Characterization of the effects of 2-methylthio-ATP and 2-chloro-ATP on brain capillary endothelial cells: similarities to ADP and differences from ATP

P2Y2 receptor desensitization on single endothelial cells

Receptor desensitization, or decreased responsiveness of a receptor to agonist stimulation, represents a regulatory process with the potential to have a significant impact on cell behavior. P2Y(2), a G-protein-coupled receptor activated by extracellular nucleotides, undergoes desensitization at many tissues, including the vascular endothelium. Endothelial cells from a variety of vascular beds are normally exposed to extracellular nucleotides released from damaged cells and activated platelets. The purpose of the present study was to compare P2Y(2) receptor desensitization observed in endothelial cells derived from bovine retina, a model of microvascular endothelium, and human umbilical vein endothelial cells (HUVECs), a model of a large blood vessel endothelium. P2Y(2) receptor desensitization was monitored by following changes in UTP-stimulated intracellular free Ca(2 +) in single cells using fura-2 microfluorometry. Both endothelial cell models exhibited desensitization of the P2Y(2) receptor after stimulation with UTP. However, the cells differed in the rate, dependence on agonist concentration, and percentage of maximal desensitization. These results suggest differential mechanisms of P2Y(2) receptor desensitization and favors heterogeneity in extracellular nucleotide activity in endothelial cells according to its vascular bed origin.

Altered endothelial Ca2+ regulation after ischemia/reperfusion produces potentiated endothelium-derived hyperpolarizing factor-mediated dilations

BACKGROUND AND PURPOSE

Endothelium-derived hyperpolarizing factor (EDHF)-mediated dilations are potentiated after several pathologies, including ischemia/reperfusion (I/R). However, no study to date has addressed the mechanism by which this potentiation occurs. This study tested the hypothesis that potentiated EDHF-mediated dilations are due to altered endothelial Ca2+ handling after I/R.

METHODS

Rat middle cerebral arteries (MCAs) were isolated after 2 hours of MCA occlusion and 24 hours of reperfusion (or sham surgery). This model has been previously demonstrated to produce potentiated EDHF-mediated dilations. MCAs were studied in a pressurized/perfused vessel chamber equipped for the simultaneous measurement of endothelial Ca2+ (with fura 2) and artery diameter. Measures were made after luminal administration of UTP (P2Y2 purinoceptor agonist), 2 MeS-ATP (P2Y1 purinoceptor agonist), and Br-A23187 (receptor-independent Ca2+ ionophore) for sham and I/R MCAs.

RESULTS

I/R resulted in significantly potentiated UTP-mediated dilations (through a P2Y2 purinoceptor) and endothelial Ca2+ responses in the presence of N(G)-nitro-L-arginine methyl ester (L-NAME) and indomethacin. Endothelial Ca2+ and diameter responses were also significantly potentiated with 2 MeS-ATP (through a P2Y1 purinoceptor) when L-NAME and indomethacin were absent. Br-A23187, a receptor-independent Ca2+ ionophore, produced significantly potentiated endothelial Ca2+ responses after I/R in the presence of L-NAME/indomethacin. Evaluation of artery diameter as a function of endothelial Ca2+ demonstrated no differences between sham and I/R groups.

CONCLUSIONS

These findings demonstrate that I/R results in augmented endothelial Ca2+ responses that appear to be downstream of the receptor level. Moreover, these data suggest that this augmented Ca2+ response contributes to the potentiated EDHF-mediated dilations after I/R.

The pharmacology of nucleotide receptors on primary rat brain endothelial cells grown on a biological extracellular matrix: effects on intracellular calcium concentration

1. Brain capillary endothelial cells express a variety of nucleotide receptors, but differences have been reported between culture models. This study reports examination of nucleotide receptors on primary cultured rat brain capillary endothelial cells (RBCEC) grown on a biological extracellular matrix (ECM) to produce a more differentiated phenotype. 2. Fura-2 fluorescence ratio imaging was used to monitor intracellular free calcium concentration [Ca(2+)](i). ATP, UTP, and 2-methylthioATP (2-MeSATP) increased [Ca(2+)](i) to similar levels, while 2-MeSADP, ADP and adenosine gave smaller responses. 3. Removal of extracellular calcium caused no significant change in the [Ca(2+)](i) response to 2-MeSATP, evidence that the response was mediated by a metabotropic (P2Y) receptor. 4. All cells tested responded to ATP, UTP, 2-MeSATP and ADP, while 63% responded to adenosine and 50% to 2-MeSADP. No cells responded to alpha, beta-methyleneATP. Cells grown on rat tail collagen instead of ECM gave smaller and less uniform [Ca(2+)](i) responses, suggesting that the differentiating effect of the ECM contributed to a more uniform receptor profile. 5. The [Ca(2+)](i) response to the P2Y(1)-selective agonist 2-MeSADP was abolished in the presence of the subtype-selective antagonist adenosine 3'-phosphate 5'-phosphosulphate (PAPS). 6. The P2Y(2) antagonist suramin completely blocked the response to ATP and inhibited the response to UTP by 66%. 7. The A(1) subtype-selective adenosine receptor agonist N(6)-Cyclopentyladenosine (CPA) gave a small but characteristic [Ca(2+)](i) response, while A(2A) and A(2B) subtype-selective agonists failed to generate [Ca(2+)](i) changes. 8. The results are consistent with the presence on RBCEC of a P2Y(2)-like receptor coupled to phospholipase C, and a P2Y(1)-like receptor mobilizing intracellular Ca(2+). The role of multiple nucleotide receptors in the function of the brain endothelium is discussed.

Diadenosine polyphosphates as antagonists of the endogenous P2Y(1) receptor in rat brain capillary endothelial cells of the B7 and B10 clones

1. Diadenosine polyphosphates (Ap(n)As, n=2 - 7) are considered as stress mediators in the cardiovascular system. They act both via identified P2 purinoceptors and via yet to be characterized receptors. This study analyses the actions of Ap(n)As in clones of rat brain capillary endothelial cells that express P2Y(1) receptors (B10 cells) or both P2Y(1) and P2Y(2) receptors (B7 cells). 2. B10 cells responded to Ap(3)A with rises in intracellular Ca(2+) concentration ([Ca(2+)](i)). This response was prevented by adenosine-3'-phosphate-5'-phosphate, an antagonist of P2Y(1) receptors. It was largely suppressed by a treatment with apyrase VII or with creatine phosphokinase/creatine phosphate to degrade contaminating ADP. 3. Ap(n)As inhibited ADP induced increases in [Ca(2+)](i) mediated by P2Y(1) receptors by shifting ADP concentration-response curves to larger concentrations. Apparent K(i) values were estimated to be 6 microM for Ap(4)A, 10 microM for Ap(5)A and 47 microM for Ap(6)A. Ap(2)A and Ap(3)A were much less active. 4. Ap(n)As were neither agonists nor antagonists of the endogenous P2Y(2) receptor in B7 cells. 5. Ap(n)As are neither agonists nor antagonists of the G(i)-coupled, ADP receptor in B10 cells. 6. The results suggest that most actions of Ap(n)As in B7 and B10 cells can be accounted for by endogenous P2Y(1) receptors. Ap(4)A, Ap(5)A and Ap(6)A are specific antagonists of endogenous Ca(2+)-coupled P2Y(1) receptors.

Inflammatory mediators and modulation of blood-brain barrier permeability

1. Unlike some interfaces between the blood and the nervous system (e.g., nerve perineurium), the brain endothelium forming the blood-brain barrier can be modulated by a range of inflammatory mediators. The mechanisms underlying this modulation are reviewed, and the implications for therapy of the brain discussed. 2. Methods for measuring blood-brain barrier permeability in situ include the use of radiolabeled tracers in parenchymal vessels and measurements of transendothelial resistance and rate of loss of fluorescent dye in single pial microvessels. In vitro studies on culture models provide details of the signal transduction mechanisms involved. 3. Routes for penetration of polar solutes across the brain endothelium include the paracellular tight junctional pathway (usually very tight) and vesicular mechanisms. Inflammatory mediators have been reported to influence both pathways, but the clearest evidence is for modulation of tight junctions. 4. In addition to the brain endothelium, cell types involved in inflammatory reactions include several closely associated cells including pericytes, astrocytes, smooth muscle, microglia, mast cells, and neurons. In situ it is often difficult to identify the site of action of a vasoactive agent. In vitro models of brain endothelium are experimentally simpler but may also lack important features generated in situ by cell:cell interaction (e.g. induction, signaling). 5. Many inflammatory agents increase both endothelial permeability and vessel diameter, together contributing to significant leak across the blood-brain barrier and cerebral edema. This review concentrates on changes in endothelial permeability by focusing on studies in which changes in vessel diameter are minimized. 6. Bradykinin (Bk) increases blood-brain barrier permeability by acting on B2 receptors. The downstream events reported include elevation of [Ca2+]i, activation of phospholipase A2, release of arachidonic acid, and production of free radicals, with evidence that IL-1 beta potentiates the actions of Bk in ischemia. 7. Serotonin (5HT) has been reported to increase blood-brain barrier permeability in some but not all studies. Where barrier opening was seen, there was evidence for activation of 5-HT2 receptors and a calcium-dependent permeability increase. 8. Histamine is one of the few central nervous system neurotransmitters found to cause consistent blood-brain barrier opening. The earlier literature was unclear, but studies of pial vessels and cultured endothelium reveal increased permeability mediated by H2 receptors and elevation of [Ca2+]i and an H1 receptor-mediated reduction in permeability coupled to an elevation of cAMP. 9. Brain endothelial cells express nucleotide receptors for ATP, UTP, and ADP, with activation causing increased blood-brain barrier permeability. The effects are mediated predominantly via a P2U (P2Y2) G-protein-coupled receptor causing an elevation of [Ca2+]i; a P2Y1 receptor acting via inhibition of adenyl cyclase has been reported in some in vitro preparations. 10. Arachidonic acid is elevated in some neural pathologies and causes gross opening of the blood-brain barrier to large molecules including proteins. There is evidence that arachidonic acid acts via generation of free radicals in the course of its metabolism by cyclooxygenase and lipoxygenase pathways. 11. The mechanisms described reveal a range of interrelated pathways by which influences from the brain side or the blood side can modulate blood-brain barrier permeability. Knowledge of the mechanisms is already being exploited for deliberate opening of the blood-brain barrier for drug delivery to the brain, and the pathways capable of reducing permeability hold promise for therapeutic treatment of inflammation and cerebral edema.

Regulation of cyclic AMP by extracellular ATP in cultured brain capillary endothelial cells

1 In primary unpassaged rat brain capillary endothelial cell cultures (RBECs), using reverse-transcriptase PCR with primers specific for P2Y receptor subtypes, we detected mRNA for P2Y2, P2Y4 and P2Y6, but not P2Y1 receptors. 2 None of the various nucleotides tested reduced forskolin elevated cyclic AMP levels in RBECs. ATP and ATPgammaS, as well as adenosine, enhanced cyclic AMP accumulation in the presence of forskolin. 3 Comparison of the concentration response curves to ATPgammaS with those for ATP and adenosine, at different incubation times, indicated that the response to purine nucleotides was not wholly dependent on conversion to adenosine. Adenosine deaminase abolished the response to adenosine but only reduced the response to ATP by about 50%. These results suggest the participation of a receptor responsive to nucleotides. 4 Isobutylmethylxanthine and 8-sulphophenyltheophylline prevented the cyclic AMP response, while neither 8-cyclopentyl-1, 3-dipropylxanthine nor SCH58261 were effective antagonists. 2-chloradenosine gave a robust response, but neither 2-chloro-N6-cyclopentyladenosine nor CGS 21680 were agonists. 5 These results show that adenosine and ATP can elevate the cyclic AMP levels of brain endothelial cells by acting on receptors which have a pharmacology apparently distinct from known P2Y and adenosine receptors.

Benzoyl ATP is an antagonist of rat and human P2Y1 receptors and of platelet aggregation

The effects of 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) on intracellular Ca2+ mobilization and cyclic AMP accumulation were investigated using rat brain capillary endothelial cells which express an endogenous P2Y1 receptor, human platelets which are known to express a P2Y1 receptor, and Jurkat cells stably transfected with the human P2Y1 receptor. In endothelial cells, BzATP was a competitive inhibitor of 2-methylthio ADP (2-MeSADP) and ADP induced [Ca2+]i responses (Ki = 4.7 microM) and reversed the inhibition by ADP of adenylyl cyclase (Ki = 13 microM). In human platelets, BzATP inhibited ADP-induced aggregation (Ki = 5 microM), mobilization of intracellular Ca2+ stores (Ki = 6.3 microM), and inhibition of adenylyl cyclase. In P2Y1-Jurkat cells, BzATP inhibited ADP and 2-MeSADP-induced [Ca2+]i responses (Ki = 2.5 microM). It was concluded that BzATP is an antagonist of rat and human P2Y1 receptors and of platelet aggregation. In contrast to other P2Y1 receptor antagonists (A2P5P and A3P5P) which inhibit only ADP-induced Ca2+ mobilization, BzATP inhibits both the Ca2+- and the cAMP-dependent intracellular signaling pathways of ADP. These results provide further evidence that P2Y1 receptors contribute to platelet ADP responses.

Na+-Ca2+ exchange and its implications for calcium homeostasis in primary cultured rat brain microvascular endothelial cells

1. The role of Na+-Ca2+ exchange in the regulation of the cytosolic free Ca2+ concentration ([Ca2+]i) was studied in primary cultured rat brain capillary endothelial cells. [Ca2+]i was measured by digital fluorescence imaging in cells loaded with fura-2. 2. ATP (100 microM) applied for a short period of time (6 s) caused a rise in [Ca2+]i from 127 +/- 3 (n = 290) to 797 +/- 25 nM, which then declined to the resting level, with a t time required for [Ca2+]i to decline to half of peak [Ca2+]i) of 5.4 +/- 0.09 s. This effect was independent of external Ca2+ and could be abolished by previously discharging the Ca2+ pool of the endoplasmic reticulum with thapsigargin (1 microM). 3. Application of thapsigargin (1 microM) or cyclopiazonic acid (10 microM) to inhibit the Ca2+-ATPase of the endoplasmic reticulum 6 s prior to ATP application did not influence the peak [Ca2+]i but greatly reduced the rate of decline of [Ca2+]i, with t values of 15 +/- 1.6 and 23 +/- 3 s, respectively. 4. In the absence of external Na+ (Na+ replaced by Li+ or N-methylglucamine) the basal [Ca2+]i was slightly elevated (152 +/- 6 nM) and the restoration of [Ca2+]i after the ATP stimulation was significantly slower (t , 7.3 +/- 0.46 s in Li+ medium, 8.12 +/- 0.4 s in N-methylglucamine medium). 5. The external Na+-dependent component of the [Ca2+]i sequestration was also demonstrated in cells stimulated by ATP subsequent to addition of cyclopiazonic acid; in a Na+-free medium [Ca2+]i remained at the peak level in 88 % of the cells after stimulation with ATP. 6. Addition of monensin (10 microM) in the presence of external Na+ increased the resting [Ca2+]i to 222 +/- 9 nM over approximately 1 min and subsequent removal of extracellular sodium resulted in a further increase in [Ca2+]i to a peak of 328 +/- 11 nM, which was entirely dependent on external Ca2+. 7. These findings indicate that a functional Na+-Ca2+ exchanger is present at the blood-brain barrier, which plays a significant role in shaping the stimulation-evoked [Ca2+]i signal and is able to work in reverse mode under pharmacological conditions.

P2 receptor subtypes in the cardiovascular system

Extracellular nucleotides have been implicated in a number of physiological functions. Nucleotides act on cell-surface receptors known as P2 receptors, of which several subtypes have been cloned. Both ATP and ADP are stored in platelets and are released upon platelet activation. Furthermore, nucleotides are also released from damaged or broken cells. Thus during vascular injury nucleotides play an important role in haemostasis through activation of platelets, modulation of vascular tone, recruitment of neutrophils and monocytes to the site of injury, and facilitation of adhesion of leucocytes to the endothelium. Nucleotides also moderate these functions by generating nitric oxide and prostaglandin I2 through activation of endothelial cells, and by activating different receptor subtypes on vascular smooth muscle cells. In the heart, P2 receptors regulate contractility through modulation of L-type Ca2+ channels, although the molecular mechanisms involved are still under investigation. Classical pharmacological studies have identified several P2 receptor subtypes in the cardiovascular system. Molecular pharmacological studies have clarified the nature of some of these receptors, but have complicated the picture with others. In platelets, the classical P2T receptor has now been resolved into three P2 receptor subtypes: the P2Y1, P2X1 and P2TAC receptors (the last of these, which is coupled to the inhibition of adenylate cyclase, is yet to be cloned). In peripheral blood leucocytes, endothelial cells, vascular smooth muscle cells and cardiomyocytes, the effects of classical P2X, P2Y and P2U receptors have been found to be mediated by more than one P2 receptor subtype. However, the exact functions of these multiple receptor subtypes remain to be understood, as P2-receptor-selective agonists and antagonists are still under development.

Analysis of the influence of nucleotidases on the apparent activity of exogenous ATP and ADP at P2Y1 receptors

1. ADP is a potent agonist of rat and human P2Y1 purinoceptors. ATP is a weak competitive antagonist. This study analyses the situation in which P2Y1 receptors are exposed to ATP in the presence of exogenous ecto-nucleotidases (apyrases) that have high or low ATPase/ADPase activity ratio. 2. Rat brain capillary endothelial cells of the B10 clone express P2Y1 receptors that couple to intracellular Ca2+ mobilization. They have low endogenous ecto-ATPase and ecto-ADPase activities. 3. ATP did not raise intracellular Ca2+ in B10 cells. Addition of apyrases III or VII (1 u ml(-1)) to ATP treated cells induced large intracellular Ca2+ transients. Apyrases had no action in the absence of ATP. 4. A 1 u ml(-1) apyrase III solution generated 20 microM ADP from 0.1 mM ATP within 15 s. This concentration of ADP was sufficient to produce maximal activation of P2Y1 receptors. 5. ATP was a full agonist of P2Y1 receptors in the presence of 1 u ml(-1) apyrase III. Dose response curves for the apparent actions of ATP were bell shaped in the presence of 0.1 u ml(-1) apyrase III. Apyrase III did not alter ADP dose response curves when coincubated with ADP for 15 s. 6. Apyrase VII (1 u ml(-1)) shifted dose response curves for the actions of ADP to larger concentrations. It induced a bell shaped ATP dose response curve. 7. Results suggest that ATPDases prevent P2Y1 receptor activation by degrading ADP but may contribute to P2Y1 receptor activation by generating ADP from ATP.

The P2Y1 Receptor Is Necessary for Adenosine 5′-Diphosphate–Induced Platelet Aggregation

The human P2Y1 receptor heterologously expressed in Jurkat cells behaves as a specific adenosine 5′-diphosphate (ADP) receptor at which purified adenosine triphosphate (ATP) is an ineffective agonist, but competitively antagonizes the action of ADP. This receptor is thus a good candidate to be the elusive platelet P2T receptor for ADP. In the present work, we examined the effects on ADP-induced platelet responses of two selective and competitive P2Y1 antagonists, adenosine-2′-phosphate-5′-phosphate (A2P5P) and adenosine-3′-phosphate-5′-phosphate (A3P5P). Results were compared with those for the native P2Y1 receptor expressed on the B10 clone of rat brain capillary endothelial cells (BCEC) and for the cloned human P2Y1 receptor expressed on Jurkat cells. A2P5P and A3P5P inhibited ADP-induced platelet shape change and aggregation (pA2 = 5) and competitively antagonized calcium movements in response to ADP in fura-2–loaded platelets, B10 cells, and P2Y1-Jurkat cells. In contrast, these compounds had no effect on ADP-induced inhibition of adenylyl cyclase in platelets or B10 cells, whereas known antagonists of platelet activation by ADP such as Sp-ATPαS were effective. These identical signaling responses and pharmacologic properties suggest that platelets and BCEC share a common P2Y1 receptor involved in ADP-induced aggregation and vasodilation, respectively. This P2Y1 receptor coupled to the mobilization of intracellular calcium stores was found to be necessary to trigger ADP-induced platelet aggregation. The present results, together with data from the literature, also point to the existence of another as yet unidentified ADP receptor, coupled to adenylyl cyclase and responsible for completion of the aggregation response. Thus, the term, P2T, should no longer be used to designate a specific molecular entity.

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.

PPADS inhibits P2Y1 purinoceptors in rat brain capillary endothelial cells and in rat ileal myocytes by an indirect mechanism

P2Y1 receptor-like responses were analyzed in rat ileal myocytes and in rat brain capillary endothelial cells. In endothelial cells, pyridoxal phosphate-6-azophenyl-2',4'disulfonic acid (PPADS) inhibits ADP induced intracellular Ca2+ transients with a half maximum effect at 3 microM. PPADS shifts ADP dose response curves to larger concentrations. Yet PPADS is inactive when added at the same time as ADP. A preequilibration of the cells with PPADS is necessary to observe its inhibitory action. Similarly in ileal myocytes, PPADS has no action on ADP responses when it is applied at the same time as ADP. Actions of PPADS require a preequilibration with the cells and are fully reversible. These results suggest that PPADS is not a competitive antagonist of P2Y1 receptors and caution about its usefulness to distinguish subtypes of P2Y1 receptors.

Fidelity in functional coupling of the rat P2Y1 receptor to phospholipase C

1. The rat homologue of the P2Y1 receptor has been heterologously expressed in 1321N1 human astrocytoma cells and in C6 rat glioma cells. 2. As has been shown previously for the turkey and human P2Y1 receptors, the rat P2Y1 receptor expressed in either cell type responded to 2MeSATP with increases in inositol phosphate accumulation that were competitively blocked by the antagonist PPADS. Neither of the wild type cell lines exhibited inositol phosphate responses to P2Y1 receptor agonists. 3. Expression of the rat P2Y1 receptor did not confer a capacity of 2MeSATP to inhibit adenylyl cyclase activity in 1321N1 cells. Moreover, the inhibition of adenylyl cyclase mediated by an endogenous P2Y receptor of C6 glioma cells was not enhanced by expression of the rat P2Y1 receptor. The P2Y receptor-mediated inhibition of adenylyl cyclase in C6 glioma cells expressing both the endogenous P2Y receptor and the rat P2Y1 receptor remained unaffected by PPADS. 4. Since the P2Y receptor responsible for inhibition of adenylyl cyclase in C6 glioma cells does not share the pharmacological or functional properties of the P2Y1 receptor, even when both receptors originate from the same species and are simultaneously expressed in the same cell line, it is concluded that the P2Y1 receptor is distinct from an endogenous P2Y receptor in C6 cells that couples to inhibition of adenylyl cyclase.

Regulation of brain capillary endothelial cells by P2Y receptors coupled to Ca2+, phospholipase C and mitogen-activated protein kinase

1. The blood-brain barrier is formed by capillary endothelial cells and is regulated by cell-surface receptors, such as the G protein-coupled P2Y receptors for nucleotides. Here we investigated some of the characteristics of control of brain endothelial cells by these receptors, characterizing the phospholipase C and Ca2+ response and investigating the possible involvement of mitogen-activated protein kinases (MAPK). 2. Using an unpassaged primary culture of rat brain capillary endothelial cells we showed that ATP, UTP and 2-methylthio ATP (2MeSATP) give similar and substantial increases in cytosolic Ca2+, with a rapid rise to peak followed by a slower decline towards basal or to a sustained plateau. Removal of extracellular Ca2+ had little effect on the peak Ca2+-response, but resulted in a more rapid decline to basal. There was no response to alpha,beta-MethylATP (alpha,beta MeATP) in these unpassaged cells, but a response to this P2X agonist was seen after a single passage. 3. ATP (log EC50 -5.1+/-0.2) also caused an increase in the total [3H]-inositol (poly)phosphates ([3H]-InsPx) in the presence of lithium with a rank order of agonist potency of ATP=UTP=UDP>ADP, with 2MeSATP and alpha,beta MeATP giving no detectable response. 4. Stimulating the cells with ATP or UTP gave a rapid rise in the level of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), with a peak at 10 s followed by a decline to a sustained plateau phase. 2MeSATP gave no detectable increase in the level of Ins(1,4,5)P3. 5. None of the nucleotides tested affected basal cyclic AMP, while ATP and ATPgammaS, but not 2MeSATP, stimulated cyclic AMP levels in the presence of 5 microM forskolin. 6. Both UTP and ATP stimulated tyrosine phosphorylation of p42 and p44 mitogen-activated protein kinase (MAPK), while 2MeSATP gave a smaller increase in this index of MAPK activation. By use of a peptide kinase assay, UTP gave a substantial increase in MAPK activity with a concentration-dependency consistent with activation at P2Y2 receptors. 2MeSATP gave a much smaller response with a lower potency than UTP. 7. These results are consistent with brain endothelial regulation by P2Y2 receptors coupled to phospholipase C, Ca2+ and MAPK; and by P2Y1-like (2MeSATP-sensitive) receptors which are linked to Ca2+ mobilization by a mechanism apparently independent of agonist stimulated Ins(1,4,5)P3 levels. A further response to ATP, acting at an undefined receptor, caused an increase in cyclic AMP levels in the presence of forskolin. The differential MAPK coupling of these receptors suggests that they exert fundamentally distinct influences over brain endothelial function.

The P2Y purinoceptor in rat brain microvascular endothelial cells couple to inhibition of adenylate cyclase

1. B10 cells, a clonal line of rat brain capillary endothelial cells, exhibit a single P2 purinoceptor, activation of which leads to increases in free intracellular calcium. In the current study the identity of this P2Y receptor was determined by its binding parameters for a range of purinoceptor ligands and by its complementary DNA (cDNA) sequence. The signal transduction mechanism activated by this receptor was also investigated. 2. The radioligand [35S]-dATP alpha S bound with high affinity (Kd = 9.8 nM) to the P2Y purinoceptor expressed on B10 cells, which was found to be extremely abundant (Bmax = 22.5 pmol mg-1 protein). The calculated Ki values of a range of P2 purinoceptor agonists which competitively displaced binding of [35S]-dATP alpha S led to the rank order of affinity: dATP alpha S (Ki 3.4 nM) > 2-chloroATP (2-ClATP) (13 nM), ATP (22 nM) > ATP gamma S (43 nM) > 2-methylthioATP (2-MeSATP) (88 nM) > ADP (368 nM) > > UTP, L-beta,gamma-methyleneATP (both > 10,000 nM). The P2 purinoceptor antagonists, Reactive blue 2 and suramin, were also able to displace binding, with Ki values of 833 and 1358 nM respectively. In contrast pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium (PPADS) was able to displace only 20% of [35S]-dATP alpha S binding at a concentration of 100 microM. 3. 2-ClATP (EC50 = 0.22 microM), 2-MeSATP (0.54 microM), ADP (7.9 microM) and ATP (a partial agonist), but not UTP, inhibited the cyclic AMP formation stimulated by cholera toxin, in a manner that was prevented by pertussis toxin. The purinoceptor antagonist, PPADS, was found to be inactive at a concentration of 100 microM. 4. A P2Y receptor cDNA was derived from mRNA from B10 cells and from C6-2B, a rat glioma cell line known to possess a P2Y receptor that is coupled to the inhibition of adenylate cyclase. Sequence analysis of the entire coding region revealed that both were 100% identical to the rat P2Y1 purinoceptor cDNA. No other P2Y-type receptor mRNA could be detected in B10 cells. Exactly the same sequence was isolated from rat brain cortical astrocytes, where 2-MeSATP has been shown to increase phospholipase C activity. 5. Since the receptor responsible for the transduction shares with the aforementioned binding site significant pharmacological features, including a strong activity of 2-MeSATP (characteristic of P2Y1 receptors alone among all known P2Y purinoceptors) and an unusual insensitivity to PPADS, and since abundant mRNA is present of the P2Y1 receptor but not of any other type resembling the known P2Y receptors, it is concluded that a P2Y1 receptor on rat brain microvascular endothelial cells can account for all of the observations. This single P2Y1 receptor, therefore, appears to couple in different native cell types to either adenylate cyclase inhibition or to phospholipase C activation.

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.

P2Y purinoceptors in gastric gland plasma membranes

This autoradiographic study of sections of the rabbit stomach fundus labelled with [35S]dATP alpha S, a radioligand for P2Y purinoceptors, has demonstrated a discrete pattern of distribution of the binding sites, i.e., the specific binding was only over the mucosa, but not over the muscular layer. Radioligand binding assays carried out on gastric gland plasma membranes showed that the binding process was saturable and a high density of a homogeneous population of binding sites was observed. These binding sites presented high affinity with a value of Kd = 4.1 +/- 0.8 nM and the maximum density of the binding sites was 16.8 +/- 1.6 pmol/mg protein. The displacement by purinoceptor ligands showed the following order of potency: ATP = 2-methylthio ATP > > alpha, beta-methylene ATP > > adenosine. Neither UTP nor pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) were able to displace the binding. The data support the presence of P2Y purinceptors in rabbit gastric glands.

The effect of PPADS as an antagonist of inositol (1,4,5)trisphosphate induced intracellular calcium mobilization

1. Brain capillary endothelial cells responded to uridine 5'-triphosphate (UTP) and adenosine 5'-triphosphate (ATP) by activation of phospholipase C and by large changes in [Ca2+]i. These cells expressed mRNA sequences identical to the sequence of the P2Y2-purinoceptor of rat pituitaries. 2. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) at 100 microM did not prevent UTP and ATP induced accumulations of total [3H]-inositol (poly)phosphates. It inhibited UTP and ATP induced intracellular Ca2+ mobilization (IC50 = 30 microM) by non competitive mechanism. 3. PPADS (100 microM) inhibited endothelin-1 induced accumulation of total [3H]-inositol (poly)phosphates by less than 20% and prevented most of endothelin-1 induced intracellular Ca2+ mobilization (IC50 = 30 microM). 4. PPADS (100 microM) had no action on ionomycin induced intracellular Ca2+ mobilization. 5. Microinjection of inositol (1,4,5)trisphosphate (InsP3) into Xenopus oocytes induced large Ca2+ activated Cl- currents that were prevented by heparin and by PPADS. 6. It is concluded that PPADS does not recognize rat P2Y2-purinoceptors and prevents UTP and ATP induced intracellular Ca2+ mobilization by a non-specific mechanism that could involve the inhibition of InsP3 channels.

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.

Some P2 purinergic agonists increase cytosolic calcium but not inositol 1,4,5-trisphosphate in isolated rat hepatocytes

Based on the capacity to increase IP3 (inositol 1,4,5-trisphosphate), P2 purinergic agonists can be subdivided into two classes: ATP, ADP, UTP, 2deoxyATP, NAD and GTP significantly increased IP3 levels whereas ADP beta S, 2MeSATP, NADP, alpha, beta MeATP, beta, gamma MeATP and ATP alpha S had only a minor, non-significant effect. Irrespective of their potency to increase IP3, all agonists were full glycogenolytic agonists and they all increased cytosolic calcium. With ATP and NAD, IP3 increasing agonists, and 2MeSATP and ADP beta S, non-IP3 increasing agonists, we found that the initial calcium response appeared to be an 'all or none' phenomenon, small amounts of the agonists being either ineffective or equally effective as high amounts. The minimal amount of an agonist needed to initiate a calcium increase and to promote glycogenolysis was very similar. In the absence of extracellular calcium, both groups of purinergic agonists (tested with ATP and 2MeSATP) were equally able to release calcium from intracellular stores. Cells with emptied intracellular calcium stores rapidly took up extracellular calcium upon treatment with ATP or 2MeSATP, the latter being the most potent. It seems therefore that all nucleotides tested increased cytosolic calcium and activated phosphorylase in a very similar way but some nucleotides had no effect on the levels of IP3.

Actions of ADP, but not ATP, on cytosolic free Ca2+ in single rat hepatocytes mimicked by 2-methylthioATP

1. Aequorin-injected, single rat hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway, including ADP and ATP. We have previously described differences in the [Ca2+]i responses of aequorin-injected hepatocytes to ADP and ATP. 2. The effects of the phosphorothioate analogue of ATP, 2-methylthioATP (2-meSATP), have been examined on single rat hepatocytes. This analogue is belived to be the most potent agonist at the P2Y1 subclass of purinoceptor. 3. The [Ca2+]i transients induced by 2-meSATP were indistinguishable from those induced by ADP, and in contrast to those induced by ATP. 4. At hig concentrations, 2-meSATP and ADP both induced transients at high frequency. In contrast, hepatocytes responded to high concentrations of ATP with an initial rapid rise in [Ca2+]i, followed by a slowly decaying fall. 5. The modulatory effects of elevated intracellular cyclic AMP concentration were the same on both 2-meSATP- and ADP-induced [Ca2+]i transients; the peak height and frequency of transients were enhanced. ATP-induced transients, however, underwent either an increase in duration or conversion into a sustained rise in [Ca2+]i. 6. ATP-induced transients were specifically potentiated by the co-addition of alpha, beta-methyleneATP, whereas 2-meSATP- and ADP-induced transients were unaffected by this treatment. 7. We conclude that 2-meSATP acts at the same receptor as ADP on rat hepatocytes, and that this is distinct from teh receptor(s) mediating the effects of ATP.

ATP, a partial agonist of atypical P2Y purinoceptors in rat brain microvascular endothelial cells

1. Purinoceptor responses were analyzed in B10 cells, a clonal population of rat brain capillary endothelial cells. 2. B10 cells lack P2U receptors as evidenced by the lack of UTP responses and the failure to amplify P2U-related sequences by polymerase chain reaction. 3. B10 cells responded to adenine nucleotides by large increases in [Ca2+]i. Half maximum effective concentrations were 2-methylthio-ATP: 180 nM > 2-chloro-ATP: 310 nM = ADP: 330 nM > adenosine 5'-O-(3-thiotrisphosphate): 2.3 microM = ATP: 2.7 microM. The maximum response to ATP was only 55% of that to ADP while that to ATP derivatives was 75%. 4. The actions of adenine nucleotides were not associated with a measurable activation of phospholipase C. 5. Cross desensitizations of the actions of ADP and ATP were observed. 6. In additivity experiments, ADP superposed its action on top of that of ATP and ATP partially inhibited the action of ADP. 7. It is concluded that ATP acts as a partial agonist of the P2Y-like receptor of brain capillary endothelial cells.

Characteristics of nucleotide receptors that cause elevation of cytoplasmic calcium in immortalized rat brain endothelial cells (RBE4) and in primary cultures

1. A dual-wavelength microfluorimetric method using Fura-2 as calcium indicator was applied to cells from an immortalized cell line of rat brain microvascular endothelial cells (RBE4), and to primary cultured rat brain endothelial cells. 2. In RBE4 cells, a brief (20 s) pulse of extracellular ATP (100 microM) induced a transient increase in the cytoplasmic calcium level ([Ca2+]i). Control responses to 100 microM ATP consisted of a ratio increase of 0.64 +/- 0.03 (mean +/- s.e., n = 51). Responses were seen at a concentration of 2.5 microM and were maximal at 100-1000 microM. When extracellular calcium was chelated with EGTA, the transient increase in [Ca2+]i was not affected. The results are consistent with Ca2+ mobilization from intracellular stores. 3. The purinoceptor involved belongs to the P2 subtype, since the agonist potency order among the adenine nucleotides was ATP > ADP > AMP. Moreover, the increase in [Ca2+]i evoked by ATP was partially inhibited by the P2 antagonist, suramin but was not affected by 8-phenyltheophylline, a P1-purinoceptor antagonist. The strong desensitization observed with repeated applications of ATP is also typical of a P2 receptor. 4. 2-Methylthio-ATP (2meS-ATP 100 microM), a P2Y agonist, elevated [Ca2+]i in only 17% of the cells tested; however, 2meS-ATP was found to antagonize the effect of ATP in all cells tested. The increase in [Ca2+]i evoked by ATP was inhibited by 500 s application of the P2Y purinoceptor antagonist, Reactive Blue 2 at 10 microM, while 60 s application of 100 microM was ineffective. 5. The uracil nucleotide, UTP (100 microM) was as effective as ATP in increasing [Ca2+]i. The effects of ATP and UTP were not additive. Cells desensitized to the action of ATP (or UTP) were unable to respond to UTP (or ATP).6. alpha,beta Methylene-ATP (alpha,beta meATP 100 microM), a P2x, agonist, elevated [Ca2+], in only 40% of the cells tested. In these cells it was less effective than ATP in increasing [Ca2+]i.7. Cells desensitized to the action of ADP responded, to a smaller extent, to ATP. In contrast, cells desensitized to the action of ATP were unable to respond to ADP.8. On primary cultures of brain endothelial cells the increase in [Ca2+]i in response to extracellular ATP(100 microM) and UTP (100 microM) was of an equivalent amplitude, and similar to the response in RBE4 cells.The pattern of desensitization was also similar to that in RBE4 cells.9 This comparative study indicates that in well-characterized brain microvascular endothelial cells that retain brain endothelial characteristics, the major class of nucleotide receptor is of the P2mu type. The implications for physiology are discussed.

Sensitization by calyculin A of brain capillary endothelial cells to endothelin-1

1. Cultured brain capillary endothelial cells of the rat respond to endothelin-1 (ET-1) by an increased activity of the Na+,K+,2Cl-, cotransporter and a mobilization of intracellular Ca2+ stores. 2. Calyculin A (1-30 nM), but not okadaic acid, sensitizes up to 100 fold the Na+,K+,2Cl- cotransporter to the action of ET-1. 3. Calyculin A (30 nM) does not modify the binding properties of ET-1 to ETA receptors. 4. Calyculin A (30 nM) inhibits ET-1 induced intracellular Ca2+ mobilization. 5. It is concluded that inhibition of protein phosphatase 1 selectively modifies the repertoire of intracellular actions of ET-1 and favours actions that are unrelated to the phospholipase C signalling cascade.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.