Relative contribution of P2U- and P2Y-purinoceptors to endothelium-dependent vasodilatation in the golden hamster isolated mesenteric arterial bed

Effects of Dual Purinoceptor-dependent Approach on Release of Vascular Endothelial Growth Factor From Human Microvascular Endothelial Cell (HMEC-1) and Endothelial Cell Condition

In the recent years, the awareness of the role purinergic signaling plays as a therapeutic target has increased considerably. The purinoceptor allows the action of extracellular nucleotides (P2 receptors) and intermediary products of their metabolism, such as adenosine (P1 receptors), regulating pivotal processes occurring in the cardiovascular system. This study focuses on a dual purinoreceptor-dependent approach, based on the activation of adenosine P1 receptors with the simultaneous inhibition of P2Y12 receptors that can be used as novel platelet inhibitors in antithrombotic therapy. Endothelial cells are directly exposed to the drugs circulating in the bloodstream. That is why effects of our concept on human microvascular endothelial cells (HMEC-1) were examined in in vitro studies, such as enzyme-linked immunosorbent assay and scratch assays. In response to adenosine receptor agonists, levels of secreted vascular endothelial growth factor varied. Two of them, 5'-N-ethylcarboxamidoadenosine and MRE0094 remarkably increased vascular endothelial growth factor release. The elevated levels were reduced when used together with the P2Y12 receptor antagonist. Also, rates of wound closure in a scratch assay were significantly reduced in these cases. The results suggest that the proposed treatment does not impair endothelial cell condition. In addition, it is suggested as a collateral benefit, namely solving the problem of excessive activation of endothelial cells during antiplatelet therapy.

P2Y₂ receptor activation decreases blood pressure via intermediate conductance potassium channels and connexin 37

AIMS

Nucleotides are important paracrine regulators of vascular tone. We previously demonstrated that activation of P2Y₂ receptors causes an acute, NO-independent decrease in blood pressure, indicating this signalling pathway requires an endothelial-derived hyperpolarization (EDH) response. To define the mechanisms by which activation of P2Y₂ receptors initiates EDH and vasodilation, we studied intermediate-conductance (KCa3.1, expressed in endothelial cells) and big-conductance potassium channels (KCa1.1, expressed in smooth muscle cells) as well as components of the myoendothelial gap junction, connexins 37 and 40 (Cx37, Cx40), all hypothesized to be part of the EDH response.

METHODS

We compared the effects of a P2Y₂/₄ receptor agonist in wild-type (WT) mice and in mice lacking KCa3.1, KCa1.1, Cx37 or Cx40 under anaesthesia, while monitoring intra-arterial blood pressure and heart rate.

RESULTS

Acute activation of P2Y₂/₄ receptors (0.01-3 mg kg(-1) body weight i.v.) caused a biphasic blood pressure response characterized by a dose-dependent and rapid decrease in blood pressure in WT (maximal response % of baseline at 3 mg kg(-1) : -38 ± 1%) followed by a consecutive increase in blood pressure (+44 ± 11%). The maximal responses in KCa3.1(-/-) and Cx37(-/-) were impaired (-13 ± 5, +17 ± 7 and -27 ± 1, +13 ± 3% respectively), whereas the maximal blood pressure decrease in response to acetylcholine at 3 μg kg(-1) was not significantly different (WT: -53 ± 3%; KCa3.1(-/-) : -52 ± 3; Cx37(-/-) : -53 ± 3%). KCa1.1(-/-) and Cx40(-/-) showed an identical biphasic response to P2Y2/4 receptor activation compared to WT.

CONCLUSIONS

The data suggest that the P2Y2/4 receptor activation elicits blood pressure responses via distinct mechanisms involving KCa3.1 and Cx37.

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta-amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein-coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.

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.

P2Y receptors in the mammalian nervous system: pharmacology, ligands and therapeutic potential

P2Y receptors for extracellular nucleotides are coupled to activation of a variety of G proteins and stimulate diverse intracellular signaling pathways that regulate functions of cell types that comprise the central nervous system (CNS). There are 8 different subtypes of P2Y receptor expressed in cells of the CNS that are activated by a select group of nucleotide agonists. Here, the agonist selectivity of these 8 P2Y receptor subtypes is reviewed with an emphasis on synthetic agonists with high potency and resistance to degradation by extracellular nucleotidases that have potential applications as therapeutic agents. In addition, the recent identification of a wide variety of subtype-selective antagonists is discussed, since these compounds are critical for discerning cellular responses mediated by activation of individual P2Y receptor subtypes. The functional expression of P2Y receptor subtypes in cells that comprise the CNS is also reviewed and the role of each subtype in the regulation of physiological and pathophysiological responses is considered. Other topics include the role of P2Y receptors in the regulation of blood-brain barrier integrity and potential interactions between different P2Y receptor subtypes that likely impact tissue responses to extracellular nucleotides in the CNS. Overall, current research suggests that P2Y receptors in the CNS regulate repair mechanisms that are triggered by tissue damage, inflammation and disease and thus P2Y receptors represent promising targets for the treatment of neurodegenerative diseases.

Mechanisms underlying altered extracellular nucleotide-induced contractions in mesenteric arteries from rats in later-stage type 2 diabetes: effect of ANG II type 1 receptor antagonism

Little is known about the vascular contractile responsiveness to, and signaling pathways for, extracellular nucleotides in the chronic stage of type 2 diabetes or whether the ANG II type 1 receptor blocker losartan might alter such responses. We hypothesized that nucleotide-induced arterial contractions are augmented in diabetic Goto-Kakizaki (GK) rats and that treatment with losartan would normalize the contractions. Here, we investigated the vasoconstrictor effects of ATP/UTP in superior mesenteric arteries isolated from GK rats (37-42 wk old) that had or had not received 2 wk of losartan (25 mg·kg(-1)·day(-1)). In arteries from GK rats (vs. those from Wistar rats), 1) ATP- and UTP-induced contractions, which were blocked by the nonselective P2 antagonist suramin, were enhanced, and these enhancements were suppressed by endothelial denudation, by cyclooxygenase (COX) inhibitors, or by a cytosolic phospholipase A(2) (cPLA(2)) inhibitor; 2) both nucleotides induced increased release of PGE(2) and PGF(2α); 3) nucleotide-stimulated cPLA(2) phosphorylations were increased; 4) COX-1 and COX-2 expressions were increased; and 5) neither P2Y2 nor P2Y6 receptor expression differed, but P2Y4 receptor expression was decreased. Mesenteric arteries from GK rats treated with losartan exhibited (vs. untreated GK) 1) reduced nucleotide-induced contractions, 2) suppressed UTP-induced release of PGE(2) and PGF(2α), 3) suppressed UTP-stimulated cPLA(2) phosphorylation, 4) normalized expressions of COX-2 and P2Y4 receptors, and 5) reduced superoxide generation. Our data suggest that the diabetes-related enhancement of ATP-mediated vasoconstriction was due to P2Y receptor-mediated activation of the cPLA(2)/COX pathway and, moreover, that losartan normalizes such contractions by a suppressing action within this pathway.

P2Y₂ receptor activation decreases blood pressure and increases renal Na⁺ excretion

ATP and UTP are endogenous agonists of P2Y(2/4) receptors. To define the in vivo effects of P2Y(2) receptor activation on blood pressure and urinary excretion, we compared the response to INS45973, a P2Y(2/4) receptor agonist and UTP analog, in wild-type (WT) and P2Y(2) receptor knockout (P2Y(2)-/-) mice. INS45973 was administered intravenously as a bolus injection or continuous infusion to determine effects on blood pressure and renal function, respectively. Within seconds, bolus application of INS45973 (0.1 to 3 mg/kg body wt) dose-dependently decreased blood pressure in WT (maximum response -35 ± 2 mmHg) and to a similar extent in endothelial nitric oxide synthase knockout mice. By contrast, blood pressure increased in P2Y(2)-/- (maximum response +18 ± 1 mmHg) but returned to basal levels within 60 s. Continuous infusion of INS45973 (25 to 750 μg·min(-1)·kg(-1) body wt) dose-dependently increased urinary excretion of Na(+) in WT (maximum response +46 ± 15%) but reduced Na(+) excretion in P2Y(2)-/- (maximum responses of -45 ± 15%) mice. In renal clearance experiments, INS45973 did not affect glomerular filtration rate but lowered blood pressure and increased fractional excretion of fluid, Na(+), and K(+) in WT relative to P2Y(2)-/- mice. The blood pressure responses to INS45973 are consistent with P2Y(2) receptor-mediated NO-independent vasodilation and implicate responses to endothelium-derived hyperpolarizing factor, and P2Y(2) receptor-independent vasoconstriction, probably via activation of P2Y(4) receptors on smooth muscle. Systemic activation of P2Y(2) receptors thus lowers blood pressure and inhibits renal Na(+) reabsorption, effects suggesting the potential utility of P2Y(2) agonism in the treatment of 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.

P2Y1 and P2Y2 receptor distribution varies along the human placental vascular tree: role of nucleotides in vascular tone regulation

The expression of purinergic P2Y receptors (P2YRs) along the cord, superficial chorionic vessels and cotyledons of the human placenta was analysed and functional assays were performed to determine their vasomotor activity. Immunoblots for the P2Y(1)R and P2Y(2)R revealed a 6- to 8-fold increase in receptor expression from the cord to the chorionic or cotyledon vessels. In the cord and chorionic vessels the receptor distribution was mainly in the smooth muscle, whereas in the cotyledon vessels these receptors were equally distributed between the endothelium and smooth muscle cells. An exception was the P2Y(2)R at the umbilical artery, which was distributed as in the cotyledon. mRNA coding for the P2Y(1)R and P2Y(2)R were detected by RT-PCR and the mRNA coding for the P2Y(4)R, P2Y(6)R and P2Y(11)R was also identified. Application of 2-MeSADP and uridine triphosphate (UTP), preferential P2Y(1)R and P2Y(2)R ligands, respectively, resulted in contraction of isolated rings from umbilical and chorionic vessels. The vasoconstriction was blocked in a concentration-dependent manner by 10-100 nm indomethacin or 10 nm GR32191, suggesting the involvement of thromboxane receptors. MRS 2179, a selective P2Y(1)R antagonist, reduced the 2-MeSADP- but not the UTP-evoked contractions. Perfusion of cotyledons with 2-MeSADP or UTP evoked concentration-dependent reductions in perfusion pressure mediated by the NO-cGMP pathway. Blockade of NO synthase abolished the vasodilatation and the rise in luminal NO elicited by either agonist. MRS 2179 antagonized the dilatation and rise in luminal NO evoked by 2-MeSADP but not by UTP. In summary, P2Y(1)R and P2Y(2)R are unevenly distributed along the human placental vascular tree; both receptors are coupled to different signalling pathways in the cord/chorionic vessels versus the cotyledon leading to opposing vasomotor responses.

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.

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.

Characterization of the NTPDase activities in the mesentery pre- and post-capillary circuits of the guinea pig

NTPDase is one of the principal enzymes involved in the sequential hydrolysis of ATP. In the present study, the presence and functionality of NTPDase in the mesenteric vein and artery were examined. Adenosine triphosphate (ATP) (0.01-1000 pmol) induces a dose-dependent vasodilation in the isolated arterial and venous mesenteric vasculatures of the guinea pig. Adenosine diphosphate (ADP) (0.01-1000 pmol) but not adenosine monophosphate (AMP) (0.01-1000 pmol) induces a similar response in the mesenteric vascular circuit. L-NAME, a nitric oxide synthase inhibitor (200 microM, 30 min), significantly reduces the arterial dilatory effect of ATP and abolishes the responses to ADP and AMP. Complete removal of the endothelium with 3-[(3-cholamidopropyl) dimethylammonio]-1-propansulfonate (CHAPS) (20 mM, 2 x 45 s) abolishes ATP-induced responses. Infusion of ATP in the vascular circuit generated detectable amounts of ADP and AMP, as measured by HPLC. CHAPS treatment significantly reduced the level of ATP and the production of AMP in the arterial mesenteric circuit. In contrast to the arterial mesenteric vasculature, endothelium removal in the venous circuit triggered a marked potentiation of ADP release and, interestingly, a marked reduction in the release of AMP. Moreover, a specific inhibitor of NTP diphosphohydrolase, 1-hydroxynaphthlene-3,6-disulfonic acid BGO 136 (10 mM for 20 min), significatively reduced AMP production in both vascular preparations. These results confirm that the endothelium contributes to the vasoactive properties of ATP, ADP, and AMP. Our data also demonstrated a significant role of endothelium in NTPDase activity on ADP and AMP production prior to exogenous administration of ATP. The activity of this particular enzyme appears to be different from the reaction products viewpoint (i.e., the production of ADP) in the pre- and post-mesenteric circuits, suggesting two different isoforms with different substrate specificities.

Role of erythrocyte in regulating local O2 delivery mediated by hemoglobin oxygenation

The release of ATP from red blood cells (RBC) in response to low O2 levels is linked to ATP production and the oxygenation state of hemoglobin. Because O2 is unloaded from the RBC, the concentration of deoxygenated hemoglobin increases, displacing phosphofructokinase from the cytoplasmic domain of band 3. We hypothesize that the ATP molecules produced through this glycolytic stimulation at the membrane surface result in the release of ATP from the RBC. Rat whole blood exposed to 5 min of low PO2 in vitro increased plasma [ATP] by 1.0 miccroM (+45%). This increase was reduced to 0.1 microM (+12%, P < 0.05) after citrate incubation and reversed after fluoride treatment (both glycolytic inhibitors) by -0.2 microM (-23%, P < 0.05). Plasma [ATP] of control RBC decreased -0.3 microM (-12%) when 8% CO (P < 0.05) was added to the chamber. Because CO and O2 bind competitively to heme, these results support our hypothesis that the release of ATP from RBC is linked to ATP production through the oxygenation state of the hemoglobin molecule.

Cyclic GMP-dependent relaxation of isolated rat renal glomeruli induced by extracellular ATP

The relaxing effect of extracellular ATP on renal glomeruli has been investigated by applying ATP and its analogues to suspensions of angiotensin II-precontracted rat renal glomeruli. Based on changes of glomerular [3H]inulin space (GIS) the relaxation of glomeruli was analysed in the presence of agonists: ATP, ADP, AMP, UTP, 2-methylthio-ATP (P2Y agonist), beta,gamma-methylene-ATP (P2X agonist) and adenosine. ATP, 2-methylthio-ATP, ADP and UTP induced concentration-dependent relaxation whereas AMP, beta,gamma-methylene-ATP and adenosine had no effect. The rank order of relaxation potency was 2-methylthio-ATP > ATP > ADP > UTP. An inhibitor of constitutive nitric oxide synthase (NOS), Nomega-nitro-L-arginine (NNA) prevented the ATP-induced increased accumulation of L-citrulline and the relaxation effect of ATP. An inhibitor of the neuronal isoform of NOS, 7-nitroindazole, had no effect on the relaxation effect of ATP. The relaxing effect of ATP was prevented in the presence of inhibitors of cyclic guanylyl cyclase: methylene blue (MB) and the more specific inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ). ATP stimulated an accumulation of cGMP that was diminished in the presence of MB. We indicated that extracellular ATP may relax the glomeruli via activation of P2Y receptors with the subsequent activation of the endothelial isoform of nitric oxide synthase and soluble guanylyl cyclase. We suggest that, based on the described mechanism, extracellular ATP may increase the filtration surface which, in turn, may influence the glomerular filtration rate.

Cardiovascular and pulmonary response to oral administration of ATP in rabbits

Extracellular purines such as ATP and adenosine participate in the regulation of cardiovascular and respiratory functions through specific P1 and P2 purine receptors. These properties have mainly been described after intravenous infusion. Experiments reported herein were designed to explore the possible effect of oral ATP administration (3 or 20 mg. kg(-1). day(-1)) on vascular, cardiac, and pulmonary functions in rabbits. Whereas a unique oral dose of ATP has no effect, chronic supplementation during 14 days reduces peripheral vascular resistance, pulmonary resistance, and respiratory frequency and increases arterial PO(2). No effect on central blood pressure and heart rate is observed, but an increase of the left ventricular work index is noticed subsequent to the diminution of vascular resistance. Rather similar cardiovascular modifications are observed in rabbits given 20 mg. kg(-1). day(-1) adenosine for 14 days but without variation of respiratory parameters. These original effects of repeated oral treatment with ATP may result from an adaptive metabolic response to nucleoside supplementation that might affect the turnover of extracellular purines leading to P1- and/or P2-receptor activation.

Sympathoinhibition by adenosine A(1) receptors, but not P2 receptors, in the hamster mesenteric arterial bed

The aim of the present study was to determine whether there are prejunctional inhibitory P2 purine receptors on sympathetic nerves in the hamster isolated perfused mesenteric arterial bed. Adenosine 5'-O-(3-thiotriphosphate (ATPgammaS; 10 microM), adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS; 100 microM) and AMP (10 microM) had no significant effect on neurogenic contractions to electrical field stimulation. In contrast, P1 receptor agonists attenuated sympathetic vasoconstriction with a potency order of N(6)5'-(Nadenosine. The pEC(50) value for CPA was 7.5+/-0.1 (n=7). The concentration-inhibitory effect curve to CPA was shifted to the right by the adenosine A(1) receptor antagonist, 8-cyclopentyl-1, 3-dipropyl-xanthine (DPCPX; 10 nM; apparent pK(B) 9.6; n=6-7). In methoxamine raised-tone mesenteries CPA (0.001-10 microM) did not elicit vasorelaxation, and NECA and adenosine were only weak vasorelaxants. These results indicate that adenosine A(1) receptors, but not P2 receptors, inhibit prejunctionally sympathetic neurotransmission in the hamster mesenteric arterial bed.

ATP released from perivascular nerves hyperpolarizes smooth muscle cells by releasing an endothelium-derived factor in hamster mesenteric arteries

1. The interaction between perivascular nerves and endothelium was investigated by measuring the changes in smooth muscle membrane potentials using intracellular microelectrode techniques in hamster mesenteric thin (100-150 microm) and thick (300-350 microm) arteries. 2. In both arteries, nerve stimulation evoked excitatory junction potentials (EJPs) which were strongly inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (0.5-5 microM). This result indicated that the EJPs were induced by the activation of P2X receptors. 3. Transient hyperpolarizations were evoked by trains of pulses at 20 Hz in PPADS (5 microM)-pre-treated thin arteries, but not in the thick arteries. ATP (100 microM) applied to adventitial surfaces mimicked the hyperpolarizations. Both the ATP- and nerve stimulation-induced hyperpolarizations were blocked by cibacron blue F3GA (2-100 microM) and were also abolished after endothelium removal, indicating that the neurally released ATP evoked transient hyperpolarization through the activation of P2Y receptors located on the endothelium. 4. In endothelium-intact preparations, intimal application of uridine 5'-triphosphate (UTP 100 microM), a P2Y2-like receptor agonist, but not 2-methylthio ATP (7 microM), hyperpolarized the smooth muscle. The UTP-induced hyperpolarization was significantly inhibited by cibacron blue F3GA and was abolished after endothelium removal. 5. These results suggest that ATP released from the perivascular nerves may reach the endothelium and activate P2Y2-like receptors to induce the release of an endothelium-derived hyperpolarizing factor in thin arteries.

ATP-induced Ca2+ response mediated by P2U and P2Y purinoceptors in human macrophages: signalling from dying cells to macrophages

The activation of macrophages by various stimuli leading to chemotactic migration and phagocytosis is known to be mediated by an increase in intracellular Ca2+ concentration ([Ca2+]i). We measured changes in [Ca2+]i using a Ca2+ imaging method in individual human macrophages differentiated from freshly prepared peripheral blood monocytes during culture of 1-2 days. A transient rise in [Ca2+]i (duration 3-4 min) occurred in 10-15 macrophages in the vicinity of a single tumour cell that was attacked and permeabilized by a natural killer cell in a dish. Similar Ca2+ transients were produced in 90% of macrophages by application of supernatant obtained after inducing the lysis of tumour cells with hypo-osmotic treatment. Ca2+ transients were also evoked by ATP in a dose-dependent manner between 0.1 and 100 microm. The ATP-induced [Ca2+]i rise was reduced to less than one-quarter in Ca2+-free medium, indicating that it is mainly due to Ca2+ entry and partly due to intracellular Ca2+ release. UTP (P2U purinoceptor agonist) was more potent than ATP or 2-chloro-ATP (P2Y agonist). Oxidized ATP (P2Z antagonist) had no inhibitory effect. Both cell lysate- and ATP-induced Ca2+ responses were inhibited by Reactive Blue 2 (P2Y and P2U antagonist) to the same extent, but were not affected by PPADS (P2X antagonist). Sequential stimuli by cell lysate and ATP underwent long-lasting desensitization in the Ca2+ response to the second stimulation. The present study supports the view that macrophages respond to signal messengers discharged from damaged or dying cells to be ingested, and ATP is at least one of the messengers and causes a [Ca2+]i rise via P2U and P2Y receptors.

Effects of P2 purinoceptor agonists on membrane potential and intracellular Ca2+ of human cardiac endothelial cells

Vasoactive agonists like adenosine-5'-triphosphate (ATP) increase intracellular Ca2+ ([Ca2+]i) in vascular endothelial cells with an initial peak due to inositol 1,4,5-triphosphate-mediated Ca2+ release from intracellular stores followed by a sustained plateau that is dependent on the presence of extracellular Ca2+, thus leading to an increased synthesis and release of prostacyclin and nitric oxide. We studied the effects of nucleotides on membrane potential and [Ca2+]i in confluent human microvascular cardiac endothelial cells obtained from patients with dilated cardiomyopathy. The whole-cell configuration of the patch-clamp technique and a confocal laser scanning microscope employing fluo-3 as a Ca2+ indicator were used. Both uridine-5'-triphosphate (UTP) and 2-methylthioadenosine-5'-triphosphate (2MeSATP) induced depolarizations in human microvascular cardiac endothelial cells and increased [Ca2+]i with a rank order of potency 2MeSATP>ATP=UTP (EC50 values (in microM) were 0.084 2MeSATP, 0.67 ATP and 1.1 UTP). This suggests that both P2u and P2y purinoceptors are present on human microvascular cardiac endothelial cells. Maximal [Ca2+]i responses of confluent human microvascular cardiac endothelial cell monolayers to UTP were lower when compared to 2MeSATP. Nucleotide-induced increases in [Ca2+]i consisted of a transient peak, which was also observed in the absence of extracellular Ca2+, and a sustained [Ca2+]i plateau. This plateau, which was not observed in all monolayers studied, was not markedly influenced by increasing extracellular [K+]. Previous incubation with thapsigargin abolished ATP-induced increases of [Ca2+]i. It is concluded that human microvascular cardiac endothelial cells express both P2y and P2u purinoceptors. P2 purinoceptor agonists release Ca2+ from intracellular thapsigargin-sensitive stores and stimulate capacitative Ca2+ influx pathways. K+ efflux through Ca2+-dependent K+ (K(Ca)) channels does not play a major role in the regulation of nucleotide-induced Ca2+ influx in human microvascular cardiac endothelial cells, which might be related to an impaired function of the cells.

Conducted vascular responses: communication across the capillary bed

Conducted vasomotor responses are important for the effective distribution of blood flow, although the mechanism by which these responses are initiated is not well understood. ATP, a substance which is released from circulating red blood cells in response to low PO2 and low pH, two conditions which are associated with decreased supply relative to demand, has been shown to initiate conducted vasodilation following its intraluminal application in first and second order arterioles. Since such low PO2 and low pH conditions would most likely occur on the venous side of the vasculature, we evaluated the response of the arteriolar and capillary networks to application of ATP into venules in the Saran-covered hamster cheek pouch retractor muscle using in vivo video microscopy. Intraluminal application of 40 and 400 pl of 10(-6) M ATP resulted in dose-dependent increases in arteriolar diameter > 450 microm upstream from the site of application. These changes in arteriolar diameter were accompanied by significant increases in red blood cell flux. In capillaries, red blood cell flux doubled in response to ATP administration. Since NO was previously determined to be involved in the vascular response to intraluminal ATP in arterioles, we evaluated its role in these responses. We found that systemic administration of l-NAME prior to ATP application eliminated any conducted response and this effect of l-NAME was reversed by the systemic administration of l-arginine. These data suggest that ATP, which is released from red blood cells in response to low PO2 and low pH, conditions which would be found in the venular microvasculature, may serve a role in distributing perfusion in response to alterations in supply.

Extracellular nucleotide signaling in the inner ear

Extracellular nucleotides, particularly adenosine 5'-triphosphate (ATP), act as signaling molecules in the inner ear. Roles as neurotransmitters, neuromodulators, and as autocrine or paracrine humoral factors are evident. The diversity of the signaling pathways for nucleotides, which include a variety of ATP-gated ion channels (assembled from different subtypes of P2X-receptor subunit) and also different subtypes of G protein-coupled nucleotide receptors (P2Y receptors) supports a major physiological role for ATP in the regulation of hearing and balance. Almost invariably both P2X and P2Y receptor expression is apparent in the complex tissue structures associated with the inner-ear labyrinth. However P2X-receptor expression, commonly associated with fast neurotransmission, is apparent not only with the cochlear and vestibular primary afferent neurons, but also appears to mediate humoral signaling via ATP-gated ion channel localization to the endolymphatic surface of the cochlear sensory epithelium (organ of Corti). This is the site of the sound-transduction process and recent data, including both electrophysiological, imaging, and immunocytochemistry, has shown that the ATP-gated ion channels are colocalized here with the mechano-electrical transduction channels of the cochlear hair cells. In contrast to this direct action of extracellular ATP on the sound-transduction process, an indirect effect is apparent via P2Y-receptor expression, prevalent on the marginal cells of the stria vascularis, a tissue that generates the standing ionic and electrical gradients across the cochlear partition. The site of generation of these gradients, including the dark-cell epithelium of the vestibular labyrinth, may be under autocrine or paracrine regulation mediated by P2Y receptors sensitive to both purines (ATP) and pyrimidines such as UTP. There is also emerging evidence that the nucleoside adenosine, formed as a breakdown product of ATP by the action of ectonucleotidases and acting via P1 receptors, is also physiologically significant in the inner ear. P1-receptor expression (including A1, A2, and A3 subtypes) appear to have roles associated with stress, acting alongside P2Y receptors to enhance cochlear blood flow and to protect against the action of free radicals and to modulate the activity of membrane conductances. Given the positioning of a diverse range of purinergic-signaling pathways within the inner ear, elevations of nucleotides and nucleosides are clearly positioned to affect hearing and balance. Recent data clearly supports endogenous ATP- and adenosine-mediated changes in sensory transduction via a regulation of the electrochemical gradients in the cochlea, alterations in the active and passive mechanical properties of the cells of the sensory epithelium, effects on primary afferent neurons, and control of the blood supply. The field now awaits conclusive evidence linking a physiologically-induced modulation of extracellular nucleotide and nucleoside levels to altered inner ear function.

Vasoconstrictor responses via P2X-receptors are selectively antagonized by NF023 in rabbit isolated aorta and saphenous artery

1. The effects of NF023, the symmetrical 3'-urea of 8-(benzamido)naphthalene-1,3,5-trisulphonic acid), and its parent compound suramin were investigated on vasoconstrictor responses to alpha, beta-methylene ATP in rabbit isolated saphenous artery and vasodilator responses to ATP in noradrenaline-precontracted rabbit isolated thoracic aorta. 2. In rabbit isolated saphenous artery, alpha, beta-methylene ATP-induced vasoconstrictor responses via P2X-receptors were concentration-dependently and competitively antagonised by NF023 (30-300 microM; pA2 = 5.69 +/- 0.04). Suramin (100-1000 microM) also competitively blocked vasoconstrictor responses to alpha, beta-methylene ATP, albeit with lower potency (pA2 = 4.79 +/- 0.05). In contrast, NF023 (100 microM) did not significantly affect contractile responses to noradrenaline or histamine in the saphenous artery. 3. In noradrenaline-precontracted rabbit isolated thoracic aorta preparations, ATP (3-3000 microM) concentration-dependently induced relaxations via endothelium-dependent or smooth muscle P2Y-receptor subtypes. NF023 (30-300 microM) failed to block relaxant responses to ATP at endothelium-dependent P2Y-receptors, whereas suramin (100-1000 microM) did antagonise endothelium-dependent vasodilator responses to ATP. Neither NF023 (100 microM) nor suramin (300 microM) influenced vasorelaxant responses to ATP via endothelium-independent P2Y-receptors. 4. In conclusion, this study outlines the selectivity of NF023 as an effective P2X-receptor antagonist in rabbit isolated blood vessels without affecting endothelium-dependent or endothelium-independent P2Y-receptor subtypes, adrenoceptors or histamine receptors.

Involvement of ATP in the non-adrenergic non-cholinergic inhibitory neurotransmission of lamb isolated coronary small arteries

1. The involvement of non-adrenergic non-cholinergic (NANC) transmitters, such as nitric oxide (NO) and adenosine 5'-triphosphate (ATP), in the neurogenic relaxation of lamb coronary small arteries was investigated in vessel segments with an internal lumen diameter of 200-550 microns, isolated from the left ventricle of the heart, and suspended for isometric tension recording in microvascular myographs. 2. In both endothelium-intact and -denuded coronary small arteries treated with phentolamine (3 x 10(-6) M), propranolol (3 x 10(-6) M), and atropine (10(-6) M) and contracted to 3 x 10(-7) M of the thromboxane analogue U46619, electrical field stimulation (EFS) evoked frequency-dependent relaxations, which were markedly reduced in the presence of tetrodotoxin (10(-6) M). 3. Exogenous NO added as acidified sodium nitrite (10(-6)-10(-3) M) and L-nitrosocysteine induced potent relaxations of lamb coronary small arteries. However, both inhibition of NO synthase with NG- nitro-L-arginine (L-NOARG, 3 x 10(-5) M), and mechanical endothelial cell removal increased rather than inhibited relaxations to EFS. In small arteries processed for NADPH-diaphorase histochemistry, activity was only observed within endothelial cells. 4. In arteries contracted to U46619, exogenously added ATP caused concentration-dependent relaxations with pD2 and maximum responses of 4.72 +/- 0.12 and 89.6 +/- 3.8% (n = 12), respectively. ADP and the P2Y-agonist, 2-methylthio-ATP, induced relaxations equipotent to ATP, while the P2x-agonist, alpha, beta-methylene ATP (10(-9)-10(-4) M), and the P2U-agonist, UTP (10(-9)-10(-4) M) only caused small transient relaxations at the highest concentrations (10(-4) and 10(-3) M). 5. ATP and EFS-induced relaxations were unchanged in the presence of the P1-purinoceptor antagonist, 8-phenyltheophylline (10(-5) M), while this antagonist inhibited the concentration-dependent relaxations to adenosine. In contrast, the P2-purinoceptor antagonist, suramin (3 x 10(-5) M), markedly reduced the relaxations to EFS. 6. After desensitization of P2x-purinoceptors with alpha, beta-methylene ATP (2 x 10(-5) M), the relaxations to exogenous added ATP were enhanced, but this procedure did not influence the relaxations to EFS. In contrast, the P2y-purinoceptor antagonist, basilen blue E-3G (3 x 10(-5) M, earlier named reactive blue 2) significantly inhibited the concentration-relaxation curves to ATP and almost abolished the EFS-induced relaxations. 7. Mechanical removal of the endothelium significantly inhibited ATP-induced maximal relaxations without affecting sensitivity, pD2 and maximum relaxations being 4.72 +/- 0.12 and 89.7 +/- 3.8% (n = 10), and 5.45 +/- 0.38 and 48.0 +/- 8.6% (P < 0.05, paired t test, n = 10) in endothelium-intact and -denuded coronary small arteries, respectively. However, incubation with L-NOARG did not change relaxations elicited by ATP. 8. The present study suggests that in NANC conditions neurogenic relaxations of coronary small arteries are mediated by ATP, which relaxes coronary small arteries through P2Y-purinoceptors. A prejunctional modulation of these relaxations by endothelial-derived NO cannot be excluded.

Innervation and nitric oxide modulation of mesenteric arteries of the golden hamster

Immunohistochemical and pharmacological techniques were used to examine perivascular nerves, endothelium and the effects of inhibition of nitric oxide synthesis on responses in mesenteric arteries/perfused mesenteric arterial beds of the Golden hamster. Frequency-dependent vasoconstrictions to electrical field stimulation and dose-dependent vasoconstrictions to noradrenaline were significantly augmented by NG-nitro-L-arginine methyl ester (10(-5) M), an inhibitor of nitric oxide synthase. In preparations with tone raised with methoxamine (10 microM) dose-dependent relaxations to ATP, but not to acetylcholine, were blocked by NG-nitro-L-arginine methyl ester. In the presence of guanethidine (5 microM) to block sympathetic neurotransmission there was no neurogenic relaxation to electrical field stimulation. Furthermore, the sensory neurotoxin capsaicin (0.05-5 nmol) did not elicit relaxation. Immunohistochemical studies demonstrated dense plexuses of fibres immunoreactive for tyrosine hydroxylase and neuropeptide Y, a plexus of moderate density for calcitionin gene-related peptide and an absence of fibres immunoreactive for substance P and vasoactive intestinal polypeptide. Of particular interest is the finding that whereas sympathetic perivascular nerves and nitric oxide regulate the function of hamster mesenteric arteries, there is no apparent motor function of calcitonin gene-related peptide-containing sensory nerves.