Sodium-dependent uptake of nucleosides by dissociated brain cells from the rat

Uridine prevents the glucose deprivation-induced death of immunostimulated astrocytes via the action of uridine phosphorylase

We previously reported that in immunostimulated astrocytes, glucose deprivation induced cell death via the loss of ATP, reduced glutathione, and mitochondrial transmembrane potential. The cytotoxicity was due to reactive nitrogen and oxygen species and blocked by adenosine, a purine nucleoside, via the preservation of cellular ATP. Here, we investigated whether uridine, a pyrimidine nucleoside, could prevent the glucose deprivation-induced cytotoxicity in LPS+IFN-gamma-treated (immunostimulated) astrocytes. Glucose deprivation induced the death of immunostimulated cells, which was significantly reduced by uridine. Glucose deprivation rapidly decreased cellular ATP levels in immunostimulated astrocytes, which was also reversed by uridine. The inhibition of cellular uptake of uridine by S-(4-nitrobenzyl)-6-thioinosine attenuated the protective effect of uridine. mRNA and protein expression for uridine phosphorylase, an enzyme catalyzing reversible phosphorolysis of uridine, were observed in rat brain as well as primary astrocytes. 5-(Phenylthio)acyclouridine (PTAU), a specific inhibitor of uridine phosphorylase, inhibited the protective effect of uridine. Additionally, the loss of mitochondrial transmembrane potential and reduced glutathione by glucose deprivation in immunostimulated cells was attenuated by uridine, which was also reversed by PTAU. These results provide the first evidence that uridine protects immunostimulated astrocytes against the glucose deprivation-induced death by preserving intracellular ATP through the action of uridine phosphorylase.

Trophic effects of purines in neurons and glial cells

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

Adenosine and the nervous system: pharmacological data and therapeutic perspectives

1. Adenosine acts on a family of G-protein-coupled receptors called purinoreceptors. 2. Four subtypes have been cloned and pharmacologically characterized. 3. The principal pharmacological data and structure-function relations for agonist interactions with P1 receptors are presented. 4. We conclude that the potent role of adenosine in the nervous system may be interesting for the development of drugs targeted at purines and their receptors.

Demonstration of the existence of mRNAs encoding N1/cif and N2/cit sodium/nucleoside cotransporters in rat brain

Nucleoside transport may be involved in the regulation of extracellular levels of adenosine, an inhibitory neuromodulator in the central nervous system. Previous reports have provided functional evidence for Na+-dependent nucleoside transport in rat brain. We isolated total RNA from various regions of rat brain and tested for the presence of mRNA for two recently cloned Na+/nucleoside cotransporters using reverse transcriptase PCR (RT-PCR). Messenger RNA for a pyrimidine-selective Na+/nucleoside cotransporter mRNA (rCNT1) was detected in samples from each brain region tested by RT-PCR amplification of a 309-bp DNA product. Southern blot and sequence analysis confirmed that this product was derived from rCNT1 mRNA. A purine-selective Na+/nucleoside cotransporter mRNA (rCNT2, also termed SPNT) was detected throughout brain by amplifying a 235-bp DNA product, the sequence of which was identical to that published. These experiments demonstrate the presence of both rCNT1 and rCNT2 mRNA in rat brain.

Identification of a saturable uptake system for deoxyribonucleosides at the blood-brain and blood-cerebrospinal fluid barriers

Substances can enter the brain either directly across the blood-brain barrier or indirectly across the choroid plexuses and arachnoid membrane (blood-CSF barrier) into the CSF and then by diffusion into the brain. Earlier studies have demonstrated a saturable thymidine uptake across the blood-CSF barrier, but not across the blood-brain barrier. In this study transport of [3H]thymidine across both barriers was measured in vivo by means of a bilateral vascular brain perfusion technique in the anaesthetised guinea-pig. This method allows simultaneous and quantitative measurement of slowly penetrating solutes into both brain and CSF, under controlled conditions of arterial inflow. The results of the present study carried out over perfusion periods of up to 30 min indicated a progressive uptake of [3H]thymidine into brain and CSF, which was found to be significantly greater than the transport of D-[14C]mannitol (a plasma space marker). Furthermore, the addition of 1 mM unlabelled thymidine in the perfusate caused saturation of [3H]thymidine uptake into both brain and CSF. In conclusion, these findings suggest that thymidine can cross both the blood-brain and blood-CSF barriers in the guinea-pig by carrier-mediated transport systems.

Characterization of nucleoside transport activity in rabbit cortical synaptosomes

Rabbit central nervous system (CNS) preparations have been used to study the central effects of adenosine, but little is known about the specific uptake mechanisms in rabbit brain involved in the regulation of extracellular adenosine concentrations. The present study assessed the kinetic and pharmacological characteristics of the uptake of [3H]uridine (a poorly metabolized substrate for adenosine transporters) by rabbit cortical synaptosomes, to define the transporter subtypes involved and to evaluate species variability in transporter characteristics. [3H]Uridine transport into rabbit cortical synaptosomes was mediated by two saturable, facilitated diffusion systems with characteristics compatible with the es and ei transporter subtypes identified in other mammalian species. About 65% of the total transport was mediated by the es system, and Km estimates of 320 and 94 microM were determined for [3H]uridine uptake by the es and ei transporter, respectively. These results differ significantly from the subtype ratio and kinetic characteristics reported for rat and guinea pig cortical synaptosomes, where most of the transport was mediated by an ei subtype. Dipyridamole, dilazep, nitrobenzylthioinosine, R75231, soluflazine, and mioflazine were relatively more effective as inhibitors of es-mediated uptake (compared with ei), while the substrates adenosine, cytidine, and guanosine did not distinguish between the es and ei transporters in rabbit cortical synaptosomes. These results highlight the significant species-tissue variability in nucleoside transporter characteristics and subtype expression, and emphasize the need to characterize the transporters in human CNS tissue to allow the rational development of CNS-active therapeutics based on inhibition of nucleoside transport.

Effects of exogenous linoleic acid on fatty acid composition, receptor-mediated cAMP formation, and transport functions in rat astrocytes in primary culture

We have examined the effects of culturing neonatal rat-brain astrocytes in medium containing delipidated serum, with or without added linoleic acid (LA, 18:2 omega 6), on membrane fatty-acid composition and functions. After 18-21 days in culture, polyunsaturated fatty acids (PUFA) constituted approximately equal to 24 mol% of the total fatty acids in the astrocytes grown in delipidated media ("controls'); these proportions were increased by 35-40% to approximately equal to 33 mol% when the cells were supplemented with 35 microM LA. Notable differences in the PUFA profiles of the cells cultured with or without added LA included: (a) higher proportions of omega 6 PUFA in the LA-supplemented astrocytes (approximately equal to 25%, relative to approximately equal to 10% in controls) that were accompanied by an increase in the ratio of omega 6/omega 3 PUFA (from < 2 in controls to approximately equal to 5), and (b) higher proportions of 20:3 omega 9 and 22:3 omega 9 in the control astrocytes (> 5%) relative to the LA-supplemented cells (approximately equal to 1%). The major metabolites in the omega 6 PUFA-enriched cells were arachidonic (20:4 omega 6), adrenic (22:4 omega 6) and docosapentaenoic (22:5 omega 6) acids (15, 5 & 3 mol%, respectively). Enrichment of the astrocytes in omega 6 PUFA did not alter basal levels of cAMP, nor did it affect the amounts of cAMP formed in response to forskolin, isoproterenol, adenosine or histamine. However, dopamine-dependent increases in cAMP formation in the presence of the phosphodiesterase inhibitor, Ro 20-1724, were reduced by approximately equal to 25% relative to those in controls. LA supplementation modified uptake of [3H]adenosine into the astrocytes; values for Kt for a high affinity transport were increased relative to controls, and maximum capacity of a lower affinity process was reduced. Uptake of [3H]glutamate was not altered in the omega 6 PUFA-enriched astrocytes. This study demonstrated that cultured astrocytes take up exogenous linoleic acid and incorporate its metabolites into phospholipid, and that the resulting changes in membrane PUFA composition modify only specific cell functional properties.

Choline substitution for sodium triggers glutamate and adenosine release from rat hippocampal slices

Substituting choline for sodium resulted in an extracellularly recorded transient depolarization and increased efflux of glutamate and adenosine from rat hippocampal slices. The depolarization was blocked by a combination of NMDA and non-NMDA antagonists, MK-801 (10 microM) and 6,7-dinitroquinoxaline-2,3-dione (DNQX, 50 microM). Adenosine release was blocked by MK-801 alone. These data suggest that glutamate released as a result of the sodium removal acts at NMDA and non-NMDA glutamate receptors to trigger the depolarization and acts at NMDA receptors to trigger adenosine release.

Ionic dependence of adenosine uptake into cultured astrocytes

Adenosine uptake in cultured astrocytes is dependent on various ions and energy metabolism. The Na(+)-gradient plays an important role, since nigericin, ouabain, amiloride and substitution of Na+ with choline inhibited adenosine uptake. The proton-gradient was of importance, since carbonylcyanide m-chlorophenylhydrozone (CCCP) and omeprazole also inhibited adenosine uptake. Furthermore, adenosine uptake was dependent on Cl- anion. Substitution of Cl- with isethionate, as well as DIDS or furosemide inhibited adenosine uptake. Adenosine uptake was also sensitive to Ca2+ gradient, removal of extracellular Ca2+ and calcimycin inhibited adenosine uptake. Adenosine uptake was not dependent on extracellular K+ and was not affected by valinomycin. Although, K(+)-channel openers (BRL 34195 and nicorandil) as well as the K(+)-channel antagonist, glyburide, inhibited adenosine uptake, the inhibitory effect of BRL 34915 was not antagonized by glyburide. Rotenone and 2,4-dinitrophenol also inhibited adenosine uptake. Ionic dependence and metabolic energy dependence of adenosine uptake suggest that uptake is primarily an active process.

Dipyridamole increases oxygen-glucose deprivation-induced injury in cortical cell culture

BACKGROUND AND PURPOSE

Adenosine transport inhibitors attenuate ischemic central neuronal damage in vivo, but the locus of this protective action is presently unknown. To help address the question of whether adenosine transport inhibitors have a protective effect directly on brain parenchyma, we tested the effect of the adenosine transport inhibitor dipyridamole on neuronal loss induced by oxygen-glucose deprivation in vitro.

METHODS

Murine cortical cultures were exposed to combined oxygen and glucose deprivation, N-methyl-D-aspartate, or kainate. The extracellular concentrations of glutamate and adenosine were measured by high-performance liquid chromatography; neuronal cell death was assessed by morphological examination and measurement of lactate dehydrogenase release.

RESULTS

Cultures exposed to oxygen-glucose deprivation for 30 to 75 minutes exhibited an insult-dependent increase in extracellular adenosine, followed shortly by an increase in extracellular glutamate and 24 hours later by neuronal death. Addition of the A1 receptor antagonist 8-cyclopentyltheophylline during oxygen-glucose deprivation enhanced both glutamate release and neuronal damage. Addition of 10 mumol/L dipyridamole decreased extracellular adenosine and also enhanced extracellular glutamate and neuronal death. In contrast, dipyridamole increased the levels of extracellular adenosine stimulated by N-methyl-D-aspartate or kainate.

CONCLUSIONS

These results are consistent with the idea that endogenous adenosine has a neuroprotective effect directly on cortical cells exposed to oxygen-glucose deprivation. However, inhibition of adenosine transport with dipyridamole was surprisingly not an effective strategy for enhancing this protective effect. The beneficial effects of adenosine transport inhibitors observed in vivo may be mediated indirectly--for example, by effects on the vasculature.

Inhibitory effects of propentofylline on [3H]adenosine influx. A study of three nucleoside transport systems

The neuroprotective effects of adenosine are well-recognized. Recently, propentofylline, a xanthine derivative, has been shown to increase extracellular concentrations of adenosine in ischemic brain and to limit the extent of neuronal damage in experimental models of cerebral ischemia. Since the concentration of adenosine in brain is controlled, in part, by nucleoside transporter proteins, the action of propentofylline was proposed to be due to inhibition of mediated transfer of adenosine across cell membranes. To determine the likelihood of this mechanism, we examined the inhibitory effects of propentofylline on [3H]adenosine transport by the three best-characterized nucleoside transport processes, es, ei, and cif in cultured cell lines under conditions where only a single transporter type was operative. Propentofylline inhibited [3H]adenosine uptake by each of the three transport processes in a concentration-dependent manner. The greatest inhibitory potency was for es transporters (L1210/B23.1 cells), with an IC50 value of 9 microM, followed by ei transporters, with IC50 values of 170 microM (L1210/C2 cells) and 166 microM (Walker 256 cells). Propentofylline was a weak inhibitor of cif transporter, with an IC50 value of 6 mM. These results demonstrate that propentofylline is an inhibitor of adenosine transport processes and suggest that its neuroprotective effects may be due to an increase in extracellular concentrations of adenosine by virtue of inhibition of es transporter function.

CSF-1 stimulates nucleoside transport in S1 macrophages

We have examined nucleoside transport (NT) in a cell line derived from primary day 7 murine bone marrow macrophages (S1 macrophages) in response to the macrophage growth factor, colony-stimulating factor 1 (CSF-1). Adenosine and uridine transport in quiescent S1 macrophages occurred primarily by two facilitated diffusional routes, one that was sensitive and one that was relatively resistant to the inhibitor nitrobenzylthioinosine (NBMPR). Addition of CSF-1 to quiescent cultures resulted in increased adenosine and uridine transport with biphasic kinetics with respect to the cell cycle. Basal NT activity was elevated (about twofold) within 15 min of CSF-1 addition, returned to near basal levels by 1 h, and then increased again (three- to fourfold) 8-12 h later, returning again to basal levels by 48 h post CSF-1 stimulation. We propose that the large increase in NT activity at 8-12 h corresponded with the time when cultures synchronously began to enter the S phase of the cell cycle. In addition to these changes in the absolute rates, the proportions of NBMPR-sensitive and NBMPR-insensitive transport also change after CSF-1 addition. Quiescent cultures exhibited primarily NBMPR-insensitive transport while logrithmically growing cultures exhibited primarily NBMPR-sensitive nucleoside transport activity. The increase in the NBMPR-sensitive component of the transport process paralleled a similar increase in the number of high-affinity NBMPR binding sites, suggesting that the mechanism for upregulating NBMPR-sensitive NT activity involves increases in the number of NBMPR-sensitive transporter sites. Interestingly, we were unable to detect Na(+)-dependent concentrative uptake of adenosine, uridine, or formycin-B either in the S1 macrophage cell line or in primary (day 7) murine macrophages. Thus these bone marrow derived macrophages did not display the characteristically large Na(+)-dependent transport systems observed by others in peritoneal macrophages, implying that these two populations of macrophages are, indeed, functionally distinct.

N-methyl-D-aspartate- and non-N-methyl-D-aspartate-evoked adenosine release from rat cortical slices: distinct purinergic sources and mechanisms of release

Excitatory amino acids, acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors, release the inhibitory neuromodulator adenosine from superfused rat cortical slices. This study was initiated to investigate the possible purinergic sources and mechanisms of release for the adenosine release evoked by NMDA and non-NMDA receptor activation. Inhibition of the bidirectional nucleoside transporter with dipyridamole greatly enhanced adenosine release evoked by glutamate. NMDA, kainate, and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Inhibition of ecto-5'-nucleotidase with alpha, beta-methylene ADP and GMP had no effect on either kainate- or AMPA-evoked adenosine release, but it decreased glutamate- and NMDA-evoked adenosine release by 23 and 68%, respectively. A similar inhibition of NMDA-evoked adenosine release was observed with alpha, beta-methylene ADP alone, indicating that the inhibitory effect was not due to the reported competitive inhibition of NMDA receptors by GMP. Finally, NMDA-evoked adenosine release, but not kainate- or AMPA-evoked release, was Ca2+ dependent. These results indicate that activation of non-NMDA receptors releases adenosine per se in a Ca(2+)-independent manner. In contrast, NMDA receptor activation releases primarily a nucleotide that is subsequently converted extracellularly to adenosine; in this case, release is Ca2+ dependent. Although neither NMDA- nor non-NMDA-evoked adenosine release occurs via the nucleoside transporter, this transporter does appear to be a major route for removal of adenosine from the extracellular space.

Neuroprotective role of adenosine in cerebral ischaemia

Several lines of evidence suggest that adenosine may be an endogenous protective agent in cerebral ischaemia. Adenosine is normally present in the extracellular fluid in most tissues of the body, including the brain, and its level increases dramatically following hypoxia or ischaemia. The rate of adenosine production is enhanced when the energy demand is larger than the rate of energy supply. Adenosine acts on specific receptors that are present in most cells in the body and that produce cellular effects that tend to antagonize a number of pathological events thought to be instrumental for ischaemic nerve cell death. Karl Rudolphi and colleagues review evidence for the neuroprotective potential of adenosine and indicate some targets for drug development.

Developmental regulation of adenosine A1 receptors, uptake sites and endogenous adenosine in the chick retina

Although adenosine A1 receptors mediate the inhibition of dopamine-dependent stimulation of adenylate cyclase activity in the developing chick retina, their localization and function are unknown. We have examined the localization of these receptors, and of endogenous adenosine and adenosine uptake sites at several stages of chick retinal development. A1 receptors were already localized predominantly to plexiform regions by embryonic day 12 (E12) with no gross changes at subsequent stages. Adenosine immunoreactivity was absent from retina at E8 but was detected at E12 in the ganglion cell layer, as well as cells in the inner nuclear cell layer and photoreceptors. At more advanced developmental stages the immunoreactivity was greater, but displayed similar localizations. Uptake sites labeled with [3H]nitrobenzylthioinosine (NBI) were detected even earlier using binding and autoradiographic methods. [3H]NBI binding was saturable, and Scatchard analysis demonstrated a single class of sites with a Kd of 0.91 nM and Bmax of 298 fmol/mg protein in E15 retinal membranes. The binding was displaced by unlabeled NBI and dipyridamole. NBI binding sites differentiated earlier than adenosine A1 receptors or endogenous adenosine immunoreactivity, showing a diffuse distribution at E8, but predominating in the plexiform layers of more developed retinas. The results indicate that elements of a putative purinergic system differentiate at specific localizations early in retinal development.

Further studies of the mechanism(s) of polyunsaturated-fatty-acid-mediated increases in intracellular cAMP formation in N1E-115 neuroblastoma cells

Following earlier observations that increasing the polyunsaturated fatty-acid (PUFA) content of N1E-115 neuroblastoma cells elevated basal and adenosine (Ado)-stimulated intracellular cyclic AMP (cAMP) formation, we carried out studies to determine the mechanism(s) by which PUFA exerted their modulatory effects. Basal increases in cAMP in the PUFA-enriched (PUFA+) cells were evident with short (60 sec) exposure to a phosphodiesterase inhibitor (Ro 20-1724), and increased to a maximum at 20 min; they were not observed in the absence of Ro 20-1724. Forskolin-stimulated cAMP formation in the presence of the Ro compound was 2- to 3-fold higher in the PUFA+ cells. Basal elevations in cAMP were reduced by approximately 70% by exposing the PUFA+ cells to Ado deaminase (ADA) or to an Ado antagonist, and were further increased by inhibiting ADA, which suggested that they could be producing endogenous Ado that activated stimulatory Ado receptors. However, this did not appear to involve PUFA-mediated stimulation of 5'-nucleotidase activity or inhibition of [3H]Ado uptake. Overall, the results of this study indicated that multiple mechanisms are involved in PUFA modulation of cAMP formation.

The distribution of enzymes involved in purine metabolism in rat kidney

Adenosine produced from 5'-AMP has been proposed as a mediator of intrinsic renal regulation. The rates of 5'-AMP and adenosine metabolism are dependent on the activities of enzyme involved in purine metabolism. The activities of adenosine kinase (AK), adenosine deaminase (ADA), 5'-nucleotidase (5'-NT), AMP deaminase, xanthine oxidase and purine nucleoside phosphorylase were measured in cytosolic and membrane fractions from glomeruli, cortical tubules, medullary thick ascending limb of Henle (MTAL) and collecting duct prepared from rat kidney by combinations of sieving and sucrose density gradient centrifugation techniques. In the cytoplasm of glomeruli cells, the activity ratios of ADA/AK and AMP deaminase/5'-NT were 70 and 2.4, respectively. The highest activity of 5'-NT was found in membrane fractions of cortical tubules where it was equally distributed between luminal and antiluminal membranes. Membrane fractions of MTAL did not contain detectable amounts of adenosine deaminase activity. The highest activity of xanthine oxidase and purine nucleoside phosphorylase was in the cytoplasm fraction of glomeruli. These results suggest that deamination of AMP and adenosine may be favored in the cytoplasm of glomeruli cells. In contrast, in the extracellular space of glomeruli and especially in the cortical tubule, AMP can be converted preferentially to adenosine by 5'-NT.

NMDA-induced increase in [Ca2+]i and45Ca2+ uptake in acutely dissociated brain cells derived from adult rats

A preparation of acutely dissociated brain cells derived from adult (3-month-old) rat has been developed under conditions preserving the metabolic integrity of the cells and the function of N-methyl-D-aspartate (NMDA) receptors. The effects of glutamate and NMDA on [Ca2+]i measured with fluo3 and 45Ca2+ uptake have been studied on preparations derived from hippocampus and cerebral cortex. Glutamate (100 microM) and N-methyl-DL-aspartate (200 microM) increased [Ca2+]i by 26-12 nM and 23-9 nM after 90 s in cerebral cortex and hippocampus, and stimulated 45Ca2+ uptake about 16-10% in the same regions. The increases in [Ca2+]i and 45Ca2+ uptake were inhibited by 40% in the presence of 1 mM MgCl2 and by 90-50% in the presence of MK-801. The results indicate (a) that a large fraction of the [Ca2+]i response to glutamate in freshly dissociated brain cells from the adult rat involves NMDA receptors, (b) when compared with results in newborn rats, there is a substantial blunting of the [Ca2+]i increase in adult age.

Non-eicosanoid functions of essential fatty acids: regulation of adenosine-related functions in cultured neuroblastoma cells

Studies have demonstrated that augmenting the omega 6 polyunsaturated-fatty-acid (PUFA) content of N1E-115 neuroblastoma cells by media supplementation with linoleic acid results in greater than or equal to 2-fold increases in basal levels of intracellular cyclic AMP (cAMP). Data suggested some involvement of increased production of adenosine from endogenous metabolites; however, increases in adenosine were not related to increased activity of 5'-nucleotidase or decreased uptake of extracellular adenosine. PUFA-dependent elevations in basal cAMP were evident within 1 min of exposure to a phosphodiesterase inhibitor; this phenomenon did not appear to be due to PUFA-dependent changes in Ca2+ uptake or to increases in sensitivity of adenylate cyclase to Ca2+. Forskolin-stimulated cAMP formation was 3-fold higher in PUFA-enriched cells than in control cells, which suggested a direct effect on the functioning of the catalytic unit. Linoleic acid supplementation resulted in a 2-fold increase in the maximum amounts of cAMP produced in response to the stable adenosine analogue, 5'-N'ethylcarboxy-amidoadenosine (NECA). The altered stimulatory response did not involve eicosanoid formation, but may have been related to an increase in the number of stimulatory adenosine receptors, as judged by binding of [3H]NECA. These studies indicate that membrane PUFA modulate adenosine-related functions in neuroblastoma cells, and suggest that a complex series of mechanisms is involved in this regulation.

Endo-oligopeptidase A, a putative enkephalin-generating enzyme, in the vertebrate retina

Endo-oligopeptidase A, EC 3.4.22.19, converts small enkephalin-containing peptides into the corresponding enkephalins in vitro. We investigated the presence of endooligopeptidase A in the retina and its possible colocalization with enkephalins in retinal neurons. The specific activity of endo-oligopeptidase. A found in pigeon retinae (30.3 +/- 7.3 mU/mg, mean +/- standard deviation) was four times higher than in rabbit retinae (7.0 +/- 1.1 mU/mg). The enzyme activity was not modified by EDTA, but it was enhanced by dithiothreitol and inhibited by zinc and 5,5'-dithiobis(2-nitrobenzoic acid). Immunohistochemical experiments with a purified antiserum against rabbit endo-oligopeptidase A revealed labeled neurons in both the inner nuclear layer and the ganglion cell layer of pigeon and rabbit retinae. Double-labeling immunofluorescence experiments demonstrated that about 90% of neurons containing endo-oligopeptidase A-like immunoreactivity also contained [Leu5]-enkephalin-like immunoreactivity. These colocalization results may represent an important step toward the demonstration of the possible involvement of endo-oligopeptidase A in enkephalin generation in vivo.

Extracellular metabolism of adenine nucleotides and adenosine in the innervated skeletal muscle of the frog

The effects of coformycin, alpha,beta-methylene ADP, dipyridamole in the absence and presence of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), nitrobenzylthioinosine (NBTI), mioflazine and ouabain on the metabolic pathways of exogenously applied ATP and its metabolites in the frog innervated sartorius muscle were investigated. ATP catabolism yielded ADP, AMP, IMP, adenosine and inosine; the ecto-ATPase in situ was shown to be Ca(2+)- or Mg(2+)-activated with a Kmapp for ATP of 767 +/- 48 microM. AMP catabolism yielded IMP, adenosine and inosine; inosine was formed from either exogenous IMP or exogenous adenosine. Catabolism of AMP into IMP was blocked by coformycin, which enhanced adenosine and inosine formation from AMP. alpha,beta-Methylene ADP blocked adenosine formation from AMP and inosine formation from IMP; formation of IMP from AMP was enhanced by alpha,beta-methylene ADP. Complete blockade of AMP degradation was achieved with the simultaneous use of coformycin and alpha,beta-methylene ADP. Dipyridamole attenuated but did not completely block extracellular adenosine removal and inosine appearance in the bath. EHNA, applied in the presence of dipyridamole, did not cause any further attenuation of extracellular adenosine removal. Mioflazine, NBTI and ouabain did not affect adenosine disappearance from the bath. The results suggest that, in the frog innervated sartorius muscle, ATP can be sequentially catabolized into AMP which is then catabolized either into IMP or into adenosine. This extracellular degradation of AMP into IMP might then constitute a shunt-like mechanism to control the levels of adenosine formed from adenine nucleotides.

L-[3H]adenosine, a new metabolically stable enantiomeric probe for adenosine transport systems in rat brain synaptoneurosomes

The stereoenantimers D-[3H]adenosine and L-[3H]adenosine were used to study adenosine accumulation in rat cerebral cortical synaptoneurosomes. L-Adenosine very weakly inhibited rat brain adenosine deaminase (ADA) activity with a Ki value of 385 microM. It did not inhibit rat brain adenosine kinase (AK) activity, nor was it utilized as a substrate for either ADA or AK. The rate constants (fmol/mg of protein/s) for L-[3H]adenosine accumulation measured in assays where transport was stopped either with inhibitor-stop centrifugation or with rapid filtration methods were 82 +/- 14 and 75 +/- 10, respectively. Using the filtration method, the rates of L-[3H]adenosine accumulation were not significantly different from the value of 105 +/- 15 fmol/mg of protein/s measured for D-[3H]adenosine transport. Unlabeled D-adenosine and nitrobenzylthiolnosine, both at a concentration of 100 microM, reduced the levels and rates of L-[3H]adenosine accumulation by greater than 44%. These findings suggest that L-adenosine, a metabolically stable enantiomeric analog, and the naturally occurring D-adenosine are both taken up by rat brain synaptoneurosomes by similar processes, and as such L-adenosine may represent an important new probe with which adenosine uptake may be studied.

The accumulation of [3H]phenylisopropyl adenosine ([3H]PIA) and [3H]adenosine into rabbit retinal neurons is inhibited by nitrobenzylthioinosine (NBI)

Uptake of [3H]adenosine and [3H]R-phenylisopropyladenosine (R-PIA) into retinal cells was assessed autoradiographically, in the presence and absence of the purine nucleoside transport inhibitor, nitrobenzylthioinosine (NBI). Under control conditions, both purine nucleosides were accumulated in cell bodies localized to the ganglion cell layer, and the inner nuclear layer. In the presence of NBI, significantly less accumulation of nucleosides within cell bodies was observed, particularly within the inner nuclear layer, suggesting that most of the uptake occurred via the transport of both substrates. The stereoisomer of adenosine, L-[3H]adenosine, was not accumulated into retinal cells consistent with the view that the accumulation of both adenosine and R-PIA occurs via the purine nucleoside transporter.

Physiological and pharmacological properties of adenosine: therapeutic implications

Adenosine is a nucleoside which has been shown to participate in the regulation of physiological activity in a variety of mammalian tissues, and has been recognized as a homeostatic neuromodulator. It exerts its actions via membrane-bound receptors which have been characterized using biochemical, electrophysiological and radioligand binding techniques. Adenosine has been implicated in the pharmacological actions of several classes of drugs. A number of studies strongly suggest that the nucleoside may regulate cellular activity in many pathological disorders and, in that respect, adenosine derivatives appear as promising candidates for the development of new therapeutic compounds, such as anticonvulsant, anti-ischemic, analgesic and neuroprotective agents.

A selective assay for endooligopeptidase A based on the cleavage of fluorogenic substrate structurally related to enkephalin

A novel quenched fluorescence substrate, QF-ERP7 (Abz-G-G-F-L-R-R-V-EDDn), structurally related to enkephalins, proved to be suitable for assaying the endooligopeptidase A (E.C.3.4.22.19) activity. The enzyme only splits the L-R bond (Km 1.75 microM, Kcat 8.25 s-1), a reaction efficiently blocked by anti-endooligopeptidase A antibodies and by inhibitor and alternative substrates of the enzyme. Evidences based on the action of inhibitors and on the analysis of QF-ERP7 fragments demonstrated that endooligopeptidase A contributes with 100% of the QF-ERP7 cleaving activity found in the cytosol of rabbit brain homogenates and with 85% of that recovered in the membrane fraction. Homologous substrates, Abz-G-G-F-L-R-R-EDDn and Abz-G-G-F-L-R-EDDn, were resistant to hydrolysis. The convenience and sensitivity of the fluorimetric assay based on the QF-ERP7 moiety offers several advantages compared with previously described painstaking procedures for endooligopeptidase A activity measurements, what will certainly contribute to further our understanding of the role of this enzyme on the peptide hormone metabolism.

Neural release of ATP and adenosine

Release of ATP can be evoked from noradrenergic nerve varicosities isolated from guinea pig ileal myenteric plexus by depolarization with K+ and veratridine and during exposure to acetylcholine or 5-HT. Clonidine, however, modulates the release of [3H]noradrenaline without affecting the release of ATP. ATP is also released from noradrenergic sympathetic nerves in the vas deferens, where it mediates the initial depolarization and contraction in the smooth muscle. Factors that apparently modulate the release of noradrenaline do not produce corresponding effects on ATP release. The above results are best explained by the hypothesis that ATP and noradrenaline are stored in separate populations of vesicles within sympathetic nerves and that these pools are subject to differential presynaptic modulation. Depolarization of rat brain synaptosomes releases adenosine by a process that is mediated, at least in part, by efflux on the nucleoside transporter. Drugs that block the nucleoside transport (such as dipyridamole) reduce evoked adenosine release and may thereby diminish, rather than augment, the actions of adenosine at its receptors. Release of adenosine does not appear to be uniformly distributed throughout the brain insofar as release varies from synaptosomes prepared from different regions. Although the distribution of several markers for possible adenosine pathways in the brain, including adenosine release, do not show any consistent correlations, the non-uniform distribution for these markers suggests that adenosine may have differential functions in various brain regions.

Analysis of adenosine immunoreactivity, uptake, and release in purified cultures of developing chick embryo retinal neurons and photoreceptors

We have investigated the presence of endogenous adenosine and of mechanisms for adenosine uptake and release in chick embryo retinal neurons and photoreceptors grown in purified cultures in the absence of glial cells. Simultaneous autoradiographic and immunocytochemical analysis showed that endogenous adenosine and the uptake mechanism for this nucleoside colocalize in practically all the photoreceptors, but only in approximately 20% of the neurons. Approximately 25% of the neurons showed either immunocytochemical labeling or autoradiographic labeling, while greater than 50% of the neurons were unlabeled with both techniques. [3H]Adenosine uptake was saturable and could be inhibited by nitrobenzylthioinosine and dipyridamole and by pretreatment of the [3H]adenosine with adenosine deaminase. Although these observations indicate that the uptake is specific for adenosine, only 35% of accumulated radioactivity was associated with adenosine, with the remaining 65% representing inosine, hypoxanthine, and nucleotides plus uric acid. Adenosine as well as several of its metabolites were released by the cells under basal as well as K(+)-stimulated conditions. Potassium-enhanced release was blocked by 10 mM CoCl2 or in Ca2(+)-free, Mg2(+)-rich solutions. The results indicate that retinal cells that synthesize, store, and release adenosine differentiate early during embryogenesis and are therefore consistent with a hypothetical role for adenosine in retinal development.

Adenosine transport systems on dissociated brain cells from mouse, guinea-pig, and rat

The kinetics and sodium dependence of adenosine transport were determined using an inhibitor-stop method on dissociated cell body preparations obtained from mouse, guinea-pig and rat brain. Transport affinity (KT) values for the high affinity adenosine transport systems (KT(H] were significantly different between these three species; mean +/- SEM values were 0.34 +/- 0.1 in mouse, 0.9 +/- 0.2 in rat, and 1.5 +/- 0.5 microM in guinea-pig. The KT values for the low affinity transport system (KT(L) were not different between the three species. Brain cells from rat displayed a significantly greater maximal capacity to accumulate [3H]adenosine (Vmax) than did mouse or guinea-pig for the high affinity system, or than did mouse for the low affinity system. When sodium chloride was replaced in the transport medium with choline chloride, the KT(H) values for guinea-pig and rat were both increased by approximately 100%; only in rat did the change reach statistical significance. The sodium-dependence of adenosine transport in mouse brain was clearly absent. The differences between KT(H) values in mouse and those in guinea-pig or rat were accentuated in the absence of sodium. The differences in kinetic values, ionic requirements, and pharmacological characteristics between adenosine transporters in CNS tissues of mouse, guinea-pig and rat may help account for some of the variability noted among species in terms of their physiological responses to adenosine.

Adenosine transport by rat and guinea pig synaptosomes: basis for differential sensitivity to transport inhibitors

Adenosine transport by rat and guinea pig synaptosomes was studied to establish the basis for the marked differences in the potency of some transport inhibitors in these species. An analysis of transport kinetics in the presence and absence of nitrobenzylthioinosine (NBTI) using synaptosomes derived from several areas of rat and guinea pig brain indicated that at least three systems contributed to adenosine uptake, the Km values of which were approximately 0.4, 3, and 15 microM in both species. In both species, the system with the Km of 3 microM was potently (IC50 of approximately 0.3 nM) and selectively inhibited by NBTI. This NBTI-sensitive system accounted for a greater proportion of the total uptake in the guinea pig than in the rat and was inhibited by dipyridamole, mioflazine, and related compounds more potently in the guinea pig. Preliminary experiments with other species indicate that adenosine transport in the mouse is similar to that in the rat, whereas in the dog and rabbit, it is more like that in the guinea pig. In the rat, none of the systems appeared to require Na+, but the two systems possessing the higher affinities for adenosine were inhibited by veratridine- and K(+)-induced depolarization. The transport systems were active over a broad pH range, with maximal activity between pH 6.5 and 7.0. Our results are consistent with the possibility that adenosine transport systems may be differentiated into uptake and release systems.