Overview. Purinergic mechanisms

Neuronal release of soluble nucleotidases and their role in neurotransmitter inactivation

Efficient control of synaptic transmission requires a rapid mechanism for terminating the actions of neurotransmitters. For amino acids and monoamines, this is achieved by their uptake into the cell by specific high-affinity transporters; acetylcholine is first broken down in the extracellular space and then choline is taken up by the cell. Because ATP is hydrolysed to adenosine by membrane-bound enzymes (ectonucleotidases) that are present in most tissues, it has been assumed that these enzymes terminate the neurotransmitter actions of ATP in the brain and in the periphery. We show here, however, that stimulation of sympathetic nerves innervating the guinea-pig vas deferens releases not only neuronal ATP, but also soluble nucleotidases that break down this ATP to adenosine, indicating that inactivation of ATP is increased by nerve activity. This release of specific nucleotidases together with ATP represents a new mechanism for terminating the actions of a neurotransmitter.

Characterization and localization of an ATP diphosphohydrolase activity (EC 3.6.1.5) in sarcolemmal membrane from rat heart

In the present report we describe an ATP diphosphohydrolase (apyrase EC 3.6.1.5) in rat cardiac sarcolemma. It is Ca2+ dependent and is insensitive to ouabain, orthovanadate, N-ethylmaleimide (NEM), lanthanum, and oligomycin that are classical ATPase inhibitors. Sodium azide that is a mitochondrial inhibitor at low concentrations, did not affect the enzyme activity at 5.0 mM or below. In contrast, at high concentrations (> 10 mM) sodium azide inhibited the enzyme. Levamisole, a specific inhibitor of alkaline phosphatase and P1, P5-di(adenosine 5'-)pentaphosphate (Ap5A), a specific inhibitor of adenylate kinase did not inhibit the enzyme. Mercury chloride showed a parallel inhibition of the hydrolysis of both substrates of apyrase. Similar inhibition profiles are powerful evidence for a common catalytic site for the hydrolysis of both substrates. The enzyme has an optimum pH range of 7.5-8.0 and catalyzes the hydrolysis of triphospho- and diphosphonucleosides other than ATP or ADP. The apparent Km (Michaelis constant) and Vmax (maximal velocity) are 62.1 +/- 5.2 microM and 1255.7 +/- 178 micromol inorganic phosphate liberated/min/mg with ATP and 59.4 +/- 4.3 microM and 269.2 +/- 39 micromol inorganic phosphate liberated/min/mg with ADP. Enzyme markers indicated that this apyrase is associated with the plasma membrane. A deposition of lead phosphate granules on the outer surface of the sarcolemmal vesicles was observed by electron microscopy in the presence of either ATP or ADP as substrate. It is suggested that the ATP diphosphohydrolase could regulate the concentration of extracellular adenosine, and thus is important in the control of vascular tone and coronary flow.

Adaptation to oxidative stress: quinone-mediated protection of signaling in rat lung epithelial L2 cells

Cells can respond to a sublethal oxidative stress by up-regulating their intracellular glutathione (GSH) pool. Such increased GSH concentration is likely to be protective against further oxidative challenge, and, in fact, pre-exposure to low levels of oxidants confers increased cellular resistance to subsequent greater oxidative stress. Previously, we have shown that pretreatment of rat lung epithelial L2 cells with sublethal concentrations of tert-butylhydroquinone (TBHQ) increases intracellular GSH concentration in a concentration- and time-dependent manner. This increase resulted from up-regulation of both gamma-glutamyltranspeptidase (GGT) and gamma-glutamylcysteine synthetase (GCS). Therefore, we investigated whether such increased GSH concentration protected these cells against a subtle loss in function caused by a subsequent challenge with sublethal concentrations of tert-butyl hydroperoxide (tBOOH) (< or = 200 microM), mimicking a physiological oxidative stress. Activation of L2 cell purinoreceptors with 100 microM ADP caused an elevation of intracellular Ca2+. This response was suppressed by a brief pre-exposure to tBOOH. The inhibition, however, was alleviated dramatically by a 16-hr pretreatment with 50 microM TBHQ. The same TBHQ pretreatment also protected the cells from ATP-depletion induced by tBOOH. L-Buthionine S,R-sulfoximine (BSO), an irreversible inhibitor of GCS, prevented the increase in intracellular GSH and also completely removed the protection by TBHQ in maintaining the ATP level. Thus, pre-exposure to a sublethal level of TBHQ results in protection of cell functions from hydroperoxide toxicity. This protection appears to depend on alteration of the intracellular GSH pool, the modulation of which constitutes an adaptive response to oxidative stress.

Attenuation of contractile responses to sympathetic co-transmitters in veins from subjects with essential hypertension

Neuropeptide Y (NPY), noradrenaline (NA) and ATP are cotransmitters of the sympathetic nervous system and exert vasocontractile effects. The aim of this study was to determine the role of these sympathetic co-transmitters in human hypertension. Subcutaneous vessels from 12 patients with essential hypertension and 12 matched controls were studied in vitro. Vascular contractile responses to NPY, NA, alpha,beta-methylene ATP (alpha,beta-mATP) and potassium were studied in isolated arteries and veins (diameter 0.1-1.1 mm) with intact endothelium. The dilatory effect of acetylcholine was used to test the endothelial function. There was no difference in potency (pD2) or contractile response to NPY, NA or alpha,beta-mATP between hypertensive and control arteries. In veins, however, the contractile response to NPY was significantly reduced in hypertensives and the responses to NA were unchanged. Furthermore, the sensitivity (pD2) to alpha,beta-mATP was significantly reduced in veins from hypertensives. There was no difference in the dilatory response to acetylcholine between the hypertensives and the controls, neither in the arteries nor in the veins, indicating that the observed changes in vascular reactivity to NPY, NA and alpha,beta-mATP were not endothelium-dependent. In conclusion, the postjunctional contractile effect of NPY and sensitivity (pD2) to alpha,beta-mATP, co-transmitters of the peripheral sympathetic nervous system, are attenuated in veins in essential hypertension.

Vasovagal syncope and skeletal muscle vasodilatation: the continuing conundrum

During vasovagal syncope, profound bradycardia and hypotension occur. Atropine administration can prevent the bradycardia but not the hypotension, suggesting that marked peripheral vasodilation is a major cause of the fall in arterial pressure. This concept has been confirmed since vasovagal syncope can be seen in patients who have undergone heart transplantation and also in patients subject to cardiac pacing. In both cases, there is no bradycardia but hypotension during the syncopal attacks. The major site of the vasodilation is in skeletal muscle and muscle sympathetic nerve activity is suppressed just prior to and during vasovagal attacks, indicating that sympathetic withdrawal contributes to the dilation. However, the skeletal muscle vasodilation seen during syncope is greater than that caused by sympathetic withdrawal alone, and it is absent in limbs that have undergone surgical sympathectomy, or local anesthetic nerve block. These observations suggest a role for neurally mediated "active" vasodilation during syncope. The afferent neural pathways that evoke the profound vasodilation during vasovagal attacks remain the subject of debate. The neural pathways responsible for the active component of the dilation are also unknown. Recent evidence has demonstrated that cholinergic, beta-adrenergic, and nitroxidergic (nitric oxide) vasodilator mechanisms are not essential to observe the dilation, demonstrating that the mechanisms responsible for it remain a continuing conundrum.

Electrophysiological consequences of purinergic receptor stimulation in isolated rat pulmonary arterial myocytes

Neither the electrophysiological effects of purinergic receptor stimulation nor the role of ATP in regulating the tone of pulmonary arterial smooth muscle has been determined. Therefore, we investigated the effects of purine nucleotides on acutely dissociated smooth muscle cells from rat small pulmonary arteries using the patch-clamp recording technique. Extracellular application of ATP activated a fast transient inward current (which decayed in the continued presence of the nucleotide) and produced sustained periodic oscillations of predominantly inward current. Pharmacological and anion substitution experiments revealed that the transient inward current was carried by the movement of cations. In contrast, the periodic oscillations of current were due primarily to a Ca(2+)-activated Cl- current (ICl,Ca) dependent on the release of Ca2+ from intracellular stores. Experiments using ATP analogues revealed the following order of potency for activation of the fast transient inward current: 2-methylthio ATP (2-meSATP) > ATP > alpha,beta-methylene ATP (alpha,beta-meATP) > > ADP > UTP = adenosine. Cross desensitization was seen between applications of ATP, alpha,beta-meATP, and 2-meSATP, suggesting that these agonists act via a common site. The order of potency for activation of ICl,Ca was UTP = ATP > > ADP > or = 2-meSATP > alpha,beta-meATP = adenosine. Both the fast transient inward current and ICl,Ca evoked by ATP and its analogues were abolished by the nonselective P2 purinoceptor antagonist suramin. These results show the existence of P2x and P2U purinoceptor subtypes in pulmonary arterial smooth muscle cells. Stimulation of these receptors results in activation of a fast transient inward cation current and ICl,Ca, respectively. It is likely that ATP acts via these receptor subtypes to regulate pulmonary arterial tone under physiological or pathological conditions.

Extracellular adenosine 5' triphosphate involvement in the death of LAK-engaged human tumor cells via P2X-receptor activation

This study reports that extracellular ATP is a critical factor involved in LAK cell-mediated cytotoxicity. Human colon carcinoma LoVo cells were resistant to LAK cells as well as to ATP, while their multidrug resistant (MDR-1+) derivative, LoVo-Dx cells, were sensitive to both LAK and ATP. LoVo-Dx cells, became resistant to LAK cells and ATP after 48 h pretreatment with Phorbol 12-Myristate-13-Acetate (PMA), while 48 h pretreatment with verapamil in parallel sensitized LoVo cells to LAK cells and to ATP as well. The sensitivity to ATP and LAK cells was not related to the expression of extracellular ecto-ATPase activity on cell targets membranes. Conversely, apyrase, an enzyme with powerful ecto-ATPase activity, abolished the LAK- and ATP-mediated cytotoxicity. Furthermore, ADP-beta-S, an antagonist of ATP, abolished both LAK and ATP-mediated cell killing. Purine binding sites have been detected by radioreceptor assays with ADP-beta[35S] on the cell surface of ATP and LAK-sensitive LoVo-Dx cells. By contrast, no nucleotide receptor was found on the ATP and LAK-resistant cells. Such a putative cytotoxic purinoreceptor has been categorized as P2x purinergic receptor by a panel of synthetic nucleotides. These results demonstrate that extracellular ATP is needed for an efficient LAK cell-mediated killing of tumor cells. We propose that ATP acts as a natural amplifier of physical, or immune cytotoxic damages since it may be released in large amounts from target cells injured by several cytotoxic mediators secreted by LAK effectors.

Activation of P2x-purinoceptors in the nucleus tractus solitarius elicits differential inhibition of lumbar and renal sympathetic nerve activity

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of alpha,beta-methylene ATP (alpha,beta-MeATP) elicits large dose-dependent decreases in mean arterial pressure (MAP) and heart rate (HR) and preferential dilation of the iliac vascular bed in comparison to renal and mesenteric vascular beds. We investigated whether sympathoinhibition contributes to the depressor responses and whether differential changes in regional sympathetic output occur. In 43 chloralose/urethane anesthetized male Sprague-Dawley rats, MAP, HR, renal (RSNA) and lumbar sympathetic nerve activity (LSNA) were recorded. Data were analyzed as both the maximum decrease and the integral of the decrease over the duration of the depressor response. Microinjection of alpha,beta-MeATP (25 and 100 pmol in 50 nl volume) into the subpostremal NTS caused significant and dose-dependent decreases in MAP, HR, RSNA and LSNA. However, the changes in RSNA were significantly greater than those observed in LSNA for both doses and both methods of analysis of data (maximum responses in delta %: 84 +/- 3 vs 62 +/- 4, and 93 +/- 3 vs 74 +/- 4 for low and high dose of alpha,beta-MeATP, respectively; integral responses in delta % x min: 32 +/- 4 vs 18 +/- 3 and 179 +/- 7 vs 134 +/- 14 for low and high dose of alpha,beta-MeATP, respectively). Blockade of P2-purinoceptors in the NTS by the specific P2-receptor antagonist suramin abolished responses to 100 pmol alpha,beta-MeATP and microinjections of vehicle did not alter neural nor hemodynamic parameters. We conclude that activation of P2x-purinoceptors in the NTS inhibits sympathetic nerve activity and evokes differential regional sympathetic responses. However, differential sympathoinhibition does not explain differential vascular responses to the activation of P2x-purinoceptors in the NTS.

Models of Alzheimer's disease: cellular and molecular aspects

Glucose metabolism in the brain is an important process that influences many normal cellular processes, from neurotransmitter synthesis to ATP production. While cortisol and insulin have opposing effects on glucose metabolism, desensitization of the neuronal insulin receptor results in metabolic abnormalities. In the normal aging brain, glucose/energy metabolism is decreased slightly. In the majority of cases. Alzheimer's disease is sporadic and has a late onset. Therefore, age-related variations in cellular metabolism following the principle of self-organized criticality may come into focus with respect to the etiopathogenesis of this neurodegenerative disorder. As a possible primary abnormal event in late-onset sporadic DAT, a prolonged desensitization of the neuronal insulin receptor is assumed to be responsible for cascade-like abnormalities in oxidative energy metabolism and related metabolism with impacts on amyloid formation.

Mechanism of resistance to alpha-adrenergic receptor antagonists of renal nerve stimulation-induced vasoconstriction at low frequencies

To determine why renal vasoconstriction elicited by periarterial nerve stimulation (RNS) at lower frequencies (< 4 Hz) is resistant to alpha-adrenergic receptor blockade in the rat kidney, we reevaluated the effect of alpha-receptor antagonists on the vasoconstrictor response to norepinephrine (NE) and to RNS and on the release of adrenergic transmitter. The alpha-receptor antagonist prazosin (PZ) at 0.2 and 7 nM reduced the vasoconstrictor response to NE, and 2.4 microM PZ abolished it. PZ (0.2 or 7 nM) reduced RNS-induced vasoconstriction without altering the fractional tritium overflow. PZ (2.4 microM) enhanced fractional tritium overflow and reduced the vasoconstrictor response to RNS at 2-10 Hz, but not at 0.5 or 1 Hz. The effect of 0.2 nM PZ to reduce RNS-induced vasoconstriction was reversed by increasing the concentration to 2.4 microM. Corynanthine (COR; 2.6 microM), a preferential alpha-receptor blocker, or phenoxybenzamine (PBZ; 30 nM) abolished the vasoconstrictor response to NE but only partially reduced response to RNS and enhanced the fractional tritium overflow. Rauwolscine (RW; 2.5 nM), a preferential alpha 2-receptor antagonist, did not alter the vasoconstrictor response to NE but potentiated RNS-induced vasoconstriction and fractional tritium overflow. RW (7.7 microM) inhibited NE-induced vasoconstriction but potentiated the vasoconstrictor response to RNS and fractional tritium overflow. PZ (7 nM) abolished the potentiation by RW and reduced the vasoconstrictor response to RNS. These data suggest that a component of RNS-induced vasoconstriction in the rat kidney is attributable to co-release of a nonadrenergic transmitter with NE. The diminished effect of alpha-receptor antagonists at higher concentrations (e.g., PZ 2.4 microM) to reduce RNS-induced vasoconstriction is caused by their prejunctional action to enhance co-release of the nonadrenergic transmitter.

Extracellular nucleotides as signalling molecules for renal mesangial cells

1. Glomerular diseases frequently cause chronic renal failure which ultimately requires dialysis and kidney transplantation. The events leading to destruction of the glomerular filtration apparatus include injury of glomerular cells, aggregation of thrombocytes and infiltration of immune cells into the glomerulus. 2. Nucleotides (e.g. ATP and UTP) are present in all glomerular cell types as well as in thrombocytes. The release of nucleotides into the extracellular space occurs after damage of glomerular cells and aggregation of thrombocytes. Several in vitro and in vivo findings indicate that extracellular nucleotides may play a role as pro-inflammatory mediators in glomerulonephritis. 3. A hallmark finding in kidney biopsies from patients with glomerulonephritis is proliferation of glomerular mesangial cells. Cell culture studies demonstrated that extracellular ATP (10-300 microM) stimulated growth of mesangial cells. The mitogenic effect of ATP was potentiated in the presence of multiple growth factors. 4. Nucleotide-induced signalling in mesangial cells included an increase of intracellular calcium, activation of phosphatidylinositol-specific phospholipase C and phospholipase D, inhibition of adenylylcyclase, stimulation of mitogen-activated protein kinase and increased expression of the immediate early genes, c-fos, c-jun and Egr-1. 5. In previous studies of experimental mesangioproliferative glomerulonephritis, exogenously given ADP beta S and ATP gamma S have been shown to aggravate the course of the disease, while 2-chloroadenosine had beneficial effects. 6. Taken together, these findings support the concept that nucleotides may function as proinflammatory mediators in glomerulonephritis while adenosine may have antiinflammatory effects.

Implication of ATP receptors in brain functions

The possible implication of P2-purinoceptors in brain functions is reviewed. Involvement of P2-purinoceptors in memory and learning (Section 2) is suggested by ATP release from hippocampal slices [Wieraszko, A., Goldsmith, G. and Seyfried, T. N. (1989) Brain Res. 485, 244-250], induction of fast synaptic currents in cultured hippocampal neurons [Inoue, K., Nakazawa, K., Fujimori, W. and Takanaka, A. (1992a) Neurosci. Lett. 134, 294-299] and long-lasting enhancement of the population spikes [Wieraszko, A. and Seyfried, T. N. (1989) Brain Res. 491, 356-359; Nishimura, S., Mohri, M., Okada, Y. and Mori, M. (1990) Brain Res. 525, 165-169; Fujii, S., Kato, H., Furuse, H., Ito, K., Osada, H., Hamaguchi, T. and Kuroda, Y. (1995) Neurosci, Lett. 187, 130-132], as well as ATP release on glutamate stimulation to evoke an increase in intracellular Ca2+ in hippocampal cells [Inoue, K., Koizum, S. and Nakazawa, K. (1995) NeuroReport 6, 437-440]. Moreover, mRNAs for certain types of P2x-purinoceptors are present in the hippocampus [Collo, G., North, R. A., Kawashima, E., Merlo-Pich, E., Neidhart, S., Surprenant, A. and Buell, G. (1996) J. Neurosci. 16, 2495-2507]. It is likely, therefore, that ATP may be involved in modulation of synaptic efficiency in the hippocampus. The implication of ATP in schizophrenia is suggested by the fact that antipsychotic drugs inhibit ATP-evoked responses in PC12 cells [Koizumi, S., Ikeda, M., Nakazawa, K., Inoue, K., Ito, K. and Inoue, K. (1995b) Biochem. Biophys. Res. Commun. 210, 624-630] without blocking the action of dopamine D2 receptors. Involvement of P2-purinoceptors in Sections 4 ("Pain and cognition") and 5 ("Central regulation of the autonomic system") are also discussed.

Molecular cloning and sequencing of a novel human P2 nucleotide receptor

A novel human P2 nucleotide receptor has been cloned from a T-cell cDNA library. The predicted amino acid sequence shows characteristics of a G-protein-coupled receptor, and shares 88% homology with a recently characterised rat P2 nucleotide receptor sequence. Distinctive features include an extremely short cytoplasmic tail with only one putative protein kinase C phosphorylation site. Northern blot analysis revealed a 1.9 kb transcript expressed in the placenta.

P2X purinoceptors in cultured myenteric neurons of guinea-pig small intestine

1. Fast excitatory postsynaptic currents (fEPSCs) and responses to exogenously applied purinoceptor agonists were studied in primary cultures of myenteric neurons from guinea-pig small intestine. Whole-cell and outside-out configurations of the patch clamp technique were used. Hexamethonium (100 microM) partly inhibited fEPSCs in 28% of neurons. Hexamethonium-resistant fEPSCs were inhibited by 97 +/- 2% by the P2X receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM). 2. ATP caused two types of inward currents. In 92% of neurons (n = 123), ATP caused a slowly desensitizing current that declined with a double exponential time course (tau 1 = 7.1 +/- 2.0 s; tau 2 = 57 +/- 7.4 s, n = 4). The rank order potency for purinoceptor agonists in these neurons was ATP > 2-methylthio-ATP (2-MeSATP) > > alpha, beta-methylene ATP (alpha, beta-me ATP) > beta, gamma-meATP > ADP. The EC50 values for ATP and 2-MeSATP were 40 and 65 microM, respectively. alpha, beta-MeATP acted as a partial agonist at these receptors. In 8% of neurons (n = 11), ATP-induced currents desensitized rapidly with a double exponential time course (tau 1 = 0.13 +/- 0.015 s; tau 2 = 2.2 +/- 1.3 s, n = 4); alpha, beta-meATP caused similar responses in these cells. Both types of ATP-induced current were associated with an increased conductance and an inwardly rectifying I-V relationship (Erev = 10 mV). Halving [Na+]o shifted the reversal potential of ATP currents by -22 +/- 6 mV. 3. ATP activated single channel currents in outside-out patches. The single channel I-V relationship was linear between -120 and 60 mV (Erev approximately 0 mV). Single channel conductance between -100 and -60 mV was 25 +/- 2 pS. Single channel open probability was voltage dependent and decreased from 0.05 +/- 0.01 at -100 mV to 0.007 +/- 0.002 at +40 mV. 4. These data show that P2X purinoceptors mediate some fEPSCs in cultured myenteric neurons. Myenteric neurons express the fast-desensitizing alpha, beta-me ATP-sensitive subtype of P2X receptor that has the properties of cloned P2X1 receptors and is similar to native receptors in smooth muscle cells. Myenteric neurons also express a P2X receptor that desensitized slowly and was alpha, beta-meATP-insensitive. This receptor has the properties of cloned P2X2 or P2X5 receptors and is similar to native receptors found in PC-12 cells and superior cervical ganglion neurons. The known distribution of P2X2 and P2X5 receptors suggests that myenteric neurons are likely to express predominantly P2X2 receptors.

Signaling by extracellular nucleotides

ATP and other nucleotides can be released from cells through regulated pathways, or following the loss of plasma membrane integrity. Once outside the cell, these compounds take on new roles as intercellular signaling molecules that elicit a broad spectrum of physiological responses through the activation of numerous cell surface receptor subtypes. This review summarizes recent advances in the molecular characterization of ATP receptors and discusses roles for cloned receptors in established and novel physiological processes.

Kindled seizures do not affect adenosinergic inhibition of DA or ACh release in rat accumbens or PFC

Epileptic seizures are thought to terminate largely as a result of the extracellular accumulation of the purinergic neuromodulator, adenosine, released by discharging neurons. However, the postictal surge in extracellular adenosine and its widespread inhibitory effects are limited in time to only a few minutes and cannot directly account for increased resistance to seizures and the complex behavioural and motivational effects that may persist for hours or days after a seizure. The present study examined whether kindled seizures might alter the sensitivity or efficacy of inhibitory presynaptic adenosine receptors, and thereby induce more enduring changes in downstream transmitter systems. Rats were kindled in the amygdala of the dominant cerebral hemisphere, contralateral to the preferred direction of rotation, and their brains were removed either 2 h or 28 days after completion of kindling. Inhibition of electrically stimulated release of dopamine (DA) and acetylcholine (ACh) by the A1 adenosine-receptor agonist, R-phenylisopropyladenosine (R-PIA) was then measured in the prefrontal cortex (PFC) and nucleus accumbens. R-PIA (1.0 microM) inhibited [1H]DA release from PFC and nucleus accumbens tissue, and [14C]ACh release from nucleus accumbens tissue, but release was unaffected by prior kindling, regardless of the intervening interval. These results do not support suggestions that DA or ACh might mediate the effects of seizure-induced changes in purinergic inhibitory tone so as to cause long-term shifts in seizure threshold and postictal behavior.

Fading and rebound of currents induced by ATP in PC12 cells

1. Patch clamp recording (whole cell configuration) was used to study the action of ATP on rat phaeochromocytoma (PC12) cells usually held at -70 mV and rapidly superfused with buffered saline. ATP (0.5, 1 or 5 mM), applied from micropipettes by pressure application with brief (< or = 50 ms) pulses, induced inward currents with rapid onset and decay. ADP and alpha, beta-methylene ATP were ineffective. 2. ATP (5 mM) applied with pulses > 200 ms long elicited a complex current response characterized by a rapid peak which faded and was followed by a strong current rebound (lasting several s) as soon as the application was terminated. This type of response was readily replicated as long as ATP applications were spaced at 2-3 min intervals. The amplitude of peak and rebound currents was dependent on the length of pressure pulse and was similarly depressed by bath application of a threshold dose (25 microM) of ATP. Rapid fading and rebound of ATP-induced membrane currents were also observed when the Y-tube method was used for applying this agonist. 3. The reversal potential for peak and rebound currents was the same while the time constant values for peak fading and rebound onset were insensitive to changes in membrane potential between -70 and -40 mV. When ATP was applied to a cell clamped at depolarized potential, no current was observed but rapid return of the membrane potential to -70 mV immediately at the end of ATP application was associated with a large rebound current. 4. Brief (20 ms) application of ATP during the onset of the rebound current strongly and transiently suppressed it. The same application performed during the gradual decay of the rebound wave elicited a transient inward current which was much smaller and shorter than the one observed when the cell was in its resting state. Application of 2 s ATP pulses at 20 s intervals equally reduced the initial peak and rebound currents which recovered at the same rate. 5. The present data are interpreted according to a scheme which suggests two types of ATP receptor desensitization. The first one (D1) would be characterized by fast kinetics and low agonist affinity; rapid recovery from D1 would then be manifested as current rebound presumably due to receptor reactivation. The second desensitized state (D2) has slow kinetics and high affinity for the agonist: it is therefore typically seen with sustained application of a low dose of ATP. It is proposed that desensitization and its recovery can influence the time course of membrane responses mediated by purinoceptors.

The assisted self-association of ATP4- by a poly(amino acid) [poly(Lys)] and its significance for cell organelles that contain high concentrations of nucleotides

The occurrence of high concentrations of ATP in certain cell organelles prompted us to study the self-association of ATP via the concentration dependence of the 1H-NMR chemical shifts for H2, H8 and H1' in D2O at pD 8.4 (25 degrees C) in the range 0.0025-0.4 M in the presence and absence of poly(alpha, L-lysine), where [Lys units] was 0.4 M. The experiment in the presence of poly(Lys) was repeated at pD 12.1. At pD 8.4, the poly(amino acid) is protonated, i.e. poly(H.Lys)n+, whereas at pD 12.1 only approximately 10% of the epsilon-amino groups are positively charged. The results in all three systems are consistent with the isodesmic model of indefinite non-cooperative stacking. The stacking tendency follows the series: ATP4- (K = 1.3 M-1; pD 8.4) < ATP4-/poly(H.Lys)n+ (K = 11.5 M-1; pD 8.4) > ATP4-/90% poly(Lys)/10% poly(H.Lys)n+ (K = 3.1 M-1; pD 12.1). It is evident that poly(H.Lys)n+ assists the association of ATP by a factor of approximately 10, and it is suggested that, via its positively charged epsilon-ammonium groups, poly(H.Lys)n+ acts as a matrix by aligning ATP4- ions via ionic interactions with the negatively charged phosphate residues. The intragranular concentrations of various constituents of several storage or secretory cell organelles, as reported in the literature, are tabulated. The chromaffin granules of the adrenal medulla and the dense granules of blood platelets contain particularly high concentrations of nucleotides ([ATP] is approximately 0.14 M in the chromaffin granules and 0.5 M in the dense granules of rabbit blood platelets) and amines, such as epinephrine or 5-hydroxytryptamine. These granules, and probably also the storage vesicles of certain neurons (which seem to have a similar composition), appear, if the total concentrations of the various solutes are considered, to be osmotically unstable, which means that the intragranular solutes must be associated. This aggregation is discussed, especially with regard to the nucleotides.

The involvement of an ATP-gated ion channel, P(2X1), in thymocyte apoptosis

In the immune system, apoptosis is involved in intrathymic elimination of self-reactive thymocytes and in peripheral T cell tolerance to exogenous antigens. Here, we describe the role in T cell apoptosis of P(2x1), a nonselective cation channel activated by ATP. P(2X1) molecules are up-regulated in thymocytes during dexamethasone-induced apoptosis, and antagonists to these receptors protect thymocytes from cell death. Moreover, P(2X1) mRNA and protein levels increase in thymocytes induced to die in vivo by the superantigen staphylococcal enterotoxin B. In contrast, T cells undergoing apoptosis in the periphery do not express P(2X1). The demonstration that P(2X1) ion channels play a role in the apoptosis of thymocytes but not peripheral T cells illustrates a novel mechanism contributing to thymocyte cell death and opens new possibilities for investigating clonal deletion in the thymus.

Differential distribution of two ATP-gated channels (P2X receptors) determined by immunocytochemistry

Several P2X receptor subunits were recently cloned; of these, one was cloned from the rat vas deferens (P2X1) and another from pheochromocytoma (PC12) cells differentiated with nerve growth factor (P2X2). Peptides corresponding to the C-terminal portions of the predicted receptor proteins (P2X1 391-399 and P2X2 460-472) were used to generate antisera in rabbits. The specificities of antisera were determined by staining human embryonic kidney cells stably transfected with either P2X1 or P2X2 receptors and by absorption controls with the cognate peptides. In the vas deferens and the ileal submucosa, P2X1 immunoreactivity (ir) was restricted to smooth muscle, whereas P2X2-ir was restricted to neurons and their processes. Chromaffin cells of the adrenal medulla and PC12 cells contained both P2X1- and P2X2-ir. P2X1-ir was also found in smooth muscle cells of the bladder, cardiac myocytes, and nerve fibers and terminals in the superficial dorsal horn of the spinal cord. In contrast, P2X2-ir was observed in scattered cells of the anterior pituitary, neurons in the hypothalamic arcuate and paraventricular nuclei, and catecholaminergic neurons in the olfactory bulb, the substantia nigra, ventral tegmental area, and locus coeruleus. A plexus of nerve fibers and terminals in the nucleus of the solitary tract contained P2X2-ir. This staining disappeared after nodose ganglionectomy, consistent with a presynaptic function. The location of the P2X1 subunit in smooth muscle is consistent with its role as a postjunctional receptor in autonomic transmission, while in neurons, these receptors appear in both postsynaptic and presynaptic locations.

Selected P2 purinoceptor modulators prevent glutamate-evoked cytotoxicity in cultured cerebellar granule neurons

Primary cultures of granule neurons derived from cerebella of postnatal rats are endowed with Glu receptors. Glu receptor agonists exert a trophic influence on differentiating granule cells but, with maturation, the cells become vulnerable to excitatory amino acids. Here we show that the P2 purinoceptor antagonist basilen blue abolishes in rat cerebellar granule neurons the cytotoxic action of glutamate with an IC50 in the 10-20 microM range. Within the same concentrations, basilen blue inhibits binding of [3H] ATP to cerebellar granule cells, glutamate-evoked release (but not uptake) of [3H] D-aspartate and Ca2+ uptake. Furthermore, the extracellular phosphorylation of a major 45-kDa endogenous ecto-protein substrate of cerebellar granule neurons is inhibited with an IC50 of about 1 microM. Similar effects are elicited by 5-adenylylimidodiphosphate, a P2 purinoceptor agonist, when supplied to the neurons for 8 days previously to the addition of glutamate. Our data point to the use of P2 purinoceptor modulators as novel elements for understanding and controlling glutamate-mediated excitatory neurotoxicity and neurotransmission. We suggest a possible involvement of P2 purinoceptors in these actions.

Modulation by purines of calcium-activated non-selective cation channels in the outer hair cells of the guinea-pig cochlea

1. The cell-attached and cell-free configurations of the patch-clamp technique were used to investigate whether external ATP and its derivatives modulate channel activity in outer hair cells freshly isolated from the guinea-pig cochlea. 2. Submicromolar concentrations of ATP stimulated a non-selective cation channel with a conductance of about 25 pS. The ATP-elicited stimulation was partly blocked by the membrane-permeant blocker 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC), and mimicked by the calcium ionophore, ionomycin, suggesting that the channel activated by ATP is identical to a previously reported calcium-activated non-selective (CAN) cation channel. 3. The P2x agonist beta, gamma-methylene-ATP (beta, gamma-MeATP, 10 microM) and the P2Y agonist 2-methyl-thio-ATP (2-MeSATP, 1 microM) both activated CAN channels. The effect of ATP was inhibited by the P2 antagonist suramin but not by the P2Y antagonist Reactive Blue 2. These results suggest that both purinergic receptors are involved in the ATP-evoked response and that internal calcium acts as a second messenger for opening CAN channels. 4. In contrast, adenosine inhibited CAN channels. This effect was reproduced by the A2 agonist 5'-N-ethylcarboxyamidoadenosine (NECA) and the permeant cAMP analogue 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP), but not by the A1 agonist N6-cyclo-hexyladenosine (CHA). CAN channels were also inhibited when the catalytic subunit of protein kinase A was applied internally on inside-out patches, suggesting that adenosine A2 receptor downregulates CAN channels via a cAMP-dependent phosphorylation.

Activation of purinergic receptors triggers oscillatory contractions in adult rat ventricular myocytes

Extracellular ATP is an important neurotransmitter that modulates cardiac function by activation of purinergic receptors. In this study, the effect of P2 purinergic receptor activation on contractions and on [Ca2+]i was investigated in adult rat ventricular myocytes. Fura 2 was used to measure [Ca2+]i, and video edge detection was used to measure contraction. Superfusion of 2-methylthio-adenosine-5'-triphosphate (2-M-S-ATP) over quiescent myocytes induced oscillations in contraction and in [Ca2+]i. The frequency of the oscillatory contractions increased with increasing concentrations of 2-M-S-ATP, but the amplitude of contractions varied from cell to cell and was independent of the concentration of 2-M-S-ATP. During electrical stimulation, activation of purinergic receptors in myocytes potentiated the amplitude of contraction and induced arrhythmias. In populations of quiescent myocytes, the plateau phase of the [Ca2+]i signal evoked by 2-M-S-ATP could be shown to represent summed oscillations in [Ca2+]i in individual cells. Pretreatment of quiescent myocytes with thapsigargin or caffeine reduced or abolished the oscillations in contractions and in [Ca2+]i triggered by 2-M-S-ATP, indicating a dependence of the oscillations on uptake and release of Ca2+ by the sarcoplasmic reticulum. These data demonstrate the novel phenomenon that activation of purinergic receptors in quiescent myocytes stimulates oscillations in [Ca2+]i and contraction. In electrically stimulated myocytes, activation of purinergic receptors triggers oscillatory contractions and potentiates the amplitude of electrically triggered contractions.

Molecular cloning and functional expression of a novel rat heart P2X purinoceptor

Here we describe a novel purinergic receptor, the P2X5 receptor, cloned from rat heart. The full-length cDNA encodes a protein 455 amino acids long which shares an overall identity of 40-47% with other members of the P2X purinergic receptor family. P2X5 mRNA transcripts are found predominantly in rat heart but are also present in brain, spinal cord and adrenal gland. Functional expression of the recombinant receptor in HEK-293 cells shows a current that resembles mostly the P2X2 phenotype: the ATP-activated current reveals little agonist desensitization, is not activated by alpha,beta-meATP and is completely blocked by suramin and PPADS.

Hydrolysis of P2-purinoceptor agonists by a purified ectonucleotidase from the bovine aorta, the ATP-diphosphohydrolase

Pharmacologists are becoming more and more aware of the possibility that certain ATP analogues currently used to classify the P2-purinoceptors are dephosphorylated by ectonucleotidases. In this study, we provide evidence that in the vascular system, these purine analogues are hydrolysed by an ATP-diphosphohydrolase (ATPDase). This enzyme is known as the major plasma membrane nucleotidase of endothelial and smooth muscle cells, and is believed to dephosphorylate extracellular triphospho- and diphosphonucleosides. Assays were conducted with a purified ATPDase from smooth muscle cells of bovine aorta. At a concentration of 250 microM, adenosine 5'-(alpha,beta-methylene) triphosphonate (alpha,beta-metATP), adenosine 5'-(beta,gamma-methylene) triphosphonate (beta,gamma-metATP), adenosine 5'-(alpha,beta-methylene) disphosphonate (alpha,beta-metADP), adenylyl 5'-(beta,gamma-imido) diphosphonate (beta,gamma-imidoATP) and adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) all resisted dephosphorylation, whereas 2-chloroadenosine triphosphate (2-chloroATP), 2-methylthioadenosine triphosphate (2-MeSATP) and 8-bromoadenosine triphosphate (8-bromo-ATP) were hydrolysed at 99, 63, and 20% of the rate of ATP hydrolysis, respectively. All the non-hydrolysable analogues tested, except alpha,beta-metADP, competed with ATP and ADP for the ATPDase catalytic site, reducing their hydrolysis by 35-50%. Apparent Km values for ATP and ADP were estimated at 14.1 and 12.0 microM, respectively, whereas apparent Km and Ki values for the purine analogues ranged from 12 to 28 microM. These results strongly support the view that (1) the ATPDase is expected to reduce substantially the P2-response induced by ATP, ADP, and some hydrolysable agonists; and (2) by competing with the hydrolysis of endogenously released ATP and ADP, non-hydrolysable analogues could alter the amplitude or direction of the cellular response induced by these natural substrates.

Combined effects of ATP and its analogs on the membrane permeability in transformed mouse fibroblasts

Extracellular ATP (0.6 mM) induces a marked decrease in the membrane potential, followed by an increase in cell membrane permeability in transformed mouse fibroblasts. The effects of the ATP analogs, p[CH2]ppA and p[NH]ppA (0.6 mM), on the membrane potential and permeability are much less pronounced. ATP at 0.05 mM has no effect by itself, but markedly increases the analog-induced membrane potential dissipation and permeability. The data suggest that ATP-induced membrane permeation is composed of two processes: One is common to ATP and its analogs and appears to be a receptor-mediated process. The second is unique for ATP, effective even at low concentration (0.05 mM), and might be mediated by cell surface enzymes, for which ATP, but not its analogs, serves as a substrate.

Neuronal mediators of inhibitory junction potentials and relaxation in the guinea-pig internal anal sphincter

1. Inhibitory junction potentials (IJPs) and relaxations evoked in response to field stimulation (supramaximal voltage, 0.1 ms, single stimulus and 5 stimuli at 5-40 Hz) of non-adrenergic non-cholinergic (NANC) nerves with atropine and phentolamine (each 1 microM) were measured in the guinea-pig internal anal sphincter (gpIAS). The mean resting membrane potential was -44.2 +/- 0.2 mV (n = 1119 cells from 260 preparations). 2. NANC nerve stimulation evoked frequency-dependent IJPs (19.7 +/- 1.1 mV, n = 165, 33 tissues to a single stimulus) and relaxations. IJPs consisted of two tetrodotoxin (1 microM)-sensitive components: one was abolished by apamin (0.3 microM) and the P2-purinoceptor antagonist suramin (100 microM); the other, smaller in amplitude, was sensitive to inhibitors of nitric oxide synthase (NOS, e.g. L-NAME, 100 microM) and the nitric oxide (NO) scavenger oxyhaemoglobin (HbO, 10 microM). 3. ATP (1 mM), vasoactive intestinal polypeptide (VIP, 0.01-0.25 microM) and pituitary adenylate cyclase-activating peptide (PACAP(1-27), 0.84 microM) each hyperpolarized and relaxed the gpIAS; only ATP responses resembled the evoked IJPs in time course. 4. The guanylyl cyclase inhibitor LY83583 (10 microM) abolished apamin-insensitive IJPs and relaxations. The cGMP phosphodiesterase inhibitor M&B 22948 (30 microM) and 8-Br-cGMP (100 microM) each hyperpolarized the gpIAS. 5. Two components comprise the IJP and relaxation evoked in response to NANC nerve stimulation in the gpIAS. One, sensitive to apamin, resembles the response to ATP and is modulated by purinoceptor antagonists; the other, apamin and suramin insensitive, is inhibited by NO antagonists.

Activation of nucleotide receptors inhibits high-threshold calcium currents in NG108-15 neuronal hybrid cells

A P2U (UTP-sensitive) nucleotide receptor has previously been cloned from NG108-15 neuroblastoma x glioma hybrid cells and it has been shown that activation of this receptor inhibits the M-type K+-current. We now report that UTP also inhibits Ca2+-currents in differentiated NG108-15 cells, but probably through a different nucleotide receptor. UTP (100 microM) inhibited the peak of the high-threshold current by 28.4 +/- 3.1% (n = 28) with no effect on the low-threshold current. Two components of high-threshold current were identified: one inhibited by 100 nM omega-conotoxin (CgTx) and one inhibited by 2 microM nifedipine and enhanced by 1 microM BAY K8644. UTP inhibited the former by 31.0 +/- 3.1%, with an IC50 of 2. 8 +/- 1.1 microM, and the latter 34.2 +/- 6.1% with an IC50 of 1.7 +/- 1.3 microM. Pertussis toxin pretreatment prevented inhibition of the CgTx-sensitive, nifedipine-resistant but not CgTx-resistant current. Inhibition was not prevented by intracellular BAPTA (20 mM) or cAMP (1mM). Effects of UTP on both currents were imitated by UDP, ATP, ADP, AP4A and ATPgammaS but weakly or not at all by 2-MeSATP, GTP, AMP-CPP or ITP. Since the receptors which inhibit Ca2+-currents are activated by ATP, it is suggested that they might mediate auto-inhibition of transmitter release by ATP if present on purinergic nerve terminals.

KEYWORDS

nucleotides, UTP, ATP, calcium currents, neuroblastoma cells

Adenosine 5'-triphosphate (ATP) receptors induce intracellular calcium changes in mouse leydig cells

Cytoplasmic calcium ([Ca(2+)](i)) changes evoked by adenosine 5(1)-triphosphate (ATP) were recorded in cultured individual Leydig cells within 10-18 h after cell dispersion. [Ca(2+)](i) was monitored using Fura-2AM loaded cells with a digital ratio imaging system. Five micromolars ATP induced biphasic [Ca(2+)](i) responses in most cells (94%,n=100), characterized by a fast increase from a basal level (126±5 nMSE,n=60 cells) to a peak (5-7 times above basal levels) within seconds, followed by a slow decrease toward a plateau level (2-3 times above basal) within 5 min. The peak phase of the [Ca(2+)](i) response increased with ATP concentrations (1-100 μM ATP) in a dose-dependent manner with an IC(50) of 5.9±1.2 μM, and it desensitized in a reversible manner with repeated application of 5 μM ATP at <5-min intervals. The [Ca(2+)](i) peak response was dependent on Ca(2+) release from an intracellular pool, whereas the plateau phase was dependent on extracellular [Ca(2+)]. ATP did not appear to induce formation of nonspecific membrane pores, since stimulation for 10 min with ATP (10-100 μM) in the presence of extracellular Lucifer yellow (LY) (5 mg/mL) did not result in dye loading of the cells. [Ca(2+)](i) transients were elicited by other adenosine nucleotides with an order of potencies (ATP>Adenosine diphosphate [ADP]>Adenosine> Adenosine monophosphate [AMP]) that was compatible with the expression of P(2) receptors. [Ca(2+)](i) responses were suppressed by the purinergic P(2) receptor antagonist, suramin. These results provide functional evidence for the expression of purinergic P(2) receptors in Leydig cells.

Characterization of a P2X-purinoceptor in cultured neurones of the rat dorsal root ganglia

1. The electrophysiological actions of the P2-purinoceptor agonists, adenosine 5'-triphosphate (ATP), 2-methylthio ATP (2-meSATP) and alpha, beta-methyleneATP (alpha, beta-meATP) and of uridine 5'-triphosphate (UTP) were studied under concentration and voltage-clamp conditions in dissociated neurones of 1-6 day old rat dorsal root ganglia. 2. ATP (10 nM-100 microM) applied rapidly via a U-tube perfusion system (equilibration time < 10 ms) activated concentration-dependent inward currents with a latency to onset of a few ms, an EC50 of 719 nM and a Hill slope of 1.47. 3. 2-meSATP (10 nM- 100 microM) and alpha, beta-meATP (100 nM - 100 microM) also evoked transient inward currents. The EC50 and Hill slopes were 450 nM and 1.58 for 2-meSATP and 1.95 microM and 1.53 for alpha, beta-meATP respectively. There was no significant difference between the maximum currents evoked by the three agonists. 4. As the concentration of ATP increased so the rate of rise and decay of the currents also increased. At 100 and 300 nM ATP the decay of the current was best fitted by a single exponential, but at 1 microM and above two exponentials were required. Log-log plots of the rise time or time constants of decay versus concentration were linear. Currents evoked by 2-meSATP and alpha, beta-meATP showed a similar concentration-dependence in their kinetics. 5. Inward currents evoked by ATP, 2-meSATP and alpha, beta-meATP (300 nM) were abolished by the P2-purinoceptor antagonist, suramin (100 microM). 6. UTP (10 microM) evoked similar transient inward currents, which were sensitive to suramin (100 microM). ATP (10 microM), applied 2 min beforehand, reduced the response to UTP (10 microM) by 80 +/- 10%. 7. This study shows that ATP, 2-meSATP and alpha, beta-meATP act via a suramin-sensitive P2x-purinoceptor to evoke rapid, transient inward currents in dissociated neurones of rat dorsal root ganglia. The pyrimidine nucleotide, UTP, was also active. It is likely that the agonists were acting at the P2x3-subtype to produce these effects.

ATP and nitric oxide: inhibitory NANC neurotransmitters in the longitudinal muscle-myenteric plexus preparation of the rat ileum

1. The nature of neurotransmitter(s) involved in non-adrenergic non-cholinergic (NANC) relaxations induced by electrical stimulation (10 s trains, 1-8 Hz) was investigated in the precontracted longitudinal muscle-myenteric plexus preparation of the rat ileum. 2. Electrical stimulation of the tissue induced complex responses, consisting of a primary contraction, a primary relaxation, an off-relaxation and a rebound contraction, which were all tetrodotoxin(TTX)-sensitive. 3. Vasoactive intestinal polypeptide (VIP) and carbon monoxide (CO) did not induce relaxations. alpha-Chymotrypsin did not reduce the relaxations induced by electrical stimulation, while zinc protoporphyrin IX had non-specific effects. 4. Nitric oxide (NO) induced concentration-dependent relaxations. NG-nitro-L-arginine methylester (L-NAME) abolished the primary contractions and off-relaxations, while it partially reduced the primary relaxations. 5. ATP induced relaxations and ATP-desensitization of the tissues partially reduced the primary relaxations. Suramin and reactive blue 2 did not consistently influence the primary relaxations. 6. The ATP-induced relaxations were not influenced by L-NAME or TTX. The inhibitory effect of ATP-desensitization and L-NAME did not summate. 7. The cyclic AMP content of the tissue did not increase upon electrical stimulation or after addition of NO or ATP. The cyclic GMP content of the tissue increased upon electrical stimulation and addition of NO, but not after addition of ATP. 8. It is concluded that the relaxation induced by electrical stimulation consists of two types of responses. The off-relaxation is completely nitrergic, while the primary relaxation is mediated by NO, ATP and an as yet unknown transmitter which is not VIP or CO.

Tyrphostin inhibition of ATP-stimulated DNA synthesis, cell proliferation and fos-protein expression in vascular smooth muscle cells

1. We and others have shown that extracellular ATP (adenosine triphosphate), released from sympathetic nerves and platelets, stimulates growth of vascular smooth muscle cells (SMC). To study the importance of tyrosine kinases for ATP-mediated proliferation in vascular smooth muscle cells we used tyrphostins, a recently developed group of highly specific inhibitors of tyrosine kinases. 2. ATP induced a powerful concentration-dependent increase in DNA synthesis measured by [3H]-thymidine incorporation in rat aorta SMC (RASMC) and an increase in total cell number after 72 h of incubation as measured by an enzymatic cell proliferation assay. Tyrphostin 25 (10(-5) M) had no effect per se on basal DNA synthesis but reduced ATP-stimulated DNA synthesis and increase in cell number in a dose-dependent manner. Higher concentrations of ATP could not reverse the inhibitory effect of tyrphostin 25. The potency of several (six) other tyrphostins was also examined and found to be slightly greater than tyrphostin 25 with equal efficacy. 3. When RASMC were incubated with 10(-5) M ATP for 2 h, nearly all of the cells (87 +/- 5%) were intensely stained with an antibody to the Fos protein while in the controls only 1 +/- 2% of the cells were weakly stained. Tyrphostin 25 greatly reduced the Fos-protein staining (14 +/- 2%). 4. ATP induced a concentration-dependent increase in 45Ca(2+)-influx and formation of inositol phosphates (IPtotal) in RASMC. These effects were not inhibited by tyrphostin 25. 5. Tyrphostin 25 did not alter ATP-induced contraction in ring segments of rat aorta. 6. In conclusion, tyrphostin 25 inhibited ATP-induced DNA synthesis, cell proliferation and Fos-protein expression, but not ATP-induced 45Ca(2+)-influx, inositolphosphate-production or vasoconstriction. This indicates that the mitogenic effect of ATP on vascular smooth muscle cells is dependent on tyrosine kinases in contrast to the contractile effect of ATP in blood vessels.

Characterization of P2Z purinergic receptors on phagocytic cells of the thymic reticulum in culture

The thymic microenvironment is under intrinsic and extrinsic control circuits by several elements including hormones, neuropeptides, lymphokines, innervation and cell contact. P2 purinergic receptors have been described in a number of cells including macrophages, thymocytes, and other cells of the immune-inflammatory system. Here, we use the whole-cell patch-clamp technique and dye permeabilization assays to investigate the presence of ionic channels and purinergic receptors in one microenvironmental thymic component, namely the phagocytic cell of the thymic reticulum. At holding potentials ranging from -30 to -60 mV, applications of extracellular ATP in the vicinity of the cell membrane induce a transient and fast-activating inward current followed in most cells by an outward current. The whole event lasts 5-20 s. The inward current has a reversal potential close to 0 mV and the outward current can be ascribed to a Ca2+ -dependent K+ conductance. Both currents are inhibited by Mg2+, suggesting that the phenomenon is mediated by ATP4-. ATP-gamma-S can also induce both inward and outward currents. Exposure of phagocytic cells of the thymic reticulum to 5 mM ATP for 10 min induced permeabilization to lucifer yellow but not to the larger dyes trypan blue and rhodamine-dextran, suggesting a molecular weight cut-off smaller than 900. These observations lead us to conclude that phagocytic cells of the thymic reticulum express P2Z purinergic receptors that can mobilize Ca2+, induce the opening of ionic channels and permeabilize the cell membrane.

ATP receptors in sickness, pain and death

Extracellular ATP elicits biological responses ranging from cell death to synaptic transmission. Recent gene-cloning efforts have uncovered a family of cell-surface ATP receptors, which are potential targets for the development of novel drugs to treat airway and cardiovascular diseases, inflammation and pain.

Oxidative metabolism deficiencies in brains of patients with Alzheimer's disease

Glucose metabolism in the brain has an important influence on many normal cellular processes. It contributes to the synthesis of acetylcholine, glutamate, aspartate, gamma-aminobutyric acid, glycine, and ATP production (the driving force behind almost all cellular and molecular activity). Neuronal glucose metabolism is controlled antagonistically by insulin and cortisol. Desensitization of the neuronal insulin receptor causes abnormalities in oxidative energy metabolism. During normal aging, the cerebral energy pool is slightly diminished, but its level increases after stressful events. In age-related sporadic late-onset dementia of the Alzheimer type (SDAT), glucose metabolism and formation of cellular energy are severely reduced. Desensitization of the neuronal insulin receptor seems to be an early event in the pathogenesis or even etiology of SDAT causing disturbances in oxidative glucose metabolism and energy failure in insulin-sensitive brain structures. These abnormalities appear to induce a cascade of disturbances that leads to abnormal APP processing and amyloid formation, membrane damage, and neuronal death.

Inhibition of ADP-induced platelet activation by 7-chloro-4-nitrobenz-2-oxa-1,3-diazole: covalent modification of aggregin, a putative ADP receptor

ADP-induced platelet responses play an important role in the maintenance of hemostasis. There has been disagreement concerning the identity of an ADP receptor on the platelet surface. The chemical structure of 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-CI) shows considerable resemblance to that of the adenine moiety of adenine-based nucleotides. The reagent has been previously used by other investigators as an affinity label for adenine nucleotide-requiring enzymes, such as mitochondrial ATPase and the catalytic subunit of cAMP-dependent protein kinase. Since ADP-induced platelet responses depend on the binding of ADP to its receptor, we investigated the effect on ADP-induced platelet responses and the nature of ADP-binding protein modified by NBD-CI. NBD-CI inhibited ADP-induced shape change and aggregation of platelets in platelet-rich plasma in a concentration- and time-dependent manner. NBD-CI also inhibited ADP-induced shape change, aggregation, exposure of fibrinogen binding sites, secretion, and calcium mobilization in washed platelets. NBD-CI did not act as an agonist for platelet shape change and aggregation. Covalent modification of platelets by NBD-CI blocked the ability of ADP to antagonize the increase in intracellular levels of cAMP mediated by iloprost (a stable analogue of prostaglandin I2). NBD-CI was quite specific in inhibiting platelet aggregation by those agonists, e.g., ADP, collagen, and U44619 (a thromboxane mimetic), that completely or partially depend on the binding of ADP to its receptor. Autoradiogram of the gel obtained by SDS-PAGE of solubilized platelets modified by [14C]-NBD-CI showed the presence of a predominant radiolabeled protein band at 100 kDa corresponding to aggregin, a putative ADP receptor. The intensity of this band was considerably decreased when platelets were either preincubated with ADP and ATP or covalently modified by a sulfhydryl group modifying reagent before modification by [14C]-NBD-CI. These results (1) indicate that covalent modification of aggregin by NBD-CI contributed to loss of the ADP-induced platelet responses, and (2) suggest that there is a sulfhydryl group in the ADP-binding domain of aggregin.

Platelet activation by 2-(4-bromo-2,3-dioxobutylthio)adenosine 5'-diphosphate is mediated by its binding to a putative ADP receptor, aggregin

Platelet responses induced by ADP are mediated by a unique P21-purinergic receptor. Although a variety of ADP analogs, substituted at C2, have been used to delineate pharmacological properties of the ADP-binding site(s), the identity of the receptor protein has not been firmly established. 2-(4-Bromo-2,3-dioxobutylthio)- ADP [2-BrCH2(CO)2CH2-S-ADP], a well-characterized ADP analog, has been previously used as an affinity label to examine the structure/function relationship of ADP-requiring enzymes [Kapetanovic, E., Bailey, J.B. & Colman, R.F. (1985) Biochemistry 24, 7586-7593]. We found that it induced platelet shape change, aggregation, exposure of fibrinogen binding sites, secretion and mobilization of intracellular calcium, but was less potent than ADP. Under non-stirring conditions, incubation of platelets with this analog for longer time periods blocked ADP-induced shape change, aggregation, and the ability to ADP to antagonize the rise in intracellular levels of cAMP induced by iloprost (a prostaglandin I2 analog). Of a variety of agonists examined, only ADP-induced aggregation was almost completely inhibited in platelets irreversibly modified by the analog. An autoradiogram of the gel obtained by SDS/PAGE of solubilized platelets modified by the ADP analog followed by reduction of the dioxo group by NaB[3H], showed the presence of a single radiolabeled protein band at 100 kDa. Platelets incubated first with either ADP, ATP, or 2-methylthio-ADP were not labeled by 2-BrCH2(CO)2CH2S-ADP and NaB[3H]4-8-BrCH2(CO)2CH2-S-ADP was previously shown by us to irreversibly antagonize ADP-induced platelet responses by selectively modifying aggregin. Incubation of platelets with 2-BrCH2(CO)2CH2S-ADP completely blocked labeling of aggregin in platelets by 8-BrCH2(CO)2CH2S-[32P]ADP. These results show that 2-BrCH2(CO)2CH2S-ADP initially interacts reversibly with aggregin (100kDa), a putative ADP receptor, and induces platelet shape change and aggregation, and at longer periods of incubation reacts irreversibly to block the ability of ADP to antagonize stimulated adenylate cyclase activity. In contrast, 6-BrCH2(CO)2CH2S-ADP was found to be a weak and reversible inhibitor of ADP-induced platelet aggregation. Prior incubation of platelets with the latter analog reduced labeling of aggregin by 8-BrCH2(CO)2CH2S-[32P]ADP. Taken together, the results further show that substitution by the BrCH2(CO)2CH2 group at the C2 and C8 positions is tolerated, while the presence of a free amino function at the C6 position is essential for its interaction with aggregin.

Cytoplasmic Ca2+ concentrations in intact Merkel cells of an isolated, functioning rat sinus hair preparation

An isolated, functioning sinus hair preparation was developed to investigate cytoplasmic Ca2+ concentrations in intact Merkel cells using microfluorimetric techniques. Intracellular Ca2+ levels were monitored by means of photon counters in small groups of Merkel cells loaded with the calcium fluorescent indicators fura-2 or fluo-3. Mechanical stimulation of Merkel cells with fine glass rods resulted in small transient increases in intracellular Ca2+ levels (by about 20%) in the group of Merkel cells around the stimulating probe. A rise in Ca2+ is presumed to be essential for the postulated synaptic transmission to the afferent nerve terminal. Depolarization with a high concentration of potassium chloride (100 mM) caused increases in intracellular Ca2+ concentrations in Merkel cells (by about 70%) only in the presence of extracellular Ca2+, indicating an influx of Ca2+ through voltage-gated channels. The Ca2+ response was abolished neither by (+)-BayK8644 nor omega-conotoxin, suggesting that the Ca2+ channels are different from the classical L- or N-type channels. Extracellular application of ATP (10 microM to 5 mM) caused dose-dependent increases in intracellular Ca2+ levels in Merkel cells of up to sevenfold from the basal level of about 100 nM. Similar responses to ATP were also measured during superfusion with Ca(2+)-free medium, suggesting intracellular stores as the main Ca2+ source. Pre-incubation of Merkel cells with the purinoceptor antagonist suramin (100 microM) for 30 min reduced the Ca2+ responses to ATP by about 50% compared with control conditions. In conclusion, the results have demonstrated that a rise in intracellular Ca2+ in Merkel cells can be evoked by mechanical stimulation, membrane depolarization and chemical stimulation by ATP. These observations strongly suggest a possible contribution of Ca2+ to the normal responsiveness of Merkel cell mechanoreceptors, in turn supporting the hypothesis that Merkel cells are involved in the mechano-electric transduction process in sinus hair type I mechanoreceptors.

Enhancement of sympathetic purinergic neurotransmission in the guinea-pig isolated vas deferens by the novel ecto-ATPase inhibitor ARL 67156

1. Field stimulation of the sympathetic nerves of the guinea-pig isolated vas deferens with trains of pulses of 20 s at 1-8 Hz produced characteristic biphasic contractions. The effect of the novel ecto-ATPase inhibitor, 6-N,N-diethyl-D-beta, gamma-dibromomethyleneATP (ARL 67156, formerly known as FPL 67156), on the magnitude of the initial, predominantly purinergic peak of this response was studied in order to determine the influence of enzymatic degradation of adenosine 5'-triphosphate (ATP) on its action as a neurotransmitter. 2. The peak magnitude of the response to nerve stimulation was significantly increased in a concentration-dependent manner by ARL 67156 (5-100 microM) and the size of the neurogenic response at 4 Hz was approximately doubled in the presence of ARL 67156 (100 microM). 3. ARL 67156 (100 microM) has a rapid onset of action. The enhancing effect on neurogenic contractions was maximal after 10 min, was well maintained for at least 30 min and was rapidly reversed, with responses returning to control levels 10 min after washout. 4. The neurogenic contraction in the presence of prazosin (0.1 microM) was purely purinergic, as it was abolished by the P2-purinoceptor antagonist, PPADS (100 microM). ARL 67156 (100 microM) produced a similar degree of enhancement of neurogenic responses in the absence and presence of prazosin, supporting the view that the enhancing effects of ARL 67156 on neurogenic contractions result from potentiation of the action of ATP. 5. Exogenous ATP and alpha, beta-methyleneATP produced rapid transient contractions. Responses to ATP were increased in magnitude and duration in the presence of ARL 67156 (100 microM), whereas those to the stable analogue, alpha, beta-methylene ATP were not significantly affected. 6. Contractions to exogenous noradrenaline (10 microM) and KCl (40 mM) were significantly enhanced by ARL 67156 (100 microM), but this potentiation was abolished by PPADS (100 microM). Therefore, this effect of the ecto-ATPase inhibitor may be due to a build up of endogenous ATP, increasing the sensitivity of the smooth muscle to other agonists. 7. It is concluded that ARL 67156 potentiates the action of ATP, and that when ATP acts as a neurotransmitter its postjunctional actions are greatly attenuated by enzymatic degradation.

Effect of vesamicol on the release of ATP from cortical synaptosomes

The aim of the present experiments was to test whether vesamicol alters the evoked release of ATP from nerve terminals. Continuous or cumulative release of ATP evoked by 33 mM KC1 from rat cerebrocortical synaptosomes was largely calcium-dependent. Vesamicol interfered with release of ATP from synaptosomes depolarized with KCl (33 mM) in a dose-dependent and stereoselective way. The (-)-vesamicol decreased the output of ATP in doses much lower than (+)-vesamicol. The release of the major excitatory neurotransmitter glutamate from depolarized nerve endings was not impaired by vesamicol. We suggest that vesamicol may alter the release of ATP specifically, probably by interacting with a protein similar to the vesamicol receptor found in cholinergic synaptic vesicles.

Cloning of the rat P2u receptor and its potential role in coronary vasodilation

We cloned and sequenced the cDNA as well as the genomic DNA of the P2u receptor gene from the rat. The coding region of the gene is not interrupted by introns. P2u is expressed in a variety of rat organs with pronounced differences of expression intensities. Highest expression was found in liver and testis, while no expression could be detected in the brain. High P2u expression was found in primary microvascular endothelial cells from the rat heart, but not in cardiac myocytes. By in situ analysis, we localized P2u expression in epithelial cells of esophagus and bronchi. Functional analysis revealed that, in isolated perfused rat hearts, the P2u ligands UTP and ATP induce a pronounced vasodilation of coronary blood vessels. In contrast, UMP and uridine, the degradative products of UTP, act as potent vasoconstrictors. Our experiments suggest that, in the rat heart, endothelial P2u receptors are involved in the ATP/UTP-mediated vasodilation of coronary blood vessels.

A cation non-selective channel induced by extracellular ATP in macrophages and phagocytic cells of the thymic reticulum

Extracellular ATP4- can bind to P2Z purinergic receptors including depolarization and cytoplasmic membrane permeabilization to small molecular weight solutes in macrophages, thymocytes, mast cells, phagocytic cells of the thymic reticulum and other cell types. An ATP(4-)-induced cation current has been described in whole-cell records of some of these cells but it is currently not clear whether these currents and the phenomenon of membrane permeabilization are a consequence of only one type of P2Z-associated channel/pore or two different phenomena triggered by one or more receptors. Here we use the outside-out patch-clamp technique to describe a single channel associated with this cation current in two murine phagocytic cells: intraperitoneal macrophages and phagocytic cells of the thymic reticulum. Multi channel currents could be readily observed in 77% of the outside-out patches of macrophages. Single channels of 7.8 pS could usually be resolved only in tail currents. Reversal potential measurements and ion replacement experiments indicated a lack of cation selectivity, similarly to what has already been described for the ATP(4-)-induced whole-cell inward current. No large-conductance channels that could explain the permeabilization to small molecular weight studies solutes was observed under our experimental conditions. A single channel of approx. 5 pS was also observed in phagocytic cells of the thymic reticulum under similar conditions. We conclude that the channel here described is the main carrier of cation current usually associated with the binding of ATP4- to P2Z receptors in whole-cell and outside-out patch-clamp experiments.

Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells

In addition to their well-established roles as neurotransmitters and neuromodulators, growing evidence suggests that nucleotides and nucleosides might also act as trophic factors in both the central and peripheral nervous systems. Specific extracellular receptor subtypes for these compounds are expressed on neurons, glial and endothelial cells, where they mediate strikingly different effects. These range from induction of cell differentiation and apoptosis, mitogenesis and morphogenetic changes, to stimulation of synthesis or release, or both, of cytokines and neurotrophic factors, both under physiological and pathological conditions. Nucleotides and nucleosides might be involved in the regulation of development and plasticity of the nervous system, and in the pathophysiology of neurodegenerative disorders. Receptors for nucleotides and nucleosides could represent a novel target for the development of therapeutic strategies to treat incurable diseases of the nervous system, including trauma- and ischemia-associated neurodegeneration, demyelinating and aging-associated cognitive disorders.

A novel P2-purinoceptor expressed by a subpopulation of astrocytes from the dorsal spinal cord of the rat

1. Astrocytes from the dorsal spinal cord express P2-purinoceptors which, when stimulated, produce a rise in the intracellular level of free Ca2+ ([Ca2+]i). Previously we have found that the P2Y class of receptor is expressed by nearly all astrocytes from the dorsal horn. To determine whether other metabotropic P2-purinoceptor classes are also present, in this study we investigated the effects of UTP. 2. Application of UTP (1-500 microM, 5-20 s) produced a transient rise in [Ca2+]i in a subpopulation of astrocytes. The magnitude of the peak increase in [Ca2+]i was dependent upon UTP concentration and the EC50 was found to be 5.2 +/- 0.2 microM. Ca2+ responses were maximum at 100 microM UTP. 3. The rise in [Ca2+]i in response to UTP was not affected by removal of extracellular Ca2+. On the other hand, application of the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, abolished responses to UTP. These findings indicate that UTP stimulates the release of Ca2+ from a thapsigargin-sensitive intracellular pool. 4. The Ca2+ response to UTP was unaffected by treatment with pertussis toxin, suggesting that UTP responses may be mediated via a pertussis toxin-insensitive G protein. 5. While all cells tested (n = 52) responded to the P2Y-purinoceptor agonist, 2-methylthio-ATP, only a subpopulation of astrocytes (n = 67/93) was responsive to UTP. The presence of UTP-sensitive and UTP-insensitive cells requires the existence of two discrete types of receptor. One receptor, expressed by UTP-insensitive cells, appears to be activated selectively by 2-methylthio-ATP. 6. To investigate whether UTP and 2-methylthio-ATP activate a common type of receptor in UTP-responsive cells, a cross-desensitization strategy was used. Desensitization with prolonged exposure to a high concentration of 2-methylthio-ATP failed to affect responses to UTP and vice versa, indicating that receptors activated by UTP are distinct from those activated by 2-methylthio-ATP. 7. The P2-purinoceptor antagonist, suramin (100 microM), blocked Ca2+ responses to UTP and to 2-methylthio-ATP. 8. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), has been reported to block responses mediated by P2X- and P2Y-purinoceptors in other systems and therefore we investigated its effects on responses to 2-methylthio-ATP and to UTP. PPADS was found to block Ca2+ responses to 2-methylthio-ATP in a concentration-dependent manner with an IC50 of 0.92 +/- 0.1 microM. PPADS also blocked UTP-evoked responses and the IC50 was 7.2 +/- 1.9 microM. At a concentration of 10 microM, PPADS produced a rightward shift in the dose-response curve for UTP and did not affect the maximum response. 9. Calcium responses evoked by the muscarinic agonist, carbachol, were unaffected either by suramin (100 microM) or by PPADS (50 microM). 10. The present results indicate the presence of a novel class of metabotropic P2U-purinoceptor in dorsal spinal astrocytes. In contrast to P2Y-purinoceptors, the P2U-purinoceptor is expressed only by a subpopulation of astrocytes and its sensitivity to suramin and PPADS distinguish this receptor from P2U-purinoceptors found in other tissues.

Molecular cloning and functional analysis of a novel P2 nucleotide receptor

The cDNA encoding a novel P2 receptor was isolated from rat aortic smooth muscle cell library and functionally characterized. The cloned P2 receptor exhibits structural features characteristic of the G protein-coupled receptor family and shows 44 and 38% amino acid identity with previously cloned rat P2U and chicken P2Y receptors, respectively. The cloned P2 receptor is functionally coupled to phospholipase C but not to adenylate cyclase in C6 rat glioma cells transfected with the cloned P2 expression vector. The rank order of agonist potency as judged by intracellular Ca2+ mobilization responses is UTP > ADP = 2-methylthioATP > ADP beta S > ATP = ATP gamma S, which is not compatible with any of the previously characterized P2 receptor subtypes. The nonselective P2 antagonists, suramin and reactive blue-2, inhibit nucleotide-induced phospholipase C activation in cells expressing the cloned P2 receptor. The cloned P2 receptor mRNA is abundantly expressed in various rat tissues including lung, stomach, intestine, spleen, mesentery, heart, and, most prominently, aorta. The results indicate that the novel metabotropic P2 receptor has pharmacological characteristics distinct from any of P2 receptor subtypes thus far identified and suggest the existence of a novel regulatory system by extracellular nucleotides of potential significance.

A comparison of the effects of exogenous and endogenous prostaglandins on fast and slow contractions of field-stimulated guinea-pig vas deferens

1. This study has compared the effects of exogenous and endogenous prostaglandins on the two phases of contraction of the guinea-pig vas deferens produced by electrical field stimulation. Prostaglandin E2 (PGE2), sulprostone and arachidonic acid dose-dependently and completely inhibited the first (fast) phase of contraction, with IC50s of 2.6 nM, 0.65 nM and 2.2 microM, respectively. 2. Following desensitization of the receptor for adenosine triphosphate (ATP) with alpha, beta-methylene ATP, PGE2, sulprostone and arachidonic acid dose-dependently inhibited the second (slow) phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation, but the inhibition was incomplete (up to only 30%). Indomethacin (2.8 microM) reduced the effect of arachidonic acid. On its own, indomethacin (0.3 to 6.0 microM) had no consistent effect although, on some tissues, a slight potentiation of the contractions was seen. 3. Cicaprost (a PGI2 analogue) at low concentrations (0.5 to 30 nM) potentiated the first phase of contraction but even at high concentrations, had no consistent effect on the second phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation. 4. PGE2, sulprostone and cicaprost potentiated contractions of the guinea-pig vas deferens produced by exogenous ATP. PGE2 and sulprostone also potentiated contractions produced by exogenous noradrenaline, whereas cicaprost had no consistent effect on the response to noradrenaline. 5. These findings indicate that prostaglandins of the E-series inhibit the second phase of contraction as well as the first phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation. However, the extent of the inhibition is much less for the second phase than for the first phase. The reasons for this differential action of PGE are not clear. 6. Cicaprost potentiates the first phase but not the second phase of contraction. Since cicaprost potentiates the contractions produced by exogenous ATP, but not by exogenous noradrenaline, by an action presumably on post-junctional IP receptors, the potentiating action of cicaprost on the first phase of contraction produced by electrical field stimulation would appear to be satisfactorily explained through the action of cicaprost on these post-junctional IP receptors. 7. Exogenous arachidonic acid is apparently converted predominantly to PGE2 by the vas deferens, since the action of arachidonic acid mimicked that of PGE2 and was reduced by indomethacin. However, there was little evidence that sufficient PGE2 is generated during a short period (15 s) of sympathetic nerve stimulation for it to have any significant inhibitory effect on the size of the contractions produced.

Agonists for endothelial P2 purinoceptors trigger a signalling pathway producing Ca2+ responses in lymphocytes adherent to endothelial cells

Recirculation of lymphocytes through the body involves their frequent adhesion to endothelial cells but little is known of the signalling pathways between these two cell types. Lymphocytes from patients with chronic lymphocytic leukaemia were loaded with the Ca(2+)-sensitive indicator, fura 2, and allowed to adhere to either glass or monolayers of human umbilical-vein endothelial cells. Addition of ATP or UTP (1-10 microM) to the superfusate produced a transient rise in cytosolic Ca2+ concentration in the lymphocytes adherent to endothelium (24 of 35 cells). In contrast, ATP or UTP (1-10 microM) had no effect on the cytosolic Ca2+ of lymphocytes attached to glass. As the only lymphocyte receptor for ATP (P2Z class) requires higher ATP concentrations ( > 50 microM) for Ca2+ influx and is unresponsive to UTP, the involvement of a lymphocyte P2Z purinoceptor is unlikely. Various agonists including ATP, UTP, 2-methylthioATP, ADP and histamine all stimulated increases in endothelial cytosolic Ca2+ but only ATP and UTP (both agonists for endothelial P2U purinoceptors) triggered Ca2+ transients in adherent lymphocytes. Removal of extracellular Ca2+ did not abolish the ATP-induced rise in cytosolic Ca2+ concentration in lymphocytes adherent to endothelial cells. These findings show that stimulation of endothelial P2U purinoceptors triggers an endothelial-lymphocyte signalling pathway which releases internal Ca2+ in adherent lymphocytes.