P2-purinoceptors

Classification of P2 purinoceptors has been limited by a lack of potent, selective, and bioavailable antagonists. Nonetheless a rational scheme for P2 purinoceptor nomenclature divides P2 receptors into two superfamilies P2Y5 LGIC family having four subclasses and P2Y) a GPCR family having seven subclasses. A third receptor type, designated the P22) is a nonselective ion pore. 

Sympathetic arc involved in blood pressure regulation and sites where drugs may act to influence the system. A. Receptors on effector cell. 6. Adrenergic varicosity. C. Nicotinic receptors (postganglionic fibers). D. Brainstem nuclei. NTS, nucleus of the tractus solitarii VMC, vasomotor center ACh, acetylcholine NE, norepinephrine a, a-adrenoceptors (3, 13-adrenoceptors P2, P2-purinoceptors ATR adenosine triphosphate. 

Driessen, B., Reimann, W., Selve, N., Friderichs, E. and Bultmann, R. Antinociceptive effect of intrathecally administered P2-purinoceptor antagonists in rats, Brain Res. 1994, 666, 182-188. 

Iredale PA, Hill SJ, Iredale PA, Hill SJ (1993) Increases in intracellular calcium via activation of an endogenous P2-purinoceptor in cultured CHO-K1 cells. Br J Pharmacol 110(4) 1305-1310... 

Suzuki H., Ikeda K, Furukawa M., Takasaka T. 1997. P2 purinoceptor of the globular substance in the otoconial membrane of the guinea pig inner ear. Am J Physiol 273 C1533-40. 

Wiebe SH, Sims SM, Dixon SJ. 1999. Calcium signalling via multiple P2 purinoceptor subtypes in rat osteoclasts. Cell Physiol Biochem 9 323-37. 

Burnstock G, Kennedy C (1985) Is there a basis for distinguishing two types of P2-purinoceptor Gen Pharmacol 16 433 10... 

Koizumi S, Fujishita K, Tsuda M et al (2003) Dynamic inhibition of excitatory synaptic transmission by astrocyte-derived ATP in hippocampal cultures. Proc Natl Acad Sci USA 100 11023-8 Kubista H, Boehm S (2006) Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors. Pharmacol Ther 112 213 42 Kukulski F, Sevigny J, Komoszynski M (2004) Comparative hydrolysis of extracellular adenine nucleotides and adenosine in synaptic membranes from porcine brain cortex, hippocampus, cerebellum and medulla oblongata. Brain Res 1030 49-56 Kurokawa M, Koga K, Kase H et al (1996) Adenosine A2a receptor-mediated modulation of striatal acetylcholine release in vivo. J Neurochem 66 1882-8 Kurz K, von Ktigelgen I, Starke K (1993) Prejunctional modulation of noradrenaline release in mouse and rat vas deferens contribution of PI- and P2-purinoceptors. Br J Pharmacol 110 1465-72... 

Accumulating findings indicate that nucleotides play an important role in neuron-to-glia communication through P2 purinoceptors. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (seven types P2X]-P2X7) contain intrinsic pores that open... 

Inscho EW, Cook AK,MuiV, Miller J Direct assessment of renal microvascular responses to P2-purinoceptor agonists. Am.J.Physiol 274 F718-F727,1998... 

As a result of coupling considerations and structural information derived from cloning and expression studies, the P2 purinoceptors are now divided into two main classes. These are (1) the metabotropic P2Y receptors, which are G-protein-coupled, and (2) the ionotropic P2X receptors, which are intrinsic ion channel receptors. [It should be noted that details of nomenclature for these receptor have changed several times recently, particularly with regard to use of subscripts. The version adopted here, is that currently recommended by the NC-IUPHAR Nomenclature Subcommittee for Purinoceptors.]... 

Currently, new subtypes of P2-purinoceptors are being cloned or otherwise discovered at a considerable rate, and some changes to the above scheme may be anticipated. 

Dunn. P.M. etal. (1988) Suramin a reversible P2-purinoceptor antagonist in the mouse vas deferens. Br. J. Pharmacol.. 93.243-245. 

Design and Pharmacological Characterization of Selective P2 purinoceptor Antagonists... 

At least five distinct P2-purinoceptor subtypes have been characterized to date, based on the rank order of potency of several ATP analogues P2X> P2Y> 2U> 2T 2Z-... 

In conclusion, PPADS and NF023 are specific P2-purinoceptor antagonists showing a high selectivity for the P2X Subtype. These two compounds may prove to be useful starting points in the synthesis of novel, highly potent and selective antagonists at P2-purinoceptor subtypes. 

Intracellular nucleotides play an important role in enzyme and ion channel regulation as well as in energy metabolism and nucleic acid synthesis. There is now widespread appreciation that ATP (and other nucleotides) may so be released into the extracellular fluid by exocytosis from nerve terminals or secretory cells. Thus, extracellular ATP can act as a neurotransmitter or modulator in a variety of peripheral tissues and cells, in autonomic ganglia and in the central nervous system [1-3]. The responses to extracellular ATP are mediated via membrane-bound receptors, termed P2-purinoceptors. Evidence has accumulated indicating heterogeneity of P2-purinoceptors, and it has become apparent that ATP acts on at least five P2-purinoceptor subtypes, i.e. P2X> P2Y> P2U> 2T 2Z... 

Introduced in 1922, suramin has been used in the treatment of trypanosomiasis and onchocerciasis. More recently, this compound has gained clinical interest for the treatment of acquired immunodeficiency syndrome, hepatitis B virus infection and metastatic carcinomas [21], Suramin has been shown to inhibit the receptor binding of a number of growth factors and cytokines, and it inhibits several nuclear and cytoplasmic enzymes as well as ecto-nucleotidases [33-41]. These latter enzymes are of special interest to the field of P2-purinoceptor research (see INTRODUCTION). 

In order to gain some insights into the structural features that contribute to the antagonistic properties of suramin at P2-purinoceptors, we have synthesized and pharmacologically characterized a series of suramin analogues (Figure 1). We hoped to alter the activity profile of suramin in favour of one of the P2-purinoceptor subtypes. 

Taken together, the results confirm that the parent compound, suramin, does not differentiate between P2X" P2Y receptor-mediated events. In contrast, the suramin analogue, NF023, is a specific P2-purinoceptor antagonist showing a high selectivity for the P2X-subtype. 

In order to assess the selectivity of PPADS for different P2-purinoceptor subtypes, its effects on the relaxant responses to adenine nucleotides were examined in the rat duodenum [31], rat mesenteric arterial bed [30], rabbit isolated blood vessels [29] and guinea-pig taenia coli [31], tissues that are endowed with the archetypal P2Y-purinoceptor. 

The effect of PPADS on P2ij-purinoceptors was investigated in the rat mesenteric arterial bed at basal tone and at tone raised by methoxamine. These experiments demonstrated the ineffectiveness of PPADS (10 pM) at both vasoconstriction- and vasodilatation-mediating P2XJ-receptors [30]. Similar results were obtained in the rat pulmonary vascular bed [62], In addition, PPADS (10 - 100 pM) was not an antagonist at P2-purinoceptors that mediate UTP-evoked depolarization of the rat superior cervical ganglion, but it produced a concentration-dependent depression of depolarizations evoked by a,3-methylene ATP [59], All these observations substantiate the proposal (see above) that PPADS is a selective antagonist at purine- (P2X receptors) rather than at pyrimidine-nucleotide receptors. 

To test the specificity of PPADS, we compared its blocking activity on P2-purinoceptor-mediated responses with its effects on responses mediated by a -adrenoceptors in rat vas deferens, histamine H]-receptors and muscarinic M3-receptors in guinea-pig ileum, adenosine A j-receptors and muscarinic M2-receptors in guinea-pig atria and adenosine A2-receptors and muscarinic Mj-receptors in rat duodenum. PPADS (100 pM) had no significant effect on either the potency or maximum responses to the respective agonists used in the various receptor preparations. These results demonstrate that the antagonistic effects of PPADS against purine-nucleotides at P2-purinoceptors are specific. 

It is noteworthy, that an isomer of PPADS, pyridoxalphosphate-6-azophenyl-2, 5 -disulfonic acid (iso-PPADS), acts as an antagonist of P2X pu oceptor-mediated responses with similar potency as PPADS [59, 63, 64]. However, very little is known about the P2-purinoceptor selectivity of this compound. 

Design and pharmacological characterization of selective P2-purinoceptor antagonists... 

More recently, studies have been performed to determine the responsiveness of rat juxtameduUary afferent arterioles to receptor-selective P2- purinoceptor agonists [246]. Experiments were performed in vitro using the blood perfused juxtameduUary nephron technique, combined with videomicroscopy. The presence of multiple P2 receptors on juxtameduUary afferent arterioles and the classification of these receptors as members of the P2X- and P2Y2 (P2U)-receptor subtypes was demonstrated. In another study, the relative contributions of adenosine A1 and A2a receptors to the responsiveness of the renal microvasculature to adenosine was investigated [247]. The presence of adenosine A1 and A2a receptors on afferent and efferent arterioles of juxtameduUary nephrons was demonstrated. Also, adenosine A2a receptor-mediated vasodilation partially buffered adenosine-induced vasoconstriction in both pre- and postglomerular segments of the renal microvasculature. 

Corr, L. and Burnstock, G. (1994) Analysis of P2-purinoceptor subtypes on the smooth muscle and endothelium of rabbit coronary artery. Journal of Cardiovascular Pharmacology, 23 709-715. 

Evans, R.J. and Kennedy, C. (1994) Characterization of P2-purinoceptors in the smooth muscle of the rat tail artery a comparison between contractile and electro-physiological responses. British Journal of Pharmacology, 113 853-860. 

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