Stimulated formation of adenosine 3',5'-cyclic phosphate in cerebral cortex: synergism between electrical activity and biogenic amines

Adenosine deaminase of cultured brain cells

Two types of adenosine deaminase (EC 3.5.4.4) were found in cultured cells of central-nervous-system origin. The predominant and more active enzyme was obtained in soluble form from the cytosol of mouse neuroblastoma (N-18), neonatal hamster astrocytes (NN), human oligodendroglioma (HOL) and human astrocytoma (Cox Clone). Particulate adenosine deaminase was probably associated with the plasma membrane. When radioactive adenosine was added to superfusates of monolayer cultures it was rapidly converted into inosine and hypoxanthine. The metabolic conversion required adenosine uptake by the cells, a probable transition through the intracellular ATP pool(s) and a rapid excretion into the superfusate of the catabolic products. We discuss the evidence that points to adenosine and its derivatives as neurohumoral modulators of central-nervous-system function.

Release of adenosine and ATP during ischemia and epilepsy

Eighty years ago Drury & Szent-Györgyi described the actions of adenosine, AMP (adenylic acid) and ATP (pyrophosphoric or diphosphoric ester of adenylic acid) on the mammalian cardiovascular system, skeletal muscle, intestinal and urinary systems. Since then considerable insight has been gleaned on the means by which these compounds act, not least of which in the distinction between the two broad classes of their respective receptors, with their many subtypes, and the ensuing diversity in cellular consequences their activation invokes. These myriad actions are of course predicated on the release of the purines into the extracellular milieu, but, surprisingly, there is still considerable ambiguity as to how this occurs in various physiological and pathophysiological conditions. In this review we summarise the release of ATP and adenosine during seizures and cerebral ischemia and discuss mechanisms by which the purines adenosine and ATP may be released from cells in the CNS under these conditions.

Adenosine Receptors: The Contributions by John W. Daly

John Daly played an important role in defining adenosine receptors as an important target for drug discovery. His systematic work characterized the effects of adenosine analogues on cyclic AMP in the brain that were antagonized by methylxanthines. He also played a decisive role in establishing these receptors as bona fide biochemical entities and contributed to the discovery of receptor heterogeneity. This brief review will cover some of his important early discoveries in the pharmacology and medicinal chemistry of adenosine receptors.

Persistent signalling and changes in presynaptic function in long-term potentiation

Long-term potentiation (LTP) is an example of a persistent change in synaptic function in the mammalian brain, thought to be essential for learning and memory. At the synapse between hippocampal CA3 and CA1 neurons LTP is induced by a Ca2+ influx through glutamate receptors of the NMDA (N-methyl-D-aspartate) type (see Collingridge et al 1992, this volume). How does a rise in [Ca2+]i lead to enhancement of synaptic function? We have tested the popular hypothesis that Ca2+ acts via a Ca(2+)-dependent protein kinase. We found that long-lasting synaptic enhancement was prevented by prior intracellular injection of potent and selective inhibitory peptide blockers of either protein kinase C (PKC) or Ca2+/calmodulin-dependent protein kinase II (CaMKII), such as PKC(19-31) or CaMKII(273-302), but not by control peptides. Evidently, activity of both PKC and CaMKII is somehow necessary for the postsynaptic induction of LTP. To determine if these kinases are also involved in the expression of LTP, we impaled cells with microelectrodes containing protein kinase inhibitors after LTP had already been induced. Strikingly, established LTP was not suppressed by a combination of PKC and CaMKII blocking peptides, or by intracellular postsynaptic H-7. However, established LTP remained sensitive to bath application of H-7. Thus, the persistent signal may be a persistent kinase, but if so, the kinase cannot be accessed within the postsynaptic cell. Evidence for a presynaptic locus of expression comes from our studies of quantal synaptic transmission under whole-cell voltage clamp. We find changes in synaptic variability expected to result from enhanced presynaptic transmitter release, but little or no increase in quantal size. Furthermore, miniature synaptic currents in hippocampal cultures are increased in frequency but not amplitude as a result of a glutamate-driven postsynaptic induction. The combination of postsynaptic induction and presynaptic expression necessitates a retrograde signal from the postsynaptic cell to the presynaptic terminal.

Use of the A(2A) adenosine receptor as a physiological immunosuppressor and to engineer inflammation in vivo

Inflammation must be inhibited in order to treat, e.g., sepsis or autoimmune diseases or must be selectively enhanced to improve, for example, immunotherapies of tumors or the development of vaccines. Predictable enhancement of inflammation depends upon the knowledge of the "natural" pathways by which it is down-regulated in vivo. Extracellular adenosine and A(2A) adenosine (purinergic) receptors were identified recently as anti-inflammatory signals and as sensors of excessive inflammatory tissue damage, respectively (Ohta A and Sitkovsky M, Nature 2001;414:916-20). These molecules may function as an important part of a physiological "metabolic switch" mechanism, whereby the inflammatory stimuli-produced local tissue damage and hypoxia cause adenosine accumulation and signaling through cyclic AMP-elevating A(2A) adenosine receptors in a delayed negative feedback manner. Patterns of A(2A) receptor expression are activation- and differentiation-dependent, thereby allowing for the "acquisition" of an immunosuppressive "OFF button" and creation of a time-window for immunomodulation. Identification of A(2A) adenosine receptors as "natural" brakes of inflammation provided a useful framework for understanding how tissues regulate inflammation and how to enhance or decrease (engineer) inflammation by targeting this endogenous anti-inflammatory pathway. These findings point to the need of more detailed testing of anti-inflammatory agonists of A(2A) receptors and create a previously unrecognized strategy to enhance inflammation and targeted tissue damage by using antagonists of A(2A) receptors. It is important to further identify the contributions of different types of immune cells at different stages of the inflammatory processes in different tissues to enable the "tailored" treatments with drugs that modulate the signaling through A(2A) purinergic receptors.

Effect of 5'-deoxy-5'-isobutylthioadenosine on formation and release of adenosine from neonatal and adult rat ventricular myocytes

1. Studies in rat polymorphonuclear leucocytes have suggested that 5'-deoxy-5'-isobutylthioadenosine (IBTA), an inhibitor of the IMP-selective cytosolic 5'-nucleotidase, may be used to test its role in adenosine formation in intact cells. We investigated adenosine formation in neonatal and adult rat cardiomyocytes. 2. 2-Deoxyglucose (30 mM) with oligomycin (2 micrograms/ml) induced a 90-100% fall in ATP concentration in 10 min in neonatal and 60 min in adult heart cells. Adenosine accumulation was substantially increased, accounting for 13% of the fall in ATP concentration in neonatal cells and 56% in adult cells. 3. Anti-(rat liver ecto-5'-nucleotidase) serum did not inhibit adenosine accumulation. Furthermore, dipyridamole (10 microM), a nucleoside-transport blocker, inhibited by 80% the appearance of the newly formed adenosine in the medium, showing that adenosine is produced intracellularly by both adult and neonatal-rat myocytes in response to inhibition of oxidative metabolism. 4. IBTA (3 mM) inhibited by 80% the appearance of adenosine in the medium, but did not inhibit total adenosine accumulation by neonatal-rat myocytes and only modestly inhibited total adenosine accumulation by adult myocytes. 5. IBTA, like dipyridamole, inhibited incorporation of extracellular adenosine (10 microM) into neonatal and adult ventricular myocyte nucleotides by 60-70%. Transport of IBTA (100 microM) into the cells did not appear to be inhibited by dipyridamole (30 microM). 6. We conclude that IBTA acted primarily to inhibit adenosine release from myocytes. The small effect on adenosine formation rates implies that the IMP-selective cytosolic 5'-nucleotidase plays a minor role in this tissue.

The evidence for astrocytes as a target for central noradrenergic activity: expression of adrenergic receptors

Our recognition and understanding of adrenergic receptor expression by astrocytes and their cultured counterparts, astroglia, has occurred primarily over the past 2 decades. The advances in our knowledge have come about largely through the advent of new techniques with which to study neurotransmitter receptors, coupled with improvements in our ability to isolate, purify, and identify this central nervous system (CNS) cell type. The development of pharmacological tools such as second messenger assays, iodinated ligands, autoradiography, and intracellular electrophysiological recordings, paralleled that of cultured clonal cells lines of glial origin, purified astroglial primary cultures, isolations of astrocytes from adult tissues, and immunocytochemical staining for the astrocyte-specific glial fibrillary acidic protein (GFAP). As these techniques were combined and applied to the study of astrocyte pharmacology, our understanding of adrenergic receptor expression by these cells deepened. This review is an account of how these events have shaped our understanding of astrocytic adrenergic receptor expression.

A cyclic AMP mechanism mediates the serotonin-induced increase in glutamic acid decarboxylase activity in the preoptic area and hypothalamus

Previous studies have shown that the injection of 5-hydroxytryptamine (5-HT) into the third ventricle of rats on the afternoon of proestrus increases glutamic acid decarboxylase (GAD) activity in the preoptic area and the hypothalamus. In the present report we examine the adenylate cyclase-cyclic AMP (cAMP) system as mediator of that effect. The increase in GAD activity induced by intraventricular injection of 5-HT was completely blocked by injecting an antiserum against cAMP into the third ventricle 30 min earlier, whereas an injection of serum from normal rabbits was ineffective. On the contrary, activation of adenylate cyclase activity by intraventricular injection of forskolin increased GAD activity, an effect that was also blocked by anti-cAMP serum. Anti-cAMP serum also lowered GAD activity in the preoptic area and hypothalamus when injected on the morning of proestrus but not when injected in the afternoon, when the values of GAD activity were already low. The results suggest that a cAMP mechanism may be involved in the changes in preoptic-area and hypothalamic GAD activity such as the rise in enzyme activity induced by intraventricular injection of 5-HT.

A simple and sensitive radioimmunoassay for adenosine

We developed a simple and sensitive radioimmunoassay (RIA) for adenosine. The RIA is based on the double antibody method with adenosine 2', 3'-0-disuccinyl-3-[125I]-iodotyrosine methyl ester (diSc-adenosine-[125I]-TME) as a tracer. Anti-adenosine antiserum for the RIA was raised in rabbits immunized with diSc-adenosine conjugated to human serum albumin (diSc-adenosine-HSA). All samples and standards were succinylated prior to assay. The present immunoassay allows detection of 6.25-400 pmol/ml of adenosine in sample. Values obtained by the RIA and by a HPLC analysis showed a high correlation with correlation coefficient of 0.997. In order to determine adenosine in plasmas, blood cells must be separated in the presence of 6 mM EDTA, 0.006% dipyridamole (Dip) and 23 microM 2'-deoxycoformycin (dCF) at 2 degrees C. Adenosine in plasma could be accurately determined by the proposed method even without any pretreatments by deproteinizing. The adenosine levels with or without EDTA-treated normal human plasmas determined were 26.2 +/- 7.26 and 100 +/- 3.62 pmol/ml (mean +/- SEM), respectively.

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.

Direct injection of blood samples into a high-performance liquid chromatographic adenine analyser to measure adenine, adenosine, and the adenine nucleotides with fluorescence detection

Adenine (Ade), adenosine (Ado) and its nucleotides such as AMP, cAMP, ADP and ATP in blood or plasma were determined by a high-performance liquid chromatographic (HPLC) adenine analyser with fluorescence detection. In order to inject samples directly into the HPLC system without pretreatment except dilution, the analyser consisted of two systems each, having three columns (pre-, mini- and analytical). A precolumn with an inlet filter of pore size 40 microns was common to both systems and packed with Butyl-Toyopearl 650-M to remove hydrophobic compounds and blood cell membranes. In the system for analysis of the nucleotides, a mini-column of Hitachi anion-exchange gel 3013-N was used for adsorbing AMP, cAMP, ADP and ATP. The adsorbed nucleotides were separated by the Hitachi gel 3013-N analytical column. In the other system for analysis of Ado and Ade, they were adsorbed on a Develosil ODS-5 mini-column and separated by an Asahipak GS-320H size-exclusion analytical column. The adenine compounds in each eluate were derivatized on-line in a 15-m reaction coil at 115 degrees C with bromoacetaldehyde as the fluorescent reagent in each mobile phase for the analytical column, and detected by spectrofluorimetry. ATP, ADP and AMP were accurately determined by the direct injection of hamster, rat and human whole blood. Authentic Ade and Ado were well separated and Ado in human plasma was determined, but it was difficult to determine it in rat plasma owing to interference from an unknown compound.

Accumulation of cyclic AMP elicited by vasoactive intestinal peptide is potentiated by noradrenaline, histamine, adenosine, baclofen, phorbol esters, and ouabain in mouse cerebral cortical slices: studies on the role of arachidonic acid metabolites and protein kinase C

In mouse cerebral cortical slices, noradrenaline (NA) potentiates cyclic AMP (cAMP) accumulation elicited by vasoactive intestinal peptide (VIP) through alpha 1-adrenergic receptors. This synergism is inhibited by indomethacin, and the prostaglandins E2 and F2 alpha mimic the effect of NA. In the present study, we observed that the synergism between VIP and NA is not inhibited by the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) or the diacylglycerol-lipase inhibitor RHC 80267, thus further stressing the role of phospholipase A2 activation. Various neuroactive agents that potentiate the stimulatory effect of VIP on cAMP formation were also examined. As with NA, the potentiation by histamine and adenosine is inhibited by indomethacin. In contrast to NA, histamine, and adenosine, the synergistic interaction between phorbol esters and VIP on cAMP formation is abolished by H-7 but not by indomethacin. The potentiation by baclofen, a gamma-aminobutyric acidB receptor agonist, is partially inhibited by the 5-lipoxygenase inhibitor nafazatrom. The synergism between ouabain and VIP is reduced by H-7 but not by indomethacin and nafazatrom. These data indicate that the stimulation of cAMP formation elicited by VIP is under the modulation of various neuroactive agents that trigger diverse intracellular mechanisms to potentiate the effect of the peptide.

Effect of acute lung injury on metabolism of adenine nucleotides in rat perfused lung

1. The hydrolysis of adenosine di- and monophosphate (ADP, AMP) was studied in perfused lungs isolated from rats treated with alpha-naphthylthiourea (ANTU) to induce acute lung injury. This injury is associated with damage to the endothelium, the locus of the ADP and AMP hydrolysing enzymes. 2. Treatment with ANTU did not change the proportion of [3H]-ADP surviving a single passage through the pulmonary circulation, at any time up to 50 h after ANTU. Less than 8% and 2% respectively of 1 or 0.1 mumol ADP, given as a bolus, appeared in lung effluent. 3. The metabolites of ADP, AMP and adenosine in lung effluent were increased fro 2 h after ANTU. 4. Metabolism of [3H]-AMP as substrate was always low but, following ANTU treatment, the adenosine content of lung effluent increased four fold. 5. It appears that, in spite of considerable endothelial cell damage, as demonstrated by pulmonary oedema, the ectoenzymes catalysing ADP and AMP hydrolysis were relatively little affected by ANTU.

Formation of second messengers in response to activation of ion channels in excitable cells

1. Depolarization of excitable cells of the central nervous system results in the formation of the second messengers cyclic AMP, cyclic GMP, inositol phosphates, and diacylglycerides. 2. Depolarization-evoked accumulation of cyclic AMP in brain preparations can be accounted for mainly by the release of adenosine, which subsequently interacts with stimulatory adenosine receptor linked to adenylate cyclase. 3. Depolarization-evoked formation of cyclic GMP in brain preparations is linked to activation of voltage-dependent calcium channels, presumably leading to activation of guanylate cyclase by calcium ions. 4. In brain slices depolarization-evoked stimulation of phosphoinositide breakdown and subsequent formation of inositol phosphates and diacylglycerides are linked to activation of voltage-dependent calcium channels, which are sensitive to dihydropyridines, presumably leading to activation of phospholipase(s) C by calcium ions. 5. In the synaptoneurosome preparation depolarization-evoked stimulation of phosphoinositide breakdown does not involve activation of dihydropyridine-sensitive calcium channels and, instead, appears to be regulated primarily by the intracellular concentration of sodium ions. Thus, agents that induce increases in intracellular sodium--such as toxins that open or delay inactivation of voltage-dependent sodium channels; ouabain, an inhibitor of Na+/K+ ATPase that transports sodium outward and a sodium ionophore--all stimulate phosphoinositide breakdown. Mechanistically, increases in intracellular sodium either might directly affect phospholipase(s) C or might lead to influx of calcium ions through Na+/Ca2+ transporters. 6. Depolarization-evoked stimulation of cyclic AMP formation and phosphoinositide breakdown can exhibit potentiative interactions with responses to receptor agonists, thereby providing mechanisms for modulation of receptor responses by neuronal activity. 7. Since all these second messengers can induce phosphorylation of ion channels through the activation of specific kinases, it is proposed that depolarization-evoked formation of second messengers represents a putative feedback mechanism to regulate ion fluxes in excitable cells.

Effects of pulmonary oedema on pharmacokinetics of adenosine in rat isolated lungs

1 Pulmonary oedema, assessed by decreases in the lung dry weight:wet weight ratio, was induced in rats by a single i.p. injection of alpha-naphthylthiourea (ANTU). The oedema reached a peak at 4 h after ANTU and had completely resolved after 28 h. 2 Pulmonary pharmacokinetics of adenosine were measured in isolated, perfused lungs using radiolabelled adenosine and sucrose, injected into the perfusate as a single bolus. 3 By 1 h after ANTU the 1 min efflux of tracer for adenosine increased to over 60% and remained high until 16 h after ANTU. The time for 50% of injected radioactivity to appear in lung effluent (t1/2) for adenosine was reduced from its normal value of greater than 120 s to a minimum of 27 s at 1 h after ANTU. The proportion of adenosine in lung effluent did not change until 16 h after ANTU treatment but returned to normal by 50 h. 4 There were only minimal changes in the T1/2 and 1 min efflux for sucrose following ANTU treatment. 5 It appears that both the uptake and metabolism of adenosine are affected by ANTU-induced lung damage. The early effects are chiefly on uptake with metabolism remaining normal. Later (after 16 h) metabolism is decreased with uptake recovering to normal levels. 6 The effects on adenosine uptake paralleled the development and the resolution of oedema, suggesting that this variable might provide a biochemical index of the physical processes leading to lung oedema.

Compartmentation of intracellular nucleotides in mammalian cells

The important role of nucleotides in cellular metabolism requires that serious consideration be given to the question of the homogeneity or inhomogeneity of nucleotide pools in cells. The purpose of this review is to summarize the existing evidence for compartmentation of nucleotide pools, discuss the limitations of this evidence, and to discuss the implications of compartmentation for the interpretation of nucleotide concentration measurements. Evidence for nucleotide compartmentation comes from the following types of evidence: compartmentation of RNA precursors; compartmentation of deoxynucleoside triphosphates; mitochondrial compartmentation; the existence of tightly bound nucleotides; pools derived from alternative synthetic routes; compartmentation in cyclic nucleotide metabolism; channeling in the synthesis of pyrimidine nucleotides; and others. The types of evidence adduced for compartmentation will be considered critically and in detail, and alternative explanations considered, as well. Implications of the data and hypotheses on nucleotide compartmentation for the interpretation of nucleotide pool measurements in various types of experiments will be discussed.

Release of adenosine, inosine and hypoxanthine from rabbit non-myelinated nerve fibres at rest and during activity

The composition of the efflux from desheathed rabbit vagus nerve, loaded with radioactivity by incubation in [3H]adenosine, was studied at rest and during electrical activity and after application of inhibitors of ecto-enzymes and modifications of intermediary metabolism. In addition, the degradation of externally applied ATP and adenosine was examined. [3H]ATP applied to the incubation medium was degraded to ADP, AMP, adenosine and inosine. The hydrolysis to nucleosides was inhibited by alpha, beta-methylene ADP; the appearance of AMP and nucleosides was slowed by beta, gamma-methylene ATP. Deamination of [3H]adenosine was blocked by 2-deoxycoformycin. The effluent from resting and stimulated preparations showed the presence of large amounts of inosine and hypoxanthine, smaller amounts of adenosine and adenine and traces of nucleotides. The composition of the effluent was not significantly altered by addition of alpha, beta-methylene ADP; beta, gamma-methylene ATP or 2-deoxycoformycin. Application of glucose-free solutions caused a large release of adenosine instead of inosine and hypoxanthine and a small increase in resting and stimulated efflux of 3H. Addition of 2-deoxyglucose produced a large increase in resting efflux and increased liberation of adenosine. Cyanide, 2,4-dinitrophenol, arsenate or salicylate increased the resting efflux of adenosine, inosine and hypoxanthine, and the effect of activity. It is concluded that electrical activity leads to release of adenosine, inosine and hypoxanthine, in various proportions depending on metabolic state, and that there is practically no liberation of nucleotides from nerve axons.

In vivo release of [3H]-purines by quinolinic acid and related compounds

In vivo release of [3H]-purines from the cortex of anaesthetized rats was measured and the actions of excitatory amino acids and analogues investigated. High KCl, N-methyl-DL-aspartate (NMDLA) and quinolinic acid produced a large increase in basal release of labelled materials. Glutamate, quisqualate and kainate had less effect. The N-methyl-D-aspartic acid (NMDA)-preferring receptor antagonist, 2-amino-7-phosphonohepatanoic acid, significantly reduced the release evoked by NMDLA and quinolinate but not that produced by the other agonists. Kynurenic acid, a compound metabolically related to quinolinic acid, reduced the release due to NMDLA and quinolinate but not glutamate. The results add further support to the suggestion that quinolinic acid acts on the NMDA-preferring receptor.

The control of adenosine concentration in polymorphonuclear leucocytes, cultured heart cells and isolated perfused heart from the rat

Rat polymorphonuclear leucocytes or neonatal-rat heart cells in culture were treated with 2'-deoxycoformycin and 5-iodotubercidin at concentrations that inhibited adenosine deaminase (EC 3.5.4.4) and adenosine kinase (EC 2.7.1.20) inside the intact cells, and the rate of adenosine accumulation was determined. The basal rate of adenosine formation was 2% (polymorphonuclear leucocytes) or 9% (heart cells) of the maximal activity of adenosine kinase also measured in intact cells. Greatly increased rates of adenosine formation were observed during adenine nucleotide catabolism. This condition also led to a decrease in adenosine kinase activity. When isolated rat hearts were perfused with 5-iodotubercidin alone at a concentration which inhibited adenosine kinase, no increase in tissue or perfusate adenosine or inosine concentration was observed. However, perfusion with hypoxic buffer or infusion of adenosine into the coronary circulation at a rate (20 nmol/min) equivalent to 40% of the activity of adenosine kinase caused large increases in effluent perfusate adenosine and inosine concentrations. These data argue unanimously against the existence of a substrate cycle controlling adenosine concentration. They suggest instead that an increase in the rate of adenosine formation is the principal cause of elevations in adenosine concentration during ATP catabolism.

Effect of streptozotocin-induced diabetes on the metabolism of ADP, AMP and adenosine in the pulmonary circulation of rat isolated lung

The uptake and metabolism of radioactive ADP, AMP and adenosine was investigated in the pulmonary circulation of lungs taken from rats with streptozotocin-induced diabetes. Between 0.26 and 500 mumol/l ADP was extensively hydrolysed to AMP equally in control and diabetic lungs. At 1 mmol/l there was less ADP breakdown in diabetic lungs. Hydrolysis of AMP to adenosine was also less in diabetic lungs at 10 mumol/l and 1 mmol/l substrate concentration, but adenosine metabolism and uptake at these concentrations was not affected by diabetes. The results indicate that formation of the anti-aggregatory adenosine and, to a lesser extent, breakdown of the pro-aggregatory ADP were decreased in diabetic lungs and may contribute to the platelet hyperreactivity associated with diabetes.

Metabolism and uptake of adenosine in rat isolated lung and its inhibition

The metabolism of adenosine perfused through the pulmonary circulation of isolated lungs from rats was investigated radiochemically. Following a 10 s infusion of radioactive [14C]- or [3H]-adenosine, the recovery of radioactivity in effluent from the lung after 1 min increased from 30% at 0.5 microM to 80% at 1 mM adenosine. Unchanged adenosine comprised the major radioactive species in effluent, being about a third of the total up to 100 microM. Uptake of radioactivity was saturable at high concentrations with an apparent Km of 215 microM. Radioactivity retained in lung comprised over 80% as ATP and about 2% as adenosine at all concentrations. Perfusion of lungs with Krebs solution containing dipyridamole (1-100 microM) or adenine (10 microM) increased the rate of radioactive efflux, decreased uptake of radioactivity by lung and decreased metabolites of adenosine (inosine and hypoxanthine) in the effluent. Dipyridamole (10 microM) was more potent in decreasing uptake in guinea-pig lungs than in rat lungs. From these results we conclude that the pulmonary circulation in rat lung exhibits a significant inactivation process for adenosine. The isolated lung provides a convenient preparation for studying in situ pharmacological or pathological modifications of this vascular inactivation process.

Effects of ouabain on carotid body chemoreceptor activity in the cat

The effects of infusions of ouabain on chemoreceptor activity recorded from the peripheral end of a sectioned carotid sinus nerve were studied in cats anaesthetized with pentobarbitone. Ouabain caused a marked increase in chemoreceptor discharge followed by a decline in discharge to frequencies near or below the pre-ouabain level; during the latter period further administration of ouabain had no effect. Infusion of ouabain during hypoxia further increased the chemoreceptor discharge, but this effect was short-lasting. On intracarotid administration ouabain was less effective in cats with the ganglioglomerular (sympathetic) nerves cut, whereas on intravenous administration no significant difference was observed. Following intravenous administration of ouabain the chemoreceptor peak discharge occurred with dose levels similar to those needed to cause cardiac arrhythmias, but following intracarotid administration the chemoreceptor discharge peaked at doses about 40% of those causing arrhythmias. During ouabain-induced excitation the stimulatory action of NaCN, CO2-equilibrated Locke solution and acetylcholine was potentiated, as was the chemo-inhibition induced by dopamine. During the post-excitatory period the responses evoked by these substances were reduced or abolished. Neither mecamylamine, a nicotinic antagonist, nor physostigmine, an anti-cholinesterase, affected the response of the carotid chemoreceptors to ouabain. The major finding of this study was that ouabain initially 'sensitizes' the carotid body chemoreceptors and then 'desensitizes' them. The most likely mechanism responsible for these effects is the well established Na+--K+-ATPase-inhibiting property of ouabain.

Swelling and ion uptake in cat cerebrocortical slices: control by neurotransmitters and ion transport mechanisms

Cat cerebrocortical slices incubating in medium containing normal K+ concentrations were exposed to a number of different transmitters. Norepinephrine, histamine and adenosine or 2-chloroadenosine caused increased swelling of the slices associated with an increased Na+ and Cl- content. These effects were seen only when both Cl- and HCO3- were present in the medium, and were inhibited by a number of anion transport inhibitors. These characteristics were identical to those of the HCO3(-)-dependent component of the swelling induced by high K+ levels in the medium. Other transmitters, namely 5-hydroxytryptamine, dopamine, and gamma-amino butyric acid, were ineffective. The effects of norepinephrine, histamine and 2-chloroadenosine were antagonised by propranolol and phentolamine, chlorpheniramine and diphenhydramine, and theophylline respectively. These antagonists also inhibited HCO3(-)-dependent, K+-stimulated swelling. The transmitters which induced swelling also stimulated the carbonic anhydrase activity of cerebrocortical slices. We conclude from these data that the HCO3(-)-dependent component of K+-stimulated swelling may be due to K+-stimulated release of transmitters. Furthermore, the fact that the transmitters which induce swelling have also been reported to be most effective in increasing cAMP content in both brain slices or cultured astrocytes is consistent with the swelling response being mediated via cAMP-induced changes and being predominantly localized to astrocytes.

Nucleoside exchange catalysed by the cytoplasmic 5'-nucleotidase

The inhibition of the cytoplasmic 5'-nucleotidase (EC 3.1.3.5) by its product, inosine, was studied with a partially purified preparation of the enzyme from rat liver. Inhibition of Pi production was found to be due to exchange of the inosine moiety between inosine and IMP. Exchange was not catalysed by reversal of the hydrolytic reaction, suggesting, instead, the mediation of an enzyme-phosphate intermediate. Two models for the catalytic mechanism are proposed and rate equations for the dependence of Pi production on inosine concentration are derived. The experimentally determined dependence was consistent with a mechanism in which hydrolysis of the enzyme-phosphate intermediate occurred only when it was unoccupied by inosine. This conclusion suggests that inosine analogues that cannot participate in exchange should inhibit the enzyme. Such inhibitors might be useful in defining the enzyme's physiological role or as pharmacological agents to decrease breakdown of purine nucleotides. The possibility that nucleoside exchange provides an alternative route for the phosphorylation of mutagenic or cytotoxic nucleoside analogues should also be considered.

Highly activatable adenylate cyclase in [2-3H]adenine-prelabeled vesicles prepared from guinea pig cerebral cortex by a simplified procedure

An improved procedure utilizing a simple fractionation technique is described for the preparation and use of [2-3H]adenine-prelabeled vesicles from guinea pig cerebral cortex containing highly responsive adenylate cyclase activity. Adenosine consistently increased activity 1500--2000%, contrasted with activations of 200--300% previously reported by other investigators. Adenosine at 5 microM was more active in our system than at 20 times this concentration in studies by other investigators, increasing activity by 580--840%. Experimental conditions were explored, and Ca2+ was found to be necessary during tissue homogenization, but not during subsequent vesicle incubation. However, neither the higher Ca2+ concentration used by us (2.5 mM) nor the method of tissue homogenization could adequately explain the high activity of our preparations. The size of the incubation vessel was critical for both low basal activity and high activity in the presence of adenosine. Our preparations were similar to others in that combinations of neurohormones, which included histamine and epinephrine, elicited higher activities than individual neurohormones. Inspection of our vesicle preparations by scanning electron microscopy also showed them to be compatible with previously described preparations.

Uptake of adenosine and release of adenine derivatives in mammalian non-myelinated nerve fibres at rest and during activity

1. Influx of adenosine into rabbit non-myelinated nerve fibres was measured using [2-(3)H]adenosine. The uptake of radioactivity increased linearly with duration of incubation for up to 60 min and adenosine concentration up to 200 mum. The uptake at different adenosine concentrations showed a saturable component with a half-maximal activation at 17.1 mum and a linear part.2. The radioactivity taken up was rapidly incorporated into AMP, ADP and ATP. Isotopic equilibrium between the nucleotides was achieved within 15 min.3. The uptake of (3)H from 0.2 mum-adenosine was almost completely inhibited by addition of 200 mum-adenosine and to a similar extent by 200 mum-tubercidin and AMP; a 70% inhibition was found with ATP and ADP; alpha, beta methylene-ADP had no effect.4. ATP, ADP and AMP added to the extracellular medium of a desheathed vagus were slowly hydrolysed.5. In preparations loaded with [2-(3)H]adenosine and then washed with adenosine and label-free solution there was a steady efflux of radioactivity amounting to 0.18 x 10(-3)/min. Addition of adenosine or tubercidin transiently increased the efflux.6. Electrical stimulation caused an extra release of radioactivity. The extra fractional loss was 21.8 x 10(-6)/impulse in preparations that had rested for several hours; it decreased to 2.3 x 10(-6)/impulse when stimulation was applied after a 30 min rest.7. The radioactivity of the resting efflux and of the extra efflux after stimulation was found mostly in inosine and hypoxanthine; adenosine and adenine accounted for only 3%, and the nucleotides for less than 1% of the efflux.8. Adenosine added to the external medium of a desheathed nerve was slowly deaminated.9. It is concluded that inosine and hypoxanthine found in the effluent from desheathed vagus nerve trunk result from release of these compounds from nerve fibres and not from extracellular breakdown of released ATP or adenosine.10. Electrical activity in non-myelinated nerve fibres of the nerve trunk thus causes the release of metabolites (inosine and hypoxanthine) together with small amounts of adenosine and adenine, while release of ATP and other nucleotides is almost completely absent.

Electroshock seizures in mice: effect on brain adenosine and its metabolites

Adenosine and its immediate metabolites, inosine and hypoxanthine, were measured in mouse brain following the induction of electroshock seizures and after a subconvulsive series of electric shocks. Electroshock seizures resulted in a marked and prolonged rise in inosine, with maximal values at 5 min. Hypoxanthine increased more slowly but reached high levels by 10 min. Adenosine was unchanged. Phenytoin and to a lesser extent phenobarbital reduced these effects. Following the subconvulsive stimulus, 15 single shocks over an interval of 5 sec, inosine increased rapidly, adenosine rose slightly, and hypoxanthine did not change. Both phenytoin and phenobarbital blocked these increases in adenosine and inosine. Early elevations in inosine may play some role in seizure generation and propagation. The high levels of inosine and hypoxanthine found after recovery may be involved in the termination of epileptic activity, possibly by interacting with the benzodiazepine receptor for which they are ligands.

The effects of morphine and methionine-enkephalin on the release of purines from cerebral cortex slices of rats and mice

1 Slices of cerebral cortex from Wistar rats, TO mice or C57 mice were preincubated with [3H]-adenosine, and labelled purines were subsequently releases by electrical stimulation or by perfusing with ouabain, 100 micro M. 2 Electrically-evoked purine release was substantially reduced when the Ca2+ concentration in the medium was lowered from 2.4 to 0.1 mM. In both rats and mice, the electrically-evoked release was increased by morphine and methionine-enkephalin (Met-enkephalin), 10 micro M, and in rats and TO mice by morphine 1 micro M, both drug effects being prevented by naloxone. 3 Purine release evoked by ouabain was also increased by morphine 1 and 10 micro M, though not by Met-enkephalin, from slices of rat cortex. Ouabain-induced release from TO mice was reduced by morphine, and from C57 mice was unchanged. 4 The enhancement by morphine of electrically-evoked purine release may indicate that purines mediate some effects of morphine in the CNS.

Adenosine-stimulated astroglial swelling in cat cerebral cortex in vivo with total inhibition by a non-diuretic acylaryloxyacid derivative

The intact cerebral cortices of cats were exposed in vivo under normothermic conditions and superfused with isotonic artificial cerebrospinal fluid containing added 0.125 mM adenosine. This resulted in chloridecation-rich cerebrocortical swelling which was shown by electron microscopy to be associated with an expanded astroglial compartment. The addition of DCPIB, a non-diuretic acylaryloxyacid analogue of ethacrynic acid and an inhibitor of coupled chloride-cation transport in cerebral cortex in vitro, totally blocked astroglial swelling and the concomitant increases in tissue ion contents. These studies support our previous experiments on the mechanism of formation of astroglial swelling. The pathological consequences of astroglial swelling and the clinical applications of these findings are discussed.

Effects of aspirin and dipyridamole on the degradation of adenosine diphosphate by cultured cells derived from bovine pulmonary artery

To improve understanding of the mechanisms by which ADP is degraded during passage through the pulmonary vascular bed, we examined cultured endothelial and smooth muscle cells of bovine pulmonmary artery for their abilities to metabolize [8-14C]ADP. ADP is rapidly converted to AMP and then to adenosine, hypoxanthine, and inosine. Inosine is the major metabolite produced by endothelial cells. Radioactivity (5-10%) is accumulated intracellularly primarily as ATP. Medium containing 50 micro M ADP incubated with endothelial cells rapidly loses its ability to aggregate platelets and becomes antiaggregatory under conditions in which prostacyclin is absent. The antiaggregatory activity is probably the result of accumulated adenosine. 10 micro M dipyridamole inhibits cellular uptake of radioactivity by greater than 90%, and inosine in the medium is largely replaced by adenosine. This is accompanied by increased anti-aggregatory activity of conditioned medium, which can be matched by authentic adenosine at the same concentration. 1 mM aspirin had no effect on the metabolism of ADP by endothelial cells. Our results suggest: (a) Metabolism of ADP during passage through the lung is mainly the result of endothelial ADPase. (b) ADP released from aggregating platelets can be converted to the antiaggregatory substance, adenosine. Dipyridamole may exert some of its antithrombotic actions by preventing the intracellular uptake of adenosine, thereby increasing its concentration near the site of thrombus formation. (c) The ability of the vessel wall to degrade ADP should not be compromised by the use of aspirin as an antithrombotic drug. (d) Endothelium may retain some of its antithrombogenicity when prostacyclin generation is impaired.

Characteristics of the release of adenosine from slices of rat cerebral cortex

1. The characteristics of the release of adenosine have been examined from slices of rat cerebral cortex after incubation with [3H]adenine or [3H]adenosine. 2. Increasing the potassium concentration of the extracellular medium to 36 or 54 mM did not evoke any release, but release was observed in the first post-potassium sample. This occurred whether potassium was present for 2 or 10 min. 3. Calcium-free solutions or verapamil prevented the post-potassium release of tritium. Tetraethylammonium bromide and 4-aminopyridine had no effect. 4. Ouabain (100 microM) induced the release of tritium, and did not prevent an additional increment of release after potassium stimulation. Ouabain induced release did not occur in calcium-free or sodium-free media, but was increased in low calcium (0.1 mM) medium. 5. It is concluded that the characteristics of adenosine release are unlike those of conventional neurotransmitters. It is suggested that the release is associated with the influx of sodium and calcium ions through sodium channels.

Effects of phenytoin on the release of 14C-adenine derivatives

The release of 14C-containing compounds from rat cortical slices prelabeled with 14C-adenine consisted largely of adenosine (6-7%), inosine (13-18%), and hypoxanthine (70-74%), with small amounts of nucleotides including cyclic AMP and adenine. This efflux was increased by both ouabain (0.1 mM) and veratridine (0.05 mM), the increment in released radioactivity consisting almost entirely of these three compounds. However, relatively more inosine than adenosine output was evoked by ouabain while the reverse was true with veratridine. Phenytoin partially reversed the effect of both depolarizing agents. After prelabeling, the efflux from astrocytoma cell cultures contained predominantly inosine (74%) and hypoxanthine (23%) with little adenosine. Ouabain increased the release of 14C-adenine derivatives, and this increase was diminished by phenytoin. Preliminary studies with neuroblastoma cell cultures have shown considerable variability in the composition of the effluent, with hypoxanthine the prevalent compound and almost no adenosine. Ouabain enhanced the efflux from these cells, and this effect was apparently reversed by phenytoin.