Respiratory System
Lung
P2Y2, P2Y4 and P2Y6 receptor subtypes are the predominant receptor subtypes expressed by airway smooth muscle and epithelial cells. In addition, the dominant functional P2X receptor subtypes are of the P2X4 and P2X5 subtypes, where ATP is involved in mucociliary clearance and surfactant release. All of the P1 receptor subtypes are expressed by the lungs, with A1, A2B and A3 receptors involved in adenosine-induced bronchoconstriction, and A2A receptors involved in bronchodilation.
Trachea
The dominant receptors expressed by tracheal smooth muscle and epithelial cells are of the P2Y1 and P2Y2 subtypes. Functional P2X1 receptors are also expressed by smooth muscle, where they mediate contraction, and functional P2X4 and P2X7 receptors are expressed by epithelium, activating ciliary function and increasing [Ca2+]i, respectively.
Neuroepithelial Bodies
To date, only P2X3 and P2X2 receptors have been identified on neuroepithelial bodies, and it has been suggested that the functional receptor is a P2X2/3 heteromeric receptor, possibly involved in mechanosensory transduction and O2 sensing.
Nasal Respiratory Epithelium
P2X3 receptors are expressed by primary olfactory neurones located both in the olfactory epithelium and on subepithelial nerve fibres in the respiratory region. P2X5 and P2X7 receptors are expressed by epithelial cells of the rat nasal mucosa. P2Y1, P2Y2, P2Y6 and P2Y11 receptors are found to be expressed by respiratory epithelium. While adenosine receptors are known to be expressed by apical epithelial cells, as yet they have only been characterized as A2 receptors.
View chapterPurchase book
Pharmacology of Purine and Pyrimidine Receptors
Ivar von Kügelgen, T. Kendall Harden, in Advances in Pharmacology, 2011
C P2Y4 Receptor
As discussed above, the human P2Y4 receptor is activated by UTP, but not by ATP. UDP and ADP also are inactive (Nicholas et al., 1996). Selective agonists for the P2Y4 receptor are not available, although the agonist 2′-azido-2′-deoxy-UTP shows some preference for this receptor (Jacobson et al., 2009).
Suramin does not block the P2Y4 receptor even at high concentrations (Table II). PPADS reduced maximal agonist responses of the human P2Y4 receptor but was without effect at the rat P2Y4 receptor (Wildman et al., 2003). Reactive blue-2 caused a modest reduction of agonist-induced responses of the human P2Y4 receptor and abolished responses at the rat P2Y4 receptor (Bogdanov et al., 1998; Wildman et al., 2003).
View chapterPurchase book
Purines and Purinoceptors: Molecular Biology Overview☆
G. Burnstock, in Reference Module in Biomedical Sciences, 2014
P2Y4 receptors
Human, rat, and mouse P2Y4 receptors have been cloned and characterized. UTP is the most potent activator of the recombinant human P2Y4 receptor. In contrast, the recombinant rat and mouse P2Y4 receptors are activated equipotently by ATP and UTP. Up4U (INS365) and dCp4U (INS37217) are agonists of the human P2Y4 receptor. Reactive blue 2 effectively blocks rat P2Y4 receptors, but only partially blocks human P2Y4 receptors. Suramin is a weak antagonist at the P2Y4 receptor. The structural determinants of agonism versus antagonism by ATP are located in the N-terminal domain and the second extracellular loop.
In the human and the mouse, P2Y4 mRNA and protein was most abundant in the intestine, but was also detected in other organs. P2Y4-null mice have apparently normal behavior, growth, and reproduction, but the chloride secretory response of the jejunal epithelium to apical UTP and ATP is abolished.
View chapterPurchase book
Volume 1
David G. Shirley, ... Robert J. Unwin, in Seldin and Giebisch's The Kidney (Fifth Edition), 2013
Proximal Tubule
Immunohistochemical studies have identified apical expression of P2Y1 and P2X5 receptors in the S3 segment of the rat pars recta, and basolateral expression of P2Y4 and P2X6 receptors in the proximal convoluted tubule (PCT); low-level expression of P2X4 protein was also seen in the PCT, although the membrane domain was not identified.171 Western blot analysis has additionally shown the presence of P2Y1 receptors in brush-border membrane vesicles from the S2 segment of rat PCT6 (see Figure 18.3). Messenger RNA expression has been assessed for only four P2 receptor subtypes: P2Y1, 2, 4 and P2Y6 are all present in rat proximal tubule.4,5 In terms of Ca2+ transients following application of P2 receptor agonists of varying selectivity, supportive evidence has been provided for apical P2Y1-like receptors in an immortalized cell line with a proximal phenotype,76 and for basolateral P2Y1 receptors in native rat PCT4,17; Bailey and colleagues5 also reported that basolateral UDP was effective in increasing [Ca2+]i, corroborating the presence of P2Y6 receptors. Finally, ATP and UTP were equally effective in stimulating Ca2+ transients when applied to rat or rabbit basolateral membranes,4,196 implying mediation by P2Y2 or P2Y4 receptors; the immunohistochemical evidence in rats favors P2Y4 receptors.171
Using a stationary microperfusion technique in rat PCT in vivo, Bailey2 showed that addition of adenosine nucleotides to the lumen inhibited bicarbonate reabsorption. ADP was more effective than ATP, implicating P2Y1 receptors; this was supported by the observation that the P2Y1 agonist 2 meSADP also had a potent inhibitory effect, which was blocked by the P2Y1-selective antagonist MRS2179. (When the tubule was perfused with MRS2179 alone, a small increase in bicarbonate reabsorption was seen, suggesting a tonic inhibitory effect of endogenous nucleotides acting via P2Y1 receptors.) The P2Y1-mediated effect on bicarbonate reabsorption involved inhibition of the Na+/H+ exchanger NHE3, since it was not additive to that of EIPA. The effect was blocked by either U73122 or H89, indicating involvement of phospholipase C and protein kinase A. In apparent contrast to these findings from intraluminal perfusions, Diaz-Silvester et al.31 found that addition of ATP to peritubular capillaries perfused in vivo caused an increase in transepithelial bicarbonate reabsorption in rat PCT. Conceivably, given the presence of ectonucleotidases in peritubular capillaries and the peritubular space (vide infra), degradation of ATP through to the nucleoside adenosine (which stimulates proximal tubular bicarbonate reabsorption29) could not be ruled out. However, increasing the viscosity of the peritubular perfusate also stimulated bicarbonate reabsorption, and this effect was blocked by peritubular suramin, suggesting P2 receptor mediation. (Shear stress was proposed as the activating factor.) Interestingly, the increase in bicarbonate reabsorption induced by ATP or by raised viscosity could be blocked by a nitric oxide synthase inhibitor.
In a preliminary study of membrane transporters in the tubules of P2Y2 receptor knockout mice, Listhrop et al.99 reported increased expression of NaPT2 protein in the proximal tubule (but no change in NHE3 abundance). In line with this, ATP has been shown to inhibit phosphate uptake (and mRNA for NaPT2) in primary cultures of rabbit PCTs.94 Interestingly, in the same preparation, ATP stimulates sodium-glucose co-transport by increasing both SGLT1 and SGLT2 protein expression.93
A renal clearance study in rats, using lithium clearance as an index of end-proximal tubular fluid delivery,165 reported remarkable effects of the naturally occurring diadenosine polyphosphate Ap4A. When infused intravenously, Ap4A increased lithium clearance almost two-fold, despite a fall in GFR, indicating a profound reduction in fractional proximal tubular reabsorption.154 Although a fascinating observation, it is debatable whether intravenous infusion of relatively high-dose exogenous nucleotide provides physiologically useful information about normal autocrine/paracrine control by endogenous agents. It is also difficult to know which P2 receptor(s) is/are involved, since Ap4A can stimulate a number of subtypes, including P2Y1 and P2Y4 receptors,143,185 which are both expressed in the rat proximal tubule (P2Y1 apically, P2Y4 basolaterally); intravenous delivery of the agonist does not allow differentiation between these possibilities.
In addition to effects on proximal tubular transport, both adenine-based and uracil-based nucleotides can stimulate gluconeogenesis, an important metabolic function of this nephron segment.16,109 Diadenosine polyphosphates also have this effect.35 As these experiments were performed using tubule suspensions or isolated tubules, the agonists will presumably have gained access to both apical and basolateral membranes; moreover, ectonucleotidase-mediated metabolism of the nucleotides is a possibility, hindering identification of the receptor subtype(s) responsible. However, ATP and UTP were equipotent in stimulating gluconeogenesis, implicating P2Y2 or P2Y4 receptors.109 Although these authors plumped for P2Y2 mediation, the fact that P2Y2 receptors have not been found in rat proximal tubules, whereas P2Y4 receptors have (vide supra), makes a basolateral P2Y4-mediated effect more likely.
View chapterPurchase book
Calcium Waves: Purinergic Regulation
M.L. Cotrina, M. Nedergaard, in Encyclopedia of Neuroscience, 2009
The P2X7 Pore-Forming Receptor and mCa2+ Signaling
The family of purinergic receptors is composed of two types, ionotropic (P2X) receptors and metabotropic (P2Y) receptors. It is known that astrocytes express P2Y1, P2Y2, and P2Y4 and P2X7 receptors, and it is likely that several purinergic receptors contribute to wave propagation in astrocytes.
Astrocytes depend on P2Y1 and P2Y2 receptors for the propagation of Ca2+ waves. Overexpression studies have shown that the properties of intercellular Ca2+ waves depend on the particular P2Y receptor subtype involved, differentially affecting the efficacy, velocity, amplitude, range, and 'saltatory' nature of the Ca2+ wave propagation.
Among P2X receptors, P2X7 is particular in that it forms large ion channels in response to ATP or other ligands and is present in many cell types, including astrocytes. P2X7 receptors can mediate the release of excitatory amino acids, like glutamate and aspartate, disclosing a novel, ligand-stimulated, nonvesicular astrocytic route for glutamate release. P2X7 receptors are activated by high concentrations of ATP, do not desensitize, and, more importantly, depend on extracellular Ca2+ ions.
As P2X7 activation can promote Ca2+ signals in optic nerve glia and Ca2+ waves among bone cells, an alternative hypothesis that could explain ATP-mediated astrocytic Ca2+ waves is that P2X7 receptors, and not connexin hemichannels, are the main pathway for the release of ATP. Using spinal cord astrocytes from Cx43 and/or P2X7 receptor-deficient mice, it has been recently found that, contrary to previous ideas, P2X7 receptors, and not connexin proteins, could be responsible for intercellular Ca2+ wave amplification when cells are exposed to solutions with low concentrations of divalent cations. Interestingly, the same study reported that many of the compounds that block connexin hemichannels, and that have been used to document the involvement of hemichannels as ATP-releasing pathways, are also antagonists of the P2X7 receptor, raising the question of the true contribution of connexin hemichannels in ATP release and Ca2+ wave propagation. Nonetheless, it is worth noting that the absence of P2X7 receptor in the model used in this study did not prevent the occurrence of astrocytic Ca2+ waves that were similar, in radius and number of cells engaged in the wave, to what was seen under normal conditions (Figure 1). So, the development of agents that can be used specifically to block one pathway versus the other is clearly needed to elucidate the exclusive or overlapping roles of these ATP-releasing pathways in astrocytes.

Sign in to download full-size image
Figure 1. An astrocytic Ca2+ wave evoked by focal electrical field stimulation in a hippocampal slice. The white arrowhead indicates the position of the stimulating electrode. The slice was loaded with the fluorescent Ca2+ indicator dye Fluo-4/AM for 1 h before two-photon laser scanning microscopy. The electrical stimulation evoked a Ca2+ wave that engaged multiple neighboring astrocytes expanding over a radius of 150–250 μm (ΔF/F, fluorescence ratio).
As P2X7 receptors are not easily activated, their expression is low or absent in some brain regions, and low Ca2+ conditions are mostly attained under pathological conditions, it has been suggested that these receptors could be major contributors of Ca2+ signaling during inflammation, ischemia, or other pathological states. Support for this idea comes from different fronts. First, ATP release and astrocytic Ca2+ signaling are both triggered by traumatic injury. Second, injury is also associated with a decrease in extracellular Ca2+, which enhances ATP release and Ca2+ signaling and increases the affinity of the P2X7 receptor. Third, P2X7 receptor can mediate cell death. On the basis of these findings, the contribution of the P2X7 receptors in the amplification of cell damage has been analyzed in a model of spinal cord injury. Using a novel bioluminescence technique in the intact, live animal, it has been found that the peritraumatic area after acute spinal cord injury exhibits unusually high levels of ATP release, which is accompanied by the subsequent cell death of spinal neurons that are highly immunoreactive for P2X7 receptors. Further studies in this model are needed to establish the source of ATP release during the injury period.
View chapterPurchase book
Adenosine Metabolism
Jacqueline A. Hubbard, Devin K. Binder, in Astrocytes and Epilepsy, 2016
Human Tissue Studies
P2X7 receptors were found in neurons, astrocytes, and microglia in human patients [159]. Surgically resected tissue from patients with TLE [129] or FCD [159] had elevated P2X7 receptor expression. P2Y1, P2Y2, and P2Y4 receptor expression levels were also elevated in resected epileptic tissue from patients with FCD, tuberous sclerosis complex (TSC), or low-grade astrocytoma compared to autopsy control brains of either patients without epilepsy or patients with gliosis but no seizures [158]. A missense single nucleotide polymorphism (SNP) was commonly found in the P2X7 receptor gene in Caucasian children with febrile seizures compared to febrile cases without seizures [160]. In addition, SNPs were found in toll-like receptor 4 (TLR4), interleukin-6 (IL-6) receptor, and prostaglandin E receptor 3 subtype EP3 (PTGER3) genes, suggesting a link between P2X7 receptors and inflammation [160].
View chapterPurchase book
Pharmacology of Purine and Pyrimidine Receptors
Laszlo Köles, ... Peter Illes, in Advances in Pharmacology, 2011
B P2Y Receptors
The basic agonist profile of the P2Y receptors was mentioned earlier. The most potent agonists at the P2Y1 receptors are ADP and its analogs (such as ADP-β-S). The P2Y2 receptor is activated approximately equally by both ATP and UTP. The P2Y4 receptors show UTP preference, while UDP is the most potent agonist at P2Y6 receptors. P2Y11 receptors are activated by ATP, P2Y12 and P2Y13 receptors prefer ADP and its analogs, and the P2Y14 receptor is preferentially activated by UDP-glucose (Abbracchio et al., 2009; Köles et al., 2008).
The classical antagonists show the following profile: suramin antagonizes most P2Y receptors but not P2Y4; PPADS antagonizes most potently the P2Y1 receptors, but it blocks other P2Y receptors only weakly, or not at all; reactive blue-2 is not effective as an antagonist at P2Y2 receptors but more (P2Y6) or less effectively (other P2Y receptors) antagonizes the residual ones.
Regarding the antagonists, MRS2179 is a specific antagonist of P2Y1 receptors, AR-C126313 is selective for P2Y2 receptors, MRS2578 is specific for P2Y6, NF157 antagonizes P2Y11, CT50547, clopidogrel, or the related antithrombotic compounds are selective for P2Y12, while MRS2211 for P2Y13. AR-C69931MX is a specific antagonist for both P2Y12 and P2Y13 receptors. For more detailed pharmacology, selective agonists, and antagonists of the P2Y receptors, see recent reviews (Fischer & Krügel, 2007; Jacobson et al., 2009; von Kügelgen, 2006).
View chapterPurchase book
Purinergic Neurotransmission and Nucleotide Receptors
Geoffrey Burnstock, in Primer on the Autonomic Nervous System (Third Edition), 2012
P2Y Receptors
At present, there are eight accepted human P2Y receptors: P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13 and P2Y14 (see [16] and Table 18.1). The missing numbers represent either non-mammalian orthologs, or receptors having some sequence homology to P2Y receptors, but for which there is no functional evidence of responsiveness to nucleotides. In contrast to P2X receptors, P2Y receptor genes do not contain introns in the coding sequence, except for the P2Y11 receptor. Site-directed mutagenesis of the P2Y1 and P2Y2 receptors has shown that some positively charged residues in TM3, TM6 and TM7 are crucial for receptor activation by nucleotides. From a phylogenetic and structural (i.e., protein sequence) point of view, two distinct P2Y receptor subgroups characterized by a relatively high level of sequence divergence have been identified. The first subgroup includes P2Y1,2,4,6,11 and the second subgroup encompasses the P2Y12,13,14 subtypes. Selective antagonists have been identified for some P2Y receptor subtypes (see Table 18.1). P2Y1, P2Y2, P2Y4 and P2Y6 receptors couple to G proteins to increase inositol triphosphate (IP3) and cytosolic calcium. Activation of the P2Y11 receptor by ATP leads to a rise in both cAMP and in IP3, whereas activation by UTP produces calcium mobilization without IP3 or cAMP increase. The P2Y13 receptor can simultaneously couple to G16, Gi and, at high concentrations of ADP, to Gs. The activation of several P2Y receptors is commonly associated with the stimulation of several mitogen-activated protein kinases, in particular extracellular signal regulated protein kinase 1/2. In most species, ADP is a more potent agonist than ATP at P2Y1 receptors. Site-directed mutagenesis studies on the human P2Y1 receptor have shown that amino acid residues in TM3, TM6 and TM7 are critical determinants in the binding of ATP. P2Y2 receptors are fully activated by ATP and UTP, whereas ADP and UDP are much less effective agonists. Expression of P2Y2 receptor mRNA and protein has been detected in many peripheral tissues. UTP is the most potent activator of the recombinant human P2Y4 receptor. In human and mouse, P2Y4 mRNA and protein was most abundant in the intestine, but was also detected in other organs. The mouse, rat and human P2Y6 receptors are UDP receptors. A wide tissue distribution of P2Y6 mRNA and protein has been demonstrated, with the highest expression in spleen, intestine, liver, brain and pituitary. ATPγS is a more potent agonist at the P2Y11 receptor than ATP. ADP is the natural agonist of the P2Y12 receptor. It is heavily expressed in platelets where it is the molecular target of the active metabolite of the antiplatelet drug clopidogrel. The P2Y13 ADP-sensitive receptor is strongly expressed in the spleen, followed by placenta, liver, heart, bone marrow, monocytes, T-cells, lung and various brain regions. The P2Y14 receptor is activated by UDP, UDP-glucose as well as UDP-galactose, UDP-glucuronic acid and UDP-N-acetylglucosamine.
The formation of oligomers by P2Y receptors is likely to be widespread and to greatly increase the diversity of purinergic signaling. P2X receptors are often expressed in the same cells as P2Y receptors. Thus, there is the possibility of bi-directional cross-talk between these two families of nucleotide-sensitive receptors.
P2X receptors in general mediate fast neurotransmission, but are sometimes located prejunctionally to mediate increase in release of cotransmitters, for example glutamate in terminals of primary afferent neurons in the spinal cord. P2Y receptors are particularly involved in prejunctional inhibitory modulation of transmitter release, as well as cell proliferation. P2Y1,2,4,6 receptors have been described on subpopulations of sympathetic neurons, P2Y2 and P2Y4 receptors in intracardiac ganglia, P2Y1 and P2Y2 receptors on sensory neurons while P2Y1 receptors appear to be the dominant subtype on enteric neurons. P2Y2 (and/or P2Y4) receptors are expressed on enteric neurons P2Y2 (and/or P2Y4) receptors are expressed on enteric glial cells.