Functional evidence for a novel suramin-insensitive pyrimidine receptor in rat small pulmonary arteries

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.

Role of the Purinergic P2Y2 Receptor in Pulmonary Hypertension

Pulmonary arterial hypertension (PAH), group 1 pulmonary hypertension (PH), is a fatal disease that is characterized by vasoconstriction, increased pressure in the pulmonary arteries, and right heart failure. PAH can be described by abnormal vascular remodeling, hyperproliferation in the vasculature, endothelial cell dysfunction, and vascular tone dysregulation. The disease pathomechanisms, however, are as yet not fully understood at the molecular level. Purinergic receptors P2Y within the G-protein-coupled receptor family play a major role in fluid shear stress transduction, proliferation, migration, and vascular tone regulation in systemic circulation, but less is known about their contribution in PAH. Hence, studies that focus on purinergic signaling are of great importance for the identification of new therapeutic targets in PAH. Interestingly, the role of P2Y2 receptors has not yet been sufficiently studied in PAH, whereas the relevance of other P2Ys as drug targets for PAH was shown using specific agonists or antagonists. In this review, we will shed light on P2Y receptors and focus more on the P2Y2 receptor as a potential novel player in PAH and as a new therapeutic target for disease management.

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.

Investigation of the functional expression of purine and pyrimidine receptors in porcine isolated pancreatic arteries

Receptors for purines and pyrimidines are expressed throughout the cardiovascular system. This study investigated their functional expression in porcine isolated pancreatic arteries. Pancreatic arteries (endothelium intact or denuded) were prepared for isometric tension recording and preconstricted with U46619, a thromboxane A(2) mimetic; adenosine-5'-diphosphate (ADP), uridine-5'-triphosphate (UTP) and MRS2768, a selective P2Y(2) agonist, were applied cumulatively, while adenosine-5'-triphosphate (ATP) and αβ-methylene-ATP (αβ-meATP) response curves were generated from single concentrations per tissue segment. Antagonists/enzyme inhibitors were applied prior to U46619 addition. ATP, αβ-meATP, UTP and MRS2768 induced vasoconstriction, with a potency order of αβ-meATP > MRS2768 > ATP ≥ UTP. Contractions to ATP and αβ-meATP were blocked by NF449, a selective P2X(1) receptor antagonist. The contraction induced by ATP, but not UTP, was followed by vasorelaxation. Endothelium removal and DUP 697, a cyclooxygenase-2 inhibitor, had no significant effect on contraction to ATP but attenuated that to UTP, indicating actions at distinct receptors. MRS2578, a selective P2Y(6) receptor antagonist, had no effect on contractions to UTP. ADP induced endothelium-dependent vasorelaxation which was inhibited by MRS2179, a selective P2Y(1) receptor antagonist, or SCH58261, a selective adenosine A(2A) receptor antagonist. The contractions to ATP and αβ-meATP were attributed to actions at P2X(1) receptors on the vascular smooth muscle, whereas it was shown for the first time that UTP induced an endothelium-dependent vasoconstriction which may involve P2Y(2) and/or P2Y(4) receptors. The relaxation induced by ADP is mediated by P2Y(1) and A(2A) adenosine receptors. Porcine pancreatic arteries appear to lack vasorelaxant P2Y(2) and P2Y(4) receptors.

Transient transcriptional regulation of the CYS-C1 gene and cyanide accumulation upon pathogen infection in the plant immune response

Cyanide is produced concomitantly with ethylene biosynthesis. Arabidopsis (Arabidopsis thaliana) detoxifies cyanide primarily through the enzyme β-cyanoalanine synthase, mainly by the mitochondrial CYS-C1. CYS-C1 loss of function is not toxic for the plant and leads to an increased level of cyanide in cys-c1 mutants as well as a root hairless phenotype. The classification of genes differentially expressed in cys-c1 and wild-type plants reveals that the high endogenous cyanide content of the cys-c1 mutant is correlated with the biotic stress response. Cyanide accumulation and CYS-C1 gene expression are negatively correlated during compatible and incompatible plant-bacteria interactions. In addition, cys-c1 plants present an increased susceptibility to the necrotrophic fungus Botrytis cinerea and an increased tolerance to the biotrophic Pseudomonas syringae pv tomato DC3000 bacterium and Beet curly top virus. The cys-c1 mutation produces a reduction in respiration rate in leaves, an accumulation of reactive oxygen species, and an induction of the alternative oxidase AOX1a and pathogenesis-related PR1 expression. We hypothesize that cyanide, which is transiently accumulated during avirulent bacterial infection and constitutively accumulated in the cys-c1 mutant, uncouples the respiratory electron chain dependent on the cytochrome c oxidase, and this uncoupling induces the alternative oxidase activity and the accumulation of reactive oxygen species, which act by stimulating the salicylic acid-dependent signaling pathway of the plant immune system.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

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.

Distribution of ecto-nucleotidases in mouse sensory circuits suggests roles for nucleoside triphosphate diphosphohydrolase-3 in nociception and mechanoreception

Nucleotide-activated P2X channels and P2Y metabotropic receptors participate in nociceptive signaling. Agonist availability is regulated by nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), -2, -3, and -8, a family of enzymes that hydrolyze extracellular ATP to generate ADP (a P2Y agonist) and AMP. They provide a major source of extracellular AMP, the substrate for adenosine production by ecto-5'-nucleotidase (NT5E), and thereby regulate adenosine (P1) receptor signaling. NTPDases vary in their efficiency of tri- and diphosphate hydrolysis; therefore, which family members are expressed impacts nucleotide availability and half-life. This study employed enzyme activity histochemistry to examine the distribution of ATPase activity and immunohistochemistry for NTPDase1, 2, 3, and 8 in dorsal root ganglion (DRG) and spinal cord. Nucleotidase activity was robust in spinal dorsal horn, confirming that nociceptive pathways are a major site of nucleotide transmission. In DRG, extensive staining revealed ATPase activity in a subset of neurons and in non-neuronal cells. mRNA for NTPDase1-3, but not NTPDase8, was detected in lumbar DRG and spinal cord. Immunoreactivity for NTPDase3 closely matched the distribution of ATPase activity, labeling DRG central projections in the dorsal root and superficial dorsal horn, as well as intrinsic spinal neurons concentrated in lamina II. In DRG, NTPDase3 co-localized with markers of nociceptors and with NT5E. In addition, labeling of a subset of larger-diameter neurons in DRG was consistent with intense staining of Meissner corpuscle afferents in glabrous skin. Merkel cells and terminal Schwann cells of hair follicle afferents were also labeled, but the axons themselves were negative. We propose that NTPDase3 is a key regulator of nociceptive signaling that also makes an unexpected contribution to innocuous tactile sensation.

NTPDase1 (CD39) controls nucleotide-dependent vasoconstriction in mouse

AIMS

Extracellular nucleotides are vasoactive molecules. The concentrations of these molecules are regulated by ectonucleotidases. In this study, we investigated the role of the blood vessel ectonucleotidase NTPDase1, in the vasoconstrictor effect of nucleotides using Entpd1(-/-) mice.

METHODS AND RESULTS

Immunofluorescence, enzyme histochemistry, and HPLC analysis were used to evaluate both NTPDase expression and activity in arteries and isolated vascular smooth muscle cells (VSMCs). Vascular reactivity was evaluated in vitro and mean arterial blood pressure was recorded in anesthetized mice after nucleotide i.v. infusion. Expression of nucleotide receptors in VSMCs was determined by RT-PCR. Entpd1(-/-) mice displayed a dramatic deficit of nucleotidase activity in blood vessel wall in situ and in VSMCs in comparison to control mice. In aortic rings from Entpd1(-/-) mice, UDP and UTP induced a potent and long-lasting constriction contrasting with the weak response obtained in wild-type rings. This constriction occurred through activation of P2Y(6) receptor and was independent of other uracil nucleotide-responding receptors (P2Y(2) and P2Y(4)). UDP infusion in vivo increased blood pressure and this effect was potentiated in Entpd1(-/-) mice. In addition, pressurized mesenteric arteries from Entpd1(-/-) mice displayed an enhanced myogenic response, consistent with higher local concentrations of endogenously released nucleotides. This effect was inhibited by the P2 receptor antagonist RB-2.

CONCLUSION

NTPDase1 is the major enzyme regulating nucleotide metabolism at the surface of VSMCs and thus contributes to the local regulation of vascular tone by nucleotides.

Knockout mice reveal a role for P2Y6 receptor in macrophages, endothelial cells, and vascular smooth muscle cells

P2Y receptors are G-protein-coupled receptors activated by extracellular nucleotides. The P2Y(6) receptor is selectively activated by UDP, and its transcript has been detected in numerous organs, including the spleen, thymus, intestine, blood leukocytes, and aorta. To investigate the biological functions of this receptor, we generated P2Y(6)-null mice by gene targeting. The P2Y(6) knockout (KO) mice are viable and are not distinguishable from the wild-type (WT) mice in terms of growth or fertility. In thioglycollate-elicited macrophages, the production of inositol phosphate in response to UDP stimulation was lost, indicating that P2Y(6) is the unique UDP-responsive receptor expressed by mouse macrophages. Furthermore, the amount of interleukin-6 and macrophage-inflammatory protein-2, but not tumor necrosis factor-alpha, released in response to lipopolysaccharide stimulation was significantly enhanced in the presence of UDP, and this effect was lost in the P2Y(6) KO macrophages. The endothelium-dependent relaxation of the aorta by UDP was abolished in KO P2Y(6) mice. The contractile effect of UDP on the aorta, observed when endothelial nitric-oxide synthase is blocked, was also abolished in P2Y(6)-null mice. In conclusion, we generated P2Y(6)-deficient mice and have shown that these mice have a defective response to UDP in macrophages, endothelial cells, and vascular smooth muscle cells. These observations might be relevant to several physiopathological conditions such as atherosclerosis or hypertension.

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.

Up4A stimulates endothelium-independent contraction of isolated rat pulmonary artery

Extracellular nucleotides, such as ATP, UDP, and UTP, regulate pulmonary vascular tone through P2X and P2Y receptors. Recently, uridine adenosine tetraphosphate (Up(4)A) was reported as a novel endothelium-derived vasoconstrictive factor. Up(4)A contains both purine and pyrimidine moieties, which potentially activate P2X and P2Y receptors. The present study examined the effect of Up(4)A on contractility of isolated rat pulmonary artery. Up(4)A at 1-100 microM stimulated contraction in a concentration-dependent manner. Up(4)A was equipotent as UTP and UDP in the endothelium-denuded artery while much more effective than UTP and UDP in endothelium-intact preparations. The vasoconstrictor effect of Up(4)A was inhibited by suramin but not IP(5)I or desensitization of P2X receptors with alpha,beta-methylene-ATP (alpha,beta-Me-ATP). Up(4)A-induced contraction was also inhibited by pretreatment with thapsigargin, nitrendipine, or EGTA but unaffected by H1152. Furthermore, unlike ATP and UTP, Up(4)A did not induce relaxation of endothelium-intact preparations precontracted with phenylephrine. These results suggest that Up(4)A is a potent vasoconstrictor, but not a vasodilator, of the rat pulmonary artery. Up(4)A likely acts through a suramin-sensitive P2Y receptor. The contractile effect of Up(4)A involves the entry of extracellular Ca(2+) and release of Ca(2+) from intracellular stores but not Ca(2+) sensitization via the RhoA/Rho kinase pathway. Up(4)A, therefore, potentially plays an important role in the regulation of pulmonary vascular tone.

P2 receptors in atherosclerosis and postangioplasty restenosis

Atherosclerosis is an immunoinflammatory process that involves complex interactions between the vessel wall and blood components and is thought to be initiated by endothelial dysfunction [Ross (Nature 362:801–09, 1993); Fuster et al. (N Engl J Med 326:242–50, 1992); Davies and Woolf (Br Heart J 69:S3–S11, 1993)]. Extracellular nucleotides that are released from a variety of arterial and blood cells [Di Virgilio and Solini (Br J Pharmacol 135:831–42, 2002)] can bind to P2 receptors and modulate proliferation and migration of smooth muscle cells (SMC), which are known to be involved in intimal hyperplasia that accompanies atherosclerosis and postangioplasty restenosis [Lafont et al. (Circ Res 76:996–002, 1995)]. In addition, P2 receptors mediate many other functions including platelet aggregation, leukocyte adherence, and arterial vasomotricity. A direct pathological role of P2 receptors is reinforced by recent evidence showing that upregulation and activation of P2Y₂ receptors in rabbit arteries mediates intimal hyperplasia [Seye et al. (Circulation 106:2720–726, 2002)]. In addition, upregulation of functional P2Y receptors also has been demonstrated in the basilar artery of the rat double-hemorrhage model [Carpenter et al. (Stroke 32:516–22, 2001)] and in coronary artery of diabetic dyslipidemic pigs [Hill et al. (J Vasc Res 38:432–43, 2001)]. It has been proposed that upregulation of P2Y receptors may be a potential diagnostic indicator for the early stages of atherosclerosis [Elmaleh et al. (Proc Natl Acad Sci U S A 95:691–95, 1998)]. Therefore, particular effort must be made to understand the consequences of nucleotide release from cells in the cardiovascular system and the subsequent effects of P2 nucleotide receptor activation in blood vessels, which may reveal novel therapeutic strategies for atherosclerosis and restenosis after angioplasty.

P2 receptors in atherosclerosis and postangioplasty restenosis

Atherosclerosis is an immunoinflammatory process that involves complex interactions between the vessel wall and blood components and is thought to be initiated by endothelial dysfunction [1-3]. Extracellular nucleotides that are released from a variety of arterial and blood cells [4] can bind to P2 receptors and modulate proliferation and migration of smooth muscle cells (SMC), which is known to be involved in intimal hyperplasia that accompanies atherosclerosis and postangioplasty restenosis [5]. In addition, P2 receptors mediate many other functions, including platelet aggregation, leukocyte adherence, and arterial vasomotoricity. A direct pathological role of P2 receptors is reinforced by recent evidence showing that up-regulation and activation of P2Y(2) receptors in rabbit arteries mediates intimal hyperplasia [6]. In addition, up-regulation of functional P2Y receptors also has been demonstrated in the basilar artery of the rat double-hemorrhage model [7] and in coronary arteries of diabetic dyslipidemic pigs [8]. It has been proposed that up-regulation of P2Y receptors may be a potential diagnostic indicator for the early stages of atherosclerosis [9]. Therefore, particular effort must be made to understand the consequences of nucleotide release from cells in the cardiovascular system and the subsequent effects of P2 nucleotide receptor activation in blood vessels, which may reveal novel therapeutic strategies for atherosclerosis and restenosis after angioplasty.

Multiple P2Y receptors couple to calcium-dependent, chloride channels in smooth muscle cells of the rat pulmonary artery

BACKGROUND

Uridine 5'-triphosphate (UTP) and uridine 5'-diphosphate (UDP) act via P2Y receptors to evoke contraction of rat pulmonary arteries, whilst adenosine 5'-triphosphate (ATP) acts via P2X and P2Y receptors. Pharmacological characterisation of these receptors in intact arteries is complicated by release and extracellular metabolism of nucleotides, so the aim of this study was to characterise the P2Y receptors under conditions that minimise these problems.

METHODS

The perforated-patch clamp technique was used to record the Ca2+-dependent, Cl- current (I(Cl,Ca)) activated by P2Y receptor agonists in acutely dissociated smooth muscle cells of rat small (SPA) and large (LPA) intrapulmonary arteries, held at -50 mV. Contractions to ATP were measured in isolated muscle rings. Data were compared by Student's t test or one way ANOVA.

RESULTS

ATP, UTP and UDP (10(-4) M) evoked oscillating, inward currents (peak = 13-727 pA) in 71-93% of cells. The first current was usually the largest and in the SPA the response to ATP was significantly greater than those to UTP or UDP (P < 0.05). Subsequent currents tended to decrease in amplitude, with a variable time-course, to a level that was significantly smaller for ATP (P < 0.05), UTP (P < 0.001) and UDP (P < 0.05) in the SPA. The frequency of oscillations was similar for each agonist (mean approximately to 6-11.min(-1)) and changed little during agonist application. The non-selective P2 receptor antagonist suramin (10(-4) M) abolished currents evoked by ATP in SPA (n = 4) and LPA (n = 4), but pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (10(-4) M), also a non-selective P2 antagonist, had no effect (n = 4, 5 respectively). Currents elicited by UTP (n = 37) or UDP (n = 14) were unaffected by either antagonist. Contractions of SPA evoked by ATP were partially inhibited by PPADS (n = 4) and abolished by suramin (n = 5). Both antagonists abolished the contractions in LPA.

CONCLUSION

At least two P2Y subtypes couple to ICl,Ca in smooth muscle cells of rat SPA and LPA, with no apparent regional variation in their distribution. The suramin-sensitive, PPADS-resistant site activated by ATP most resembles the P2Y11 receptor. However, the suramin- and PPADS-insensitive receptor activated by UTP and UDP does not correspond to any of the known P2Y subtypes. These receptors likely play a significant role in nucleotide-induced vasoconstriction.

Impaired NO-dependent inhibition of store- and receptor-operated calcium entry in pulmonary vascular smooth muscle after chronic hypoxia

We have recently demonstrated that chronic hypoxia (CH) attenuates nitric oxide (NO)-mediated decreases in pulmonary vascular smooth muscle (VSM) intracellular free calcium concentration ([Ca2+]i) and promotes NO-dependent VSM Ca2+ desensitization. The objective of the current study was to identify potential mechanisms by which CH interferes with regulation of [Ca2+]i by NO. We hypothesized that CH impairs NO-mediated inhibition of store-operated (capacitative) Ca2+ entry (SOCE) or receptor-operated Ca2+ entry (ROCE) in pulmonary VSM. To test this hypothesis, we examined effects of the NO donor, spermine NONOate, on SOCE resulting from depletion of intracellular Ca2+ stores with cyclopiazonic acid, and on UTP-induced ROCE in isolated, endothelium-denuded, pressurized pulmonary arteries (213 +/- 8 microm inner diameter) from control and CH (4 wk at 0.5 atm) rats. Arteries were loaded with fura-2 AM to continuously monitor VSM [Ca2+]i. We found that the change in [Ca2+]i associated with SOCE and ROCE was significantly reduced in vessels from CH animals. Furthermore, spermine NONOate diminished SOCE and ROCE in vessels from control, but not CH animals. We conclude that NO-mediated inhibition of SOCE and ROCE is impaired after CH-induced pulmonary hypertension.

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.

P2X(1) receptor-deficient mice establish the native P2X receptor and a P2Y6-like receptor in arteries

The contribution of P2 receptors to vasoconstriction of mouse mesenteric arteries was determined using wild-type (WT) and P2X(1) receptor-deficient (KO) animals. alpha,beta-methylene ATP (alpha,beta-meATP) and ATP evoked transient inward currents and constrictions of WT mesenteric arteries. In contrast, alpha,beta-meATP (100 microM) and ATP (100 microM) failed to evoke responses in KO arteries from a range of vascular beds. Nerve stimulation (100 pulses at 10 Hz) evoked constrictions of mesenteric arteries. For WT arteries, the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2'-5'-disulfonate (PPADS) (30 microM) reduced the amplitude of response by approximately 50%; the residual constriction was abolished by prazosin (0.1 microM). In KO mice, vasoconstriction induced by nerve stimulation was reduced in amplitude by approximately 50%, unaffected by PPADS, but was abolished by prazosin. ADP (1 mM) (a P2Y(1), P2Y(12), and P2Y(13) receptor agonist) was ineffective. Because ATP had no effect on mesenteric artery tone from KO mice, this rules out the contribution of P2Y(2) receptors. The P2Y(4) receptor agonist ITP also failed to contract mesenteric arteries. However, UTP and UDP evoked sustained contractions of mesenteric arteries with similar potency (EC(50) approximately 10 microM). Complementary studies using reverse-transcriptase polymerase chain reaction showed that mesenteric arteries express P2Y(1), P2Y(2), and P2Y(6) receptors. These results demonstrate that homomeric P2X(1) receptors underlie the artery smooth muscle P2X receptor phenotype and contribute approximately 50% to sympathetic neurogenic vasoconstriction and indicate the presence of a UTP- and UDP-sensitive P2Y(6)-like receptor, but not vasoconstrictor P2Y(2) or P2Y(4) receptors, on mouse mesenteric arteries.

Regional variation in P2 receptor expression in the rat pulmonary arterial circulation

The P2 receptors that mediate contraction of the rat isolated small (SPA, 200-500 micro m i.d.) and large (LPA, 1-1.5 mM i.d.) intrapulmonary arteries were characterized. 2 In endothelium-denuded vessels the contractile order of potency was alpha,beta-methyleneATP (alpha,beta-meATP)>>UDP=UTP=ATP=2-methylthioATP>ADP in the SPA and alpha,beta-meATP=UTP>or=UDP>2-methylthioATP, ATP>>ADP in the LPA. alpha,beta-meATP, 2-methylthioATP and ATP had significantly greater effects in the SPA than the LPA (P<0.001), but there was no difference in the potency of UTP or UDP between the vessels. 3 In the SPA, P2X1 receptor desensitisation by alpha,beta-meATP (100 microM) inhibited contractions to alpha,beta-meATP (10 nM-300 microM), but not those to UTP or UDP (100 nM-300 microM). In the LPA, prolonged exposure to alpha,beta-meATP (100 microM) did not desensitize P2X receptors. 4 Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), suramin and reactive blue 2 (RB2) (30-300 microM) inhibited contractions evoked by alpha,beta-meATP. UTP and UDP were potentiated by PPADS, unaffected by RB2 and inhibited, but not abolished by suramin. 1 and 3 mM suramin produced no further inhibition, indicating suramin-resistant components in the responses to UTP and UDP. 5 Thus, both P2X and P2Y receptors mediate contraction of rat large and small intrapulmonary arteries. P2Y agonist potency and sensitivity to antagonists were similar in small and large vessels, but P2X agonists were more potent in small arteries. This indicates differential expression of P2X, but not P2Y receptors along the pulmonary arterial tree.

ATP-induced endothelium-independent enhancement of lymphatic vasomotion in guinea-pig mesentery involves P2X and P2Y receptors

1. The present study has investigated mechanisms underlying ATP-induced endothelium-independent enhancement of vasomotion in guinea-pig mesenteric lymphatic vessels. 2. Lymphatic vasomotion, vessel tone and smooth muscle [Ca(2+)](i) showed similar ATP concentration-response curves. 3. ATP, at 0.1 mM, caused a biphasic increase in tonic [Ca(2+)](i) and superimposed vasomotion-associated Ca(2+) transients. All ATP-induced [Ca(2+)](i) changes were abolished by incubating the smooth muscle with suramin (0.1 mM). 4. alpha,beta-MeATP (0.1 mM) and UTP (0.1 mM) caused similar changes in [Ca(2+)](i) but the responses to these agonists were smaller than to ATP. 5. The actions of alpha,beta-MeATP (0.1 mM) were inhibited by suramin (0.1 mM) and PPADS (30 micro M) but not by reactive blue 2 (30 micro M). 6. In the presence of alpha,beta-MeATP (0.1 mM), the increases in tonic [Ca(2+)](i) and vasomotion-associated Ca(2+) transients induced by ATP (0.1 mM) were inhibited by U73122 (5 micro M), CPA (20 micro M) and heparin, whereas U73343 (5 micro M) and pre-treatment with PTx (100 ng ml(-1)) had no significant effects. 7. Depletion of the intracellular stores with CPA (20 micro M) caused an increase in [Ca(2+)](i), which was not blocked by desensitization of P(2X) receptors with alpha,beta-MeATP. 8. The data indicate that ATP, at relatively high concentrations increases lymphatic smooth muscle [Ca(2+)](i) and vasomotion through activation of P(2X1) and P(2Y2) purinoceptors present on lymphatic smooth muscle. The increase in [Ca(2+)](i) is likely to result from Ca(2+) release from inositol-1,4,5-trisphosphate-sensitive stores as well as Ca(2+) influx through store-operated channels and P(2X)-gated channels.

Analysis of purine- and pyrimidine-induced vascular responses in the isolated rat cerebral arteriole

Effects of extraluminal UTP were studied and compared with vascular responses to ATP and its analogs in rat cerebral-penetrating arterioles. UTP, UDP, 2-methylthio-ATP, and alpha,beta-methylene-ATP dilated arterioles at the lowest concentration and constricted them at high concentrations. Low concentrations of ATP dilated the vessels; high concentrations caused a biphasic response, with transient constriction followed by dilation. Endothelial impairment inhibited ATP- and UTP-mediated dilation and potentiated constriction to UTP but not to ATP. ATP- and 2-methylthio-ATP- but not UTP-mediated constrictions were inhibited by desensitization with 10(-6) M alpha,beta-methylene-ATP or 3 x 10(-6) M pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). PPADS at 10(-4) M abolished the UTP-mediated constriction and induced vasodilation in a dose-dependent manner but did not affect the dilation to ATP. These results suggest that in rat cerebral microvessels 1) ATP and 2-methylthio-ATP induce transient constriction via smooth muscle P(2X1) receptors in the cerebral arteriole, 2) UTP stimulates two different classes of P(2Y) receptors, resulting in constriction (smooth muscle P(2Y4)) and dilation (possibly endothelial P(2Y2)), and 3) ATP and UTP produce dilation by stimulation of a single receptor (P(2Y2)).

Properties of P2X and P2Y receptors are dependent on artery diameter in the rat mesenteric bed

P2 receptor mediated contractile responses have been characterized in different diameter arteries from the rat mesenteric arterial vasculature (first, second to third and fifth to sixth order for large, medium and small arteries) using wire myograph and diamtrak video imaging. alpha,ss-methylene ATP (alpha,beta-meATP) evoked transient concentration-dependent contractions in mesenteric arteries with EC(50) values of 0.4, 2.5 and 107 microM for small, medium and large arteries respectively. Suramin (10 - 100 microM) produced substantial parallel rightward shifts of the concentration-response curve to alpha,beta-meATP in small and medium-sized arteries with pA(2) of 5.1. Responses in large vessels were unaffected by suramin. Immunohistochemical analysis of arterial sections revealed no substantial differences in expression patterns of P2X receptors between different sizes of artery. P2X(1) receptors were expressed at high levels, P2X(4) and P2X(5) receptors were also detected on smooth muscle. The P2X receptor response is dominated by P2X(1) receptor in small and medium arteries but the nature of the receptor mediating the suramin insensitive alpha,beta-meATP mediated response in large arteries is unclear. The P2Y receptor agonist UTP was significantly more potent in small than in medium or large arteries (EC(50) values: 15.0 microM small, 88.5 microM diamtrak medium 1.6 mM myography medium and 1.4 mM large). Responses in both small and medium-sized vessels were reduced by suramin (30 - 100 microM). The sensitivity to UTP and suramin indicates the presence of P2Y(2) receptors. This study shows that P2 receptors do not have a homogenous phenotype throughout the mesenteric vascular bed and that the properties depend on artery size.

P2 purinoceptor-mediated control of rat cerebral (pial) microvasculature; contribution of P2X and P2Y receptors

Purine and pyrimidine nucleotides evoke changes in the vascular tone of medium to large cerebral vessels through the activation of P2 purinoceptors. We have applied P2 receptor drugs to rat pial arterioles and measured changes in arteriole diameter (o.d. 40-84 micrometer at rest), and recorded currents from arteriolar smooth muscle cells using patch-clamp techniques. Transient vasoconstrictions and rapidly inactivating currents were evoked by alpha,beta-methylene ATP (0.1-30 micrometer) and were sensitive to the P2 receptor antagonists suramin and iso-PPADS. UTP and UDP (0.1-1000 micrometer) evoked sustained suramin-sensitive vasoconstrictions. ATP (0.1-1000 micrometer) and 2-methylthioATP (2MeSATP, 300 micrometer) evoked transient vasoconstrictions followed by sustained vasodilatations. ADP application resulted in only vasodilatation (EC50 approximately 4 micrometer). Vasodilator responses to ATP, 2MeSATP or ADP were unaffected by suramin (100 micrometer). RT-PCR analysis indicated that P2X1-7 and P2Y1,2,6 RNA can be amplified from the pial sheet. Our results provide direct evidence for the presence of functional P2X receptors with a phenotype resembling the P2X1 receptor subtype on cerebral resistance arterioles. The pharmacological properties of the pyrimidine-evoked responses suggest that a combination of P2Y2- and P2Y6-like receptors are responsible for the sustained vasoconstrictions. It is therefore likely that the nucleotides and their associated receptors are involved in a complicated regulatory system to control cerebral blood pressure.

The stable pyrimidines UDPbetaS and UTPgammaS discriminate between the P2 receptors that mediate vascular contraction and relaxation of the rat mesenteric artery

The contractile and relaxant effects of the different P2 receptors were characterized in the rat isolated mesenteric artery by use of extracellular nucleotides, including the stable pyrimidines uridine 5'-O-thiodiphosphate (UDPbetaS) and uridine 5'-O-3-thiotriphosphate (UTPgammaS). The selective P2X receptor agonist, alphabeta-methylene-adenosine triphosphate (alphabeta-MeATP) stimulated a potent (pEC(50)=6.0) but relatively weak contraction (E:(max)=57% of 60 mM K(+)). The contractile concentration-response curve of adenosine triphosphate (ATP) was biphasic when added in single concentrations. The first part of the response could be desensitized by alphabeta-MeATP, indicating involvement of P2X receptors, while the second part might be mediated by P2Y receptors. The contractile P2Y receptors were further characterized after P2X receptor desensitization with 10 microM alphabeta-MeATP. Uridine diphosphate (UDP), uridine triphosphate (UTP) and ATP stimulated contraction only in high concentrations (1 - 10 mM). The selective P2Y(6) agonist, UDPbetaS, and the P2Y(2)/P2Y(4)-receptor agonists UTPgammaS and adenosine 5'-O-3-thiotriphosphate (ATPgammaS) were considerably more potent and efficacious (E:(max) approximately 250% of 60 mM K(+)). Adenosine 5'-O-thiodiphosphate (ADPbetaS) was inactive, excluding contractile P2Y(1) receptors. After precontraction with 1 microM noradrenaline, UTP, ADP and ATP induced relaxations with similar potencies (pEC(50) approximately 5.0). UTPgammaS, ADPbetaS and ATPgammaS were approximately one log unit more potent indicating the presence of endothelial P2Y(1) and P2Y(2)/P2Y(4) receptors. The P2Y(6) receptor agonist, UDPbetaS, had no effect. UDPbetaS and UTPgammaS are useful tools when studying P2 receptors in tissue preparations with ectonucleotidase activity. Contractile responses can be elicited by stimulation of P2Y(6) and, slightly less potently, P2Y(2)/P2Y(4) receptors. The P2X response was relatively weak, and there was no P2Y(1) response. Stimulation of P2Y(1) and P2Y(2)/P2Y(4) receptors elicited relaxation, while P2Y(6) did not contribute.

Vasoconstriction of intrapulmonary arteries to P2-receptor nucleotides in normal and pulmonary hypertensive newborn piglets

1. The vasoconstrictor responses of isolated intrapulmonary arteries (IPA) to P2-receptor agonists was investigated during adaptation to extrauterine life in the normal piglet and the effect of pulmonary hypertension was studied following exposure of newborn animals to chronic hypobaric hypoxia (51 kPa) for 3 days. 2. At resting tone, alpha,beta-methyleneATP (alpha,beta-meATP) (P2X-receptor agonist) contracted intrapulmonary arteries from adult, but not immature pigs, and repeated application desensitized the response. 3. Adenosine 5'-triphosphate (ATP) induced endothelium-independent relaxation at low concentrations at all ages, a variable contractile response to high concentrations developed by 3 days, becoming larger and consistent by 14 days of age. 4. Uridine 5'-triphosphate (UTP) evoked a contractile response in normal intrapulmonary arteries from foetal to adult life, the magnitude of the response increasing with age. Endothelial removal and pre-incubation with Nomega-nitro-L-arginine methyl ester (L-NAME) (100 microM) increased the contractile response of adult vessels. 5. Pre-incubation with alpha,beta-meATP (100 microM), increased the contractile response to UTP in both newborn and adult vessels. ATP-induced relaxations were reduced in newborn vessels but there was no effect on the responses of adult vessels. 6. Responses to UTP, ATP and alpha,beta-meATP of intrapulmonary arteries from newborn piglets exposed to chronic hypobaric hypoxia for 3 days were normal. 7. In summary, UTP elicited marked vasoconstriction of porcine IPA at all ages. UTP and ATP responses were consistent with activation of the P2Y4-receptor recently identified in vascular smooth muscle by others. alpha, beta-meATP induced a small vasoconstriction in the adult probably via the P2X1-receptor. Responses remained normal in neonatal pulmonary hypertension.

Vasodilatation of intrapulmonary arteries to P2-receptor nucleotides in normal and pulmonary hypertensive newborn piglets

1. The vasodilator responses of isolated intrapulmonary arteries (IPA) to P2-receptor agonists were investigated during adaptation to extrauterine life in the piglet. The effect of pulmonary hypertension on the normal response was determined after exposing newborn animals to chronic hypobaric hypoxia (51 kPa) for 3 days. 2. Adenosine 5'-triphosphate (ATP), 2-methylthioATP (2-meSATP), adenosine 5-O-(2-thiodiphos-phate) (ADPbetaS) and uridine 5'-triphosphate (UTP) induced a relaxation in normal newborn piglet IPA pre-contracted with prostaglandin F2alpha (PGF2alpha). The relaxations were not affected by removal of the endothelium. The responses to ATP and ADPbetaS increased significantly with age. 3. The relaxation responses of IPA to ATP, 2-meSATP and ADPbetaS continued to increase normally after birth in an hypoxic environment. 4. The results of the study show that vasodilatation of porcine intrapulmonary vessels to nucleotides increased during development from foetus to adult; that the vasodilatation to purines was mediated by P2Y-receptors on the vascular smooth muscle rather than on the endothelium; and that the P2Y-receptor mediated relaxation of IPA remained normal in the pulmonary hypertensive neonate.