Extracellular adenosine 5'-triphosphate-evoked norepinephrine secretion not relating to voltage-gated Ca channels in pheochromocytoma PC12 cells

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

Nerve growth factor-stimulated neuronal differentiation induces changes in P2 receptor expression and nucleotide-stimulated catecholamine release

Extracellular nucleotides modulate synaptic transmission and neuronal communication by activating purinergic 2 (P2) (nucleotide) receptors. Here, we assessed changes in the regulation by nucleotides and their receptors of an important physiological response - release and uptake of catecholamines - that accompanies sympathoadrenal neuronal differentiation. Nerve growth factor (NGF)-promoted differentiation of pheochromocytoma 12 (PC12) cells enhanced the ability of the non-hydrolyzable ATP analog, ATPgammaS, to stimulate catecholamine (norepinephrine, NE) release and this enhancement occurred without a significant alteration in NE uptake. In addition to ATPgammaS, 2-MeSATP and alphabetaMeATP, P2X receptor-selective agonists, caused greater NE release from NGF-differentiated than from undifferentiated PC12 cells. NGF-differentiated PC12 cells had altered mRNA expression of several P2Y and P2X receptors but protein expression was only increased for P2X, in particular P2X(1-4,) receptors and P2X, but not P2Y, receptor inhibitors blunted the NGF-promoted enhancement in nucleotide-regulated catecholamine release. Surprisingly, siRNA directed against P2X(2), the receptor with the highest expression, failed to alter NE release by ATPgammaS. These findings indicate that sympathetic neuronal differentiation by NGF increases both the expression of P2X receptor sub-types and their regulation of catecholamine release. NGF-promoted increased expression of P2X receptors thus appears to be a physiologically important response that characterizes sympathetic neuronal differentiation.

Retinoic acids increase P2X2 receptor expression through the 5'-flanking region of P2rx2 gene in rat phaeochromocytoma PC-12 cells

The P2X2 receptor is a subtype of ionotropic ATP receptor and plays a significant role in regulating fast synaptic transmission in the nervous system. Because the expression level of the P2X2 receptor is known to determine its channel properties and functional interactions with other neurotransmitter channels, elucidating the mechanisms underlying the regulation of P2X2 receptor expression in neuronal cells is important. Here, we identified three motifs that correspond to the retinoic acid response element in the 5'-flanking region of the rat P2X2 gene. In rat pheochromocytoma PC-12 cells, treatment with 9-cis-retinoic acid as well as all-trans-retinoic acid significantly increased the mRNA and protein level of P2X2 receptor. In addition, in PC-12 cells transiently transfected with a luciferase reporter gene driven by the promoter region of the rat P2X2 gene, both 9-cis-retinoic acid and all-trans-retinoic acid increased the luciferase activity, whereas their effects were diminished by truncation of the retinoic acid response elements in the promoter. Furthermore, 9-cis-retinoic acid enhanced the ATP-evoked whole cell currents and intracellular Ca2+- and ATP-evoked dopamine release, indicating the up-regulation of functional P2X2 receptors on the plasma membrane. These results provide the molecular mechanism underlying the transcriptional regulation of P2X2 receptors and suggest that retinoid is an important factor in regulating P2X2 receptors in the nervous system.

Characterization of Exocytotic Events From Single PC12 Cells: Amperometric Studies in Native PC12h, DA-Loaded PC12h and Bovine Adrenal Chromaffin Cells

Exocytotic events from rat pheochromocytoma (PC12) cells were characterized by amperometric analysis. For single-cell amperometric recordings, PC12h cells cultured onto poly-L-lysine corted glass-base dish were incubated with 1 mM dopamine (DA) for 60 min. Amperometric recordings, with a carbon fiber microelectrode (5 mum diameter), of catecholamine release from the individual cells were conducted under an inverted microscope at 25 degrees C. To characterize a single exocytotic event that is detected as a single spike current, the spike number, spike parameters (rise time, middle width and area) and spike shape were analyzed. Exposure of DA-loaded PC12h cells to 60 mM KCl (1000 hps) for 5 min and for 4 s evoked a train of events with the event number of 114+/-19 (spikes/response for 5 min) and 12+/-3 (spikes/response for 15 s), respectively. We observed distinctive kinetics in the events (rise time=0.83+/-0.19 ms, middle width=2.89+/-0.62 ms, area=62+/-7.6 fC and the spikes with a "foot"=15.4+/-2.7% of total spikes). The number and mean height of the events were 3- to 4-fold higher than that in DA-unloaded cells, and the values of rise time and middle width in DA-loaded PC12h cells were approx. 5- and 10-fold less than those observed in cultured adrenal chromaffin cells. The successful application of amperometry to monitor DA released from secretory vesicles in DA-loaded PC12h cell suggest that this technique is applicable to characterize exocytotic events in neurons.

Factors affecting habituation of PC12 cells to ATP

Extracellular ATP triggers catecholamine secretion from PC12 cells by activating ionotropic purine receptors. Repeated stimulation by ATP leads to habituation of the secretory response. In this paper, we use amperometric detection to monitor the habituation of PC12 cells to multiple stimulations of ATP or its agonist. Cells habituate to 30 microm ATP slower than they do to 300 or 600 microm ATP. Modifying external Mg2+ affects the response of cells to 30 microm ATP, but does not affect habituation, suggesting that habituation does not necessarily correspond to either stimulus intensity or cellular response. Mg2+ affects the initial response of PC12 cells to 2MeSATP in a manner similar to ATP. Increasing external [Mg2+] to 3.0 mm, however, eliminates habituation to 2MeSATP. This habituation can be partially restored by costimulation with 100 microm UTP. Background application of UTP increases habituation to both ATP and 2MeSATP. This suggests that ATP-sensitive metabotropic (P2Y) receptors play a role in the habituation process. Finally, although Ca2+ influx through voltage-operated calcium channels does not appear to contribute to secretion during ATP stimulation, blocking these channels with nicardipine increases habituation. This suggests a role for voltage-operated calcium channels in the habituation process.

Selective modulation of ligand-gated P2X purinoceptor channels by acute hypoxia is mediated by reactive oxygen species

Purinergic excitatory synapses use ATP to mediate fast synaptic transmission via activation of P2X receptor cation channels, and this response can be altered by acute hypoxia. This study examined the effect of acute hypoxia on cloned homo- and heteromeric P2X2 and P2X3 receptors expressed in human embryonic kidney 293 cells. In cells expressing homomeric P2X2 receptors, perfusion of 5 microM ATP (EC25) induced an inward whole-cell current that showed little desensitization during repeated exposures under continuously normoxic conditions. Exposure to a hypoxic ATP solution (pO2, 25-40 mm Hg) significantly reduced the whole-cell current to 49% of normoxic control. This hypoxic inhibition of P2X2-mediated inward current was maintained across all potentials when a voltage-step protocol was applied. In contrast, currents mediated by homomeric P2X3 receptors or heteromeric P2X(2/3) receptors were insensitive to an acute hypoxic challenge. One mechanism whereby hypoxia may modulate P2X2 channels is via the production of reactive oxygen species (ROS). H2O2 (1.8 mM) reversibly reduced homomeric P2X2 whole-cell currents to 38% of control. Furthermore, H2O2 attenuated the effect of hypoxia on homomeric P2X2 whole-cell currents. Inhibitors of the mitochondrial electron transport chain that reduce (rotenone and myxothiazol) or increase (antimycin A) the production of ROS altered the magnitude of P2X2-mediated currents. In summary, this is the first report indicating that acute hypoxia is able to regulate the activity of any ligand-gated ion channel. Furthermore, our data show that acute hypoxia selectively modulates the P2X2 receptor and that the response of P2X2 receptor subunits to hypoxia is mediated through the mitochondrial production of ROS.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Mechanisms underlying the neuronal calcium sensor-1-evoked enhancement of exocytosis in PC12 cells

Neuronal calcium sensor-1 (NCS-1) or the originally identified homologue frequenin belongs to a superfamily of EF-hand calcium binding proteins. Although NCS-1 is thought to enhance synaptic efficacy or exocytosis mainly by activating ion channel function, the detailed molecular basis for the enhancement is still a matter of debate. Here, mechanisms underlying the NCS-1-evoked enhancement of exocytosis were investigated using PC12 cells overexpressing NCS-1. NCS-1 was found to have a broad distribution in the cells being partially distributed in the cytosol and associated to vesicles and tubular-like structures. Biochemical and immunohistochemical studies indicated that NCS-1 partially colocalized with the light synaptic vesicle marker synaptophysin. When stimulated with UTP or bradykinin, agonists to phospholipase C-linked receptors, NCS-1 enhanced the agonist-mediated elementary and global Ca2+ signaling and increased the levels of downstream signals of phosphatidylinositol 4-kinase. NCS-1 enhanced the UTP-evoked exocytosis but not the depolarization-evoked Ca2+ responses or exocytosis, suggesting that the enhancement by NCS-1 should involve phospholipase C-linked receptor-mediated signals rather than the Ca2+ channels or exocytotic machinery per se. Taken together, NCS-1 enhances phosphoinositide turnover, resulting in enhancement of Ca2+ signaling and exocytosis. This is a novel regulatory mechanism of exocytosis that might involve the activation of phosphatidylinositol 4-kinase.

Release of dopamine and ATP from PC12 cells treated with dexamethasone, reserpine and bafilomycin A1

The amounts and time courses of dopamine and ATP released from perfused PC12 cells were examined using a simultaneous on-line recording system. High KCl (60 mM) caused dopamine and ATP release with similar time courses. The relative amount of dopamine to ATP in the effluent was 9.5. In PC12 cells cultured with dexamethasone, reserpine or bafilomycin A1 for 2 days, these drugs did not affect increases of intracellular Ca2+ in response to high KCl. Dexamethasone doubled the amount of dopamine release induced by high KCl without changing the amount of ATP release. High KCl failed to cause dopamine release in reserpine-treated cells but evoked ATP release. Bafilomycin A1 decreased both high KCl-induced dopamine and ATP release. The ratio of released ATP to total adenine nucleotides and adenosine in response to high KCl was not changed by treatment with the drugs. These results suggest that dopamine and ATP are simultaneously released from secretory vesicles of PC12 cells, in which they are stored via different pathways. Similar to dopamine uptake into secretory vesicles, the H+-gradient across the vesicular membrane developed by vacuolar ATPase may play an important role in the vesicular uptake of ATP.

Stimulation of noradrenaline release by T-588, a cognitive enhancer, in PC12 cells

Previously, we reported that (R)-(-)-1-(benzo[b]thiophen-5-yl)-2-[2-(N,N- diethylamino)ethoxy] ethanol hydrochloride (T-588), a novel putative cognitive enhancer, stimulated noradrenaline (NA) release from rat cerebral cortical slices. In this study, we investigated the effects of T-588 compared to other secretagogues on NA release from PC12 cells. Addition of as little as 10 microM T-588 stimulated [3H]NA release in a dose-dependent and an extracellular Ca(2+)-independent manner from PC12 cells. Ten micromolar ionomycin-, 300 microM adenosine-5'-O-(gamma-thiotriphosphate)- and 10 microM forskolin-induced extracellular Ca(2+)-dependent [3H]-NA release was further enhanced by 30 microM T-588. Cytosolic synaptophysin and 25-kDa synaptosome-associated protein immunoreactivity was increased by addition of T-588 in a dose-dependent manner. Interestingly, increases in synaptic vesicle-related proteins triggered by T-588 had a 4-min lag time and were completely dependent on extracellular CaCl2. These findings suggest that T-588 stimulates NA release from PC12 cells in a Ca(2+)-independent manner. T-588 also induced the translocation of synaptic vesicles in a Ca(2+)-dependent manner.

Rat chromaffin cells lack P2X receptors while those of the guinea-pig express a P2X receptor with novel pharmacology

1. Whole-cell patch-clamp recording was used to determine the functional expression and pharmacological properties of P2X receptors in chromaffin cells dissociated from adrenal medullae of rats and guinea-pigs. 2. In rat chromaffin cells maintained in culture for 1 - 7 days, ATP and UTP failed to evoke any detectable response. 3. Guinea-pig chromaffin cells responded to ATP (100 microM) with a rapidly activating inward current. The amplitude of the response to ATP increased over the period cells were maintained in culture and so did the number of cells giving a detectable response, with 69% of cells responding after >/=4 days of culture. 4. The response to ATP desensitized slowly, and had a reversal potential of 2.5 mV. The EC50 for ATP was 43 microM. The potency order for ATP analogues was 2-MeSATP>ATP>ADP. Adenosine, UTP and alpha,beta-meATP were inactive. 5. Suramin (100 microM) and Cibacron blue (50 microM) inhibited the ATP (100 microM)-activated current by 51 and 47%, respectively. PPADS antagonized the response to ATP (100 microM) with an IC50 of 3.2 microM. 6. The ATP concentration-response curve shifted to the left at pH 6.8 (EC50, 19 microM) and right at pH 8.0 (EC50, 96 microM), without changing the maximal response. Zn2+ inhibited the response to ATP (100 microM) with an IC50 of 48 microM. 7. This study indicates that expression of ATP-gated cation channels in chromaffin cells is species dependent. The P2X receptors in guinea-pig chromaffin cells show many characteristics of the P2X2 receptor subtype.

Interaction between purinoceptor subtypes on hippocampal serotonergic transmission using in vivo microdialysis

The effects of purinoceptor subtypes on hippocampal extracellular serotonin levels were determined by using in vivo microdialysis. Perfusion with adenosine-5'-triphosphate (ATP) for 20 min produced concentration-dependent changes in hippocampal extracellular serotonin levels, which consisted of an initial rise phase, with levels increasing to 309% of control with 100 microM ATP, followed by a later rebound reduction phase, with levels decreasing to 6% of control. The P2X1-7 active P2 purinoceptor agonist, 2-methylthioATP (2-MeSATP: 100 microM) increased the extracellular serotonin level drastically (638%), while the P2X1,3 active P2 purinoceptor agonist, alpha, beta-methylene-L-ATP (alpha, beta-meATP: 100 microM) produced a small increase (132%) in the serotonin level. The P2X1,2,3,5,7 active P2 purinoceptor antagonist, suramin (100 microM), reduced the basal serotonin level (86%) and the ATP-evoked initial rise phase (from 309 to 254%) without affecting the late reduction phase. The adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT: 50 microM) potentiated the rising phase (167%) and abolished the subsequent ATP-evoked reduction phase. Perfusion with CPT and an adenosine A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX: 10 microM), reduced the ATP-evoked initial rise (to 181%) and abolished the late reduction phases of serotonin release. These results indicate that ATP-evoked hippocampal serotonin release is composed of an initial rise phase and a later reduction phase. The ATP-evoked initial rise phase might be produced by an activation of P2X purinoceptor function, whereas the late reduction phase was modulated by the activation of adenosine A1 receptor function by adenosine, metabolized from ATP in the synaptic cleft.

ATP-stimulated activation of the mitogen-activated protein kinases through ionotrophic P2X2 purinoreceptors in PC12 cells. Difference in purinoreceptor sensitivity in two PC12 cell lines

Extracellular purine nucleotides elicit a diverse range of biological responses through binding to specific cell surface receptors. The ionotrophic P2X subclass of purinoreceptors respond to ATP by stimulation of calcium ion permeability; however, it is unknown how P2X purinoreceptor activation is linked to intracellular signaling pathways. We report that stimulation of PC12 cells with ATP results in the activation of the mitogen-activated protein (MAP) kinases ERK1 and ERK2 and was wholly dependent upon extracellular calcium ions. Treatment of the cells with adenosine, AMP, ADP, UTP, or alpha,beta-methylene ATP was without effect; however, MAP kinase activation was abolished by pretreatment with suramin and reactive blue 2. The calcium-activated tyrosine kinase, Pyk2, acts as an upstream regulator of the MAP kinases and became tyrosine phosphorylated following treatment of the cells with ATP. We have ruled out the involvement of depolarization-mediated calcium influx because specific blockers of voltage-gated calcium channels did not affect MAP kinase activation. These data provide direct evidence that calcium influx through P2X2 receptors results in the activation of the MAP kinase cascade. Finally, we demonstrate that a different line of PC12 cells respond to ATP through P2Y2 purinoreceptors, providing an explanation for the conflicting findings of purine nucleotide responsiveness in PC12 cells.

Inhibition by imipramine of ATP-evoked responses in rat pheochromocytoma cells

The effect of imipramine on the ATP-evoked release of dopamine was analyzed in parallel with its effects on the rise in the intracellular Ca2+ concentration ([Ca2+]i) and current induced by ATP in rat pheochromocytoma PC12 cells. Imipramine (10-300 microM) inhibited the ATP-evoked release of dopamine and rise in [Ca2+]i in a concentration-dependent fashion though the effect of imipramine on the release was slightly more obvious. Imipramine also inhibited the ATP-activated inward current at a similar concentration range. These results show a new pharmacological profile of imipramine, namely the inhibition of P2X2 receptors.

Activation of P2Y2 receptors by UTP and ATP stimulates mitogen-activated kinase activity through a pathway that involves related adhesion focal tyrosine kinase and protein kinase C

We examined downstream signaling events that followed the exposure of PC12 cells to extracellular ATP and UTP, and we compared the effects of these P2 receptor agonists with those of growth factors and other stimuli. Based on early findings, we focused particular attention on the mitogen-activated protein (MAP) kinase pathway. ATP and/or UTP produced increases in tyrosine phosphorylation of multiple proteins, including p42 MAP (ERK2) kinase, related adhesion focal tyrosine kinase (RAFTK) (PYK2, CAKbeta), focal adhesion kinase (FAK), Shc, and protein kinase Cdelta (PKCdelta). MAP (ERK2) kinase activity (quantified by substrate phosphorylation) was increased by UTP, ATP, phorbol 12-myristate 13-acetate, ionomycin, and growth factors. UTP and ATP were equipotent (EC50 approximately 25 microM) in stimulating MAP kinase activity, suggesting that these effects were mediated via the Gi-linked P2Y2 (P2U) receptor. Consistent with this, the UTP- and ATP-promoted activation of MAP kinase was diminished in pertussis toxin-treated cells. Treatment of cells with pertussis toxin also reduced both the UTP-dependent increases in intracellular calcium ion concentration ([Ca2+]i) and the tyrosine phosphorylation of RAFTK. Similarly, when [Ca2+]i elevation was prevented using BAPTA and EGTA, the activation of MAP kinase by UTP and ionomycin was blocked, and the tyrosine phosphorylation of RAFTK was reduced. The UTP-promoted increase in MAP kinase activity was partially reduced in cells in which PKC was down-regulated, suggesting that both PKC-dependent and PKC-independent pathways were involved. PKCdelta, which increases MAP kinase activity in some systems, became tyrosine-phosphorylated within 15 s of exposure of cells to ATP or UTP; but epidermal growth factor, nerve growth factor, and insulin had little effect. UTP also promoted the association of Shc with Grb2. These results suggest that the P2Y2 receptor-initiated activation of MAP kinase was dependent on the elevation of [Ca2+]i, involved the recruitment of Shc and Grb2, and was mediated by RAFTK and PKC.

The effect of the neuropeptide substance P on desensitization of ATP receptors of PC12 cells

1. Patch clamp recording (whole cell configuration) was employed to investigate the modulatory action of substance P on inward currents elicited by adenosine 5'-triphosphate (ATP, focally applied via a pressure pipette) from phaechromocytoma (PC12) cells usually held at -70 mV. 2. Bath-applied substance P (0.2-20 microM) had no effect on baseline membrane current but reversibly reduced ATP peak currents in a concentration-dependent fashion. The depressant effect was not associated with a change in the ATP current reversal potential. 3. Equiamplitude peak responses induced by 50 microM or 5 mM ATP were differentially affected by substance P which preferentially reduced currents evoked by 5 mM ATP. In the presence of substance P a conditioning pulse of ATP evoked a stronger depression to subsequent test pulses of the same agonist. 4. Combined patch clamping and confocal laser imaging of intracellular Ca2+ ([Ca2+]i) of single PC12 cells showed that substance P (applied by a pressure pipette) per se had no effect on [Ca2+]i or current baseline, although it reduced the inward current and associated [Ca2+]i rise elicited by ATP. 5. These results are interpreted as due to facilitation by substance P of desensitization of ATP-gated P2x2 receptors of PC12 cells. It is proposed that the novel modulation by this peptide of ATP responses may serve as a model for further studies aimed at elucidating the action of substance P on purinergic neurotransmission.

Role of intracellular calcium in fast and slow desensitization of P2-receptors in PC12 cells

1. Combined whole-cell patch clamp recording and confocal laser scanning microscopy of [Ca2+]i transients were performed on single PC12 cells to study any correlation between membrane currents induced by ATP and elevation in [Ca2+]i. ATP was applied by pressure from micropipettes near the recorded PC12 cells continuously superfused at a fast rate. 2. Brief (20 ms) pulses of ATP elicited monophasic inward currents and [Ca2+]i increases. Long applications (2 s) of ATP (5 mM) evoked peak currents which rapidly faded during the pulse and were followed by a large rebound current, interpreted as due to rapid desensitization and recovery of P2-receptors. The associated [Ca2+]i increase grew monotonically to a peak reached only after the occurrence of the current rebound, indicating that it is unlikely this cation has a role in fast desensitization. 3. Both membrane currents and [Ca2+]i transients were linearly dependent on holding membrane potential, suggesting that Ca2+ influx is the predominant cause of [Ca2+]i elevation. This view was supported by experiments carried out in Ca(2+)-free solution. 4. Brief pulses of ATP applied after a desensitizing pulse (2 s) of the same elicited smaller inward currents and [Ca2+]i rises indicating a role for [Ca2+]i in controlling slow desensitization of P2-receptors. 5. This notion was confirmed in experiments with various [Ca2+]i chelators which differentially affected slow desensitization in relation to their buffering capacity, while sparing fast receptor desensitization. 6. These results suggest a role for [Ca2+]i in slow rather than fast desensitization of P2-receptors, thus proposing this divalent cation as an intracellular factor able to provide an efficient and reversible control over receptor activity induced by ATP.

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.

Cu2+ potently enhances ATP-activated current in rat nodose ganglion neurons

Several lines of evidence suggest a physiological role for Cu2+ in regulating nervous system function. In the present study using whole-cell patch-clamp recording, Cu2+ greatly enhanced current activated by 10 microM ATP in the majority of rat nodose ganglion neurons. The enhancement was concentration-dependent between 1 and 50 microM Cu2+, and had an EC50 of 6.1 microM. Cu2+ shifted the ATP concentration-response curve to the left in a parallel manner. However, Cu2+ did not enhance ATP-activated current in the presence of a maximally-effective concentration of Zn2+. The observations suggest that Cu2+ increases the affinity of the receptor for ATP by acting at the Zn2+ modulatory site. In addition, a subset of neurons in the nodose ganglion express ATP-gated receptor-channels that are insensitive to modulation by physiological concentrations of Cu2+, Zn2+ and protons.

P2 purinoceptor-mediated stimulation of adenylyl cyclase in PC12 cells

PC12 pheochromocytoma cells have P2 purinoceptors which are activated by ATP and coupled to Ca2+ influx and catecholamine release. Also PC12 cells have adenosine receptors coupled positively to adenylyl cyclase, and cyclic AMP regulates cell functions such as catecholamine release. The effects of ATP and ATP analogs on cyclic AMP accumulation in PC12 cells were investigated in this study. ATP and adenosine 5'-0-(3-thiotriphosphate) stimulated cyclic AMP accumulation at low concentrations up to 300 microM but showed inhibitory effects above this concentration. 2',3'-O-(4-Benzoyl)benzoyl ATP and 2-methylthio ATP showed similar effects, although the responses were very limited. Addition of adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) or beta, gamma-methylene ATP, but not alpha, beta-methylene ATP, stimulated cyclic AMP accumulation markedly without causing an inhibitory phase. The effects of ATP, ADP beta S and beta, gamma-methylene ATP were not inhibited by adenosine deaminase or specific antagonists to A1 and A2 adenosine receptors. Neither ADp beta S nor beta, gamma-methylene ATP showed any effect on Ca2+ influx or noradrenaline release. Suramin, a P2 receptors antagonists, had no inhibitory effect against ATP analog-stimulated cyclic AMP accumulation, although reactive blue 2 inhibited the beta, gamma-methylene ATP-stimulated reaction but not that up-regulated by ADP beta S. These findings suggest that the pharmacological characteristics of these ATP receptors coupled to adenylyl cyclase are clearly different from those of ligand-gated ion channels defined by P2X purinoceptors, which have been cloned and shown to be coupled to Ca2+ influx and catecholamine release in PC12 cells. The existence of a new type of P2 purinoceptor-mediating stimulation of adenylyl cyclase is proposed in PC12 cells.

Brain synaptosomes display a diadenosine tetraphosphate (Ap4A)-mediated Ca2+ influx distinct from ATP-mediated influx

Studies undertaken to compare the effects of Ap4A and ATP on altering intrasynaptosomal Ca2+ levels from deermouse brain reveal that both ligands induce a rapid influx of extracellular Ca2+. The Ca2+ profile elicited by 167 microM Ap4A is "spike-like" (half-time for decline to baseline, 19.1 +/- 1.2 sec), in contrast to the gradual decline observed with ATP (104.0 +/- 7.4 sec). DIDS (4-4'-diisothiocyano-2,2'-disulfonic acid stilbene) and suramin preincubation alter only the ATP-induced Ca2+ profile. Cross-desensitization studies indicate that prior application of ATP does not significantly affect the Ca2+ influx elicited by Ap4A, and that prior application of Ap4A does not affect the Ca2+ influx elicited by ATP. These results demonstrate that extracellular Ap4A and ATP elicit distinct intrasynaptosomal Ca2+ influx profiles, and suggest that these two nucleotides may be interacting with distinct purinoceptor subclasses or purinoceptor-effector complexes. Subjecting the synaptosomes simultaneously to depolarization and Ap4A, or to depolarization and ATP, induces an additive effect on Ca2+ influx. Preincubation with verapamil negates the effects of depolarization without modifying the ligand-elicited Ca2+ fluxes. These results indicate the presence of Ap4A and ATP ligand-gated channels that may function as modulators of neuronal activity.

ATP receptor activation potentiates a voltage-dependent Ca channel in hippocampal neurons

Activation of a purinergic P2 receptor by adenosine 5'-triphosphate (ATP) has previously been shown to open a non-selective cation channel with a reversal potential of approximately 0 mV. We examined the effect of P2 receptor activation on voltage-gated ionic currents in acutely isolated CA3 pyramidal neurons from guinea pig hippocampus using the whole-cell-patch technique. Under conditions designed to isolate current through voltage-dependent Ca channels (ICa), ATP (50 microM) potentiated ICa by 36%. This increase in ICa desensitized back to control levels within 4 min. In contrast to the non-selective cation channel, ICa elicited from a holding potential (HP) of -100 mV showed significant potentiation in response to ATP when depolarized to a test potential (TP) of -10 mV but showed no effect on ICa when the same neuron was alternately depolarized to TP = -70 mV. No change in holding current at HP = -100 mV occurred. Tail currents were unaffected by ATP exposure suggesting that ICa potentiation was not due to modulation of L-type Ca channels. This potentiation was also observed either with ATP-gamma-s, the slowly hydrolyzable ATP analog, or with ATP in the presence of alpha, beta-methylene-ADP, an ectonucleotidase inhibitor, indicating that the effects observed were not due to activation of an adenosine receptor that required ATP hydrolysis. The potentiation of ICa was not observed with the P2X agonist, beta, gamma-methylene-ATP. These results suggest that ATP receptors can modulate voltage- as well as ligand-gated channels permeable to calcium and may play an important role in the dynamics of intracellular Ca2+ in these neurons.

Potassium current inhibition by nonselective cation channel-mediated sodium entry in rat pheochromocytoma (PC-12) cells

Under physiological conditions, nonselective cation (NSC) channels mediate the entry of cations into cells, the most important being Na+ and Ca2+. In contrast to the Ca(2+)-dependent signaling mechanisms, little is known about the consequences and the spatial distribution of intracellular [Na+] elevation. In this study we demonstrate that Na+ entry, during the opening of ATP-activated NSC channels, leads to an inhibition of voltage-dependent K+ currents (IK) in cromaffin-like undifferentiated PC-12 cells. The effect was dependent on the charge carrier as well as on the density of the ATP-activated current. Extracellular alkali cations (Na+, Li+) were more efficient than NH4+ in suppressing IK. Intracellular infusion of Na+ had the same effect as Na+ influx through ATP-activated NSC channels. The inhibition of IK persisted when the total ATP-induced Na+ entry was reduced by membrane depolarization, suggesting a spatial restriction of the required Na+ accumulation. Our results indicate that NSC channels influence the function of other ion channels by changing local intracellular ion concentrations.

Potentiation by cadmium ion of ATP-evoked dopamine release in rat phaeochromocytoma cells

1. The effects of cadmium ion (Cd2+) on release of dopamine and on an inward current evoked by extracellular ATP were investigated in rat phaeochromocytoma PC12 cells. 2. Cd2+ (100 microM-3 mM) potentiated the dopamine release evoked by 30 microM ATP from the cells. Cd2+ (100 microM) shifted the concentration-response curve of ATP-evoked dopamine release to the left without affecting the maximal response. 3. Suramin (30 microM) completely abolished the dopamine release evoked by 30 microM ATP but only partially inhibited the release evoked by 100 microM ATP consistent with its role as a competitive antagonist. The response evoked by 30 microM ATP in the presence of Cd2+ (300 microM) was comparable to that observed with 100 microM ATP alone; however, only the former was almost completely inhibited by suramin. 4. Cd2+ (100 microM) potentiated an inward current activated by 30 microM ATP alone. A higher concentration of Cd2+ (300 microM) had a smaller effect on amplitude potentiation but significantly prolonged the duration of the current. 5. The time-course of the ATP-evoked dopamine release was investigated using a real-time monitoring system for dopamine release. Although Cd2+ (300 microM) had little effect on the time-course of activation the ATP-evoked dopamine release, it produced a long-lasting dopamine release which slowly returned to the baseline. 6. Taken together, these observations suggest that Cd2+ enhances ATP-evoked dopamine release by affecting P2-purinoceptor/channels. The enhancement may be attributed to a Cd(2+)-dependent increase in sensitivity to ATP.

Characteristics of ATP-induced catecholamine secretion from adrenal chromaffin cells of the guinea-pig

The effect of ATP on catecholamine secretion and intracellular Ca2+ concentration ([Ca2+]i) was examined using perfused adrenal glands and dispersed chromaffin cells of the guinea-pig. The application of ATP (2 to 10 mM) for 2 min caused a dose-dependent increase in catecholamine secretion from perfused adrenal glands. ADP, but neither AMP nor adenosine, was also effective in increasing catecholamine secretion, though its potency was much less than that of ATP. 3. The ATP-induced secretory response was observed even under Na(+)-deficient conditions, but was reversibly abolished by removal of extracellular Ca2+. 4. In dispersed chromaffin cells, ATP (0.5 mM) caused increases in catecholamine secretion and [Ca2+]i, both of which were abolished after the removal of extracellular Ca2+. 5. These results indicate that ATP released from adrenal chromaffin cells together with catecholamine may exert a positive feedback influence on the following exocytotic catecholamine secretion. This effect may be accomplished by increasing the entry of extracellular Ca2+, but not by mobilizing intracellular Ca2+ stores in adrenal chromaffin cells of the guinea-pig.

ATP increases extracellular dopamine level through stimulation of P2Y purinoceptors in the rat striatum

The effect of ATP on release of dopamine (DA) from rat striatum was studied using in vivo microdialysis. ATP increased the striatal extracellular levels of DA dose-dependently. These analogs produced an increase in DA according to this order of potency: 2-methylthio ATP > ATP > or = alpha,beta-methylene ATP > ADP > AMP > adenosine. Adenosine 5'-[beta, gamma imido]-triphosphate had a more prolonged effect on the increase in DA level than ATP. The ATP-induced increase in DA was inhibited by adding suramin, a nonselective P2 purinoceptor antagonist, and reactive blue 2, a P2Y purinoceptor antagonist, but not inhibited by xanthine amine congener, an adenosine receptor antagonist. Pertussis toxin reduced the increase in DA produced by ATP, which suggests that the P2 purinoceptor may be coupled with a G-protein in the rat striatum. Results suggest that P2Y purinoceptors may involve an ATP-induced increase in DA. The ATP-induced release of DA was tetrodotoxin-sensitive, Ca(2+)-dependent and was abolished by omega-conotoxin GVIA, indicating that the opening of voltage-sensitive Na+ channel and the Ca2+ influx through the N-type voltage-dependent calcium channel are both required for the ATP-induced increase in DA. The ATP-induced increase in DA is presumably due to the release of DA via the stimulation of P2Y purinoceptors in the rat striatum.

Cytosolic calcium responses to extracellular adenosine 5',5" '-P1,P4-tetraphosphate in PC12 cells

Binding of adenosine 5',5" '-P1,P4-tetraphosphate (Ap4A) to a purinoceptor on nerve growth factor-differentiated (NGF) pheochromacytoma (PC12) cells modulated cytosolic Ca2+ levels. Both Ap4A and ATP elicited an influx of extracellular Ca2+, but both the sensitivity of the response and the flux profile were different. Preincubation of the PC12 cells with the compounds adenosine 5'-0-(2-thio)diphosphate (ADP-beta-S) and periodate-oxidized ATP had differential effects upon the Ap4A and ATP-induced response. These results indicate that Ap4A and ATP were either interacting with distinct purinoceptor subclasses or with the same purinoceptor with differing affinities. Simultaneous depolarization and application of either Ap4A or ATP to the PC12 cells induced an additive effect on the calcium flux. Preincubation with verapamil negated the effects of depolarization without significantly modifying the ligand-elicited Ca2+ fluxes, suggesting the presence of Ap4A ligand-gated channels that may function as modulators of PC12 cell function.

ATP receptor-mediated increase of Ca ionophore-stimulated arachidonic acid release from PC12 pheochromocytoma cells

Phospholipase A2 has recently been proposed as the effector enzyme involved in the receptor-mediated release of arachidonic acid (AA). Released AA and its metabolites have been demonstrated to play an important role in the regulation of cell functions. [3H]AA release from prelabeled PC12 cells was stimulated by a Ca ionophore such as ionomycin or A23187. Although ATP and its effective analog, adenosine 5'-O-(3-thiotrisphosphate) (ATP gamma S), 2-methylthio ATP and 3'-O-(4-benzoyl)benzoyl ATP, did not stimulate [3H]AA release on their own, they did enhance Ca ionophore-stimulated [3H]AA release. The effect of ATP analogs was dose-dependent. ADP, UTP, GTP, ITP, alpha beta-methylene ATP, beta gamma-methylene ATP and 8-bromo ATP showed no effect or very limited effect. The effect of ATP gamma S was antagonized by suramin, a putative P2Y receptor antagonist. The effective ATP analogs also increased [Ca2+]i (cytosolic free Ca2+ concentration) via Ca2+ influx. However, the addition of 50 mM KCl or 10 microM bradykinin, which are well-known to increase [Ca2+]i by different pathways, did not stimulate [3H]AA release, either with or without the Ca ionophore. The addition of phorbol 12-myristate 13-acetate, an activator of protein kinase C, showed no effect on [3H]AA release, either with or without the Ca ionophore. These data suggest that 1) ATP increased Ca ionophore-stimulated AA release via a P2Y-like ATP receptor, and that 2) the elevation of [Ca2+]i by ATP does not quantitatively explain the ATP-stimulated AA release in PC12 cells.

Inhibition by Zn2+ of uridine 5'-triphosphate-induced Ca(2+)-influx but not Ca(2+)-mobilization in rat phaeochromocytoma cells

1. Uridine 5'-triphosphate (UTP)-evoked increase in intracellular Ca2+ concentration ([Ca]i) and release of dopamine were investigated in rat phaeochromocytoma PC12 cells. UTP (1-100 microM) evoked an increase in [Ca]i in a concentration-dependent manner. This response was decreased to about 30% by extracellular Ca(2+)-depletion, but not abolished. This [Ca]i rise was mimicked by 100 microM ATP but not by 100 microM 2-methyl-thio-ATP or alpha,beta-methylene-ATP in the absence of external Ca2+, suggesting that the response was mediated by P2U purinoceptors, a subclass of P2-purinoceptors. 2. The UTP-evoked [Ca]i rise consisted of two components; a transient and a sustained one. When external Ca2+ was removed, the sustained component was abolished while the transient component was decreased by about 70% but did not disappear. These results suggest that UTP induces Ca(2+)-mobilization and, subsequently, Ca(2+)-influx. 3. The UTP-evoked increase in [Ca]i was not affected by Cd2+ (100 and 300 microM) or nicardipine (30 microM), inhibitors of voltage-gated calcium channels, but was significantly inhibited by Zn2+ (10-300 microM) in the presence of external Ca2+. Zn2+, however, did not affect the Ca2+ response to UTP in the absence of external Ca2+. 4. UTP (30 microM-1 mM) evoked the release of dopamine from the cells in a concentration-dependent manner. This dopamine release was abolished by Ca(2+)-depletion or Zn2+ but not by Cd2+ or nicardipine. 5. Taken together, the data demonstrate that UTP stimulates P2U-purinoceptors and induces a rise in [Ca]i both by Ca(2+)-mobilization and Ca(2+)-influx in PC12 cells. The dopamine release evoked by UTP requires external Ca2+ which may enter the cells through pathways sensitive to Zn2+ but insensitive to Cd2+ or nicardipine.

Cultured chick sympathetic neurons: ATP-induced noradrenaline release and its blockade by nicotinic receptor antagonists

The ATP-induced increase in tritium outflow from cultured chick sympathetic neurons prelabelled with [3H]-noradrenaline was investigated. Seven days-old dissociated cell cultures of embryonic paravertebral ganglia, loaded with [3H]-noradrenaline (0.05 microM), were superfused in the presence of (+)-oxaprotiline and exposed to ATP, ATP-analogues, or 1,1-dimethyl-4-piperazinium (DMPP) for 2 min. ATP (3 microM-3 mM), 2-methylthio-ATP (3-100 microM), as well as DMPP (10 and 100 microM) induced a significant overflow of tritium. The EC50-value of ATP was 20 microM. Both the ATP-induced and the DMPP-induced tritium overflow was Ca(2+)-dependent and sensitive to tetrodotoxin (0.3 microM) and omega-conotoxin (0.1 microM); in addition, it was inhibited by the alpha 2-adrenoceptor agonist 5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline (UK-14,304; 1 microM). The effects of ATP and DMPP were not additive. The ATP-induced as well as the DMPP-induced overflow of tritium was diminished by the P2-purinoceptor antagonists suramin (300 microM) and reactive blue 2 (3 microM); in all 4 cases, the inhibition amouted to approximately 40%. The tritium overflow induced by ATP or DMPP was almost abolished by the nicotinic receptor antagonist mecamylamine (10 microM) and markedly inhibited by hexamethonium (100 microM). Neither ATP nor electrical stimulation caused an overflow of tritium from cultures loaded with [3H]-choline. The results suggest that ATP at mumolar concentrations induces noradrenaline release from cultured chick sympathetic neurons via an action on a subclass of the nicotinic cholinoceptor.

ATP-evoked membrane current in guinea pig adrenal chromaffin cells

Adenosine 5'-triphosphate (ATP) evoked an inward current in whole-cell voltage-clamped adrenal chromaffin cells of the guinea pig. The reversal potential (Erev) of ATP-evoked current was about 0 mV in normal external solution and was shifted towards negative potentials by substituting Tris+ or sucrose, but not Ca2+, for the extracellular Na+. This current was mediated by the activation of non-selective cation channels and had some different properties from nicotinic current. It is suggested that these channels may function as a part of the ATP-induced Ca2+ influx pathway in guinea pig chromaffin cells.

Enhancement by zinc of ATP-evoked dopamine release from rat pheochromocytoma PC12 cells

The effects of zinc (Zn2+) on ATP-evoked dopamine release was investigated in rat pheochromocytoma PC12 cells. Zn2+ potentiated the dopamine release evoked by 30 microM ATP in a concentration-dependent manner over a concentration range from 3 to 300 microM. High concentration of Zn2+ (> 1 mM) inhibited the release. Zn2+ (10 microM) shifted the concentration-response curve of the ATP-evoked dopamine release to the left without affecting the maximal response. The dopamine release evoked by 40 mM KCl was not affected by Zn2+ (1-100 microM), whereas high concentration of Zn2+ ( > 300 microM) attenuated the response. The dopamine release evoked by 30 microM ATP in the presence of 10 microM Zn2+ were suppressed by suramin (30 microM), an antagonist to P2-purinoceptors, to an extent similar to that in the absence of Zn2+. Zn2+ (1-100 microM) enhanced the ATP-evoked increase in intracellular Ca2+ concentration ([Ca]i) in the cells. The Ca2+ responses to ATP in the presence and absence of Zn2+ were abolished by external Ca(2+)-depletion. Under whole-cell voltage-clamp, Zn2+ (10 microM) augmented by two-fold the peak amplitude of an inward current evoked by 30 microM ATP. Taken together, it is suggested that Zn2+ enhances the ATP-evoked dopamine release by increasing sensitivity to ATP. The enhancement may be due to the augmentation of ATP-gated Ca(2+)-influx, but not due to modulation of cellular machinery downstream to [Ca]i rise. The enhancement of the ATP-mediated responses may underlie modulation by Zn2+ of physiological functions in various types of neuronal cells.

Selective and competitive antagonism by suramin of ATP-stimulated catecholamine-secretion from PC12 phaeochromocytoma cells

1. Suramin, a putative P2-antagonist, (10 to 300 microM) inhibited the adenosine 5'-triphosphate (ATP)-stimulated secretion of [3H]-noradrenaline or endogenous dopamine from phaeochromocytoma PC12 cells in a concentration-dependent manner. Suramin (300 microM) did not affect the dopamine-secretion stimulated by high K+ or nicotine. 2. Suramin shifted the concentration-response curve for ATP to the right. The antagonism was competitive with a pA2 value of 4.52. 3. ATP also stimulated an increase in intracellular Ca2+ concentration as determined by fura-2 methods. Suramin antagonized this effect over the same concentration range that antagonized the ATP-stimulated catecholamine secretion. 4. These results suggest that suramin can be used as a selective and competitive antagonist of ATP in experiments concerning mechanisms of catecholamine-secretion.

Accentuation by pertussis toxin of the 5-hydroxytryptamine-induced potentiation of ATP-evoked responses in rat pheochromocytoma cells

We previously demonstrated that 5-hydroxytryptamine (5-HT) enhances the cationic current activated by extracellular ATP in rat pheochromocytoma PC12 cells. We report here that pertussis toxin (PTX) modulates this 5-HT-dependent enhancement in these cells. 5-HT potentiated ATP-evoked intracellular Ca2+ concentration ([Ca]i) rise and dopamine release over a concentration range from 1 to 100 microM. When cells were pre-treated with PTX, this potentiation was accentuated. Pretreatment with PTX also accentuated the 5-HT-dependent enhancement of the ATP-activated current. These results suggest that the enhancement by 5-HT of the ATP-evoked responses is negatively regulated by a mechanism mediated through PTX-sensitive GTP-binding protein.

Effect of intraperitoneal ATP on tumor growth and bone marrow radiation tolerance

Transplants of a spontaneous murine fibrosarcoma (FSaII) treated with intraperitoneal ATP were studied in vitro, and in both C3H and nu/nu mice. Daily ATP treatment prolonged tumor volume doubling time in vivo and in vitro. Daily ATP treatments at the maximally tolerated dose (2 mmol/kg i.p.) did not significantly affect the pH or the PCr/Pi, or beta ATP/Pi ratios (measured by MRS). In contrast to the reduced tumor growth rate, there was no change in bone marrow recovery after whole body irradiation. ATP is minimally toxic to animals at active dose levels. It slows tumor growth rate without adversely affecting bone marrow radiation tolerance. ATP might therefore be useful as a biological modifier of chemotherapy or radiation therapy.

Antidiuretic effects of ATP induced by microinjection into the hypothalamic supraoptic nucleus in water-loaded and ethanol-anesthetized rats

The effects of microinjection of purinoceptor agonists into the hypothalamic supraoptic nucleus (SON) on urination were examined in water-loaded and ethanol-anesthetized rats. Adenosine triphosphate (ATP), but neither adenosine diphosphate (ADP), adenosine monophosphate (AMP) nor adenosine, concentration-dependently decreased the urine outflow with concomitant increase in the urine osmotic pressure. The ED50 value for ATP was approx. 60 nmol. The antidiuretic effect of ATP was blocked either by prior injection of theophylline (an antagonist of the P1-type purinoceptor) or by intravenous administration d(CH2)5-D-Tyr(Et)-valine-arginine-vasopressin (VAVP). These results suggest that ATP injected into the SON has antidiuretic effects due to release of AVP through an activation of theophylline-sensitive purinoceptors.

Noradrenaline release from rat sympathetic neurons evoked by P2-purinoceptor activation

The effects of ATP and analogues on the release of previously incorporated 3H-noradrenaline were studied in cultured sympathetic neurons derived from superior cervical ganglia of neonatal rats. Electrical field stimulation (40 mA at 3 Hz) of the neurons for 10 s markedly enhanced the outflow of tritium. ATP applied for 5 s to 2 min at concentrations of 0.01 to 1 mmol/l caused a time- and concentration-dependent overflow with half maximal effects at about 10 s and 100 mumol/l, respectively. 2-Methylthio-ATP was equipotent to ATP in inducing 3H-overflow. ADP (100 mumol/l), when applied for 2 min, also caused a small 3H-overflow, but alpha, beta-methylene-ATP (100 mumol/l), AMP (100 mumol/l), R(-)N6-(2-phenylsiopropyl)-adenosine (R(-)-PIA; 10 mumol/l) and 5'-N-ethylcarboxamidoadenosine (NECA; 1 mumol/l) did not. The 3H-overflow induced by 10 s applications of 100 mumol/l ATP was abolished by suramin (100 mumol/l) and reduced by about 70% by reactive blue 2 (3 mumol/l). Electrically evoked overflow, in contrast, was slightly enhanced by suramin, but not modified by reactive blue 2. Xanthine amine congener (10 mumol/l) and hexamethonium (10 mumol/l) did not alter ATP-evoked release. Removal of extracellular Ca2+ from the medium reduced ATP- and electrically induced overflow by about 95%. Tetrodotoxin (1 mumol/l) abolished electrically evoked 3H-overflow but inhibited ATP-induced overflow by only 70%. The alpha 2-adrenoceptor agonist UK 14,304 at a concentration of 1 mumol/l diminished both electrically and ATP-evoked tritium overflow by approximately 70%. These results indicate that activation of P2-purinoceptors stimulates noradrenaline release from rat sympathetic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of capsaicin on acetylcholine-evoked responses in rat phaeochromocytoma cells

1. The effects of capsaicin on cellular responses evoked by acetylcholine (ACh) and those by adenosine 5'-triphosphate (ATP) were investigated in rat phaeochromocytoma PC12 cells. 2. Capsaicin (1 to 30 microM) suppressed dopamine release and the intracellular Ca2+ increase evoked by 100 microM ACh. The concentration-dependence of the ACh-evoked release of dopamine was not shifted but the maximal response was reduced by capsaicin. Dopamine release evoked by 80 mM KCl was also suppressed by capsaicin (3 and 30 microM), but the extent of suppression was smaller than that of the ACh-evoked release. 3. Under whole-cell voltage-clamp, capsaicin (1 to 30 microM) reversibly inhibited the inward current activated by ACh (30 to 300 microM). The inhibition exhibited dependence on the concentration of ACh, and the current activated by a higher concentration of ACh was less inhibited. Voltage-dependence of block by capsaicin was not observed when it was tested either by applying a ramp pulse during the current activation by ACh or by eliciting the current in cells held at various potentials. 4. High concentrations of capsaicin (30 to 100 microM) enhanced the inward current as well as dopamine release evoked by 30 microM ATP. 5. The results suggest that low concentrations of capsaicin selectively antagonize responses mediated by nicotinic receptor-channels without affecting those mediated by purinoceptor-coupled channels. As the antagonism by capsaicin of the ACh-evoked responses was observed at concentrations as low as 1 microM, the influence on nicotinic receptors should be taken into account when this compound is used as a pharmacological tool to deplete neuropeptides.

Dual modulation by adenosine of ATP-activated channels through GTP-binding proteins in rat pheochromocytoma PC12 cells

Effects of adenosine on inward current activated by extracellular ATP were examined in rat pheochromocytoma PC12 cells. Adenosine induced two types of modulation on the current activated by 30 microM ATP; a low concentration of adenosine (1 microM) inhibited the current whereas a high concentration (> 10 microM) enhanced the current. Neither the inhibition nor the enhancement was observed in cells pretreated with pertussis toxin (PTX), or in cells dialyzed with guanosine 5'-O-(2-thiotriphosphate) trilithium salt (GDP beta S). In contrast, dialysis with K-252a, a protein kinase inhibitor, abolished the inhibition, but not the enhancement. Adenosine induced similar inhibition and enhancement on ATP-evoked increase in intracellular free Ca2+ concentration. The results suggest that adenosine produces dual modulation on the ATP-activated channels through different mechanisms involving PTX-sensitive GTP-binding proteins.

Potentiation by adenosine of ATP-evoked dopamine release via a pertussis toxin-sensitive mechanism in rat phaeochromocytoma PC12 cells

1. The effects of adenosine on adenosine 5'-triphosphate (ATP)-evoked dopamine release from rat phaeochromocytoma PC12 cells was investigated to determine whether adenosine exerts a regulatory effect on the ATP-evoked response. Adenosine potentiated ATP (30 microM)-evoked dopamine release in a concentration-dependent manner over a concentration-range of 1 to 100 microM. Adenosine (100 microM) shifted the concentration-dependence of the ATP-evoked response to the left without affecting the maximal response. 2. Aminophylline, a non-selective adenosine receptor antagonist, and CP66713, a selective antagonist at the A2 subclass of adenosine receptors, abolished the adenosine-induced potentiation. Furthermore, 8-cyclopentyltheophylline, a selective antagonist at the adenosine A1 receptor partially inhibited the adenosine-evoked potentiation. CGS22492, a selective A2 receptor agonist, potentiated ATP-evoked dopamine release whereas N6-cyclohexyladenosine (CHA), a selective A1 receptor agonist, had no effect. 3. Pertussis toxin (PTX), a bacterial exotoxin which catalyzes the ADP-ribosylation of guanosine 5'-triphosphate (GTP)-binding proteins (G-proteins), inhibited the adenosine-induced potentiation of dopamine release. Dibutyryl cyclic AMP (db cyclic AMP), an analogue of cyclic AMP, had no effect on the release on the ATP-evoked response. 4. Adenosine potentiated the ATP-evoked rise in intracellular Ca2+ concentration ([Ca]i) in PC12 cells. This potentiation was also observed with CGS 22492 but not with CHA. PTX completely inhibited the adenosine-induced potentiation of the rise in [Ca]i. 5. On the basis of these findings, we suggest that the adenosine-induced potentiation of ATP-evoked dopamine release was due to an increase in [Ca]i in the cells. Although the potentiation is most likely mediated by a subclass of A2 receptors, the subclass may be different from those previously reported since the potentiation was sensitive to PTX and was not reproduced by db cyclic AMP.

Extracellular ATP triggers two functionally distinct calcium signalling pathways in PC12 cells

We have investigated the effects of extracellular ATP on Ca2+ signalling, and its relationship to secretion in rat pheochromocytoma (PC12) cells. In single cells, extracellular ATP evoked two very distinct subcellular distributions of intracellular calcium concentration ([Ca2+]i), only one of which could be mimicked by the pyrimidine nucleotide UTP, suggesting the involvement of more than one cell surface receptor in mediating the ATP-induced responses. ATP and UTP were equipotent in activating a receptor leading to inositol phosphate production and the mobilisation of intracellular Ca2+. In some cells (19%) this rise in [Ca2+]i initiated at a discrete site and then propagated across the cell in the form of a Ca2+ wave. In addition to mobilising intracellular Ca2+ through a ‘nucleotide’ receptor sensitive to ATP and UTP, the results indicate that ATP also activates divalent cation entry through an independent receptor-operated channel. Firstly, ATP-induced entry of Ca2+ or Mn2+ was independent of Ca2+ mobilisation, as prior treatment of cell populations with UTP abolished the ATP-evoked release of intracellular Ca2+ stores, but left the Ca(2+)- and Mn(2+)-entry components uneffected. Secondly, although UTP and ATP were equally effective in generating inositol phosphates, only ATP stimulated divalent cation entry, indicating that ATP-activated influx was independent of phosphoinositide turnover. Thirdly, single cell experiments revealed a subpopulation of cells that responded to ATP with divalent cation entry without mobilising Ca2+ from intracellular stores. Lastly, the dihydropyridine antagonist, nifedipine, reduced the ATP-induced rise in [Ca2+]i by only 24%, suggesting that Ca2+ entry was largely independent of L-type voltage-operated Ca2+ channels. The Ca2+ signals could also be distinguished at a functional level. Activation of ATP-induced divalent cation influx was absolutely required to evoke transmitter release, because ATP triggered secretion of [3H]dopamine only in the presence of external Ca2+, and UTP was unable to promote secretion, irrespective of the extracellular [Ca2+]. The results suggest that the same extracellular stimulus can deliver different Ca2+ signals into the same cell by activating different Ca2+ signalling pathways, and that these Ca2+ signals can be functionally distinct.

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

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

Stimulation of secretion in permeabilized PC12 cells by adenosine 5'-[gamma-thio]triphosphate: possible involvement of nucleoside diphosphate kinase

The addition of Ca2+, adenosine 5'-[gamma-thio]triphosphate (ATP[S]) or guanosine 5'-[gamma-thio]triphosphate (GTP[S]) to digitonin-permeabilized PC12 cells stimulates noradrenaline secretion. Both ATP[S] and GTP[S] stimulate release in the absence of Ca2+. Whereas ADP and adenosine 5'-[beta gamma-imido]triphosphate inhibited ATP[S]-stimulated release, they did not inhibit Ca(2+)-stimulated release even in the absence of added ATP. This suggests that the kinase which uses ATP[S] to induce secretion may not play an essential role in Ca(2+)-stimulated release. As GTP[S]-stimulated and ATP[S]-stimulated secretions were not additive, it seemed possible that stimulation by ATP[S] might in part result from the thiophosphorylation of GDP by nucleoside-diphosphate (NDP) kinase to form GTP[S]. The following results are consistent with this possibility. (1) A low concentration of GDP increased ATP[S]-stimulated secretion, but not GTP[S]-stimulated or Ca(2+)-stimulated secretion. (2) A variety of ribo- and deoxyribo-nucleoside di- and tri-phosphates inhibited ATP[S]-stimulated secretion, but not GTP[S]-stimulated or Ca(2+)-stimulated secretion. Thus, like NDP kinase, the kinase which uses ATP[S] to cause noradrenaline release appears to have a very low specificity for ATP. (3) Incubation of permeabilized cells in a sucrose-containing buffer resulted in the preferential loss of ATP[S]-stimulated secretion and a decrease in the level of NDP kinase. The addition of rat liver NDP kinase to those depleted cells partially restored ATP[S]-stimulated secretion.

ATP-induced secretion in PC12 cells and photoaffinity labeling of receptors

Secretion of catecholamines by rat PC12 cells is strongly stimulated by extracellular ATP via a P2-type purinergic receptor. ATP-induced norepinephrine release was inhibited 80% when extracellular Ca2+ was absent. Only four nucleotides, ATP, ATP gamma S, benzoylbenzoyl ATP (BzATP), and 2-methylthio-ATP, gave substantial stimulation of norepinephrine release from PC12 cells. ATP-induced secretion was inhibited by Mg2+, and this inhibition was overcome by the addition of excess ATP suggesting that ATP4- was the active ligand. ATP-induced secretion of catecholamine release was enhanced by treatment of cells with pertussis toxin or 12-O-tetradecanoylphorbol 13-acetate. The stimulatory effects of 12-O-tetradecanoylphorbol 13-acetate and pertussis toxin on norepinephrine release were additive. After brief exposure of intact cells to the photoaffinity analog, [alpha-32P]BzATP, two major proteins of 44 and 50 kDa and a minor protein of 97 kDa were labeled. An excess of ATP gamma S and BzATP but not GTP blocked labeling of the proteins by [32P]BzATP. Labeling of the 50-kDa protein was more sensitive to competition by 2-methylthio-ATP than the other labeled proteins, suggesting that the 50-kDa protein represents the P2 receptor responsible for ATP-stimulated secretion in these cells.

Organic hydroperoxide-induced activation of liver microsomal glutathione S-transferase of rats in vitro

The effect of t-butyl hydroperoxide (t-BuOOH), cumene hydroperoxide (CuOOH) or linoleic acid hydroperoxide (linoleic-OOH) on liver microsomal glutathione S-transferase of rats was studied in vitro. When microsomes were incubated with either 100 microM t-BuOOH or 25 microM CuOOH, glutathione S-transferase activity was increased 1.5-fold; activity was further increased to 2.2-fold in the presence of small amounts of glutathione. The same amounts of dithiothreitol or cysteine did not enhance the t-BuOOH or CuOOH-induced increase in transferase activity. The transferase activity was also increased 1.4-fold by 10 microM linoleic-OOH plus 1 microM glutathione. The increase in microsomal glutathione S-transferase activity after treatment of microsomes with t-BuOOH in the presence of glutathione was completely reversed by addition of dithiothreitol, whereas the activation of the transferase caused by t-BuOOH in the absence of glutathione was not reversed. Although microsomal glutathione S-transferase also possesses glutathione peroxidase activity, only transferase activity was increased by t-BuOOH in either the presence or absence of glutathione. These data indicate that microsomal glutathione S-transferase is activated by organic hydroperoxides in either the absence or presence of small amounts of glutathione, suggesting an activation of the transferase by thiol oxidation of the cysteine residue.