Species differences in the binding of [3H]nitrobenzylthioinosine to the nucleoside transport system in mammalian central nervous system membranes: evidence for interconvertible conformations of the binding site/transporter complex

Effectivity of Two Cell Proliferation Markers in Brain of a Songbird Zebra Finch

Simple Summary

The present study compared the effectivity of two cell proliferation markers, BrdU and EdU, in the brain neurogenic zone of the songbird zebra finch. It shows their saturation doses, that BrdU labels more cells than the equimolar dose of EdU, and that both markers can be reliably detected in the same brain.

Abstract

There are two most heavily used markers of cell proliferation, thymidine analogues 5-bromo-2′-deoxyuridine (BrdU) and 5-ethynyl-2′-deoxyuridine (EdU) that are incorporated into the DNA during its synthesis. In neurosciences, they are often used consecutively in the same animal to detect neuronal populations arising at multiple time points, their migration and incorporation. The effectivity of these markers, however, is not well established. Here, we studied the effectivity of equimolar doses of BrdU and EdU to label new cells and looked for the dose that will label the highest number of proliferating cells in the neurogenic ventricular zone (VZ) of adult songbirds. We found that, in male zebra finches (Taeniopygia guttata), the equimolar doses of BrdU and EdU did not label the same number of cells, with BrdU being more effective than EdU. Similarly, in liver, BrdU was more effective. The saturation of the detected brain cells occurred at 50 mg/kg BrdU and above 41 mg/kg EdU. Higher dose of 225 mg/kg BrdU or the equimolar dose of EdU did not result in any further significant increases. These results show that both markers are reliable for the detection of proliferating cells in birds, but the numbers obtained with BrdU and EdU should not be compared.

Transgenic expression of human equilibrative nucleoside transporter 1 in mouse neurons

Transgenic mice that express human equilibrative nucleoside transporter subtype 1 (hENT1) under the control of a neuron-specific enolase promoter have been generated. Southern blot and PCR revealed the presence of the transgene in five founder mice. Mice from each founder line were examined by reverse transcriptase (RT)-PCR and found to express hENT1 in RNA isolated from whole brain, cerebral cortex, striatum, hippocampus, and cerebellum but not liver, kidney, heart, lung or skeletal muscle. Cortical synaptosomes prepared from transgenic mice had significantly increased [(3)H]adenosine uptake and [(3)H]nitrobenzylthioinosine binding, relative to samples from wild-type mice. In behavioral tests, transgenic mice had altered responses to caffeine and ethanol, two drugs that inhibit and enhance, respectively, adenosine receptor activity. Caffeine-induced locomotor stimulation was attenuated whereas the hypnotic effect of ethanol was enhanced in transgenic mice. Caffeine was more potent in inhibiting ethanol-induced motor incoordination in wild-type than in transgenic mice. No differences in expression of mouse genes for adenosine receptors, nucleoside transporters, or purine metabolizing enzymes were detected by RT-PCR analyses. These data indicate that expression of hENT1 in neurons does not trigger adaptive changes in expression of adenosine-related genes. Instead, hENT1 expression affects dynamic changes in endogenous adenosine levels, as revealed by altered behavioral responses to drugs that affect adenosine receptor signalling.

Pharmacological analysis and molecular cloning of the canine equilibrative nucleoside transporter 1

We studied the binding of [3H]nitrobenzylthioinosine (NBMPR) and the uptake of [3H]formycin B by the es (equilibrative inhibitor-sensitive) nucleoside transporter of Madin Darby Canine Kidney (MDCK) cells. NBMPR inhibited [3H]formycin B uptake with a Ki of 2.7+/-0.6 nM, and [3H]NBMPR had a KD of 1.3+/-0.3 nM for binding to these cells; these values are significantly higher than those obtained in human and mouse cell models. In contrast, other recognized es inhibitors, such as dipyridamole, were significantly more effective as inhibitors of [3H]NBMPR binding and [3H]formycin B uptake by MDCK cells relative to that seen for human cells. We isolated a cDNA encoding the canine es nucleoside transporter (designated cENT1), and assessed its function by stable expression in nucleoside transport deficient PK15NTD cells. The PK15-cENT1 cells displayed inhibitor sensitivities that were comparable to those obtained for the endogenous es nucleoside transporter in MDCK cells. These data indicate that the dog es/ENT1 transporter has distinctive inhibitor binding characteristics, and that these characteristics are a function of the protein structure as opposed to the environment in which it is expressed.

Interaction of the Novel Adenosine Uptake Inhibitor 3-[1-(6,7-Diethoxy-2-morpholinoquinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione Hydrochloride (KF24345) with the es and ei Subtypes of Equilibrative Nucleoside Transporters

Nucleosides such as adenosine, as well as many nucleoside-based drugs, permeate cell membranes via a family of equilibrative nucleoside transporters (ENTs). We assessed the effects of (3-[1-(6,7-diethoxy-2-morpholino-quinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione hydrochloride (KF24345), a novel anti-inflammatory agent that potentiates the actions of adenosine, on the es (inhibitor-sensitive) and ei (inhibitor-resistant) subtypes of ENTs in human, mouse, and rat cells. KF24345 was similar to the prototypical high-affinity inhibitor nitrobenzylthioinosine (NBMPR) for blocking the human es transporter (K(I) of approximately 0.4 nM), but was 50-fold more effective than NBMPR at blocking the human ei transporter (K(I) of approximately 100 nM). KF24345 displayed significantly less species heterogeneity in its affinity for the es transporter than did dipyridamole, a widely used inhibitor of nucleoside transport; KF24345 may thus prove useful as an inhibitor for studies of nucleoside metabolism in a range of animal models. Furthermore, KF24345 seemed to act as a noncompetitive inhibitor of both [(3)H]NBMPR binding and [(3)H]nucleoside uptake by human es transporters, and these kinetics were consistent with an observed slow dissociation of KF24345 from the inhibitor binding site. KF24345 also exhibited unusual biphasic profiles for inhibition of [(3)H]NBMPR binding to membranes prepared from a recombinant human es transporter model (PK15-hENT1), suggesting the presence of multiple populations of NBMPR binding proteins in these membranes. The atypical tight binding interaction of KF24345 with the es transporter may prove useful for the molecular delineation of inhibitor binding domains and will facilitate its use as an in vivo inhibitor of nucleoside transport in studies focused on the biological effects of adenosine.

Complex effects of sulfhydryl reagents on ligand interactions with nucleoside transporters: evidence for multiple populations of ENT1 transporters with differential sensitivities to N-ethylmaleimide

Functional studies have implicated cysteines in the interaction of ligands with the ENT1 nucleoside transporter. To better define these interactions, N-ethylmaleimide (NEM) and p-chloromercuribenzylsulfonate (pCMBS) were tested for their effects on ligand interactions with the [(3)H] nitrobenzylthioinosine (NBMPR) binding site of the ENT1 transporters of mouse Ehrlich ascites cells and human erythrocytes. NEM had biphasic, concentration-dependent effects on NBMPR binding to intact Ehrlich cells, plasma membranes, and detergent-solubilized membranes, with about 35% of the binding activity being relatively insensitive to NEM inhibition. NBMPR binding to human erythrocyte membranes also displayed heterogeneity in that about 33% of the NBMPR binding sites remained, albeit with lower affinity for NBMPR, even after treatment with NEM at concentrations in excess of 1 mM. However, unlike that seen for Ehrlich cells, no "reversal" in NBMPR binding to human erythrocyte membranes was observed at the higher concentrations of NEM. pCMBS inhibited 100% of the NBMPR binding to both Ehrlich cell and human erythrocyte membranes, but had no effect on the binding of NBMPR to intact cells. The effects of NEM on NBMPR binding could be prevented by coincubation of membranes with nonradiolabeled NBMPR, adenosine, or uridine. Treatment with NEM and pCMBS also decreased the affinity of other nucleoside transport inhibitors for the NBMPR binding site, but enhanced the affinities of nucleoside substrates. These data support the existence of at least two populations of ENT1 in both erythrocyte and Ehrlich cell membranes with differential sensitivities to NEM. The interaction of NEM with the mouse ENT1 protein may also involve additional sulphydryl groups not present in the human ENT1.

Increase in adenosine sensitivity in the nucleus accumbens following chronic morphine treatment

There is a growing body of evidence suggesting that the neuromodulator adenosine is involved in drug addiction and withdrawal and that adenosine signaling pathways may offer new targets for therapeutic treatments of addiction. Recent studies have suggested that chronic exposure to drugs of abuse may alter adenosine metabolism in the nucleus accumbens, a brain region critically involved in drug addiction and withdrawal. The present study examined the effects of chronic morphine treatment on the ability of adenosine to inhibit excitatory postsynaptic currents in nucleus accumbens medium spiny neurons. It was found that chronic morphine treatment via subcutaneous implantation of morphine pellets in rats for 1 wk did not alter the level of adenosine-mediated tonic inhibition of nucleus accumbens excitatory synapses. However, chronic morphine treatment did induce a leftward shift in the adenosine dose-response curve, indicating an increase in the sensitivity of synaptic currents to exogenously applied adenosine. This shift was not due to a change in adenosine receptors or their effectors, because chronic morphine treatment had no effect on the dose-response relationship of a nonmetabolized adenosine receptor agonist. When adenosine transport was blocked, the ability of chronic morphine to shift the adenosine dose-response curve was eliminated. These experiments suggest that the increase in the sensitivity of nucleus accumbens synapses to the inhibitory effects of adenosine may be due to a decrease in adenosine transport. The identification of these changes in the adenosine system after chronic drug exposure may help identify new therapeutic strategies aimed at easing withdrawal from opioids.

Distribution of equilibrative, nitrobenzylthioinosine-sensitive nucleoside transporters (ENT1) in brain

Nucleoside transport processes may play a role in regulating endogenous levels of the inhibitory neuromodulator adenosine in brain. The cDNAs encoding species homologues of one member of the equilibrative nucleoside transporter (ENT) gene family have recently been isolated from rat (rENT1) and human (hENT1) tissues. The current study used RT-PCR, northern blot, in situ hybridization, and [3H]nitrobenzylthioinosine autoradiography to determine the distribution of mRNA and protein for ENT1 in rat and human brain. Northern blot analysis indicated that hENT1 mRNA is widely distributed in adult human brain. 35S-labeled sense and antisense riboprobes, transcribed from a 153-bp segment of rENT1, were hybridized to fresh frozen coronal sections from adult rat brain and revealed widespread rENT1 mRNA in pyramidal neurons of the hippocampus, granule neurons of the dentate gyrus, Purkinje and granule neurons of the cerebellum, and cortical and striatal neurons. Regional localization in rat brain was confirmed by RT-PCR. Thus, ENT1 mRNA has a wide cellular and regional distribution in brain, indicating that this nucleoside transporter subtype may be important in regulating intra- and extracellular levels of adenosine in brain.

Sensitization by chronic diazepam treatment of A2A adenosine receptor-mediated relaxation in rat pulmonary artery

The effects of a 10-day i.p. treatment of rats with diazepam on responses to subtype selective adenosine receptor agonists were studied 3 h, 2 and 8 days after termination of diazepam treatment in isolated cardiovascular tissues possessing distinct adenosine receptors. After long-lasting diazepam exposure, the relaxation elicited by the specific A2A receptor agonist CGS 21680 was enhanced in rat main pulmonary arteries (a tissue containing A2A adenosine receptors). The increased sensitivity of A2A receptors observed 3 h and 2 days after withdrawal of diazepam was completely restored by the 8th day of the wash-out period. N6-cyclopentyladenosine (CPA)-induced suppression in mechanical activity of electrically stimulated rat atrial myocardium (a tissue containing A1 adenosine receptors) was not altered following diazepam treatment. In order to reveal the possible role of inhibition of membrane adenosine transport in the effects of diazepam (a moderate inhibitor of membrane adenosine transport), the action of a 10-day treatment with dipyridamole or S-(p-nitrobenzyl)-6-thioinosine (NBTI; prototypic adenosine uptake inhibitors) was also studied. Dipyridamole or NBTI treatment, like diazepam, increased the responsiveness of rat pulmonary artery to CGS 21680, but did not influence the cardiodepressive effect of CPA in electrically driven left atrial myocardium. The CGS 21680-induced relaxations were significantly antagonized by 10 nM ZM 241385 (a selective A2A adenosine receptor antagonist) in vessels of diazepam-treated rats. The relaxation responses to verapamil were unaltered in pulmonary arteries obtained from animals chronically treated with diazepam, dipyridamole or NBTI. These results suggest that chronic diazepam treatment is able to enhance the A2A adenosine receptor-mediated vascular functions, but does not modify the responses mediated via A1 receptors of rat myocardium, where nucleoside transport inhibitory sites of membrane are of a very low density. It is possible that sensitization of A2A adenosine receptor-mediated vasorelaxation is due to a long-lasting inhibition of membrane adenosine transporter during diazepam treatment.

Saturation kinetics, specificity and NBMPR sensitivity of thymidine entry into the central nervous system

It was not until the development of a technique that could measure the brain uptake of slowly moving substrates, that the saturable transport system at the blood-brain barrier (BBB) for the pyrimidine deoxyribonucleoside, thymidine, was demonstrated. The aim of this present study was to further characterize this saturable uptake system at the blood-brain and blood-CSF barriers in terms of specificity, 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine (NBMPR) sensitivity and saturation kinetics by means of the in situ brain perfusion technique in anaesthetized guinea pigs. The results indicated that the transport system identified for [3H]thymidine can also transport other pyrimidine deoxyribonucleosides (deoxycytidine) and pyrimidine ribonucleosides (uridine) and is partially NBMPR-sensitive. In addition, guanosine, monocarboxylic acids, hexoses or amino acids were not substrates for the transport system. Further studies revealed that the transport system for [3H]thymidine at the BBB has a low affinity (Km 0.20 +/- 0.06 mM), but a relatively high capacity (Vmax 1.06 +/- 0.08 nmol min(-1) g(-1)). Overall, this study is indicative of a NBMPR-sensitive (es) facilitative transport system for [3H]thymidine and the likely presence of a NBMPR-insensitive and/or sodium-dependent transport system of the N2 (cit) type at the blood-brain and blood-CSF barriers of the guinea pig.

Markers of adenosine removal in normotensive and hypertensive rat nervous tissue

Adenosine mechanisms are altered in brain stem nuclei associated with cardiovascular control in spontaneously hypertensive rats (SHR). Therefore, in the present study we used a number of techniques to compare the binding of the adenosine transport inhibitor [3H]nitrobenzylthioinosine ([3H]NBMPR) as well as adenosine deaminase immunoreactivity (ADA-IR) in brain stems and nodose ganglia of SHR and age-matched normotensive Donryu rats (DRY). Saturation binding revealed a single class of [3H]NBMPR binding sites in the dorsal brain stem of both strains, with Kd and Bmax values of 65 +/- 9 pmol/L and 282 +/- 31 fmol/mg protein, respectively, in SHR and 129 +/- 2 pmol/L and 217 +/- 23 fmol/mg protein in DRY. The Kd for [3H]NBMPR was significantly lower in SHR than in DRY. In competition assays, NBMPR, dilazep, dipyridamole, and adenosine displaced [3H]NBMPR binding, with Kd values of 0.21 +/- 0.04, 57.16 +/- 16.20, 1340 +/- 100, and 87000 +/- 12500 nmol/L, respectively, in DRY and 0.17 +/- 0.04, 28.24 +/- 3.60, 621 +/- 100, and 32000 +/- 6820 in SHR. Kd values for all displacers were lower in SHR; however, only values for dipyridamole and adenosine reached statistical significance. Autoradiography of adenosine transport sites with [3H]NBMPR revealed that unilateral nodose ganglionectomy reduced [3H]NBMPR binding on the denervated side of the nucleus tractus solitarius by 20.6 +/- 1.1% in DRY and 18.7 +/- 2.3% in SHR. The density of [3H]NBMPR binding in nodose ganglia was significantly lower in SHR (0.99 +/- 0.06 Bq/mm2) than in DRY (1.25 +/- 0.08). Immunohistochemical studies demonstrated ADA-IR in the dorsal vagal complex, associated with both nerve cells and fibers. Measurement of ADA-IR in the dorsal vagal complex with an 125I-labeled secondary antibody revealed a significantly higher level of ADA-IR in SHR (122%) than in DRY. In the nodose ganglia, ADA-IR was associated with a population of vagal perikarya. The present study helps provide a molecular explanation for the previously reported impaired cardiovascular responses to intra-nucleus tractus solitarius microinjection of adenosine in hypertensive rats.

Autoradiographic study on the pharmacological characteristics of [3H]3-OH-PCP binding sites in rat brain

The pharmacological characteristics and the regional distribution of [3H]3-OH-PCP (1-[1(3-hydroxyphenyl)-cyclohexyl]piperidine) binding were investigated in rat brain by quantitative autoradiography. Kinetic analysis of [3H]3-OH-PCP binding revealed fast and slow components, in the association and dissociation studies. The regional distribution of binding closely corresponded to those of binding sites labeled by [3H]N-[l-(2-thienyl)-cyclohexyl]3,4-piperidine (TCP) and [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate (MK 801). High densities of [3H]3-OH-PCP binding sites were found in the stratum radiatum and orients of field CA1 in the hippocampus and in the outer layers of cerebral cortices. In contrast, low levels of binding were seen in the brain stem and the granular cell layer of the cerebellum. [3H]3-OH-PCP binding was strongly inhibited by MK 801 and 3-OH-PCP, while the potency of (+)-SKF 10047 in inhibiting [3H]3-OH-PCP binding was less in the cerebral cortex and hippocampus. The antagonists for the glutamate, glycine and polyamine recognition sites at the NMDA/PCP receptor complex displaced [3H]3-OH-PCP binding sites with a potency similar to that of [3H]MK 801. These findings suggest that the [3H]3-OH-PCP binding site is similar or identical to the PCP binding site labeled by [3H]TCP and [3H]MK 801.

[3H]adenosine transport in DDT1 MF-2 smooth muscle cells: inhibition by metabolites of propentofylline

Adenosine receptor signal transduction mechanisms have previously been characterized in Syrian hamster smooth muscle DDT1 MF-2 cells but adenosine transport in these cells has not. DDT1 MF-2 cells possess a high density (370,000 sites/cell) of high affinity (Kd value of 0.26 nM) binding sites for [3H]nitrobenzylthioinosine, a marker for the equilibrative and inhibitor-sensitive subtype of nucleoside transporters. Transport of [3H]adenosine was insensitive to Na+ and was inhibited by the nucleoside transport inhibitors nitrobenzylthioinosine, dilazep and dipyridamole with IC50 values of 1, 13 and 270 nM, respectively. Propentofylline, a neuroprotective compound that can inhibit nucleoside transporters, is rapidly metabolized in vivo to the racemate (+/-)-A720287. Based on recent findings that some transport inhibitors exhibit marked stereoselectivity, we tested the degree to which individual stereoisomers of (+/-)-A720287 affect adenosine transport. Propentofylline inhibited [3H]adenosine transport in DDT1 MF-2 cells with an IC50 value of 24 microM. (+/-)-A720287 and the individual stereoisomers (+)-833791 and (-)-844261 had similar potency to propentofylline for inhibition of [3H]adenosine transport in DDT1 MF-2 cells as well as in clonal mouse leukemia L1210/B23.1 cells, cells which possess only the equilibrative and inhibitor-sensitive subtype of nucleoside transporters. Thus, the neuroprotective effects of propentofylline may be due, in part, to the primary metabolites of propentofylline.

GTP and guanosine synergistically enhance NGF-induced neurite outgrowth from PC12 cells

Six per cent of rat pheochromocytoma (PC12) cells extended neurites (processes greater than one cell diameter in length) in the presence of 300 microM extracellular GTP or 300 microM guanosine for 48 hr, compared to only 2.5% of cells in control cultures. In the presence of 40 ng/ml of 2.5S NGF, about 20-35% of PC12 cells had neurites after 48 hr, and the addition of 300 microM guanosine or GTP together with NGF synergistically increased the proportion of cells with neurites to 40-65%. GTP and guanosine also increased the average number of branches per neurite, from 0.6 in NGF-treated cultures to 1.2 (guanosine) or 1.5 (GTP). Neurites formed after exposure to NGF alone had axonal characteristics as determined by immunocytochemistry with antibody, SMI-31, against axonal-specific polyphosphorylated neurofilament epitopes. Neurites generated with the addition of both guanosine or GTP had the same characteristics. GTP probably did not exert its effects via the P2X or P2Y purinoceptors because the adenine nucleotides ATP, ATP gamma S, ADP beta S, and ADP, which are all agonists of these receptors, inhibited rather than enhanced, NGF-induced neurite outgrowth. UTP also enhanced the proportion of cells with neurites, although not to the same degree as did GTP. This may indicate activity through a P2U-like nucleotide receptor. However, the response profile obtained, GTP > UTP >> ATP, does not fit the profile of any known P2Y, P2X or P2U receptor. The poorly hydrolyzable GTP analogues, GTP gamma S and GDP beta s were also unable to enhance the proportion of cells with neurites. This implied that GTP may produce its effects through a GTP-specific ectoenzyme or kinase. This idea was supported by results showing that another poorly hydrolyzable analogue, GMP-PCP, competitively inhibited the effects of GTP on neurite outgrowth. GTP did not exert its effects after hydrolysis to guanosine since the metabolic intermediates GDP and GMP were also ineffective in enhancing the proportion of cells with neurites. Moreover, the effects of GTP and guanosine were mutually additive, implying that these two purines utilized different signal transduction mechanisms. The effects of guanosine were not affected by the nucleoside uptake inhibitors nitrobenzylthioinosine (NBTI) and dipyridamole, indicating that a transport mechanism was not involved. Guanosine also did not activate the purinergic P1 receptors, because the A2 receptor antagonists, 1,3-dipropyl-7-methylxanthine (DPMX) or CGS15943, and the A1 receptor antagonist, 1,3-dipropyl-8-(2-amino-4-chloro)xanthine (PACPX) did not inhibit its reaction. Therefore guanosine enhanced neurite outgrowth by a signal transduction mechanism that does not include the activation of the P1 purinoceptors. The enhancement of the neuritogenic effects of NGF by GTP and guanosine may have physiological implications in sprouting and functional recovery after neuronal injury in the CNS, due to the high levels of nucleosides and nucleotides released from dead or injured cells.

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.

Immunolocalisation of nucleoside transporters in human placental trophoblast and endothelial cells: evidence for multiple transporter isoforms

Polyclonal antibodies raised against the human erythrocyte nucleoside transporter were used to investigate the distribution of the nucleoside transporters in the placenta. Immunoblots of brush-border membranes isolated from the human syncytiotrophoblast revealed a cross-reactive species that co-migrated with the erythrocyte nucleoside transporter as a broad band of apparent M(r) 55,000. In contrast, no labelling was detected in basal membranes containing a similar number of equilibrative nucleoside transporters as assessed by nitrobenzylthioinosine (NBMPR)-binding. The absence of cross-reactive epitopes in basal membranes and their presence in brush-border membranes was confirmed by confocal immunofluorescence microscopy. These results suggest that at least two isoforms of the NBMPR-sensitive nucleoside transporter are present in the human placenta. The lumenal surfaces of fetal capillaries, small placental vessels and umbilical vein were also strongly labelled by the antibody, a finding that suggests that the high fetal-placental adenosine uptake previously reported is due to endothelial transporters.

Effect of cellular differentiation on nucleoside transport in human neuroblastoma cells

The nucleoside transport characteristics of undifferentiated and differentiated LA-N-2 human neuroblastoma cells were compared through measurement of the cellular accumulation of [3H]formycin B in the absence and presence of specific nucleoside transport blockers such as dipyridamole and nitrobenzylthioinosine (NBMPR). [3H]NBMPR was also used as a high affinity probe to obtain an estimate of the number of NBMPR-sensitive nucleoside transport proteins. Undifferentiated LA-N-2 cells accumulated [3H]formycin B (25 microM) via a NBMPR/dipyridamole sensitive, Na(+)-independent, nucleoside transport system (Vi = 1.52 pmol/microliters/s; maximum intracellular concentration = 45 pmol/microliters cell water). The undifferentiated cells also had a high density of site-specific [3H]NBMPR binding sites (135,000 sites/cell; KD = 0.4 nM). When cell differentiation was induced by exposure to a serum-free defined medium, the initial rate of transporter-mediated [3H]formycin B uptake increased to 1.92 pmol/microliters/s, and the steady-state intracellular concentration of [3H]formycin B also increased significantly to 73 pmol/microliters. However, there was no concomitant change in the number of [3H]NBMPR binding sites, and the additional uptake was not Na(+)-dependent. This enhanced uptake in the differentiated cells appeared to be due, in part, to an increased functional expression of a NBMPR-resistant form of facilitated nucleoside transporter. Approximately 18% of the transporter-mediated uptake in the differentiated cells was resistant to inhibition by NBMPR at concentrations that blocked transport completely in the undifferentiated cells. This cell model may prove useful for basic studies on regulation of nucleoside transporter subtype expression in neural tissues, and for evaluation of the efficacy and potential host toxicity of cytotoxic nucleoside analogues (+/- specific transport blockers) in the treatment of neuroblastoma.

High expression of nitrobenzylthioinosine-insensitive dipyridamole binding sites in postmortem human ependymal tissue

The presence of both nitrobenzylthioinosine-sensitive and nitrobenzylthioinosine-insensitive dipyridamole binding sites in postmortem human ependymal tissue is reported. Displacement studies using 15 nM [3H]dipyridamole revealed 50-60% of the sites were sensitive to nitrobenzylthioinosine. Non-linear analysis of binding isotherms to estimate the density of nitrobenzylthioinosine-sensitive and -insensitive sites revealed a maximum number of nitrobenzylthioinosine-sensitive binding sites (Bmax) of 395 +/- 19 fmol/mg protein and a nitrobenzylthioinosine-insensitive Bmax of 3910 +/- 700 fmol/mg protein (corresponding Kd values of 0.1 nM and 114 nM respectively). Thus there are approximately 10 times as many nitrobenzylthioinosine-insensitive sites as nitrobenzylthioinosine-sensitive [3H]dipyridamole binding sites in human ependymal membranes.

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.

Interaction of the mioflazine derivative R75231 with the nucleoside transporter: evidence for positive cooperativity

This study investigated the interaction of the mioflazine derivative R75231 with the nucleoside transport system of rabbit cortical synaptosomes, and assessed the binding of [3H]R75231 to human erythrocyte ghost membranes. R75231 was a potent inhibitor of [3H]nitrobenzylthioinosine binding and [3H]uridine uptake in synaptosomes (Ki < 10 nM). This inhibition was evident even after extensive washing of the synaptosomes, subsequent to exposure to R75231. In addition to its tight binding characteristics, R75231 was shown to be a 'mixed' type inhibitor of [3H]nitrobenzylthioinosine binding (increased KD, decreased Bmax). [3H]R75231 bound with high affinity (KD = 0.4 nM) to erythrocyte membranes with a Bmax of 44 pmol/mg protein, which is comparable to the number of [3H]nitrobenzylthioinosine binding sites in this preparation. Binding of [3H]R75231 to these membranes was reversible, but the rate of dissociation was dependent upon the displacer used. Nitrobenzylthioinosine and dipyridamole each induced a complete dissociation of site-bound [3H]R75231 at rates not significantly different from those observed using a protocol involving a 100-fold dilution with buffer (no displacer). However, R75231 and mioflazine slowed the rate of dissociation of [3H]R75231 and actually caused an initial increase in the amount of site-bound [3H]R75231. Adenosine, on the other hand, enhanced the rate of [3H]R75231 dissociation. These results indicate that R75231 binding to the nucleoside transporter is a complex reaction, which may involve multiple interacting sites demonstrating positive cooperativity.

Differential uptake of [3H]guanosine by nucleoside transporter subtypes in Ehrlich ascites tumour cells

Intracellular metabolism of [3H]guanosine was minimal (< 15%) during the first 22 s of incubation, and hence reasonable estimates of initial-rate influx kinetics could be derived by using metabolically active cells. Na(+)-dependent concentrative [3H]guanosine uptake was not observed. Data suggest that [3H]guanosine was accumulated primarily via the nitrobenzylthioguanosine (NBTGR)-sensitive subtype of facilitated nucleoside transporter. Incubation of cells with 100 nM-NBTGR significantly decreased the potency of guanosine as an inhibitor of [3H]uridine influx. The Vmax. for [3H]guanosine influx (9.2 pmol/s per microliters) was significantly lower than that for [3H]uridine influx (16 pmol/s per microliters). The Km for transporter-mediated [3H]guanosine influx determined in the presence of 100 nM-NBTGR was 16-fold higher (1780 microM) than that determined in its absence, whereas the Km for [3H]uridine influx was shifted by only 2-fold. In other respects, the cellular accumulations of [3H]guanosine and [3H]uridine were similar; both had Km values of approx. 140 microM for total mediated influx, and both were inhibited similarly by other nucleosides and transport inhibitors. These characteristics, and the fact that guanosine is an endogenous nucleoside, suggest that [3H]guanosine may prove useful as a poorly metabolized, relatively selective, substrate for study of the NBTGR-sensitive nucleoside transport systems of mammalian cells.

Heterogeneity of [3H]dipyridamole binding to CNS membranes: correlation with [3H]nitrobenzylthioinosine binding and [3H]uridine influx studies

The relationship between the nucleoside transport system and the nitrobenzylthioinosine-sensitive and -resistant [3H]dipyridamole binding sites was examined by comparing the characteristics of [3H]dipyridamole binding with those of [3H]nitrobenzylthioinosine binding and [3H]-uridine influx in rabbit and guinea pig cerebral cortical synaptosomes. Two distinct high-affinity synaptosomal membrane-associated [3H]dipyridamole binding sites, with different sensitivities to inhibition by nitrobenzylthioinosine, were characterized in the presence of 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS, 0.01%) to prevent [3H]dipyridamole binding to glass tubes and filters. The nitrobenzylthioinosine-resistant [3H]-dipyridamole binding sites represented a greater proportion of the total membrane sites in guinea pig than in rabbit (40 vs. 10% based on inhibition studies). In rabbit, nitrobenzylthioinosine-sensitive [3H]dipyridamole binding (KD = 1.4 +/- 0.2 nM) and [3H]nitrobenzylthioinosine binding (KD = 0.30 +/- 0.01 nM) appeared to involve the same membrane site associated with the nitrobenzylthioinosine-sensitive nucleoside transporter. By mass law analysis, [3H]-dipyridamole binding in guinea pig could be resolved into two components based on sensitivity to inhibition by 1 microM nitrobenzylthioinosine. The nitrobenzylthioinosine-resistant [3H]dipyridamole binding sites were relatively insensitive to inhibition by all of the nucleoside transport substrates and inhibitors tested, with the exception of dipyridamole itself. In guinea pig synaptosomes, 100 microM dilazep blocked nitrobenzylthioinosine-resistant [3H]uridine transport completely but inhibited the nitrobenzylthioinosine-resistant [3H]dipyridamole binding component by only 20%. Furthermore, a greater percentage of the [3H]dipyridamole binding was nitrobenzylthioinosine resistant in guinea pig compared with rabbit, yet both species had a similar percentage of nitrobenzylthioinosine-resistant [3H]uridine transport.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Further characterization of adenosine transport in renal brush-border membranes

Adenosine transport has been further characterized in rat renal brush-border membranes (BBM). The uptake shows two components, one sodium-independent and one sodium-dependent. Both components reflect, at least partly, translocation via a carrier mechanism, since the presence of adenosine inside the vesicles stimulates adenosine uptake in the presence as well as in the absence of sodium outside the vesicles. The sodium-dependent component is saturable (Km adenosine = 2.9 microM, Vmax = 142 pmol/min per mg protein) and is abolished at low temperatures. The sodium-independent uptake has apparently two components: one saturable (Km = 4-10 microM, Vmax = 174 pmol/min per mg protein) and one non-saturable (Vmax = 3.4 pmol/min per mg protein, Km greater than 2000 microM). Inosine, guanosine, 2-chloroadenosine and 2'-deoxyadenosine inhibit the sodium-dependent and -independent transport, as shown by trans-stimulation experiments, probably because of translocation via the respective transporter. Uridine and dipyridamole inhibited only the sodium-dependent uptake. Other analogs of adenosine showed no inhibition. The kinetic parameters of the inhibitors of the sodium-dependent component were further investigated. Inosine was the most potent inhibitor with a Ki (1.9 microM) less than the Km of adenosine. This suggests a physiological role for the BBM ecto-adenosine deaminase (enzyme which extracellularly converts adenosine to inosine), balancing the amount of nucleoside taken up as adenosine or inosine by the renal proximal tubule cell.

Binding of [G-3H]6-(4-nitrobenzylmercapto)purine ribonucleoside to isolated membranes. Inhibitory effect of mioflazine and its derivatives

The binding of [G-3H]-6-(4-nitrobenzylmercapto)purine ribonucleoside [( G-3H]NBMPR) was investigated using a centrifugation assay with membrane preparations from hamster tissues including liver, lung, kidney and heart. Only liver and lung membranes showed high specific binding, with dissociation constants (Kd) values of 2.4 +/- 0.4 and 0.44 +/- 0.05 nM, and maximal binding (Bmax) of 3.7 +/- 0.4 and 1.04 +/- 0.01 pmol/mg, respectively. The binding of [G-3H]NBMPR was inhibited in a concentration dependent manner by unlabelled NBMPR, dilazep and a new group of chemically related nucleoside transport inhibitors, mioflazine, soluflazine and R57974, the latter being the most potent derivative. R57974 displaced bound [G-3H]NBMPR as effectively as unlabelled NBMPR suggesting a common binding site. The assay procedure used appears useful for the rapid screening of the effectiveness of nucleoside transport inhibitors which will be of value for the selection of inhibitors suitable for combination with cytotoxic nucleosides in the treatment of selected cancers or parasitic diseases.

Inhibition of nucleoside transport by a new series of compounds related to lidoflazine and mioflazine

A new series of compounds related to the nucleoside transport inhibitors, lidoflazine and mioflazine, is introduced. The influence of these derivatives on nucleoside-specific transport proteins was studied in two ways. First, a rapid, non-radioactive assay was developed for the screening of this type of material for actual transport inhibition in human erythrocytes. The method is based on the dose-dependent reversal of the inhibition of inorganic phosphate release induced by inosine when human erythrocytes are suspended in a phosphate-free medium. It enables the estimation of the potency and specificity of this new series of nucleoside transport inhibitors, most of which are highly active (EC50 values as low as 13 nM). Second, the displacement of a radiolabeled transport inhibitor, [3H]nitrobenzylthioinosine, was examined. All compounds were capable of displacing specific [3H]nitrobenzylthioinosine binding to crude and solubilized plasma membranes of calf lung tissue, displaying affinities in the nanomolar range. Pseudo-Hill coefficients derived from the shape of the displacement curves were significantly greater than unity for most derivatives, in contrast to values of approximately unity obtained for dipyridamole and analogs. These findings were incorporated in a mathematical model describing the interaction of mioflazine analogs with the transport protein, suggesting that one molecule of mioflazine is capable of displacing two or more molecules of [3H]nitrobenzylthioinosine at a time. The consequences of this model regarding the nature of the transport protein are discussed.

Solubilization and reconstitution of a nucleoside-transport system from Ehrlich ascites-tumour cells

Uptake of [3H]uridine by Ehrlich cells was mediated by both nitrobenzylthioinosine (NBMPR)-sensitive (75%) and NBMPR-insensitive (25%) mechanisms. Each cell contained approx. 26,000 high-affinity (KD = 0.19 nM) recognition sites for [3H]NBMPR, and binding was inhibited by dipyridamole and adenosine at concentrations similar to those required for inhibition of [3H]uridine uptake. Calculations show that each cell contains a total of about 35,000 nucleoside transporters. Photoaffinity labelling of a partially purified preparation of plasma membranes with [3H]NBMPR resulted in a single broad 3H-labelled band on SDS/polyacrylamide gels, with an apparent molecular-mass peak of 42 kDa. This is in contrast with human erythrocyte membranes, where [3H]NBMPR photolabelled two broad bands with peaks at 55 and 80 kDa. Treatment of photoaffinity-labelled membranes with endoglycosidase F decreased the apparent molecular masses of both the Ehrlich-cell and erythrocyte [3H]NBMPR-labelled proteins to approx. 40 kDa. These results suggest that the human erythrocyte [3H]NBMPR-binding polypeptides are more extensively glycosylated than the corresponding Ehrlich-cell polypeptides. Octyl beta-D-glucopyranoside [1.0% (w/v) + asolectin] solubilized over 90% of the [3H]NBMPR-binding sites, with near-complete retention of [3H]NBMPR-binding characteristics. The only major change was a 65-fold decrease in affinity for dipyridamole, which was partly reversed upon incorporation of the solubilized proteins into asolectin membranes. Proteoliposomes, prepared by using asolectin and the octyl glucoside-solubilized plasma membranes, were capable of accumulating [3H]uridine via a protein-dependent dipyridamole/nitrobenzylthioguanosine/dilazep-sensitive mechanism. We have thus demonstrated the efficient solubilization and functional reconstitution of a nucleoside-transport system from Ehrlich ascites-tumour cells.

A kinetic analysis of kappa-opioid agonist binding using the selective radioligand [3H]U69593

The interaction of the nonselective opioid ligand [3H]bremazocine and of the kappa-opioid [3H]U69593 with the kappa-receptor was investigated in guinea-pig cortical membranes. Each radioligand bound to a single population of high-affinity sites, although [3H]U69593 apparently recognised only 70% of those sites labelled by [3H]bremazocine. Naloxone and the kappa-selective ligands U69593 and PD117302 exhibited full inhibition of the binding of both radioligands. Kinetic analysis demonstrated biphasic rates of association and dissociation for both [3H]bremazocine and [3H]U69593. Detailed analysis of the binding of [3H]U69593 revealed that the fast rate of association was dependent on radioligand concentration, in contrast to the slow rate, which was independent of ligand concentration. Guanylyl-5'-imidodiphosphate (GppNHp) inhibited binding of [3H]U69593; saturation analysis demonstrated that the inhibitory effects of GppNHp resulted in a decrease in affinity without any significant change in binding capacity. GppNHp attenuated the formation of the slow component of [3H]U69593 binding, while accelerating the fast component. The data are consistent with the formation of a high-affinity complex between the kappa-receptor and a guanine nucleotide binding protein. Guanine nucleotides promote the dissociation of this ternary complex and the stabilisation of a lower-affinity state of the receptor.

Heterogeneity of nucleoside transport inhibitory sites in heart: a quantitative autoradiographical analysis

1. The distribution of nucleoside transport inhibitory sites in rat and guinea-pig cardiac sections was investigated by use of [3H]-nitrobenzylthioinosine ([3H]-NBMPR) autoradiography. The distribution of these sites was heterogeneous in guinea-pig sections and homogeneous in rat sections. 2. The areas of high density of nucleoside transport inhibitory sites found in guinea-pig cardiac sections were compared to the distribution of an endothelial cell marker, von Willebrand Factor, determined by radioimmunocytochemistry. These two markers were co-localized suggesting that coronary endothelial cells from guinea-pig have a high density of NBMPR binding sites and thus may have a high nucleoside transport capacity. 3. Nucleoside transporter subtypes with differing affinity for NBMPR or dipyridamole were investigated by quantitative autoradiography. Sites in rat tissues had high affinity for NBMPR (KD = 0.6 nM) but were of low sensitivity to dipyridamole (Ki = 3.1 microM). In guinea-pig sections, areas of high and low [3H]-NBMPR binding site density were analyzed separately. In both areas, sites had high affinity for NBMPR (KD = 1.4 nM, 4.5 nM, respectively) and dipyridamole (Ki = 108 nM, 245 nM, respectively). 4. While differences in density of nucleoside transport inhibitory sites are detectable between distinct regions of the heart, subtypes differing in affinity for NBMPR or dipyridamole were not evident. Therefore, more detailed structure activity studies are required to determine if subtypes of these sites exist within a single heart.

Subtypes of nucleoside transport inhibitory sites in heart: a quantitative autoradiographical analysis

We evaluated the interaction of several nucleoside transport inhibitors and substrates with the binding of [3H]nitrobenzylthioinosine ([3H]NBMPR) to nucleoside transport sites in guinea pig cardiac sections. Using quantitative autoradiography, we determined inhibition constants for inhibition of [3H]NBMPR binding to both coronary endothelial cells and cardiac myocytes. We studied the interactions of NBMPR, nitrobenzylthioguanosine, dipyridamole, dilazep, hexobendine, lidoflazine, mioflazine, soluflazine, adenosine, inosine and uridine for these two cell types. Of the compounds tested in this study, lidoflazine (8.2X) and hexobendine (6.3X) have the greatest selectivity for coronary endothelial cell nucleoside transporters. All other compounds had 3-fold or less selectivity. Therefore, there is evidence of nucleoside transporter subtypes between endothelial cells and myocytes. This heterogeneity of transport inhibitory sites on nucleoside transporters may allow the development of agents to modulate selectively some of the cardiovascular effects of adenosine.

Discrimination between CCK receptors of guinea-pig and rat brain by cyclic CCK8 analogues

The properties of high affinity CCK8 binding sites of guinea-pig and rat brain cortex were compared using [3H]pCCK8. Large differences were observed, with the KD value being significantly higher in the rat (KD = 1.25 nM) than in guinea-pig brain (KD = 0.18 nM). Both sites exhibited different specificities for various CCK8 analogues, the selectivity factors KI rat/KI guinea-pig varied from 0.9 for CCK4 to 64 for cyclic CCK8-related compounds. Significant differences in the inhibition of [3H]pCCK8 binding by monovalent and nucleotides cations were also observed. These results could be explained by a difference in receptor environment or by a species difference in the proportion of CCK8 receptor-subtypes.

Pharmacological characterization of rapidly accumulated adenosine by dissociated brain cells from adult rat

Mechanically dissociated brain cells from adult rats were used to study biochemically and pharmacologically their capacity to accumulate rapidly [3H]adenosine. The assay, which used an inhibitor-stop method to prevent further uptake into cells, was characterized with respect to protein and optimal substrate concentrations, and incubation times that ranged from 5 to 180 s. The accumulation of [3H]adenosine using 15-s incubation periods, conditions under which less than 10% of accumulated [3H]adenosine was metabolized, was best described kinetically by a two-component system with Km and Vmax values for the high-affinity component of 0.8 microM and 6.2 pmol/mg protein/15 s and for the low-affinity component 259 microM and 2,217 pmol/mg protein/15 s, respectively. The potencies with which nucleosides, adenosine deaminase resistant adenosine receptor agonists, and nucleoside uptake inhibitors competed for these uptake components were determined. Of the nucleosides examined, adenosine was the "preferred" substrate for the uptake site. The Ki value of adenosine for the high-affinity component was 10.7 microM. Inosine and uridine competed for a single lower affinity uptake system: Ki values were 142 and 696 microM, respectively. Nucleoside uptake inhibitors--nitrobenzylthioinosine, dipyridamole, and dilazep--were the most potent inhibitors of [3H]adenosine accumulation tested: the Ki values for the high-affinity system were 0.11, 1.3, and 570 nM, respectively. The adenosine analogs S-phenylisopropyladenosine, R-phenylisopropyladenosine, and cyclohexyladenosine inhibited the high-affinity component with Ki values of 2.3, 9.3, and 14.5 microM, respectively. N-Ethylcarboxamidoadenosine competed for a single lower affinity uptake system: Ki, 292 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine transporters in chromaffin cells: subcellular distribution and characterization

Adenosine transporters in freshly isolated and cultured chromaffin cells were quantified by the [3H]dipyridamole binding technique, showing a maximal bound capacity of 0.4 +/- 0.05 pmol/10(6) cells (240,000 +/- 20,000 transporters by cell). Scatchard analysis showed a similar affinity for [3H]dipyridamole in isolated cells and subcellular fractions (Kd = 5 +/- 0.6 nM). For enriched plasma membrane preparations and chromaffin granule membranes, the maximal binding capacities were also very similar, 2.3 +/- 0.3 and 1.8 +/- 0.4 pmol/mg protein, respectively. When [3H]nitrobenzylthioinosine was employed as a radioligand, the maximal bound capacity in cultured chromaffin cells was 0.053 +/- 0.004 pmol/10(6) cells (32,000 +/- 3000 transporters per cell) with a high affinity constant (Kd = 0.25 +/- 0.03 nM); similar values were obtained in all subcellular fractions (Kd = 0.1 +/- 0.01). Also, plasma and chromaffin granule membranes showed similar maximal binding values (0.4 +/- 0.06 pmol/mg protein). Photoincorporation studies with [3H]nitrobenzylthioinosine into plasma membrane polypeptides showed the presence of three molecular species of 115 +/- 10; 58 +/- 6 and 42 +/- 5 kDa. Chromaffin granule membranes showed only the 105 +/- 9 and 51 +/- 4 molecular species.

Nucleoside transport in rat cerebral-cortical synaptosomes. Evidence for two types of nucleoside transporters

The transport of [U-14C]uridine was investigated in rat cerebral-cortical synaptosomes using an inhibitor-stop filtration method. Under these conditions the rapid efflux of uridine from the synaptosomes is prevented and uridine is not significantly metabolized in the synaptosome during the first 1 min of uptake. The dose-response curve for the inhibition of uridine transport by nitrobenzylthioinosine (NBMPR) was biphasic: approx. 40% of the transport activity was inhibited with an IC50 (concentration causing half-maximal inhibition) value of 0.5 nM, but the remaining activity was insensitive to concentrations as high as 1 microM. Similar biphasic dose-response curves were observed for dilazep inhibition, but both transport components were equally sensitive to dipyridamole inhibition. Uridine influx by both components was saturable (Km 300 +/- 51 and 214 +/- 23 microM, and Vmax. 12 +/- 3 and 16 +/- 3 pmol/s per mg of protein, for NBMPR-sensitive and NBMPR-insensitive components respectively), and inhibited by other nucleosides such as 2-chloroadenosine, adenosine, inosine, thymidine and guanosine with similar IC50 values for the two components. Inhibition of uridine transport by NBMPR was associated with high-affinity binding of NBMPR to the synaptosome membrane (Kd 58 +/- 15 pM). Binding of NBMPR to these sites was competitively blocked by uridine and adenosine and inhibited by dilazep and dipyridamole, with Ki values similar to those measured for inhibiting NBMPR-sensitive uridine influx. These results demonstrate that there are two components of nucleoside transport in our rat synaptosomal preparation that differ in their sensitivity to inhibition by NBMPR. Thus conclusions regarding nucleoside transport in rat brain based only on NBMPR-binding activity must be viewed with caution.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

Subcellular, regional and immunohistochemical localization of adenosine deaminase in various species

Immunohistochemical and subcellular fractionation techniques were employed to compare the cellular and subcellular localization of adenosine deaminase (ADA) in various brain regions of several mammalian species. A relatively restricted distribution of ADA-immunoreactive neurons in rat brain was previously reported. Mouse brain exhibited a pattern similar in many respects to rat and, in addition, contained intensely immunostained neurons in lateral habenula and hippocampus. Glial immunostaining was absent or very light in rat but evident in mouse. Prominent immunoreactive fibers and neurons were observed in hamster spinal cord and anterior hypothalamus, respectively. ADA-immunostaining in guinea-pig was localized to presumptive fibers in the superficial layers of spinal cord dorsal horn and to glial cells throughout the brain. Demonstration of specific immunostaining in rabbit was not possible. ADA activity was far more heterogeneously distributed in rat and most brain areas in guinea-pig and rabbit contained up to 5-fold and 10-fold higher levels of activity, respectively, compared with rat. Crude synaptosomal (P2) fractions of rat cortex contained a greater proportion of ADA activity than those of rabbit cortex. Within rat, relatively high activity was found in P2 fractions of whole hypothalamus, cerebellum, and hippocampus. ADA activity was greater in P2 fractions of rat anterior compared with whole hypothalamus and the greatest proportion of the enzyme in this fraction was localized to purified synaptosomes. The large variations in the activity and cellular location of ADA in the animals examined suggest species differences in mechanisms governing adenosine metabolism in brain and possible differences in the relationships between cellular metabolism, ADA and the neuroregulatory role of adenosine in the CNS.

Quantitative [3H]dipyridamole autoradiography: evidence for adenosine transporter heterogeneity in guinea pig brain

[3H] Dipyridamole binding in guinea pig brain slices has been characterized. Binding of [3H] dipyridamole to guinea pig forebrain slices was found to be rapid, reversible and saturable. Saturation experiments revealed a class of high affinity binding sites with a Bmax value of 592 +/- 118 fmol/mg protein and Kd value of 10.8 nM +/- 2.1 nM in the analysed concentration range. In competition experiments, the adenosine transport inhibitors hexobendine and dipyridamole itself were the most potent displacers (inhibition constants of 4.6 nM +/- 1 nM and 11.5 nM +/- 3 nM) with "pseudo-Hill" coefficients close to 1. Competition curves with nitrobenzylthioinosine, another adenosine transport inhibitor, however, showed a biphasic profile with a "pseudo-Hill" coefficient of 0.33 +/- 0.04. Just 42% +/- 4% of [3H] dipyridamole binding were inhibited by nanomolar concentrations of nitrobenzylthionosine and only micromolar concentrations displaced the remainder. Subsequent quantitative autoradiography demonstrated regional differences in the inhibition of [3H] dipyridamole binding by submicromolar concentrations of nitrobenzylthioinosine. While in cortical areas of cerebrum and cerebellum 500 nM nitrobenzylthioinosine displaced binding of [3H] dipyridamole to only about one-third of its sites (in the Purkinje cell layer less than 10%), it showed similar potency as dipyridamole in various areas of the brainstem and hypothalamus. This biphasic and regionally heterogenous interaction of nitrobenzylthioinosine with [3H] dipyridamole binding sites in guinea pig brain slices strongly suggests heterogeneity of adenosine transporters.

[3H]dipyridamole binding to guinea pig brain membranes: possible heterogeneity of central adenosine uptake sites

The binding of [3H]dipyridamole ([3H]DPR) to guinea pig brain membranes is described and compared to that of [3H]nitrobenzylthioinosine ([3H]NBI). The binding of [3H]DPR is saturable, reversible, and specific with pharmacologic evidence indicating that this ligand is binding to the adenosine uptake site. Compared to [3H]NBI the binding of [3H]DPR is of higher capacity (Bmax = 208 +/- 16 fmol/mg protein for [3H]NBI and 530 +/- 40 fmol/mg protein for [3H]DPR) and lower affinity (KD = 0.35 +/- 0.02 nM for [3H]NBI and 7.6 +/- 0.7 nM for [3H]DPR). The adenosine uptake inhibitors are the most potent inhibitors of binding (Ki of 10(-8)-10(-7) M) whereas adenosine receptor ligands such as cyclohexyladenosine, 2-chloroadenosine, and various methylxanthines are several orders of magnitude less potent (Ki 10(-5)-10(-2). The inhibition of [3H]DPR binding by NBI is biphasic, with only 40% of binding being susceptible to inhibition of NBI concentrations less than 10(-5) M. The tissue distribution of [3H]DPR binding parallels that of [3H]NBI although in most cases significantly more sites are observed with [3H]DPR. Calcium channel blocking agents such as nifedipine, nimodipine, and verapamil are also inhibitors of [3H]DPR binding with potencies in the micromolar range. The data are consistent with [3H]DPR being a useful additional ligand for the adenosine uptake site and provide evidence that multiple uptake binding sites exist of which only about 40% are NBI-sensitive.

[3H]dipyridamole binding to nucleoside transporters from guinea-pig and rat lung

Membranes from guinea-pig lung exhibited high-affinity binding of [3H]dipyridamole, a potent inhibitor of nucleoside transport. Binding (apparent KD 2 nM) was inhibited by the nucleoside-transport inhibitors nitrobenzylthioinosine (NBMPR), dilazep and lidoflazine and by the transported nucleosides uridine and adenosine. In contrast, there was no detectable high-affinity binding of [3H]dipyridamole to lung membranes from the rat, a species whose nucleoside transporters exhibit a low sensitivity to dipyridamole inhibition. Bmax. values for high-affinity binding of [3H]dipyridamole and [3H]NBMPR to guinea-pig membranes were similar, suggesting that these structurally unrelated ligands bind to the NBMPR-sensitive nucleoside transporter with the same stoichiometry.

A comparative study on characterization and distribution of cholecystokinin binding sites among the rat, mouse and guinea pig brain

Using the in vitro quantitative receptor autoradiographical technique, [propionyl-3H]propionylated cholecystokinin octapeptide ([3H]pCCK-8) binding sites were investigated in tissue sections of rat, mouse and guinea pig brains. In all the tested animals, [3H]pCCK-8 bound very slowly to the tissue sections. Dissociation was also slow, and had a biphasic profile suggesting CCK-8 binding sites are heterogeneous. Dissociation rate constants were, however, unequal among these species. In the saturation binding studies, both Bmax and (Kd)app values varied among the animal species. The autoradiograms revealed marked species differences in [3H]pCCK-8 binding sites in the brain among 3 closely related species of rodents. [3H]pCCK-8 binding sites were undetectable in the nucleus accumbens/caudate-putamen and the amygdaloid complex of the mouse brain, and scarcely found in the ventromedialis of the hypothalamus of the mouse and guinea pig brain. Furthermore, moderate-to-high densities of [3H]pCCK-8 binding sites were observed in the cerebella of the mouse and guinea pig, whereas in the rat cerebellum the binding sites were undetected. The above-mentioned observations suggest the existence of species differences in the binding pattern of CCK-like peptides among closely related animal species. Furthermore, it would appear that CCK-like peptides in the brain may play different physiological roles among animal species.

Nucleoside transporter of cerebral microvessels and choroid plexus

The nucleoside transporter of cerebral microvessels and choroid plexus was identified and characterized using [3H]nitrobenzylthioinosine (NBMPR) as a specific probe. [3H]NBMPR bound reversibly and with high affinity to a single specific site in particulate fractions of cerebral microvessels, choroid plexus, and cerebral cortex of the rat and the pig. The dissociation constants (KD 0.1-0.7 nM) were similar in the various tissue preparations from each species, but the maximal binding capacities (Bmax) were about fivefold higher in cerebral microvessels and choroid plexus than in the cerebral cortex. Nitrobenzylthioguanosine and dipyridamole were the most potent competitors for [3H]NBMPR binding. Several naturally occurring nucleosides displaced specific [3H]NBMPR binding to cerebral microvessels in vitro, in a rank order that correlated well with their ability to cross the blood-brain barrier in vivo. Adenosine analogues and theophylline were less effective in displacing [3H]NBMPR binding than in displacing adenosine receptor ligands. Photoactivation of cerebral microvessels and choroid plexus bound with [3H]NBMPR followed by solubilization and polyacrylamide gel electrophoresis labeled a protein(s) with a molecular weight of approximately 60,000. These results indicate that cerebral microvessels and choroid plexus have a much higher density of the nucleoside transporter moiety than the cerebral cortex and that this nucleoside transporter has pharmacological properties and a molecular weight similar to those of erythrocytes and other mammalian tissues.

Effects of purine nucleotides on the binding of [3H]cyclopentyladenosine to adenosine A-1 receptors in rat brain membranes

Adenine nucleotides displace the binding of the selective adenosine A-1 receptor ligand [3H]cyclopentyladenosine (CPA) to rat brain membranes in a concentration-dependent manner, with the rank order of activity being ATP greater than ADP greater than AMP. Binding was also displaced by GTP, ITP, adenylylimidodiphosphate (AppNHp), 2-methylthioATP, and the beta-gamma-methylene isostere of ATP, but was unaffected by the alpha-beta-methylene isosteres of ADP and ATP, and UTP. At ATP concentrations greater than 100 microM, the inhibitory effects on CPA binding were reversed, until at 2 mM ATP, specific binding of CPA was identical to that seen in controls. Concentrations of ATP greater than 10 mM totally inhibited specific binding. Inclusion of the catabolic enzyme adenosine deaminase in the incubation medium abolished the inhibitory effects of ATP, indicating that these were due to adenosine formation, presumably due to ectonucleotidase activity. The inhibitory effects were also attenuated by the alpha-beta-methylene isostere of ATP, an ectonucleotidase inhibitor. Adenosine deaminase, alpha-beta-methylene ATP (100 microM), and beta-gamma-methylene ATP (100 microM) had no effect on the "stimulatory" phase of binding, although GTP (100 microM) slightly attenuated it. Comparison of the binding of [3H]CPA in the absence and presence of 2 mM ATP by saturation analysis showed that the KD and apparent Bmax values were identical. Examination of the pharmacology of the control and "ATP-dependent" CPA binding sites showed slight changes in binding of adenosine agonists and antagonists. The responses observed with high concentrations of ATP were not observed with GTP, AppNHp, the chelating agents EDTA and EGTA, or inorganic phosphate. The divalent cations Mg2+ and Ca2+ at 10 mM attenuated the stimulatory actions of high (2 mM) concentrations of ATP, whereas EGTA and EDTA (10 mM) enhanced the "stimulatory" actions of ATP. EDTA (10 mM) abolished the inhibitory effects of ATP, indicating a specific dependence on Mg2+ for the inhibitory response. The effects of ATP on [3H]CPA binding were reversible for antagonists but not agonists. The mechanism by which ATP reverses its own inhibitory action on adenosine A-1 radioligand binding is unclear, and from the observed actions of the divalent cations and chelating agents probably does not involve a phosphorylation-dependent process.