P2-purinoceptor-mediated inhibition of noradrenaline release in rat atria

Endothelial and Neuronal Nitric Oxide Activate Distinct Pathways on Sympathetic Neurotransmission in Rat Tail and Mesenteric Arteries

Nitric oxide (NO) seems to contribute to vascular homeostasis regulating neurotransmission. This work aimed at assessing the influence of NO from different sources and respective intracellular pathways on sympathetic neurotransmission, in two vascular beds. Electrically-evoked [³H]-noradrenaline release was assessed in rat mesenteric and tail arteries in the presence of NO donors or endothelial/neuronal nitric oxide synthase (NOS) inhibitors. The influence of NO on adenosine-mediated effects was also studied using selective antagonists for adenosine receptors subtypes. Location of neuronal NOS (nNOS) was investigated by immunohistochemistry (with specific antibodies for nNOS and for Schwann cells) and Confocal Microscopy. Results indicated that: 1) in mesenteric arteries, noradrenaline release was reduced by NO donors and it was increased by nNOS inhibitors; the effect of NO donors was only abolished by the adenosine A₁ receptors antagonist; 2) in tail arteries, noradrenaline release was increased by NO donors and it was reduced by eNOS inhibitors; adenosine receptors antagonists were devoid of effect; 3) confocal microscopy showed nNOS staining in adventitial cells, some co-localized with Schwann cells. nNOS staining and its co-localization with Schwann cells were significantly lower in tail compared to mesenteric arteries. In conclusion, in mesenteric arteries, nNOS, mainly located in Schwann cells, seems to be the main source of NO influencing perivascular sympathetic neurotransmission with an inhibitory effect, mediated by adenosine A₁ receptors activation. Instead, in tail arteries endothelial NO seems to play a more relevant role and has a facilitatory effect, independent of adenosine receptors activation.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

Presynaptic adenosine and P2Y receptors

Adenine-based purines, such as adenosine and ATP, are ubiquitous molecules that, in addition to their roles in metabolism, act as modulators of neurotransmitter release through activation of presynaptic P1 purinoceptors or adenosine receptors (activated by adenosine) and P2 receptors (activated by nucleotides). Of the latter, the P2Y receptors are G protein-coupled, whereas the P2X receptors are ligand-gated ion channels and not covered in this review.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

ATP release and its prejunctional modulation

We studied some properties of the release of noradrenaline and ATP in isolated sympathetically innervated tissues. Release was elicited by electric stimulation and assessed as overflow of tritiated compounds (after labelling with [3H]noradrenaline) and enzymically measured ATP, respectively. Evans blue, which inhibits ectonucleotidases, greatly increased the evoked overflow of ATP, indicating that a major part of the ATP was metabolized after release. Much of the ATP was postjunctional in origin. The neural fraction was isolated when postjunctional release was suppressed by prazosin (alpha 1-adrenoceptor antagonist) and suramin (P2 purinoceptor antagonist). Comparison of neural ATP and [3H]-noradrenaline release showed that prostaglandin E2 reduced the release of both co-transmitters to a similar extent. Activation of prejunctional alpha 2-adrenoceptors, however, preferentially reduced the release of [3H]noradrenaline, and activation of prejunctional A1 purinoceptors reduced preferentially the release of ATP. Nucleotides such as ATP depressed the release of [3H]noradrenaline through two receptors: the well-known prejunctional A1 receptors and a separate group of prejunctional P2 purinoceptors. P2 antagonists increased the release of [3H]-noradrenaline. Overall, the results indicate differential storage, release and modulation of release of the two sympathetic co-transmitters. They also indicate that postganglionic sympathetic axons possess receptors for both co-transmitters: alpha 2 and P2 autoreceptors.

Adenosine receptors involved in modulation of noradrenaline release in isolated rat tail artery

Adenosine receptors involved in the modulation of noradrenaline release from postganglionic sympathetic nerves in rat tail artery were characterized by studying the effects of adenosine-receptor agonists and antagonists on electrically evoked tritium overflow (100 pulses, 5 Hz) and by immunohistochemistry. The adenosine A1 receptor-selective agonist N6-cyclopentyladenosine (CPA; 1-100 nM) and the non-selective adenosine receptor agonist N-ethylcarboxamidoadenosine (NECA; 1-10 microM) decreased tritium overflow. These effects were blocked by the adenosine A1 receptor-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 30 nM). The adenosine A(2A) receptor-selective agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine (CGS 21680; 1-100 nM) enhanced tritium overflow, an effect blocked by the adenosine A(2A) receptor-selective antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261; 20 nM) but not changed by the adenosine A(2B) receptor-selective antagonist N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl) phenoxy]acetamide (MRS 1706; 20 nM). In the presence of DPCPX (30 nM), NECA enhanced tritium overflow, an effect abolished by MRS 1706 but not influenced by SCH 58261. Immunohistochemistry revealed immunoreactivity for all adenosine-receptor subtypes. Areas of co-localization were found for neurofilament with adenosine A1, A(2A) and A(2B) but not A3 receptors. In conclusion, the present study provides functional and morphological evidence for the occurrence of multiple adenosine receptor-mediated modulation of noradrenaline release in the rat tail: inhibition mediated by adenosine A1 receptors and facilitation mediated by both adenosine A(2A) and A(2B) receptors.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Adenosine A2A receptor-mediated facilitation of noradrenaline release involves protein kinase C activation and attenuation of presynaptic inhibitory receptor-mediated effects in the rat vas deferens

In the epididymal portion of rat vas deferens, facilitation of noradrenaline release mediated by adenosine A2A receptors, but not that mediated by beta2-adrenoceptors or by direct activation of adenylyl cyclase, was attenuated by blockade of alpha2-adrenoceptors and abolished by simultaneous blockade of alpha2-adrenoceptors, adenosine A1 and P2Y receptors. The adenosine A2A receptor-mediated facilitation was not changed by inhibitors of protein kinase A, protein kinase G or calmodulin kinase II but was prevented by inhibition of protein kinase C with chelerythrine or bisindolylmaleimide XI. Activation of protein kinase C with phorbol 12-myristate 13-acetate caused a facilitation of noradrenaline release that was abolished by bisindolylmaleimide XI and reduced by antagonists of alpha2-adrenoceptors, adenosine A1 and P2Y receptors. Activation of adenosine A2A receptors attenuated the inhibition of noradrenaline release mediated by the presynaptic inhibitory receptors. This effect was mimicked by phorbol 12-myristate 13-acetate and prevented by bisindolylmaleimide XI. It is concluded that adenosine A2A receptors facilitate noradrenaline release by a mechanism that involves a protein kinase C-mediated attenuation of effects mediated by presynaptic inhibitory receptors, namely alpha2-adrenoceptors, adenosine A1 and P2Y receptors.

P2Y receptor regulation of cultured rat cerebral cortical cells: calcium responses and mRNA expression in neurons and glia

1 We have investigated increases in cytosolic Ca(2+) in response to nucleotides in mixed rat cerebrocortical cultures (neurons and glia in similar numbers) and in essentially neuron-free glial cultures. 2 In both cultures, the agonist-response profile was 2-methylthioADP(2MeSADP)>2-methylthioATP(2MeSATP)>ADP>ATP>adenosine 5'-O-(3-thiotriphosphate), consistent with a P2Y(1) receptor. The maximal responses to 2MeSADP, 2MeSATP and ADP were identical, but that to ATP was higher. 3 Suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid, reactive blue 2 (RB2), and adenosine biphosphate (A3P5P) were antagonists with apparent pA(2) values of 5.5 for suramin, 6.4 for RB2, and 4.7 for A3P5P. 4 Single cell imaging divided the cells from the mixed neuronal-glial cultures into two populations: responsive (neurons) and unresponsive (glial cells) to high [K(+)]. The response of cells to nucleotides was almost exclusively limited to those not responsive to high K(+). 5 In the presence of extracellular Mn(2+), the response of the mixed cultures to 30 mM K(+) and 20 micro M Bay K 8644 was attenuated. However, when 2MeSADP was added there was no reduction in response in cultures previously loaded with Mn(2+). This further indicated that the 2MeSADP response was not in the neurons. 6 Reverse transcriptase-polymerase chain reaction studies detected transcripts for P2Y(1), P2Y(4) and P2Y(6) in RNA preparations from embryonic rat cortex, and from both mixed and glial cultures. P2Y(2) transcripts were not detected in the embryonic cortex. 7 Based on this and previous work, it is proposed that the principal P2Y influences in the brain are on cytosolic Ca(2+) in glial cells and presynaptic sites on neurons.

Facilitation of noradrenaline release by adenosine A(2A) receptors in the epididymal portion and adenosine A(2B) receptors in the prostatic portion of the rat vas deferens

The adenosine-receptor modulation of noradrenaline release was compared in prostatic and epididymal portions of rat vas deferens. In both portions, tritium overflow elicited by electrical stimulation (100 pulses/8 Hz) was reduced by the adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine, and enhanced by the nonselective receptor agonist, 5'-N-ethylcarboxamidoadenosine, in the presence of the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 20 and 100 nM). The adenosine A(2A) receptor agonist, 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine, increased tritium overflow, but only in the epididymal portion. The enhancement caused by NECA was prevented by the adenosine A(2A) receptor antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385; 20 nM), in the epididymal and by the adenosine A(2B) receptor antagonist, alloxazine (1 microM), in the prostatic portion. Inhibition of adenosine uptake enhanced tritium overflow in both portions, an effect blocked by ZM 241385 in the epididymal and by alloxazine in the prostatic portion. The results indicate that adenosine exerts an adenosine A(1) receptor-mediated inhibition, in both portions, and facilitation mediated by adenosine A(2A) receptors in the epididymal and by A(2B) receptors in the prostatic portion.

Release inhibitory receptors activation favours the A2A-adenosine receptor-mediated facilitation of noradrenaline release in isolated rat tail artery

1. Interactions between A(2A)-adenosine receptors and alpha(2)-, A(1)- and P2- release-inhibitory receptors, on the modulation of noradrenaline release were studied in isolated rat tail artery. Preparations were labelled with [(3)H]-noradrenaline, superfused with desipramine-containing medium, and stimulated electrically (100 pulses at 5 Hz or 20 pulses at 50 Hz). 2. Blockade of alpha(2)-autoreceptors with yohimbine (1 microM) increased tritium overflow elicited by 100 pulses at 5 Hz but not by 20 pulses at 50 Hz. 3. The selective A(2A)-receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 1-100 nM) enhanced tritium overflow elicited by 100 pulses at 5 Hz. Yohimbine prevented the effect of CGS 21680, which was restored by the A(1)-receptor agonist N(6)-cyclopentyladenosine (CPA; 100 nM) or by the P2-receptor agonist 2-methylthioadenosine triphosphate (2-MeSATP; 80 microM). 4. CGS 21680 (100 nM) failed to increase tritium overflow elicited by 20 pulses at 50 Hz. The alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304; 30 nM), the A(1)-receptor agonist CPA (100 nM) or the P2-receptor agonist 2-MeSATP (80 microM) reduced tritium overflow. In the presence of these agonists CGS 21680 elicited a facilitation of tritium overflow. 5. Blockade of potassium channels with tetraethylammonium (TEA; 5 mM) increased tritium overflow elicited by 100 pulses at 5 Hz to values similar to those obtained in the presence of yohimbine but did not prevent the effect of CGS 21680 (100 nM) on tritium overflow. 6. It is concluded that, in isolated rat tail artery, the facilitation of noradrenaline release mediated by A(2A)-adenosine receptors is favoured by activation of release inhibitory receptors.

Co-transmitter function of ATP in central catecholaminergic neurons of the rat

Intracellular recordings were made in a mid-pontine slice preparation of the rat brain containing the nucleus locus coeruleus. Focal electrical stimulation evoked biphasic synaptic potentials consisting of early depolarizing (d.p.s.p.) and late hyperpolarizing (i.p.s.p.) components. The alpha(2)-adrenoceptor antagonist idazoxan inhibited the i.p.s.p. without altering the d.p.s.p. All of the following experiments were carried out in the presence of kynurenic acid and picrotoxin to block the glutamatergic and GABAergic fractions of the d.p.s.p., respectively. Guanethidine, which is known to inhibit noradrenaline and ATP release from nerve terminals of postganglionic sympathetic nerves, depressed both the d.p.s.p. and the i.p.s.p. in a concentration-dependent manner. Damage of catecholaminergic nerve terminals by 6-hydroxydopamine also decreased both the d.p.s.p. and the i.p.s.p. The P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) depressed the d.p.s.p., whereas the i.p.s.p. remained unaffected. The further application of PPADS did not increase the depression of the d.p.s.p. by guanethidine. Superfusion with the mixed alpha-adrenoceptor agonist noradrenaline or the selective P2 receptor agonist adenosine 5'-O-(2-thiodiphosphate) inhibited both the d.p.s.p. and the i.p.s.p. The inhibitory effects of these agonists were prevented by the respective antagonists idazoxan or suramin. In the presence of suramin noradrenaline failed to inhibit the residual d.p.s.p. Superfused noradrenaline potentiated rather than inhibited responses to pressure-applied alpha,beta-methylene-ATP; superfused adenosine 5'-O-(2-thiodiphosphate) did not interact with pressure-applied noradrenaline. In conclusion, we present electrophysiological evidence for the co-release of ATP and catecholamines in the CNS. At the cell somata of neurons in the locus coeruleus, noradrenaline and ATP activate inhibitory alpha(2)-adrenoceptors and excitatory P2 receptors, respectively. In addition, inhibitory presynaptic autoreceptors of the alpha(2) and P2 types appear to regulate release of the two co-transmitters.

Characterization of prejunctional purinoceptors inhibiting noradrenaline release in rat mesenteric arteries

The effects of purinoceptor agonists on noradrenaline NA release by electrical stimulation in rat mesenteric arteries were examined to clarify the pharmacological properties of prejunctional purinoceptors on adrenergic nerves. Adenosine and the other P1-receptor agonists, 5'-(N-ethylcarboxamido) adenosine and 2-chloroadenosine, significantly inhibited the release of NA. Also beta,gamma-methylene ATP and 2-methylthio ATP, P2-receptor agonists, significantly inhibited NA releases. The inhibitory effect of adenosine was significantly reduced by adenosine deaminase, but those of beta,gamma-methylene ATP and 2-methylthio ATP were not affected. This suggests that the inhibitory effects of P2-receptor agonists are not due to conversion into adenosine. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX), a P1 (A1)-receptor antagonist, significantly reduced the inhibitory effects of not only the P1- but also P2-receptor agonists. Therefore, DPCPX appears to act on both prejunctional P1- and P2-receptor as an antagonist. Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a P2-receptor antagonist, significantly reduced the inhibitory effects of the P2-receptor agonists, but not those of the P1-receptor agonists. From these findings in the rat mesenteric artery, the P1-receptor agonist-induced inhibition of NA-release appears to be mediated via a well-known prejunctional P1-receptor of the A1-subtype, but the P2-receptor agonist-induced inhibition appears to be mediated via an unidentified purinoceptor that is blocked not only by P2-receptor antagonists but also by P1-receptor antagonists.

Local regulation of [(3)H]-noradrenaline release from the isolated guinea-pig right atrium by P(2X)-receptors located on axon terminals

In this study the regulation of cardiac sympathetic outflow by presynaptic P(2X) receptor-gated ion channels was examined. ATP (30 microM - 1 mM) and other P2-receptor agonists elicited [(3)H]-noradrenaline ([(3)H]-NA) outflow from the isolated guinea-pig right atrium with the potency order of ATP>2-methyl-thioATP>alpha,beta-methylene-ATP=ADP, whereas ss, gamma-methylene-L-ATP was inactive. Ca(2+)-free conditions abolished both electrical field stimulation (EFS)- and ATP-evoked release of tritium. Unlike from EFS-induced outflow, ATP-induced [(3)H]-NA outflow was not reduced by omega-Conotoxin-GVIA (100 nM), Cd(2+) (100 microM) and tetrodotoxin (1 microM). The rapid extracellular decomposition of ATP was revealed by HPLC analysis. However, the effect of ATP to promote [(3)H]-NA release was not prevented by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 250 nM), 3, 7-dimethyl-1-propargylxanthine (DMPX, 250 nM), or by reactive blue 2 (RB2, 10 microM), antagonists of A(1)-, A(2)- and inhibitory P(2) receptors. Zn(2+) (50 microM), the P(2X)-receptor modulator potentiated, and P(2X) receptor antagonists, i.e. suramin (300 microM), pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM) and 2'-o-(trinitrophenyl)-adenosine 5'-triphosphate (TNP-ATP, 30 microM) antagonized the ATP (1 mM)-evoked response. RT - PCR study revealed the expression of P(2X2) and P(2X3) receptor mRNAs in guinea-pig superior cervical ganglion. PPADS (30 microM) significantly reduced the EFS-induced [(3)H]-NA outflow in the presence DPCPX (250 nM) and RB2 (10 microM). In summary a P(2X)-type purinoceptor regulates noradrenaline release from the isolated right atrium of the guinea-pig. The pharmacological profile of the receptor resemble to homo-oligomeric P(2X3) or hetero-oligomeric P(2X2)/P(2X3) complexes, and provide a new target to intervene on sympathetic neuroeffector transmission at the presynaptic site.

The effect of nucleotides and adenosine on stimulus-evoked glutamate release from rat brain cortical slices

Evidence has previously been presented that P1 receptors for adenosine, and P2 receptors for nucleotides such as ATP, regulate stimulus-evoked release of biogenic amines from nerve terminals in the brain. Here we investigated whether adenosine and nucleotides exert presynaptic control over depolarisation-elicited glutamate release. Slices of rat brain cortex were perfused and stimulated with pulses of 46 mM K(+) in the presence of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (0.2 mM). High K(+) substantially increased efflux of glutamate from the slices. Basal glutamate release was unchanged by the presence of nucleotides or adenosine at concentrations of 300 microM. Adenosine, ATP, ADP and adenosine 5'-O-(3-thiotriphoshate) at 300 microM attenuated depolarisation-evoked release of glutamate. However UTP, 2-methylthio ATP, 2-methylthio ADP, and alpha,beta-methylene ATP at 300 microM had no effect on stimulated glutamate efflux. Adenosine deaminase blocked the effect of adenosine, but left the response to ATP unchanged. The A(1) antagonist 8-cyclopentyl-1, 3-dipropylxanthine antagonised the inhibitory effect of both adenosine and ATP. Cibacron blue 3GA inhibited stimulus-evoked glutamate release when applied alone. When cibacron blue 3GA was present with ATP, stimulus-evoked glutamate release was almost eliminated. However, this P2 antagonist had no effect on the inhibition by adenosine. These results show that the release of glutamate from depolarised nerve terminals of the rat cerebral cortex is inhibited by adenosine and ATP. ATP appears to act directly and not through conversion to adenosine.

P2Y purinoceptor activation mobilizes intracellular Ca2+ and induces a membrane current in rat intracardiac neurones

1. The mobilization of Ca2+ by purinoceptor activation and the relative contributions of intra- and extracellular sources of Ca2+ were investigated using microfluorimetric measurements of fura-2 loaded in cultured neurones from rat intracardiac ganglia. 2. Reverse transcriptase-polymerase chain reaction (RT-PCR) revealed expression of mRNA for the G protein-coupled P2Y2 and P2Y4 receptors. 3. Brief application of either 300 microM ATP or 300 microM UTP caused transient increases in [Ca2+]i of 277 +/- 22 nM and 267 +/- 39 nM, respectively. Removal of external Ca2+ did not significantly reduce these [Ca2+]i responses. 4. The order of purinoceptor agonist potency for [Ca2+]i increases was ATP = UTP > 2-MeSATP > ADP >> adenosine, consistent with the profile for P2Y2 purinoceptors. ATP- and UTP-induced rises in [Ca2+]i were completely and reversibly blocked by 10 microM PPADS (a P2 purinoceptor antagonist) and partially inhibited by 100 microM suramin (a relatively non-specific purinoceptor antagonist). 5. In the presence of the endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (10 microM) in Ca2+-free media, the [Ca2+]i responses evoked by ATP were progressively decreased and abolished. 6. ATP- and UTP-induced [Ca2+]i rises were insensitive to pertussis toxin, caffeine (5 mM) and ryanodine (10 microM) but were significantly reduced by U-73122, a phospholipase C (PLC) inhibitor. 7. In fura-2-loaded cells, perforated patch whole-cell recordings show that ATP and UTP evoked slow outward currents at -60 mV, concomitant with the rise in [Ca2+]i, in approximately 30 % of rat intracardiac neurones. 8. In conclusion, these results suggest that in r intracardiac neurones, ATP binds to P2Y2 purinoceptors to transiently raise [Ca2+]i and activate an outward current. The signalling pathway appears to involve a PTX-insensitive G protein coupled to PLC generation of IP3 which triggers the release of Ca2+ from a ryanodine-insensitive Ca2+ store(s).

P2 receptors in the central and peripheral nervous systems modulating sympathetic vasomotor tone

Arterial pressure depends on the level of activity of sympathetic vasoconstrictor outflow to blood vessels. This activity is generated in the central nervous system, and involves inputs from a variety of brain regions projecting to sympathetic preganglionic neurones. Of especial interest are a group of neurones in the rostral ventrolateral medulla (RVLM), as they have been demonstrated to have a fundamental role in reflex regulation of the cardiovascular system, and in generation of tonic drive to sympathetic outflow. Sympathetic outflow to blood vessels is additionally modulated at sympathetic ganglia, and at the peripheral terminals of sympathetic nerves. This review considers the role of P2 purine receptors in this neural pathway. Ionotropic P2X receptors are expressed in the RVLM, in sympathetic ganglia, and at the sympathetic neuromuscular junction, and mediate fast excitatory neurotransmission, indicating a general role for ATP as a regulator of sympathetic vasomotor tone. P2Y receptors couple to G proteins and mediate slower signalling to ATP; they have been reported to inhibit prejunctionally neurotransmission at the peripheral terminals of sympathetic nerves, but little is known about their possible role in the central nervous system and in sympathetic ganglia.

Presynaptic P2 receptors?

Although the emphasis in ATP research has been on postjunctional receptors, there is also evidence for presynaptic receptors regulating transmitter release in the autonomic nervous system. Recent work has attempted to identify similar mechanisms in the central nervous system. Some of the existing results can be explained by the metabolism of nucleotides to adenosine or adenosine 5'-monophosphate (AMP). However, studies of presynaptic effects using sensitive electrophysiological tests such as paired-pulse interactions indicate that nucleotides can act at presynaptic sites, but that their effects may be mediated by a release of adenosine. Results are also described which indicate that, under some conditions, nucleotides can mediate phenomena such as long-term potentiation, which probably involves a significant presynaptic element. In part these effects may involve a nucleotide-induced release of adenosine and the simultaneous activation of P1 and P2 receptors.

Sympathoinhibition by adenosine A(1) receptors, but not P2 receptors, in the hamster mesenteric arterial bed

The aim of the present study was to determine whether there are prejunctional inhibitory P2 purine receptors on sympathetic nerves in the hamster isolated perfused mesenteric arterial bed. Adenosine 5'-O-(3-thiotriphosphate (ATPgammaS; 10 microM), adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS; 100 microM) and AMP (10 microM) had no significant effect on neurogenic contractions to electrical field stimulation. In contrast, P1 receptor agonists attenuated sympathetic vasoconstriction with a potency order of N(6)5'-(Nadenosine. The pEC(50) value for CPA was 7.5+/-0.1 (n=7). The concentration-inhibitory effect curve to CPA was shifted to the right by the adenosine A(1) receptor antagonist, 8-cyclopentyl-1, 3-dipropyl-xanthine (DPCPX; 10 nM; apparent pK(B) 9.6; n=6-7). In methoxamine raised-tone mesenteries CPA (0.001-10 microM) did not elicit vasorelaxation, and NECA and adenosine were only weak vasorelaxants. These results indicate that adenosine A(1) receptors, but not P2 receptors, inhibit prejunctionally sympathetic neurotransmission in the hamster mesenteric arterial bed.

Suramin-sensitive suppression of paired-pulse inhibition by adenine nucleotides in rat hippocampal slices

In order to assess the possible presence of presynaptic P2 receptors for nucleotides in the hippocampus, adenosine triphosphate and betagamma-methyleneATP have been examined on paired-pulse inhibition in rat hippocampal slices. Both compounds reproduced the effects of adenosine and reduced the amount of paired-pulse inhibition at an interpulse interval of 10 ms and increased the amount of facilitation at intervals of 20 and 50 ms. These effects were prevented by 8-phenyltheophylline and adenosine deaminase, indicating their mediation by adenosine. The effects were also reduced by suramin at 50 microM, suggesting the possible activation of P2 receptors. It is suggested that a population of P2 receptors may exist which promote the release of endogenous adenosine in the hippocampus.

Two different P2Y receptors linked to steroidogenesis in bovine adrenocortical cells

Both extracellular adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) induced corticoid production (steroidogenesis) concentration-dependently in bovine adrenocortical cells (BA cells). Pertussis toxin (PTX, approx. 2 microg/ml) partially inhibited (approx. 55% inhibition) extracellular ATP (100 microM)-induced steroidogenesis in BA cells. However, PTX did not inhibit extracellular UTP (100 microM)-induced steroidogenesis. Both ATP- and UTP-induced steroidogeneses were significantly inhibited by suramin (50-200 microM). These effects were inhibited significantly by reactive blue-2 (more than 100 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (more than 100 microM). Both nucleotides (1-100 microM) induced inositol phosphates accumulation and intracellular Ca2+ mobilization, but PTX did not inhibit them. The RT-PCR procedure identified only P2Y2-receptor mRNA in BA cells. These results suggest that extracellular ATP induces steroidogenesis via a unique P2 receptor linked to PTX-sensitive guanine nucleotide-binding protein (G-protein), while extracellular UTP induces steroidogenesis via P2 receptor linked to PTX-insensitive G-protein. Thus, it was concluded that at least two different P2Y-like receptors linking to steroidogenesis exist in BA cells.

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

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

P2-receptor modulation of noradrenergic neurotransmission in rat kidney

1. ATP has previously been shown to act as a sympathetic cotransmitter in the rat kidney. The present study analyses the question of whether postganglionic sympathetic nerve endings in the kidney possess P2-receptors which modulate noradrenaline release. Rat kidneys were perfused with Krebs-Henseleit solution containing the noradrenaline uptake blockers cocaine and corticosterone and the alpha2-adrenoceptor antagonist rauwolscine. The renal nerves were electrically stimulated, in most experiments by 30 pulses applied at 1 Hz. The outflow of endogenous noradrenaline (or, in some experiments, of ATP and lactate dehydrogenase) as well as the perfusion pressure were measured simultaneously. 2. The P2-receptor agonist adenosine-5'-O-(3-thiotriphosphate) (ATPgammaS, 3-30 microM) reduced the renal nerve stimulation (RNS)-induced outflow of noradrenaline (estimated EC50 =8 microM). The P2-receptor antagonist cibacron blue 3GA (30 microM) shifted the concentration-inhibition curve for ATPgammaS to the right (apparent pKB value 4.7). 3. Cibacron blue 3GA (3-30 microM) and its isomer reactive blue 2 (3-30 microM) significantly increased RNS-induced outflow of noradrenaline in the presence of the P1-receptor antagonist 8-(p-sulphophenyl)theophylline (8-SPT, 100 microM) by about 70% and 90%, respectively. The P2-receptor antagonist suramin (30-300 microM) only tended to enhance RNS-induced outflow of noradrenaline. When the nerves were stimulated by short pulse trains consisting of 6 pulses applied at 100 Hz (conditions under which autoinhibition is inoperative), reactive blue 2 did not affect the RNS-induced outflow of noradrenaline. 4. RNS (120 pulses applied at 4 Hz) induced the outflow of ATP but not of the cytoplasmatic enzyme lactate dehydrogenase. 5. ATPgammaS (3-30 microM) concentration-dependently reduced pressor responses to RNS at 1 Hz. Cibacron blue 3GA, reactive blue 2 as well as suramin also reduced pressor responses to RNS (maximally by 50 to 70%). 6. This study in rat isolated kidney, in which the release of endogenous noradrenaline was measured, demonstrates that renal sympathetic nerves possess prejunctional P2-receptors that mediate inhibition of transmitter release. These prejunctional P2-receptors are activated by endogenous ligands, most likely ATP, released upon nerve activity. Both, P2-receptor agonists and P2-receptor antagonists reduced pressor responses to RNS either by inhibiting transmitter release or by blocking postjunctional vasoconstrictor P2-receptors.

Autonomic neural control of cardiac function: modulation by adenosine and adenosine 5'-triphosphate

Adenosine and adenosine 5'-triphosphate (ATP) are found in every cell of the human body. These molecules are released from cells into the extracellular fluid under physiologic and pathophysiologic conditions. Outside of cells, adenosine and ATP act as physiologic regulators of cells, tissues, and organs. In the heart, extracellular adenosine and ATP exert pronounced inotropic, lusitropic, electrophysiologic, and metabolic effects, which are mediated by specific cell surface receptors. In addition, both compounds can modulate sympathetic and parasympathetic input to the heart by interacting with neural elements within and without the heart, thereby modulating autonomic neural control of cardiac functions. This article briefly reviews these indirect, neurally-mediated actions of adenosine and ATP.

P2-purinoceptors on postganglionic sympathetic neurones

1. Postganglionic sympathetic neurones possess both excitatory and inhibitory P2-purinoceptors. 2. The mechanisms of action of excitatory P2-purinoceptors have recently been studied on cultured sympathetic neurones of the rat. The receptors mediate fast increases in intracellular Ca2+ levels and a release of noradrenaline. They are likely to belong to the neuronal types of P2X-purinoceptors and to be located on the sympathetic nerve cell bodies or their dendrites. 3. Inhibitory P2-purinoceptors have been shown to operate at sympathetic axon terminals in isolated tissues. Adenine nucleotides decreased the stimulation-evoked release of noradrenaline by activation of these receptors. The receptors are likely to belong to the group of G-protein-coupled P2Y-purinoceptors. They mediate a negative feedback in which co-transmitter ATP inhibits subsequent sympathetic transmitter release.

Purinergic modulation of neural control of cardiac function

1. The purine nucleotide adenosine 5'-triphosphate (ATP) and its related nucleoside, adenosine (Ado), exert pronounced electrophysiologic, inotropic, lusitropic and metabolic effects in the mammalian heart. 2. These effects are the result of direct actions of these compounds on cardiac myocytes and endothelial cells, mediated by cell surface receptors. 3. In addition, ATP and Ado can stimulate neural elements inside and outside the heart and thereby modulate neural control of cardiac function. These latter actions of ATP and Ado are briefly reviewed and their hypothetical physiological role is outlined.

Purinoceptor modulation of noradrenaline release in rat tail artery: tonic modulation mediated by inhibitory P2Y- and facilitatory A2A-purinoceptors

1. The effects of analogues of adenosine and ATP on noradrenaline release elicited by electrical stimulation (5 Hz, 2700 pulses) were studied in superfused preparations of rat tail artery. The effects of purinoceptor antagonists, of adenosine deaminase and of adenosine uptake blockade were also examined. Noradrenaline was measured by h.p.l.c. electrochemical detection. 2. The A1-adenosine receptor agonist, N6-cyclopentyladenosine (CPA; 0.1-100 nM) reduced, whereas the A2A-receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 3-30 nM) increased evoked noradrenaline overflow. These effects were antagonized by the A1-adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 20 nM) and the A2-adenosine receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX; 100 nM), respectively. The P2Y-purinoceptor agonist, 2-methylthio-ATP (1-100 microM) reduced noradrenaline overflow, an effect prevented by the P2-purinoceptor antagonist, cibacron blue 3GA (100 microM) and suramin (100 microM). 3. Adenosine deaminase (2 u ml-1), DMPX (100 nM) and inhibition of adenosine uptake with S-(p-nitrobenzyl)-6-thioinosine (NBTI; 50 nM) decreased evoked noradrenaline overflow. DPCPX alone did not change noradrenaline overflow but prevented the inhibition caused by NBTI. The P2Y-purinoceptor antagonist, cibacron blue 3GA (100 microM) increased evoked noradrenaline overflow as did suramin, a non-selective P2-antagonist. 4. It is concluded that, in rat tail artery, inhibitory (A1 and P2Y) and facilitatory (A2A) purinoceptors are present and modulate noradrenaline release evoked by electrical stimulation. Endogenous purines tonically modulate noradrenaline release through activation of inhibitory P2Y and facilitatory A2A purinoceptors, whereas a tonic activation of inhibitory A1 purinoceptors seems to be prevented by adenosine uptake.

Vasoconstrictor responses to the P2x-purinoceptor agonist beta, gamma-methylene-L-ATP in human cutaneous and renal blood vessels

1. Strips of human saphenous veins and of human renal arteries and veins were superfused with Krebs-Henseleit solution at 37 degrees C. Constrictor responses were elicited by exogenous noradrenaline and the P2x-purinoceptor-selective agonist, beta, gamma-methylene-L-ATP. 2. In human saphenous veins, beta, gamma-methylene-L-ATP (0.3-30 microM; EC50 2.2 microM) induced marked constrictor responses. The maximal response to beta, gamma-methylene-L-ATP was similar to the maximal response to noradrenaline. The P2-purinoceptor antagonist suramin (30 microM) shifted the concentration-response curve of beta, gamma-methylene-L-ATP to the right (apparent pKB value 4.8); suramin (100 microM) markedly inhibited the responses to beta, gamma-methylene-L-ATP. The preferential P2x-purinoceptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 3 microM) slightly reduced the response to beta, gamma-methylene-L-ATP. At a ten times higher concentration (30 microM), PPADS almost abolished the responses to beta, gamma-methylene-L-ATP. PPADS (30 microM), in contrast, caused no significant change in the concentration-response curve of noradrenaline. 3. In extrarenal and intrarenal arteries, EC50 values and maximal responses to noradrenaline were similar when compared with responses to noradrenaline in saphenous veins. Noradrenaline also constricted extrarenal veins. However, in contrast to the results obtained on saphenous veins, beta, gamma-methylene-L-ATP caused almost no constrictor responses in extrarenal veins and arteries and only moderate responses in intrarenal arteries. 4. The results demonstrate marked differences in responsiveness of human blood vessels to the selective P2x-purinoceptor agonist, beta, gamma-methylene-L-ATP, suggesting tissue differences in the occurrence or operation of P2x-purinoceptors in human vascular tissues. Moreover, the results indicate that PPADS blocks P2x-purinoceptors in human isolated blood vessels as previously demonstrated in animal blood vessels.