellular adenosine 5′-triphosphate and mechanical stimulation in airway epithelial cells

Airway epithelial cells in culture respond to extracellular adenosine 5′-triphosphate (ATP) by increasing their intracellular Ca2+ concentration ([Ca2+]i). The effective concentration of ATP that elicited a Ca2+ response equal to 50% of the maximal response (EC50) was 0.5 microM. Release of ATP from a pipette to form a local gradient of ATP increased [Ca2+]i of individual cells in a sequential manner. Cells closest to the pipette showed an immediate increase in [Ca2+]i while more distal cells displayed a delayed increase in [Ca2+]i. This response to the local release of ATP appeared as a wave of increasing [Ca2+]i that spread to several cells and, in this respect, was similar to the intercellularly communicated Ca2+ waves initiated by mechanical stimulation in airway epithelial cells (Sanderson et al., Cell Regul. 1, 585–596, 1990). In the presence of a unidirectional fluid flow, the Ca2+ response to a local release of ATP was biased such that virtually all the cells responding with an increase in [Ca2+]i were downstream of the release site. By contrast, an identical fluid flow did not bias the radial propagation of intercellular Ca2+ waves induced by mechanical stimulation. Suramin, a P2-purinergic receptor antagonist, did attenuate the Ca2+ response induced by ATP but did not block the propagation of mechanically induced Ca2+ waves. Cells from young cultures (3-5 days) or those at the leading edge of an outgrowth elevated their [Ca2+]i in response to ATP. However, these cells do not respond to mechanical stimulation by the propagation of a Ca2+ wave. From these results we conclude that the intercellular Ca2+ waves elicited by mechanical stimulation are not the result of ATP or another compound released from the stimulated cell, diffusing through the extracellular fluid. This conclusion is consistent with previous experimental evidence suggesting that intercellular Ca2+ signaling in epithelial cells is mediated by the movement of inositol trisphosphate through gap junctions.

PPADS selectively antagonizes P2X-purinoceptor-mediated responses in the rabbit urinary bladder

1. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), an inhibitor of P2X-purinoceptor-mediated responses in rabbit vas deferens, was investigated for its ability to antagonize contractions evoked by alpha,beta-methylene ATP (alpha,beta-MeATP), carbachol and electrical field stimulation in the rabbit urinary bladder detrusor muscle. 2. PPADS. (1-30 microM) caused concentration-dependent inhibition of contractions to the stable P2X-purinoceptor agonist, alpha,beta-MeATP, decreasing the maximum response to alpha,beta-MeATP (30 microM) at concentrations of 3-30 microM. The pD2 value for alpha,beta-MeATP in the absence of PPADS was 6.52 +/- 0.10 (8). In the presence of PPADS at concentrations of 1, 3, 10 and 30 microM the negative log concentrations of alpha,beta-MeATP that cause the same contractile response as the pD2 value were significantly different from control, being respectively 6.17 +/- 0.09 (8), 5.64 +/- 0.12 (7), 5.15 +/- 0.23 (7) and 4.78 +/- 0.22 (5). 3. PPADS (1-30 microM) caused concentration-dependent inhibition of contractions to stimulation of intramural purinergic nerves (1-32 Hz). There was a greater inhibition at lower frequencies (1-8 Hz) than at higher frequencies (16-32 Hz). PPADS, 30 microM, did not produce significantly greater antagonism than 10 microM. 4. PPADS (30 microM) had no significant influence on the contractile potency of carbachol: the pD2 values of carbachol in the absence and presence of PPADS were not significantly different being 6.42 +/- 0.16 (5) and 6.33 +/- 0.18 (5), respectively. However, PPADS caused a small, but significant, suppression of the maximal response of carbachol, reducing it by approximately 9%. 5. Radioligand binding studies carried out on rabbit bladder membranes with [3H]-alpha,beta-methylene ATP([3H]-alpha,beta-MeATP) showed that PPADS concentration-dependently inhibited the binding of [3H]-alpha,beta-MeATP to P2X-purinoceptors, while the binding of [3H]-quinuclidinyl benzilate to muscarinic cholinoceptors was not affected.6. Thus, PPADS (1-30 microM) antagonized responses mediated via P2X-purinoceptors in the rabbit urinary bladder. It was selective for P2-purinoceptor-mediated contractions rather than those mediated via muscarinic receptors. Binding studies demonstrated that the antagonistic effect of PPADS is via a direct interaction with P2x-purinoceptors.

Suramin increases the efficacy of ATP to activate an inward current in myenteric neurons from guinea-pig ileum

Previous reports have indicated that suramin antagonizes, in a competitive manner, the actions of adenosine 5'-triphosphate (ATP) mediated by P2 purinoceptors. In neurons from the myenteric plexus, however, suramin has different effects on an ATP-activated conductance. During whole-cell recordings, ATP (3-1000 microM) evoked a rapidly desensitizing inward current in a concentration-dependent manner. The amplitude of the ATP-activated current was increased, in a reversible manner, when suramin was applied previous to or during ATP stimulation. This effect was increased by raising the suramin concentration. Higher concentrations of suramin were required for higher concentrations of ATP; the EC50 values of suramin were 5 +/- 1.1 microM and 143 +/- 48 nM when ATP concentrations of 30 and 10 microM were used. No change in the holding current was observed with suramin alone. These observations suggest that a different subtype of P2 purinoceptors could be expressed in the myenteric neurons.

Identification of potent, selective P2Y-purinoceptor agonists: structure-activity relationships for 2-thioether derivatives of adenosine 5'-triphosphate

Study of P2-purinoceptor subtypes has been difficult due to the lack of potent and selective ligands. With the goal of developing high affinity P2-purinoceptor-selective agonist, we have synthesized a series of analogues of adenine nucleotides modified on the purine ring as chain-extended 2-thioethers or as N6-methyl-substituted compounds. Chemical functionality incorporated in the thioether moiety included cyanoalkyl, nitroaromatic, amino, thiol, cycloalkyl, n-alkyl, and olefinic groups. Apparent affinity of the compounds for P2Y-purinoceptors was established by measurement of P2Y-purinoceptor-promoted phospholipase C activity in turkey erythrocyte membranes and relaxation of carbachol-contracted smooth muscle in three different preparations (guinea pig taenia coli, rabbit aorta, and rabbit mesenteric artery). Activity at P2X-purinoceptors was established by measurement of contraction of rabbit saphenous artery and of the guinea pig vas deferens and urinary bladder. All 11 of the 2-thioethers of ATP stimulated the production of inositol phosphates with K0.5 values of 1.5-770 nM, with an (aminophenyl)ethyl derivative being most potent. Two adenosine diphosphate analogues were equipotent to the corresponding ATP analogues. Adenosine monophosphate analogues were full agonists, although generally 4 orders of magnitude less potent. ATP 2-thioethers displayed pD2 values in the range of 6-8 in smooth muscle assay systems for activity at P2Y-receptors. There was a significant correlation for the 2-thioether compounds between the pK0.5 values for inositol phosphate production and the pD2 values for relaxation mediated via the P2Y-purinoceptors in the guinea pig taenia coli, but not for the vascular P2Y-receptors or for the P2X-receptors. At P2X-receptors, no activity was observed in the rabbit saphenous artery, but variable degrees of activity were observed in the guinea pig vas deferens and bladder depending on distal substituents of the thioether moiety. N6-Methyl-ATP was inactive at P2X-receptors, and approximately equipotent to ATP at taenia coil P2Y-receptors. This suggested that hybrid N6-methyl and 2-thioether ATP derivatives might be potent and selective for certain P2Y-receptors, as was shown for one such derivative, N6-methyl-2-(5-hexenylthio)-ATP.

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.

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

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

Characterization of purinoceptors mediating depolarization of rat isolated vagus nerve

1. As part of a broader study to characterize neuronal purinoceptors, the effects of adenosine 5'-triphosphate (ATP) and a range of ATP analogues were investigated on the extracellularly recorded membrane potential of the rat isolated vagus nerve, using a 'grease-gap' technique. 2. ATP evoked depolarization of the rat vagus nerve. The concentration-effect curve to ATP was not monophasic: at the lower concentrations (1 x 10(-5)-1 x 10(-3) M) the curve was shallow (< 50% of the near maximal response to 5-hydroxytryptamine (5-HT)) whilst at higher concentrations the relationship between concentration and amplitude of depolarization was steeper (> 135% of the response to 5-HT at the highest concentration tested, 1 x 10(-2) M). On washout of the high drug concentrations large after-hyperpolarizations were often observed. 3. alpha,beta-methylene ATP (1 x 10(-6)-3 x 10(-4) M), beta,gamma-methylene ATP (1 x 10(-6)-1 x 10(-3) M), and 5'-adenylylimidodiphosphate (beta,gamma-imido ATP; 1 x 10(-6)-1 x 10(-3) M) were all more potent than ATP and produced large depolarizations of the rat vagus nerve at the highest concentrations tested (> 150% of the response to 5-HT). The overall rank order of potency was alpha,beta-methylene ATP > beta,gamma-methylene ATP = beta,gamma-imido ATP > ATP. 4. In contrast, 2-methylthio ATP (1 x 10(-6)-1 x 10(-3) M) produced relatively small depolarizations (< 100% of the response to 5-HT). As was the case with low concentrations of ATP, the concentration-effect curve to 2-methylthio ATP was very shallow. 5. Adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP), adenosine and adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-s; all 1 x 10-6 1 x 10-3M) evoked only small depolarizations of the vagus nerve, amounting to 47 +/- 2.5%, 40.8 +/- 7.8%, 33.7 +/- 3.3% and 62.4 +/- 12.7% of the response to 5-HT, respectively. Uridine 5'-triphosphate (UTP; 1 X 10-6 1 X 10-3M) was inactive.6. The P2 purinoceptor antagonist, suramin (1 x 10-5-M-1 X 10-4 M), antagonized responses to alpha-beta-methylene ATP. The nature of this antagonism was not, however, consistent with simple competitive kinetics between agonist and antagonist. Depolarizations produced by beta,gamma-methylene ATP and beta,gamma-imido ATP were also attenuated by suramin (1 x 10-4 M), but in contrast, suramin had no effect on responses to ADP, 2-methylthio ATP, ADP-beta-S or 5-HT.7. In addition to its antagonist effects, suramin (10-4 M) markedly increased the maximum amplitude of the depolarization produced by ATP.8. It is concluded that a heterogeneous receptor population mediates depolarization of the rat vagus nerve by purine nucleotides. Importantly, the large amplitude depolarizations to alpha,beta-methylene ATP,beta,gamma-methylene ATP and beta,gamma-imido ATP are mediated via receptors that share many characteristics of the classical P2, receptor. In contrast, the relatively small depolarizing effects of ADP, ADP-beta-S and 2-methylthio ATP were suramin-resistant. Although it appears that other purinoceptors are present,these data suggest that the rat vagus nerve may serve as a useful preparation for studying the pharmacology of neuronal P2x receptors.

Characterization of the adenosine receptor mediating contraction in rat colonic muscularis mucosae

1. The objective of this study was to characterize the adenosine receptor mediating contraction in rat isolated colonic muscularis mucosae (RCMM). 2. Sequential additions of the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA; 0.01-10 microM) elicited reproducible, concentration-related contractions in RCMM. The effects of NECA were mimicked by the adenosine A1 receptor-selective agonists cyclopentyladenosine (CPA), R-phenylisopropyladenosine (R-PIA) and N-[1S, trans)2-hydroxycyclopentyl] adenosine (GR79236) and by S-PIA (the stereoisomer of R-PIA). The adenosine A2 agonists N-[(2-methylphenyl)methyl] adenosine (metrifudil) and 2-[p-(2-carboxyethyl)phenethylamine]-5'-N-ethylcarboxamidoadenosine (CGS21680) also produced contractions in RCMM but were 54 and 165 times less potent respectively than NECA. The rank order of agonist potency for contraction of RCMM was CPA > or = GR79236 = R-PIA > or = NECA > > S-PIA = metrifudil > CGS21680, which is identical to that reported for the inhibition of spontaneous rate in rat isolated right atria and inhibition of lipolysis in rat isolated adipocytes by these same agonists. 3. R-PIA, S-PIA and metrifudil behaved as partial agonists in RCMM. 4. The adenosine A1 receptor-selective antagonist 8-cyclopentyl-1,3- dipropylxanthine (DPCPX) inhibited the contractions produced by all the adenosine agonists tested, with pKB values between 9.2 and 9.5. The non-selective adenosine antagonist 8-phenyltheophylline (8-PT) antagonized the effects of NECA but also markedly potentiated (by 93.0 +/- 10.2% at 3 microM) the maximum contractile response to NECA in RCMM. Neither 8-PT (3 microM) nor DPCPX (0.1 microM) had any effect on the contractions produced by carbachol. 5. The contractile responses to NECA in RCMM were not affected by atropine (1 microM), tetrodotoxin(0.3 microM) or the P2 antagonist, suramin (100 microM).6. The present study confirms that contractions to adenosine agonists in the RCMM are mediated via adenosine Al receptors.

Stimulation of lumbar sympathetic nerves evokes contractions of cat colon circular muscle mediated by ATP and noradrenaline

1. The action of the lumbar sympathetic nerves to cat colon was studied in vitro using isolated muscle strips with attached lumbar colonic nerves (LCN) orientated in the axis of circular muscle layer. Electrical stimulation of LCN caused frequency-dependent increases in resting tension and in amplitude of spontaneous contractions. Contractile responses were abolished by tetrodotoxin (3 microM) and by guanethidine (30 microM), indicating that they were neurogenic, involving the release of neurotransmitter from sympathetic fibres. 2. Propranolol (1-9 microM), a beta-adrenoceptor antagonist, caused a concentration-dependent potentiation of LCN-evoked contractile responses. Propranolol (3 microM) potentiated contractile responses to exogenously applied noradrenaline but not to phenylephrine. 3. Phentolamine (1-9 microM), an alpha-adrenoceptor antagonist, and prazosin (1-9 microM), an alpha 1-adrenoceptor antagonist, caused a concentration-dependent reduction of amplitude but did not abolish LCN-evoked contractile responses. Prazosin (3 microM) or phentolamine (3 microM) antagonized contractile responses to noradrenaline and phenylephrine. 4. Desensitization of purinoceptors with the P2x-receptor agonist, alpha,beta-methylene ATP, caused a decrease in amplitude of LCN-evoked contractile responses and abolished contractile responses to ATP. In muscle strips where alpha 1-adrenoceptors were blocked with prazosin (3 microM) and P2-purinoceptors were desensitized with alpha,beta-methylene ATP, the amplitude of contractile responses was reduced by 82-100%. 5. The P2x-purinoceptor antagonists, arylazido amino propyl adenosine triphosphate (ANAPP3) and 5. The P2x-purinoceptor antagonists, arylazido amino propyl adenosine triphosphate (ANAPP3) and suramin, affected LCN-evoked contractile responses. ANAPP3 (50-100 microM) caused a concentration-dependent reduction in the amplitude of contractile response. Suramin (100 microM) caused a small reduction in amplitude of contractile responses but potentiated their amplitude at a concentration of 500 microM. 6. ANAPP3 (100 microM) irreversibly inhibited contractions to alpha,beta-methylene ATP or ATP. Suramin (100-500 microM) inhibited contractions to alpha,beta-methylene ATP (0.5-1 microM) or low concentrations of ATP (10-50 microM) but potentiated contractions at higher concentrations. ANAPP3 (100 microM) and suramin (100, 500 microM) had no effect on contractile responses to noradrenaline. 7. Clonidine (0.05-1 microM), a selective alpha 2-adrenoceptor agonist, caused a concentration-dependent reduction in amplitude of LCN-evoked contractile responses, at 10 Hz, while yohimbine (0.1-1 microM), a selective alpha 2-adrenoceptor antagonist, increased them. At 1 microM, both compounds affected LCN-evoked contractions at all frequencies. This suggests that prejunctional alpha 2-receptors are involved in autoinhibition at sympathetic terminals. 8. In summary, LCN-evoked contractile responses involve the corelease of noradrenaline and ATP or a related purine nucleotide from sympathetic fibres. It is likely that the neurogenic responses are mediated through excitatory postjunctional alpha 1-adrenoceptors, excitatory suramin-sensitive and suramin-insensitiveP2X-purinoceptors and inhibitory beta-adrenoceptors. Also, autoinhibitory prejunctional alpha2-adrenoceptors regulate the LCN excitatory pathway to cat colon circular muscle.

ATP and endogenous agonists inhibit evoked [3H]-noradrenaline release in rat iris via A1 and P2y-like purinoceptors

Effects of ATP, adenosine and purinoceptor antagonists on field stimulation-evoked (3 Hz, 2 min) [3H]-noradrenaline overflow were investigated in the rat isolated iris. ATP and adenosine inhibited the evoked overflow of [3H]-noradrenaline. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX) shifted the concentration-response curve of ATP to the right in a concentration-dependent manner, but with a potency (-log KB = 7.88) much lower than expected for an A1 adenosine receptor. In the continuous presence of DPCPX, the ATP-induced prejunctional inhibition was unaffected by suramin (100 mumol/l) and DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 50 mumol/l) but was antagonized by the P2Y-receptor antagonist cibacron blue (= reactive blue 2; 30 and 100 mumol/l, -log KB = 4.7) and alpha,beta-methylene-ATP (10 mumol/l). Whereas the evoked [3H]-noradrenaline overflow was unaffected by suramin and DIDS, cibacron blue and alpha,beta-methylene-ATP caused a small and transient increase. Cibacron blue at 30 mumol/l failed to antagonize the inhibition of evoked [3H]-noradrenaline overflow that adenosine produced in the absence of DPCPX. Basal [3H]-noradrenaline overflow was enhanced by cibacron blue, not changed by alpha,beta-methylene-ATP and DIDS, and decreased by suramin. The results show that exogenous ATP inhibits sympathetic neurotransmission in the rat iris via A1 and P2Y-like purinoceptors. The latter have a low apparent affinity for cibacron blue and probably are blocked by alpha,beta-methylene-ATP. Under the present conditions, endogenous purines exert a tonic inhibition not only via A1- but also via these P2Y-receptors.

Desensitization of the P2-purinoceptors on the rat colon muscularis mucosae

1. Adenosine 5'-triphosphate (ATP) and adenosine have been shown to contract the rat colon muscularis mucosae, and the receptors at which they act have been classified as P2Y and A1 respectively. Uridine 5'-triphosphate (UTP) also contracts this tissue, and desensitization was used to investigate the receptors by which it acts, in the light of recent suggestions that specific pyrimidinoceptors may exist for UTP, or that nucleotide receptors may exist which are responsive to both ATP and UTP but not to some ATP analogues such as 2-methylthioadenosine 5'-triphosphate (2-MeSATP). 2. ATP, UTP and adenosine each contracted the rat colon muscularis mucosae in a concentration-dependent manner over the concentration range 0.3-300 microM, although maximal responses to ATP and UTP were not obtained. ATP was approximately 4 times as potent as UTP and approximately equipotent with adenosine although the maximal response to adenosine appeared to be less than that to ATP or UTP. 3. Desensitization of the tissue with ATP (200 microM) given immediately before each concentration of the agonists reduced subsequent contractions induced by ATP itself and also by UTP, but did not reduce contractions induced by adenosine. Desensitization of the tissues with UTP (200 microM) also reduced contractions induced by ATP and UTP but not by adenosine, whereas desensitization with adenosine (200 microM) reduced contractions induced by adenosine itself but not by ATP or UTP. 4. Desensitization of the tissue with 2-MeSATP (200 microM), which is a more potent agonist than ATP at P2Y-purinoceptors, greatly reduced the responses to ATP and to UTP, but had no effect on responses induced by adenosine. Attempts to desensitize the tissue with adenosine 5'-(alpha,beta-methylene)triphosphonate(AMPCPP), which is a more potent agonist than ATP at P2X-purinoceptors but is less potent atP2y-purinoceptors, were unsuccessful.5. These results show that cross desensitization to ATP and UTP occurred and was specific for these agonists rather than being due to a general decrease in the ability of the muscle to contract. This implies that ATP and UTP act at the same receptor, which does not support the existence of specificpyrimidinoceptors but which could be taken as evidence for the existence of a nucleotide receptor on this tissue. However, the ability of 2-MeSATP, which is inactive at the proposed nucleotide receptors,also selectively to desensitize this receptor indicates instead that ATP and UTP are both acting at a purinoceptor of the P2Y type in this tissue.

Axon terminal P2-purinoceptors in feedback control of sympathetic transmitter release

Extracellular ATP acts on P2-purinoceptors of peripheral effector cells, and this is the basis for its function as a (co-)transmitter in peripheral efferent neurons. ATP also acts on P2-receptors of neuronal cell bodies or dendrites, and this is the basis for its function as a fast excitatory transmitter at neuroneural synapses. A third site of action is axon terminals. In the vas deferens of the mouse, noradrenaline and ATP are postganglionic sympathetic co-transmitters, and exogenous ATP acts on P2-purinoceptors of the sympathetic terminals to inhibit release of noradrenaline. Here we show that two P2 antagonists, suramin and Reactive Blue 2, increase the release of noradrenaline in mouse vas deferens. The increase is only obtained when there has been preceding nerve activity and is largely independent of the postjunctional response. These findings indicate a physiological function for axon terminal P2-purinoceptors: they mediate a novel prejunctional negative feedback in which released ATP inhibits subsequent transmitter release.

Evidence for nitric oxide as an inhibitory neurotransmitter in rabbit isolated anococcygeus

The role of nitric oxide (NO) and ATP as putative inhibitory non-adrenergic, non-cholinergic (NANC) neurotransmitters was investigated in rabbit isolated anococcygeus after block of adrenergic and cholinergic responses, and raising tone with histamine. NANC nerve stimulation produced rapid relaxations which were completely abolished by tetrodotoxin. The magnitude of the NANC inhibitory responses was significantly reduced by the nitric oxide synthase inhibitors NG-nitro-L-arginine (NO-Arg) and NG-nitro-L-arginine methyl ester (L-NAME). This effect could be partially reversed by L-arginine but not by D-arginine. Oxyhaemoglobin inhibited NANC nerve responses and sodium nitroprusside mimicked the effects of NANC nerve stimulation. NO-Arg also reduced the magnitude of the inhibitory junction potentials recorded from the smooth muscle cells during NANC nerve stimulation. Exogenously applied ATP and adenosine each produced concentration dependent relaxations which were unaffected by the NO-synthase inhibitor NO-Arg. Relaxations to adenosine were virtually abolished by the P1 purinoceptor antagonist 8-(p-sulphophenyl)theophylline. Relaxations to ATP were also significantly reduced, indicating that part of the response to exogenous ATP is due to its breakdown to adenosine and subsequent action on P1 purinoceptors. Relaxations of the tissue to ATP and adenosine were unaffected by the P2 purinoceptor antagonist suramin. NANC nerve mediated responses were not significantly changed by either 8-(p-sulphophenyl)theophylline or suramin. These results suggest that NO is involved in inhibitory NANC neurotransmission in the rabbit isolated anococcygeus, but do not support a role for ATP as a NANC neurotransmitter in this tissue.

Blockade by glibenclamide of the flow-evoked endothelial release of ATP that contributes to vasodilatation in the pulmonary vascular bed of the rat

1. The effect of step augmentation of flow rate on the level of adenosine -5'-triphosphate (ATP) measured in the Krebs perfusate was investigated, and the effect of glibenclamide on the release of ATP was tested in the rat pulmonary vascular bed. 2. For flow rates between 10.38 +/- 1.18 and 28.88 +/- 2.08 ml min-1 (n = 8) 1 microM suramin, a P2-purinoceptor antagonist, significantly (P < 0.05) increased vascular resistance under conditions of step augmentation of flow rate. This suggests that endogenous ATP released during increases in flow rate dilates pulmonary vessels. 3. In response to a step augmentation in flow rate from 9.13 +/- 0.97 to 18.3 +/- 1.69 ml min-1 (n = 4) ATP levels were up to 23 fold higher (P < 0.05) for 15 s, and gradually dropped to a level of about half the initial rise. Once the ATP levels had stabilized, another step augmentation of flow rate to 27.00 +/- 3.49 ml min-1 was able to evoke a corresponding increase of ATP release. The ability of the vascular bed to respond with increased ATP release after the initial ATP responses had tapered, demonstrates that the drop in ATP levels after the initial rise is not due to depletion of ATP. Furthermore, the maximal ATP response directly precedes the vasodilatation observed following each jump in perfusion pressure produced with each step increase in flow rate. 4. In response to two 3 fold step augmentations of flow rate (8.41-27.29 ml min-1) spaced 30 min apart there were two increases in the level of ATP which were not significantly different from each other.However, perfusion with 1 microM glibenclamide between the first and the second step augmentation of flow rate (8.08-24.67 ml min-1) significantly (P<0.05; n = 6) blocked the increase in ATP release. This suggests that the release of intracellular ATP is mediated by glibenclamide-sensitive K+ channels.5. A concentration of 1 microM glibenclamide perfused for 30 min was without effect on vascular pressure at constant flow. However, under conditions where flow was augmented in a stepwise manner (between 11.50 and 36.45 ml min-1) perfusing with 1 microM glibenclamide increased vascular resistance (P <0.10).6. It is concluded that flow-induced ATP release is mediated by a glibenclamide-sensitive K+ channel,and that the release of ATP from endothelial cells probably functions to vasodilate the pulmonary vascular bed of the rat.

Contractile response to neuropeptide Y of rat isolated duodenum

The response of isolated duodenum to neuropeptide Y (NPY) was studied isotonically in neonatal and adult rats. Neuropeptide Y (10(-8) to 10(-6) M) elicited a biphasic contraction of isolated duodenum from neonatal rats, but monophasic and weak contraction of adult duodenum. The first phase of NPY-induced contraction of neonatal duodenum was concentration dependent and partially inhibited by preincubation with tetrodotoxin, a Na+ channel blocker, hyoscine, a muscarinic antagonist, suramin, a P2 purinoceptor antagonist, and indomethacin, an inhibitor for prostaglandin biosynthesis. Neuropeptide Y(13-36), a specific Y2 NPY receptor agonist, elicited a concentration-dependent contraction of neonatal rat duodenum. The duodenal response to NPY thus changes during development in rats. Both cholinergic and purinergic transmission and prostaglandin biosynthesis may be involved in the NPY-induced contraction of neonatal duodenum. Neuropeptide Y-induced contraction may be mediated through presynaptic Y2 receptors.

Nucleotides mobilize intracellular calcium stores of renal proximal cells in primary culture: existence of a suramin-sensitive mechanism

Changes of intracellular calcium concentrations [Ca2+]i were measured in primary cultured rabbit proximal convoluted tubules (PCT). A dual-excitation, digital-imaging inverted microscope was used to monitor the fura-2 fluorescence. The basal calcium level was 106 +/- 11 nM (n = 36). The stimulatory effects of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine were studied. ATP and ADP induced transient increases of [Ca2+]i (1059 +/- 115% of the resting level (n = 29), and 659 +/- 134% (n = 10), respectively) by releasing calcium from cytoplasmic stores. Adenosine had less effect (279 +/- 48% of the resting level, n = 3). In the same conditions the ATP antagonist suramin (100 microM) inhibited the action of ATP and ADP to 231 +/- 52% (n = 3), and 308 +/- 29% (n = 4) of the resting level, respectively, but did not modify that of adenosine (281 +/- 72%, n = 3). A pretreatment (500 ng/ml for 2 h at 37 degrees C) of the culture with the toxin of Bordetella pertussis completely blocked the ATP response. Our results are evidence for the presence of a functional suramin-sensitive ATP and ADP puriceptor in cultured renal proximal cells. A pertussis-toxin-sensitive G protein is linked to the transduction mechanism. This receptor is distinct from an adenosine puriceptor also found in the proximal monolayer.

The regulation of aortic endothelial cells by purines and pyrimidines involves co-existing P2y-purinoceptors and nucleotide receptors linked to phospholipase C

1. We have examined the phospholipase C responses in bovine aortic endothelial cells to purines (ATP, ADP and analogues) and the pyrimidine, uridine triphosphate (UTP). 2. The cells responded to purines in a manner consistent with the presence of P2y purinoceptors; both 2-methylthioadenosine 5'-triphosphate (2MeSATP) and adenosine 5'-0-(2-thiodiphosphate) (ADP beta S) were potent agonists (EC50 0.41 microM and 0.85 microM respectively) while beta, gamma-methylene ATP at 300 microM was not. 3. The cells also responded to UTP. The maximal response to UTP was less than that for either 2MeSATP and ADP beta S while adenosine 5'-0-(3-thiotriphosphate) (ATP gamma S) gave the largest maximal response. 4. The concentration-effect curve to UTP was additive in the presence of either 2MeSATP or ADP beta S. However, the concentration-effect curves to ATP gamma S reached the same maximum in the presence or absence of UTP. 5. Suramin, at concentrations between 10 microM and 100 microM was a competitive antagonist for the response to ADP beta S and 2MeSATP but not the response to UTP. 6. The results show that there are two separate, co-existing, receptor populations: P2y-purinoceptors (responding to purines) and nucleotide receptors (responding to both purines and pyrimidines). We conclude that purines such as ATP/ADP may regulate aortic endothelial cells by interacting with two phospholipase C-linked receptors.