Distribution and Roles of Purinoceptor Subtypes

Myosin light chain kinase and Rho-kinase participate in P2Y receptor-mediated acceleration of permeability through the endothelial cell layer

We have shown that P2Y receptor stimulation accelerates macromolecular permeation through the endothelial cell layer. To elucidate the mechanism of this acceleration, we examined the effects of ML-9, a myosin light chain kinase inhibitor, and Y-27632, a Rho-kinase inhibitor, on fluorescein isothiocyanate dextran (FD-4) permeation across the human umbilical vein endothelial cell monolayer. FD-4 permeation was analysed by high-performance liquid chromatography fluorescence detection. A P2Y receptor agonist, 2meS-ATP, enhanced the permeability of FD-4, which was inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a P2Y-receptor antagonist. The 2meS-ATP-induced increase in the permeability of FD-4 was significantly inhibited by ML-9. Also, Y-27632 prevented the 2meS-ATP-induced increase in the permeability of FD-4. Neither ML-9 nor Y-27632 influenced the spontaneous permeation of FD-4. These results suggest that phosphorylation of the myosin light chain may play an important role in the purinergic regulation of macromolecular permeation through the vascular endothelium.

P2Y receptor-mediated Ca(2+) signaling increases human vascular endothelial cell permeability

We investigated the effects of P2-receptor agonists on cell size, intracellular calcium levels ([Ca(2+)](i)), and permeation of FITC-labeled dextran (FD-4) as well as the relationship between these effects in human umbilical vein endothelial cells (HUVEC). FD-4 concentration, cell size, and [Ca(2+)](i) were analyzed by HPLC with fluorescence, phase contrast microscopic imaging, and fluorescent confocal microscopic imaging, respectively. The P2Y(1)-receptor agonists 2-methylthio ATP (2meS-ATP) and ADP decreased cell size and increased [Ca(2+)](i) in HUVEC. The P2Y(2)-receptor agonist UTP increased [Ca(2+)](i), but did not influence cell size. The P2X-receptor agonist alpha,beta-methylene ATP did not induce either response. The decrease in size and increase in [Ca(2+)](i) by 2meS-ATP were blocked by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, P2Y(1)-antagonist), thapsigargin (Ca(2+)-pump inhibitor), and U73122 (phospholipase C inhibitor). Furthermore, 2meS-ATP (P2Y(1)-receptor agonist) enhanced permeation of FD-4 through the endothelial cell monolayer. The 2meS-ATP-induced enhancement of the permeation was also prevented by PPADS, thapsigargin, and U73122. These results indicate that activation of P2Y receptors induces a decrease in cell size, an increase in [Ca(2+)](i), and may participate in facilitating macromolecular permeability in HUVEC.

P2Y receptor-mediated enhancement of permeation requires Ca2+ signalling in vascular endothelial cells

1. We investigated the effects of 2-methylthioATP (2meS-ATP; a P2Y receptor agonist) on the permeation of fluorescein isothiocyanate (FITC)-labelled dextran, transendothelial electrical resistance (TEER) and intracellular calcium levels ([Ca2+]i) in cultured endothelial cells isolated from the rat caudal artery. 2. The cellular transport of FITC-labelled dextran was enhanced and TEER of the endothelial monolayer was reduced by 2meS-ATP. Both these effects were prevented by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, a P2Y receptor antagonist, which also inhibited the increase in [Ca2+]i induced by 2meS-ATP in endothelial cells. 3. The increase in [Ca2+]i induced by 2meS-ATP was inhibited by thapsigargin (a Ca2+ pump inhibitor) and by U-73122 (a phospholipase C inhibitor). 4. These findings suggested that activation of the P2Y receptor enhances the passage of material in the endothelium, which is associated with Ca2+ signalling in endothelial cells.

P2Y-receptor regulates size of endothelial cells in an intracellular Ca2+ dependent manner

The effects of P2 receptor agonists on cell size and intracellular calcium levels, [Ca(2+)](i), was investigated using cultured endothelial cells isolated from the caudal artery of male Wistar rats. Cell size and [Ca(2+)](i) were measured using a phase-contrast and fluorescent confocal microscopic image analyzer and a Calcium Green fluorescence probe. P2Y receptor agonists, 2-methylthio ATP (2meS-ATP), ADP, UTP and ATP decreased the cell size and increased [Ca(2+)](i) in endothelial cells from rat caudal artery. However, alpha,beta-methylene ATP, a P2X receptor agonist, did not induce these responses. The decrease in size and the increase in [Ca(2+)](i), by 2meS-ATP were blocked by PPADS (P2-antagonist), suramin (P2-antagonist), thapsigargin (Ca(2+) pump inhibitor) and U-73122 (phospholipase C inhibitor). The present results show that activation of P2Y receptors, not P2X receptors, induces a decrease in cell size and an increase in [Ca(2+)](i), and the pharmacological properties of these two responses are the same. We concluded that the size of endothelial cells is regulated by P2Y receptors via intracelluar Ca(2+) derived from Ca(2+) stores.

Participation of ATP in cell volume regulation in the endothelium after hypotonic stress

1. The role of ATP in the regulatory volume decrease (RVD) after hypotonic cell swelling was examined in cultured endothelial cells isolated from the rat caudal artery. 2. Hypotonic stress increased [Ca2+]i in addition to increasing the overflow of ATP and cell volume. The hypotonicity induced increase in [Ca2+]i was prevented by pyridoxalphosphate-6-azophenyl-1-2',4'-disulphonic acid (PPADS; a P2 purinoceptor antagonist), U-73122 (a phospholipase C inhibitor) and thapsigargin (a Ca2+ pump inhibitor). However, the hypotonicity induced increase in cell volume was potentiated by PPADS, U-73122 and thapsigargin. 3. Similar changes were observed in cells treated with 2-methylthioATP, a P2Y purinoceptor agonist, but not by alpha,beta-methylene ATP, a P2X purinoceptor agonist. Thus, it appears that the responses observed following hypotonic stress are mediated by activation of P2Y purinoceptors. 4. On the basis of these findings, it is suggested that ATP, which is released by hypotonicity, may participate in the RVD as a substantial regulator or initiator via P2 purinoceptor-induced increases in [Ca2+]i.

Involvement of astrocytes in purine‐mediated reparative processes in the brain

Astrocytes are involved in multiple brain functions in physiological conditions, participating in neuronal development, synaptic activity and homeostatic control of the extracellular environment. They also actively participate in the processes triggered by brain injuries, aimed at limiting and repairing brain damages. Purines may play a significant role in the pathophysiology of numerous acute and chronic disorders of the central nervous system (CNS). Astrocytes are the main source of cerebral purines. They release either adenine-based purines, e.g. adenosine and adenosine triphosphate, or guanine-based purines, e.g. guanosine and guanosine triphosphate, in physiological conditions and release even more of these purines in pathological conditions. Astrocytes express several receptor subtypes of P1 and P2 types for adenine-based purines. Receptors for guanine-based purines are being characterised. Specific ecto-enzymes such as nucleotidases, adenosine deaminase and, likely, purine nucleoside phosphorylase, metabolise both adenine- and guanine-based purines after release from astrocytes. This regulates the effects of nucleotides and nucleosides by reducing their interaction with specific membrane binding sites. Adenine-based nucleotides stimulate astrocyte proliferation by a P2-mediated increase in intracellular [Ca2+] and isoprenylated proteins. Adenosine also, via A2 receptors, may stimulate astrocyte proliferation, but mostly, via A1 and/or A3 receptors, inhibits astrocyte proliferation, thus controlling the excessive reactive astrogliosis triggered by P2 receptors. The activation of A1 receptors also stimulates astrocytes to produce trophic factors, such as nerve growth factor, S100beta protein and transforming growth factor beta, which contribute to protect neurons against injuries. Guanosine stimulates the output of adenine-based purines from astrocytes and in addition it directly triggers these cells to proliferate and to produce large amount of neuroprotective factors. These data indicate that adenine- and guanine-based purines released in large amounts from injured or dying cells of CNS may act as signals to initiate brain repair mechanisms widely involving astrocytes.

Regional and cellular distribution of the P2Y(1) purinergic receptor in the human brain: striking neuronal localisation

The biological actions of extracellular nucleotides are exerted via two families of P2 receptors, P2X and P2Y. The metabotropic P2Y receptors comprise at least 7 distinct subtypes, which have been cloned from a number of species. However, none of the P2Y receptor proteins have been visualised yet in human brain. In the present study, the regional and cellular distribution of the P2Y(1) receptor was investigated in the human brain by using immunohistochemistry. Polyclonal antibodies were raised against a synthetic peptide from the C-terminus of the P2Y(1) protein. Immunoblot analysis demonstrated that P2Y(1) antiserum specifically recognised a 63-kDa band in human and rat brain membranes. Similarly, the antiserum specifically detected the human P2Y(1) receptor in transfected 1321N1 cells. Immunohistochemical analysis on perfusion-fixed human brain tissue showed a widespread distribution for this receptor throughout the brain. At the cellular level, the P2Y(1) receptor was strikingly localised to neuronal structures of the cerebral cortex, cerebellar cortex, hippocampus, caudate nucleus, putamen, globus pallidus, subthalamic nucleus, red nucleus, and midbrain. Expression of the P2Y(1) receptor was not detected in other non-neuronal cell types. These results provide the first characterisation of the cellular distribution of a P2Y receptor in the human brain. The widespread and abundant distribution of the P2Y(1) receptor suggests its involvement in a number of important functions within the human brain. The neuronal localisation of this receptor points towards a possible role in neurotransmission, and also highlights a major role for extracellular nucleotides as signaling molecules within the brain.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

The hypotensive effect of docosahexaenoic acid is associated with the enhanced release of ATP from the caudal artery of aged rats

Fish oils have been shown to lower blood pressure in hypertensive subjects. To determine the mechanism of this hypotensive effect, we examined the effects of docosahexaenoic acid (DHA), one of the (n-3) polyunsaturated fatty acids in fish oil, on blood pressure and on the release of adenyl purines, such as ATP, ADP, AMP and adenosine, from the caudal arteries of aged rats. Aged female Wistar rats (100 wk) were fed a high cholesterol diet and were administered intragastrically ethyl all-cis-4,7,10,13,16,19-docosahexaenoate [300 mg/(kg.d)] for 12 wk (DHA group) or vehicle alone (control group). Compared with the controls, rats supplemented with DHA had significantly greater (10.1%) DHA concentrations in the caudal arteries. This was associated with more total (n-3) arterial fatty acids, a greater unsaturation index of arterial fatty acids, 43.9% lower plasma noradrenaline levels and the repression of the elevation in blood pressure observed with advancing age. The amount of purines released, both spontaneously and in response to noradrenaline, from arterial segments of DHA-supplemented rats was significantly higher than that released from tissues of control rats. Regression analysis revealed significant negative relationships between the total amount of purines released from the artery and the systolic (SBP) and diastolic (DBP) blood pressures. These results suggest that in aged rats, supplementation with DHA alters the membrane fatty acid composition as well as the amount of ATP released from vascular endothelial cells and decreases plasma noradrenaline, and that these factors may ameliorate the rise in blood pressure normally associated with advancing age.

Novel variant of the P2X2 ATP receptor from the guinea pig organ of Corti

ATP functions as a neurotransmitter and a neuromodulator in various tissues by acting on metabotropic (P2Y) and ionotropic (P2X) receptors. Evidence suggests that ATP activates P2X receptors on several cell types in the organ of Corti of guinea pig including outer hair cells (OHCs), Deiters' cells, Hensen's cells, pillar cells and inner hair cells (IHCs). Determining the sequence and structure of P2X receptors in guinea pig organ of Corti is important for understanding the function of ATP in the cochlea. We screened a guinea pig organ of Corti cDNA library for P2X2 ATP receptors using rat P2X2 cDNA as a probe. We sequenced three P2X2 variants which were found to be abundant in this library. One is a novel P2X2 isoform (P2X2-3) created by a retained intron coding for an additional 27 amino acids (81 bp) in the putative extracellular domain. We have also sequenced a variant (P2X2-2) that lacks both the 81-bp sequence and a 192-bp sequence in the 3' intracellular domain. A third variant (P2X2-1) contains the intracellular 192-bp sequence but not the extracellular 81-bp sequence found in P2X2-3. The multiple transcripts arise from alternative intron and exon splicing events. In situ hybridization with a probe common to the three variants localized P2X2 to many of the cells of the organ of Corti.

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.

Differential degradation of extracellular adenine nucleotides by folliculated oocytes of Xenopus laevis

The degradation of extracellular ATP and ADP by Xenopus oocytes was studied to investigate whether one or two ecto-enzymes are responsible for breakdown of both nucleotides. At a concentration of 100 microM, the half-life of ATP and ADP was 33 and 40 min, respectively. Degradation of ATP caused an initial fast and then a sustained accumulation of ADP in the buffer, while the concentration of AMP in the buffer increased slowly, but progressively, in a relatively linear manner. The rates of degradation of ATP and ADP were similar at pH levels between 7 and 10, but the velocity of breakdown of ATP was significantly higher than that of ADP at pH of 5-6. In divalent cation-free buffer, the addition of 0.1 mM of Ca2+, but not equimolar Mg2+, significantly potentiated the degradation of ATP by oocytes while, in the case of ADP, each of these divalent cations were able to potentiate its degradation. The rate of hydrolysis of ATP and its kinetic constants were not significantly different in the presence or absence of ADP (50 microM). In conclusion, differences in pH- and cation-dependency, and absence of inhibitory effect of ADP on degradation of ATP suggest that degradation of ATP and ADP by oocytes is provided by separate enzymes, namely Ca2+/Mg(2+)-dependent ecto-ATPase and ecto-ADPase, rather than by one ecto-enzyme.

Evidence for a P2-purinoceptor mediating vasoconstriction by UTP, ATP and related nucleotides in the isolated pulmonary vascular bed of the rat

1. The vasoconstrictor effects of uridine 5'-triphosphate (UTP), uridine 5'-diphosphate (UDP), uridine 5'-monophosphate (UMP) and uridine were tested in the isolated pulmonary vascular bed of the rat. Comparison was made with the effects of adenine nucleotides, adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and with adenosine. The effect of P2x-purinoceptor desensitization and blockade was compared on the vascular responses to uracil and adenine nucleotides. 2. At doses ranging from 10(-8) to 10(-5) mol, UTP elicited dose-dependent vasoconstriction. UDP was equiactive to UTP, while UMP and uridine did not show vasomotor activity. Similarly, ATP showed dose-related vasoconstrictor activity. ADP was less potent than ATP in eliciting vasoconstriction, while AMP was active only at the higher doses tested and adenosine was ineffective. 3. Vasoconstriction was produced by ATP analogues with the following order of potency: alpha, beta-methylene ATP > ATP gamma S > beta, gamma-methylene ATP > 2-methylthio ATP > or = ATP. 4. Desensitization of P2x-purinoceptors by the selective agonist alpha, beta-methylene ATP did not modify the vasoconstrictor activity of UTP and UDP and only partially reduced vasoconstrictor responses to ATP, while it abolished vascular responses to alpha, beta-methylene ATP itself. 5. The antagonists of P2-purinoceptors, suramin and pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS), did not affect vascular responses to UTP and UDP, but reduced vasoconstriction evoked by beta, gamma-methylene ATP and ATP by about 70 and 30%, respectively. 6. This study demonstrates that uracil nucleotides, UTP and UDP, elicit vasoconstriction in the rat pulmonary vascular bed. In addition to confirming the presence of classical P2x-purinoceptors, these results also suggest the presence of a distinct purinoceptor subtype which mediates UTP- and ATP- evoked vasoconstriction in the rat pulmonary circulation.

Diadenosine polyphosphates regulate cytosolic calcium in human fibroblast cells by interaction with P2x purinoceptors coupled to phospholipase C

The effects of diadenosine pentaphosphate (AP5A), and diadenosine hexaphosphate (AP6A) on the cytosolic-free Ca2+ concentration ([Ca2+]i) were evaluated in cultured human fibroblast cells (HF cells) using the fluorescent dye technique. AP5A, and AP6A concentration-dependently increased [Ca2+]i in HF cells. The addition of 10 mumol/1 AP5A and AP6A significantly increased [Ca2+]i in HF cells from 71 +/- 3 nmol/1 (n = 184) to 241 +/- 39 nmol/1 (n = 11; P < 0.001 compared to resting value) and to 227 +/- 26 nmol/1 (n = 23; P < 0.001), respectively. The purinoceptor P2 blockers, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), inhibited the diadenosine polyphophate-induced [Ca2+]i increase, whereas the P2y purinoceptor blocker, reactive blue, had no effect. Adenosinetriphosphate (ATP) and the P2x agonist, alpha 1 beta-methylene-ATP also significantly increased [Ca2+]i in HF cells, whereas the P2y agonist methylthio-ATP showed only a small [Ca2+]i response. Diadenosine polyphosphates mainly induced transplasmamembrane Ca2+ influx as was confirmed by experiments in the absence of extracellular Ca2+ or by manganese quenching studies. Organic (verapamil) and inorganic Ca2+ channel blockers (NiCI2) significantly reduced the AP6A induced transplasmamembrane Ca2+ influx. The inhibitor of phosphatidylcholine-specific phospholipase C, D609, significantly reduced the effect of diadenosine polyphosphates on [Ca2+]i in HF cells. It is concluded that diadenosine polyphosphates regulate transplasmamembrane Ca2+ influx after occupation of P2x receptors via activation of phosphatidylcholine-specific phospholipase C and hence of voltage-operated Ca2+ channels.

Evidence for two distinct P2-purinoceptors subserving contraction of the rat anococcygeus smooth muscle

1. The effects of the P2-purinoceptor agonists, adenosine 5'-triphosphate (ATP), alpha, beta-methylene ATP (alpha, beta-MeATP), beta, gamma-methylene ATP (beta, gamma-MeATP), L-beta, gamma-methylene ATP (L-beta, gamma-MeATP), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), and 2-methylthio ATP (2-MeSATP) were investigated on the isometric tension of the rat anococcygeus muscle. 2. Non-cumulative additions of ATP (100-1500 microM), alpha, beta-MeATP (1-300 microM), beta, gamma-MeATP (10-300 microM), L-beta, gamma-MeATP (3-100 microM) and ADP beta S (1-100 microM) produced concentration-dependent contractions, whereas 2-MeSATP (1-100 microM) had no effect. The rank order of potency was alpha, beta-MeATP > L-beta, gamma-MeATP > or = ADP beta S > beta, gamma-MeATP > > ATP > 2-MeSATP. 3. Contractions to cumulative additions of ATP, alpha, beta-MeATP, beta, gamma-MeATP and L-beta, gamma-MeATP were subject to desensitization whilst those to ADP beta S were unaffected. 4. Contractions to ATP, alpha, beta-MeATP, beta, gamma-MeATP and ADP beta S were abolished by the non-selective P2X/. P2Y-purinoceptor antagonist, suramin (100 microM). In contrast, contractions to ATP, alpha, beta-MeATP and beta, gamma-MeATP were not affected by the non-selective P1-purinoceptor antagonist 8-(p-sulphophenyl)-theophylline (8SPT, 30 microM). Blockade of P2X-purinoceptors with the selective P2X-purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM) or desensitization with L-beta, gamma-MeATP (10 microM) abolished contractions to alpha, beta-MeATP, but enhanced those to ADP beta S. The P2Y-purinoceptor antagonist, reactive blue 2 (RB2, 100 microM) enhanced contractions to ATP and alpha, beta-MeATP but abolished those to ADP beta S. 5. Simultaneous addition of alpha, beta-MeATP and ADP beta S produced an additive contraction. 6. The findings suggest that in the rat anococcygeus, smooth muscle cells are endowed with two distinct P2-purinoceptors which subserve contractions: a P2X-purinoceptor activated by ATP and its analogues, and another type of P2-purinoceptor activated by ADP beta S.

Partial characterization of a new nucleotide binding glycoprotein of hepatocyte plasma membrane

Hepatocyte plasma membranes contain a glycosylated 230-kDa Ca(2+) -dependent, Mg(2+)-stimulated ATPase (pgp230), which consists of two subunits, one of 120 kDa and the other of 110 kDa. pgp230 can be enriched by the use of affinity chromatography on Concanavalin A-Sepharose, wheat germ lectin-Sepharose, and 5'-AMP-Sepharose. It has a high-affinity Ca2+ binding site. In the presence of Ca2+, it forms a phosphorylated intermediate by autocatalytic transfer of the terminal phosphate residue from ATP. Maximal Ca(2+)-dependent autophosphorylation is observed at pH 5-6. Photoaffinity labeling using 8-azido-[alpha-32P]ATP or [y-32P]ATP confirms the presence of ATP binding sites. Incubation with [alpha-32P]ATP leads to a rapid but transient labeling of pgp230. Various nucleotides, nucleotide receptor agonists, or antagonists inhibit Ca(2+)-dependent phosphorylation by [y-32P]ATP. The concentrations of half-maximal inhibition range from 10(-7) M to 10(-3) M. The rank order of inhibitory potency is: ATP > alpha,beta-methylene-ATP > CTP = TTP > y-4-amino-phenyl-ATP = 2-methyl-thio-ATP > UTP = GTP > GDP = ADP = beta,y-methylene-ATP = beta, y-methylene-TTP = beta,y-methylene-GTP = adenosine-5'-O-2-thiodiphosphate = CMP = AMP > adenosine > cytidine > guanosine = suramin > Reactive blue 2 > iso-butyl-methyl-xanthine > thymidine > uridine. These data suggest a nucleotide binding capacity of this new hepatocyte membrane glycoprotein. Further investigations should be carried out to reveal its biological function.

Regional differences of endogenous ATP release in rabbit arteries

The releases of endogenous ATP from ear, femoral, renal and pulmonary arteries of rabbit were compared. Adenyl purines, such as ATP, ADP, AMP and adenosine, and norepinephrine (NE) were quantified by HPLC-fluorescence detection and HPLC-electrochemical detection, respectively. In all four blood vessels, electrical stimulation (ES) at 16 Hz significantly increased overflow of adenyl purine and NE. ATP was the main largest among adenyl purine released; release of adenosine was very small. The rank order of amounts of total purine released by ES was ear artery > renal artery = femoral artery >> pulmonary artery. There was no significant difference among the amounts of NE release induced by ES from these four arteries. ES-induced purine release was notably reduced by denudation of endothelium and prazosin at 1 muM. Methoxamine, alpha 1-adrenoceptor agonist, also produced release of adenyl purines in the four arteries. The rank order of amount of total purine released was ear artery > renal artery = femoral artery >> pulmonary artery. These suggest that the sources of ATP released by sympathetic nerve stimulation, which seems to be alpha 1-adrenoceptors on endothelial cells, not distributed homogeneously in the vasculature.

High potassium-evoked release of ATP from rabbit pulmonary artery via endogenous noradrenaline

1. High potassium 30 mM and noradrenaline at 10 microM significantly evoked the release of ATP and produced remarkable vasoconstriction in the rabbit pulmonary artery. 2. Phentolamine and prazosin at 0.1 microM inhibited ATP release but not vasoconstriction by 30 mM potassium. 3. 30 mM potassium significantly evoked the release of noradrenaline. 4. There was a significant positive correlation between the amounts of release of total purines, sum of ATP, ADP, AMP and adenosine, and noradrenaline evoked by 30 mM potassium. 5. 30 mM potassium-evoked ATP release was significantly reduced by denudation of endothelium.

Activation of cell growth inhibitor by ectoprotein kinase-mediated phosphorylation in transformed mouse fibroblasts

Our previous studies have shown that exogenous ATP induces cell growth inhibition in transformed mouse fibroblasts, 3T6 cells, whereas the growth of their nontransformed counterparts, Swiss 3T3 cells, is only slightly affected. In this study a similar selective, ATP-induced growth inhibition was found in Balb/c SV40-3T3 cells and in primary cultures of adenovirus-transformed murine fibroblasts. The inhibitory activity was found in the conditioned media of ATP-treated cultures. Several lines of evidence have shown that ectoprotein kinase (ecto-PK) plays a major role in the ATP-induced growth inhibition. (a) There is a good correlation between the activity of ecto-PK and the ability of ATP to induce cell growth inhibition. (b) The removal of the ecto-PK from the cell surface prevents the ATP-induced growth inhibition. (c) Addition of the removed enzyme to the cell culture reconstitutes the ability of ATP to induced growth inhibition. (d) Serum-containing, or serum-free, conditioned media from untreated cultures gain an inhibitory activity after their phosphorylation, and dephosphorylation of conditioned media from ATP-treated cultures results in the loss of the inhibitory activity. (e) Growth medium by itself does not inhibit cell proliferation after its phosphorylation. The findings described in d and e indicate, as well, that the ATP-induced growth inhibitor is produced by the cells. The putative inhibitor was found to be a protein, with an apparent molecular mass of 13 kDa. The selectivity of the inhibition for transformed cells is due to the higher level of ecto-PK in these cells, as well as to their higher susceptibility to the inhibitor, as compared with their non-transformed counterparts.

Regional difference of endogenous ATP release in the pulmonary artery of rabbits

1. The release of adenyl purines such as ATP, ADP, AMP and adenosine, from the pulmonary artery trunk (PAT), extrapulmonary artery (EPA) and intrapulmonary artery (IPA) were compared. 2. The amount of basal overflow of adenyl purines from the PAT was significantly smaller than those from EPA and IPA. There was no significant difference between the amount of the overflow from EPA and PAT. 3. Methoxamine, an alpha 1-adrenoceptor agonist, significantly increased the overflow of adenine nucleotides from the PAT, EPA and IPA, but did not increase those of adenosine. Methoxamine-induced release of adenyl purines from IPA was significantly larger than those from EPA and PAT. 4. These results suggest that an alpha 1-adrenoceptor-mediated mechanism for ATP-release is not homogeneously distributed in the pulmonary artery and a larger amount of ATP may be released in the peripheral part of the artery.

Current state of purinoceptor research

It is hoped that this summary of the history and current status of purinoceptors will convince readers that receptors for purines are now established alongside other well-known extracellular messenger systems. These receptors are primitive, widespread and serve many different systems. Receptors of adenosine (P1-purinoceptors) are clearly different from receptors of ATP (P2-purinoceptors). As for other major transmitters such as acetylcholine, GABA, glutamate and 5-HT, receptors of two major families are activated by ATP, one (the P2X-purinoceptor family) mediates fast responses via ligand-gated ion channels, while the other (the P2Y-purinoceptor family) mediates slower responses via G-proteins (see Table 3). Subclasses of these two families have been suggested on the basis of recent molecular biology studies and the development of new selective agonists and antagonists (Abbracchio and Burnstock, 1994). It would indeed be helpful if the work on purinoceptors could be extended to studies of their chemical structure employing crystallography.

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.

Evidence for the presence of both pre- and postjunctional P2-purinoceptor subtypes in human isolated urinary bladder

1. In order to characterize P2-purinoceptor(s) in human urinary bladder the contractile effects of ATP and its slowly-hydrolyzable analogues alpha, beta-methylene ATP (alpha, beta-MeATP) and beta, gamma-methylene ATP (beta, gamma-MeATP) were investigated on human detrusor strips taken from patients undergoing cystectomy for bladder carcinoma. 2. Serial concentration-response curves (SCRC) for ATP, alpha, beta-MeATP and beta, gamma-MeATP were constructed with an interval of 25 min between two successive doses to avoid tachyphylaxis. ATP (10 microM-10 mM) induced a phasic contraction, which was very rapid in onset. The dose-response curve to ATP appeared not to be monophasic: at the lower concentrations (10-300 microM) the curve was shallow, whilst at high concentrations (1-10 mM) the curve was steeper. The magnitude of the response obtained at the highest concentration tested (10 mM) was only 21.1 +/- 2.8% (mean +/- s.e. mean; n = 4) of the KCl (100 mM)-induced contraction. 3. alpha, beta-MeATP (0.3 microM-1 mM) and beta, gamma-MeATP (10 microM-1 mM) elicited a phasic contraction with a time course similar to that exhibited by ATP. The magnitude of the response obtained at the highest concentration tested (1 mM) was 70.3 +/- 6.3% for alpha, beta-MeATP (n = 10) and 27.9 +/- 4.5% for beta, gamma-MeATP (n = 8) of KCl (100 mM)-induced contraction. The rank order of potency was alpha, beta-MeATP > beta, gamma-MeATP > ATP. A plateau of response could not be achieved by any of these agonists. 4. The P2-purinoceptor antagonist, suramin (10-300 microM), dose-dependently antagonized only the lower part of alpha,beta-MeATP dose-response curve. Data were analysed in terms of dose-ratio estimated at two levels of response (10% and 35% of KC1 100 mM-induced contraction). At 10% of KCl response the Schild plot slope was 0.98 and the estimated pKB was 5.85, whereas using the dose-ratio at the 35% level of the KCl response, the Schild plot was not linear suggesting an interaction of alpha,beta-MeATP with a heterogeneous receptor population.5. The putative P2-purinoceptor antagonist, Coomassie Brilliant Blue G (CB-G) at 0.3 and 1 l micro M(n = 5), shifted to the left the alpha,beta-MeATP SCRC. The response at the highest concentration of agonist was potentiated, being equal to 78.8 +/- 11.7% of the KCl (100 mM) response (n = 5). CB-G at 0.3 microM also shifted to the left the beta,upsilon-MeATP SCRC and significantly potentiated the response at 1 mM up to 46.3 +/- 5.6% of KCl 100 mM response (n = 4).6. Pretreatment with terodotoxin (TTX) at 1 microM shifted to the left the alpha,beta-MeATP SCRC but the response to the highest concentration of the agonist was not potentiated, being 73.6 +/- 9.9% of the KCl(100 mM) response (n = 5). TTX (1 micro M) shifted to the left the beta,upsilon-MeATP SCRC and significantly potentiated the response at 1 mM (61.6 +/- 3.1% of KCl response; n = 4).7. The NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) at 100 micro M did not modify the SCRC to either alpha, beta or beta,upsilon-MeATP.8. We conclude that in human detrusor muscle there is a heterogeneity of purinoceptors. The complex antagonism exhibited by suramin suggests the presence not only of Ph-purinoceptors but also of another contractile P2-purinoceptor subtype insensitive to suramin. Moreover, the activity of CB-G and TTX seems to support the existence of a prejunctional P2-purinoceptor subtype inducing the release of one or more inhibitor neurotransmitters.

A new type of Ca(2+)-dependent, Mg(2+)-stimulated ATPase of rat liver plasma membrane

Incubation of a glycoprotein fraction obtained from rat liver plasma membrane which has been previously well characterized using [gamma-32P]ATP results in the phosphorylation of a 230-kDa glycoprotein (pgp230). It is composed of a 120-kDa subunit (pgp120) and a 110-kDa subunit (pgp110) linked by interchain disulfide bonds. Peptide maps of pgp120 and pgp110 suggest extensive similarity in their polypeptide chains. Glycan analysis reveals between four and six hybrid-type oligosaccharide chains for both phosphoproteins. Immunoblotting using monoclonal antibodies and endoglycosidase digestion exclude an identity of pgp120 or pgp110 with the hepatocyte plasma membrane glycoproteins dipeptidylpeptidase IV or the taurocholate transport protein, which co-purify and co-migrate in SDS/PAGE. Protein phosphorylation is Ca(2+)-dependent (K0.5(Ca2+) = 0.35 microM, in the absence of Mg2+). In the presence of Mg2+, the glycoprotein undergoes rapid cycles of phosphorylation and dephosphorylation, resulting in ATPase activity. Analysis of phosphorylated amino acids identifies phosphothreonine as the major one. Photoaffinity labeling with 8-azido-[alpha-32P]ATP demonstrates the presence of one or more ATP binding site(s). Preincubation of pgp230 with various purine or pyrimidine nucleotides (ATP, UTP, TTP, ADP, GDP, AMP, CMP) or known P2-purinoceptor agonists or antagonists (adenosine 5'-[alpha,beta-methylene]triphosphate, 2-methyl-thio-adenosine 5'-triphosphate, suramin) inhibits its phosphorylation by [gamma-32P]ATP. The biological function of pgp230 is unknown at present. Several findings of the present study are compatible with the idea that pgp230 may be involved in a P2-purinoceptor function of the hepatocyte. Following this concept, a mechanism is discussed where a cytosolically exposed high-affinity Ca(2+)-binding site of pgp230 would allow for receptor feedback control, via phosphorylation and dephosphorylation, by sensing changes in cytosolic Ca2+ concentration.

In vitro studies of release of adenine nucleotides and adenosine from rat vascular endothelium in response to alpha 1-adrenoceptor stimulation

1. Noradrenaline-induced release of endogenous adenine nucleotides (ATP, ADP, AMP) and adenosine from both rat caudal artery and thoracic aorta was characterized, using high-performance liquid chromatography with fluorescence detection. 2. Noradrenaline, in a concentration-dependent manner, increased the overflow of ATP and its metabolites from the caudal artery. The noradrenaline-induced release of adenine nucleotides and adenosine from the caudal artery was abolished by bunazosin, an alpha 1-adrenoceptor antagonist, but not by idazoxan, an alpha 2-adrenoceptor antagonist. Clonidine, an alpha 2-adrenoceptor agonist, contracted caudal artery smooth muscle but did not induce release of adenine nucleotides or adenosine. 3. Noradrenaline also significantly increased the overflow of ATP and its metabolites from the thoracic aorta in the rat; however, the amount of adenine nucleotides and adenosine released from the aorta was considerably less than that released from the caudal artery. 4. Noradrenaline significantly increased the overflow of ATP and its metabolites from cultured endothelial cells from the thoracic aorta and caudal artery. The amount released from the cultured endothelial cells from the thoracic aorta and caudal artery. The amount released from the cultured endothelial cells from the aorta was also much less than that from cultured endothelial cells from the caudal artery. In cultured smooth muscle cells from the caudal artery, a significant release of ATP or its metabolites was not observed. 5. These results suggest that there are vascular endothelial cells that are able to release ATP by an alpha 1-adrenoceptor-mediated mechanism, but that these cells are not homogeneously distributed in the vasculature.

Purinergic and cholinergic components of bladder contractility and flow

The role of ATP as a neurotransmitter/neuromodulator in the urinary tract has been the subject of much study, particularly whether ATP has a functional role in producing urine flow. Recent studies suggested significant species variation, specifically a variation between cat and other species. This study was performed to determine the in vivo response of cat urinary bladder to pelvic nerve stimulation (PNS) and to the exogenous administration of cholinergic and purinergic agents. In anesthetized cats, bladder contractions and fluid expulsion was measured in response to PNS and to the exogenous administration of cholinergic and purinergic agents. Fluid was instilled into the bladder and any fluid expelled by bladder contractions induced by PNS or exogenous agents was collected in a beaker. The volume was measured in a graduated cylinder and recorded. PNS, carbachol and APPCP produced sustained contractions with significant expulsion of fluid. ATP, ACh and hypogastric nerve stimulation did not produce any significant expulsion of fluid. Atropine, a cholinergic antagonist, inhibited PNS contractions and fluid expulsion with no effect on purinergic actions. There was a significant relationship between the magnitude of the contraction, duration of the contractions and volume of fluid expelled. The data and information from other studies, strongly suggests a functional role for ATP as a cotransmitter in the lower urinary tract different from ACh's role. ATP stimulation of a specific purinergic receptor plays a role in initiation of bladder contractions and perhaps in the initiation of urine flow from the bladder. ACh's role is functionally different and appears to be more involved in maintenance of contractile activity and flow.

Effects of exogenous ATP and related analogues on the proliferation rate of dissociated primary cultures of rat astrocytes

The effects of ATP (5-500 microM) were evaluated on the proliferation rate of cultured astrocytes by measuring 3H-thymidine incorporation and by flow cytometric analysis of the cell cycle. Determinations after 16 hours showed that ATP present in the culture medium for the whole period caused a dose-dependent reduction of cell proliferation, while if the exposure to ATP was limited to the first 8 hours, the proliferation was increased (always in a dose-dependent manner). A time course study of 3H-thymidine incorporation showed that, in the presence of ATP, 3H-thymidine was incorporated at a slower rate than in controls; the replacement of the culture medium with an ATP-free fresh medium, at the 8th hour, was followed by a 3H-thymidine incorporation occurring at such a fast rate to overshoot the control values. High performance liquid chromatography (HPLC) analysis, carried out to identify purine compounds present in the culture medium during cell exposure to ATP, indicated that more than 95% of the added ATP was metabolized within 1 hr. Conversely, an increase of purine metabolites was measured, this accumulation being greater at the highest concentrations of added ATP. The presence of high levels of extracellular ATP catabolites suggested that these compounds may act on the regulation of cell replication via the different purine receptors. This hypothesis was tested and confirmed by using agonists and antagonists selective for the P1 and the P2 sites. One hundred microM 2methylthio-ATP (2MeSATP), a P2Y agonist metabolized as fast as ATP, reproduced effects very similar to the ATP-induced ones. On the other hand, the nonhydrolisable ATP analogue, adenosine 5'-(beta, gamma-imido)-triphosphate (AMP-PNP) at 100 microM, induced a mitogenic effect as well as the A2 site stimulation. On the contrary, the activation of A1 receptors by 5 microM R-phenyl-isopropyladenosine (R-PIA) inhibited astrocyte proliferation; moreover, 100 nM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an A1 site antagonist, reversed the ATP-induced inhibition of cell proliferation. These results indicate that exogenous ATP, as a consequence of its rapid extracellular breakdown, exerts a dual influence on astrocyte proliferation by the involvement of both P1 and P2Y receptors. These findings might be relevant to such pathological conditions of the central nervous system (CNS), as seizures, hypoxia or ischemia, in which great amounts of purines released in the brain can influence a reactive astrocyte proliferative response to injury.

Release of endogenous ATP from the caudal artery in rats with arteriosclerosis

Noradrenaline significantly increased (by a prazosin-sensitive mechanism) the overflow of ATP and its metabolites from the caudal arteries of rats treated with excess vitamin D2 and a high-cholesterol diet (arteriosclerotic rats), although the amount of the overflow was smaller than that in the normal rats. The arteries from the arteriosclerotic rats showed a marked increase in the calcium content and there was a significant negative correlation between the noradrenaline-induced overflow of ATP and the arterial calcium content. These findings indicate that ATP release from arteriosclerotic rat caudal arteries mediated by alpha 1-adrenoceptors is impaired in proportion to the extent of arterial calcification.

Permeabilization of cells of hemopoietic origin by extracellular ATP4-: elimination of osteoclasts, macrophages, and their precursors from isolated bone cell populations and fetal bone rudiments

Skeletal tissues contain, apart from cells of the osteogenic and chondrogenic lineage, cells of hemopoietic origin, e.g., macrophages, osteoclasts, and their precursors. In the present study we examined the sensitivity for extracellular ATP4- of the above-mentioned cell types in freshly isolated, bone-derived cell populations and in explanted fetal metatarsal bones. Cells of hemopoietic origin reacted to the presence of ATP4- with an increased permeability for impermeant cytotoxic molecules, e.g., ethidium bromide (EB), thiocyanate (KSCN), and an increased non-ion selective membrane conductance. As a consequence, these cells could be killed by a short treatment with adenosine-5' triphosphate (ATP)+KSCN. On the other hand, cells of nonhemopoietic origin (e.g., osteoblasts, chondrocytes) were found to be insensitive to ATP4- in this respect. These cells survived the treatment without apparent damage to their alkaline phosphatase activities, osteogenic potentials, and osteoclast induction capacities. The elimination of the endogenous cells of hemopoietic origin from bone tissue or cell populations derived therefrom offers the possibility to study the properties and functions of osteogenic or chondrogenic cells without interference by the presence of cells of hemopoietic origin. It also allows the study of interactions between osteogenic cells and selected cell populations of hemopoietic origin in coculture experiments.

Contractions mediated by alpha 1-adrenoceptors and P2-purinoceptors in a cat colon circular muscle

1. The postjunctional excitatory and inhibitory effects of adrenoceptor and purinoceptor agonists and antagonists were studied in circular smooth muscle strips of cat colon. 2. In the presence of tetrodotoxin (0.5 microM), noradrenaline caused contraction or relaxation of circular smooth muscle at resting tension or with raised tone, respectively. The noradrenaline-evoked contractions were potentiated and the noradrenaline-evoked relaxations were antagonized by propranolol (1 microM), suggesting beta-adrenoceptor involvement. 3. At resting tension, noradrenaline, adrenaline and the selective alpha 1-adrenoceptor agonist, phenylephrine, caused concentration-dependent contractile responses, with EC50 values of 1.8 +/- 0.2 microM, 1.9 +/- 0.4 microM and 4.3 +/- 1.7 microM, respectively. The EC50 values and the amplitude of maximal responses were not significantly different from one another. Clonidine (0.1-500 microM), a selective alpha 2-adrenoceptor agonist, was not effective. 4. Prazosin (0.1-9 microM), competitively antagonized the contractile effects of noradrenaline with an estimated pA2 value of 6.93 and a slope of 1.07 +/- 0.03. The Kb values, estimated from a single shift (0.1 microM prazosin) of the concentration-response curves to noradrenaline, adrenaline and phenylephrine were 92.8 +/- 9.3 nM, 108.7 +/- 6.4 nM and 18.4 +/- 3.1 nM, respectively. 5. At resting tension, adenosine 5' triphosphate (ATP, 5-1000 microM), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP, 0.05-50 microM), beta,gamma-methylene adenosine 5'-triphosphate (beta,gamma-MeATP, 0.5-100 microM), and 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 1-500 microM) caused concentration-dependent contractions with EC50 values of 60.5 +/- 15.9 microM, 0.7 +/- 0.1 microM, 7.6 +/- 0.1 microM and 25.3 +/- 12.8 microM, respectively. The maximal responses to alpha, beta-MeATP and beta,gamma-MeATP were greater than maximal responses to 2-MeSATP and ATP.6. Suramin (50-500 microM), competitively antagonized the contractile responses of alpha,beta-MeATP with an estimated pA2 value of 4.92 and a slope of 1.08 +/- 0.04. The Kb values, estimated from a single shift(1I00 microM suramin) of the concentration-response curves to ATP, alpha,beta-MeATP, beta,gamma-MeATP and 2MeSATPwere 52.3 +/- 20.2 microM, 25.2 +/- 4.5 microM, 21.7 +/- 11.0 microM and 11.6 +/- 2.7 microM, respectively.7. At resting tension, reactive blue 2 (100 microM), a selective antagonist of the P2Y-purinoceptor, and 8-(p-sulphophenyl)-theophylline (8-SPT) (1 microM), a selective antagonist of the PI-purinoceptor, did not antagonize the contractile responses to alpha,beta-MeATP (0.5-5 microM). Contractile responses to ATP(50-500 microM) were not altered by 8-SPT (I microM) but were potentiated by reactive blue 2 (100 microM).8. With raised tone, ATP and 2-MeSATP caused a relaxant effect. This effect of ATP was not altered by either tetrodotoxin (TTX) (0.5 microM) or suramin (100 microM), but was antagonized by reactive blue 2(100 microM) and 8-SPT (1 microM), suggesting that inhibitory P2y- and P1-purinoceptors are involved. In contrast, alpha,beta-MeATP and Beta,gamma-MeATP caused only contractions. This contractile effect of alpha,beta-MeATPwas resistant to TTX (0.5 microM) and antagonized by suramin (100 microM).9. In summary, cat colon circular muscle contains postjunctional alpha 1-adrenoceptors and P2X purinoceptors which mediate contractions and P2y- and PI-purinoceptors which mediate relaxation.Postjunctional alpha2-adrenoceptors appear not to be present.

Demonstration of an ectoATP-diphosphohydrolase (E.C.3.6.1.5.) in non-vascular smooth muscles of the bovine trachea

An ectoATP-diphosphohydrolase (ATPDase) is put in evidence in non-vascular smooth muscles of the bovine trachea. The enzyme has an optimum pH of 7.0 and catalyzes the hydrolysis of the gamma- and beta-phosphate residues from extracellular triphospho- and diphosphonucleosides. It requires either Ca2+ or Mg2+ and is insensitive to ouabain, oligomycin and Ap5A. Sodium azide (20 mM), mercuric chloride (10 microM) and gossypol (35 microM) inhibit the enzyme activity by more than 45%. Polyacrylamide gel electrophoresis under non-denaturing conditions and kinetic properties, namely pH dependency profiles, heat inactivation and 60Co gamma-irradiation-inactivation curves, support the view that the same catalytic site is responsible for the hydrolysis of ATP and ADP to AMP. Accordingly, when both ATP and ADP were combined, reaction rates were not additive. With ATP, Km,app and Vmax,app were estimated at 15 +/- 2 microM and 1.9 +/- 0.1 mumol inorganic phosphate/min per mg of protein, respectively. From 60Co gamma-irradiation-inactivation curves, the molecular mass of the enzyme was estimated at 71 +/- 5 kDa. Enzyme markers indicate that the ATPDase is associated with the plasma membrane. Enzyme assays on trachea smooth muscle cells in suspension confirm that the catalytic site of this ATPDase is localized on the outer surface of the plasma membrane. Analysis of the biochemical properties shows many points of similarity between the tracheal ATPDase and the ATPDase recently described in the bovine lung.

Activation of specific ATP receptors induces a rapid increase in intracellular calcium ions in rat hypothalamic neurons

We have used real-time dynamic video imaging of Fura-2 fluorescence to study the acute effects of external ATP on [Ca2+]i in cultured rat hypothalamic neurons. The addition of ATP at microM concentrations, but not adenosine, AMP, ADP or GTP, produced a rapid, dose-dependent increase in cytosolic Ca2+. The hydrolysis-resistant ATP analogues 3-thio-ATP and beta,gamma-imido-ATP produced a similar response but alpha,beta-methylene ATP had much lower efficacy. The ATP response was inhibited by 10 microM nifedipine, abolished by 50 microM cadmium and by the absence of extracellular Ca2+, but was unaffected by ryanodine or omega-conotoxin GVIA. The P2-purinoceptor antagonist suramin reversibly and selectively inhibited the ATP response but had no effect on other neurotransmitter-induced Cai2+ responses. Antagonists to muscarinic, nicotinic, NMDA, non-NMDA, GABA, 5-HT and adenosine receptors had no effect on the ATP response. Thus the Ca2+ response of hypothalamic neurons to ATP is mediated by specific suramin-sensitive ATP-receptors, activation of which is independent of ATP hydrolysis and results in an influx of extracellular Ca2+ largely through high voltage-gated Ca2+ channels. These findings support the assertion that ATP acts in the CNS as an excitatory neurotransmitter.

Methoxamine-induced release of endogenous ATP from rabbit pulmonary artery

Methoxamine, an alpha 1-adrenoceptor agonist, significantly increased the overflow of ATP, ADP and AMP, but not adenosine, by a prazosin-sensitive mechanism in the rabbit pulmonary artery. Among the adenine nucleotides released, the amount of ATP was larger than those of the other two. Such release of adenine nucleotides was not induced by clonidine, an alpha 2-adrenoceptor agonist, and isoproterenol, a beta-adrenoceptor agonist. Methoxamine-induced release was observed in the absence of extracellular calcium, but was not observed at a low temperature, 27 degrees C. This suggests an extracellular calcium-independent and temperature-dependent ATP-releasing mechanism coupled with alpha 1-adrenoceptors in rabbit pulmonary artery.

Effects of ATP and UTP in pheochromocytoma PC12 cells: evidence for the presence of three P2 receptors, only one of which subserves stimulation of norepinephrine release

1. In pheochromocytoma PC12 cells ATP and, to a lesser extent, 2-methylthioATP stimulate phosphoinositide breakdown, release of intracellular calcium, and influx of external calcium, leading to stimulation of norepinephrine release. In contrast, although UTP also stimulates phosphoinositide breakdown, release of intracellular calcium, and influx of external calcium, there is no stimulation of norepinephrine release. 2. 2-MethylthioATP, presumably acting at P2y receptors, and UTP, presumably acting at P2u receptors, in combination elicit a phosphoinositide breakdown greater than that elicited by either alone. Intracellular levels of calcium measured with Fura-2 increase to greater levels with ATP than with UTP and are sustained, while the UTP intracellular levels of calcium rapidly return to basal values. Both ATP and UTP cause a similar influx of 45 Ca2+ presumably by stimulation of a P2 receptor directly linked to a cation channel. 3. It is proposed that PC12 cells contain two distinct G protein-coupled P2 receptors that activate phospholipase C and a P2 receptor linked to a cation channel. The P2y receptor sensitive to ATP (and to 2-methylthioATP) causes the depletion of a pool of intracellular calcium, sufficient to activate so-called "receptor-operated calcium entry". The sustained elevation of intracellular calcium after ATP treatment is proposed to result in stimulation of norepinephrine release and activation of calcium-dependent potassium channels and sodium-calcium exchange pathways. 4. The P2u receptor sensitive to UTP (and to ATP) causes only a transient elevation in levels of intracellular calcium, perhaps from a different pool, insufficient to activate so-called receptor-operated calcium entry. Further sequelae do not ensue, and the functional role of the UTP-sensitive P2u receptor is unknown.

Purinoceptors: are there families of P2X and P2Y purinoceptors?

There has been an exponential growth in interest in purinoceptors since the potent effects of purines were first reported in 1929 and purinoceptors defined in 1978. A distinction between P1 (adenosine) and P2 (ATP/ADP) purinoceptors was recognized at that time and later, A1 and A2, as well as P2x and P2y subclasses of P1 and P2 purinoceptors were also defined. However, in recent years, many new subclasses have been claimed, particularly for the receptors to nucleotides, including P2t, P2z, P2u(n) and P2D, and there is some confusion now about how to incorporate additional discoveries concerning the responses of different tissues to purines. The studies beginning to appear defining the molecular structure of P2-purinoceptor subtypes are clearly going to be important in resolving this problem, as well as the introduction of new compounds that can discriminate pharmacologically between subtypes. Thus, in this review, on the basis of this new data and after a detailed analysis of the literature, we propose that: (1) P2X(ligand-gated) and P2Y(G-protein-coupled) purinoceptor families are established; (2) four subclasses of P2X-purinoceptor can be identified (P2X1-P2X4) to date; (3) the variously named P2-purinoceptors that are G-protein-coupled should be incorporated into numbered subclasses of the P2Y family. Thus: P2Y1 represents the recently cloned P2Y receptor (clone 803) from chick brain; P2Y2 represents the recently cloned P2u (or P2n) receptor from neuroblastoma, human epithelial and rat heart cells; P2Y3 represents the recently cloned P2Y receptor (clone 103) from chick brain that resembles the former P2t receptor; P2Y4-P2Y6 represent subclasses based on agonist potencies of newly synthesised analogues; P2Y7 represents the former P2D receptor for dinucleotides. This new framework for P2 purinoceptors would be fully consistent with what is emerging for the receptors to other major transmitters, such as acetylcholine, gamma-aminobutyric acid, glutamate and serotonin, where two main receptor families have been recognised, one mediating fast receptor responses directly linked to an ion channel, the other mediating slower responses through G-proteins. We fully expect discussion on the numbering of the different receptor subtypes within the P2X and P2Y families, but believe that this new way of defining receptors for nucleotides, based on agonist potency order, transduction mechanisms and molecular structure, will give a more ordered and logical approach to accommodating new findings. Moreover, based on the extensive literature analysis that led to this proposal, we suggest that the development of selective antagonists for the different P2-purinoceptor subtypes is now highly desirable, particularly for therapeutic purposes.

Growth inhibition of breast cancer cells induced by exogenous ATP

Addition of ATP (> 0.1 mM) to cultures of human breast cancer T47D cells resulted in an inhibition of cell proliferation. The inhibition was found to be specific for ATP, and dependent on its concentration. Growth inhibition continued for at least three days, although ATP and its hydrolysis products were metabolized within one day. Conditioned medium from ATP-treated cultures (CM+) was found to inhibit the growth of cells that were not exposed to ATP. This is an indication that extracellular factors, besides ATP, are involved in the inhibition process. The inhibition was maintained after dialysis of the CM+, using an 8 kDa cut-off membrane. Conditioned medium from untreated cultures (CM-), however, only slightly affected cell growth. The data suggest that the CM(+)-induced cell growth inhibition is mediated by an ATP-activated growth inhibiting factor. Flow microfluorometry and thymidine incorporation experiments have shown that the growth arrest is mainly due to the elongation of the S-phase of the cell cycle.

pH-dependent binding of chromogranin B and secretory vesicle matrix proteins to the vesicle membrane

Contrary to the notion that the soluble intravesicular matrix proteins of the secretory vesicles of adrenal medullary chromaffin cells freely float in the vesicle, several vesicle matrix proteins of the secretory vesicles, including chromogranins A and B, bound to the vesicle membrane at intravesicular pH (5.5) and were freed from it when the pH was raised to a near physiological pH (7.5). Estimation of the fraction of vesicle matrix proteins that might remain bound to the vesicle membrane in the vesicle suggested that the majority (> 50-80%) of chromogranins A and B, as well as several other proteins, will stay bound to the membrane in the vesicle. Comparison of the amino-acid sequences of chromogranins A and B revealed two highly conserved regions, i.e., one near the N-terminus and the other being the C-terminal region. Since it has been demonstrated with chromogranin A that the conserved near N-terminal region of chromogranin A exhibited the pH-dependent membrane-binding activity (Yoo, S. H. (1993) Biophys. J., 64, A195), the same region in chromogranin B (residues 17-36) was tested using a synthetic chromogranin B peptide, and found to exhibit the pH-dependent membrane-binding activity. The pH-dependent binding of the matrix proteins at pH 5.5 and the automatic untethering at a physiological pH accord well with the rapid release and circulation of the vesicular contents in the bloodstream.

Stimulation of lumbar sympathetic nerves evokes contractions of cat colon circular muscle mediated by ATP and noradrenaline

1. The action of the lumbar sympathetic nerves to cat colon was studied in vitro using isolated muscle strips with attached lumbar colonic nerves (LCN) orientated in the axis of circular muscle layer. Electrical stimulation of LCN caused frequency-dependent increases in resting tension and in amplitude of spontaneous contractions. Contractile responses were abolished by tetrodotoxin (3 microM) and by guanethidine (30 microM), indicating that they were neurogenic, involving the release of neurotransmitter from sympathetic fibres. 2. Propranolol (1-9 microM), a beta-adrenoceptor antagonist, caused a concentration-dependent potentiation of LCN-evoked contractile responses. Propranolol (3 microM) potentiated contractile responses to exogenously applied noradrenaline but not to phenylephrine. 3. Phentolamine (1-9 microM), an alpha-adrenoceptor antagonist, and prazosin (1-9 microM), an alpha 1-adrenoceptor antagonist, caused a concentration-dependent reduction of amplitude but did not abolish LCN-evoked contractile responses. Prazosin (3 microM) or phentolamine (3 microM) antagonized contractile responses to noradrenaline and phenylephrine. 4. Desensitization of purinoceptors with the P2x-receptor agonist, alpha,beta-methylene ATP, caused a decrease in amplitude of LCN-evoked contractile responses and abolished contractile responses to ATP. In muscle strips where alpha 1-adrenoceptors were blocked with prazosin (3 microM) and P2-purinoceptors were desensitized with alpha,beta-methylene ATP, the amplitude of contractile responses was reduced by 82-100%. 5. The P2x-purinoceptor antagonists, arylazido amino propyl adenosine triphosphate (ANAPP3) and 5. The P2x-purinoceptor antagonists, arylazido amino propyl adenosine triphosphate (ANAPP3) and suramin, affected LCN-evoked contractile responses. ANAPP3 (50-100 microM) caused a concentration-dependent reduction in the amplitude of contractile response. Suramin (100 microM) caused a small reduction in amplitude of contractile responses but potentiated their amplitude at a concentration of 500 microM. 6. ANAPP3 (100 microM) irreversibly inhibited contractions to alpha,beta-methylene ATP or ATP. Suramin (100-500 microM) inhibited contractions to alpha,beta-methylene ATP (0.5-1 microM) or low concentrations of ATP (10-50 microM) but potentiated contractions at higher concentrations. ANAPP3 (100 microM) and suramin (100, 500 microM) had no effect on contractile responses to noradrenaline. 7. Clonidine (0.05-1 microM), a selective alpha 2-adrenoceptor agonist, caused a concentration-dependent reduction in amplitude of LCN-evoked contractile responses, at 10 Hz, while yohimbine (0.1-1 microM), a selective alpha 2-adrenoceptor antagonist, increased them. At 1 microM, both compounds affected LCN-evoked contractions at all frequencies. This suggests that prejunctional alpha 2-receptors are involved in autoinhibition at sympathetic terminals. 8. In summary, LCN-evoked contractile responses involve the corelease of noradrenaline and ATP or a related purine nucleotide from sympathetic fibres. It is likely that the neurogenic responses are mediated through excitatory postjunctional alpha 1-adrenoceptors, excitatory suramin-sensitive and suramin-insensitiveP2X-purinoceptors and inhibitory beta-adrenoceptors. Also, autoinhibitory prejunctional alpha2-adrenoceptors regulate the LCN excitatory pathway to cat colon circular muscle.

Cloning and functional expression of a brain G-protein-coupled ATP receptor

A cDNA encoding a novel member of the G-protein-coupled receptor (GCR) superfamily, an ATP receptor, has been isolated from an embryonic chick whole brain cDNA library by hybridization screening. The encoded protein has a sequence of 362 amino acids (41 kDa) and shares no more than 27% amino acid identity with any known GCR. When expressed as a complementary RNA (cRNA) in Xenopus oocytes a slowly-developing inward current was observed in response to application of ATP. The pharmacology of this expressed protein defines it as a P2Y purinoceptor.

GTP[S] stimulates migration of electropermeabilized neutrophils via a pertussis toxin-sensitive G-protein

Electropermeabilized neutrophils were used to study the role of G-proteins in neutrophil migration. Rabbit neutrophils, under specific conditions, retained their ability to migrate after electropermeabilization. Introduction of guanosine-5'-[3-thio] triphosphate (GTP[S]) into the cell interior stimulated random migration and enhanced migration activated by a suboptimal concentration of formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) (10(-11) M). GTP[S] had no effect on random migration by intact cells, or on migration of intact cells activated with a suboptimal concentration of fMet-Leu-Phe, indicating that the effect of GTP[S] was intracellular. The effects of GTP[S] were inhibited by pertussis toxin and by guanosine-5'-[2-thio] diphosphate (GDP beta S) indicating that a pertussis toxin-sensitive G-protein was involved. GTP stimulated random migration to the same extent as GTP[S], but had only a small effect on migration activated by a suboptimal concentration of fMet-Leu-Phe (10(-11) M). Several other nucleotides tested had no effect on random migration or migration activated with 10(-11) M fMet-Leu-Phe. The results show that neutrophil migration can be potentiated by direct activation of a pertussis toxin-sensitive G-protein, and the results obtained with GTP suggest that possibly more than one G-protein is involved in this process.

Triphosphate, the key structure of the ATP molecule responsible for interaction with P2X-purinoceptors

1. A radioligand binding assay was carried out to explore the key structure in molecules of ATP and its analogues responsible for the binding to P2x-purinoceptors. 2. It was found that adenosine, adenine and xanthine had no significant effect on [3H]alpha, beta-methylene ATP binding to membrane fractions prepared from rat urinary bladder, while pentasodium triphosphate and disodium pyrophosphate could effectively displace the binding. Sodium orthophosphate was shown to displace the binding only at much higher concentration. 3. Apart from ATP, several other nucleotides could also fully displace the specific binding, but with potencies lower than that of ATP. 4. The results indicate that the phosphate side chain in molecules of ATP and its analogues is the key structure responsible for the binding to P2x-purinoceptors.

Ap4A and ADP-beta-S binding to P2 purinoceptors present on rat brain synaptic terminals

1. Diadenosine tetraphosphate (Ap4A) a dinucleotide stored and released from rat brain synaptic terminals presents two types of affinity binding sites in synaptosomes. When [3H]-Ap4A was used for binding studies a Kd value of 0.10 +/- 0.014 nM and a Bmax value of 16.6 +/- 1.2 fmol mg-1 protein were obtained for the high affinity binding site from the Scatchard analysis. The second binding site, obtained by displacement studies, showed a Ki value of 0.57 +/- 0.09 microM. 2. Displacement of [3H]-Ap4A by non-labelled Ap4A and P2-purinoceptor ligands showed a displacement order of Ap4A > adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S) > 5'-adenylyl-imidodiphosphate (AMP-PNP) > alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP) in both sites revealed by the Ki values of 0.017 nM, 0.030 nM, 0.058 nM and 0.147 nM respectively for the high affinity binding site and values of 0.57 microM, 0.87 microM, 2.20 microM and 4.28 microM respectively for the second binding site. 3. Studies of the P2-purinoceptors present in synaptosomes were also performed with [35S]-ADP-beta-S. This radioligand showed two binding sites the first with Kd and Bmax values of 0.11 +/- 0.022 nM and 3.9 +/- 2.1 fmol mg-1 of protein respectively for the high affinity binding site obtained from the Scatchard plot. The second binding site showed a Ki of 0.018 +/- 0.0035 microM obtained from displacement curves. 4. Competition studies with diadenosine polyphosphates of [35S]-ADP-beta-S binding showed a displacement order of Ap4A > Ap5A > Ap6A in the high affinity binding site and Ki values of 0.023 nM, 0.081 nM and 5.72 nM respectively. The second binding site potency order was Ap5A> Ap4A > Ap6A,with Ki values of 0.28 microM, 0.53 microM and 5.32 microM respectively.5. Displacement studies of [35S]-ADP-beta-S with P2-purinoceptor agonists showed the following potency pattern: ADP-beta-S > AMP-PNP >alpha,beta-MeATP with Ki values of 0.021 nM, 0.029 nM 0.215 nM respectively in the high affinity binding site. 2-Methylthio-adenosine 5'-triphosphate (2MeSATP) was unable to displace [35S]-ADP-beta-S in this binding site. The second binding site showed a profile of ADP-beta-S> a,beta-MeATP> AMP-PNP > 2MeSATP and Ki values of 0.0 18 microM, 0.212 microM, 0.481 microM and 18.04 microM respectively.6. These studies suggest the presence of a new P2-purinoceptor in rat brain synaptosomes with high affinity for diadenosine polyphosphates which we tentatively designate as P2d.