Control of endothelial function by nucleotides: multiple receptors and transduction mechanisms

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Vasodilator responses to ATP and UTP are not dependent on nitric oxide release, K+ATP channel activation, or the release of vasodilator prostaglandins in the hindlimb vascular bed of the cat

The effects of the purinergic agonists, ATP, ATPγS, UTP, and 2-Met-Thio AP, were investigated in the hindlimb vascular bed of the cat. Under constant-flow conditions, injections of the purinergic agonists into the perfusion circuit elicited dose-related decreases in perfusion pressure. The order of potency was 2-Met-Thio ATP > ATPγS > ATP > UTP. In contrast, injections of GTPγS, cAMP, UDP, and UMP had no effect. Vasodilator responses to ATP, ATPγS, UTP, and 2-Met-Thio ATP were increased in duration by the cAMP phosphodiesterase inhibitor rolipram, whereas the cGMP phosphodiesterase inhibitor zaprinast had no effect. Responses to the purinergic agonists were not altered by nitric oxide synthase inhibitors, K+ATP channel antagonists, cyclooxygenase inhibitors, or agents that interfere with the actions of the adrenergic nervous system. These data suggest that ATP, ATPγS, UTP, and 2-Met-Thio ATP dilate the hindlimb vascular bed by a direct cAMP-dependent mechanism, and that the release of nitric oxide, vasodilator prostaglandins, K+ATP channel opening, or an inhibitory effect on the adrenergic nervous system play little, if any, role in mediating or modulating responses to the purinergic agonists in the hindlimb circulation of the cat.Key words: purinergic agonists, P2 purinergic receptors, cAMP-dependent vasodilator activity, adrenergic system, nitric oxide prostaglandins.

Nucleotide-evoked relaxation of human coronary artery

Endothelium-dependent dilation of coronary blood vessels in response to ATP and related nucleotides has been demonstrated in various animal species. The aim of the present study was to investigate a possible relaxant effect of ATP, the adenine nucleotides 2-methylthio ATP (MeSATP) and adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), and the pyrimidine nucleotide UTP in isolated human coronary artery. In endothelium-intact rings of human coronary artery precontracted with K+ (20-40 mM), the nucleotides caused relaxation. Average maximal percentage relaxations and average EC50 values (concentrations causing half-maximal relaxation) were 89% and 47.1 microM for ATP, 28% and 0.3 microM for MeSATP, 35% and 0.6 microM for ADPbetaS, and 49% and 1.6 microM for UTP. For each of the four agonists, the potency to elicit relaxation varied greatly between individual rings, so that equi-relaxing concentrations spanned several orders of magnitude. Moreover, the sensitivities to ATP and UTP, when tested in the same ring, were not correlated. Mechanical removal of the endothelium as well as NG-nitro-L-arginine methyl ester (L-NAME; 30 microM), an inhibitor of nitric oxide synthase, abolished the relaxation caused by MeSATP, ADPbetaS and UTP and greatly attenuated the response to lower concentrations of ATP (3.2-320 microM), but high concentrations of ATP (320 and 1000 microM) caused relaxation also in endothelium-denuded preparations and in the presence of L-NAME. High concentrations of ADPbetaS (32 and 100 microM) and UTP (320 and 1000 microM) caused contraction of endothelium-denuded preparations. Thus, extracellular nucleotides cause endothelium-dependent, primarily nitric oxide-mediated relaxation of human coronary artery. ATP in addition causes endothelium-independent relaxation. The receptors activated by the nucleotides appear to be unevenly distributed on the coronary endothelium.

Differences in nucleotide effects on intracellular pH, Na+/H+ antiport activity, and ATP-binding proteins in endothelial cells

Bovine (BPAEC) and human (HPAEC) pulmonary artery endothelial cell monolayers were incubated with either ATP, ATP analogues, or UTP, followed by measurement of intracellular pH (pHi) and the rate of recovery from acidosis. ATP increased baseline pHi and the rate of acid recovery in BPAEC. This response was inhibited by the amiloride analogue, methyisobutylamiloride, demonstrating that activation of the Na+/H+ antiport was responsible for the increase in baseline pHi and the recovery from acidosis. This response had the features of both a P2Y and P2U purinergic receptor, based on the responses to a series of ATP analogues and UTP. In contrast, none of the nucleotides had any significant effect on pHi and Na+/H+ antiport activity in HPAEC. This difference in the response to extracellular nucleotides was not due to a difference in ATP metabolism between cell types, since the ectonucleotidase-resistant analogue. ATP gamma S, also had no effect on HPAEC. Analogues of cAMP had no effect on pHi or acid recovery in either cell type. Incubation of BPAEC and HPAEC with the photoaffinity ligand [32P] 8-AzATP indicated that both BPAEC and HPAEC possess an ATP-binding protein of 48 kDa. However, BPAEC exhibited an additional binding protein of 87 kDa. Thus, the contrasting response to extracellular ATP between bovine and human pulmonary artery endothelial cells may be related to differences in the signal transduction pathway leading to antiport activation, including different ATP-binding sites on the cell membrane.

Evidence of ectokinase-mediated phosphorylation of osteopontin and bone sialoprotein by osteoblasts during bone formation in vitro

Osteopontin (OPN) and bone sialoprotein (BSP) are phosphorylated glycoproteins that, together with osteonectin/secreted protein, acidic, rich in cysteine (SPARC) and osteocalcin, comprise the major non-collagen proteins of bone. Although phosphorylation of OPN and BSP, which is known to influence the biological properties of these proteins, has been shown to occur intracellularly, recent studies have demonstrated ectokinase activity in bone cell populations [Mikuni-Takagaki, Kakai, Satoyoshi, Kawano, Suzuki, Kawase and Saito (1995) J. Bone Miner. Res. 10, 231-241]. To determine whether OPN and BSP are phosphorylated by ectokinase activity we have used [gamma-32P]ATP and [gamma-32P]GTP as cell-impenetrable phosphate donors to analyse for ectokinase activity in osteoblastic UMR106.06 cells and fetal rat calvarial cells (FRCCs). By pulse-labelling confluent cells with radiolabelled nucleotides, the phosphorylation of endogenous and exogenously added OPN and BSP was demonstrated together with the labelling of a number of cell surface proteins. These phosphorylation reactions were inhibited by a cell-impermeable ectokinase inhibitor, K252b, and cell surface phosphorylation was also inhibited by exogenously added OPN and BSP substrates, indicating competition for the ectokinase enzyme. However, phosphorylation of OPN and BSP, both of which can mediate cell attachment through Arg-Gly-Asp (RGD) motifs, was not inhibited by an RGD peptide, suggesting that binding of OPN and BSP to cell surface integrins is not required. In similar experiments, ectokinase-mediated phosphorylation of OPN and BSP was demonstrated during mineralized tissue formation by FRCCs in vitro. These studies demonstrate that OPN and BSP secreted by bone cells are phosphorylated by a casein kinase II-like ectokinase present on the surface of osteoblastic cells.

P2U agonists induce chemotaxis and actin polymerization in human neutrophils and differentiated HL60 cells

Human neutrophils or HL60 cells express P2U receptors and respond to micromolar concentrations of ATP, adenosine 5'-O-(thiotriphosphate) (ATPgammaS), or UTP with immediate increases in intracellular Ca2+ through activation of phosphoinositide phospholipase C (Cowen, D. S., Lazarus, H. M., Shurin, S. B., Stoll, S. E., and Dubyak, G. R. (1989) J. Clin. Invest. 83, 1651-1660). P2U agonists reportedly induce limited enzyme secretion and enhance the respiratory burst in response to chemotactic factors. We demonstrate here that P2U agonists are chemotactic for neutrophils or differentiated HL60 cells. Rhodamine phalloidin staining indicates that ATPgammaS treatment induces actin polymerization and shape changes similar to those seen when these cells are treated with chemotactic peptide fMet-Leu-Phe. Although undifferentiated HL60 cells fail to mount a rise in Ca2+ when challenged with fMet-Leu-Phe, they increase Ca2+ in response to P2U agonists. However, functional expression of phospholipase C-coupled receptors is not sufficient for chemotaxis since HL60 cell migration in response to these agonists or to fMet-Leu-Phe occurs only after exposure to differentiating agents such as BT2cAMP. In addition to the well known G protein-linked receptors for lipid or peptide chemotactic factors, neutrophils apparently also can utilize G protein-linked purino/pyrimidino receptors to recognize nucleotides as chemoattractants. High concentrations of ATP and UTP generated at sites of platelet aggregation and tissue injury could thus be important mediators of inflammation.

Mechanisms of intracellular calcium regulation in adult astrocytes

Microfluorimetric techniques were used to measure changes in intracellular calcium in astrocytes cultured from the forebrain of the adult rat. Application of ATP consistently raised intracellular calcium. The response persisted in the absence of extracellular calcium, but then quickly declined upon repeated agonist application. Thapsigargin abolished responses to nucleotides following depletion of the endoplasmic reticular calcium stores. Calcium release was inhibited by caffeine, but was dramatically increased through inositol phosphate receptor sensitization by the sulphydryl reagent thimerosal. Responses to repeated nucleotide applications resulted in a gradual decline of peak calcium concentrations, suggesting a (post)receptor-mediated desensitization or gradual depletion of the internal calcium stores. Subsequent application of ionomycin suggested intracellular calcium depletion as the relevant mechanism. Depletion of the internal calcium stores with ATP, ionomycin or thapsigargin failed to reveal a calcium influx pathway. These results suggest that the capacitative mechanism of calcium entry does not operate in response to nucleotide receptor activation in these cells, and that the immediate refilling of the internal calcium stores is primarily determined by re-uptake of cytosolic calcium into the endoplasmic reticulum. A complete refilling of this calcium store by extracellular calcium may be a much slower process. Control of these signal transduction pathways is crucial to the maintenance of the calcium/energy homeostasis of the adult astrocyte in the central nervous system.

Osteoclast ATP receptor activation leads to a transient decrease in intracellular pH

Application of extracellular adenosine triphosphate (ATP) induces a pulsed decrease in osteoclast intracellular pH (pHi), as measured with seminaphthofluorescein (SNAFL)-calcein on a laser scanning confocal microscope. Adenosine diphosphate also produces a pHi decrease, but adenosine monophosphate, uridine triphosphate, 2-methylthio-ATP, and beta, gamma-methylene-ATP have little effect on pHi. The ATP-induced pHi decrease is largely inhibited by suramin, a P2 purinergic receptor blocker. Clamping intracellular free [Ca2+] ([Ca2+]i) with BAPTA/AM does not affect the ATP-induced pHi change, showing that this pHi decrease is not caused by the increased intracellular [Ca2+]i that is produced by activation of osteoclast purinergic receptors. We show that an increase in [Ca2+]i by itself will produce a pHi increase. The ATP effect is not blocked by inhibition of Na+/H+ exchange by either Na(+)-free bathing medium or amiloride. Two inhibitors of the osteoclast cell membrane proton pump, N-ethylmaleimide and vanadate, produce partial inhibition of the ATP-induced pHi decrease. Two other proton pump inhibitors, bafilomycin and N,N'-dicyclohexylcarbodiimide, have no influence on the ATP effect. None of the proton pump inhibitors but vanadate has a direct effect on pHi. Vanadate produces a transient pHi increase upon application to the bathing medium, possibly as a result of its known effect of stimulating the Na+/H+ exchanger. Inhibition of Cl-/HCO3- exchange by decreasing extracellular Cl- gives a pronounced long-term pHi increase, supporting the hypothesis that this exchange has an important role in osteoclast pHi homeostasis. In Cl(-)-free extracellular medium, there is a greatly reduced effect of extracellular ATP on pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

Review: Ca(2+)-mobilizing receptors for ATP and UTP

Extracellular nucleotides are potent Ca2+ mobilizing agents. A variety of receptors for extracellular ATP are recognised. Some are involved in fast neuronal transmission and operate as ligand-gated ion channels. Others are involved in the paracrine or autocrine modulation of cell function. Many receptors of this type are coupled to phosphoinositide-specific phospholipase C and, in some cases, other phospholipases. One of these receptors (P2z), however, also appears to operate, at least in part, as a ligand-gated ion channel. Pharmacological data suggest that one nucleotide receptor subtype (currently designated P2U) responds selectively to either a purine nucleotide, ATP, or a pyrimidine nucleotide, UTP. According to an alternative view, ATP and UTP recognise distinct receptors. Because of the diversity of receptors for extracellular nucleotides this may be the case in some cells. Nevertheless, a G-protein coupled receptor that confers both ATP and UTP sensitivity has been cloned, expressed in cultured cell lines and sequenced. This receptor appears to have two ligand binding domains that may partially overlap. The nature of this overlap is discussed and a simple model presented. Activation of the receptor protein via one or other ligand binding domain may underlie some of the more subtle differences between the effects of ATP and UTP.

Mechanisms of ATP-induced Ca2+ signaling in osteoclasts

We investigate the mechanisms underlying the intracellular calcium pulse that occurs in response to extracellular adenosine triphosphate (ATP) in osteoclasts. We find that pre-loading of GDP-beta-S abolishes the response in Ca(2+)-free medium, demonstrating an internal release of Ca2+ via a pathway that involves a G protein. GDP-beta-S does not block in normal Ca(2+)-containing medium, suggesting that ATP also induces a Ca2+ influx across the cell membrane. We confirmed this using the Mn2+ quenching technique, which shows significant opening of Ca2+ channels. We find a smaller response to adenosine diphosphate (ADP) and 2-methylthio-ATP (2-MeSATP), but no response to beta, gamma-methylene-ATP (AMP-PCP), adenosine monophosphate (AMP) or uridine triphosphate (UTP). Prior application of AMP and UTP, but not AMP-PCP, blocks the response to ATP. Our results indicate that the receptor is a P2 subtype that is not characteristic of any previously reported P2 receptor or combination of P2 receptors.

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

1. Brain capillary endothelial cells responded to 2-methylthio-ATP (2MeSATP) by large increases in [Ca2+]i (EC50 = 27 nM) that were partially dependent on the presence of extracellular Ca2+ and that were not associated with a measurable production of inositol phosphates. 2. 2-chloro-ATP (2ClATP) raised [Ca2+]i in a biphasic manner. At low concentrations, intracellular Ca2+ mobilization was not associated with a measurable production of inositol phosphates. At concentrations > 30 microM, 2ClATP activated phospholipase C. 3. The actions of 2ClATP, 2MeSATP and ADP on [Ca2+]i were additive to those of ATP and UTP. Non-additive actions of 2MeSATP and of low concentrations of ADP or of 2ClATP were observed. 4. Cross desensitizations of the actions of ADP, 2MeSATP and 2ClATP were observed. None of them desensitized cells to the action of ATP. 5. It is concluded that 2MeSATP and low concentrations of 2ClATP and ADP induce intracellular Ca2+ mobilization by acting via an atypical P2y purinoceptor that is not coupled to phospholipase C. At high concentrations, 2ClATP also activates phospholipase C and further increases [Ca2+]i probably by acting on P2u purinoceptors.