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

Intracellular adenosine released from THP-1 differentiated human macrophages is involved in an autocrine control of Leishmania parasitic burden, mediated by adenosine A2A and A2B receptors

Leishmania infected macrophages have conditions to produce adenosine. Despite its known immunosuppressive effects, no studies have yet established whether adenosine alter Leishmania parasitic burden upon macrophage infection. This work aimed at investigating whether endogenous adenosine exerts an autocrine modulation of macrophage response towards Leishmania infection, identifying its origin and potential pharmacological targets for visceral leishmaniasis (VL), using THP-1 differentiated macrophages. Adenosine deaminase treatment of infected THP-1 cells reduced the parasitic burden (29.1 ± 2.2%, P < 0.05). Adenosine A_2A and A_2B receptor subtypes expression was confirmed by RT-qPCR and by immunocytochemistry and their blockade with selective adenosine A_2A and A_2B antagonists reduced the parasitic burden [14.5 ± 3.1% (P < 0.05) and 12.3 ± 3.1% (P < 0.05), respectively; and 24.9 ± 2.8% (P < 0.05), by the combination of the two antagonists)], suggesting that adenosine A₂ receptors are tonically activated in infected THP-1 differentiated macrophages. The tonic activation of adenosine A₂ receptors was dependent on the release of intracellular adenosine through equilibrative nucleoside transporters (ENT1/ENT2): NBTI or dipyridamole reduced (~25%) whereas, when ENTs were blocked, adenosine A₂ receptor antagonists failed to reduce and A₂ agonists increase parasitic burden. Effects of adenosine A₂ receptors antagonists and ENT1/2 inhibitor were prevented by L-NAME, indicating that nitric oxide production inhibition prevents adenosine from increasing parasitic burden. Results suggest that intracellular adenosine, released through ENTs, elicits an autocrine increase in parasitic burden in THP-1 macrophages, through adenosine A₂ receptors activation. These observations open the possibility to use well-established ENT inhibitors or adenosine A₂ receptor antagonists as new therapeutic approaches in VL.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

Purinergic signaling and blood vessels in health and disease

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

Combined contribution of endothelial relaxing autacoides in the rat femoral artery response to CPCA: an adenosine A2 receptor agonist

We examined the contribution of endothelial relaxing factors and potassium channels in actions of CPCA, potent adenosine A(2) receptor agonist, on isolated intact male rat femoral artery (FA). CPCA produced concentration-dependent relaxation of FA, which was notably, but not completely, reduced after endothelial denudation. DPCPX, A(1) receptor antagonist, had no significant effect, while SCH 58261 (A(2A) receptor antagonist) notably reduced CPCA-evoked effect. Pharmacological inhibition of nitric oxide synthase or cyclooxygenase comparably reduced CPCA-evoked action, still in a lesser degree than after denudation. In the presence of buffer with high K(+) (100  mM), CPCA-produced relaxations were almost abolished. TEA (nonselective K(Ca) blocker), glibenclamide (K(ATP) blocker), Ba(++) (K(IR) blocker), or ouabain (Na(+)/K(+)-ATPase inhibitor) did not change CPCA-induced relaxation. Concentration-response curve for CPCA was significantly shifted to the right after the incubation of apamin (SK channel blocker). CPCA produced concentration-dependent relaxation of FA that was partly dependent on endothelial cells. Endothelium-related portion of CPCA-elicited effect was mediated by combined action of endothelial NO, prostacyclin, and EDHF after activation of endothelial A(2A) receptors. Small conductance K(Ca) channels were involved in this action.

Functional crosstalk of prejunctional receptors on the modulation of noradrenaline release in mesenteric vessels: A differential study of artery and vein

The role of angiotensin II receptors, bradykinin receptors and β-adrenoceptors in the modulation of noradrenaline release and the influence of α(2)-autoinhibition in these effects was investigated in the mesenteric artery and vein. Rings of mesenteric vessels of male Wistar rats were labelled with [(3)H]-noradrenaline and the effects of modulators on tritium overflow evoked by 100 pulses at 2Hz (marked α(2)-autoinhibition) and by 20 pulses at 50Hz or 100 pulses at 2Hz plus yohimbine (1μM; reduced α(2)-autoinhibition) were evaluated. Angiotensin II and bradykinin enhanced noradrenaline release evoked by 100 pulses at 2Hz, in a concentration-dependent manner, in both vessels. These effects were attenuated under conditions of reduced α(2)-autoinhibition. The attenuation was partially reversed by activation of adenosine A(1) receptors in both vessels and by activation of P2Y receptors in the vein. Isoprenaline and the selective β(2)-adrenoceptor agonist formoterol enhanced tritium overflow independently of α(2)-autoinhibition in the vein. In the artery, the enhancement by formoterol was only observed under reduced α(2)-autoinhibition. Pharmacological characterization of the β-adrenoceptors indicated that in the artery the effect of isoprenaline was mediated by the β(1)-subtype under marked α(2)-autoinhibition and by the β(2)-subtype under reduced α(2)-autoinhibition whereas in the vein the effect was independent of α(2)-autoinhibition. The results indicate that α(2)-autoinhibition is a key determinant of the magnitude of facilitation caused by angiotensin II and bradykinin in both types of mesenteric vessels and regulates the effects mediated by β(1)-and β(2)-adrenoceptors which co-exist in the artery.

The P2Y(1) and P2Y(12) receptors mediate autoinhibition of transmitter release in sympathetic innervated tissues

In the sympathetic nervous system, ATP is a co-transmitter and modulator of transmitter release, inhibiting noradrenaline release by acting on P2Y autoreceptors, but in peripheral tissues the subtypes involved have only scarcely been identified. We investigated the identity of the noradrenaline release-inhibiting P2Y subtypes in the epididymal portion of vas deferens and tail artery of the rat. The subtypes operating as autoreceptors, the signalling mechanism and cross-talk with alpha(2)-autoreceptors, was also investigated in the epididymal portion. In both tissues, the nucleotides 2-methylthioATP, 2-methylthioADP, ADP and ATP inhibited noradrenaline release up to 68%, with the following order of potency: 2-methylthioADP=2-methylthioATP>ADP=ATP in the epididymal portion and 2-methylthioADP=2-methylthioATP=ADP>ATP in the tail artery. The selective P2Y(1) antagonist 2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate (30microM) and the P2Y(12) antagonist 2,2-dimethyl-propionic acid 3-(2-chloro-6-methylaminopurin-9-yl)-2-(2,2-dimethyl-propionyloxymethyl)-propyl ester (30microM) increased noradrenaline release per se by 25+/-8% and 18+/-3%, respectively, in the epididymal portion but not in tail artery. Both antagonists attenuated the effect of nucleotides in the epididymal portion whereas in tail artery only the P2Y(1) antagonist was effective. The agonist of P2Y(1) and P2Y(12) receptors, 2-methylthioADP, caused an inhibition of noradrenaline release that was not prevented by inhibition of phospholipase C or protein kinase C but was abolished by pertussis toxin. 2-methylthioADP and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were less potent at inhibiting noradrenaline release under marked influence of alpha(2)-autoinhibition. In both tissues, nucleotides modulate noradrenaline release by activation of inhibitory P2Y(1) receptors but in the epididymal portion P2Y(12) receptors also participate. P2Y(1) and P2Y(12) receptors are coupled to G(i/o)-proteins and operate as autoreceptors in the vas deferens where they interact with alpha(2)-adrenoceptors on the modulation of noradrenaline release.

Protective effect of octreotide and infliximab in an experimental model of indomethacin-induced inflammatory bowel disease

Indomethacin administration in animals increases permeability of the small intestine, leading to inflammation that mimics Crohn's disease. Nonsteroidal anti-inflammatory drugs increase the permeability of the intestinal epithelial barrier and should therefore be used with caution in patients with Crohn's disease. We analyzed the protective effects of octreotide and the tumor necrosis factor-alpha inhibitor infliximab in a rat model of indomethacin-induced enterocolitis. Male Wistar rats received 20 mg of infliximab or 10 mug of octreotide 24 h prior to injection with indomethacin. Intestinal permeability was analyzed using Cr-51-ethylenediaminetetraacetic acid clearance. No microscopic or macroscopic alterations were observed in the rats receiving infliximab or octreotide, both of which increased permeability (P < 0.001 versus controls). Our macroscopic and microscopic findings might be related to the low specificity of infliximab and suggest that cytokines affect the intestinal epithelial barrier, as evidenced by the protective effect that infliximab had on the permeability parameters evaluated.

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.

Increased expression of adenosine A2A receptors in patients with spontaneous and head-up-tilt-induced syncope

BACKGROUND

Adenosine may play a role in the triggering of neurocardiogenic syncope, but no information on adenosine receptors is available at the present time.

OBJECTIVE

The purpose of this study was to investigate whether adenosine A2A receptors expression is altered in patients with neurocardiogenic syncope.

METHODS

Adenosine plasma levels (APLs), the expression of A2A receptors, were measured (mean +/- standard error of the mean) during tilt testing. Expression of receptors was assessed on mononuclear cells using a selective receptor ligand.

RESULTS

At baseline, the APLs of 16 patients with a positive test were higher than those of 17 patients with a negative test and of those of a control group (2.10 +/- 0.30 vs. 0.40 +/- 0.05 and 0.41 +/- 0.06 muM, respectively; P <.0001). The number of receptors was higher in patients tested positive than in patients tested negative or in the control group (122 +/- 10 vs. 38 +/- 4 and 44 +/- 4 fmol/g of proteins, respectively; P <.0001). No difference was found in the affinity or synthesis among the three groups.

CONCLUSION

This study showed an increased number and an up-regulation of adenosine A2A receptors in patients with spontaneous syncope and a positive head-up tilt, which in the context of high APLs may play a role in the recurrence of syncopal episodes.

A2A adenosine-receptor-mediated facilitation of noradrenaline release in rat tail artery involves protein kinase C activation and betagamma subunits formed after alpha2-adrenoceptor activation

This work aimed to investigate the molecular mechanisms involved in the interaction of alpha2-adrenoceptors and adenosine A2A-receptor-mediated facilitation of noradrenaline release in rat tail artery, namely the type of G-protein involved in this effect and the step or steps where the signalling cascades triggered by alpha2-adrenoceptors and A2A-receptors interact. The selective adenosine A2A-receptor agonist 2-p-(2-carboxy ethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 100 nM) enhanced tritium overflow evoked by trains of 100 pulses at 5 Hz. This effect was abolished by the selective adenosine A2A-receptor antagonist 5-amino-7-(2-phenyl ethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo [1,5-c]pyrimidine (SCH 58261; 20 nM) and by yohimbine (1 microM). CGS 21680-mediated effects were also abolished by drugs that disrupted G(i/o)-protein coupling with receptors, PTX (2 microg/ml) or NEM (40 microM), by the anti-G(salpha) peptide (2 microg/ml) anti-G(betagamma) peptide (10 microg/ml) indicating coupling of A2A-receptors to G(salpha) and suggesting a crucial role for G(betagamma) subunits in the A(2A)-receptor-mediated enhancement of tritium overflow. Furthermore, phorbol 12-myristate 13-acetate (PMA; 1 microM) or forskolin (1 microM), direct activators of protein kinase C and of adenylyl cyclase, respectively, also enhanced tritium overflow. In addition, PMA-mediated effects were not observed in the presence of either yohimbine or PTX. Results indicate that facilitatory adenosine A2A-receptors couple to G(salpha) subunits which is essential, but not sufficient, for the release facilitation to occur, requiring the involvement of G(i/o)-protein coupling (it disappears after disruption of G(i/o)-protein coupling, PTX or NEM) and/or G(betagamma) subunits (anti-G(betagamma)). We propose a mechanism for the interaction in study suggesting group 2 AC isoforms as a plausible candidate for the interaction site, as these isoforms can integrate inputs from G(salpha) subunits (released after adenosine A2A-receptor activation; prime-activation), G(betagamma) subunits (released after activation of G(i/o)-protein coupled receptors) which can directly synergistically stimulate the prime-activated AC or indirectly via G(betagamma) activation of the PLC-PKC pathway.

Endogenous adenosine inhibits CNS terminal Ca(2+) currents and exocytosis

Bursts of action potentials (APs) are crucial for the release of neurotransmitters from dense core granules. This has been most definitively shown for neuropeptide release in the hypothalamic neurohypophysial system (HNS). Why such bursts are necessary, however, is not well understood. Thus far, biophysical characterization of channels involved in depolarization-secretion coupling cannot completely explain this phenomenon at HNS terminals, so purinergic feedback mechanisms have been proposed. We have previously shown that ATP, acting via P2X receptors, potentiates release from HNS terminals, but that its metabolite adenosine, via A(1) receptors acting on transient Ca(2+) currents, inhibit neuropeptide secretion. We now show that endogenous adenosine levels are sufficient to cause tonic inhibition of transient Ca(2+) currents and of stimulated exocytosis in HNS terminals. Initial non-detectable adenosine levels in the static bath increased to 2.9 microM after 40 min. These terminals exhibit an inhibition (39%) of their transient inward Ca(2+) current in a static bath when compared to a constant perfusion stream. CPT, an A(1) adenosine receptor antagonist, greatly reduced this tonic inhibition. An ecto-ATPase antagonist, ARL-67156, similarly reduced tonic inhibition, but CPT had no further effect, suggesting that endogenous adenosine is due to breakdown of released ATP. Finally, stimulated capacitance changes were greatly enhanced (600%) by adding CPT to the static bath. Thus, endogenous adenosine functions at terminals in a negative-feedback mechanism and, therefore, could help terminate peptide release by bursts of APs initiated in HNS cell bodies. This could be a general mechanism for controlling transmitter release in these and other CNS terminals.

The effect of an adenosine A1 receptor agonist in the treatment of experimental subarachnoid hemorrhage-induced cerebrovasospasm

BACKGROUND

Adenosine is a potent vasodilator and an important modulator of cardiovascular function. It has been postulated that nitric oxide (NO) is involved in adenosine-induced vasodilation. This study was designed to examine the effect of an adenosine A1 agonist, N6-cyclopentyladenosine (CPA), in the prevention of subarachnoid haemorrhage (SAH)-induced vasospasm. Method. Experimental SAH was induced in Sprague-Dawley rats by injecting 0.3 mL autogenous blood into the cisterna magna. Intraperitoneal injections of CPA (0.003 mg/kg), or vehicle were administered 5 min and 24 hours after induction of SAH. The degree of vasospasm was determined by averaging the cross sectional areas of the basilar artery 2 days after SAH. Expressions of endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) in basilar artery were evaluated. Findings. There were no significant differences among the control and treated groups in physiological parameters recorded before sacrifice. When compared with animals in the control group, cross-sectional area of basilar arteries areas in the SAH only, SAH plus vehicle and SAH plus CPA groups were reduced by 19% (p < 0.01), 22% (p < 0.01), and 9% (p = 0.133), respectively. The cross-sectional areas of the CPA-treated group differed significantly from those of the SAH only and SAH plus vehicle group (p < 0.05). Induction of iNOS-mRNA and protein in basilar artery by SAH was not significantly diminished by CPA. The SAH-induced suppression of eNOS-mRNA and protein were relieved by CPA treatment. Conclusions. This is the first evidence to show an adenosine A1 receptor agonist is effective in partially preventing SAH-induced vasospasm without significant cardiovascular complications. The mechanisms of adenosine A1 receptor agonists in attenuating SAH-induced vasospasm may be, in part, related to preserve the normal eNOS expression after SAH. Inability in reversing the increased iNOS expression after SAH may lead to the incomplete anti-spastic effect of CPA.

Receptors subtypes involved in adenosine-mediated modulation of norepinephrine release from cardiac nerve terminals

The objective of this study was to determine which adenosine receptor subtypes were involved in the modulation of norepinephrine release from cardiac nerve terminals. In addition, the persistence of adenosine-mediated effects was evaluated. Rat hearts attached to the stellate ganglion were isolated and perfused. The ganglion was electrically stimulated twice (S1 and S2), allowing 10 min between the stimulations. To determine adenosine receptor subtypes, selective and nonselective adenosine agonists and antagonists were infused following S1 and until the end of S2. To evaluate the persistence of adenosine-mediated effect on norepinephrine release, the stellate ganglion was stimulated a third (S3) and fourth (S4) time. Coronary effluents were collected to determine norepinephrine content. Adenosine and a selective A1 receptor agonist, CCPA, inhibited norepinephrine release by 49% and 54%, respectively. This effect was reversed by simultaneous infusion of nonspecific (8-SPT) and specific (DPCPX) A1 receptor antagonists. Selective A2A (CGS 21680) and A3 (AB-MECA) receptor agonists had no discernible effect on norepinephrine release. Similarly, adenosine A2A receptor antagonists CSC and DMPX did not alter the dose-response relation between norepinephrine release and adenosine. Finally, the inhibitory effects of adenosine on norepinephrine release did not persist 10 min subsequent to the removal of adenosine. Adenosine inhibited norepinephrine release primarily via the adenosine A1 receptor. This effect of adenosine was of short duration. Adenosine A2A and A3 receptors were either absent or functionally insignificant in the regulation of norepinephrine release in the rat heart.

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.

Animal models for the study of adenosine receptor function

Adenosine receptors represent a family of G-protein coupled receptors that are ubiquitously expressed in a wide variety of tissues. This family contains four receptor subtypes: A1 and A3, which mediate inhibition of adenylyl cyclase; and A2a and A2b, which mediate stimulation of this enzyme. Currently, all receptor subtypes have been genetically deleted in mouse models except for the A2b adenosine receptor, and some have been overexpressed in selective tissues of transgenic mice. Studies involving these transgenic mice indicated that receptor levels are rate limiting, as effects were amplified upon increases in receptor level. The knockout models pointed to clusters of activities related to the physiologies of the cardiovascular and the nervous systems, which are either reduced or enhanced upon specific receptor deletion. Interestingly, the trend of effects on these systems is similar in the A1 and A3 adenosine receptor knockout mice and opposite to the effects observed in the A2a adenosine receptor knockout model. This review summarizes in vitro studies on pathways affected by each adenosine receptor, and primarily focuses on the above in vivo models generated to investigate the physiologic role of adenosine receptors. Furthermore, it illustrates the need for multiple adenosine receptor subtype deficiency studies in mice and the deletion of the A2b subtype.

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.

Analysis of adenosine vascular effect in isolated rat aorta: possible role of Na+/K+-ATPase

The present experiments were undertaken in order to examine the effect of adenosine in isolated rat aorta, to investigate the possible role of intact endothelium and endothelial relaxing factors in this action and to determine which population of adenosine receptors is involved in rat aorta response to adenosine. Adenosine (0.1-300 microM) produced concentration-dependent (intact rings: pD2=4.39+/-0.09) and endothelium-independent (denuded rings: pD2=4.52+/-0.12) relaxation of isolated rat aorta. In the presence of high concentration of K+ (100 mM) adenosine-evoked relaxation was significantly reduced (maximal relaxation in denuded rings: control - 92.1+/-9.8 versus K+- 54.4+/-5.0). Similar results were obtained after incubation of ouabain (100 microM) or glibenclamide (1 microM). In K+-free solution, K+ (1-10 mM)-induced rat aorta relaxant response was significantly inhibited by ouabain (100 microM). Application of indomethacin (10 microM), NG-nitro-L-arginine (10 microM) or tetraethylammonium (500 microM) did not alter the adenosine-elicited effect in rat aorta. 8-(3-Chlorostyril)-caffeine (0.3-3 microM), a selective A2A-receptor antagonist, significantly reduced adenosine-induced relaxation of rat aorta in a concentration-dependent manner (pKB=6.57). Conversely, 1,3-dipropyl-8-cyclopentylxanthine (10 nM), an A1-receptor antagonist, did not affect adenosine-evoked dilatation. These results indicate that in isolated rat aorta, adenosine produces endothelium-independent relaxation, which is most probably dependent upon activation of smooth muscle Na+/K+-ATPase, and opening of ATP-sensitive K+ channels, to a smaller extent. According to receptor analysis, vasorelaxant action of adenosine in rat aorta is partly induced by activation of smooth muscle adenosine A2A receptors.

Contribution of adenosine to the depression of sympathetically evoked vasoconstriction induced by systemic hypoxia in the rat

Previous studies have shown that systemic hypoxia evokes vasodilatation in skeletal muscle that is mediated mainly by adenosine acting on A1 receptors, and that the vasoconstrictor effects of sympathetic nerve activity are depressed during hypoxia. The aim of the present study was to investigate the role of adenosine in this depression. In anaesthetised rats, increases in femoral vascular resistance (FVR) evoked by stimulation of the lumbar sympathetic chain with bursts of impulses at 40 or 20 Hz were greater than those evoked by continuous stimulation at 2 Hz with the same number of impulses (120) over 1 min. All of these responses were substantially reduced by infusion of adenosine or by graded systemic hypoxia (breathing 12, 10 or 8 % O2), increases in FVR evoked by continuous stimulation at 2 Hz being most vulnerable. Blockade of A1 receptors ameliorated the depression caused by adenosine infusion of the increase in FVR evoked by 2 Hz only and did not ameliorate the depression caused by 8 % O2 of increases in FVR evoked by any pattern of sympathetic stimulation. A2A receptor blockade accentuated hypoxia-induced depression of the increase in FVR evoked by burst stimulation at 40 Hz, but had no other effect. Neither A1 nor A2A receptor blockade affected the depression caused by hypoxia (8 % O2) of the FVR increase evoked by noradrenaline infusion. These results indicate that endogenously released adenosine is not responsible for the depression of sympathetically evoked muscle vasoconstriction caused by systemic hypoxia; adenosine may exert a presynaptic facilitatory influence on the vasoconstrictor responses evoked by bursts at high frequency.

Role of endogenous adenosine as a modulator of syncope induced during tilt testing

BACKGROUND

Previous reports that used head-up tilt testing and adenosine administration have suggested that adenosine may be an important endogenous mediator that may trigger a vasovagal response in susceptible patients. However, little is known regarding endogenous adenosine plasma levels (APLs) during vasovagal syncope provoked by tilt testing. The aim of this study was to determine whether APLs differ in patients with a positive head-up tilt test compared with those with a negative test and whether APLs are modified during tilt-induced vasovagal syncope.

METHODS AND RESULTS

APLs (mean+/-SEM) were measured during head-up tilt test in 26 patients who presented with unexplained syncope. In the 15 patients with a negative test, APLs were 0.39+/-0.03 micromol/L at baseline, 0.22+/-0.03 micromol/L immediately after tilting, and 0.44+/-0.03 micromol/L after 45 minutes. APLs were significantly higher in the 11 patients with a positive test (2.66+/-0.67 micromol/L at baseline and 3.22+/-0.85 micromol/L immediately after tilting) than in those with a negative test. During tilt testing-induced syncope, APLs increased to reach 4.03+/-0.66 micromol/L (ie, a 52% increase compared with baseline levels; P<0.02). Furthermore, we observed that the higher the APL during syncope, the shorter the time to appearance of symptoms.

CONCLUSIONS

This study showed that APLs were higher in patients with a positive tilt test than in patients with a negative test and that they increased during tilt testing-induced syncope. These observations suggest that adenosine release may be involved in the triggering mechanism of syncope induced during tilt testing.

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.

Adenosine-mediated hypotension in in vivo guinea-pig: receptors involved and role of NO

1. Adenosine produced a biphasic lowering of the mean BP with a drastic bradycardic effect at the highest doses. The first phase hypotensive response was significantly reduced by the nitric oxide (NO) synthase inhibitor L-NAME. 2. The A(2a)/A(2b) agonist NECA produced hypotensive and bradycardic responses similar to those elicited by adenosine, which were not significantly modified by the A(2b) antagonist enprofylline. 3. The A(2a) agonist CGS 21680 did not significantly influence basal HR while induced a hypotensive response antagonized by the A(2a) selective antagonist ZM 241385, and reduced by both L-NAME and the guanylate cyclase inhibitor methylene blue. 4. The A(1) agonist R-PIA showed a dose-dependent decrease in BP with a drastic decrease in HR at the highest doses. The A(1) selective antagonist DPCPX significantly reduced the bradycardic activity and also the hypotensive responses obtained with the lowest doses while it increased those obtained with the highest ones. 5. The A(1)/A(3) agonist APNEA, in the presence of the xanthinic non-selective antagonist 8-pSPT, maintained a significant hypotensive, but not bradycardic, activity, not abolished by the histamine antagonist diphenhydramine. 6. The selective A(3) agonist IB-MECA revealed a weak hypotensive and bradycardic effect, but only at the highest doses. 7. In conclusion, in the systemic cardiovascular response to adenosine two major components may be relevant: an A(2a)- and NO-mediated hypotension, and a bradycardic effect with a consequent hypotension, via atypical A(1) receptors. Finally, an 8-pSPT-resistant hypotensive response not attributable to A(3) receptor-stimulation or to release of histamine by mastocytes or other immune cells was observed.

Modulation of catecholamine release by endogenous adenosine in the rat adrenal medulla

Adenosine was shown to inhibit norepinephrine (NE) release from sympathetic nerve endings. The purpose of this study was to examine whether endogenous adenosine restrains NE and epinephrine release from the adrenal medulla. The effects of an adenosine receptor antagonist, 1,3-dipropyl-8-(p-sulfophenyl) xanthine (DPSPX), on epinephrine and NE release induced by intravenous administration of insulin in conscious rats were examined. Plasma catecholamines were measured by HPLC with an electrochemical detector. DPSPX significantly increased plasma catecholamine in both control rats and rats treated with insulin. The effect of DPSPX on plasma catecholamine was significantly greater in rats treated with insulin. Additional experiments were performed in adrenalectomized rats to investigate the contribution of the adrenal medulla to the effect of DPSPX on plasma catecholamine. The effect of DPSPX and insulin on epinephrine in adrenalectomized rats was significantly reduced compared with that of the controls. Finally, we tested whether endogenous adenosine restrains catecholamine secretion partially through inhibiting the renin-angiotensin system. The effect of DPSPX on plasma catecholamine in rats pretreated with captopril (an angiotensin-converting enzyme inhibitor) was reduced. These results demonstrate that under basal physiological conditions, endogenous adenosine tonically inhibits catecholamine secretion from the adrenal medulla, and this effect is augmented when the sympathetic system is stimulated. The effect of endogenous adenosine on catecholamine secretion from the adrenal medulla is achieved partially through the inhibitory effect of adenosine on the renin-angiotensin system.

Differential effects of omega-conotoxin GVIA, tetrodotoxin and prolonged cold storage on purinergic and adrenergic transmission in isolated canine splenic artery

Double-peaked vasoconstrictions (biphases of vasoconstrictions) were readily induced in the conditions of 30 s trains of pulses at 1 Hz in the isolated, perfused canine splenic artery. P2X purinoceptors have previously been shown to be involved mainfy in the first-peaked response and alpha1-adrenoceptors mostly in the second. The treatment with 10 nM omega-conotoxin GVIA (omega-CTX) produced a parallel inhibitory effect on the first- and second-peaked vasoconstrictor responses to nerve stimulation. A submaximal concentration of tetrodotoxin (TTX) (3 nM) did not affect the first peak of constriction, but strongly inhibited the second peak, although a larger dose of TTX (30 nM) abolished either the first- or second-peaked response. On the other hand, after cold storage at 4 degrees C for 7 days, the first-peaked vasoconstriction markedly decreased, whereas the second-peaked response was not significantly modified.

IN CONCLUSION

(1) omega-CTX-sensitive calcium channels may produce a parallel modulation of purinergic and adrenergic components of sympathetic cotransmission; (2) TTX-sensitive sodium channels may have a more important role in controlling the adrenergic rather than purinergic transmission; and (3) the function of purinergic transmission of sympathetic nerve might be affected more strongly than that of adrenergic transmission in the cold-stored canine splenic artery.

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.

Characterization of adenosine action in isolated rat renal artery. Possible role of adenosine A(2A) receptors

Adenosine (0.1-300 microM) induced concentration- and endothelium-dependent relaxation of rat renal artery (RRA). N(G)-Nitro-L-arginine (L-NOARG, 10 microM) significantly reduced adenosine-elicited dilatation, but not the application of indomethacin (10 microM), ouabain (100 microM) or tetraethylammonium (TEA, 500 microM). In the presence of high concentration of K(+) (100 mM) or glibenclamide (1 microM), adenosine-evoked relaxation was almost abolished. 8-(3-Chlorostyril)caffeine (CSC, 0.3-3 microM), a selective A(2A)-antagonist, significantly reduced adenosine-evoked dilatation in a concentration-dependent manner (pA(2)=7.29). Conversely, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM), an A(1)-antagonist, did not alter adenosine-induced relaxation. These results indicate that adenosine produces endothelium-dependent relaxation of isolated RRA. Dilatation evoked by adenosine is mediated by predominant releasing of endothelium-derived hiperpolarizing factor (EDHF) and also in one part of nitric oxide (NO) from endothelial cells. The obtained results also suggest that RRA response to adenosine is most likely initiated by activation of endothelial adenosine A(2A) 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.

Prejunctional effects of angiotensin II and bradykinin in the heart and blood vessels

1. Angiotensin and bradykinin facilitate the release of noradrenaline from sympathetic nerve terminals and cause positive inotropy in rat isolated atria and ventricles. The effect of bradykinin was enhanced by the ACE inhibitor, ramiprilat. 2. The facilitated release of noradrenaline in rat ventricle by bradykinin was blocked by the beta2-receptor antagonist HOE-140. This response is also reduced by removing the endocardium, suggesting the release of a mediator from the endocardium. 3. The facilitated noradrenaline release by angiotensin II and bradykinin was blocked by the angiotensin receptor antagonist saralasin to the same extent. In contrast, losartan caused only minor blockade in a range of vascular and cardiac tissues. This suggests that angiotensin and bradykinin exert these responses by interacting with a prejunctional receptor different from the established AT1 subtype. 4. These results suggest that bradykinin mediates facilitation of noradrenaline release via the local release of angiotensin onto an atypical AT1 receptor.

Noradrenaline release from cultured mouse postganglionic sympathetic neurons: autoreceptor-mediated modulation

The possible existence of alpha2-autoreceptors, P2-autoreceptors, and adenosine A1- or A2A-receptors was studied in cultured thoracolumbar postganglionic sympathetic neurons from mice. The cells were preincubated with [3H]noradrenaline and then superfused. The selective alpha2-adrenoceptor agonist UK 14,304 reduced the electrically evoked overflow of tritium. When the cultures were stimulated by trains of increasing pulse number, ranging from a single pulse to 72 pulses at 3 Hz, the concentration-inhibition curve of UK 14,304 was shifted progressively to the right and the maximal inhibition obtainable became progressively smaller. Six alpha-adrenoceptor antagonists shifted the concentration-inhibition curve of UK 14,304 in a parallel manner to the right. Neither ATP (3-300 microM), adenosine (0.01-100 microM), the selective A1-receptor agonist cyclopentyladenosine (1-1,000 nM), nor the selective A2A-receptor agonist CGS-21680 (1-10,000 nM) changed the basal or the electrically evoked overflow of tritium. It is concluded that the cultured neurons possess presynaptic, release-inhibiting alpha2-autoreceptors. As in intact tissues, the effectiveness of presynaptic alpha2-adrenergic inhibition depends on the "strength" of the releasing stimulus. The pK(D) values of the six antagonists against UK 14,304 indicate that the autoreceptors belong to the pharmacological alpha2D and hence the genetic alpha(2A/D) subtype of alpha2-adrenoceptor. Neither P2-autoreceptors nor receptors for adenosine, the degradation product of ATP, were detected.

Systemic adenosine increase during cold pressor test is dependent on sympathetic activation

1. Following local vasoconstriction-inducing stimuli, such as the cold pressor test (CPT), significant changes occur in haemodynamics, with a rise in arterial blood pressure and heart rate (HR) due to the activation of the sympathetic nervous system. Among the compensatory mechanisms to local ischaemia, the endogenous nucleoside adenosine (ADO) has been suggested to play a relevant role by contributing to sympathetic stimulation. The possibility was investigated that CPT-induced increases in plasma ADO levels were not only an expression of the increased production of ADO in the ischaemic area, but also a consequence of systemic sympathoexcitatory mechanisms, thus showing a bidirectional involvement of the mechanisms of ADO formation. 2. The CPT was performed in 15 volunteers and mean arterial blood pressure (MABP) and HR were evaluated, together with plasma levels of noradrenaline (NA) and ADO in the tested and contralateral arm. The 15 subjects were then divided into three groups of five that were treated with either 5 mg transdermal clonidine weekly, 100 mg atenolol daily or 600 mg aminophylline twice daily. After 1 week treatment, the same test was repeated in the respective groups. 3. The CPT induced a rise in MABP and HR and an increase in plasma levels of NA and ADO. Increases in ADO were more pronounced in the tested arm. Clonidine blunted the haemodynamic response and NA release, while increases in ADO increase were reduced to a greater extent in the contralateral arm rather than the tested arm. Atenolol only affected MABP and HR without any effect on NA and ADO levels. Theophylline did not show any effect on CPT-induced changes. 4. In conclusion, local vasoconstriction and ischaemia induced in one arm following CPT are associated with haemodynamic changes dependent on the activation of the sympathetic system. The observed increase in plasma levels of ADO seems to be, in part, a direct expression of local responses to ischaemia (pre-dominant in the tested arm), but also appears as the consequence of systemic sympathoexcitatory mechanisms. Such increases in ADO are not dependent on a beta 1-adrenoceptor-mediated mechanism. Finally, theophylline, at a therapeutic dose, has no effect on the response to CPT.

Differential blocking effects of tetrodotoxin on double-peaked vasoconstrictor responses to periarterial nerve stimulation in canine isolated, perfused splenic artery

1. In the present study, we investigated the effects of progressive inhibition of neuronal sodium channels by increasing concentrations of tetrodotoxin (TTX; 1-30 nmol/L) on the double-peaked vasoconstrictor responses to electrical periarterial nerve stimulation in the canine isolated and perfused splenic artery. 2. Double-peaked vasoconstrictions (biphasic vasoconstrictor responses) were consistently observed in following electrical stimulation with 30 s trains of pulses at 1-10 Hz. At low frequencies of stimulation (1-3 Hz), a submaximal concentration of 3 nmol/L TTX had no effect on the first phase of the contractile response, but almost completely inhibited the second-phase response. At high frequencies (6-10 Hz), the two vasoconstrictor phases were almost equally inhibited by 50% by 3 nmol/L TTX. A three-fold increase in the concentration of TTX used (10 nmol/L) abolished the second-phase vasoconstriction at all stimulation frequencies tested, whereas this concentration of TTX failed to block the first-phase response. Further increasing the concentration of TTX to 30 nmol/L completely blocked the remaining first-phase response. 3. Treatment with 0.1 mumol/L prazosin did not modify the first-phase response to any of the stimulation frequencies in the presence of 3 nmol/L TTX. Moreover, 0.1 mumol/L prazosin had no affect on the second-phase response at low frequencies (1-3 Hz), while at high frequencies (6-10 Hz) it slightly, but significantly inhibited the second-phase response. The vasoconstrictor responses that persisted after 3 nmol/L TTX and 0.1 mumol/L prazosin were completely suppressed by subsequent application of 1 mumol/L alpha, beta-methylene ATP at all stimulation frequencies (1-10 Hz). 4. In conclusion, progressive inhibition of sodium channels by increasing the concentration of TTX may exert a more preferential inhibition on adrenergic rather than purinergic components, suggesting that TTX-sensitive sodium channels may have a more important role in determining the adrenergic rather than purinergic transmission of sympathetic nerves.

Adrenergic-purinergic interactions on vasoconstrictor responses to periarterial electric nerve stimulation in canine splenic arteries

1. Periarterial nerve electrical stimulation caused a double peaked vasoconstriction in isolated perfused canine splenic arterial preparations. At low frequencies (1-3 Hz), the 1st peak responses were significantly inhibited by alpha,beta-methylene ATP. On the other hand, at high frequencies (8-10 Hz), the responses were not completely inhibited by alpha,beta-methylene ATP but the remaining response was abolished by an additional treatment of prazosin. 2. Concerning the 2nd peak responses, at low frequencies (1-3 Hz), the response was mostly suppressed by alpha,beta-methylene ATP, but at high frequencies (6-10 Hz), the response was not significantly modified by it, although the remaining responses were completely blocked by prazosin. Thus, at high frequencies an adrenergic and purinergic interaction may exist presynaptically, to prevent the inhibition by alpha,beta-methylene ATP. 3. At 1 Hz, rauwolscine, an alpha2-adrenoceptor antagonist, caused a potentiation of electrical stimulation-induced responses (both 1st and 2nd peaked responses) which were inhibited by prazosin, and the remaining ones were abolished by alpha,beta-methylene ATP. On the other hand, at 10 Hz, rauwolscine did not cause any potentiation of the double peaked responses. 4. The biphasic responses at 1 Hz were strongly inhibited by exogenously applied ATP, and its inhibition was reversed in part by a P1 receptor antagonist 8-phenyltheophylline (8-PT). On the other hand, the biphasic vasoconstrictions at 10 Hz were only slightly depressed by ATP, and a subsequent administration of 8-PT produced a partial recovery of the 1st phase response but not that of the 2nd one. 5. From these results, it is concluded that (1) at low frequencies the double peaked responses are mostly mediated via P2X receptor, presynaptic P1 receptors may also modulate the release of ATP, and presynaptic alpha2-adrenergic mechanism may tonically participate in the release of noradrenaline (2) at high frequencies the responses are mostly mediated via alpha1-adrenoceptors and presynaptic P2 receptors may exert its action to inhibit the release of noradrenaline from adrenergic nerve terminals.

Effects of A1-adenosine receptor antagonists on purinergic transmission in the guinea-pig vas deferens in vitro

1. Intracellularly recorded excitatory junction potentials (ej.ps) were used to study the effects of adenosine receptor antagonists on neurotransmitter release from postganglionic sympathetic nerve terminals in the guinea-pig vas deferens in vitro. 2. The A1 adenosine receptor antagonists, 8-phenyltheophylline (10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM), increased the amplitude of e.j.ps evoked during trains of 20 stimuli at 1 Hz in the presence, but not in the absence, of the alpha2-adrenoceptor antagonist, yohimbine (1 microM) or the non-selective alpha-adrenoceptor antagonist, phentolamine (1 microM). 3. Adenosine (100 microM) reduced the amplitude of e.j.ps, both in the presence and in the absence of phentolamine (1 microM). This inhibitory effect of adenosine is most likely caused by a reduction in transmitter release as there was no detectable change in spontaneous ej.p. amplitudes. 4. In the presence of phentolamine, application of the adenosine uptake inhibitor, S-(p-nitrobenzyl)-6-thioinosine (0.1 microM), had no effect on ej.p. amplitudes. 5. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (100 microM), significantly increased the amplitudes of all e.j.ps evoked during trains of 20 stimuli at 1 Hz, both in the presence and in the absence of phentolamine (1 microM). 6. These results suggest that endogenous adenosine modulates neurotransmitter release by an action at prejunctional A1 adenosine receptors only when alpha2-adrenoceptors are blocked.

Adrenergic-dependent Effect of Adenosine-induced Ventricular Fibrillation in the Isolated Rabbit Heart

BACKGROUND: The present study examined the contributory role of endogenous catecholamines in adenosine-induced ventricular fibrillation in isolation rabbit hearts. METHODS AND RESULTS: Cardiac catecholamine depletion was induced in eleven rabbits by the administration of 6-hydroxydopamine (2 x 30 mg/kg, every 12 hours intramuscularly). Hearts were removed 24 hours later, and subjected to 12 minutes of hypoxic perfusion followed by 40 minutes of reoxygenation while heart rate was maintained with atrial pacing. One of six, and one of five hearts from 6-hydroxydopamine treated rabbits developed ventricular fibrillation during hypoxia-reoxygenation when exposed to 3,7-dimethyl-1-propargylzanthine (DMPX) (10 µM) + adenosine (ADO) (1 µM) and DMPX (10 µM) + ADO (10 µM), respectively. In hearts from a control group, not exposed to 6-hydroxydopamine, ventricular fibrillation developed in each of five (100% incidence) hearts when perfused in the presence of DMPX (10 µM) + ADO (10 µM) (P <.05). Nadolol (1 µM), a beta-adrenoceptor DMPX (10 µM) + ADO (10 µM) treated hearts (n = 6, P <.05 vs DMPX + ADO treated hearts). To ensure catecholamine depletion, spontaneously beating isolated hearts from vehicle and 6-hydroxydopamine treated rabbits were perfused under normoxic conditions while exposed to increasing concentrations of tyramine (1, 3, 10 mM) and the change in heart rate was determined. A concentration-related, positive chronotorpic response to tyramine was obtained in hearts from the vehicle treated group that was absent in hearts from 6-hydroxy-dopamine treated rabbits or hearts perfused in the presence of nadolol. CONCLUSIONS: The results demonstrate that inhibition of the cardiac adenosine A(2) receptor, unmasks an adenosine A(1) receptor profibrillatory effect that is dependent upon endogenous cardiac catecholamines and beta-adrenoreceptor activation during myocardial hypoxia-reoxygenation.

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.

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.

Modulation by extracellular ATP of L-type calcium channels in guinea-pig single sinoatrial nodal cell

1. The effects of extracellular adenosine 5'-triphosphate ([ATP]zero) on the L-type Ca2+ channel currents in guinea-pig single sinoatrial nodal (SAN) cells, isolated by enzymatic dissociation, were investigated by use of whole-cell patch-clamp techniques. 2. The application of [ATP]zero (2 microM-1 mM) produced an inhibitory effect on the L-type Ca2+ channel current peak amplitude (10 mM Ba2+ as charge carrier) in a concentration-dependent and reversible manner with an IC50 of 100 microM and a Hill coefficient of 1.83. 3. The presence of the adenosine receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 microM) and 8-phenyltheophylline (10 microM) did not affect the [ATP]zero-induced inhibition of the Ca2+ channel currents. Adenosine (100 microM) had little effect on the basal Ca2+ channel currents. Adenosine 500 microM, caused 23% inhibition of the Ca2+ channel current, which was abolished by 0.1 microM DPCPX. 4. The presence of the P2-purinoceptor antagonists, suramin (1, 10 and 100 microM), reactive blue 2 (1 and 10 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 50 and 100 microM) failed to affect the inhibitory action of [ATP]zero on Ca2+ channel currents. 5. The relative rank order of potency of different nucleotides and nucleosides, at a concentration of 100 microM, on the inhibition of the Ca2+ channel currents is as follows: adenosine 5'-triphosphate (ATP) = alpha,beta-methylene-ATP (alpha,beta MeATP) > > 2-methylthioATP (2-MeSATP) > or = adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) > > uridine 5'-triphosphate (UTP) = adenosine 5'-diphosphate (ADP) > adenosine 5'-monophosphate (AMP) > or = adenosine. 6. These results suggest that [ATP]zero may play an important role in the heart beat by inhibiting the L-type Ca2+ channel currents in single SAN cells. This inhibitory effect is not due to the formation of adenosine resulting from the enzymatic degradation of [ATP]zero. Based on the relative order of inhibitory potency of different nucleotides and nucleosides on the L-type Ca2+ channel currents and the ineffectiveness of the purinoceptor antagonists tested, a novel type of purinoceptor may be involved.