P2X (purinergic) receptor distributions in rat blood vessels

Purinergic regulation of pulmonary vascular tone

Purinergic signaling is a crucial determinant in the regulation of pulmonary vascular physiology and presents a promising avenue for addressing lung diseases. This intricate signaling system encompasses two primary receptor classes: P1 and P2 receptors. P1 receptors selectively bind adenosine, while P2 receptors exhibit an affinity for ATP, ADP, UTP, and UDP. Functionally, P1 receptors are associated with vasodilation, while P2 receptors mediate vasoconstriction, particularly in basally relaxed vessels, through modulation of intracellular Ca2+ levels. The P2X subtype receptors facilitate extracellular Ca2+ influx, while the P2Y subtype receptors are linked to endoplasmic reticulum Ca2+ release. Notably, the primary receptor responsible for ATP-induced vasoconstriction is P2X1, with α,β-meATP and UDP being identified as potent vasoconstrictor agonists. Interestingly, ATP has been shown to induce endothelium-dependent vasodilation in pre-constricted vessels, associated with nitric oxide (NO) release. In the context of P1 receptors, adenosine stimulation of pulmonary vessels has been unequivocally demonstrated to induce vasodilation, with a clear dependency on the A2B receptor, as evidenced in studies involving guinea pigs and rats. Importantly, evidence strongly suggests that this vasodilation occurs independently of endothelium-mediated mechanisms. Furthermore, studies have revealed variations in the expression of purinergic receptors across different vessel sizes, with reports indicating notably higher expression of P2Y1, P2Y2, and P2Y4 receptors in small pulmonary arteries. While the existing evidence in this area is still emerging, it underscores the urgent need for a comprehensive examination of the specific characteristics of purinergic signaling in the regulation of pulmonary vascular tone, particularly focusing on the disparities observed across different intrapulmonary vessel sizes. Consequently, this review aims to meticulously explore the current evidence regarding the role of purinergic signaling in pulmonary vascular tone regulation, with a specific emphasis on the variations observed in intrapulmonary vessel sizes. This endeavor is critical, as purinergic signaling holds substantial promise in the modulation of vascular tone and in the proactive prevention and treatment of pulmonary vascular diseases.

On P2X receptors in the brain: microvessels. Dedicated to the memory of the late Professor Geoffrey Burnstock (1929-2020)

This tribute article presents selected immunocytochemical and transmission electron microscope data on the location of ATP-gated P2X receptor in the rat brain, as studied in the 1990s in Prof G. Burnstock's laboratory at University College London. There are examples of immuno-ultrastructural findings and introductory information about pre- and post-synaptic location of P2X receptors in the rat cerebellum and endocrine hypothalamus to support the concept of purinergic transmission in the central nervous system. Then findings of diverse immunoreactivity for P2X1, P2X2, P2X4, and P2X6 receptors associated with brain microvessels are shown, including vascular endothelium and pericytes as well as perivascular astrocytes and neuronal components. These findings imply the involvement of P2X receptors and hence purinergic signalling in the neurovascular unit, at least in microvessels in the rat cerebellum and hypothalamic paraventricular and supraoptic nuclei examined here. Various aspects of P2X receptors in brain microvessels are discussed.

History of Geoff Burnstock's research on P2 receptors

Geoffrey Burnstock is a purinergic signalling legend who's discoveries and conceptualisation created and shaped the field. His scientific achievements were extraordinary and sustained. They included his demonstration that ATP can act as a neurotransmitter and hence extracellular signalling molecule, which he championed despite considerable initial opposition to his proposal that ATP acts outside of its role as an energy source inside cells. He led on purine receptor classification: initially of the P1 and P2 receptor families, then the P2X and P2Y receptor families, and then subtypes of P2X and P2Y receptors. This was achieved across several decades as he conceptualised and made sense of the emerging and growing evidence that there were multiple receptor subtypes for ATP and other nucleotides. He made discoveries about short term and long term/trophic purinergic signalling. He was a leader in the field for over 50 years. He inspired many and was a great colleague and mentor. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, University College London. This review is a personal perspective of some of Geoff's research on P2 receptors carried out during that time. It is a tribute to Geoff who I regarded with enormous respect and admiration.

Purinergic signalling in the cardiovascular system-a tribute to Geoffrey Burnstock

Geoffrey Burnstock made groundbreaking discoveries on the physiological roles of purinergic receptors and led on P2 purinergic receptor classification. His knowledge, vision and leadership inspired and influenced the international scientific community. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, initially as a PhD student and then as a postdoctoral research fellow. I regarded him with enormous admiration and affection. This review on purinergic signalling in the cardiovascular system is a tribute to Geoff. It includes some personal recollections of Geoff.

Purinergic Signaling During Hyperglycemia in Vascular Smooth Muscle Cells

The activation of purinergic receptors by nucleotides and/or nucleosides plays an important role in the control of vascular function, including modulation of vascular smooth muscle excitability, and vascular reactivity. Accordingly, purinergic receptor actions, acting as either ion channels (P2X) or G protein-coupled receptors (GCPRs) (P1, P2Y), target diverse downstream effectors, and substrates to regulate vascular smooth muscle function and vascular reactivity. Both vasorelaxant and vasoconstrictive effects have been shown to be mediated by different purinergic receptors in a vascular bed- and species-specific manner. Purinergic signaling has been shown to play a key role in altering vascular smooth muscle excitability and vascular reactivity following acute and short-term elevations in extracellular glucose (e.g., hyperglycemia). Moreover, there is evidence that vascular smooth muscle excitability and vascular reactivity is severely impaired during diabetes and that this is mediated, at least in part, by activation of purinergic receptors. Thus, purinergic receptors present themselves as important candidates mediating vascular reactivity in hyperglycemia, with potentially important clinical and therapeutic potential. In this review, we provide a narrative summarizing our current understanding of the expression, function, and signaling of purinergic receptors specifically in vascular smooth muscle cells and discuss their role in vascular complications following hyperglycemia and diabetes.

ATP-Gated P2X3 Receptors Are Specialised Sensors of the Extracellular Environment

P2X3 receptors are ion channels expressed by autonomic and sensory nerves and specialised in transducing extracellular ATP signals. Structural data, together with functional and biochemical studies, suggest that conformational changes of P2X3 receptors upon agonist binding influence downstream intracellular molecular mechanisms relevant for neuronal responses. Activity of P2X3 receptors is implicated in pain, itch, asthma, cardiovascular dysfunction and other pathologies. The study of these receptors has therefore a large potential in the field of drug development and interdisciplinary efforts could clarify molecular mechanisms controlling P2X3 receptor function in different physiological or pathological contexts.

ATP is not involved in α1-adrenoceptor-mediated vasoconstriction in resistance arteries

Recent publications suggest that α1-adrenoceptor stimulation by exogenous agonists such as phenylephrine in resistance arteries cause contraction through the release of ATP from within the vascular smooth muscle cells. This ATP exits the cell through pannexin-1 channels to act back "autocrine-like" on P2 receptors on the smooth muscle that cause the contraction. In this work we directly test this hypothesis by using a selective P2X1 purinoceptor antagonist NF449 (1-10µM) against phenylephrine and ATP concentration-response curves in small mesenteric arteries of the rat and thoracodorsal arteries of the mouse. We show that NF449 is a simple competitive antagonist of ATP with a pKB of 6.43 and 6.41 in rat and mouse arteries, respectively, but did not antagonise phenylephrine concentration-response curves. This work cautions against the growing overstated role of the reputed pannexin-1/ATP release axis following α1-adrenoceptor activation in small resistance arteries.

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.

Localization of α-adrenoceptors: JR Vane Medal Lecture

This review is based on the JR Vane Medal Lecture presented at the BPS Winter Meeting in December 2011 by J.C. McGrath. A recording of the lecture is included as supporting information. It covers his laboratory's work from 1990 to 2010 on the localization of vascular α1 -adrenoceptors in native tissues, mainly arteries.

MAIN POINTS

(i) α1 -adrenoceptors are present on several cell types in arteries, not only on medial smooth muscle, but also on adventitial, endothelial and nerve cells; (ii) all three receptor subtypes (α1 A , α1 B , α1 D ) are capable of binding ligands at the cell surface, strongly indicating that they are capable of function and not merely expressed. (iii) all of these cell types can take up an antagonist ligand into the intracellular compartments to which endocytosing receptors move; (iv) each individual subtype can exist at the cell surface and intracellularly in the absence of the other subtypes. As functional pharmacological experiments show variations in the involvement of the different subtypes in contractions of different arteries, it is concluded that the presence and disposition of α1 -adrenoceptors in arteries is not a simple guide to their involvement in function. Similar locations of the subtypes, even in different cell types, suggest that differences between the distribution of subtypes in model systems do not directly correlate with those in native tissues. This review includes a historical summary of the alternative terms used for adrenoceptors (adrenergic receptors, adrenoreceptors) and the author's views on the use of colours to illustrate different items, given his partial colour-blindness.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

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.

Introduction and perspective, historical note

P2 nucleotide receptors were proposed to consist of two subfamilies based on pharmacology in 1985, named P2X and P2Y receptors. Later, this was confirmed following cloning of the receptors for nucleotides and studies of transduction mechanisms in the early 1990s. P2X receptors are ion channels and seven subtypes are recognized that form trimeric homomultimers or heteromultimers. P2X receptors are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed on many types of non-neuronal cells to mediate smooth muscle contraction, secretion, and immune modulation. The emphasis in this review will be on the pathophysiology of P2X receptors and therapeutic potential of P2X receptor agonists and antagonists for neurodegenerative and inflammatory disorders, visceral and neuropathic pain, irritable bowel syndrome, diabetes, kidney failure, bladder incontinence and cancer, as well as disorders if the special senses, airways, skin, cardiovascular, and musculoskeletal systems.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Blunted coronary vasodilator response to uridine adenosine tetraphosphate in post-infarct remodeled myocardium is due to reduced P1 receptor activation

We previously demonstrated that uridine adenosine tetraphosphate (Up4A) exerts a potent vasodilator effect in the healthy porcine coronary vasculature. Since the coronary microvascular effects of Up4A after myocardial infarction (MI) are unknown, the present study investigated the response to Up4A in coronary microvessels from post-MI remodeled porcine myocardium, and the involvement of purinergic receptor subtypes. Coronary small arteries (diameter ∼150 μm) were dissected from the apex of Sham-operated swine and swine in which MI had been produced 5 weeks earlier by transient (2h) occlusion of the left circumflex coronary artery, and mounted on Mulvany wire myographs. Up4A (10(-9)-10(-5)M) produced coronary vasodilation that was reduced in MI as compared to Sham-operated swine. Up4A-induced vasodilation was reduced by P1 blockade with 8-phenyltheophylline in Sham-operated swine and to a lesser extent in MI, while the attenuation by the A2A receptor blocker SCH58261 was similar in Sham-operated and MI swine. Up4A-induced vasodilation remained unaffected by non-selective P2 receptor antagonist PPADS, but was attenuated by selective P2X1 and P2Y1 receptor antagonists MRS2159 and MRS2179, albeit to a similar extent in Sham-operated and MI swine. These responses were paralleled by similar mRNA expression levels of A2A, P2X1 and P2Y1 receptors in MI compared to slaughterhouse control swine. Finally, attenuation of Up4A-induced coronary vasodilation by nitric oxide synthase inhibition was not attenuated in MI as compared to Sham-operated swine. In conclusion, blunted coronary vasodilation in response to Up4A in MI swine is most likely due to reduced activation of P1, rather than P2, receptors and does not involve a loss of NO bioavailability.

Removal of half the sympathetic innervation does not reduce vasoconstrictor responses in rat tail artery

Following reinnervation of denervated rat tail arteries, nerve-evoked contractions are at least as large as those evoked in normally innervated arteries despite a much lower nerve terminal density. Here nerve-evoked contractions have been investigated after transection of half the sympathetic innervation of normal tail arteries. After 1 week, the noradrenergic plexus 50-70 mm along the tail was about half as dense as control. Excitatory junction potentials recorded in smooth muscle cells of arterial segments isolated in vitro were half their normal amplitude. Surprisingly, nerve-evoked contractions of isometrically mounted segments were not reduced in amplitude, as was also the case after only 3 days. After 1 week, enhancement of nerve-evoked contractions by blocking either neuronal re-uptake of noradrenaline with desmethylimipramine or prejunctional α2-adrenoceptors with idazoxan was similar to control, suggesting that these mechanisms are matched to the number of innervating axons. The relative contribution of postjunctional α2-adrenoceptors to contractions evoked by long trains of stimuli was enhanced but that of α1-adrenoceptors was unchanged. Transiently, sensitivity to the α1-adrenoceptor agonist phenylephrine was slightly increased. After 7 weeks, amplitudes of nerve-evoked contractions remained similar to control, and sensitivity to phenylephrine had recovered but that to the α2-adrenoceptor agonist clonidine was slightly raised. The normal amplitude of nerve-evoked contractions after partial denervation is only partly explained by the greater contribution of α2-adrenoceptors. While the post-receptor mechanisms activated by nerve-released transmitter may be modified to amplify the contractions after partial denervation, our findings suggest that these mechanisms are normally saturated, at least in this artery.

Extracellular ATP signaling in equine digital blood vessels

The functional distribution of ATP-activated P2 receptors is well characterized for many blood vessels, but not in the equine digital vasculature, which is a superficial vascular bed that displays thermoregulatory functions and has been implicated in ischemia-reperfusion injuries of the hoof. Isolated equine digital arteries (EDA) and veins (EDV) were submitted to isometric tension studies, whereby electric field stimulation (EFS) and concentration-response curves to exogenously applied agonists were constructed under low tone conditions. Additionally, immunofluorescent localization of P2X and P2Y receptor subtypes was performed. EFS-induced constriction was abolished by tetrodotoxin (1 μM, n=4). Endothelium denudation did not modify the EFS-induced constriction (n=3). The EFS-induced constriction in EDA was inhibited by phentolamine (67.7±1.8%, n=6; 10 μM), and by the non-selective P2 receptor antagonist suramin (46.2±1.3%, n=6; 10 μM). EFS-induced constriction in EDV was reduced by suramin (48.2±2.4%, n=6; 10 μM), the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (58.3±4.5%, n=6; 10 μM), and phentolamine (23.2±2.5%, n=6; 10 μM). Exogenous methoxamine and ATP mimicked EFS-induced constriction in EDA and EDV. Immunostaining for P2X1, P2X2 and P2X3, and, for P2X1 and P2X7 receptor subunits were observed in EDA and EDV smooth muscle and adventitia, respectively. ATP and noradrenaline are co-transmitters in sympathetic nerves supplying the equine digital vasculature, noradrenaline being the dominant agonist in EDA, and ATP in EDV. In conclusion, P2X receptors mediate vasoconstriction in EDA and EDV, although different P2X subunits are involved in these vessels. The physiological significance of this finding in relation to thermoregulatory functions and equine laminitis is discussed.

A Ca²⁺-dependent chloride current and Ca²⁺ influx via Ca(v)1.2 ion channels play major roles in P2Y receptor-mediated pulmonary vasoconstriction

BACKGROUND AND PURPOSE

ATP, UTP and UDP act at smooth muscle P2X and P2Y receptors to constrict rat intrapulmonary arteries, but the underlying signalling pathways are poorly understood. Here, we determined the roles of the Ca²⁺ -dependent chloride ion current (I(Cl,Ca)), Ca(v)1.2 ion channels and Ca²⁺ influx.

EXPERIMENTAL APPROACH

Isometric tension was recorded from endothelium-denuded rat intrapulmonary artery rings (i.d. 200-500 µm) mounted on a wire myograph.

KEY RESULTS

The I(Cl,Ca) blockers, niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and the Ca(v)1.2 channel blocker, nifedipine, reduced peak amplitude of contractions evoked by UTP and UDP by ∼45-50% and in a non-additive manner. Ca²⁺-free buffer inhibited responses by ∼70%. Niflumic acid and nifedipine similarly depressed contractions to ATP, but Ca²⁺-free buffer almost abolished the response. After peaking, contractions to UTP and UDP decayed slowly by 50-70% to a sustained plateau, which was rapidly inhibited by niflumic acid and nifedipine. Contractions to ATP, however, reversed rapidly and fully. Tannic acid contracted tissues per se and potentiated nucleotide-evoked contractions.

CONCLUSIONS AND IMPLICATIONS

I (Cl,Ca) and Ca²⁺ influx via Ca(v)1.2 ion channels contribute substantially and equally to contractions of rat intrapulmonary arteries evoked by UTP and UDP, via P2Y receptors. ATP also activates these mechanisms via P2Y receptors, but the greater dependence on extracellular Ca²⁺ most likely reflects additional influx through the P2X1 receptor pore. The lack of a sustained response to ATP is probably due to it acting at P2 receptor subtypes that desensitize rapidly. Thus multiple signalling mechanisms contribute to pulmonary artery vasoconstriction mediated by P2 receptors.

Exhaustive swimming differentially inhibits P2X1 receptor- and α1-adrenoceptor-mediated vasoconstriction in isolated rat arteries

AIM

To investigate the effects of exhaustive swimming exercise on P2X1 receptor- and α1-adrenoceptor-mediated vasoconstriction of different types of arteries in rats.

METHODS

Male Wistar rats were divided into 2 groups: the sedentary control group (SCG) and the exhaustive swimming exercise group (ESEG). The rats in the ESEG were subjected to a swim to exhaustion once a day for 2 weeks. Internal carotid, caudal, pulmonary, mesenteric arteries and aorta were dissected out. Isometric vasoconstrictive responses of the arteries to α,β-methylene ATP (α,β-MeATP) or noradrenaline (NA) were recorded using a polygraph.

RESULTS

The exhaustive swimming exercise did not produce significant change in the EC(50) values of α,β-MeATP or NA in vasoconstrictive response of most of the arteries studied. The exhaustive swimming exercise inhibited the vasoconstrictive responses to P2X1 receptor activation in the internal carotid artery, whereas it reduced the maximal vasoconstrictive responses to α1-adrenoceptor stimulation in the caudal, pulmonary, mesenteric arteries and aorta. The rank order of the reduction of the maximal vasoconstriction was as follows: mesenteric, pulmonary, caudal, aorta.

CONCLUSION

Exhaustive swimming exercise differentially affects the P2X1 receptor- and α1-adrenoceptor-regulated vasoconstriction in internal carotid artery and peripheral arteries. The ability to preserve purinergic vasoconstriction in the peripheral arteries would be useful to help in maintenance of the basal vascular tone during exhaustive swimming exercise.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

Physiological significance of P2X receptor-mediated vasoconstriction in five different types of arteries in rats

P2X(1) receptors, the major subtype of P2X receptors in the vascular smooth muscle, are essential for α,β-methylene adenosine 5'-triphosphate (α,β-MeATP)-induced vasoconstriction. However, relative physiological significance of P2X(1) receptor-regulated vasoconstriction in the different types of arteries in the rat is not clear as compared with α(1)-adrenoceptor-regulated vasoconstriction. In the present study, we found that vasoconstrictive responses to noncumulative administration of α,β-MeATP in the rat isolated mesenteric arteries were significantly smaller than those to single concentration administration of α,β-MeATP. Therefore, we firstly reported the characteristic of α,β-MeATP-regulated vasoconstrictions in rat tail, internal carotid, pulmonary, mesenteric arteries, and aorta using single concentration administration of α,β-MeATP. The rank order of maximal vasoconstrictions for α,β-MeATP (E (max·α,β-MeATP)) was the same as that of maximal vasoconstrictions for noradrenaline (E (max·NA)) in the internal carotid, pulmonary, mesenteric arteries, and aorta. Moreover, the value of (E (max·α,β-MeATP)/E (max·KCl))/(E (max·NA)/E (max·KCl)) was 0.4 in each of the four arteries, but it was 0.8 in the tail artery. In conclusion, P2X(1) receptor-mediated vasoconstrictions are equally important in rat internal carotid, pulmonary, mesenteric arteries, and aorta, but much greater in the tail artery, suggesting its special role in physiological function.

P2X receptors in health and disease

Seven P2X receptor subunits have been cloned which form functional homo- and heterotrimers. These are cation-selective channels, equally permeable to Na(+) and K(+) and with significant Ca(2+) permeability. The three-dimensional structure of the P2X receptor is described. The channel pore is formed by the α-helical transmembrane spanning region 2 of each subunit. When ATP binds to a P2X receptor, the pore opens within milliseconds, allowing the cations to flow. P2X receptors are expressed on both central and peripheral neurons, where they are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed in most types of nonneuronal cells and mediate a wide range of actions, such as contraction of smooth muscle, secretion, and immunomodulation. Changes in the expression of P2X receptors have been characterized in many pathological conditions of the cardiovascular, gastrointestinal, respiratory, and urinogenital systems and in the brain and special senses. The therapeutic potential of P2X receptor agonists and antagonists is currently being investigated in a range of disorders, including chronic neuropathic and inflammatory pain, depression, cystic fibrosis, dry eye, irritable bowel syndrome, interstitial cystitis, dysfunctional urinary bladder, and cancer.

Dinucleoside polyphosphates: strong endogenous agonists of the purinergic system

The purinergic system is composed of mononucleosides, mononucleoside polyphosphates and dinucleoside polyphosphates as agonists, as well as the respective purinergic receptors. Interest in the role of the purinergic system in cardiovascular physiology and pathophysiology is on the rise. This review focuses on the overall impact of dinucleoside polyphosphates in the purinergic system. Platelets, adrenal glands, endothelial cells, cardiomyocytes and tubular cells release dinucleoside polyphosphates. Plasma concentrations of dinucleoside polyphosphates are sufficient to cause direct vasoregulatory effects and to induce proliferative effects on vascular smooth muscle cells and mesangial cells. In addition, increased plasma concentrations of a dinucleoside polyphosphate were recently demonstrated in juvenile hypertensive patients. In conclusion, the current literature accentuates the strong physiological and pathophysiological impact of dinucleoside polyphosphates on the cardiovascular system.

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.

Purinergic 2X1 receptors mediate endothelial dependent vasodilation to ATP

ATP is an important endogenous mediator in the cardiovascular system. It induces endothelium dependent vasodilation, but the precise receptor pathway activated in this response is currently under debate. We have used traditional bioassay techniques to show that ATP-induced vasodilation in mesenteric vessels is endothelium-dependent. Furthermore, ATP-induced vasodilation was inhibited by both suramin and 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP), consistent with a P2X(1)-, P2X(2)-, or P2X(3)-mediated event and was not potentiated by ivermectin, indicating that these responses were not P2X(4) receptor-mediated. ATP did not induce vasodilation in vessels from P2X (-/-)(1) mice, confirming an absolute requirement for this receptor. Finally, in pure cell populations of mouse mesenteric artery endothelial cells, we show that P2X(1) mRNA is specifically expressed. However, in line with observations in the brain, the P2X(1) present in endothelial cells does not seem to be recognized by conventional antibodies. Together, these results show that ATP-induced vasodilation is mediated by P2X(1) receptor activation on mesenteric arterial endothelial cells. These observations establish a critical role for P2X(1) receptors in the ATP vasodilator pathway.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Non-synaptic transmission at autonomic neuroeffector junctions

Non-synaptic transmission is characteristic of autonomic neuroeffector junctions. The structure of the autonomic neuromuscular junction is described. The essential features are that: the terminal portions of autonomic nerve fibers are varicose and mobile, transmitters being released 'en passage' from varying distances from the effector cells; while there is no structural post-junctional specialization on effector cells, receptors for neurotransmitters accumulate on cell membranes at close junctions; muscle effectors are bundles rather than single smooth muscle cells, that are connected by gap junctions which allow electrotonic spread of activity between cells. A multiplicity of transmitters are utilized by autonomic nerves, and cotransmission occurs often involving synergistic actions of the cotransmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place. It is suggested that autonomic neural control of immune, epithelial and endothelial cells also involves non-synaptic transmission.

Schwann cells in rat vascular autonomic nerves activated via purinergic receptors

Schwann cells at the somatic neuromuscular junction possess adenosine receptors that when activated by the release of endogenous transmitter modulate quantal transmitter release. Recently, purinergic receptors have been shown to exist on Schwann cells of axon varicosities in visceral smooth muscle where they are activated by endogenous transmitters to give a calcium transient, although adenosine receptors were not identified. In the present work, we show that Schwann cells associated with axon varicosities of vascular smooth muscle, namely that of mesenteric blood vessels, possess both adenosine and purinergic receptors that when activated give rise to calcium transients in these cells. Then qualitative differences exist in the extent of adenosine and purine receptors that give rise to calcium transients in Schwann cells located in these different muscles.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

P2X purinoceptor subtypes on paraventricular nucleus neurones projecting to the rostral ventrolateral medulla in the rat

The rostral ventrolateral medulla (RVLM) is essential for the generation of sympathetic nerve activity. The RVLM receives a substantial innervation from the hypothalamic paraventricular nucleus (PVN). Activation of P2X purinoceptors via ATP has been shown to mediate fast excitatory synaptic neurotransmission. There is mounting evidence to suggest the presence of P2X purinoceptors in hypothalamic nuclei, including the PVN. In this study, we determined whether P2X1-P2X6 purinoceptor subtypes were present on PVN neurones that projected to the RVLM. Injection of the retrogradely transported tracer, rhodamine-tagged microspheres, into the pressor region of the RVLM was used to identify the neurones in the PVN that innervated the RVLM. P2X1-P2X6 purinoceptors were detected by immunohistochemistry. Double-labelled neurones were quantified and expressed as a proportion of the retrogradely labelled neurones. The proportions of double-labelled neurones for each of the P2X purinoceptor subtypes varied, on average, from 14 to 29%. The P2X3 purinoceptor subtype was found to be the dominant purinoceptor subtype present on PVN neurones projecting to the RVLM. Additionally it was apparent that more than one P2X purinoceptor subtype was present on the PVN neurones projecting to the RVLM, since the sum of the average percentages of double-labelled neurones for each P2X purinoceptor subtype exceeded 100%. These findings highlight the presence of the P2X1-P2X6 purinoceptors on PVN neurones projecting to the RVLM. The results suggest a potential role for ATP in the PVN in the regulation of sympathetic nerve activity.

Localisation, function and composition of primary Ca(2+) spark discharge region in isolated smooth muscle cells from guinea-pig mesenteric arteries

Smooth muscle cells (SMCs) contain numerous calcium release domains, grouped into regions discharging as a single unit. Laser scanning confocal microscopy, voltage clamp and immunocytochemistry of single SMCs from small mesenteric arteries of guinea-pig were used to study the localisation, function and macromolecular composition of such calcium discharge regions (CDRs). Use of the Ca(2+)-sensitive fluorescent dye fluo-3 or fluo-4 with BODIPY TR-X ryanodine (BTR), a fluorescent derivative of ryanodine, showed spontaneous Ca(2+) sparks originating from regions stained by BTR, located immediately under the plasma membrane, in the arch formed by the sarcoplasmic reticulum surrounding the nucleus. Membrane depolarisation or application of noradrenaline or alpha,beta-methylene ATP, a P2X purinoceptor agonist, elicited Ca(2+) sparks from the same, spontaneous Ca(2+) spark-discharging region. The most active (primary) CDR accounted for nearly 60% of spontaneous transient outward currents at -40 mV and these were of significantly higher amplitude than the ones discharged by secondary CDRs. Immunocytochemical staining for type 1 IP(3) receptors, BK(Ca) channels, P2X(1) purinoceptors or alpha(1) adrenoceptors revealed their juxtaposition with BTR staining at the location typical of the primary CDR. These data suggest the existence of a primary calcium discharge region in SMCs; its position can be predicted from the cell's structure, it acts as a key region for the regulation of membrane potential via Ca(2+) sparks and is a potential link between the external, neurohumoral and the cell's internal, calcium signalling system.

Changes in purinergic signalling in developing and ageing rat tail artery: importance for temperature control

This study aimed to examine the expression and function of P2 receptors of the rat tail and mesenteric arteries during maturation and ageing (4, 6 and 12 weeks, 8 and 24 months). Functional studies and receptor expression by immunohistochemistry revealed a heterogeneous phenotype of P2 receptor subtypes depending on artery age. The purinergic component of nerve-mediated responses in the tail artery was greater in younger animals; similarly responses to ATP and alpha,beta-meATP and the expression of P2X1 receptors decreased with age. Contractile responses to 2-MeSADP decreased with age, and were absent at 8 and 24 months; P2Y1 receptor expression followed this pattern. UTP-induced contractions and P2Y2 receptor expression also decreased with age. The mesenteric artery contracted to UTP, responses at 4 and 6 weeks were larger than at other ages although P2Y2 receptor expression did not significantly differ with age. 2-MeSADP induced relaxation of the mesenteric artery, responses being greatest at 6 weeks and decreased thereafter, which was mimicked by the P2Y1 receptor immunostaining. We speculate that the dramatic changes in expression of P2 receptors in the rat tail artery, compared to the mesenteric artery, during development and ageing are related to the role of the tail artery in temperature regulation.

Disruption of lipid rafts inhibits P2X1 receptor-mediated currents and arterial vasoconstriction

P2X1 receptors for ATP are ligand-gated cation channels expressed on a range of smooth muscle preparations and blood platelets. The receptors appear to be clustered close to sympathetic nerve varicosities and mediate the underlying membrane potential changes and constriction following nerve stimulation in a range of arteries and resistance arterioles. In this study we have used discontinuous sucrose density gradients, Western blot analysis, and cholesterol measurements to show that recombinant and smooth muscle (rat tail artery, vas deferens, and bladder) P2X1 receptors are present in cholesterol-rich lipid rafts and co-localize with the lipid raft markers flotillin-1 and -2. Lipid rafts are specialized lipid membrane microdomains involved in signaling and trafficking. To determine whether lipid raft association was essential for P2X1 receptor channel function we used the cholesterol-depleting agent methyl-beta-cyclodextrin (10 mm for 1 h). This led to a redistribution of the P2X1 receptor throughout the sucrose gradient and reduced P2X1 receptor-mediated (alpha,beta-methylene ATP, 10 microm) currents in HEK293 cells by >90% and contractions of the rat tail artery by approximately 50%. However contractions evoked by potassium chloride (60 mm) were unaffected by methyl-beta-cyclodextrin and the inactive analogue alpha-cyclodextrin had no effect on P2X1 receptor-mediated currents or contractions. P2X1 receptors are subject to ongoing regulation by receptors and kinases, and the present results suggest that lipid rafts are an essential component in the maintenance of these localized signaling domains and play an important role in P2X1 receptor-mediated control of arteries.

P2X1 receptors and the endothelium

Adenosine triphosphate (ATP) is now established as a principle vaso-active mediator in the vasculature. Its actions on arteries are complex, and are mediated by the P2X and P2Y receptor families. It is generally accepted that ATP induces a bi-phasic response in arteries, inducing contraction via the P2X and P2Y receptors on the smooth muscle cells, and vasodilation via the actions of P2Y receptors located on the endothelium. However, a number of recent studies have placed P2X1 receptors on the endothelium of some arteries. The use of a specific P2X1 receptor ligand, alpha, beta methylene ATP has demonstrated that P2X1 receptors also have a bi-functional role. The actions of ATP on P2X1 receptors is therefore dependant on its location, inducing contraction when located on the smooth muscle cells, and dilation when expressed on the endothelium, comparable to that of P2Y receptors.

Correlation of non-uniform protein expression with variation in transmitter release probability

The strength of synaptic transmission is highly variable between different synapses. The present study examined some factors that may contribute to this variation in the strength of neurotransmission in sympathetic varicosities of the mouse vas deferens. Transmitter release was measured using a focal macropatch electrode placed over pairs of visualised varicosities. By regulating the calcium concentration of the solutions inside the recording electrode and in the bath independently of each other, transmitter release was restricted to one or two surface varicosities at each recording site. Using this technique, transmitter release probability was shown to be highly variable, even between adjacent varicosities on single axon branches. Very little variation was observed in the calcium influx following single impulse nerve stimulation between adjacent Oregon Green BAPTA-1 loaded varicosities. However, the staining intensities of three vesicular proteins, SV2, synaptophysin, and synaptotagmin 1, showed considerable variation between adjacent varicosities on single axon branches. This variation in staining intensity may be partly explained by variation in the density of synaptic vesicles. However, double staining experiments using two vesicular antigens showed some varicosities staining for one vesicular antigen, but not for the second, suggesting that the expression of these release machinery proteins is regulated locally within the varicosities. The results of the present study strengthen suggestions that synaptic strength is at least in part, regulated by variation in the expression of vesicular proteins.

Purinergic Signalling to Rat Ovarian Smooth Muscle: Changes in P2X Receptor Expression during Pregnancy

The expression of P2X and P2Y receptor subtypes in the smooth muscle of the rat ovary during the oestrus cycle and pregnancy was examined using immunohistochemistry. RT-PCR studies of P2X receptor mRNA were also carried out. In the non-pregnant rats, P2X2 receptor protein was dominant in the smooth muscle of perifollicular rings and blood vessels. P2X1 protein expression was seen on vascular smooth muscle too, but little, if any, was present on perifollicular smooth muscle. No changes in P2X1 or P2X2 receptor expression were seen during the oestrous cycle. During early and mid-late pregnancy, there was a switch from P2X2 to P2X1 receptor protein expression in the smooth muscle of the perifollicular ring; P2X1 receptors were also more prominently expressed than P2X2 receptors on ovarian vascular smooth muscle in non-pregnant animals, but during late pregnancy the expression of P2X2 receptors was found to equal that of the P2X1 receptors. There was a return to non-pregnant P2 receptor subtype distribution 2 days after birth. Ovarian vascular and perifollicular smooth muscle showed immunoreactivity for P2Y1, but not for P2X3-7, P2Y2 or P2Y4 receptors. P2Y1 receptor expression in ovarian smooth muscle of both blood vessels and follicular rings did not show significant changes during the oestrus cycle or pregnancy. RT-PCR studies indicated that P2X1 and P2X2 receptor mRNA was present in the ovary during pregnant and non-pregnant conditions. P2X4-6 receptor mRNA was also present in all stages studied, however no immunostaining showing receptor protein for these subtypes was seen on the ovarian sections examined. In summary, purinergic signalling to ovarian perifollicular smooth muscle changed from P2X2 to P2X1 receptors during pregnancy, while there was an increase in P2X2 receptor expression on vascular smooth muscle.

Acidosis abolishes the effect of repeated applications of ATP on pulmonary artery force and [Ca2+]i

The purine nucleotide, ATP, can cause receptor-mediated desensitizing contractions of smooth muscle that may be modulated by pH. We investigated in the rat the effect of acidosis upon the contractile and Ca2+ responses induced by ATP upon intrapulmonary artery (PA) smooth muscle. Four successive applications of ATP (300 microM) at pH 7.4 induced desensitising contractile responses that showed progressively decreasing peak amplitudes that correlated with decreases of [Ca2+]i. Acidosis significantly reduced the peak contractile response to the first application of ATP without modifying the rate or degree of desensitisation in response to ATP and without decreasing the [Ca2+]i. Successive applications of ATP did not further reduce contractile force nor [Ca2+]i. These results demonstrated that acidosis abolishes the effect of repeat applications of ATP on pulmonary artery force and [Ca2+]i via alteration in the desensitization-resensitisation characteristics of ATP receptor. This suggest a potentially important physiological role for changes in external pH in the regulation of ATP-mediated control of the pulmonary circulation.

Novel role for P2X receptor activation in endothelium-dependent vasodilation

1 ATP is an important vasoactive mediator, which acts via two receptor classes: P2X and P2Y. Activation of P2X receptors has traditionally been associated with the well-characterised vasoconstrictor properties of ATP. 2 In the current study, we have shown that the P2X(1 & 3) receptor ligand, alpha, beta methylene ATP, induces vasodilation of rat isolated mesenteric arteries and that P2X1 receptors are abundantly expressed in the endothelium of these vessels. 3 Second-order rat mesenteric arteries were mounted in myographs and vasomotor responses recorded. Both ATP and alpha, beta methylene ATP induced a constriction followed by a vasodilation. The dilator effects of either ATP or alpha, beta methylene ATP were slower in onset than those induced by acetylcholine. By contrast, the traditional vasodilator P2Y ligand, ADP, induced vasodilation without contraction. 4 Vasodilation induced by alpha, beta methylene ATP was endothelial dependent, but was not affected by treatment of the vessels with L-NAME plus indomethacin alone. Dilation was, however, partially inhibited by the combination of apamin plus charybdotoxin and blocked by treating vessels with all four drugs. 5 Using confocal microscopy, P2X1 receptor immunoreactivity was localised to the endothelial, smooth muscle and adventitial layers of mesenteric vessels. P2X1 protein migrated as a primary band at around 50-60 kDa in vascular tissue. 6 These results show for the first time that P2X1 receptors are expressed on the endothelium and that a selective ligand of this receptor results in vasoconstriction followed by vasodilation. These observations have important implications for our understanding of the role of purines in biological responses.

Response to ATP is accompanied by a Ca2+ influx via P2X purinoceptors in the coronary arterioles of golden hamsters

In the vascular wall, adenosine-5'-triphosphate (ATP) released along with noradrenaline from sympathetic nerve terminals is considered to play an important role in controlling intracellular calcium ion ([Ca2+]i) levels in arteries. The present study examined how vascular smooth muscle cells in coronary arterioles respond to ATP in relation to [Ca2+]i dynamics. For this purpose, the dynamics of [Ca2+]i in the coronary arterioles of golden hamsters was examined by real-time laser scanning confocal microscopy. This technique enabled the visualization of [Ca2+]i changes in response to ATP in the intact coronary arterioles, the ultrastructure of which was well preserved. It was shown that an increase in [Ca2+]i in the arteriole smooth muscle cells was elicited by ATP. While P1 purinoceptor agonists have no effect on this process, P2 purinoceptor agonists were found to induce a [Ca2+]i increase in the smooth muscle cells. Suramin (an antagonist of P2X and P2Y receptors) completely inhibited ATP-induced [Ca2+]i dynamics, but reactive blue 2 (a P2Y receptor antagonist) did not. Uridine-5'-triphosphate (a P2Y receptor agonist) had no effect on [Ca2+]i, but alpha,beta-methylene ATP (a P2X receptor agonist) caused a strong increase in [Ca2+]i. We conclude that smooth muscle cells of the hamster coronary arterioles possess P2X, but not P1 or P2Y purinoceptors. The smooth muscle cells probably respond to extracellular ATP via P2X purinoceptors, resulting in the contraction of the coronary arterioles.

Functional maturation of isolated neural progenitor cells from the adult rat hippocampus

Although neural progenitor cells (NPCs) may provide a source of new neurons to alleviate neural trauma, little is known about their electrical properties as they differentiate. We have previously shown that single NPCs from the adult rat hippocampus can be cloned in the presence of heparan sulphate chains purified from the hippocampus, and that these cells can be pushed into a proliferative phenotype with the mitogen FGF2 [Chipperfield, H., Bedi, K.S., Cool, S.M. & Nurcombe, V. (2002) Int. J. Dev. Biol., 46, 661-670]. In this study, the active and passive electrical properties of both undifferentiated and differentiated adult hippocampal NPCs, from 0 to 12 days in vitro as single-cell preparations, were investigated. Sparsely plated, undifferentiated NPCs had a resting membrane potential of approximately -90 mV and were electrically inexcitable. In > 70%, ATP and benzoylbenzoyl-ATP evoked an inward current and membrane depolarization, whereas acetylcholine, noradrenaline, glutamate and GABA had no detectable effect. In Fura-2-loaded undifferentiated NPCs, ATP and benzoylbenzoyl-ATP evoked a transient increase in the intracellular free Ca(2+) concentration, which was dependent on extracellular Ca(2+) and was inhibited reversibly by pyridoxalphosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS), a P2 receptor antagonist. After differentiation, NPC-derived neurons became electrically excitable, expressing voltage-dependent TTX-sensitive Na(+) channels, low- and high-voltage-activated Ca(2+) channels and delayed-rectifier K(+) channels. Differentiated cells also possessed functional glutamate, GABA, glycine and purinergic (P2X) receptors. Appearance of voltage-dependent and ligand-gated ion channels appears to be an important early step in the differentiation of NPCs.

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.

Unusual absence of endothelium-dependent or -independent vasodilatation to purines or pyrimidines in the rat renal artery

BACKGROUND

Adenosine triphosphate (ATP) is a cotransmitter with noradrenaline (NA) in sympathetic perivascular nerves. It has a dual role in the maintenance of vascular tone as ATP, released from endothelial cells during shear stress or hypoxia, induces vasodilatation via endothelial P2Y receptors or by direct action on smooth muscle. The role and distribution of P2 receptors is well characterized for many blood vessels but not for the rat renal artery. This study aims to determine whether ATP is a vasoconstrictor cotransmitter with NA and whether ATP induces vasodilatation via the endothelium or smooth muscle.

METHODS

On isolated rat renal arteries, electrical field stimulation (EFS) in the absence and presence of antagonists to P2X receptors and alpha1-adrenoceptors was examined. Concentration-response curves were constructed to NA, ATP, alpha,beta-methylene ATP (alpha,beta-meATP), uridine triphosphate (UTP), and 2-methylthio ADP (2-MeSADP) on low tone. Curves to acetylcholine (ACh), 2-MeSADP, and UTP were constructed on raised tone. Immunofluorescent localization of P2X and P2Y receptor subtypes was performed.

RESULTS

Electrical field stimulation induced vasoconstriction, partially inhibited by the P2X receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, and predominantly by prazosin. Exogenous NA and ATP mimicked EFS; immunostaining for P2X1 and P2X2 receptors was expressed on vascular smooth muscle. Unusually, ATP, 2-MeSADP, and UTP failed to induce vasodilatation. Acetylcholine induced vasodilatation. alpha,beta-meATP, 2-MeSADP, and UTP induced vasoconstriction via P2X1, P2Y1, and P2Y2 receptors, respectively. Immunostaining for P2X1, P2Y1, and P2Y2 receptors was expressed on the vascular smooth muscle.

CONCLUSION

Adenosine triphosphate and NA are cotransmitters in sympathetic nerves supplying the rat renal artery, NA being the dominant partner. The novel feature of this vessel is that purines and pyrimidines do not produce either endothelium-dependent or -independent vasodilatation; P2X1, P2Y1, and P2Y2 receptors on the smooth muscle all mediate vasoconstriction.

Purinergic signalling in the medullary mechanisms of respiratory control in the rat: respiratory neurones express the P2X2 receptor subunit

ATP is involved in central respiratory control and may mediate changes in the activity of medullary respiratory neurones during hypercapnia, thus playing an important role in central chemoreception. The main objective of this study was to explore further the role of ATP-mediated signalling in respiratory control and central chemoreception by characterising the profile of the P2X receptors expressed by physiologically identified respiratory neurones. In particular we determined whether respiratory neurones in the rostral ventrolateral medulla (VLM) express P2X2 receptor subunits of the ATP-gated ion channel, since ATP currents evoked at recombinant P2X2 receptors are potentiated by lowering extracellular pH. Experiments were performed on anaesthetised (pentobarbitone sodium 60 mg kg-1 I.P., then 10 mg kg-1 I.V. as required), gallamine-triethiodide-treated (10 mg kg-1 I.V., then 2-4 mg kg-1 h-1 I.V.) and artificially ventilated rats. The VLM respiratory neurones were classified according to the timing of their discharge pattern in relation to that of the phrenic nerve and by the exclusion of pump cells from the study population; these were labelled with Neurobiotin using the juxtacellular method, and visualised with fluorescence microscopy. It was found that a substantial proportion of the VLM respiratory neurones express the P2X2 receptor subunit. P2X2 receptor subunit immunoreactivity was detected in approximately 50 % (six out of 12) of expiratory neurones and in approximately 20 % (two out of 11) of neurones with inspiratory-related discharge (pre-inspiratory and inspiratory). In contrast, no Neurobiotin-labelled VLM respiratory neurones (n = 19) were detectably immunoreactive for the P2X1 receptor subunit. Microionophoretic application of ATP (0.2 M, 20-80 nA for 40 s) increased the activity of approximately 80 % (13 out of 16) of expiratory neurones and of approximately 30 % (five out of 18) of VLM neurones with inspiratory-related discharge. These effects were abolished by the P2 receptor blocker suramin (0.02 M, 80 nA), which also reduced the baseline firing in some expiratory neurones. These data indicate that modulation of P2X2 receptor function, such as that evoked by acidification of the extracellular environment during hypercapnia, may contribute to the changes in activity of the VLM respiratory neurones that express these receptors.

Identification and quantification of diadenosine polyphosphate concentrations in human plasma

OBJECTIVE

Diadenosine polyphosphates have been demonstrated to be involved in the control of vascular tone as well as the growth of vascular smooth muscle cells and hence, possibly, in atherogenesis. In this study we investigated the question of whether diadenosine polyphosphates are present in human plasma and whether a potential source can be identified that may release diadenosine polyphosphates into the circulation.

METHODS AND RESULTS

Plasma diadenosine polyphosphates (ApnA with n=3 to 6) were purified to homogeneity by affinity-, anion exchange-, and reversed phase-chromatography from deproteinized human plasma. Analysis of the homogeneous fractions with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) revealed molecular masses ([M+H]+) of 757, 837, 917, and 997 d. Comparison of the postsource decay matrix-assisted laser desorption/ionization mass spectrometry mass spectra of these fractions with those of authentic diadenosine polyphosphates revealed that these isolated substances were identical to Ap3A, Ap4A, Ap5A, and Ap6A. Enzymatic analysis showed an interconnection of the phosphate groups with the adenosines in the 5'-positions of the ribose moieties. The mean total plasma diadenosine polyphosphate concentrations (micromol/L; mean +/- SEM) in cubital veins of normotensive subjects amounted to 0.89+/-0.59 for Ap3A, 0.72+/-0.72 for Ap4A, 0.33+/-0.24 for Ap5A, and 0.18+/-0.18 for Ap6A. Cubital venous plasma diadenosine polyphosphate concentrations from normotensives did not differ significantly from those in the hypertensive patients studied. There was no significant difference between arterial and venous diadenosine polyphosphate plasma concentrations in 5 hemodialysis patients, making a significant degradation by capillary endothelial cells unlikely. Free plasma diadenosine polyphosphate concentrations are considerably lower than total plasma concentrations because approximately 95% of the plasma diadenosine polyphosphates were bound to proteins. There were no significant differences in the diadenosine polyphosphate plasma concentrations depending on the method of blood sampling and anticoagulation, suggesting that platelet aggregation does not artificially contribute to plasma diadenosine polyphosphate levels in significant amounts. The ApnA (with n=3 to 6) total plasma concentrations in adrenal veins were significantly higher than the plasma concentrations in both infrarenal and suprarenal vena cava: adrenal veins: Ap3A, 4.05+/-1.63; Ap4A, 6.18+/-2.08; Ap5A, 0.53+/-0.28; Ap6A, 0.59+/-0.31; infrarenal vena cava: Ap3A, 1.25+/-0.66; Ap4A, 0.91+/-0.54; Ap5A, 0.25+/-0.12; Ap6A, 0.11+/-0.06; suprarenal vena cava: Ap3A, 1.40+/-0.91; Ap4A, 1.84+/-1.20; Ap5A, 0.33+/-0.13; Ap6A, 0.11+/-0.07 (micromol/L; mean +/- SEM; each P<0.05 (concentration of adrenal veins versus infrarenal or suprarenal veins, respectively).

CONCLUSIONS

The presence of diadenosine polyphosphates in physiologically relevant concentrations in human plasma was demonstrated. Because in adrenal venous plasma significantly higher diadenosine polyphosphate concentrations were measured than in plasma from the infrarenal and suprarenal vena cava, it can be assumed that, beside platelets, the adrenal medulla may be a source of plasma diadenosine polyphosphates in humans.

Potent P2Y6 receptor mediated contractions in human cerebral arteries

BACKGROUND

Extracellular nucleotides play an important role in the regulation of vascular tone and may be involved in cerebral vasospasm after subarachnoidal haemorrhage. This study was designed to characterise the contractile P2 receptors in endothelium-denuded human cerebral and omental arteries. The isometric tension of isolated vessel segments was recorded in vitro. P2 receptor mRNA expression was examined by RT-PCR.

RESULTS

In human cerebral arteries, the selective P2Y6 receptor agonist, UDPbetaS was the most potent of all the agonists tested (pEC50 = 6.8 PlusMinus; 0.7). The agonist potency; UDPbetaS > alphabeta-MeATP > UTPgammaS > ATPgammaS > ADPbetaS = 0, indicated the presence of contractile P2X1 P2Y2, P2Y4 and P2Y6, but not P2Y1 receptors, in human cerebral arteries. In human omental arteries, UDPbetaS was inactive. The agonist potency; alphabeta-MeATP > ATPgammaS = UTPgammaS > ADPbetaS = UDPbetaS = 0, indicated the presence of contractile P2X1, and P2Y2 receptors, but not P2Y1 or P2Y6 receptors, in human omental arteries. RT-PCR analysis of endothelium-denuded human cerebral and omental arteries demonstrated P2X1, P2Y1, P2Y2 and P2Y6 receptor mRNA expression. There were no bands for the P2Y4 receptor mRNA in the omental arteries, while barely detectable in the cerebral arteries.

CONCLUSIONS

P2Y6 receptors play a prominent role in mediating contraction of human cerebral arteries. Conversely, no such effect can be observed in human omental arteries and previous results confirm the absence of P2Y6 receptors in human coronary arteries. The P2Y6 receptor might be a suitable target for the treatment of cerebral vasospasm.

Regional differences in sympathetic purinergic transmission along the length of the mouse vas deferens

Contraction of the smooth muscle in the mouse vas deferens is elicited by sympathetic nerves releasing at least two neurotransmitters, adenosine triphosphate (ATP) and noradrenaline (NA). Several studies have indicated the presence of regional variation in the purinergic and noradrenergic contributions to sympathetic nerve-evoked contractions in rodent vasa deferentia. We examined the relative contribution of ATP and NA to neurotransmission and contraction at the prostatic and epididymal ends of the mouse vas deferens. The success rate of recording excitatory junction currents (EJCs, extracellular indication of ATP release) from varicosities at the prostatic end of the vas deferens was eight times greater than for varicosities located at the epididymal end. Both regions of the vas deferens responded similarly to focal application of NA and ATP. Furthermore, the relative density and distribution of P2X(1)-receptor mRNA and anti-P2X(1) immunostaining did not differ between the two regions. Our results suggest that most varicosities located at the epididymal end of the vas deferens are releasing an insufficient amount of ATP to evoke detectable EJCs.

Expression level dependent changes in the properties of P2X2 receptors

The currents of P2X(2) receptors expressed in Xenopus oocytes or HEK293 cells show significant cell-to-cell variation in many properties including the rate of desensitization and the magnitude of potentiation by zinc or acidic pH. In this study, we examined whether differences in expression levels underlie this variability. We injected Xenopus oocytes with different concentrations of RNA encoding rat P2X(2) to give a wide range of maximum current amplitudes, and then measured the potentiation of responses to 10 micro M adenosine 5'-triphosphate (ATP) by zinc or acidic pH. Individual oocytes showed potentiation ratios that ranged from 1.4- to 25-fold. Oocytes with small amplitude responses to a saturating concentration of ATP tended to have larger potentiation ratios than oocytes with large amplitude responses. This phenomenon was explained by an inverse correlation between the EC(50) for ATP and the maximum current amplitude, with the EC(50) decreasing from about 37 to 7 micro M as expression level increased. In contrast, the Hill coefficient was not correlated with the maximum current amplitude. Truncated receptors lacking the last 76 amino acids also showed an inverse correlation between the EC(50) and the maximum current amplitude. Thus, the interactions that cause expression-dependent changes in P2X(2) receptor properties must involve domains proximal to position H397.