Overview. Purinergic mechanisms

Inhibition of ADP-induced platelet responses by covalent modification of aggregin, a putative ADP receptor, by 8-(4-bromo-2,3-dioxobutylthio)ADP

ADP is an important platelet agonist which initiates platelet shape change, aggregation, exposure of fibrinogen receptors, and calcium mobilization. Because of the limitations of previously used affinity analogs and photo-labeling studies as well as controversies surrounding the identity of an ADP receptor on platelets, we have used an affinity label capable of alkylating a putative exofacial receptor on platelets. We now report that 8-(4-bromo-2,3-dioxobutylthio)adenosine-5'-diphosphate (8-BDB-TADP), which is an analog of the natural ligand ADP, blocked ADP-induced platelet shape change, aggregation, exposure of fibrinogen-binding sites, secretion, and calcium mobilization. Following modification by 8-BDB-TADP, the rates of aggregation of platelets induced by thrombin, a calcium ionophore (A23187) or a stimulator of protein kinase C (phorbol myristate acetate) were minimally affected. However, the 8-BDB-TADP-modified platelets exhibited decreased rates of aggregation in response to ADP, as well as collagen and a thromboxane mimetic (U46619), both of which partially require ADP. Autoradiograms of the gels obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized platelets modified by either [beta-32P]8-BDB-TADP, or 8-BDB-TADP and NaB[3H]4 showed the presence of a single radiolabeled protein band at 100 kDa. The intensity of this band was reduced when platelets were preincubated with ADP, ATP, and 8-bromo-ADP prior to labeling by the radioactive 8-BDB-TADP. The results show that 8-BDB-TADP selectively and covalently labeled aggregin (100 kDa), a putative ADP receptor, resulting in a loss of ADP-induced platelet responses.

GNAZ in human fetal cochlea: expression, localization, and potential role in inner ear function

Dissociation of an activated alpha-subunit from the beta-gamma complex directly regulates secondary messenger proteins. To address the potential role of G proteins expressed in human fetal cochlea, degenerate oligonucleotide primers corresponding to the 3'-end of the conserved region of alpha-subunits were used for polymerase chain reaction amplification of reverse-transcribed total human fetal cochlear mRNAs; GNAZ and GNAQ were isolated. These two G proteins are unique among the G-protein family because they lack a typical pertussis modification site. GNAZ is expressed in high levels in neural tissue while GNAQ is ubiquitously expressed. We characterized GNAZ expression using Northern blots, tissue in-situ hybridization and immunohistochemistry techniques to elucidate the potential role of this protein in inner ear function. Our data suggest that GNAZ may play a role in maintaining the ionic balance of perilymphatic and endolymphatic cochlear fluids.

Selective attenuation of neuropeptide-Y-mediated contractile responses in blood vessels from patients with diabetes mellitus

Vascular smooth muscle contractile responses to neuropeptide Y, alpha,beta-methyleneATP and noradrenaline were studied in circular segments of isolated vessels with intact endothelium in vitro from 12 patients with diabetes mellitus type 2 (NIDDM) and 12 control subjects. The dilatory effect of acetylcholine was used to test the function of the endothelium. Subcutaneous arteries and veins (diameter 0.1-1.1 mm) were obtained during surgery. There was no difference in contractile responses to noradrenaline or alpha,beta-methyleneATP between diabetic and control vessels. The contractile response to neuropeptide Y, however, was markedly reduced in the diabetic group. The maximal contractile effect (46.0 +/- 14.0%, p < 0.05) but not the sensitivity to neuropeptide Y was significantly less in diabetic veins compared to control (107.5 +/- 19.6%). Thus, the attenuation of neuropeptide Y responses was present in humans as previously observed in alloxan-induced diabetes mellitus in rabbits. There was no difference in the dilator effect of acetylcholine between the diabetic and the control group in any of the vessel types, indicating that the difference in vascular reactivity to neuropeptide Y was not endothelium-dependent. In conclusion, the present study has shown that the postjunctional effects of neuropeptide Y, a co-transmitter of the peripheral sympathetic nervous system, is selectively attenuated in diabetes mellitus.

Differential modulation of voltage-activated conductances by intracellular and extracellular cyclic nucleotides in leech salivary glands

1. Two-electrode voltage clamp was used to study the effects of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) on voltage-dependent ion channels in salivary gland cells of the leech, Haementeria ghilianii. 2. Intracellular cyclic AMP specifically blocked delayed rectifier K+ channels. This was shown by use of 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor), forskolin (an activator of adenylyl cyclase) and intracellular injection of cyclic AMP and its dibutyryl and 8-bromo analogues. Cyclic AMP appeared to be the second messenger for the putative neuroglandular transmitter, 5-hydroxytryptamine. 3. Intracellular injection of cyclic GMP specifically potentiated high-voltage-activated (HVA) Ca2+ current and the effect was mimicked by zaprinast, an inhibitor of cyclic GMP-dependent phosphodiesterase. 4. Extracellularly, cyclic GMP and cyclic AMP specifically decreased the amplitude and increased the rate of inactivation of HVA Ca2+ current. These effects of the cyclic nucleotides are identical to those known for extracellular ATP, which activates a presumed purinoceptor. The pyrimidine nucleotide, UTP, was almost equipotent to ATP (threshold dose < 10(-6) M), indicative of a vertebrate-type nucleotide receptor. However, suramin (5 x 10(-5) M), a non-specific P2-receptor antagonist, failed to block the effects of 5 x 10(-6) M ATP (higher suramin doses could not be reliably tested because of the depolarization and increase in membrane conductance produced by the drug). 5. Activation of the putative purinoceptor by ATP did not affect inward rectifier Na+/K+ current which is known to be potentiated by intracellular cyclic AMP and reduced by intracellular cyclic GMP. 6. The preparation may provide a useful model for study of nucleotide actions, and interactions, in channel modulation. It has technical advantages such as large cells (1200 microns in diameter) which lack intercellular coupling and may be individually dissected for biochemical studies.

Ca2+ responses to acetylcholine and adenosine triphosphate in the otocyst of chick embryo

The action of acetylcholine and adenosine triphosphate (ATP) on cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in the otocyst epithelium of embryonic day 3 chicks with Ca(2+)-sensitive fluorescence measurements. Increases in [Ca2+]i were evoked by the bath application of acetylcholine (1 microM or higher). The rise in [Ca2+]i was due to the release of Ca2+ from intracellular Ca2+ stores, since the Ca2+ response occurred even in a Ca(2+)-free medium. The Ca2+ response to acetylcholine was mediated by muscarinic receptors. Atropine of 1 microM abolished the response to 10 microM acetylcholine; muscarine and carbamylcholine (100 microM each) evoked Ca2+ rises. Increases in [Ca2+]i were also evoked by the bath application of ATP (10 microM or higher). The Ca2+ rise by ATP was evoked even in a Ca(2+)-free medium. Adenosine (500 microM) did not cause any Ca2+ response. Suramin and reactive blue 2 (200 microM each) completely blocked the Ca2+ response to 500 microM ATP. Uridine triphosphate (500 microM) caused comparable Ca2+ responses with those to 500 microM ATP. These results suggested the involvement of P2U purinoceptors. The potentiation of Ca2+ rise was observed when acetylcholine and ATP were co-applied at submaximal concentrations (10 microM and 100 microM, respectively). We conclude that undifferentiated cells in the otocyst epithelium have Ca2+ mobilizing systems activated by acetylcholine and ATP.

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

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

Extracellular ATP inhibits agonist-induced mobilization of internal calcium in human platelets

Our previous studies have demonstrated that platelets possess ATP purinergic receptors in addition to the ADP, P2T, receptor. Occupancy of the P2 receptor by ATP inhibited agonist-induced platelet aggregation. This study demonstrated that the mechanism of inhibition may involve ATP inhibition of agonist-induced mobilization of internal calcium. Within the cardiovascular system, the ATP inhibition of calcium mobilization is unique to platelets. All other cell types in the cardiovascular system, where calcium mobilization is affected by extracellular ATP, responded with an increased mobilization as opposed to inhibition. The platelet inhibitory response to ATP was enhanced by the addition of an ATP generating system, creatine phosphate/phosphocreatine kinase. ATP and ATP analogues were found to inhibit calcium mobilization with a rank order of alpha beta-methylene ATP, beta gamma-methylene ATP approximately ATP > benzoyl ATP > 2 methylthio ATP which is a characteristic of P2x-like receptors. The inhibitory effect of ATP could be abrogated by prolonged treatment of platelets with the P2x desensitizing agent, alpha beta-methylene ATP. Also, UTP and CTP were approximately as effective inhibitors as ATP while GTP was not. ATP competition with ADP for the P2T receptor was excluded in studies with platelets derived from an aspirin-treated individual which were essentially insensitive to ADP. The agonist-induced calcium mobilization and inhibition by ATP occurred with the thromboxane A2 mimetic, U46619, collagen and thrombin; however, the kinetics of mobilization varied somewhat with the different agonists. The responses to extracellular ATP were independent of extracellular Ca2+, where 1 mM calcium or 0.3 mM EGTA was added to the reaction mixture. The inhibition of calcium mobilization coupled to inhibition of platelet aggregation by extracellular ATP may serve an important physiologic role. ATP, released from activated platelets at localized sites of vascular injury, may help to limit the size of the platelet plug-clot that, if left unregulated, could occlude the injured blood vessel.

Functional characterization of the adenosine receptor mediating inhibition of peristalsis in the rat jejunum

1. The non-selective adenosine agonist, 5'-N-ethylcarboxamidoadenosine (NECA), is a potent inhibitor of morphine withdrawal diarrhoea in rats. More recently we found that NECA exerts its antidiarrhoeal effect by inhibiting secretion in both the jejunum and ileum and also by inhibiting peristalsis in the ileum. The specific aim of this study was to characterize the receptor in the rat jejunum mediating inhibition of peristalsis via functional studies using a range of metabolically stable adenosine analogues based on the pharmacological criteria of relative agonist and antagonist potencies. 2. Peristalsis in the rat isolated jejunum was achieved by raising the pressure to between 7-11 cmH2O for 3 min followed by a 3 min rest period (pressure at zero). The mean rate of peristalsis during inflation was 7.3 +/- 0.1 peristaltic waves per 3 min and this rate remained consistent for up to 30 min, in 5 separate tissues. The inhibitory effects of the adenosine analogues were quantified by expressing their effects as a % reduction in the mean number of peristaltic contractions derived from the control tissues. 3. The rank order of agonist potency to reduce the rate of peristalsis was: N6-cyclopentyladenosine (CPA) > NECA > R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) > chloroadenosine (2-CADO) > S-PIA > 2-phenylaminoadenosine (CV-1808). This order complies well with the rank order of agonist potency that represents the activation of the A1 receptor subtype (CPA > R-PIA = CHA = > NECA > 2-CADO > S-PIA > CV-1808). 4. The selective A1 adenosine antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) and the nonselective adenosine antagonist 8-phenyltheophylline (8-PT) at their respective concentrations of 10 nM and 2 microM caused parallel rightward shifts in the concentration-response curve to the non-selective Al/A2 agonist NECA. DPCPX was significantly more potent at inhibiting NECA than 8-PT as revealed by their apparent pA2 values; DPCPX (9.5) and 8-PT (7.26). The high affinity of DPCPX relative to that of 8-PT suggests the presence of an Al and not an A2B receptor. In addition, the high affinity of DPCPX(pA2:9.37) against the selective Al agonist CPA, further confirms the presence of the Al receptor subtype.5. In this study we found that the Al adenosine receptor is involved in regulating in vitro peristalsis which is different from the adenosine receptor regulating inhibition of secretion (A2B) in the same region of intestine of the same species. We propose that A2B adenosine agonists could be of clinical value in the management of diarrhoea that is due to microbiological organisms where antimotility effects are not desired.

Modulation of potassium currents in cultured murine microglial cells by receptor activation and intracellular pathways

The electrophysiological properties of ameboid microglia from rodent brain are dominated by inwardly rectifying potassium channels and by the lack of outward currents. This channel pattern results in a distinct physiological behavior: depolarizing events, e.g. following adenosine triphosphate receptor activation, can lead to a long lasting membrane depolarization. Here we address the question whether this resting K+ channel activity can be modulated. Intracellular application of guanosine 5'-O-(3-thiotriphosphate) induced an outward current and led to a complete disappearance of the inward current inward rectifier potassium current as measured with the patch clamp technique. Moreover, an elevation in cytosolic calcium concentration (to 1.6 microM) via intracellular perfusion reversibly blocked the inward current. The inhibition of inward currents by guanosine 5'-O-(3-thiotriphosphate) could be enhanced by additional adenosine triphosphate receptor activation. Adenosine triphosphate or tumor necrosis factor receptor activation alone could lead to a transient partial block of the inward rectifier and to the transient appearance of a delayed outward current. We conclude that the activity of the microglia K+ channels and thus the physiological behavior of microglia can be modulated on a time scale of seconds by receptor activation and distinct intracellular pathways.

ATP acts as fast neurotransmitter in rat habenula: neurochemical and enzymecytochemical evidence

The release of ATP and ADP, the putative central neurotransmitters, from the isolated habenula preparation was investigated in the rat, at rest and during electrical stimulation, using the luciferin-luciferase assay and the creatine phosphokinase assay. Electrical field stimulation (2 Hz, 360 pulses) released a considerable amount of ATP (2450 +/- 280 pmol/g wet tissue) from the tissue; inhibition of the voltage Na+ entry by tetrodotoxin (1 microM) reduced significantly the evoked release (by 66.25 +/- 6.65%), but not the resting release of ATP. Endogenous ADP also appeared in the effluent, but its amount differed during resting condition and after stimulation from that of ATP, suggesting that the majority of the released compound is ATP in response to stimulation. When ATP was added to the tissue, it readily decomposed to ADP and AMP (Km = 811.6 +/- 68.88 microM, vmax = 23.1 +/- 2.75 nmol/min per prep., measured by high-performance liquid chromatography combined with ultraviolet detection), indicating that the habenula contains ectoATPases. In addition, the inactivation of extracellular ATP by the ectoATPase enzyme was also visualized by electron microscopic enzyme cytochemistry. The ectoATPase enzyme was present on the membranes of the dendrites and nerve terminals and in the synapses of the habenula. Taking into account the fact that ATP is ubiquitous in excitable cells (storage) and the findings published by Edwards et al. in 1992 ("ATP receptor-mediated synaptic currents in the central nervous system", Nature, Vol. 359, pp. 144-147), our data provides evidence for the release by axonal stimulation and extracellular decomposition of ATP, all needed for an endogenous substance qualified as a transmitter.

A study on P2X purinoceptors mediating the electrophysiological and contractile effects of purine nucleotides in rat vas deferens

1. We have studied both the electrophysiological and contractile effects of the purine nucleotide, adenosine-5'-triphosphate (ATP), as well as a number of its structural analogues as agonists at P2X purinoceptors in the rat vas deferens in vitro. 2. Electrophysiological effects were investigated by a whole cell voltage clamp technique (holding potential-70 mV) with fast flow concentration-clamp applications of agonists in single isolated smooth muscle cells. ATP, 2-methylthio adenosine-5'-triphosphate (2-MeSATP) and alpha,beta methylene adenosine-5'-triphosphate (alpha,beta-meATP) all evoked inward currents over a similar concentration range (0.3-10 microM), being approximately equipotent with similar concentrations for threshold effects (0.3 microM). ADP (10 microM) also evoked a rapid current of similar peak amplitude to that seen with ATP (10 microM). 3. alpha,beta-meATP was the most potent agonist in producing concentrations of the rat vas deferens whole tissue preparation, with a threshold concentration equal to that in the electrophysiological studies (0.3 microM). However, ATP and 2-MeSATP were at least ten times less potent in studies measuring contraction than in the electrophysiological studies. Furthermore, their concentration-effect curves were shallow with smaller maximal responses than could be achieved with alpha,beta-meATP. ADP, AMP and adenosine were inactive at concentrations up to 1 mM. The rank order of agonist potencies observed for contraction was alpha,beta-meATP >> ATP = 2-MeSATP. 4. Measurement of inorganic phosphate (iP), as a marker of purine nucleotide metabolism in the vasdeferens whole tissue preparation, indicated that ATP and 2-MeSATP were rapidly metabolized,whereas alpha,beta-meATP was stable for up to 2 h. Removal of divalent cations prevented breakdown of ATP and 2-MeSATP, suggesting that metabolism involved a Ca2+/Mg2+-dependent enzyme.5. It appears that in isolated preparations of rat vas deferens, the low potency of ATP and 2-MeSATP can be explained by rapid agonist breakdown by ectonucleotidases. However, this is not the case in the single cell studies where the use of rapid concentration-clamp applications revealed the true potency of the agonists. Under such conditions the three agonists were all equal in potency indicating that the rank order of agonist potencies of alpha,beta-meATP>> ATP = 2-MeSATP is not in fact characteristic of smooth muscle P2x-purinoceptors as commonly believed.

Electrophysiological properties of P2X-purinoceptors in rat superior cervical, nodose and guinea-pig coeliac neurones

1. Whole-cell recordings were made from guinea-pig coeliac, rat nodose and rat superior cervical (SCG) neurones in culture, and currents in response to fast-flow (concentration clamp) application of ATP and other ATP analogues were measured. 2. At a holding potential of -70 mV, ATP evoked inward currents in all neurons. ATP-induced currents reversed at approximately 0 mV and showed strong inward rectification. Halving the external sodium concentration shifted the reversal potential by -15 to -17 mV, while increasing external potassium from 2 to 20 mM produced a 6-10 mV shift in reversal potential. Latency to onset of ATP current was < 1 ms; rise time was concentration dependent with maximum time to peak of 5-20 ms in nodose and coeliac neurones but 80 ms in superior cervical neurones. 3. Threshold concentrations of ATP were 0.1 microM for nodose and coeliac neurones but 10 microM for superior cervical neurones; EC50 values were approximately 3 microM for both nodose and coeliac neurones and 43 microM for superior cervical ganglia. Hill slopes for ATP concentration-response curves were not significantly different from unity in nodose and coeliac neurones whereas the Hill slope in superior cervical neurones was two. 4. 2-MethylthioATP (2-MeSATP) acted as a full agonist in all three neuronal preparations; EC50 values were 0.4, 2.8 and 46 microM for nodose, coeliac and superior cervical neurones, respectively. alpha,beta-Methylene ATP (alpha,beta-MeATP) was a full agonist in nodose and coeliac neurones with EC50 values of 9 and 13 microM, respectively. 5. In superior cervical neurones alpha,beta-MeATP had little or no agonist action but produced a concentration-dependent attenuation of the ATP current. Thus, alpha, beta-MeATP appears to behave as a partial agonist at P2X-purinoceptors in superior cervical neurones. 6. The non-selective purinoceptor antagonists suramin (1-100 microM) and pyridoxal-5'-phosphate (30 microM), as well as the putative P2Y-selective antagonist, Cibacron Blue (30 microM), inhibited all agonist-evoked responses to a similar degree in all three neuronal populations. 7. This study demonstrates that an agonist potency profile of 2-MeSATP > or = ATP > or = alpha,beta-MeATP is characteristic of ligand-gated P2X-purinoceptors in isolated peripheral neurones. We also suggest that the P2X-receptor in superior cervical neurones may represent a distinct subtype of P2X-purinoceptor from that present in nodose and coeliac neurones.

The effects of suramin on purinergic and noradrenergic neurotransmission in the rat isolated tail artery

Intracellular microelectrode recording was used to examine the effects of suramin, a P2-purinoceptor antagonist, on the electrical responses evoked by sympathetic nerve stimulation in the rat isolated tail artery. Field stimulation (10 or 20 pulses at 0.5, 1 and 2 Hz) evoked a biphasic electrical response, consisting of fast, transient excitatory junctional potentials (e.j.p.s) and a slow, prolonged depolarisation. Suramin (100 microM) abolished the e.j.p.s and significantly increased the amplitude of the slow depolarisation at all frequencies. In contrast, phentolamine (2 microM) abolished the slow depolarisation, but had no effect on the magnitude of e.j.p.s. Neither drug altered the resting membrane potential of cells. The ability of suramin to inhibit e.j.p.s in rat tail artery is consistent with the proposal that it is a P2X-purinoceptor antagonist and supports a role for ATP as an excitatory cotransmitter from the sympathetic nerves innervating this tissue. Suramin is also able to increase the alpha-adrenoceptor-mediated slow depolarisation by an unknown mechanism.

Serum fractions and related agonists with calcium-mobilizing activity in the bovine growth plate chondrocyte

Longitudinal growth of bone involves a complex sequence of cellular events in the cartilaginous epiphysis. Whole blood serum has been shown previously to be a potent stimulus to the cells of the growth plate, as demonstrated by its ability to activate the inositol phosphate-calcium second messenger system, resulting in a rise in intracellular Ca2+. By manipulating the preparation of serum to functionally separate it into its constituent parts, we have shown that the processes of platelet lysis and activation of the clotting cascade are responsible for the generation of factors that stimulate this signaling mechanism in isolated bovine growth plate chondrocytes. Through a subsequent trial of bioactive agents generated in these processes, we identified and partially characterized several novel agonists of growth plate chondrocytes:adenosine triphosphate and adenosine diphosphate, the purine energy substrates, and bradykinin, the bioactive peptide generated in a side reaction of the clotting cascade, each induces a rise in intracellular Ca2+ via release from intracellular ion stores. Additionally, the three distinct isoforms of platelet-derived growth factor (AA, AB, and BB), also released on platelet lysis, were compared with respect to their ability to stimulate the inositol phosphate-calcium second messenger system in growth plate chondrocytes.

Activation of P2-purinoreceptors triggered Ca2+ release from InsP3-sensitive internal stores in mammalian oligodendrocytes

1. The subcellular characteristics of an ATP-induced elevation of the cytoplasmic free calcium concentration ([Ca2+]i) were studied in cultured cells of the oligodendrocyte lineage obtained from mouse cortex and rabbit retina, as well as in oligodendrocytes from mouse corpus callosum slices, using laser scanning confocal microfluorometry. 2. With the stage- and lineage-specific antibodies O4 and O10, three developmental stages within the oligodendrocyte lineage were distinguished prior to Ca2+ recording. 3. Bath application of 1-100 microM ATP induced a transient increase of [Ca2+]i in late precursors and oligodendrocytes but not in early glial precursor cells from retinal and cortical cultures and from corpus callosum slices. This effect of ATP was observed in Ca(2+)-free extracellular solution, suggesting that the ATP-mediated elevation of [Ca2+]i is due to a Ca2+ liberation from intracellular stores. 4. In both late precursors and oligodendrocytes from retina, the amplitude of ATP-induced [Ca2+]i transients was significantly higher in processes as compared with the soma; in cortical cultures such an uneven response was only observed in oligodendrocytes, while in immature cells responses in soma and processes were of similar amplitude. 5. The rank order of potency for the purine and pyrimidine nucleotides was UTP > or = ATP > ADP >> AMP = adenosine = Me-ATP for retinal oligodendrocytes, and ADP > or = ATP >> UTP = AMP = adenosine = Me-ATP for cortical oligodendrocytes. The response to ATP and related nucleotides was blocked by suramin, indicating the involvement of a P2-purinoreceptor in the ATP-mediated [Ca2+]i response. 6. ATP-induced elevation of the cytosolic Ca2+ concentration was inhibited by incubating cells with thapsigargin (10 microM) and by intracellular administration of heparin (1 microM). These findings indicate that ATP triggers a release of Ca2+ ions from InsP3-sensitive internal stores. 7. The ATP receptors may play a role in neuron-glial signal transfer; ATP is released as neurotransmitter, but also under pathological conditions from damaged cells.

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.

Capacitative Ca2+ entry exclusively inhibits cAMP synthesis in C6-2B glioma cells. Evidence that physiologically evoked Ca2+ entry regulates Ca(2+)-inhibitable adenylyl cyclase in non-excitable cells

Elevation of cytosolic free Ca2+ inhibits the type VI adenylyl cyclase that predominates in C6-2B cells. However, it is not known whether there is any selective requirement for Ca2+ entry or release for inhibition of cAMP accumulation to occur. In the present study, the effectiveness of intracellular Ca2+ release evoked by three independent methods (thapsigargin, ionomycin, and UTP) was compared with the capacitative Ca2+ entry that was triggered by these treatments. In each situation, only Ca2+ entry could inhibit cAMP accumulation (La3+ ions blocked the effect); Ca2+ release, which was substantial in some cases, was without effect. A moderate inhibition, as was elicited by a modest degree of Ca2+ entry, could be rendered substantial in the absence of phosphodiesterase inhibitors. Such conditions more closely mimic the physiological situation of normal cells. These results are particularly significant, in demonstrating not only that Ca2+ entry mediates the inhibitory effects of Ca2+ on cAMP accumulation, but also that diffuse elevations in [Ca2+]i are ineffective in modulating cAMP synthesis. This property suggests that, as with certain Ca(2+)-sensitive ion channels, Ca(2+)-sensitive adenylyl cyclases may be functionally colocalized with Ca2+ entry channels.

Responses of the aorta of the garter snake (Thamnophis sirtalis parietalis) to purines

1. Isolated aortic rings from the garter snake (Thamnophis sirtalis parietalis) were investigated in order to identify and classify responses to adenosine and adenosine 5'-triphosphate (ATP) and their analogues as part of a comparative study of vertebrate purinoceptors. 2. Adenosine, D-5'-(N-ethylcarboxamide) adenosine (NECA), R- and S-N6-(2-phenylisopropyl) adenosine (R- and S-PIA) and 2-chloroadenosine (2-CA) all concentration-dependently relaxed aorta preconstricted with noradrenaline (NA). The order of potency was: NECA > R-PIA = 2-CA > adenosine > S-PIA. Individual pD2 values for the analogues were: NECA 7.12 +/- 0.13 (9), R-PIA 5.93 +/- 0.25 (7), 2-CA 5.64 +/- 0.40 (5), adenosine 5.04 +/- 0.10 (13) and S-PIA 4.26 +/- 0.10 (7). The order of potency has characteristics of both A1 and A2 receptors and cannot satisfactorily be classified according to the P1-(adenosine) purinoceptor subtypes established in mammalian preparations. 3. ATP, alpha, beta-methylene ATP (alpha, beta-MeATP), 2-methylthio ATP (2MeSATP), beta, gamma-methylene ATP (beta, gamma,-MeATP) and uridine 5'-triphosphate (UTP) all concentration-dependently constricted the isolated aorta. The order of potency was alpha, beta-MeATP = 2MeSATP > ATP > beta, gamma-MeATP > UTP. Only ATP, alpha, beta-MeATP and 2MeSATP consistently produced a maximum response; pD2 values were: ATP 3.98 +/- 0.07 (10), alpha, beta-MeATP 5.86 +/- 0.15 (12) and 2MeSATP 6.06 +/- 0.23 (9). In vessels preconstricted with NA neither ATP nor 2MeSATP caused relaxation in the presence or absence of the endothelium. 4. Suramin (0.1 mM) inhibited vasoconstriction to ATP, alpha,beta-MeATP, 2MeSATP and beta,upsilon-MeATP;however, since contractions to ATP and analogues did not reach a maximum response in the presence of this and other antagonists, pD2 values could not be calculated.5. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 30 micro M), a P2x-purinoceptor antagonist,antagonized constrictions to alpha, beta-MeATP only. Reactive blue 2 (RB2; 30 micro M), a P2Y-purinoceptor antagonist, inhibited vasoconstrictions to 2MeSATP only.6. Indomethacin (30 micro M) inhibited vasoconstriction in response to ATP and 2MeSATP, but not alpha,beta,-MeATP, suggesting that the presence of an unaltered phosphate chain on the ATP analogue was necessary to stimulate the production of a prostanoid.7. Repeated administration of alpha,beta-MeATP (3 microM) caused desensitization of the receptor responsible for the constriction due to alpha,beta-MeATP whereas the responses to ATP and 2MeSATP were unaltered.8. In summary, both P1-purinoceptors mediating vasodilatation and P2-purinoceptors mediating vasoconstriction are present on the garter snake aorta. However, in contrast to mammalian vessels, bothP2x and P2Y subtypes mediate vasoconstriction. There was no evidence for vasodilatation to ATP or analogues. Stimulation of the P2-purinoceptor by ATP and 2MeSATP caused the synthesis of a prostanoid. In addition, the possibility of a receptor activated by ATP, separate from P2X and P2Y subtypes is discussed since contractions to ATP proved to be insensitive to both PPADS and RB2. A comparison is made of purinoceptors in the garter snake aorta with those in other vertebrate vessels.

Activation of nucleotide receptors inhibits M-type K current [IK(M)] in neuroblastoma x glioma hybrid cells

A phospholipase-C-linked nucleotide receptor, sensitive to both uridine and adenosine triphosphate (UTP and ATP) has been cloned from NG108-15 neuroblastoma x glioma hybrid cells. We have tested whether activation of this receptor could inhibit the voltage-dependent K+ current [IK(M) or "M-current"] in NG108-15 cells recorded using whole-cell patch-clamp methods. Both UTP and ATP inhibited IK(M) by 44% and 42%, respectively, at 100 microM. Mean IC50 values were: UTP, 0.77 +/- 0.27 microM; ATP, 1.81 +/- 0.82 microM. The order of nucleotide and nucleoside activity at 100 microM was: UTP = ATP > ATP [gamma S] = ITP > 2-MeSATP > ADP = GTP >> AMP-CPP, adenosine, where ATP[gamma S] is adenosine 5'-O-(3-thiotriphosphate), ITP is inosine 5'-triphosphate, 2-MeSATP is 2-methylthio ATP and AMP-CPP is alpha, beta methylene ATP. This rank order accords with their activities at the cloned P2U receptor. Effects were not inhibited by suramin (up to 500 microM) or by pre-incubation for 12 h in 500 ng.ml-1 Pertussis toxin. Inhibition of IK(M) was frequently preceded by a transient outward current, probably a Ca(2+)-activated K+ current, responding to Ca2+ mobilization. No effect on the delayed rectifier K+ current was observed. These observations match those expected from stimulating other phospholipase-C-linked receptors in NG108-15 cells.

New insights into the local regulation of blood flow by perivascular nerves and endothelium

Blood flow, particularly in the skin, is essential for the success of plastic surgical operations. This review describes recent studies of the perivascular nerves and vascular endothelial cells which regulate blood flow. Perivascular nerves, once considered simply adrenergic or cholinergic, release many types of neurotransmitters, including peptides, purines and nitric oxide. Cotransmission (synthesis, storage and release of more than one transmitter by a single nerve) commonly takes place. Some afferent nerves have an efferent (motor) function and axon reflex control of vascular tone by these "sensory-motor" nerves is more widespread than once thought. Endothelial cells mediate both vasodilatation and vasoconstriction. The endothelial cells can store and release vasoactive substances such as acetylcholine (vasodilator) and endothelin (vasoconstrictor). The origins and functions of such vasoactive substances are discussed. Endothelial vasoactive substances may be of greater significance in the response of blood vessels to local changes while perivascular nerves may be concerned with integration of blood flow in the whole organism. The dual regulation of vascular tone by perivascular nerves and endothelial cells is altered by aging and conditions such as hypertension, as well as by trauma and surgery. Studies of vascular tone in disease and after denervation or mechanical injury suggest possible trophic interactions between perivascular nerves and endothelial cells. Such trophic interactions may be important for growth and development of the two control systems, particularly in the microvasculature where neural-endothelial separation is small.

Characterization of P2-purinoceptors in the smooth muscle of the rat tail artery: a comparison between contractile and electrophysiological responses

1. The electrophysiological actions of the P2-purinoceptor agonists, adenosine 5'-triphosphate (ATP), 2-methylthioATP (2-meSATP), alpha, beta-methyleneATP (alpha, beta-meATP) and uridine 5'-triphosphate (UTP) were studied under concentration and voltage-clamp conditions in acutely dissociated rat tail artery smooth muscle cells. For comparison, their actions as vasoconstrictors were studied in intact ring preparations. 2. Rapid application of ATP (100 nM-1 microM) via a U-tube superfusion system activated concentration-dependent inward currents with a latency to onset of less than 3 ms. The inward current decayed by more than 95% during a 2 s application of 300 nM and 1 microM ATP. 3. 2-meSATP (100 mM-1 microM) and alpha, beta-meATP (100 nM-1 microM) also evoked transient inward currents. The agonist order of potency was ATP = 2-meSATP > or = alpha, beta-meATP. UTP (300 nM-1 microM) did not produce a change in the holding current. 4. A second application of ATP (300 nM and 1 microM) 10 min after the first, evoked currents which were one third of the initial amplitude. This decline was dependent upon activation of the P2-purinoceptor. Similar results were seen with 2-meSATP and alpha, beta-meATP (both 300 nM and 1 microM). Cross-desensitization was seen between ATP and 2-meSATP or alpha, beta-meATP. 5. Inward currents evoked by ATP, 2-meSATP and alpha, beta-meATP (all 1 microM) were abolished by the P2-purinoceptor antagonist suramin (100 microM). 6. Alpha, beta-meATP (100 nM-30 micro M), 2-meSATP (3 micro M- 100 micro M), ATP (3 micro M-I mM) and UTP (3 ELM-I mM)produced concentration-dependent contractions of rat tail artery rings. When measured at a level equal to 50% of the maximum response to noradrenaline, the rank order of agonist potency was alpha,beta-meATP>>2-meSATP >UTP >ATP.7. This study shows that the rank order of agonist potency at the P2X-purinoceptor which mediates contractions of the rat isolated tail artery is very different from the potency order for evoking the inward current which initiates the contractions. It is concluded that this difference may be due to the relative absence of breakdown of some of the agonists in the single cell system compared with artery rings.

New structural motif for ligand-gated ion channels defined by an ionotropic ATP receptor

The adenosine-5'-triphosphate (ATP) molecule is an extracellular messenger in neural and non-neural tissues, where it activates several cell-surface-receptor subtypes, including G-protein-coupled receptors and ligand-gated ion channels. ATP-gated channels (termed P2x receptors) have been characterized on smooth muscle cells and autonomic and sensory neurons, where they mediate membrane depolarization and, in some cases, Ca2+ entry. P2x receptors are functionally heterogeneous, but resemble acetylcholine- and serotonin-gated channels with respect to ion selectivity and kinetic parameters of channel gating. We report here that despite such close functional similarities, the deduced sequence of a cloned P2x receptor predicts an unusual subunit structure resembling voltage-insensitive cation channels. Thus, the P2x receptor provides a striking example of convergent evolution, whereby proteins have been fashioned with similar functional properties from subunits having very different structural characteristics. There is sequence similarity between the ATP receptor and RP-2, a gene activated in thymocytes undergoing programmed cell death. RP-2 may encode a receptor for ATP or another metabolite released during apoptosis.

A new class of ligand-gated ion channel defined by P2x receptor for extracellular ATP

Extracellular ATP exerts its effects through P2 purinoceptors: these are ligand-gated ion channels (P2x) or G-protein-coupled receptors (P2Y, P2U). ATP at P2x receptors mediates synaptic transmission between neurons and from neurons to smooth muscle, being responsible, for example, for sympathetic vasoconstriction in small arteries and arterioles. We have now cloned a complementary DNA encoding the P2x receptor from rat vas deferens and expressed it in Xenopus oocytes and mammalian cells. ATP activates a cation-selective ion channel with relatively high calcium permeability. Structural predictions suggest that the protein (399 amino acids long) is mostly extracellular and contains only two transmembrane domains plus a pore-forming motif which resembles that of potassium channels. The P2x receptor thus defines a new family of ligand-gated ion channels.

Effects of divalent cations on the potency of ATP and related agonists in the rat isolated vagus nerve: implications for P2 purinoceptor classification

1. By use of a 'grease-gap' technique, the depolarizing effects of adenosine 5'-triphosphate (ATP) and ATP analogues on the rat isolated vagus nerve were determined in normal and in Ca2+/Mg(2+)-free (+ 1 x 10(-3) M ethylenediamine tetraacetic acid) physiological salt solution (PSS). 2. In normal PSS, ATP produced concentration-dependent depolarization responses but the concentration-effect curve to ATP was incomplete and a maximum effect was not achieved. The threshold concentration for depolarization was 1 x 10(-5) M and at the highest concentration tested (1 x 10(-3) M) the peak amplitude of the response to ATP only amounted to 71% of the depolarization produced by a near maximal response to 5-hydroxytryptamine (5-HT, 1 x 10(-5) M). 3. In Ca2+/Mg(2+)-free PSS, ATP produced depolarization responses at much lower concentrations and of markedly larger amplitude. Under these conditions, the threshold concentration for depolarization was 1-3 x 10(-7) M and the maximal response to ATP amounted to 526% of the response to 5-HT (1 x 10(-5) M) in normal PSS. The concentration-effect curve to ATP was sigmoid, with a defined maximum effect and a mean EC50 value of 1.2 x 10(-6) M. 4. In contrast to the effects on responses to ATP, the absence of divalent cations in the PSS did not modify the effective concentrations of either alpha, beta-methylene ATP or 5-HT. However, the maximum responses to both alpha, beta-methylene ATP and 5-HT were significantly increased in Ca2+/Mg(2+)-free PSS. 5. The depolarizing effects of several analogues of ATP were determined in Ca2+/Mg2+-free PSS.ATP-gamma-S and 2-methylthioATP were of similar potency to ATP (respective equi-effective molar ratios(EMRs) of 1.9 and 1.3, where ATP = 1) and similar maximum responses were obtained. Alpha, beta-MethyleneATP, beta, gamma-methylene ATP and ,beta, gamma-imido ATP were considerably less potent than ATP, analysis yielding mean EMRs of 48.9, 85.0 and 60.0, respectively. Maximum responses to these latter three agonists were not obtained at the highest concentrations tested (1 x 10-4-3 X 10- M). Benzoyl ATP, adenosine 5'-0-(2-thiodiphosphate) and adenosine diphosphate produced only small depolarizing responses at high concentrations (>1 x 10-4 M). Adenosine monophosphate, adenosine and uridine S'-triphosphate each had little or no depolarizing effect in Ca2+/Mg2+-free PSS.6. These data demonstrate that in the absence of divalent cations the excitatory actions of some, but not all, purine nucleotides in the rat vagus nerve are markedly potentiated. In Ca2+/Mg2+-free PSS, the rank order of agonist potencies was ATP = 2-methylthioATP = ATP-gamma-S>> alpha,beta-methylene ATP = beta, gamma imido ATP = P,y-methylene ATP. These findings are in stark contrast to our previous observations in normal PSS where the rank order of agonist potencies for these nucleotides was alpha,beta-methyleneATP> ATP-gamma-S > beta,gamma-imido ATP = beta,gamma-methylene ATP> 2-methylthioATP> ATP.7. We suggest that the two different rank orders of potency can be explained by differential metabolism involving Ca2+/Mg2+-dependent ectonucleotidases. If so, these data indicate that ATP and 2-methylthioATP are inherently more potent than alpha,beta-methylene ATP as agonists at neuronal P2X purinoceptors in the rat vagus nerve. The possible implications of these findings to the present system for subclassifying P2 purinoceptors are profound.

Influence of ATP and ATP agonists on the physiology of the isolated semicircular canal of the frog (Rana pipiens)

In the present study, the influence of extracellular ATP and ATP agonists in the physiology of the vestibular organs was examined, using the in vitro model of the isolated semicircular canal of the frog (Rana pipiens). The firing activity of the afferent nerve, the d.c. nerve potential and the transepithelial potential were measured in the absence and presence of mechanical stimulation of the sensory epithelium. Administration of ATP into the perilymphatic compartment, from 10(-12) to 10(-3) M, increased the firing rate of the afferent fibers recorded in the absence of mechanical stimulation. Recordings of the d.c. nerve potential indicated that the afferent fibers were hyperpolarized. The presence of the purine also modified the transepithelial potential. During mechanical stimulation of the sensory epithelium, both the evoked afferent firing and the evoked variation of the d.c. nerve potential were reduced in the presence of ATP. However, ATP did not effect the evoked modulation of the transepithelial potential, evoked by the mechanical stimulation. Administration of the P2x purinoceptor agonists, alpha, beta-methylene-ATP and beta, gamma-methylene-ATP, at concentrations between 10(-12) and 10(-3) M, did not significantly modify the different bioelectrical activities investigated. In contrast, 2-methylthio-ATP, a P2y purinoceptor agonist, more potent and efficacious than ATP in its effect on the spontaneous firing. Concurrently, no modification of the d.c. nerve potential, the transepithelial potential and their variation during mechanical stimulation was observed. In opposition to the ATP effect, the total amplitude of the evoked firing was increased in the presence of 2-methylthio-ATP. These data suggest that extracellular ATP, present in the perilymphatic compartment, may act as a neuromodulator in the vestibular physiology. The effects of the purine appear to be mediated by the activation of a P2y subtype of purinoceptor. The absence of an effect of ATP and 2-methylthio-ATP on the evoked variation of the transepithelial potential suggest that the purine did not affect the processes responsible for the generation of the receptor potential but more likely modified the mechanisms involved in the release of the neurotransmitter from the hair cells and/or acted on the afferent endings.

Effect of extracellular ATP on breast tumor cell growth, implication of intracellular calcium

We studied the effects of purine nucleotides and particularly adenosine triphosphate (ATP) in two (one hormonosensitive, MCF7 and one hormonoinsensitive, MDA-MB 231) human breast tumor cell lines. As described in other cells, we observed that purine nucleotides produced transient elevations in intracellular calcium ions, [Ca2+]i, in both types of cells as determined from Indo-1 fluorescence of loaded cells. In the absence of external calcium the [Ca2+]i transients consisted of single narrow peaks while an extension of peak duration along with a biphasic appearance were observed in the presence of external calcium. The potency of different purine nucleotides in elevating [Ca2+]i was ATP > ADP >> AMP > adenosine (which was inefficient) proving the presence of P2 purinergic receptor subtypes. Suramin, a compound known to compete with ATP for its binding sites, nearly abolished the effect of ATP on [Ca2+]i increase. while verapamil, a calcium channel blocker, was unable to abolish such an an ATP-induced [Ca2+]i increase. The concentrations of ATP required to increase [Ca2%bdi ranged from 10(-7) M to 10(-3) M, the maximal effect being obtained with 10(-4) M ATP. At this latter concentration, ATP induced cell growth inhibition which was dose-independent as triggered only when maximal elevation of [Ca2+]i was attained. This ATP concentration also induced maximal apoptotic features in both types of cells. Together, our results highlighted an 'all or none' effect of ATP on breast tumor cell growth mediated by its effect on [Ca2+]i liberation from intracellular stores, the first rise of [Ca2+]i being further amplified by an influx of calcium from extracellular space. The attainment of sufficient [Ca2+]i level then triggers cellular events.

Extracellular ATP activates calcium entry and mobilization via P2U-purinoceptors in rat lactotrophs

Extracellular ATP has been previously shown to activate calcium signalling in pituitary cell populations [1] but the particular cell types involved have not been identified. We used video imaging of Fura-2 loaded into single rat pituitary cells and identified as lactotrophs to study the effects of extracellular ATP on [Ca2+]i. ATP does not permeabilize the cells as shown by exclusion of propidium iodide. ATP causes two types of calcium transients in lactotrophs. The most common response is a rapid increase in [Ca2+]i that decays slowly and is terminated by washout of ATP. This type of response is also seen in calcium-free medium, demonstrating mobilization of calcium stores dependent upon the presence of the agonist. ATP also stimulates calcium entry as detected by Mn(2+)-quenching of Fura-2. ATP in Mg(2+)-free medium and ATP gamma S are effective agonists suggesting ATP4- is the active form. The presence of P2-purinoceptors is apparent because ATP, ADP and AMP increase [Ca2+]i in decreasing order of potency and adenosine has no effect. ATP-induced calcium transients are reduced by the P2-purinoceptor antagonists suramin and quinidine. UTP is equipotent with ATP and defines the receptor subtype as P2U. We conclude that ATP4- acts on rat lactotrophs via P2U-purinoceptors to elevate [Ca2+]i from intracellular and extracellular sources.

The phospholipase C activating P2U purinoceptor also inhibits cyclicAMP formation in DDT1 MF-2 smooth muscle cells

The P2U purinoceptor mediated effect on cellular cAMP was investigated in DDT1 MF-2 smooth muscle cells. Stimulation of these receptors by ATP or UTP caused a pronounced decrease of about 50% in cellular cAMP levels in forskolin or isoprenaline pretreated cells. This action of the nucleotides was concentration dependent with an IC50 of 9.4 +/- 0.2 microM and 29.0 +/- 0.5 microM for UTP and ATP, respectively and was inhibited by the P2-purinoceptor antagonist suramin. The cAMP level appeared to be modified by intracellular Ca2+, represented by an initial decline in cAMP. Neither inactivation of protein kinase C by staurosporine nor elevated cytoplasmic Ca2+ concentrations interfered with the sustained decrease in cAMP levels induced by ATP or UTP, showing that this effect is not mediated via the phospholipase C pathway known to be activated after P2U purinoceptor stimulation in DDT1 MF-2 cells. Pertussis toxin inhibited the action of these nucleotides on the cellular cAMP level. It can be concluded that the P2U purinoceptor in DDT1 MF-2 cells is coupled to different G-proteins, activating phospholipase C and inhibiting adenylyl cyclase activity.

Alcohol action on a neuronal membrane receptor: evidence for a direct interaction with the receptor protein

For almost a century, alcohols have been thought to produce their effects by actions on the membrane lipids of central nervous system neurons--the well known "lipid theory" of alcohol action. The rationale for this theory is the correlation of potency with oil/water or membrane/buffer partition coefficient. Although a number of recent studies have shown that alcohols can affect the function of certain neuronal neurotransmitter receptors, there is no evidence that the alcohols interact directly with these membrane proteins. In the present study, we report that inhibition of a neuronal neurotransmitter receptor, an ATP-gated ion channel, by a series of alcohols exhibits a distinct cutoff effect. For alcohols with a molecular volume of < or = 42.2 ml/mol, potency for inhibiting ATP-activated current was correlated with lipid solubility (order of potency: 1-propanol = trifluoroethanol > monochloroethanol > ethanol > methanol). However, despite increased lipid solubility, alcohols with a molecular volume of > or = 46.1 ml/mol (1-butanol, 1-pentanol, trichloroethanol, and dichloroethanol) were without effect on the ATP-activated current. The results suggest that alcohols inhibit the function of this neurotransmitter receptor by interacting with a small hydrophobic pocket on the receptor protein.

P2 purinoceptor antagonist properties of pyridoxal-5-phosphate

The antagonist properties of pyridoxal-5-phosphate, a synthesis precursor of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, were investigated on P2 purinoceptor-mediated responses of the rat isolated vagus nerve and vas deferens. In addition, the effect of this agent was studied on high affinity tritiated alpha,beta-methylene adenosine triphosphate (alpha,beta-meATP) binding to rat vas deferens membranes, thought to represent binding to functional P2x purinoceptors. In the rat vagus nerve, pyridoxal-5-phosphate (10(-5)-10(-4) M) produced concentration-related antagonism of depolarisation responses induced by alpha,beta-meATP, measured using an extracellular recording technique. In contrast, depolarisation responses to 5-hydroxytryptamine (5-HT) were unaffected by pyridoxal-5-phosphate. In the rat vas deferens, pyridoxal-5-phosphate (10(-5)-10(-4) M) antagonised contractile responses produced by alpha,beta-meATP while contractions to phenylephrine were unaffected. However, responses of the vagus nerve and the vas deferens to alpha,beta-meATP were not antagonised by pyridoxal hydrochloride (10(-4) M). Pyridoxal-5-phosphate competed for high affinity binding of [3H]alpha,beta-meATP to homogenised membranes of the rat vas deferens with a pKi estimate of 4.91 +/- 0.12 and a Hill slope of 0.80 +/- 0.03. Pyridoxal hydrochloride only competed for binding at concentrations in excess of 10(-4) M, yielding a pKi estimate of 3.21 +/- 0.04 and a Hill slope of 1.82 +/- 0.12. These findings indicate that pyridoxal-5-phosphate acts as a specific antagonist of P2 purinoceptors in the vagus nerve and vas deferens of the rat and that the phosphate moiety is required for activity.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Adenosine triphosphate activates ion permeabilities in biliary epithelial cells

BACKGROUND/AIMS

The biliary epithelium contributes to bile formation through absorption and secretion of fluid and electrolytes. The effects of extracellular nucleotides on membrane ion transport were assessed in isolated bile duct cells from rats and Mz-ChA-1 cells from a human cholangiocarcinoma.

METHODS

The rates of efflux of 125I and 86Rb were used to assess membrane Cl- and K+ permeabilities, respectively. Patch clamp recordings of whole cell currents were used to evaluate the properties of adenosine triphosphate (ATP)-activated currents.

RESULTS

Purinergic receptor agonists ATP and uridine triphosphate stimulated 125I and 86Rb efflux about twofold above basal levels. The effects were reproduced by a nonhydrolyzable analogue of ATP (adenosine 5'-O-[3-thiophosphate]) and were unaffected by an adenosine receptor blocker xanthine amine congener. 125I efflux was also stimulated by adenosine and its receptor agonists 5'-N-ethylcarboxamidoadenosine, N6-(2-phenylisopropyl)adenosine; these effects were inhibited by xanthine amine congener, suggesting a separate adenosine receptor. ATP, adenosine 5'-O-(3-thiophosphate), and uridine triphosphate each stimulated release of Ca2+ from intracellular stores, whereas adenosine had no effect. In whole cell recordings of Mz-ChA-1 cells, ATP activated an early transient outward current consistent with a K+ conductance and a later, sustained inward current consistent with a Cl- conductance.

CONCLUSIONS

Biliary cells possess at least two classes of nucleotide receptors that modulate membrane ion permeability through Ca(2+)-dependent and -independent pathways, and ATP may be involved in the regulation of biliary secretion.

The P2Z-purinoceptor of human lymphocytes: actions of nucleotide agonists and irreversible inhibition by oxidized ATP

1. Extracellular adenosine triphosphate (ATP) is known to open a receptor-operated ion channel (P2Z class) in human lymphocytes which conducts a range of cationic permeants. The activity of a range of different agonists and inhibitors towards the P2Z-purinoceptor was investigated by measuring the agonist-induced influx of Ba2+ into fura-2 loaded lymphocytes. 2. The most potent agonist was 2' & 3'-0-(4-benzoylbenzoyl)-ATP (benzoylbenzoic ATP) which gave 2 fold greater maximum Ba2+ influx and had a 10 fold lower EC50 than for ATP. The rank order of agonist potency in K(+)-media was benzoylbenzoic ATP >> ATP = 2-methylthio ATP = 2-chloro ATP > ATP-gamma-S. ADP, UTP and alpha,beta-methylene ATP were unable to stimulate Ba2+ influx. 3. Extracellular Na+ inhibited the increment of Ba2+ influx induced by all concentrations of ATP, 2-methylthio ATP, 2-chloroATP and ATP-gamma-S. This inhibitory effect of extracellular Na+ is also reflected in the different EC50s for benzoylbenzoic ATP (8 microM in K(+)-media, 18 microM in Na(+)-media) but the maximal response to this agonist was the same in the presence or absence of Na+. 4. Treatment of lymphocytes with 2,3 dialdehyde ATP (oxidized ATP0 at 300 microM for 60 min gave total and irreversible inhibition of ATP-induced Ba2+ influx. 5'-p-Fluorosulphonyl benzoyladenosine (FSBA) also was an irreversible inhibitor but the maximal inhibition achieved was 90%. 5. It is concluded that the P2z-purinoceptor of human lymphocytes has a rank order of agonist potency which clearly distinguishes it from other P2-receptors and that oxidized ATP is a convenient irreversible inhibitor for the P2Z-purinoceptor.

Protection of mice from endotoxic death by 2-methylthio-ATP

The lethal effects of endotoxin, a bacterial product shed into the blood during bacteremia, are thought to be due to macrophage release of mediators such as tumor necrosis factor alpha and interleukin 1. Although much is known about the pathophysiology of endotoxemia, relatively little is known about the cellular signaling mechanisms that are involved. The data in this study suggest that extracellular adenine nucleotides can influence the development of endotoxin shock. An adenine nucleotide analog, 2-methylthio-ATP, inhibited the endotoxin-stimulated release of toxic mediators (i.e., tumor necrosis factor alpha and interleukin 1), and it protected mice from endotoxin-induced death. These studies suggest a fundamental and unusual role for adenine nucleotides on endotoxin action, and they provide a potentially new therapeutic approach for the control of the pathophysiology of Gram-negative septicemia.

Activation of P2-purinoceptors in the nucleus tractus solitarius mediate depressor responses

The purpose of the study was to examine the role of P2 purinergic receptors in mechanisms of cardiovascular control mediated by the nucleus tractus solitarius (NTS), a major integrative site in the brainstem involved in the reflex coordination of cardiorespiratory and visceral response patterns. Microinjections of ATP and its analogues were made into the subpostremal NTS of anesthetized, spontaneously breathing rats. ATP, alpha,beta-methylene ATP (alpha beta-meATP) and 2-methylthio-ATP (2-meSATP) produced significant dose-related reductions in arterial blood pressure. alpha beta-meATP was slightly more potent than ATP and 2-meSATP. Pretreatment with the P2 receptor antagonist, suramin (0.5 nmol/rat), into the same NTS site 10 min prior to agonist administration completely blocked pronounced depressor response pattern elicited by the highest dose of alpha beta-meATP (0.1 nmol/rat). The present findings suggest that endogenous ATP may serve as a fast transmitter substance in NTS-mediated mechanisms of cardiovascular control.

Extracellular ATP induces immediate-early gene expression but not cellular hypertrophy in neonatal cardiac myocytes

It is well-documented that norepinephrine (NE) induces the expression of immediate-early genes (IEGs), such as c-fos, c-jun, and jun-B, in cultured neonatal heart cells and leads to cell growth without cell division (ie, hypertrophy). Although purinergic receptors activated by ATP are present on cardiac myocytes and ATP is coreleased with NE from sympathetic nerve endings within the heart, the potential role of the purinergic system in the cascade of events that leads to cardiac hypertrophy is unknown. We report in the present study that stimulation of purinergic receptors by micromolar concentrations of extracellular ATP increased the levels of c-fos and jun-B mRNA as well as FOS and JUN-B proteins in neonatal cardiac myocytes. The magnitude of response to micromolar ATP was comparable to that elicited by NE. The increase in IEG expression induced by ATP was preceded by a rapid transient increase in cytosolic Ca2+. Pretreatment of myocytes with the intracellular Ca2+ chelator BAPTA-AM prevented the ATP-stimulated increase in cytosolic Ca2+ and attenuated the ATP-stimulated increase in c-fos expression. In contrast, NE did not increase cytosolic Ca2+ in quiescent myocytes, and pretreatment with BAPTA-AM did not inhibit the NE-stimulated increase in c-fos gene expression. Furthermore, although NE markedly increased [14C]phenylalanine incorporation into protein and myocyte hypertrophy measured by cell size, ATP did not. These results demonstrate that stimulation of purinergic receptors by ATP activates IEGs via a Ca(2+)-dependent pathway in cardiac myocytes that differs from the NE stimulated activation of these genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Age as risk factor for sporadic dementia of the Alzheimer type?

Normal brain aging is accompanied by a slight but persistent reduction in energy formation and an increase in energy demand. Stress conditions aggravate energy shortage particularly in old age. Mitochondrial dysfunction in both ATP formation and ATP release may be assumed to be causative for the reduced availability of energy in cerebral cells leading to diminished cellular work. In this respect, aging may be considered as a contributing risk factor for sporadic late-onset dementia of the Alzheimer type. Metabolically, this disorder seems to be characterized also by an early energy shortage in cerebral cells with the same consequences for the disturbance of cellular work. However, the causative abnormality may be seen in a perturbation of the control of cerebral glucose metabolisms, in all probability mediated by a desensitization of the neuronal insulin receptor.

Extracellular ATP-induced currents in astrocytes: involvement of a cation channel

Whole-cell currents were measured with the perforated patch clamp technique in cultured rat astrocytes to analyze the underlying ionic mechanism for a P2-purinoceptor-mediated depolarization. ATP (100 microM) induced an inward current with a mean amplitude of 130 pA and an EC50 of 17 microM. The response desensitized during a 1 min application. Replacement of extracellular Na+ with NMDG or K+ abolished the ATP-evoked inward current. Replacement of Na+ with choline, however, resulted in an ATP-evoked response of one-third the amplitude in normal solution. This is indicative of a cation rather than Na+ channel. However, due to difficulties in voltage-clamping these gap junction-coupled cells at voltages different from the membrane resting potential, the current reversal potential could not be determined. Measurements with K(+)-sensitive microelectrodes showed that 100 microM ATP lowered the intracellular K+ concentration. Replacement of extracellular Ca2+ or Cl- did not alter the ATP-induced inward currents. Fura-2 imaging experiments revealed a transient rise of the intracellular Ca2+ concentration during ATP application. Removal of extracellular Ca2+ did not influence the peak response; it did, however, shorten the time course. These results and previous observations that the permeability changes are caused by a P2x receptor are indicative of an ATP-sensitive cation conductance. In addition, cytoplasmic Ca2+ is increased by mobilization from intracellular stores, and by additional influx across the cell membrane. Extracellular ATP released by neurons could evoke K+ release from astrocytes as well as be a mediator for cation changes that signal cell activation processes when released by damaged cells.

Evoked noradrenaline release in the rabbit ear artery: enhancement by purines, attenuation by neuropeptide Y and lack of effect of calcitonin gene-related peptide

1. Adenosine (30 microM) and its analogues 5'-N-ethylcarboxaminoadenosine (5 and 30 microM) and L-phenylisopropyladenosine (5 and 30 microM), potentiated the evoked but not spontaneous release of tritiated noradrenaline in the rabbit central ear artery. 2. Prejunctional inhibition of the evoked but not spontaneous release of tritiated noradrenaline by 100 nM neuropeptide Y is greater at 2 min than at 10 min after superfusion of the peptide. 3. Calcitonin gene-related peptide (2.63 to 263 nM) did not affect the evoked or spontaneous release of tritiated noradrenaline in this preparation. 4. These results are discussed in terms of prejunctional modulation of sympathetic transmission in the rabbit central ear artery.

Presence of P2-purinoceptors in the rat pineal gland

1. The effects of noradrenaline, ATP, adenylyl-imidodiphosphate (AMP-PNP), adenosine, alpha,beta-methylene-ATP and the P2-purinoceptor antagonist, suramin on N'-acetyl-5-hydroxytryptamine production were studied in cultured denervated rat pineal glands. 2. Noradrenaline (3 nM-1 microM) increased N'-acetyl-5-hydroxytryptamine production as measured both in the gland and the culture medium. 3. In noradrenaline (10 nM)-stimulated pineal glands, ATP (0.03 nM-1 mM) or AMP-PNP (0.1 microM-1 mM) increased N'-acetyl-5-hydroxytryptamine production in a concentration-dependent manner. 4. Alpha,beta-Methylene-ATP at the concentration of 0.1 mM, but not 3 microM, attenuated the enhancement by ATP (0.1 mM) of noradrenaline (10 nM)-induced N'-acetyl-5-hydroxytryptamine production. 5. Suramin (0.1 mM) blocked the potentiating effect of ATP (0.1 mM), but not the potentiating effect of adenosine (0.1 mM) in glands incubated with noradrenaline (10 nM). 6. These findings suggest that the rat pineal gland possesses P2-purinoceptors which when stimulated potentiate the effect of noradrenaline but do not, by themselves, induce an increase in N'-acetyl-5-hydroxytryptamine production.

The use of antagonists to characterize the receptors mediating depolarization of the rat isolated vagus nerve by alpha, beta-methylene adenosine 5'-triphosphate

1. We have previously found that the P2x-purinoceptor agonist, alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-methylene ATP), depolarizes the rat cervical vagus nerve, measured with a 'grease-gap' extracellular recording technique. This effect was attenuated by the P2 purinoceptor antagonist, suramin. In the present study we have investigated in more detail the antagonism produced by suramin and have also investigated the actions of two other putative P2 purinoceptor antagonists, cibacron blue and pyridoxal-phosphate-6-azophenyl-2', 5'-disulphonic acid (iso-PPADS). Furthermore, we have studied the interactions between suramin and cibacron blue or iso-PPADS in an attempt to determine whether these antagonists act at a common receptor site. 2. Suramin (1 x 10(-5)-1 x 10(-4) M) produced reversible, concentration-related rightward displacements of the concentration-effect curve to alpha, beta-methylene ATP. Schild analysis of this antagonism yielded a pA2 value of 5.90 with a slope value of 0.47. 3. Cibacron blue (3 x 10(-5)-1 x 10(-4) M) also antagonized depolarizations induced by alpha, beta-methylene ATP. The antagonistic effects of cibacron blue were slow to reach equilibrium but could be readily reversed on washout. At low concentrations for antagonism, cibacron blue (1 x 10(-5) M and 3 x 10(-5) M) produced enhancement of the maximal response to alpha, beta-methylene ATP. At the highest concentration tested (1 x 10(-4) M) the concentration-effect curve to alpha, beta-methylene ATP was shifted to the right in a parallel manner, yielding a pKB estimate of 4.96. 4. Iso-PPADS (1 X 10-6 1 X 10-5- M) produced a concentration-related depression in the maxima ofthe concentration-effect curves to alpha,beta-methylene ATP. Analysis of these data by a double reciprocal plot yielded a pKB estimate of 6.02. This profile of insurmountable antagonism could not be attributed to irreversible binding of iso-PPADS to the receptor since the effect of iso-PPADS could be reversed on washing, albeit slowly.5. In the presence of suramin (1 x 10-4 M), cibacron blue (1 x 10-4 M) produced no further rightward displacement of the alpha,beta-methylene ATP concentration-effect curve. The mean agonist concentration ratios in the presence of suramin or cibacron blue alone (11.7 and 10.3, respectively) were not significantly different from the mean concentration-ratio in the presence of both antagonists (11.8). This finding suggests that high concentrations of alpha,beta-methylene ATP activate a receptor population which is resistant to blockade by either antagonist.6. The antagonistic effect of iso-PPADS (1 x 10-5 M) was partially attenuated by suramin (1I x 10-4 M).It is possible that this interaction reflects a slow dissociation of iso-PPADS from the receptor with which suramin and alpha,beta-methylene ATP interact.7. Suramin, cibacron blue or iso-PPADS had no marked effect on depolarization produced by 5-hydroxytryptamine (5-HT, 1 x 10-7-3 x 10-5 M), indicating their specificity in antagonizing responses to alpha, beta-methylene ATP.8. Responses to alpha,beta-methylene ATP were not antagonized by 8-para-sulphophenyltheophylline (3 x 10-5M), ondansetron (1 x 10-7 M), bicuculline (1 x I0-5 M), phentolamine (1 X 10-6 M) or hexamethonium(1 X 10-4 M), which are antagonists at P1-purinoceptors, 5-HT3 receptors, GABAA receptors, a-adrenoceptors and nicotinic cholinoceptors, respectively, thereby excluding the involvement of these receptors.Indomethacin (3 X 10-6 M) had no effect on responses to alpha,beta-methylene ATP.9. The results obtained with three purinoceptor antagonists confirm and extend our original supposition that alpha,beta-methylene ATP-induced depolarization of the rat vagus nerve is mediated predominantly via P2 purinoceptors, thought to be of the P2,X subtype. The finding that responses induced by high concentrations of agonist were resistant to blockade by suramin and cibacron blue, but could be attenuated by iso-PPADS, adds further weight to our speculation that the purinoceptor population in the rat vagus nerve is heterogeneous.

Stimulation of chloride secretion by P1 purinoceptor agonists in cystic fibrosis phenotype airway epithelial cell line CFPEo-

1. P1 purinoceptor agonists like adenosine have been shown to stimulate Cl- transport in secretory epithelia. In the present study, we investigated whether P1 agonist-induced Cl- secretion is preserved in cystic fibrosis airway epithelium and which signalling mechanism is involved. The effects of purinoceptor agonists on Cl- secretion were examined in a transformed cystic fibrosis airway phenotype epithelial cell line, CFPEo-. 2. Addition of adenosine (ADO; 0.1-1 mM) markedly increased 125I efflux rate. The rank order of potency of purinoceptor agonists in stimulating 125I efflux was ADO > AMP > ADP approximately equal to ATP. A similar order of potency was seen in transformed cystic fibrosis nasal polyp cells, CFNPEo- (ADO > ATP > AMP > ADP). These results are consistent with the activation of Cl- secretion via a P1 purinoceptor. 3. The P1 agonists tested (at 0.01 and 0.1 mM) revealed a rank order of potency of 5'-N-ethylcarboxamine adenosine (NECA) > 2-chloro-adenosine (2-Cl-ADO) > R-phenylisopropyl adenosine (R-PIA). 4. The known potent A2 adenosine receptor (A2AR) agonist, 5'-(N-cyclopropyl) carboxamidoadenosine (CPCA, 2 microM) but not the A1 adenosine receptor agonist, N6-phenyl adenosine (N6-phenyl ADO, 10 microM) markedly increased 125I efflux rate (baseline, 5.9 +/- 2.0% min-1, + CPCA, 10.9 +/- 0.6% min-1; P < 0.01). The stimulant effect of CPCA (10 microM) was abolished by addition of the A2AR antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) (100 microM; reported K(i) = 11 +/- 3 microM). These results favour the involvement of A2AR. 5. ADO (0.1-mM) and CPCA (2 microM) both induced a marked increase in intracellular [Ca2+] ([Ca2+]i); the effect of the latter was again abolished by pretreatment of the cells with DMPX. By contrast, N6-phenyl ADO did not affect [Ca2+]i. 6. In patch-clamp experiments, ADO (1 mM) induced an outwardly-rectified whole-cell Cl- current (baseline, 2.5 +/- 0.8 pA pF-1, + ADO, 78.4 +/- 23.8 pA pF-1; P < 0.02), which was largely inhibited in cells internally perfused with a selective inhibitory peptide of the multifunctional Ca2+/calmodulin-dependent protein kinase, CaMK [273-302] (20 microM), as compared to a control peptide, CaMK [284-302]. Addition of BAPTA (10 mM), a Ca2+ chelator, to the perfusion pipette also abolished the ADO-elicited Cl- current. 7. In conclusion, our results suggest that A2AR participates in regulation of airway C1 secretion via aCa2+-dependent signalling pathway, which involves CaMK and appears to be at least partially conserved in cystic fibrosis airway epithelial cells.

Cloning and expression of a human P2U nucleotide receptor, a target for cystic fibrosis pharmacotherapy

The Cl- secretory pathway that is defective in cystic fibrosis (CF) can be bypassed by an alternative pathway for Cl- transport that is activated by extracellular nucleotides. Accordingly, the P2 receptor that mediates this effect is a therapeutic target for improving Cl- secretion in CF patients. In this paper, we report the sequence and functional expression of a cDNA cloned from human airway epithelial (CF/T43) cells that encodes a protein with properties of a P2U nucleotide receptor. With a retrovirus system, the human airway clone was stably expressed in 1321N1 astrocytoma cells, a human cell line unresponsive to extracellular nucleotides. Studies of inositol phosphate accumulation and intracellular Ca2+ mobilization induced by extracellular nucleotides in 1321N1 cells expressing the receptor identified this clone as the target receptor in human airway epithelia. In addition, we independently isolated an identical cDNA from human colonic epithelial (HT-29) cells, indicating that this is the same P2U receptor that has been functionally identified in other human tissues. Expression of the human P2U receptor (HP2U) in 1321N1 cells revealed evidence for autocrine ATP release and stimulation of transduced receptors. Thus, HP2U expression in the 1321N1 cell line will be useful for studying autocrine regulatory mechanisms and in screening of potential therapeutic drugs.

Activation of chloride current by P2-purinoceptors in rat ventricular myocytes

1. Rat ventricular myocytes were dissociated and their responses to extracellularly applied ATP were recorded using patch pipettes under the whole cell configuration. 2. ATP initially induced an inward current followed by an outward current at -50 mV. With a Cs-rich pipette solution the late outward current was blocked, leaving a sustained inward current (IATPs) suggesting that a K+ conductance underlies the late response. 3. When the extracellular Cl- concentration was changed, the reversal potential of IATPs corresponded well to the shift of the Cl- equilibrium potential. IATPs was reversibly blocked by the chloride channel blocker, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). 4. The concentration-response curve of IATPs had a Hill coefficient of 0.98 and an EC50 value of 5.2 x 10(-6) M. 5. ATP was more potent than ADP, while AMP and adenosine were ineffective, suggesting that P2-purinoceptor activation induced IATPs. 6. The activation of IATPs was depressed by depleting the extracellular Mg2+ and increased by adding Mg2+. 7. Our results strongly suggest that P2-purinoceptor activation by ATP induces both a Cl(-)-conductance (IATPs) and a K(+)-conductance in rat ventricular myocytes.

Neuropeptide Y stimulates proliferation of human vascular smooth muscle cells: cooperation with noradrenaline and ATP

Since the sympathetic nervous system has been shown to exert a trophic influence on vascular smooth muscle cells (SMC), we studied the growth regulating effects of neuropeptide Y (NPY) in cooperation with the sympathetic co-transmitters noradrenaline and adenosine triphosphate (ATP) in human vascular SMC. NPY stimulated DNA synthesis in human SMC grown from subcutaneous arteries and veins (diameter: 0.4 mm) measured by [3H]thymidine incorporation. Also cell number and protein synthesis were stimulated. The effect was mediated through the Y1-receptor and not Y2 or Y3 since the Y1-selective NPY analogue Pro34-NPY and peptide YY stimulated mitogenesis in the same magnitude as NPY while the NPY-fragment NPY13-36 only had minor effects. The effect was blocked by pretreating the cells with pertussis toxin indicating a Gi/o-coupled effect. The other sympathetic co-transmitters, noradrenaline and ATP, also stimulated mitogenesis in the human SMC in a similar magnitude as NPY. When added together NPY and noradrenaline potentiated each other in the mitogenic response. ATP had mainly additive effects. This is the first demonstration that NPY, noradrenaline and ATP stimulates growth in human vascular SMC. This suggests a role of the sympathetic cotransmitters in modulating vascular tone, but also by inducing hypertrophy/hyperplasia with possible clinical consequences.

Purinergic axis in cardiac blood vessels. Agonist-mediated release of ATP from cardiac endothelial cells

Purified endothelial cells isolated from guinea pig hearts by enzymatic perfusion were grown in monolayer culture and used to test the ability of a variety of vasoactive agents to stimulate ATP release from these cells. Stimulation of endothelial cells with the peptide agonist bradykinin (1 nmol/L), acetylcholine (1 mumol/L), serotonin (1 mumol/L), or adenosine 5'-diphosphate (10 mumol/L) resulted in the rapid appearance of ATP in the incubation medium determined with the firefly luciferase assay for ATP. Addition of antagonists for muscarinic (atropine, 0.1 mumol/L) and purinergic (suramin, 100 mumol/L; reactive blue-2, 100 mumol/L) receptors suggested that ATP release from these cells was receptor-mediated. Bradykinin-induced release of ATP was rapid (peak < 30 seconds at 3 nmol/L bradykinin), dose-dependent (EC50, 0.18 nmol/L), and diminished with repeated administration of agonist. Desensitization to bradykinin also affected the ability of acetylcholine to induce release and was reversible when cells were returned to growth conditions for short periods. Measurement of released adenyl purines as their fluorescent N6-ethenopurine derivatives by high-performance liquid chromatography revealed the origin of the purine released to be ATP and confirmed its rapid dephosphorylation. Addition of the purine nucleotide analogues 2-methylthio-ATP (2-methyl-S-ATP), ADP, and beta gamma-methylene ATP to endothelial cell cultures resulted in a dose-dependent increase in the appearance of ATP measured in the medium bathing the cells at 30 seconds, suggesting the presence of ATP-induced ATP release.(ABSTRACT TRUNCATED AT 250 WORDS)