[3H]nitrobenzylthioinosine binding to the guinea pig CNS nucleoside transport system: a pharmacological characterization

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.

Chronic dipyridamole administration downregulates [3H]nitrobenzylthioinosine binding site affinity in guinea pig kidney but not heart and brain

Specific binding of the nucleoside transporter probe, [3H]nitrobenzylthioinosine, ([3H]NBMPR) was measured in washed guinea pig cardiac, renal and forebrain membranes after 14 days of treatment with dipyridamole (37.5 mg/kg, s.c., b.i.d.) or vehicle. When compared to values in vehicle-treated animals, a 100 percent increase in equilibrium dissociation constant (Kd) was observed in the kidney of dipyridamole-treated animals (0.51 +/- 0.04 to 1.0 +/- 0.06, p < 0.01). The maximal binding capacity (Bmax) was unaltered. No changes were observed in the heart or forebrain. The increase in Kd suggests that chronic dipyridamole treatment decreases the apparent binding affinity of NBMPR for kidney nucleoside transporters. Cardiac and brain nucleoside transporters may be either less susceptible to chronic dipyridamole administration or have a different adaptive mechanism.

Characterization of nitrobenzylthioinosine binding sites in the mitochondrial fraction of rat testis

The presence of [3H] NBMPR binding sites in the mitochondrial fraction of rat testis is described. The dissociation constant (KD) from saturation studies was 0.16 +/- 0.04 nM. The association and dissociation rate constants (k1 and k-1) were 3.95 +/- 0.57 x 10(8) M(-1) min(-1) and 0.025 +/- 0.002 min(-1), respectively. The number of binding sites was 2,100 +/- 163 fmols/mg protein. [3H] NBMPR binding was inhibited, in a nanomolar range, by NBMPR (KI= 0.23 +/- 0.02 nM), OH-NBMPR (KI= 2.30 +/- 0.55 nM) and HNBTG (KI= 2.58 +/- 0.33 nM). In the micromolar range, adenosine receptor ligands such as PIA (3.46 +/- 1.36 microM), 2-chloroadenosine (18.81 +/- 3.36 microM) and NECA (8.26 +/- 3.90 microM), and mitochondrial benzodiazepine receptor ligands such as Ro 5-4864 (5.15 +/- 1.82 micrmoM and PK 11195 inhibited the specific binding of [3H] NBMPR. These results suggest the existence of a nucleoside transport system in the mitochondrial fraction of rat testis.

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.

Influence of CGS 15943 A (a nonxanthine adenosine antagonist) on the protection offered by a variety of antiepileptic drugs against maximal electroshock-induced seizures in mice

CGS 15943 A (a nonxanthine adenosine antagonist) was studied on the protective efficacy of carbamazepine (60 min prior to the convulsive test), diazepam (60 min), diphenylhydantoin (120 min), phenobarbital (120 min), and valproate (30 min) against maximal electroshock-induced convulsions in mice. Moreover, the influence of the adenosine antagonist on 2-chloroadenosine (1 mg/kg, 20 min prior to the test)- and valproate (250 mg/kg, 30 min)-induced inhibitions of locomotor activity was also studied. CGS 15943 A (1 mg/kg) was given 15 min before both tests and all the drugs were administered i.p.. The adenosine antagonist (1 mg/kg) remained without influence upon the protective activity of all studied antiepileptics, reflected by their respective ED50 values against maximal electroshock. However, both 2-chloroadenosine and valproate-induced inhibitions of locomotor activity were attenuated by CGS 15943 A, which alone did not affect this parameter. However, CGS 15943 A (5 mg/kg) diminished the protection offered by diphenylhydantoin, increasing its ED50 value from 13 to 16 mg/kg. It may be concluded that the protection provided by common antiepileptic drugs against electroconvulsions seems independent of adenosine-mediated inhibition. In the case of diphenylhydantoin, one may suggest the involvement of purinergic transmission in the final anticonvulsant effect.

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

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

Characterization of 3'-phosphoadenosine 5'-phospho[35S]sulfate transport carrier from rat brain microsomes

3'-Phosphoadenosine 5'-phospho[35S]sulfate [( 35S]PAPS) specific binding properties of rat brain tissue were studied. [35S]PAPS specific binding was optimal at pH 5.8 in either Tris-maleate or potassium phosphate buffers. Association was maximal at low temperature, reaching equilibrium in 20 min. Dissociation was rapid, with a dissociation time of 80 s. Scatchard analysis of [35S]PAPS specific binding was consistent with a single site having a KD of 0.46 +/- 0.06 microM and a Bmax of 20.8 +/- 2.0 pmol/mg of protein. Low concentrations of Triton X-100 (0.025%) were effective in increasing the number of binding sites to a Bmax of 44.5 +/- 4.6 pmol/mg of protein without affecting the affinity. [35S]PAPS specific binding was enriched in crude synaptic membranes (P2) and microsomes (P3). Regional distribution of [35S]PAPS specific binding was quite homogeneous in all brain structures studied. The pharmacological profile of [35S]PAPS specific binding in rat brain microsomes was consistent with a membrane protein having a high selectivity for the 3'-O-phosphoryl group substitution on the ribose moiety. Thus, 3'-phosphoadenosine 5'-phosphate was more potent than 2'-phosphoadenosine 5'-phosphate in competing for [35S]PAPS specific binding. Adenosine 5'-phosphosulfate was a good inhibitor of [35S]PAPS specific binding. ATP and ADP were also good displacers. Dipyridamole, a highly selective marker for adenosine uptake sites, was ineffective. 4,4-Diisothiocyanostilbene-2,2-disulfonic acid, the chloride transporter inhibitor, showed an IC50 of 36 +/- 5.1 microM for inhibition of [35S]PAPS specific binding. 2,6-Dichloro-4-nitrophenol had a low selectivity in competing for the [35S]PAPS binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous adenosine can reduce epileptiform activity in the human epileptogenic cortex maintained in vitro

The effects induced by adenosine and some related compounds upon Mg2+-free epileptogenesis were studied in slices of human epileptogenic neocortex maintained in vitro. Extracellular recordings revealed stimulus-induced and spontaneous epileptiform activity within 1-2 h of perfusion with Mg2+-free medium. A 30-90% decrease of the frequency of occurrence of spontaneous epileptiform discharges was induced by 40-50 microM adenosine while the analog 2-Cl-adenosine exerted a depressant effect (greater than 75% reduction in frequency of occurrence) at 0.3-3 microM. 2-Cl-adenosine also depressed stimulus-induced epileptiform responses and often blocked spontaneous epileptiform activity. Similar effects were seen during bath application of the adenosine uptake inhibitor nitrobenzylthioinosine (10-50 microM) indicating that endogenous adenosine can by itself influence epileptogenicity. Our data demonstrate that in the human epileptogenic neocortex a purinergic mechanism can control Mg2+-free epileptiform activity.

Antiproliferative action of benzodiazepines in cultured brain cells is not mediated through the peripheral-type benzodiazepine acceptor

The benzodiazepines, Ro 5-4864, diazepam, clonazepam, and also PK-11195, inhibited, at micromolar concentrations, the proliferation of rat C6 glioma and mouse neuro-2A neuroblastoma cells in culture. The cells possessed high levels of "peripheral-type" high-affinity benzodiazepine binding sites as judged by binding assays and displacement potencies. However, the different potencies and specificities of compounds for the antiproliferative actions and binding affinities for the binding site suggest that the antiproliferative actions were not mediated through the peripheral-type binding site. In support of this, these compounds have also been shown to inhibit proliferation of some nonneuronal cultured cell lines, e.g., mouse SP2/O-Ag 14 hybridoma and rat NCTC epithelial cells, which have no detectable high-affinity peripheral-type benzodiazepine binding sites.

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.

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.

Ca++ antagonists and ACAT inhibitors promote cholesterol efflux from macrophages by different mechanisms. I. Characterization of cellular lipid metabolism

The effects of the slow Ca++ channel blocker, nifedipine, and ACAT inhibitor, octimibate, on the cholesterol metabolism of cholesterol-loaded macrophages were compared. We demonstrated that apolipoprotein A-I containing high density lipoproteins (HDL) bind to specific receptor sites on macrophages, are internalized, take up cholesterol, and are then released from the cells as native lipoproteins. The ACAT inhibitor enhances HDL receptor activity and promotes HDL-mediated cholesterol efflux from cultured mouse peritoneal macrophages. In contrast, the Ca++ antagonist increases acetyl LDL-mediated cholesterol influx, abolishes the increase in HDL binding induced by cholesterol accumulation, enhances apo E synthesis, and promotes cholesterol efflux by a mechanism independent of the presence of HDL in the surrounding medium. Concomitantly, a decrease in nucleoside transporter activity, an increase in intracellular ATP hydrolysis, adenosine and cyclic AMP concentration, and a stimulation of the activities of acid and neutral cholesteryl ester hydrolase and ACAT indicated that protein kinase A-catalyzed phosphorylation reactions might be involved in the increase in cholesterol efflux. The Ca++ antagonist-induced efflux occurred only with lysosomal-associated cholesterol, while the ACAT inhibitor acted on the formation of cytoplasmic lipid droplets. The secreted lipoprotein particles contained 68% unesterified cholesterol and 21% phospholipids, 8% esterified cholesterol, and 3% triglycerides. The phospholipid components were: 72% phosphatidylcholine, 22% sphingomyelin, and 6% phosphatidylserine, phosphatidylinositol, and phosphatidylethanolamine. We conclude that macrophages release cholesterol in two ways: 1) an HDL-mediated release of unesterified cholesterol increasing upon ACAT inhibition, and 2) an HDL-independent secretion of cholesterol which can be amplified by Ca++ antagonists.

Adenosine uptake and [3H]nitrobenzylthioinosine binding in developing rat brain

The ontogenesis of adenosine transport sites as labelled with [3H]nitrobenzylthioinosine ([3H]NBI) was examined using radioligand binding and membrane preparations from whole brain and 4 brain regions of rats between the postnatal ages of one day through to adulthood. In whole brain, cerebral cortex and cerebellum, [3H]NBI binding was two-fold higher in 6-day-old than in 50-day-old rats. In contrast, [3H]NBI binding was higher in adults than in one-day-old rats by 4-fold in hypothalamus and 8-fold in superior colliculus. In cortex and hypothalamus, the levels of [3H]NBI binding in newborn and adult rats were reflected by changes in Bmax and not Kd values. As a measure of the utility of [3H]NBI as a probe for identifying functional adenosine transport sites, we examined [3H]NBI binding to and [3H]adenosine accumulation by intact brain cells prepared from adult and newborn rats. For [3H]NBI binding to brain cells from adult rats, the values of Kd were 0.092 nM and of Bmax were 274 fmol/mg protein. For newborns, slightly higher Kd and Bmax values were observed; 0.2 nM and 395 fmol/mg protein, respectively. [3H]Adenosine accumulation was higher in brain cells from one-day-old than from adult rat brains. Kinetically this uptake was best described by a two-component model: the Vmax values for the high- and low-affinity uptake, and the Km value for the high-affinity component in one-day-old rats were greater than in adults.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of Ca2+-channel antagonists on nucleoside and nucleobase transport in human erythrocytes and cultured mammalian cells

Lidoflazine strongly inhibited the equilibrium exchange of uridine in human erythrocytes (Ki approximately 16 nM). Uridine zero-trans influx was similarly inhibited by lidoflazine in cultured HeLa cells (IC50 approximately to 80 nM), whereas P388 mouse leukemia and Novikoff rat hepatoma cells were three orders of magnitude more resistant (IC50 greater than 50 microM). Uridine transport was also inhibited by nifedipine, verapamil, diltiazem, prenylamine and trifluoperazine, but only at similarly high concentrations in both human erythrocytes and the cell lines. IC50 values ranged from about 10 microM for nifedipine and about 20 microM for verapamil to more than 100 microM for diltiazem, prenylamine and trifluoperazine. The concentrations required for inhibition of nucleoside transport are several orders higher than those blocking Ca2+ channels. Lidoflazine competitively inhibited the binding of nitrobenzylthioinosine to high-affinity sites in human erythrocytes, but did not inhibit the dissociation of nitrobenzylthioinosine from these sites on the transporter as is observed with dipyridamole and dilazep.

Nucleoside transport in guinea pig myocytes. Comparison of the affinities and transport velocities for adenosine and 2-chloroadenosine

The affinities of adenosine and 2-chloroadenosine for the nucleoside transport system of guinea pig myocytes were evaluated indirectly by studying the inhibition of the binding of [3H]nitrobenzylthioinosine and directly by measuring the influx of [3H]radiolabeled substrates. Maximal transport velocities of the two nucleosides were also obtained. [3H]Nitrobenzylthioinosine bound to a single class of high-affinity sites (KD of 0.8 nM) which possessed a maximal binding capacity (Bmax) of 870,000 sites/cell. Adenosine, 2-chloroadenosine or the nucleoside transport inhibitor, dipyridamole, competitively inhibited the site-specific binding of [3H]nitrobenzylthioinosine with Ki values of 318 microM, 22 microM and 75 nM respectively. Both [3H]adenosine and [3H]2-chloroadenosine entered myocytes in a saturable and inhibitible manner. Observed transport kinetic constants (Km and Vmax) were 146 microM and 24.2 pmoles/10(6) cells/sec, respectively, for adenosine and 36 microM and 11.7 pmoles/10(6) cells/sec, respectively for 2-chloroadenosine. Affinities of adenosine, 2-chloroadenosine, nitrobenzylthioinosine and dipyridamole for the nucleoside transport system derived from binding and influx methodologies were equivalent which confirms that [3H]nitrobenzylthioinosine binding sites are closely associated with the nucleoside transporter.

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.

Inhibition of nucleoside and nucleobase transport and nitrobenzylthioinosine binding by dilazep and hexobendine

The transport of 500 microM uridine by human erythrocytes and S49, P388 and L1210 mouse leukemia cells, Chinese hamster ovary (CHO) cells and Novikoff rat hepatoma cells was inhibited strongly by dilazep and hexobendine with similar concentration dependence, but the sensitivity of transport in the various cell types varied greatly; IC50 values ranged from 5-30 nM for human erythrocytes and S49 and P388 cells to greater than 1 microM for CHO and Novikoff cells. The binding of nitrobenzylthioinosine (NBTI) to high-affinity sites on these cells (Kd approximately equal to 0.5 nM) was inhibited by hexobendine and dilazep in a similar pattern. Furthermore, these drugs, just as dipyridamole and papaverine, inhibited the dissociation of NBTI from high-affinity binding sites but only at concentrations 10-100 times higher than those inhibiting uridine transport. In contrast, high uridine concentrations (greater than 2 mM) accelerated the dissociation of NBTI. Dilazep also inhibited the transport of hypoxanthine, but only in those cell lines whose transporter is sensitive to inhibition by uridine and dipyridamole. Adenine transport was not inhibited significantly by dilazep in any of the cell lines tested, except for a slight inhibition in Novikoff cells. [14C]Hexobendine equilibrated across the plasma membrane in human erythrocytes within 2 sec of incubation at 25 degrees, but accumulated to 6-10 times the extracellular concentration in cells of the various cultured lines. Uptake was not affected by high concentrations of uridine, NBTI or dipyridamole. Hexobendine inhibited the growth of various cell lines to a lesser extent (IC50 = greater than or equal to 100 microM) than dipyridamole (IC50 = 15-40 microM). At 40 microM, however, it completely inhibited the growth of S49 cells that had been made nucleoside dependent by treatment with methotrexate or pyrazofurin.

Nucleoside transport in rat erythrocytes: two components with differences in sensitivity to inhibition by nitrobenzylthioinosine and p-chloromercuriphenyl sulfonate

The sensitivity of nucleoside transport by rat erythrocytes to inhibition by nitrobenzylthioinosine (NBMPR) and the slowly permeating organomercurial, p-chloromercuriphenyl sulfonate (pCMBS), was investigated. The dose response curve for the inhibition of uridine transport (100 microM) by NBMPR was biphasic--35% of the transport activity was inhibited with an IC50 value of 0.25 nM, but 65% of the activity remained insensitive to concentrations as high as 1 microM. These two components of uridine transport are defined as NBMPR-sensitive and NBMPR-insensitive, respectively. Uridine influx by both components was saturable and conformed to simple Michaelis-Menten kinetics, and was inhibited by other nucleosides. The uridine affinity of the NBMPR-sensitive transport component was threefold higher than for the NBMPR-insensitive transport mechanism (apparent Km for uridine 50 +/- 18 and 163 +/- 28 microM, respectively). The two transport systems also differed in their sensitivity to pCMBS. NBMPR-insensitive uridine transport was inhibited by pCMBS with an IC50 of approximately 25 microM, while 1 mM pCMBS had little effect on NBMPR-sensitive transport by intact cells. pCMBS inhibition was reduced in the presence of uridine and adenosine and reversed by the addition by beta-mercaptoethanol, suggesting that the pCMBS-sensitive thiol group is located on the exterior surface of the erythrocyte membrane within the nucleoside binding site of the transport system. Inhibition of uridine transport by NBMPR was associated with high-affinity [3H]NBMPR binding to the cell membrane (apparent Kd 46 +/- 25 pM). Binding of inhibitor to these sites was competitively blocked by uridine and inhibited by adenosine, thymidine, dipyridamole, dilazep and nitrobenzylthioguanosine. Assuming that each NBMPR-sensitive transport site binds a single molecule of NBMPR, the calculated translocation capacity of each site is 25 +/- 6 molecules/site per sec at 22 degrees C. pCMBS had no effect on [3H]NBMPR binding to intact cells but markedly inhibited binding to disrupted membranes indicating that the NBMPR-sensitive nucleoside transporter probably has a thiol group located on the inner surface of the membrane. Exposure of rat erythrocyte membranes to UV light in the presence of [3H]NBMPR resulted in covalent radiolabeling of a membrane protein(s) (apparent Mr on SDS gel electropherograms of 62,000). Labeling of this protein was abolished in the presence of nitrobenzylthioguanosine. We conclude that nucleoside transport by rat erythrocytes occurs by two facilitated-diffusion systems which differ in their sensitivity to inhibition by both NBMPR and pCMBS.

Localization of [3H]cyclohexyladenosine and [3H]nitrobenzylthioinosine binding sites in rat striatum and superior colliculus

The localization of adenosine receptors labelled with [3H]cyclohexyladenosine ([3H]CHA) and adenosine transport sites labelled with [3H]nitrobenzylthioinosine ([3H]NBI) was examined in striatum and superior colliculus (SC) using radioligand binding and lesioning methods. Striatal kainic acid lesions significantly reduced the number (Bmax) of a single class of high affinity binding sites for [3H]CHA by 50% and that for [3H]NBI by 15% without altering Kd values for either ligand. In SC, enucleations significantly reduced both high and low affinity [3H]CHA binding sites by about 60% while levels of [3H]NBI binding were unaffected. Thus, adenosine receptors are present on striatal interneurons and retinal projections to the SC and some [3H]NBI binding sites are located on striatal interneurons.

Effects of adenosine uptake blockers and adenosine on evoked potentials of guinea-pig olfactory cortex

The olfactory cortex slice preparation from guinea-pig has been used to test compounds which inhibit the cellular uptake of adenosine. The uptake inhibitors dipyridamole (0.1-10 mumol/l), dilazep (1-10 mumol/l) nitrobenzylthioguanosine (1-10 mumol/l), nitrobenzylthioinosine (0.1-5 mumol/l), and hexobendine (1-100 mumol/l) increased the potency of adenosine (0.1-30 mumol/l) by up to 5-fold but did not potentiate cyclohexyladenosine (0.01-10 mumol/l). The benzodiazepine, diazepam (1 mumol/l) slightly increased the potency of adenosine (by 1.7-fold) whereas flurazepam (3 mumol/l) had no effect, suggesting that inhibition of adenosine uptake is probably not the major therapeutic action of these compounds. The uptake inhibitors depressed the amplitude of the monosynaptic epsp when added alone, an effect reversed by adenosine deaminase (1 unit/ml) whereas the adenosine deaminase inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine (10 mumol/l) had no effect on adenosine action. These results show that in this preparation (a) adenosine action is attenuated by an uptake mechanism and (b) endogenous adenosine release normally has no apparent effects on synaptic transmission at low stimulus rates. Nitrobenzylthioinosine and nitrobenzylthioguanosine are probably the best uptake blockers.

Localization of [3H]nitrobenzylthioinosine binding sites in rat spinal cord and primary afferent neurons

The distribution of [3H]nitrobenzylthioinosine ([3H]NBI) binding to nucleoside transport sites in rat spinal cord and spinal roots was examined using membrane binding and autoradiographic techniques. A single class of high affinity binding sites having dissociation constants (KD) between 0.42 +/- 0.05 and 0.088 +/- 0.012 nM was observed in dorsal and ventral spinal cord and their associated roots. The maximal number of binding sites (Bmax) in dorsal and ventral spinal cord was 110.1 +/- 7.1 and 73.6 +/- 7.5 fmol/mg protein, respectively. The highest levels of [3H]NBI binding were found in the dorsal grey matter of the cervical and lumbar enlargements. Autoradiographic studies showed that [3H]NBI sites were especially concentrated in the substantia gelatinosa of the dorsal spinal cord and the nucleus caudalis of the spinal trigeminal nucleus. The level of these binding sites in dorsal roots was nearly 4 times that observed in ventral roots; 98.5 and 23.0 fmol/mg protein, respectively. Adult animals depleted of unmyelinated sensory fibers by neonatal capsaicin treatment showed significantly reduced numbers of [3H]NBI sites (35%) in dorsal roots but not ventral roots, while KD values were unaffected. These results indicate that [3H]NBI sites are enriched in areas of the spinal cord and brainstem which subserve sensory functions and that these sites are located, in part, on unmyelinated primary afferent fibers.

Photoaffinity labelling of benzodiazepine receptors: lack of effect on ligand binding to the nucleoside transport system

This study was undertaken to investigate the possibility of an allosteric interaction between benzodiazepine receptors and the CNS nucleoside transport system. Irreversible (photoaffinity) labelling of the benzodiazepine receptors in guinea pig cortical membranes resulted in a marked reduction in the binding (Bmax) of both [3H]flunitrazepam (71%) and [3H]ethyl-beta-carboline-3-carboxylate (22%) to the benzodiazepine receptors but had no effect on the binding of [3H]nitrobenzylthioinosine to the nucleoside transport system. Furthermore, although photoaffinity labelling resulted in a significant decrease in the affinities of flunitrazepam (approximately equal to 16-fold) and dipyridamole (approximately equal to sevenfold) for the [3H]Ro 15-1788 binding site of the benzodiazepine receptor complex, the affinities of these compounds for the nucleoside transport system were unaltered. These results suggest that the CNS nucleoside transport system and the benzodiazepine receptor complex are distinct, noninteractive ligand recognition sites.

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

The binding of [3H]nitrobenzylthioinosine (NBMPR) to specific sites in CNS membranes was investigated using cortical tissue from a variety of mammalian species. Mass law analysis of the site-specific binding of NBMPR data revealed that rat, mouse, guinea pig, and dog cortical membranes each contained an apparent single class of high-affinity (KD 0.11-4.9 nM) binding sites for NBMPR; rabbit cortical membranes, however, exhibited two distinct classes of NBMPR binding sites with KD values of 0.4 nM and 13.8 nM. Dipyridamole, a potent inhibitor of nucleoside transport, produced a biphasic profile of inhibition of the binding of NBMPR to guinea pig, rabbit, and dog membranes (IC50 less than 20 nM and IC50 greater than 6 microM for NBMPR binding sites displaying high and low affinity for dipyridamole, respectively). These results are indicative of heterogeneity of NBMPR binding sites in mammalian cortical membranes. Rat and mouse cortical membranes appear to possess only one type of NBMPR binding site, which has low affinity for dipyridamole. Detailed analysis of inhibitor-induced dissociation of NBMPR from its sites in each species led to the conclusion that these multiple forms of NBMPR binding sites are different conformations of a single site associated with the CNS nucleoside transport system, rather than two distinct sites. It is also suggested that the affinity of dipyridamole for each conformation of NBMPR site indicates the susceptibility of that conformation of the nucleoside transport system to inhibition by dipyridamole.

Heterogeneous distribution of adenosine transport sites labelled by [3H]nitrobenzylthioinosine in rat brain: an autoradiographic and membrane binding study

A highly heterogeneous distribution of [3H]nitrobenzylthioinosine [( 3H]NBI) binding sites was observed using both autoradiographic and membrane binding methodology. Of the 24 brain regions examined in the radio-ligand binding studies, the highest levels of [3H]NBI sites were found in the thalamus, followed by midbrain, superior colliculus, olfactory cortex and hypothalamus. The thalamus contained over 5 times more sites than cerebellum which exhibited the lowest [3H]NBI binding levels. The results obtained from autoradiographic analysis agreed well with quantitative measurements and revealed that subnuclei of thalamus and hypothalamus as well as specific layers of the superior colliculus contained particularly high concentrations of [3H]NBI sites. When the [3H]NBI autoradiograms were compared with the distribution of adenosine deaminase in brain it was found that brain regions richest in neural elements immunoreactive for adenosine deaminase contained the greatest numbers of [3H]NBI sites. In contrast, a poor correlation was found between the distribution of [3H]NBI binding and adenosine receptors labelled with [3H]cyclohexyladenosine. The co-localization of [3H]NBI binding and adenosine deaminase in brain indicates the existence of neural systems having a high capacity to take up and metabolize adenosine.