Extracellular nucleotides stimulate receptor-mediated calcium mobilization and inositol phosphate production in human fibroblasts

Endothelin-mediated calcium response and inositol 1,4,5-trisphosphate release in neuroblastoma-glioma hybrid cells (NG108-15): cross talk with ATP and bradykinin

Addition of endothelins (ETs) to neuroblastoma-glioma hybrid cells (NG108-15) induced increases in cytosolic free Ca2+ ([Ca2+]i) levels of labeled inositol monophosphates and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The increases in [Ca2+]i elicited by the three ETs (ET-1, ET-2, and ET-3) were transient and did not show a sustained phase. Chelating extracellular Ca2+ in the medium by adding excess EGTA decreased the ET-mediated Ca2+ response by 40-50%. This result indicates that a substantial portion of the increase in [Ca2+]i was due to influx from an extracellular source. However, the increase in [Ca2+]i was not affected by verapamil or nifedipine (10(-5) M). A rank order potency of ET-1 > ET-2 > ET-3 is shown for the stimulated increase in [Ca2+]i, as well as labeled inositol phosphates, in these cells. ATP (10(-4) M) and bradykinin (10(-7) M) also induced the increases in [Ca2+]i and Ins(1,4,5)P3 in NG108-15 cells, albeit to a different extent. When compared at 10(-7) M, bradykinin elicited a five- to sixfold higher increase in the level of Ins(1,4,5)P3, but less than a twofold higher increase in [Ca2+]i than those induced by ET-1. Additive increases in both Ins(1,4,5)P3 and [Ca2+]i were observed when ET-1, ATP, and bradykinin were added to the cells in different combinations, suggesting that each receptor agonist is responsible for the hydrolysis of a pool of polyphosphoinositide within the membrane. ET-1 exhibited homologous desensitization of the Ca2+ response, but partial heterologous desensitization to the Ca2+ response elicited by ATP. On the contrary, ET-1 did not desensitize the response elicited by bradykinin, although bradykinin exhibited complete heterologous desensitization to the response elicited by ET-1. Taken together, these results illustrate that, in NG108-15 cells, a considerable amount of receptor cross talk occurs between ET and other receptors that transmit signals through the polyphosphoinositide pathway.

Inositol 1,4,5-trisphosphate generation and calcium mobilisation via activation of an atypical P2 receptor in the neuronal cell line, N1E-115

1. Alterations in the levels of intracellular calcium ([Ca2+]i) and D-myo-inositol-1,4,5-trisphosphate (InsP3) were measured in the murine neuroblastoma cell line clone, N1E-115, by use of the calcium-sensitive dye, fura-2 and a radioreceptor assay, respectively. 2. Exposure of the cells to ATP (100 microM) elicited rapid and transient increases in [Ca2+]i and InsP3, with both responses reaching a maximum between 10-20 s after agonist addition. 3. Investigation of concentration-response data by use of various analogues of ATP suggests the presence of an extracellular receptor which fails to fit into the current classification of purinoceptors. 4. Cross-desensitization experiments suggest that the same receptor can also be activated by the structurally different pyrimidine base, UTP. 5. Application of the tumour-promoting agent, beta-phorbol-12,13 dibutyrate (PDBu) caused a reduction in the increases in both [Ca2+]i and InsP3, suggesting a role for protein kinase C in feedback inhibition of purinoceptor responses in this cell line. 6. In summary, we present the first evidence for the existence of an atypical purinoceptor on a cell line of CNS origin. This receptor is linked to stimulation of phosphoinositide turnover and subsequent mobilisation of intracellular calcium.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Effect of nucleotides on the cytosolic free calcium activity and inositol phosphate formation in human glomerular epithelial cells

1. Glomerular epithelial cells (GEC) were cultured from human kidneys and immunologically characterized. 2. The effect of extracellular nucleotides on the cytosolic free calcium activity [Ca2+]i was investigated with the fura-2 microfluorescence method. Extracellular UTP, UDP, UMP, ATP, adenosine 5'-O-(3-thio)-trisphosphate (ATP-gamma-S), inosine-triphosphate (ITP), guanyltriphosphate (GTP), 2-methylthio-ATP, AMP, alpha,beta-methylene-ATP and adenosine led to a rapid, transient, concentration-dependent increase of [Ca2+]i, followed by a plateau above the baseline level. 3. In a calcium-free extracellular solution, the rapid increase of [Ca2+]i was still present, whereas the plateau level was abolished. 4. ATP and UTP (ED50 both: 10(-5) M) stimulated inositol trisphosphate (InsP3) formation in GEC. 5. The order of potency for the purine nucleotides in stimulating InsP3 formation was ATP = ATP-gamma-S greater than ADP greater than 2-methylthio-ATP greater than AMP = a,beta methylene-ATP = adenosine. 6. The increase of InsP3 induced by ATP (10(-5) M) could be inhibited by the P2 receptor blocker suramin (greater than 10(-4) M). Reactive blue 2 exhibited a weak stimulating effect on the InsP3 formation and only a weak inhibitory effect at a concentration of 10(-3) M was observed. 7. Protein kinase C activation by preincubation of GEC with phorbol 12-myristate 13-acetate (PMA, 100 ng ml-1, 15 min) abolished the effect of ATP (10(-5) M) on InsP3 formation. Downregulation of protein kinase C by long term incubation (18 h) with PMA had no significant effect on the phosphoinositol turnover induced by ATP.8. The results indicate that an increase of [Ca2+]i and inositol phosphate breakdown can be mediated via activation of a P2 receptor in human GEC.

A nucleotide receptor in vascular endothelial cells is specifically activated by the fully ionized forms of ATP and UTP

Extracellular ATP causes an increase in the concentration of cytoplasmic free calcium ([Ca2+]i) in bovine pulmonary-artery endothelial (BPAE) cells that results in the synthesis and release of prostacyclin (PGI2), a potent vasodilator and inhibitor of platelet aggregation. We show here that PGI2 release in BPAE cells correlates with the concentration of the fully ionized form of extracellular ATP (ATP4-) and not with the concentration of other ionic forms of ATP. Concentrations as low as 10 nM-ATP4- elicited an increase in PGI2 release [EC50 (concn. giving half-maximal stimulation) 3 microM] in BPAE cells incubated in an iso-osmotic medium, pH 7.4, lacking Ca2+ and Mg2+. When the pH or the Mg2+ concentration of the medium was varied so as to maintain a constant level of ATP4-, while varying the concentration of proton-ATP (HATP3-) or MgATP2- respectively, PGI2 release remained constant. An inhibitory effect of extracellular Mg2+ on PGI2 release could be attributed solely to a decrease in the concentration of ATP4-. In contrast with Mg2+, extracellular Ca2+ stimulated PGI2 release induced by ATP. Several results suggest that extracellular Ca2+ modulates PGI2 release by increasing Ca2+ uptake through an ATP(4-)-activated plasma-membrane channel. In BPAE cells incubated in Ca(2+)-free medium, ATP elicited a transient increase in [Ca2+]i that declined to the basal level within 60 s. In cells incubated in Ca(2+)-containing medium, ATP caused an increase in [Ca2+]i that had two components: a transient peak in [Ca2+]i (0-60 s) and a sustained increase in [Ca2+]i that was maintained for several minutes after ATP addition. Increasing the concentration of extracellular calcium from 0.25 mM to 10 mM had no effect on the transient rise in [Ca2+]i induced by ATP, but significantly enhanced the magnitude of the sustained increase in [Ca2+]i. Alterations in the magnitude of the sustained increase in [Ca2+]i would likely modulate PGI2 release, which was not complete until 2 min after ATP addition. Extracellular Ca2+ also stimulated PGI2 release induced by bradykinin. Bradykinin caused a sustained increase in [Ca2+]i in BPAE cells in the presence of extracellular Ca2+. Finally, the magnitude of PGI2 release induced by UTP, a more potent agonist than ATP, correlated with the concentration of extracellular fully ionized UTP (UTP4-). These findings support the hypothesis that nucleotide receptors in BPAE cells recognize the fully ionized form of ATP and UTP and are coupled to signal-transduction pathways involving the mobilization of intracellular Ca2+, the influx of extracellular Ca2+ and the subsequent release of PGI2.

Possible mechanism of ciliary stimulation by extracellular ATP: involvement of calcium-dependent potassium channels and exogenous Ca2+

Ciliary motility was examined optically in tissue cultures from frog palate epithelium and frog's esophagus as a function of extracellular concentration of adenosine 5'-triphosphate (ATP) and related compounds. The addition of micromolar concentration of ATP caused a strong enhancement of frequency and wave velocity in the direction of the effective stroke. Since adenosine 5'-[beta,gamma imido]-triphosphate (AMP-PNP), a nonhydrolyzable analog of ATP, produces the same effects, ATP hydrolysis is not required. The overall potency is ATP approximately equal to AMP-PNP greater than ADP much greater than adenosine greater than AMP. It is suggested that both the phosphate and the base moieties are involved in ATP binding. The enhancement of ciliary activity by extracellular ATP is dependent on the presence of extracellular Ca2+, which can be replaced by extracellular Mg2+. The effect of a number of potent inhibitors of the voltage-gated calcium channels on the stimulation of ciliary activity by ATP were examined. No effect was detected in the concentration range within which these agents are specific. On the other hand, quinidine, a potent inhibitor of K+ (calcium-dependent) channels, inhibits the effect of ATP. The following model is suggested: exogenous ATP interacts with a membrane receptor in the presence of Ca2+, a cascade of events occurs which mobilizes intracellular calcium, thereby increasing the cytosolic free Ca2+ concentration which consequently opens the calcium-activated K+ channels, which then leads to a change in membrane potential. The ciliary response to these changes is the enhancement of ciliary activity.

Evidence for P2-purinoceptors on human osteoblast-like cells

ATP released from damaged cells or by controlled secretion could be an important factor in the formation or remodeling of bone. In a variety of other tissues ATP has been shown to control cellular processes by acting on P2-purinoceptors and activating the calcium signaling pathway. Here we demonstrate for the first time that extracellular ATP increases the intracellular free calcium [Ca2+]i concentration in normal human osteoblasts and in SaOS-2 cells, a human osteosarcoma-derived cell line, but not in ROS 17/2.8 cells. The ATP-induced increase in [Ca2+]i was dose dependent, and the concentrations of ATP required were similar to those reported to regulate cellular functions in other cell types. Although ATP is metabolized rapidly by bone cells, the effects on [Ca2+]i appeared to be mediated directly by ATP rather than one of its metabolites. Adenosine 3-thiotriphosphate, a nonhydrolyzable analog of ATP, induced similar changes in [Ca2+]i. This indicates that P2-purinoceptors are present on osteoblast-like cells and that extracellular ATP from various sources might be an important factor in the regulation of osteoblast functions.

Mechanisms by which extracellular ATP and UTP stimulate the release of prostacyclin from bovine pulmonary artery endothelial cells

Extracellular ATP and UTP caused increases in the concentration of cytoplasmic free calcium ([Ca2+]i) and the intracellular level of inositol 1,4,5-trisphosphate (IP3), a second messenger for calcium mobilization, prior to the release of prostacyclin (PGI2) from cultured bovine pulmonary artery endothelial (BPAE) cells. The agonist specificity and dose-dependence were similar for nucleotide-mediated increases in IP3 levels, [Ca2+]i and PGI2 release. An increase in [Ca2+]; and PGI2 release was observed after addition of ionomycin, a calcium ionophore, to BPAE cells incubated in a calcium-free medium. The addition of ATP to the ionomycin-treated cells caused no further increase in [Ca2+]i or PGI2 release. The inability of ATP to cause an increase in [Ca2+]i or PGI2 release in ionomycin-treated cells was apparently due to the ionomycin-dependent depletion of intracellular calcium stores since the subsequent addition of extracellular calcium caused a significant increase in both [Ca2+]i and PGI2 release. Introduction of BAPTA, a calcium buffer, into BPAE cells inhibited ATP-mediated increases in [Ca2+]i and PGI2 release, further evidence that PGI2 release is dependent upon an increase in [Ca2+]i. The increase in [Ca2+]i elicited by ATP apparently caused the activation of a calmodulin-dependent phospholipase A2 since trifluoperazine, an inhibitor of calmodulin, and quinacrine, an inhibitor of phospholipase A2, prevented the stimulation of PGI2 release by ATP. Furthermore, ATP caused the specific hydrolysis of [14C]arachidonyl-labeled phosphatidylcholine and the generation of free arachidonic acid, the rate-limiting substrate for PGI2 synthesis, prior to the release of PGI2 from BPAE cells. These findings suggest that the increase in PGI2 release elicited by ATP and UTP is at least partially dependent upon a phospholipase C-mediated increase in [Ca2+]i and the subsequent activation of a phosphatidylcholine-specific phospholipase A2. ATP analogs modified in the adenine base or phosphate moiety caused PGI2 release with a rank order of agonist potency of adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) greater than 2-methylthioATP (2-MeSATP) greater than ATP, whereas alpha, beta methyleneATP and beta, gamma methyleneATP had no effect on PGI2 release.

Recent developments in the classification and functional significance of receptors for ATP and UTP, evidence for nucleotide receptors

The presence of a nucleotide receptor activated with similar potencies by UTP and ATP is suggested by recent data from a variety of different cell types. This receptor type appears distinct from previously described ATP-sensitive P2-purinoceptor subtypes and, probably, from other UTP-sensitive receptors, however further studies using selective antagonists are necessary to provide a definitive characterisation. Although the functional role of endogenous extracellular ATP has already achieved recognition there are also many diverse examples of cells and tissues which respond to UTP at micromolar or sub-micromolar concentrations. Therefore, the possible physiological importance of UTP is a fertile area for further investigation. The functional significance of ATP/UTP receptors is underlined by recent demonstrations that UTP and ATP modulate chloride ion secretion in human airways epithelium, possibly by activation of a nucleotide receptor, an effect which may have potential clinical utility in the treatment of cystic fibrosis.

Purinergic receptor activation of Cl- secretion in T84 cells

The regulation by ATP of Cl- secretion in T84 cells grown on filters was investigated by measuring short-circuit current (Isc = net Cl- secretion). ATP (greater than or equal to 10 microM) added to the basolateral side markedly stimulated Isc both in the presence and absence of forskolin-activated Isc. Fluorescence microscopy of cells loaded with the Ca2+ indicator fura-2 showed that ATP stimulated a transient increase in intracellular free Ca2+ concentration [Ca2+]i. The augmentation of forskolin-stimulated Isc by ATP was at least partly caused by mobilization of Ca2+ from an internal store because prior depletion of the store using ionomycin prevented the response. The activity sequence for stimulation of Isc in the presence of forskolin was adenosine 5'-O-(3-thiotriphosphate) = 5'-adenylylimidodiphosphate (AMP-PNP) greater than ATP greater than ADP greater than AMP, suggesting the presence of a P2 purinergic receptor. Neither beta, gamma-methyleneadenosine 5'-triphosphate nor alpha, beta-methyleneadenosine 5'-triphosphate increased the Isc. Stimulation of Isc by ATP in the absence of forskolin was at least partly due to the breakdown of ATP to AMP and adenosine, which act at P1 receptors to stimulate Isc, since 1) inhibition of the ecto-phosphohydrolase 5'-nucleotidase by alpha, beta-methylene-ADP partially inhibited stimulation of Isc by ATP, 2) the adenosine receptor antagonists caffeine and 8-phenyltheophylline markedly inhibited the ATP-stimulated Isc, and 3) AMP-PNP, a weakly hydrolyzable analogue of ATP, caused a much smaller increase in Isc compared with ATP. Adenosine had no effect on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Chick embryo muscarinic and purinergic receptors activate cytosolic Ca2+ via phosphatidylinositol metabolism

In dissociated cells from chick embryos or from chick limb buds, acetylcholine (ACh) induced an increase in cellular levels of inositol 1,4,5-trisphosphate (Ins-P3) and of inositol 1,3,4,5-tetrakisphosphate (Ins-P4). The concentration of Ins-P3 was enhanced transiently, whereas the level of Ins-P4 remained elevated for at least 20 min after addition of ACh. In most cases the increase in Ins-P4 levels was more pronounced than that of Ins-P3 levels. The inhibition of the ACh-induced inositol-phosphate response by atropine (half-maximal inhibition at 10 nM) indicates the involvement of muscarinic receptors, which in chick embryo cells induce a transient rise and a following persistent elevation of cytosolic Ca2+ activity (G. Oettling et al. (1989) J. Dev. Physiol. 12, 85-94). Adenosine 5'-triphosphate (ATP) elicited a similar transient rise in cytosolic Ca2+ activity, however, without a subsequent plateau. ATP also caused an increase in inositol-oligophosphate levels. Thus, both muscarinic and purinergic receptors in chick embryo cells are coupled to phospholipase C. The enzymatically formed Ins-P3 mediates the release of Ca2+ from internal stores. The Ca2+ signal could be involved in embryonic cell migration during morphogenesis.

Calcium-permeable channels in rat hepatoma cells are activated by extracellular nucleotides

Extracellular ATP is known to cause uptake of Ca2+ by rat liver cells. The specific pathway permitting influx of Ca2+ has not yet been identified. In the present investigations, we studied the properties of ATP-evoked 45Ca2+ uptake in rat hepatoma cell monolayers and then used patch-clamp electrophysiology to identify the channel that may account for this uptake. The results suggest that ATP-stimulated 45Ca2+ uptake occurs as a result of P2-purinergic receptor interaction because uptake was inhibited by Reactive Blue (100 microM), a blocker of this type of receptor. Furthermore, the ability of other adenine nucleotides to stimulate 45Ca2+ uptake was related to the selectivity sequence for binding to the P2-purinergic receptor. ATP-stimulated 45Ca2+ uptake occurs primarily through a conductance pore since it was inhibited by 70% upon dissipation of the membrane potential using the K+ ionophore valinomycin. The calcium channel blockers nifedipine and verapamil failed to inhibit 45Ca2+ uptake, but gadolinium (GdCl3) was an effective blocker. In cell-attached patch-clamp experiments, a single type of channel was activated with ATP (100 microM) addition to the bath in 18 of 32 trials. The current-voltage relationship of the ATP-activated channel is identical to that of the stretch-activated channel previously characterized in this laboratory as a calcium-permeable cation-nonselective channel [Am. J. Physiol. 258 (Cell Physiol. 27): C421-C428, 1990]. There are several lines of evidence which suggest that this cation-nonselective channel may account for ATP-stimulated 45Ca2+ uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of multiple phosphoinositide-linked receptors on human SK-N-MC neuroepithelioma cells

The biochemical and pharmacological characteristics of receptor-stimulated phosphoinositide (PPI) hydrolysis in human SK-N-MC neuroepithelioma cells have been examined. Of 11 ligands tested, the addition of four, i.e., norepinephrine, oxotremorine-M, endothelin-1, and ATP, each resulted in an increased release (three- to eightfold) of inositol phosphates from [3H]inositol-prelabeled cells. Agonist-stimulated PPI turnover was sustained for at least 30 min and required the addition of Ca2+ for full effect. An increased release of inositol phosphates could also be elicited by the addition of the Ca2+ ionophore, ionomycin. All four agonists enhanced the release of radiolabeled inositol mono- and bisphosphates, inositol 1,3,4-trisphosphate, and inositol tetrakisphosphate. Increases in inositol 1,4,5-trisphosphate were smaller and only consistently observed in the presence of norepinephrine or oxotremorine-M. Norepinephrine-stimulated PPI turnover was potently inhibited by prazosin, WB-4101, and 5-methylurapidil (Ki less than 2.5 nM), but was relatively insensitive to chlorethylclonidine pretreatment. This pharmacological profile is consistent with the involvement of an alpha 1A-receptor subtype. The presence of an M1 muscarinic cholinergic receptor is also indicated, because pirenzepine blocked oxotremorine-M-stimulated inositol phosphate release (Ki = 35 nM) with a 30-fold greater potency than the M2-selective antagonist, AF-DX 116. Of the three endothelins tested, only the addition of endothelin-1 and endothelin-2 promoted PPI hydrolysis, whereas endothelin-3 was essentially inactive. A P2 nucleotide receptor of broad agonist specificity is also present on these cells and activates PPI turnover in the absence of a generalized increase in plasma membrane permeability. These results indicate that SK-N-MC cells express at least four PPI-linked receptors. Because the functional coupling of three of these receptors, i.e., alpha 1A-adrenergic, endothelin, and P2 nucleotide, has not been extensively characterized previously in neural tissues, the SK-N-MC cell line may provide a useful model system for studies of these receptors and their regulation.

Adenosine triphosphate (ATP) and other nucleotides stimulate the hydrolysis of phosphatidylethanolamine in intact fibroblasts

Addition of adenosine triphosphate (ATP) to [14C]ethanolamine-prelabeled NIH 3T3 fibroblasts resulted in rapid formation of [14C]ethanolamine from the prelabeled cellular phosphatidylethanolamine (PtdEtn) pool. After 2-min exposure, 10 microM ATP had near maximal effects on PtdEtn hydrolysis. Several other nucleotides, including UTP, ITP, and the stable ATP analog adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), also had stimulatory effects on PtdEtn hydrolysis with a potency comparable to that observed with ATP. The same nucleotides which acted on PtdEtn hydrolysis also had similar stimulatory effects on the hydrolysis of phosphatidylcholine (PtdCho) in [14C]choline-labeled cells. In isolated membranes, Mg2+ greatly enhanced the stimulatory effects of ATP and ATP gamma S, but not of other nucleotides, on the hydrolysis of PtdEtn and PtdCho. Results indicate that in isolated membranes, both ATP and ATP gamma S stimulate phospholipid hydrolysis by two different mechanisms, but in intact cells only one of these mechanisms appears to be responsive to externally added nucleotides.

Transmembrane signalling by the N-formyl peptide receptor in stably transfected fibroblasts

We investigated the requirement for N-formyl peptide receptor-mediated transmembrane signalling in transfected mouse fibroblasts that express the receptor. Stably transfected cells displayed specific binding for N-formyl-Met-Leu-[3H]Phe with a dissociation constant of 3 nM. The cells responded to ligand stimulation with mobilization of calcium from intracellular stores. Calcium mobilization was ligand dose-dependent (EC50 = 3 nM fMet-Leu-Phe) and could be inhibited by pertussis toxin treatment. These results provide the first demonstration that expression of the single-chain N-formyl peptide receptor in mouse fibroblasts is sufficient for mediating ligand-induced early transmembrane signalling events, which do not appear to require other neutrophil-specific cellular components.

Regulation of transepithelial ion transport and intracellular calcium by extracellular ATP in human normal and cystic fibrosis airway epithelium

1 The role of extracellular nucleotides in regulation of ion transport activities (short circuit current, Isc) of human respiratory epithelia was studied. 2 Application of nucleotides to the apical or basolateral membrane of human nasal epithelium induced a concentration-dependent increase in Isc. 3 The rank order of potency of purine- or pyrimidine-induced changes in Isc of normal human nasal epithelium when applied to the apical membrane (UTP greater than or equal to ATP greater than ATP gamma S greater than 2MeSATP greater than ADP beta S much greater than beta gamma MeATP greater than or equal to alpha beta MeATP) or basolateral membrane (2MeSATP greater than UTP greater than ATP greater than ATP gamma S greater than alpha beta MeATP greater than beta gamma MeATP) is consistent with involvement of a P2 purinoceptor. A similar rank order of potencies was observed for nucleotide effects on intracellular calcium measured by Fura-2 fluorescence using microspectrofluorimetry. 4 Similar nucleotide potency in the regulation of ion transport and intracellular calcium in cystic fibrosis (CF) airway epithelium (UTP greater than or equal to ATP) was observed, suggesting purinoceptors might be used to stimulate ion transport processes that would promote hydration of airway secretions and facilitate their clearance from CF lungs. 5 These data provide evidence for the regulation of ion transport by P2 purinoceptors in normal and cystic fibrosis human airway epithelium.

Further subclassification of ATP receptors based on agonist studies

Recent studies using agonist analogues of ATP and other nucleotides have generated some surprising observations which may have ramifications for the classification of P2 receptors, particularly for those responses currently attributed to P2Y receptor activation. 2-MethylthioATP (2-MeSATP), the conventional P2Y receptor agonist, does not interact with ATP in the expected fashion in various models of endothelial function, suggesting that it acts by a different mechanism. Furthermore, in certain cell types where responses to ATP are mediated by phospholipase C activation, 2-MeSATP has little or no activity. Interestingly, the pyrimidine uridine triphosphate (UTP) invariably shows similar potency to ATP in systems where 2-MeSATP is inactive. In this article Steve O'Connor and colleagues discuss these data and their significance, and propose that separate receptors may be responsible: one sensitive to 2-MeSATP and the other, a 'nucleotide' receptor, sensitive to UTP.

Signal transduction by P2-purinergic receptors for extracellular ATP

Extracellular adenosine triphosphate (ATP), at micromolar/nanomolar concentrations, has been shown to induce significant functional changes in a wide variety of normal and transformed cell types. While ATP can be nonspecifically released from the cytosol of damaged cells, it is also co-packaged in certain exocytotic vesicles/granules containing conventional neurotransmitters and hormones. The diverse biologic responses to ATP appear to be mediated by a variety of so-called P2-purinergic, cell surface receptors that are activated upon binding ATP and other nucleotides. Recent physiologic, biochemical, and pharmacologic studies suggest that there are multiple ATP receptor subtypes. These include: (1) G-protein-coupled ATP receptors, which stimulate inositol phospholipid hydrolysis, Ca2+ mobilization, and activation of protein kinase C; (2) ATP receptors that directly activate nonselective cation channels in the plasma membranes of a variety of excitable cell types; and (3) ATP receptors that, via the rapid induction of surface membranes pores permeable to both ions and endogenous metabolites, can produce cytotoxic or activation responses in T lymphocytes and other immune effector cells. In addition to these functional criteria, these putative ATP receptor subtypes can be distinguished by characteristic selectivities for a variety of structurally modified ATP analogs. Current research is directed towards the identification, isolation, and structural characterization of these receptors by both biochemical and molecular biologic approaches.

Cytosolic calcium responses induced by photolytic release of 1,4,5-inositol trisphosphate in single human fibroblasts

We have used the whole cell technique to microinject human fibroblasts with either 1,4,5-inositol trisphosphate (InsP3) or 'caged' InsP3, in order to study the mechanisms of transmembrane signalling related to mitogenic stimulations. Cytosolic Ca2+ elevations in response to 1,4,5 InsP3 diffusing from the patch pipette were difficult to detect, while 1,4,5 InsP3, photoreleased after loading the cell with its inactive precursor, was capable of generating not only a single cytosolic Ca2+ rise but sometimes triggered an oscillatory calcium response, similar to that often observed under mitogenic stimulation. We estimated that less than 100 nM InsP3 was sufficient to generate Ca2+ responses. The Ca2+ rise produced by the photoreleased InsP3 could fully activate the K+ channels present in the plasma membrane of human fibroblasts.

ATP-induced Ca2+ release and Cl- current in cultured smooth muscle cells from pig aorta

1. The effect of exogenous ATP on transmembrane currents and on the cytoplasmic Ca2+ has been investigated in single cultured smooth muscle cells of pig aorta. 2. ATP applied to cells held at a potential of -50 mV evoked a transient inward current and a transient rise in [Ca2+]i. At a potential of +20 mV the ATP-induced increase in [Ca2+]i was accompanied by an outward current. 3. At a potential of -50 mV, ATP evoked in Ca(2+)-free solution an inward current which was similar to that in the presence of external Ca2+. A second application of ATP in Ca(2+)-free solution induced a much smaller current. 4. ATP induced in Ca(2+)-free solution a pronounced transient stimulation of the 45Ca2+ efflux from confluent smooth muscle monolayers. 5. The I-V curve of the ATP-activated current has a reversal potential close to 0 mV. A reduction of external Cl- shifts this reversal potential in accordance with the change of the Cl- equilibrium potential. 6. It is concluded that ATP causes a release of calcium from intracellular stores. The ensuing increase of [Ca2+]i activates a Cl- current, which can depolarize the cell membrane and thereby promote a voltage-gated Ca2+ entry.

Dual effects of ATP on phosphatidylinositol breakdown in rat hepatocyte membranes

The mechanisms whereby adenosine-5'-triphosphate (ATP) regulates the inositol phospholipid-signalling system were studied in rat hepatocytes. Intact hepatocytes respond to extracellular ATP, adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S), ADP and weakly to guanosine-5'-triphosphate (GTP), but not to other purine nucleotides (GDP or AMP). This is consistent with the idea that a P2 purinergic receptor is coupled to the phosphatidylinositol metabolism in these cells. Partially purified plasma membranes prepared from myo-[3H]inositol prelabelled hepatocytes exhibit a phosphatidylinositol-4,5-bisphosphate phospholipase C activity sensitive to ATP, ATP gamma S and guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S). Moreover the GTP gamma S effect is greatly enhanced by ATP and ATP gamma S. These potentiating effects differ according to the adenylnucleotide considered. ATP produces (1) an increase in the GTP gamma S-PLC sensitivity, (2) a potentiation of the phospholipase C (PLC) response induced by maximal dose of GTP gamma S, and (3) an increase in the inositol lipids pools. At variance, ATP gamma S, a nonhydrolysable analogue of ATP, only increases the PLC-sensitivity towards GTP gamma S. These results may signify that ATP stimulates inositol phosphate accumulation via at least two distinct mechanisms (i) a direct activation of a P2 purinergic receptor coupled to a PLC via a GTP binding protein and (ii) a stimulation of the phosphatidylinositol (PI) and phosphatidylinositol-4-phosphate (PIP) kinases which increased the pool of phospholipase C substrates.

P2-purinergic receptors activate a guanine nucleotide-dependent phospholipase C in membranes from HL-60 cells

We have previously determined that human neutrophils and monocytes, as well as neutrophil/monocyte progenitor cells, express a subtype of P2-purinergic receptors (for ATP) which activate the inositol phospholipid signalling system. In the present study, membranes prepared from HL-60 promyelocytic leukemia cells were used to examine the mechanism by which these ATP receptors activate phosphatidylinositol-specific phospholipase C (PI-PLC) under defined in vitro conditions. Micromolar concentrations of the receptor agonists ATP, UTP, and ATP gamma S stimulated the GTP-dependent formation of inositol bisphosphate (IP2) and inositol trisphosphate (IP3) in washed membranes prepared from undifferentiated HL-60 cells prelabeled with [3H]inositol. The stimulatory effects of these nucleotides on PI-PLC appeared to be mediated through a GTP binding protein since minimal inositol polyphosphate accumulation was observed in the absence of guanine nucleotides. The increased inositol polyphosphate formation triggered by these nucleotide receptor agonists did not result from inhibition of GTP breakdown. Neither was it a consequence of increased [3H]polyphosphatidylinositol levels resulting from enhanced activity of membrane-associated PI- or PIP-kinases. Instead, the stimulated phospholipase activity was apparently receptor-mediated. The rank order of potency observed in these in vitro membrane assays (ATP = UTP greater than ATP gamma S much greater than TTP greater than CTP much greater than beta, gamma-CH-ATP) was similar to that observed with intact HL-60 cells. This order of potency appears to distinguish the P2-purinergic receptors expressed by human phagocytic leukocytes from the P2 gamma-purinergic receptors which activate PI-PLC in turkey erythrocyte membranes.

Undifferentiated HL60 cells respond to extracellular ATP and UTP by stimulating phospholipase C activation and exocytosis

We have recently characterised the presence of a Ca2(+)-mobilising receptor for ATP which stimulates exocytosis in differentiated HL60 cells. Here we demonstrate that the undifferentiated HL60 cells also respond to extracellular ATP by stimulating an increase in inositol phosphates and exocytosis. Of the nucleotides (ATP, UTP, ITP, ATP gamma S, AppNHp, XTP, CTP, GTP, 8-Br-ATP and GTP gamma S) that were active in stimulating inositol phosphate formation, only UTP, ATP, ITP, ATP gamma S and AppNHp were active in stimulating secretion. On differentiation, the extent of secretion due to the purinergic agonists ATP, ITP, ATP gamma S and AppNHp remained unchanged whilst secretion due to UTP, a pyrimidine, was substantially increased. These results indicate that the effect of ATP and UTP may be mediated via separate purinergic and pyrimidinergic receptors, respectively.