Differential regulation of inositol 1,4,5-trisphosphate by co-existing P2Y-purinoceptors and nucleotide receptors on bovine aortic endothelial cells

Stretch-induced phosphorylation of focal adhesion kinase in endothelial cells: role of mitochondrial oxidants

Mechanical stretch activates a number of signaling pathways in endothelial cells, and it elicits a variety of functional responses including increases in the phosphorylation of focal adhesion kinase (FAK), a nonreceptor tyrosine kinase involved in integrin-mediated signal transduction. Stretch also triggers an increase in the generation of reactive oxygen species (ROS), which may function as second messengers in the signal transduction cascades that activate cellular responses to strain. Mitochondria represent an important source of ROS in the cell, and these organelles may release ROS in response to strain by virtue of their attachment to cytoskeletal proteins. We therefore tested whether cyclic stretch increases FAK phosphorylation at Tyr397 through a mitochondrial ROS signaling pathway in bovine pulmonary artery endothelial cells (BPAEC). Oxidant signaling, measured using 2′7′-dichlorofluorescin (DCFH), increased 152 ± 16% during 1.5 h of cyclic strain relative to unstrained controls. The mitochondrial inhibitors diphenylene iodonium (5 μM) or rotenone (2 μM) attenuated this increase, whereas l-nitroarginine (100 μM), allopurinol (100 μM), or apocynin (30 μM) had no effect. The antioxidants ebselen (5 μM) and dithiodidiethyldithiocarbamate (1 mM) inhibited the strain-induced increase in oxidant signaling, but Hb (5 μM) had no effect. These results indicate that strain induces oxidant release from mitochondria. Treatment with cytochalasin D (5 μM) abrogated strain-induced DCFH oxidation in BPAEC, indicating that actin filaments were required for stretch-induced mitochondrial ROS generation. Cyclic strain increased FAK phosphorylation at Tyr397, but this was abolished by mitochondrial inhibitors as well as by antioxidants. Strain-induced FAK phosphorylation was abrogated by inhibition of protein kinase C (PKC) with Ro-31-8220 or Gö-6976. These findings indicate that mitochondrial oxidants generated in response to endothelial strain trigger FAK phosphorylation through a signaling pathway that involves PKC.

Characterization of a Ca2+ response to both UTP and ATP at human P2Y11 receptors: evidence for agonist-specific signaling

Previous reports on heterologously-expressed human P2Y11 receptors have indicated that ATP, but not UTP, is an agonist stimulating both phosphoinositidase C and adenylyl cyclase. Consistent with these findings, we report that in 1321N1 cells expressing human P2Y11 receptors, UTP stimulation did not lead to accumulation of inositol(poly)phosphates under conditions in which ATP gave a robust, concentration-dependent effect. Unexpectedly, however, both UTP and ATP stimulated increases in cytosolic Ca2+ concentration ([Ca2+]c), with both nucleotides achieving similar EC50 and maximal responses. The responses to maximally effective concentrations of ATP and UTP were not additive. The [Ca2+]c increase in response to UTP was less dependent on extracellular Ca2+ than was the response to ATP. AR-C67085 (2-propylthio-beta,gamma-difluoromethylene-d-ATP, a P2Y11-selective agonist), adenosine 5'-O-(3-thiotriphosphate), and benzoyl ATP were all full agonists with potencies similar to those of ATP and UTP. In desensitization experiments, exposure to ATP resulted in loss of the UTP response; this response was more sensitive to desensitization than that of ATP. Pertussis toxin pretreatment attenuated the response to UTP but left the ATP response unaffected. The presence of 2-aminoethyl diphenylborate differentially affected the responses of ATP and UTP. No mRNA transcripts for P2Y2 or P2Y4 were detectable in the P2Y11-expressing cells. We conclude that UTP is a Ca2+-mobilizing agonist at P2Y11 receptors and that ATP and UTP acting at the same receptor recruit distinct signaling pathways. This example of agonist-specific signaling is discussed in terms of agonist trafficking and differential signal strength.

Attenuation of signal flow from P2Y6 receptor by protein kinase C-alpha in SK-N-BE(2)C human neuroblastoma cells

Extracellular nucleotides exert a variety of biological actions through several kinds of P2 receptors in many tissues and cell types. We found that treatment with nucleotides increases intracellular Ca2+ concentration ([Ca2+]i) in SK-N-BE(2)C human neuroblastoma cells with a following order of potency: UDP > UTP > ADP >> ATP. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that specific mRNAs coding for human P2Y1, P2Y4, and P2Y6 receptors were expressed in the cells, but Northern blot analysis revealed that P2Y6 receptors were the predominant type. Activation of protein kinase C-alpha by treatment with 1 micro m phorbol 12-myristate 13-acetate dramatically inhibited both the UDP-induced [Ca2+]i rise and inositol 1,4,5-trisphosphate (IP3) generation, whereas incubation with pertussis toxin had little effect on the responses. The UDP-induced [Ca2+]i rise and IP3 production were maintained up to 30 min after stimulation, while bradykinin-induced responses rapidly decreased to the basal level within 5 min of stimulation. Pretreatment of cells with the maximal effective concentration of UDP reduced the subsequent carbachol- or bradykinin-induced [Ca2+]i rise without inhibition of IP3 generation. Neuronal differentiation of the cells by treatment with retinoic acid for 7 days did not change the expression level of P2Y6 receptors. Taken together, the data indicate that P2Y6 receptors highly responsive to diphosphonucleotide UDP are endogenously expressed in the human neuroblastoma SK-N-BE(2)C cells and that they are involved in the modulation of other phospholipase C-coupled receptor-mediated Ca2+ mobilization by depleting the IP3-sensitive Ca2+ stores.

Effect of 5-aminoimidazole-4-carboxamide riboside (AICA-r) on isolated thoracic aorta responses in streptozotocin-diabetic rats

Diabetes mellitus alters the vascular responsiveness to several vasoconstrictors and vasodilators. 5-amino-4-imidazole-carboxamide riboside (AICA-r), a nucleoside corresponding to AICA-ribotide and an intermediate of the de novo pathway of purine biosynthesis, was recently proposed as a new insulinotropic tool in non-insulin-dependent diabetes mellitus. The aim of the present study was to define whether AICA-r affects altered vascular responsiveness to vasoconstrictors and vasodilators in the thoracic aorta of neonatal streptozotocin (STZ)-diabetic rats. The results of this study indicate that a 1-month treatment with AICA-r significantly increases the body weight in diabetic rats; significantly decreases the blood glucose level of diabetic rats (from 302+/-47 to 135+/-11 mg/dL, p<0.001); does not significantly affect the fast, slow, and total components of responses to noradrenaline in all the experimental groups; reverses the increased Emax values of noradrenaline in diabetic rats to near-control values; reverses the completely abolished responses of acetylcholine (pD2 and percent relaxation) in diabetic rats to control values; and reverses the decreased pD2 values of sodium nitroprussiate in diabetic rats to control values. In conclusion, AICA-r treatment in neonatal STZ-diabetic rats improved increased blood glucose levels, accelerated weight gain, reversed endothelial dysfunction, and normalized vascular responses.

Capacitative Ca(2+) entry in vascular endothelial cells is mediated via pathways sensitive to 2 aminoethoxydiphenyl borate and xestospongin C

1. Agonists increase endothelial cell intracellular Ca(2+), in part, by capacitative entry, which is triggered by the filling state of intracellular Ca(2+) stores. It has been suggested that depletion of endoplasmic reticulum (ER) Ca(2+) stores either leads to a physical coupling between the ER and a plasma membrane channel, or results in production of an intracellular messenger which affects the gating of membrane channels. As an axis involving the IP(3) receptor has been implicated in a physical coupling mechanism the aim of this study was to examine the effects of the putative IP(3) receptor antagonists/modulators, 2 aminoethoxydiphenyl borate (2APB) and xestospongin C, on endothelial cell Ca(2+) entry. 2. Studies were conducted in fura 2 loaded cultured bovine aortic endothelial cells and endothelial cells isolated from rat heart. 3. 2APB (30 - 300 microM) inhibited Ca(2+) entry induced by both agonists (ATP 1 microM, bradykinin 0.1 microM) and receptor-independent mechanisms (thapsigargin 1 microM, ionomycin 0.5 and 5 microM). 2APB did not diminish endothelial cell ATP-induced production of IP(3) nor effect in vitro binding of [(3)H]-IP(3) to an adrenal cortex binding protein. Capacitative Ca(2+) entry was also blocked by disruption of the actin cytoskeleton with cytochalasin (100 nM) while the initial Ca(2+) release phase was unaffected. 4. Similarly to 2APB, xestospongin C (3 - 10 microM) inhibited ATP-induced Ca(2+) release and capacitative Ca(2+) entry. Further, xestospongin C inhibited capacitative Ca(2+) entry induced by thapsigargin (1 microM) and ionomycin (0.5 microM). 5. The data are consistent with a mechanism of capacitative Ca(2+) entry in vascular endothelial cells which requires (a) IP(3) receptor binding and/or an event distal to the activation of the ER receptor and (b) a spatial relationship, dictated by the cytoskeleton, between Ca(2+) release and entry pathways.

P2y1 and P2y2 receptor-operated Ca2+ signals in primary cultures of cardiac microvascular endothelial cells

Intracellular Ca2+ signals elicited by nucleotide agonists were investigated in primary cultures of rat cardiac microvascular endothelial cells using the fura-2 technique. UTP increased the intracellular [Ca2+] in 94% of the cells, whereas 2MeSATP was active in 32%. The rank order of potency was ATP = UTP > 2MeSATP and the maximal response to 2MeSATP was lower compared to UTP and ATP. ATP and UTP showed strong homologous and heterologous desensitization. ATP fully inhibited the 2MeSATP response, while UTP abolished 2MeSATP-elicited transients in 25% of cells. 2MeSATP did not desensitize the UTP or ATP response. Adenosine 2',5'-diphosphate inhibited the response to 2MeSATP, while it did not modify the response to ATP and UTP. 2MeSATP was more sensitive to suramin than UTP and ATP. These results indicate that P(2Y1) and P(2Y2) receptors may be coexpressed in CMEC. Nucleotide-induced Ca2+ signals lacked a sustained plateau and were almost independent from extracellular Ca2+. ATP and UTP elicited Ca2+ transients longer than 2MeSATP-evoked transients. The kinetics of Ca2+ responses was not affected by bath solution stirring or ectonucleotidase inhibition. Furthermore, the nonhydrolyzable ATP analogue AMP-PNP induced Ca2+ signals similar to those elicited by ATP and UTP. These results suggest that the distinct kinetics of nucleotide-evoked Ca2+ responses do not depend on the activity of ectonucleotidases or ATP autocrine stimulation. The possibility that Ca2+ signals with different time courses may modulate different cellular responses is discussed.

Coexpression of several types of metabotropic nucleotide receptors in single cerebellar astrocytes

We have examined the expression of mRNA for several P2Y nucleotide receptors by northern blot analysis in purified type 1 cerebellar astrocyte cultures. These results suggest that different P2Y subtypes could be responsible for ATP metabotropic calcium responses in single type 1 astrocytes. To identify these subtypes we have studied the pharmacological profile of ATP calcium responses using fura-2 microfluorimetry. All tested astrocytes responded to ATP and UTP stimulations evoking similar calcium transients. Most astrocytes also responded to 2-methylthioATP and ADP challenges. The agonist potency order was 2-methylthioATP > ADP > ATP = UTP. Cross-desensitization experiments carried out with ATP, UTP, and 2-methylthioATP showed that 2-methylthioATP and UTP interact with different receptors, P2Y(1) and P2Y(2) or P2Y(4). In a subpopulation of type 1 astrocytes, ATP prestimulation did not block UTP responses, and UDP elicited clear intracellular Ca(2+) concentration responses at very low concentrations. 2-MethylthioATP and UTP calcium responses exhibited different sensitivity to pertussis toxin and different inhibition patterns in response to P2 antagonists. The P2Y(1)-specific antagonist N:(6)-methyl-2'-deoxyadenosine 3', 5'-bisphosphate (MRS 2179) specifically blocked the 2-methylthio-ATP responses. We can conclude that all single astrocytes coexpressed at least two types of P2Y metabotropic receptors: P2Y(1) and either P2Y(2) or P2Y(4) receptors. Moreover, 30-40% of astrocytes also coexpressed specific pyrimidine receptors of the P2Y(6) subtype, highly selective for UDP coupled to pertussis-toxin insensitive G protein.

Differential regulation of P2Y(11) receptor-mediated signalling to phospholipase C and adenylyl cyclase by protein kinase C in HL-60 promyelocytes

The regulatory mode of the P2Y(11) purinoceptor-mediated signalling cascades towards phospholipase C and adenylyl cyclase was studied in HL-60 promyelocytes. Treatment with the potent P2Y(11) receptor activator dATP evoked an elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) and inositol 1,4,5-trisphosphate (IP(3)) production that was sustained for longer than 30 min. However, the dATP-induced responses were significantly inhibited by the activation of protein kinase C after a short exposure to phorbol 12-myristate 13-acetate (PMA). dATP also potently stimulated cyclic AMP production with half maximum effect seen at 23+/-7 microM dATP. In addition, a 5-min pretreatment with PMA enhanced the dATP-stimulated cyclic AMP accumulation. PMA potentiated the cyclic AMP production when adenylyl cyclase was activated directly by forskolin or indirectly by G protein activation after cholera toxin treatment. dATP also enhanced the forskolin-mediated cyclic AMP generation. Treatment of the cells with 10 microM U-73122, which almost completely blocked the dATP-stimulated IP(3) production and [Ca(2+)](i) rise, had no effect on cyclic AMP accumulation, while 10 microM 9-(tetrahydro-2-furyl)adenine (SQ 22536), which inhibited the adenylyl cyclase activation, did not effect the dATP-stimulated phosphoinositide turnover. Taken together, the results indicate that P2Y(11) receptor-mediated activation of phospholipase C and adenylyl cyclase occurs through independent pathways and is differentially regulated by protein kinase C in HL-60 cells.

Diadenosine polyphosphate receptors. from rat and guinea-pig brain to human nervous system

Diadenosine polyphosphates are a family of naturally occurring nucleotidic compounds present in secretory vesicles together with other chemical messengers. The exocytotic release of these compounds permits them to stimulate receptors termed "purinoceptors" or "ATP receptors." Purinoceptors for nucleotides are named P2 in contrast with those sensitive to nucleosides (P1). P2 receptors are further subdivided into metabotropic P2Y receptors, further divided into 5 subtypes, and ionotropic P2X receptors, with 7 different subtypes. Diadenosine polyphosphates can activate recombinant P2Y(1), P2Y(2), and P2Y(4) and recombinant homomeric P2X(1), P2X(2), P2X(3), P2X(4), and P2X(6). Heteromeric P2X receptors change their sensitivity to diadenosine polyphosphates when co-assembly between different subunits occurs. Diadenosine polyphosphates can activate specific receptors termed dinucleotide receptors or P4 receptors, which are insensitive to other nucleosides or nucleotides. The P4 receptor is a receptor-operated Ca(2)+ channel present in rat brain synaptic terminals, stimulated by diadenosine pentaphosphate and diadenosine tetraphosphate. This receptor is strongly modulated by protein kinases A and C and protein phosphatases. The dinucleotide receptor is present in different brain areas, such as midbrain (in rat and guinea-pig), cerebellum (in guinea-pig), and cortex (in human).

P2Y receptor-mediated inhibition of tumor necrosis factor alpha -stimulated stress-activated protein kinase activity in EAhy926 endothelial cells

In the EAhy926 endothelial cell line, UTP, ATP, and forskolin, but not UDP and epidermal growth factor, inhibited tumor necrosis factor alpha (TNFalpha)- and sorbitol stimulation of the stress-activated protein kinases, JNK, and p38 mitogen-activated protein (MAP) kinase, and MAPKAP kinase-2, the downstream target of p38 MAP kinase. In NCT2544 keratinocytes, UTP and a proteinase-activated receptor-2 agonist caused similar inhibition, but in 13121N1 cells, transfected with the human P2Y(2) or P2Y(4) receptor, UTP stimulated JNK and p38 MAP kinase activities. This suggests that the effects mediated by P2Y receptors are cell-specific. The inhibitory effects of UTP were not due to induction of MAP kinase phosphatase-1, but were manifest upstream in the pathway at the level of MEK-4. The inhibitory effect of UTP was insensitive to the MEK-1 inhibitor PD 098059, changes in intracellular Ca(2+) levels, or pertussis toxin. Acute phorbol 12-myristate 13-acetate pretreatment also inhibited TNFalpha-stimulated SAP kinase activity, while chronic pretreatment reversed the effects of UTP. Furthermore, the protein kinase C inhibitors Ro318220 and Go6983 reversed the inhibitory action of UTP, but GF109203X was ineffective. These results indicate a novel mechanism of cross-talk regulation between P2Y receptors and TNFalpha-stimulated SAP kinase pathways in endothelial cells, mediated by Ca(2+)-independent isoforms of protein kinase C.

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.

PKCbetaI mediates the inhibition of P2Y receptor-induced inositol phosphate formation in endothelial cells

1. Bovine pulmonary artery endothelium (CPAE) expresses phospholipase C (PLC)-linked P2Y1 and P2Y2 receptors, for them 2-methylthio-ATP (2MeSATP) and UTP are respective agonists. Here, we have investigated the particular protein kinase C (PKC) isoform(s) responsible for the inhibition of P2Y1 and P2Y2 receptor-evoked inositol phosphate (IP) formation by phorbol 12-myristate 13-acetate (PMA). 2. Although short-term (20 min) pretreatment of cells with PMA attenuated 2MeSATP- and UTP-induced phosphoinositide (PI) breakdown, this inhibition was lost after 15 h. Preincubation with PMA for 24 h, on the contrary, potentiated 2MeSATP and UTP responses. The IP formation stimulated by NaF was unaltered by PMA pretreatment. 3. Western blot analysis showed that treatment of CPAE with PMA resulted in a rapid translocation of PKC isoform betaI, epsilon and mu, but not lambda, from the cytosol to the membrane fraction. 4. Pretreatment of the selective PKC inhibitor Ro 31-8220 attenuated the inhibitory effect of PMA on IP formation. Go 6976 (an inhibitor of conventional PKCalpha, beta and gamma) and LY 379196 (a selective PKCbeta inhibitor) also dose-dependently inhibited the PMA-mediated desensitization. 5. Transfection of PKCbeta-specific antisense oligonucleotide reduced PKCbetaI protein level and inhibited PMA-mediated PI reduction. 6. RT - PCR analysis showed that PMA treatment for 4 - 24 h up-regulated P2Y1 and P2Y2 receptors at the mRNA levels. 7. These results suggest that PKCbetaI may exert a negative feedback regulation on endothelial P2Y1 and P2Y2 receptor-mediated PI turnover. The down-regulation of PKCbetaI and enhanced P2Y receptor expression together might contribute to the late PI enhancing effect of PMA.

P2Y receptor-mediated Ca2+ signalling in cultured rat aortic smooth muscle cells

1. ATP, UTP, ADP and ADP-beta-S elicited Ca2+ -signals in cultured aortic smooth muscle cells although ADP, UDP and ADP-beta-S gave approximately 40% of the maximal response seen with ATP and UTP. Adenosine, AMP or alpha,beta-methylene-ATP had no effect. These responses were attributed to P2Y2/4 and P2Y1 receptors, which we assumed could be selectively activated by UTP and ADP-beta-S respectively. 2. The response to UTP was reduced (approximately 50%) by pertussis toxin, whilst this toxin had no effect upon the response to ADP-beta-S. This suggests P2Y2/4 receptors simultaneously couple to pertussis toxin-sensitive and -resistant G proteins whilst P2Y1 receptors couple to only the toxin-resistant proteins. 3. Repeated stimulation with UTP or ADP-beta-S caused desensitization which was potentiated by 12-O-tetradecanoyl phorbol-13-acetate (TPA) and attenuated by staurosporine. 4. TPA completely abolished sensitivity to ADP-beta-S but the response to UTP had a TPA-resistant component. In pertussis toxin-treated cells, however, TPA could completely abolish sensitivity to UTP and so the TPA-resistant part of this response seems to be mediated by pertussis toxin-sensitive G proteins. 5. Loss of sensitivity to UTP did not occur when pertussis toxin-treated cells were repeatedly stimulated with this nucleotide, suggesting that pertussis toxin-sensitive G proteins mediate this effect. The toxin did not, however affect desensitization to ADP-beta-S.

Pharmacological characterization of ATP receptors in ampulla from frog semicircular canal

Phosphoinositidase C activities sensitive to purine and pyrimidine nucleotides have been identified earlier in ampulla from Rana ridibunda semicircular canal. The aim of this study was to characterize the pharmacological properties of other P2 receptors borne by this structure. A microassay was developed to measure the binding of [35S]adenosine 5'-O-(2-thiodiphosphate) ([35S]ADPbetaS) to a few ampullas microdissected from frog semicircular canals. When determined at 4 degrees C in the absence of divalent cations, [35S]ADPbetaS binding was saturable with incubation time and reversible after elimination of free radioligand. The dissociation kinetics were biphasic and comprised a major component that was rapidly reversible and a minor component that dissociated slowly. [35S]ADPbetaS binding was competitively inhibited by unlabeled ADPbetaS with an apparent dissociation constant of 0.48 +/- 0.09 microM and a Hill coefficient of 0.70 +/- 0.06, and Scatchard analysis revealed a minor class of high-affinity binding sites (RT1 = 52 +/- 11 fmol [35S]ADPbetaS bound/ampulla and Kd1 = 0.15 +/- 0.04 microM) and a major class of low-affinity binding sites (RT2 = 436 +/- 79 fmol [35S]ADPbetaS bound/ampulla and Kd2 = 2.0 +/- 0.8 microM). The pattern of stereospecificity for recognition of unlabeled structural ATP analogs was ADPbetaS >/= alpha, beta-methyleneadenosine 5'-triphosphate = ADP = adenosine 5'-O-(3-thiotriphosphate) > ATP = diadenosine tetraphosphate = AMP > 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate >/= 2-methylthioadenosine 5'-triphosphate > 2-desoxythymidine 5'-triphosphate = guanosine 5'-triphosphate = inosine-5'-triphosphate = xanthosine 5'-triphosphate = cytosine 5'-triphosphate = uridine 5'-triphosphate = uridine-5'-diphosphate, whereas cAMP and adenosine were devoid of activity. For antagonists, suramin revealed competitive inhibitor potencies, whereas reactive blue 2 and DIDS acted as pure noncompetitive inhibitors. Results suggest that the population of labeled receptors is heterogeneous and contains a low number of P2Y-like receptors and a large number of P2X-like receptors whose molecular subtypes and functions in endolymph homeostasis remain to be defined.

P2Y- and P2U-mediated increases in internal calcium in single bovine aortic endothelial cells in primary culture

Increases in intracellular calcium ([Ca2+]i) to ATP, ADP, AMP, adenosine, UTP, 2-methylthio ATP (2-MeSATP), 2-methylthio ADP (2-MeSADP) and alpha,beta-methylene ATP (alpha,beta-meATP) were investigated in single bovine aortic endothelial cells (BAEC) in primary culture using Indo-1. Evidence was obtained for the presence of P2Y and P2U, but not P2X receptors. Normalized concentration-effect curves for ATP, UTP and 2-MeSATP were biphasic in shape. At 10 nM, the agonist rank order was UTP > ATP approximately 2-MeSATP, while above 1 microM, it was ATP > or = UTP > or = 2-MeSATP. No cross-desensitization between responses to P2U and P2Y receptors was observed in normal external solution. However, when internal Ca2+ stores were depleted by exposure to 2-MeSATP or UTP in Ca2+-free solution and agonists then re-applied in presence of external Ca2+, homologous but not heterologous desensitization was seen. In the same conditions, heterologous desensitization was observed for UTP after ATP but not for ATP after UTP. Taken together, the results are consistent with the coexistence of P2Y and P2U receptors in primary-cultured BAEC and suggest that upon activation, different intracellular signaling pathways could be involved in increasing [Ca2+]i.

The regulation of vascular function by P2 receptors: multiple sites and multiple receptors

Although the effects of nucleotides in the cardiovascular system have been known for almost 70 years, it is only in the past few years that some of the P2 receptors at which they act have been cloned and characterized. It is now clear that the control of cardiovascular function by nucleotides is complex, involving multiple receptors and multiple effects in the different cell types of importance. In this review Mike Boarder and Susanna Hourani summarize the P2 receptors that are present in endothelial cells, platelets, smooth muscle and nerves, the signalling pathways that they activate and the responses that are produced. They also discuss the important role of nucleotides in the interactions between the different cell types, and the implications of this in vascular disease.

P2U-receptor mediated endothelium-dependent but nitric oxide-independent vascular relaxation

1. The dilator effect of extracellular adenosine triphosphate (ATP) has mainly been characterized as a direct effect on smooth muscle or as an endothelium-dependent effect mediated by nitric oxide (NO) or prostaglandins. We tested the hypothesis that endothelium-derived hyperpolarizing factor (EDHF) may also be involved. Dilator effects were studied in vitro by continuous recording of isomeric tension in cylindrical segments of rat blood vessels precontracted by noradrenaline (NA), in the presence of indomethacin (10 microM). 2. By screening different blood vessels in the rat we found that both acetylcholine (ACh) and ATP dilate mesenteric arteries with a resting tone of 1 mN by an endothelium-dependent non-NO mechanism. With an increased resting tone (4 mN) the dilatation was mediated by NO. Thus by varying the resting tension the different dilator mechanisms could be examined. However, in the carotid artery the dilatation was solely mediated by an endothelium-dependent NO mechanism, even at different resting tones (1 and 4 mN). 3. The N-nitro-L-arginine methyl ester (L-NAME)-resistant dilatation to ACh and ATP was further inhibited by the NO-scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), indicating L-NAME insensitive NO-synthesis. 4. In carotid arteries and mesenteric arteries at high resting tones (4 mN) the ATP-dilatation was totally inhibited by endothelium removal or L-NAME (10(-3) M). In mesenteric arteries at low resting tone (1 mN) the ATP, UTP (uridine-triphosphate) and 2-MeSATP (2methylthioATP)-dilatation was totally inhibited by endothelium removal. However, L-NAME in combination with indomethacin attenuated only 5% of the UTP dilatation, 70% of the ATP dilatation but all of the 2-MeSATP-dilatation. The inhibitors of Ca2+-activated K+ channels charybdotoxin (0.5 x 10(-7) M) together with apamin (10(-6) M), and the cytochrome P450 inhibitor, SKF 525A (10(-4) M), each in combination with indomethacin. L-NAME and PTIO (0.5 x 10(-3) M) totally abolished the remaining ATP and UTP-dilatation. This indicates a dilatation mediated by an endothelium-dependent non-NO factor, probably EDHF. 5. Agonist potency (UTP>ATP>>2-MeSATP), indicates that the EDHF-mediated dilatation was stimulated by a P2U-receptor, possibly by a selective pyrimidine-receptor. In contrast, a P2Y-receptor stimulated NO-mediated dilatation (2-MeSATP=ATP>UTP). 6. In conclusion, the dilator effects of ATP and especially UTP can be mediated by an endothelium-dependent non-NO-mediated mechanism, probably EDHF, mediated by a P2U-receptor, possibly a selective pyrimidine-receptor, while NO-mediated dilatation is stimulated mainly by a P2Y1-receptor. Furthermore, the EDHF-dilatation is dependent on the resting tone of the blood vessel.

Homologous and heterologous desensitisation of somatostatin-induced increases in intracellular Ca2+ and inositol 1,4,5-trisphosphate in CHO-K1 cells expressing human recombinant somatostatin sst5 receptors

The mechanisms responsible for somatostatin (SRIF)-induced increases in intracellular Ca2+ concentration ([Ca2+]i) and subsequent desensitisation were studied in CHO-K1 cells expressing human sst5 receptors (CHOsst5 cells). To study the nature of the desensitisation, interactions with uridine triphosphate (UTP) were examined. SRIF (pEC50 7.10) and UTP (pEC50) 5.14) caused concentration-dependent increases in [Ca2+]i but the SRIF maximum was about 40% of that to UTP. SRIF-, but not UTP-, induced increases in [Ca2+]i were transient and abolished by pertussis toxin. SRIF and UTP caused sustained increases in Ins(1,4,5)P3 but the SRIF maximum was about 30% of that to UTP. Removal of [Ca2+]e attenuated the SRIF-induced peak rise in [Ca2+]i but had no effect on the peak increases in Ins(1,4,5)P3. UTP-induced increases in [Ca2+]i and Ins(1,4,5)P3 were attenuated in the absence of [Ca2+]e. Following pre-exposure to SRIF (1 microM) or UTP (100 microM) for 5 min, subsequent SRIF responses were desensitised. Similar results were obtained in the absence of [Ca2+]e. Pre-exposure to SRIF had no effect on subsequent responses to UTP but in the absence of [Ca2+]e, responses to UTP were attenuated. The results suggest that SRIF but not UTP-induced increases in [Ca2+]i in CHOsst5 cells are mediated by pertussis toxin sensitive G proteins and are caused by an entry of extracellular Ca2+ and release from an Ins(1,4,5)P3 sensitive Ca2+ store. Homologous or heterologous desensitisation of agonist-induced increases in [Ca2+]i could be demonstrated in the presence or absence of extracellular Ca2+ respectively, and the latter appeared to involve depletion of a common intracellular Ca2+ store.

P2 purinoceptors in cultured bovine middle cerebral artery endothelial cells

Extracellular adenosine triphosphate (ATP) plays an important role in the regulation of endothelial function. However, its receptors and their signal-transduction pathways in major cerebral arterial endothelial cells are largely unknown. This study was undertaken functionally to classify the P2 purinoceptors in cultured bovine middle cerebral artery endothelial cells by using [Ca2+]i microfluorimetry. The rank order of potency to increase [Ca2+]i was 2-methylthio-ATP approximately ATP approximately uridine triphosphate (UTP) > adenosine diphosphate (ADP) >> adenosine monophosphate (AMP) > alpha,beta-methylene-ATP > adenosine, suggesting that the effect was mediated by both P2y and P2u receptors. ATP, 2-methylthio-ATP, and UTP mobilized Ca2+ from intracellular stores and triggered Ca2+ entry. The effects of ATP, 2-methylthio-ATP, and UTP were reduced by phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC), but only the effects of ATP and UTP were attenuated by pertussis toxin, indicating that P2y and P2u receptors may activate the same effector mechanisms by coupling to different G proteins. The [Ca2+]i entry caused by UTP was significantly reduced by the receptor-regulated Ca2+ channel blocker SK&F 96365, by P-450 inhibitor econazole and by inorganic Ca2+ entry blocker lanthanum. P2-receptor antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), and reactive blue 2 reduced the effects of ATP and 2-methylthio-ATP, but not those of UTP, in a concentration-dependent manner. These studies suggest a coexistence of P2y and P2u receptors in cultured bovine middle cerebral artery endothelial cells.

The P2U purinoceptor obligatorily engages the heterotrimeric G protein G16 to mobilize intracellular Ca2+ in human erythroleukemia cells

To assess the role of G16, a trimeric G protein exclusively expressed in hematopoietic cells, Galpha16 antisense RNA was stably expressed in human erythroleukemia (HEL) cells. Western blot analysis showed that in transfected cell lines, the expression of endogenous Galpha16 protein was suppressed, but the expression of Galphaq/11, Galphai2, and Galphai3 remained unaffected. Suppression of Galpha16 in transfected HEL cells did not interfere with transient elevations of intracellular free Ca2+ concentrations induced by prostaglandin E1 (PGE1), platelet-activating factor, or thrombin. In parental HEL cells, UTP and ATP mobilized Ca2+ from intracellular stores with half-maximum effective concentrations of 3. 6 +/- 0.7 and 4.7 +/- 1.6 microM, respectively, apparently by stimulating P2U purinoceptors. By contrast, Ca2+ mobilization by UTP or ATP was completely abrogated in Galpha16-suppressed cells, indicating specific coupling of G16 to P2U purinoceptors. Pertussis toxin inhibited the effect of UTP in parental HEL cells by 57.6 +/- 4.9%. These data indicate that signaling by the P2U purinoceptor obligatorily requires G16 but may be modulated further by activation of Gi. Priming of HEL cells with UTP or ATP prior to stimulation with PGE1 markedly enhanced the PGE1-induced intracellular Ca2+ release. This indirect, potentiating effect of UTP and ATP was not impaired in Galpha16-suppressed cells but was inhibited by pertussis toxin, indicating that functional P2U purinoceptors are present on these cells and that the potentiating effect primarily depends on Gi. The data demonstrate (i) that Galpha16 antisense RNA selectively inhibits endogenous Galpha16 protein expression in HEL cells; (ii) that stimulation of endogenous P2U (P2Y2) purinoceptors leads to the mobilization of intracellular Ca2+ by a mechanism that strictly depends on Galpha16; and (iii) that P2U purinoceptors in HEL cells can communicate with two distinct signaling pathways diverging at the G protein level.

Dynamic regulation of intracellular Ca2+ concentration in aortic endothelial cells

In non-excitable cells, a Ca2+ entry pathway is opened after the depletion of intracellular Ca2+ store sites. We have tried to estimate the sensitivity of this pathway to Ca2+ release using bovine aortic endothelial cells. Single application of a high concentration (30 microM) of ATP released almost all stored Ca2+ in Ca(2+)-free extracellular solution, whereas a low concentration of ATP (30 nM) produced a partial (57.3 +/- 3.0%) release of Ca2+. By 10 min of Ca2+ re-perfusion, the Ca2+ store site was reloaded to 97.1% of its initial filling state. When thapsigargin was applied to this cell in Mn2+ solution, Mn(2+)-induced quenching of fura-2 dye started when 19.3 +/- 5.3% of Ca2+ release, produced by 30 nM ATP, had occurred. Therefore, Ca2+ release required for Mn2+ entry was estimated as 11.1 +/- 3.0% of stored Ca2+. These results indicate that intracellular Ca2+ concentration is controlled dynamically by simultaneously occurring Ca2+ release and entry in bovine aortic endothelial cells.

P2 purinoceptor-stimulated conversion of arginine to citrulline in bovine endothelial cells is reduced by inhibition of protein kinase C

Bovine aortic endothelial cells contain two coexisting receptors for extracellular ATP, named the P2Y and P2U purinoceptors. Previous studies have shown that these receptors are linked to phospholipase C in a manner that is modulated in part by protein kinase C (PKC). In this study, we investigate the influence of PKC in the regulation of endothelial nitric oxide synthase (NOS) by these two purinoceptors. Activation of either P2Y or P2U purinoceptors by either 2-methylthio-ATP or UTP, respectively, stimulated the formation of [3H]-citrulline in [3H]-arginine-labelled cells in a concentration-dependent manner. This stimulation was sensitive to inhibition by NG-nitro-L-arginine. Ten minutes of pretreatment with the PKC activator tetradecanoyl phorbol acetate (TPA) failed to affect NOS activity, either alone or when stimulated with 2-methylthio-ATP or UTP. However, under these conditions TPA caused almost complete translocation of PKC-alpha from the cytosol to the membrane. Ten minutes of pretreatment with the PKC inhibitor Ro 31-8220 significantly inhibited the agonist-induced stimulation of NOS. These results show that both P2Y and P2U purinoceptors stimulate endothelial NOS in a manner that is dependent on PKC activity.

Endothelial P2-purinoceptors: subtypes and signal transduction

1. Adenine nucleotides stimulate the synthesis and release of prostacyclin and nitric oxide (two potent platelet aggregation inhibitors) by endothelial cells from different origins. These responses are mediated by P2 purinergic receptors, coupled to the production of inositol (1,4,5)trisphosphate (InsP3) and to the increase of intracytoplasmic calcium concentration. 2. In bovine aortic endothelial cells (BAEC), both 2-MeSATP and UTP stimulate the production of InsP3. By experiments of additivity and cross desensitization, we have confirmed the expression of both P2Y/P2Y1 and P2U/P2Y2 receptors on these cells. Moreover, these receptors are not segregated on different subpopulations but are co-localized on the same cells. 3. The action of UTP on InsP3 production was inhibited by pertussis toxin and was unaffected by a pretreatment with phorbol 12-myristate, 13-acetate (PMA). On the other hand, the response induced by 2-MeSATP was inhibited by PMA but insensitive to pertussis toxin. These results suggest that P2Y/P2Y1 and P2U/P2Y2 receptors are respectively coupled to Gq/G11 and G1 proteins. 4. Northern blotting experiments revealed the expression of the P2Y1 (doublet of 2 and 2.2 kb) and of the P2Y2 (2.4 kb) receptor messengers in BAEC. A signal corresponding to the P2Y2 mRNA was also detectable in human umbilical vein endothelial cells. 5. These various results thus demonstrate the expression of the P2Y1 and P2Y2 receptors in vascular endothelial cells.

Phosphorylation and activation of p42 and p44 mitogen-activated protein kinase are required for the P2 purinoceptor stimulation of endothelial prostacyclin production

Extracellular ATP and ADP, released from platelets and other sites stimulate the endothelial production of prostacyclin (PGI2) by acting on G-protein-coupled P2Y2 and P2Y2 purinoceptors, contributing to the maintenance of a non-thrombogenic surface. The mechanism, widely described as being dependent on elevated cytosolic [Ca2+], also requires protein tyrosine phosphorylation. Here we show that activation of both these P2 receptor types leads to the tyrosine phosphorylation and activation of both the p42 and p44 forms of mitogen-activated protein kinase (MAPK). 2-Methylthio-ATP and UTP, selectively activating P2Y1 and P2Y2 purinoceptors respectively, and ATP, a non-selective agonist at these two receptors, stimulate the tyrosine phosphorylation of both p42mapk and p44mapk, as revealed by Western blots with an antiserum specific for the tyrosine-phosphorylated forms of the enzymes. By using separation on Resource Q columns, peptide kinase activity associated with the phosphorylated MAPK enzymes distributes into two peaks, one mainly p42mapk and one mainly p44mapk, both of which are stimulated by ATP with respect to kinase activity and phospho-MAPK immunoreactivity. Stimulation of P2Y1 or P2Y2 purinoceptors leads to a severalfold increase in PGI2 efflux; this was blocked in a dose-dependent manner by the selective MAPK kinase inhibitor PD98059. This drug also blocked the agonist-stimulated increase in phospho-MAPK immunoreactivity for both p42mapk and p44mapk but left the phospholipase C response to P2 agonists essentially unchanged. Olomoucine has been reported to inhibit p44mapk activity. Here we show that in the same concentration range olomoucine inhibits activity in both peaks from the Resource Q column and also the agonist stimulation of 6-keto-PGF1, but has no effect on agonist-stimulated phospho-MAPK immunoreactivity. These results provide direct evidence for the involvement of p42 and p44 MAPK in the PGI2 response of intact endothelial cells: we have shown that both the endothelial P2Y purinoceptors are linked to activation of MAPK, and that activation of this pathway is a requirement for the stimulation by ATP/ADP of endothelial PGI2 production.

Characterization of signaling pathways of P2Y and P2U purinoceptors in bovine pulmonary artery endothelial cells

The actions of ATP on the endothelium are mediated by P2 purinoceptors. We have shown that P2Y and P2U purinoceptors coexist in bovine pulmonary artery endothelial cells (CPAE), where they induce phosphoinositide (PI) turnover and Ca2+ mobilization. The relative order of potency (based on the threshold concentration) of nucleotide analogues (1-100 microM) in stimulating the accumulation of inositol phosphate (IP) was 2-methylthio-ATP (2MeSATP) = 2-methylthio-ADP (2MeSADP) > or = 2ClATP > UTP = ATP = ADP. alpha, beta-methylene ATP, beta, gamma-methylene ATP, UDP, adenosine-5'-tetraphospho-5'-adenosine, and adenosine-5'-pentaphospho-5'-adenosine had no effect at concentrations as high as 100 microM. At maximal concentrations, the IP responses to 2MeSATP and UTP were additive, whereas those to ATP and either 2MeSATP or UTP were not. Moreover, the maximal response to 2MeSADP was additive to that to UTP but not to that of 2MeSATP. Pretreatment with pertussis toxin slightly inhibited 2MeSATP- and UTP-stimulated IP generation by 15%. Under Ca(2+)-free conditions, UTP-induced IP formation was inhibited more markedly than that induced by 2MeSATP. Short-term treatment of the cells with phorbol 12-myristate-13-acetate (PMA) resulted in a dose-dependent inhibition of 2MeSATP-induced IP formation greater and more sensitive than that induced by UTP; similar results were obtained for the sensitivity of inhibition by suramin and reactive blue. Stimulation of the cells with either 2MeSATP or UTP induced a rapid increase in intracellular Ca2+ level, followed by a slow decrease to basal levels, followed by Ca2+ level oscillation. In the absence of extracellular Ca2+, [Ca2+]i responses were quantitatively less and did not show the slow phase and oscillation. Together these results suggest that both P2Y and P2U purinoceptors are expressed in bovine pulmonary artery endothelial cells and are coupled to phospholipase C (PLC) activation and Ca2+ mobilization through pertussis toxininsensitive G proteins.

Protein kinase C isoforms in bovine aortic endothelial cells: role in regulation of P2Y- and P2U-purinoceptor-stimulated prostacyclin release

1. Enhanced synthesis of prostacyclin (PGI2) and inositol polyphosphates in bovine aortic endothelial cells in response to ATP and ADP is mediated by co-existing P2Y- and P2U-purinoceptors. Here we examine the regulation of these responses by isoforms of protein kinase C (PKC). 2. Immunoblots with antisera specific for 8 different PKC isoforms revealed the presence of alpha, epsilon and zeta, while no immunoreactivity was found for beta, gamma, delta, eta and theta isoforms. PKC-alpha was largely cytosolic in unstimulated cells and almost all translocated to the membrane (Triton X-100 soluble) after a 1 min treatment with the PKC activating phorbol myristate acetate (PMA); PKC-epsilon was always in a Triton X-100 insoluble membrane fraction, while PKC-zeta was found in both soluble and membrane bound (Triton X-100 soluble) forms in the unstimulated cells and was unaffected by PMA. 3. Treatment with PMA for 6 h led to a 90% downregulation of PKC-alpha, while the immunoreactivity to the epsilon and zeta isoforms remained largely unchanged. 4. After either 10 min or 6 h exposure to PMA the PGI2 response to activation of both receptors was enhanced, while the inositol 1,4,5-trisphosphate response to P2Y-purinoceptor activation was substantially attenuated and the P2U-purinoceptor response was unchanged. Thus the PGI2 response to PMA under conditions when 90% of the PKC-alpha was lost resembles that seen on acute stimulation of PKC by PMA, and the PGI2 response does not correlate with phospholipase C response. 5. Inhibition of PKC with the isoform non-selective inhibitors, Ro 31-8220 and Go 6850 abolished the PGI2 response to both P2U- and P2Y-purinoceptor stimulation. However, Go 6976, which preferentially inhibits Ca2+ sensitive isoforms (such as PKC-alpha) and not Ca2+ insensitive isoforms (such as PKC-epsilon), had no effect on the PGI2 response. 6. The results show that there is a requirement for PKC in the stimulation of PGI2 production by endothelial P2Y- and P2U-purinoceptors. Both downregulation and inhibition studies show that PKC-alpha is not responsible for the regulation of the response to P2-purinergic stimulation, and imply that the response is mediated by PKC-epsilon (PKC-zeta is unresponsive to PMA), or an as yet uncharacterized PKC isoform.

Interactions between inositol trisphosphate and Ca2+ dependent Ca2+ release mechanisms on the endoplasmic reticulum of permeabilised bovine aortic endothelial cells

In this paper we describe data from cultured bovine aortic endothelial (BAE) cells demonstrating a Ca2+ induced Ca2+ release (CICR) process which appears to have pharmacological properties different from CICR mechanisms in other cell types. CICR was measured in saponin permeabilised cells in which the internal stores had been preloaded with 45Ca2+. Step increases in the free Ca2+ concentration of the bathing solution, from 10 nM up to 10 microM were found to increase 45Ca2+ loss. This process was completely inhibited by ruthenium red. Caffeine induced a small release of 45Ca2+ and the response to a subsequent stimulation with a Ca2+ step was reduced. In intact cells, ryanodine activated small oscillations in intracellular Ca2+ in the presence, but not the absence, of external Ca2+. However, in permeabilised cells, ryanodine had no effect on either basal efflux or the increased efflux of 45Ca2+ seen following a step increase in free Ca2+. These data suggest the operation of a ruthenium red sensitive but ryanodine insensitive CICR mechanism on the endoplasmic reticulum (ER) which may also be modulated by caffeine. An IP3 dependent 45Ca2+ release was also observed. In the presence of ruthenium red, the IP3 induced 45Ca2+ release was reduced suggesting that CICR may operate to amplify the magnitude of the IP3 response. The Ca2+ dependence of the IP3 induced release was also measured. Co-operativity between IP3 and Ca2+ could not be detected between 100-300 nM Ca2+. The results suggest that the regulation of IP3 induced Ca2+ release may be different in BAE cells, and point to the operation of a 'novel' CICR process and to complex interactions between Ca2+ release systems in BAE cells.

PPADS: an antagonist at endothelial P2Y-purinoceptors but not P2U-purinoceptors

1. Bovine aortic endothelial (BAE) cells contain two co-existing receptors for extracellular ATP, the P2Y and P2U-purinoceptors. Here we have determined whether the proposed P2X-purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS) could distinguish between these two receptor subtypes. 2. Cells labelled with myo-[2-3H]-inositol were stimulated with increasing concentrations of either the P2Y-agonist, 2MeSATP, or the P2U-agonist, UTP in the absence or presence of 30 microM PPADS. The accumulation of total [3H]-inositol (poly)phosphates mediated by 2MeSATP was markedly attenuated by PPADS, whereas the response to UTP was not significantly affected. 3. Stimulation of BAE cells with increasing concentrations of ATP showed a reduced response in the presence of 10 microM PPADS, but this effect of the antagonist was not significant. By contrast, inhibition of the response to ADP was profound and highly significant. 4. These observations show that PPADS is not a selective P2X-purinoceptor antagonist, but is able to distinguish between P2Y- and P2YU-purinoceptors in BAE cells, and indicate that this compound may provide a useful tool in the study of multiple subtypes of P2-purinoceptors. Furthermore the results are consistent with the hypothesis that ATP interacts with both receptor subtypes, but that the action of ADP is primarily at the P2Y-purinoceptor in these endothelial cells.

Evidence for requirement of tyrosine phosphorylation in endothelial P2Y- and P2U- purinoceptor stimulation of prostacyclin release

1. The release of prostacyclin (PGI2) from vascular endothelial cells is stimulated by ATP acting at G protein-coupled P2-purinoceptors. Here we investigate the hypothesis that tyrosine protein phosphorylations are involved in this response. 2. The use of Western blots with anti-phosphotyrosine antibodies showed that 30 microM 2MeSATP (selective for P2Y-purinoceptors), 300 microM UTP (selective for P2U-purinoceptors) and 300 microM ATP (effective at both these purinoceptors), each stimulate the tyrosine phosphorylation of proteins in bovine cultured aortic endothelial cells. Each of these agonists also stimulates 6-keto PGF1 alpha accumulation in the medium (an index of PGI2 release) in these cells in the same period. 3. The tyrosine kinase inhibitor, genistein, inhibits the 6-keto PGF1 alpha response with the same concentration-dependency (1-100 microM) as the tyrosine phosphorylation response. 4. Tyrphostin, a structurally and functionally distinct tyrosine kinase inhibitor, is also a potent inhibitor (0.1-10 microM) of the 6-keto PGF1 alpha response. 5. Neither tyrphostin nor genistein inhibit the phospholipase C response to P2-purinoceptor stimulation. Furthermore, these inhibitors do not affect the 6-keto PGF1 alpha response to ionomycin. 6. These results show that the regulation of vascular endothelial cells by ATP acting at both P2Y- and P2U-purinoceptors involves the stimulation of tyrosine phosphorylation, and suggest that this is a necessary event for the purinoceptor-mediated stimulation of PGI2 production.

Pharmacological selectivity of the cloned human P2U-purinoceptor: potent activation by diadenosine tetraphosphate

1. The human P2U-purinoceptor was stably expressed in 1321N1 human astrocytoma cells and the pharmacological selectivity of the expressed receptor was studied by measurement of inositol lipid hydrolysis. 2. High basal levels of inositol phosphates occurred in P2U-purinoceptor-expressing cells. This phenomenon was shown to be due to release of large amounts of ATP from 1321N1 cells, and could be circumvented by adoption of an assay protocol that did not involve medium changes. 3. UTP, ATP and ATP gamma S were full and potent agonists for activation of phospholipase C with EC50 values of 140 nM, 230 nM, and 1.72 microM, respectively. 5BrUTP, 2C1ATP and 8BrATP were also full agonists although less potent than their natural congeners. Little or no effect was observed with the selective P2Y-, P2X-, and P2T-purinoceptor agonists, 2MeSATP, alpha,beta-MeATP, and 2MeSADP, respectively. 4. Diadenosine tetraphosphate, Ap4A, was a surprisingly potent agonist at the expressed P2U-purinoceptor with an EC50 (720 nM) in the range of the most potent P2U-purinoceptor agonists. Ap4A may be a physiologically important activator of P2U-purinoceptors.

Oscillations in cytosolic free Ca2+ induced by ADP and ATP in single rat hepatocytes display differential sensitivity to application of phorbol ester

We have previously described differences in the oscillatory responses of cytosolic free Ca2+ concentration ([Ca2+]i) in hepatocytes to ADP and ATP, which we have interpreted as evidence that these two nucleotides are acting at distinct receptors. We show here that ADP- and ATP-induced oscillations are differentially sensitive to application of the phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB). ADP-induced [Ca2+]i oscillations are abolished by low concentrations of PDB (5-10 nM), whereas ATP-induced oscillations of long duration are refractory to PDB, even at greatly elevated concentrations (100 nM). The data illustrate a further difference in the actions of ADP and ATP, strengthening the argument that these agonists are not acting at the same receptor on rat hepatocytes.

Binding of [35S]adenosine 5'-O-(2-thiodiphosphate) to endothelial cells in culture

We have investigated the binding of [35S]adenosine 5'-O-(2-thiodiphosphate) ([35S]ADP beta S) to intact cultured bovine aortic endothelial cells which have been previously shown to co-express P2y and P2u purinoceptors and to bovine adrenal medulla endothelial cells which solely possess P2u purinoceptors. ADP beta S has been shown to stimulate phospholipase C activity in these cells via the P2y purinoceptor and does not interact with the P2u purinoceptor. We describe a simple equilibrium binding procedure designed for the study of low affinity agonists and compare these results with those obtained by separation of bound and free by filtration. Saturation analysis of equilibrium binding data revealed two sites for ADP beta S binding; one with KD = 3.3 x 10(-8) M, Bmax = 32 pmol/mg protein; and the other with KD = 4.3 x 10(-6) and Bmax = 2155 pmol/mg protein. Use of filtration did not significantly alter the KD of either of these sites, nor the Bmax of the high affinity site, but reduced the Bmax of the low affinity site by more than 95%. The rank order of agonist potency for competing for [35S]ADP beta S binding indicated that most of this was to non-P2y purinoceptor sites as beta,gamma-methylene ATP, a P2x purinoceptor agonist, was more potent than 2-methylthio ATP, a P2y purinoceptor agonist. Binding was also carried out in the presence of beta,gamma-methylene ATP, in an attempt to reduce non-P2y purinoceptor binding and produced similar results. Specific [35S]ADP beta S binding sites were also found in bovine adrenal medulla endothelial cells which do not possess P2y purinoceptors. These results indicate that [35S]ADP beta S was able to bind to endothelial cells from different parts of the vasculature but that the ligand can only be considered suitable for investigation of P2y purinoceptors on mammalian cells when specific conditions are designed to reduce the large amount of non-receptor binding.

G protein-coupled P2 purinoceptors: from molecular biology to functional responses

Nucleotides such as ATP and ADP act as intercellular messengers and exert a widespread influence on cellular function by acting on a variety of cell surface receptors. Until recently, progress has been restrained, in part, by a lack of cloned receptors. Now, however, the successful cloning of a variety of P2 purinoceptors is holding out the prospect of rapid advances in the understanding of this diverse group of receptors and the potent therapeutic resource they represent. In this article, Michael Boarder and colleagues summarize the findings of recent cloning studies, and assess the impact of these on the understanding of the function of the G protein-coupled P2 purinoceptors in several types of cells and tissues.

Coexpression of P2Y and P2U receptors on aortic endothelial cells. Comparison of cell localization and signaling pathways

Depending on the vascular bed considered, the actions of ATP on the endothelium are mediated by either P2Y or P2U receptors. The two types of receptors seem to coexist on bovine aortic endothelial cells, where they are both coupled to phospholipase C. In this study, we have investigated whether they are truly coexpressed on the same cells and whether their signaling pathways diverge beyond phospholipase C activation. Measurements of [Ca2+]i in single cells showed that almost all bovine aortic endothelial cells are responsive to both 2-methylthio-ATP (2MeSATP), an agonist of P2Y receptors, and UTP, an agonist of P2U receptors. UTP stimulated the release of prostacyclin from freshly isolated bovine aortic endothelial cells, even when they were exposed to cycloheximide at the time of their collection: this indicates that P2U receptors must already be expressed on endothelial cells in situ and do not appear during cell culture. The time course of inositol phosphate (InsP) accumulation and the relative proportion of Ins(1,4,5)P3, Ins(1,3,4,5)P4, and Ins(1,3,4)P3 were similar in cells stimulated by 2MeSATP or UTP. UTP and 2MeSATP both stimulated the hydrolysis of phosphatidylcholine by phospholipase D, as reflected by the release of [3H]choline from prelabeled cells. The responses to both agents were blocked after downregulation of protein kinase C, resulting from a prolonged exposure to phorbol 12-myristate 13-acetate: this blockade occurred at a step distal to phospholipase C activation. A single difference between the two pathways has been identified: the effect of 2MeSATP on InsP3 was significantly more inhibited after a short exposure to phorbol 12-myristate 13-acetate than that of UTP.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for Purinergic Receptors in Vestibular Dark Cell and Strial Marginal Cell Epithelia of Gerbil

Purinergic receptors have been found to modulate ion transport in several types of epithelial cells as well as excitable cells. It was of interest to determine whether vestibular dark cells and strial marginal cells contain purinergic receptors in either the apicalor basolateral membrane which modulate transepithelial ion transport. Vestibular dark cell and strial marginal cell epithelia were mounted in a micro-Ussing chamber for the measurement of the transepithelial voltage and resistance from which the equivalent short circuit current (I(sc)) was obtained. The apical and basolateral sides were independently perfused with adenosine and adenosine 5'-triphosphate (ATP). Adenosine (10(-5) M) had no effect on I(sc) at either the apical or basolateral side of vestibular dark cells and strial marginal cells, suggesting either the absence of P(1) receptors or the absence of coupling of P(1) receptors to vectorial ion transport by these epithelia. Apical perfusion of ATP (10(-8) to 10(-4) M) caused a decrease in I(sc) of both vestibular dark cells and strial marginal cells. Apical perfusion of the nucleotides uridine 5'-triphosphate (UTP), 2-methylthioadenosine triphosphate (2-meS-ATP), adenosine 5'-O-(3-thiotriphosphate) (ATPγS) and α,β-methylene adenosine 5'-triphosphate (α,β-meth-ATP) caused qualitatively similar responses with different magnitudes of response. The sequence of the magnitude of response of each compound at 10(-6) or 10(-5) M was assessed from the fractional change of I(sc). The sequence for vestibular dark cells was UTP = ATP = ATPγS ≫ 2-meS-ATP > α,β-meth-ATP, and for strial marginal cells it was UTP = ATP ≫ 2-meS-ATP, corresponding to the sequence for the P(2U) receptor. The effect of agonist on the apical membrane was reduced by the antagonist 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) but not cibacron blue or suramin. DIDS in the absence of exogenous purinergic agonist caused a sustained increase in I(sc). The effect of ATP on the apical membrane was greater in the absence of divalent cations. Basolateral perfusion of ATP led to a biphasic response of I(sc) in vestibular dark cell and strial marginal cell epithelia, consisting of an initial rapid increase followed by a slower decrease. Perfusion of the perilymphatic surface of the stria vascularis (basal cell layer) with ATP had no acute effect on I(sc). The initial increase of I(sc) in vestibular dark cell epithelium during basolateral perfusion had a sequence of 2-meS-ATP > ATP ≫ UTP = α,β-meth-ATP = ATPγS, corresponding to the sequence for the P(2Y) receptor. Subsequently, the agonists caused a sustained decrease in I(sc) with a sequence of ATPγS > 2-meS-ATP > ATP > UTP >α,β-meth-ATP. This sequence is most simply interpreted as the result of the coexistence of P(2U) and P(2Y) receptors in the basolateral membrane. Both the increase and decrease of I(sc) by ATP at the basolateral membrane were reduced by the antagonist suramin. These findings provide evidence for the regulation of transepithelial ion transport by P(2U) receptors in the apical membrane and by coexisting P(2U) and P(2Y) receptors in the basolateral membrane of K(+)-secretory epithelial cells in the inner ear and are consistent with the hypothesis that the apical receptors are part of an autocrine negative feedback system in these cells.