Binding of the A1-selective adenosine antagonist 8-cyclopentyl-1,3-dipropylxanthine to rat brain membranes

Chronic changes in thyroid hormones do not affect brain adenosine receptors

1. In this study, the authors examined the effects of chronic (14 days) changes in thyroid function on a major neuromodulatory receptor system in the brain- the adenosinergic system. While previous investigators have examined the effects of alteration in thyroid function on adenosine receptors in peripheral tissues (adipocytes), this is the first study to examine such effects in brain. 2. Three groups of male Sprague-Dawley rats were treated for 14 days with either a) oral PTU (0.00625%), iodine-free diet, and i.p. saline injections, b) i.p. saline injections, or c) i.p. triiodothyronine (25 micrograms/100 g) injections. 3. These manipulations reliably resulted in the production of hypothyroidism (TSH 30.2 +/- 8.6 ng/ml), euthyroidism (TSH 2.1 +/- 0.9), and hyperthyroidism (TSH < 0.4). 4. Treatment had no significant effect on the Bmax or Kd of [3H]DPCPX (A1-antagonist) binding to homogenates from cerebral cortex, cerebellum or hippocampus; similarly, no effect on [3H]CGS-21680 (A2-agonist) binding to striatal homogenates was noted. 5. Similarly, quantitative autoradiographic studies failed to reveal consistent regional alterations unique to either hypo- or hyperthyroidism. 6. Incubation of sections with GppNHp resulted in the expected reduction (approximately 40%) in agonist binding, but there was no differential effect seen for either the hypo- or hyperthyroid tissues. 7. These preliminary findings suggest that alterations in brain adenosine receptors or G-protein-receptor coupling are unlikely to be requisite correlates of abnormal thyroid hormone levels.

Adenosine receptor-induced second messenger production in adult guinea-pig cerebellum

1. The effects of adenosine receptor agonists on cyclic nucleotides accumulation were investigated in adult guinea-pig cerebellar slices by use of radioactive precursors. 2. Adenosine elicited a rapid and maintained increase in cyclic AMP, that was fully reversed upon addition of adenosine deaminase. Adenosine analogues stimulated cyclic AMP generation up to 40 fold with the rank order of potency: 5'-N-ethylcarboxamidoadenosine (0.6 microM) > 2-chloroadenosine (6 microM) > adenosine (13 microM). CGS 21680 (10 microM) elicited only a small stimulation (1.2 fold). 3. The cyclic AMP response to NECA was reversed by the 1,3-dipropylxanthine-based adenosine receptor antagonists 8-[4-[[[[(2-aminoethyl)amino]amino]carbonyl]methyl]oxy]- phenyl]-1,3-dipropylxanthine (XAC), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and N-[2-(dimethylamino)ethyl]N-methyl-4-(1,3-dipropylxanthine)benzene sulphonamide (PD 115,199) with estimated apparent inhibition constants of 15, 81 and 117 nM, respectively. 4. Pretreatment with adenosine also potentiated the cyclic GMP response to sodium nitroprusside, abolishing the decline in [3H]-cyclic GMP observed with sodium nitroprusside alone, and allowing [3H]-cyclic GMP levels to be maintained for at least an additional 10 min. This potentiation was fully reversed by adenosine deaminase. 5. Adenosine analogues potentiated the sodium nitroprusside-elicited cyclic GMP generation with the rank order of potency: 5'-N-ethylcarboxamidoadenosine (0.7 microM) > 2-chloroadenosine (6 microM) > adenosine (42 microM). 6. NECA potentiation of cyclic GMP formation was reversed by the antagonists XAC, DPCPX and PD 115,199 with apparent inhibition constants of 17, 102 and 242 nM, respectively. 7. The similar potencies of adenosine analogues and xanthine antagonists for stimulation of cyclic AMP and potentiation of cyclic GMP lead to the suggestion that these phenomena are mediated through the same adenosine receptor, the A2b receptor. Furthermore, we suggest that potentiation of the sodium nitroprusside-induced cyclic GMP response may be mediated at the level of phosphodiesterase hydrolysis of the cyclic nucleotides.

Characterization of the adenosine receptor mediating contraction in rat colonic muscularis mucosae

1. The objective of this study was to characterize the adenosine receptor mediating contraction in rat isolated colonic muscularis mucosae (RCMM). 2. Sequential additions of the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA; 0.01-10 microM) elicited reproducible, concentration-related contractions in RCMM. The effects of NECA were mimicked by the adenosine A1 receptor-selective agonists cyclopentyladenosine (CPA), R-phenylisopropyladenosine (R-PIA) and N-[1S, trans)2-hydroxycyclopentyl] adenosine (GR79236) and by S-PIA (the stereoisomer of R-PIA). The adenosine A2 agonists N-[(2-methylphenyl)methyl] adenosine (metrifudil) and 2-[p-(2-carboxyethyl)phenethylamine]-5'-N-ethylcarboxamidoadenosine (CGS21680) also produced contractions in RCMM but were 54 and 165 times less potent respectively than NECA. The rank order of agonist potency for contraction of RCMM was CPA > or = GR79236 = R-PIA > or = NECA > > S-PIA = metrifudil > CGS21680, which is identical to that reported for the inhibition of spontaneous rate in rat isolated right atria and inhibition of lipolysis in rat isolated adipocytes by these same agonists. 3. R-PIA, S-PIA and metrifudil behaved as partial agonists in RCMM. 4. The adenosine A1 receptor-selective antagonist 8-cyclopentyl-1,3- dipropylxanthine (DPCPX) inhibited the contractions produced by all the adenosine agonists tested, with pKB values between 9.2 and 9.5. The non-selective adenosine antagonist 8-phenyltheophylline (8-PT) antagonized the effects of NECA but also markedly potentiated (by 93.0 +/- 10.2% at 3 microM) the maximum contractile response to NECA in RCMM. Neither 8-PT (3 microM) nor DPCPX (0.1 microM) had any effect on the contractions produced by carbachol. 5. The contractile responses to NECA in RCMM were not affected by atropine (1 microM), tetrodotoxin(0.3 microM) or the P2 antagonist, suramin (100 microM).6. The present study confirms that contractions to adenosine agonists in the RCMM are mediated via adenosine Al receptors.

Characterization of the adenosine receptors of the rat superior cervical ganglion

1. Adenosine analogues caused hyperpolarization and inhibition of the depolarizing response to muscarine of the rat isolated superior cervical ganglion (SCG) measured by a 'grease gap' recording technique. The receptors mediating these responses have been characterized by use of a range of selective adenosine analogues and adenosine receptor antagonists. 2. In decreasing order of potency N6-cyclopentyladenosine (CPA), 2-chloroadenosine (2CA), adenosine, 2-phenylaminoadenosine (PAA), caused concentration-dependent hyperpolarizations whilst N6-(9-fluorenylmethyl)adenosine (PD 117,413) was inactive at up to 100 microM. 3. The order of potency of adenosine analogues in depressing depolarization caused by a submaximal concentration of muscarine (100 nM) was: CPA > R-PIA = 2CA > NECA > S-PIA > BZA > adenosine > PAA, where R- and S-PIA = R(-)- and S(+)-N6-(2-phenylisopropyl)adenosine, NECA = 5'N-ethylcarboxamidoadenosine and BZA = N6-benzyladenosine. PD 117,413 was inactive at concentrations up to 100 microM. The maximum inhibitions of the muscarine-induced depolarization by CPA, 2CA, NECA and BZA were similar. R-PIA, S-PIA and PAA produced similar maximal inhibitions which were significantly smaller than those produced by CPA. 4. Hyperpolarizations caused by adenosine were antagonized by the P1-purinoceptor selective antagonist 1,3-dimethyl-8-phenylxanthine (8PT) and by the selective A1-adenosine receptor antagonist, 1,3-dipropyl-8-(4-((2-aminoethyl)amino)carbonylmethyloxyphenyl++ +)xanthine (XAC). Hyperpolarizations caused by CPA, adenosine and PAA were antagonized by the A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2-selective antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX). 5. Inhibition of the muscarinic-induced depolarization by CPA was antagonized by 8PT and DPCPXbut not by DMPX.6. It is concluded that the neurones of the rat SCG possess P1-purinoceptors of the Al-adenosine receptor subtype which mediate hyperpolarization and inhibition of depolarization caused by muscarine.

Effects of adenosine A1-agonist and -antagonist on urinary volume and Na excretion in IAP-treated and non-treated rats

Effects of an adenosine A1-receptor agonist and antagonist were determined in pertussis toxin (IAP)-treated and non-treated rats. (-)-N6-(2-phenylisopropyl) adenosine, an adenosine A1-agonist, reduced the urine volume and sodium excretion without decreasing the glomerular filtration rate at 0.1 mg/kg (p.o.) in both IAP-treated and non-treated rats. Diuretic effects of KW-3902 (8-(noradamantan-3-yl)-1,3-dipropylxanthine) and 8-cyclopentyl-1,3-dipropylxanthine, adenosine A1-receptor antagonists, were not affected by pretreatment with IAP. These results suggest that endogenous adenosine may induce antidiuretic effects by accelerating the reabsorption of water and sodium at tubular sites via an IAP-insensitive mechanism, and that the diuretic effects of the adenosine A1-receptor antagonist may result from inhibiting this action of endogenous adenosine.

Effect of xanthine derivatives on chemotactic polypeptide-induced superoxide and enzyme release from human polymorphonuclear leucocytes

1. We investigated the effects of new xanthine derivatives, 1-methyl-3-propyl xanthine (MPX) and 1,3-dipropyl xanthine (DPX), and several other xanthine derivatives on N-formyl-methionyl-leucyl-phenylalanine-induced superoxide and lysozyme release from human polymorphonuclear leucocytes (PMN). 2. MPX and DPX at low concentrations (10(-8) - 10(-9) mol/L) inhibited superoxide release from PMN by a maximum of 31.2 +/- 10.6% and 49.8 +/- 10.4% (mean +/- s.d.), respectively, and 10(-3) mol/L concentrations completely inhibited the release reactions (4.8 +/- 1.2 and 7.6 +/- 2.5% of control level). At 10(-5) mol/L, however, the inhibition did not occur (99.9 +/- 7.3 and 110.2 +/- 15.8% of control level). When PMN was pre-incubated with adenosine deaminase (ADA, 0.1 U/mL), superoxide release from PMN was inhibited in a dose-dependent manner by MPX and DPX and the interruption of the inhibition at 10(-5) mol/L was not observed. 3. Lysozyme release from PMN was inhibited by MPX at low concentrations (10(-7) - 10(-6) mol/L) and high concentrations (10(-3) mol/L). However 10(-4) mol/L of MPX facilitated the release (23.7 +/- 27.0%). When pretreated with ADA (0.1 U/mL), MPX suppressed lysozyme release in a dose-dependent manner and the facilitation of the release at 10(-4) mol/L was not observed. 4. When comparing effects of some other xanthine derivatives on superoxide release, the interruption of the inhibition of superoxide release at 10(-5) mol/L was commonly observed among xanthine derivatives with adenosine A2 antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)

Mediation of the neuroprotective action of R-phenylisopropyl-adenosine through a centrally located adenosine A1 receptor

1. Systemic injections of kainic acid, 10 mg kg-1, into adult rats resulted in lesions in the hippocampus, as assessed by peripheral benzodiazepine ligand binding. Co-administration of clonazepam at 1 mg kg-1 or 0.2 mg kg-1 prevented major seizures associated with kainate injections, but did not alter significantly the production of hippocampal damage. 2. The co-administration of the adenosine A1 agonist R-phenylisopropyladenosine (R-PIA, 25 micrograms kg-1, i.p.) abolished the lesions induced by kainic acid. 3. The presence of the selective A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (250 or 50 micrograms kg-1, i.p.) abolished the R-PIA neuroprotective action. 4. The A1/A2 antagonist, 8-(p-sulphophenyl)theophylline (20 mg kg-1, i.p.) which cannot cross the blood brain barrier, did not alter significantly the neuroprotective action of R-PIA, indicating that the neuroprotective action of the purine may be predominantly central. 5. The time course of the neuroprotection was also examined. R-PIA was effective when administered 2 h before or after kainate administration. 6. The results emphasise the potential utility of systemically active adenosine A1 receptor ligands in reducing CNS gliosis induced by the activation of excitatory amino acid receptors.

Adenosine receptors and their modulators

The identification and characterization of adenosine receptors and the development of potent, receptor subtype-selective agonists and antagonists has been an active area of research for the past 20 years. Major recent advances in the field have been the cloning of several adenosine receptor subtypes of different species, including the discovery of a new subtype, designated A3, the discovery and development of new agonists and antagonists, particularly those with selectivity for the A2a adenosine receptor, the characterization of signal transduction pathways, and the development of agents which act indirectly on the adenosine receptor system. The present article focusses on aspects of pharmaceutical/medicinal chemistry related to adenosine receptors.

The subcellular distribution of [3H]-CGS 21680 binding sites in the rat striatum: copurification with cholinergic nerve terminals

The subcellular distribution of the adenosine A2a receptor in rat striatum has been investigated using specific binding of the A2a-selective ligand [3H]-CGS 21680. After subcellular fractionation, the distribution of [3H]-CGS 21680 binding was similar to that of the cholinergic nerve terminal marker acetylcholinesterase rather than the more general membrane marker 5'-nucleotidase, with 42% of binding associated with the synaptosomal sub-fraction and 19% with a light membrane fraction. Binding of [3H]-CGS 21680 was also found to co-purify with the cholinergic nerve terminal marker choline acetyltransferase during immunoaffinity purification of striatal cholinergic nerve terminals. These results demonstrate that some adenosine A2a receptors are present on cholinergic nerve terminals in rat striatum.

Species comparison of adenosine and beta-adrenoceptors in mammalian atrial and ventricular myocardium

The antagonist radioligand 1,3-[3H]dipropyl-8-cyclopentylxanthine ([3H]DPCPX) was used to characterize adenosine A1 receptors in membrane preparations from atrial and ventricular myocardium of rat, rabbit, guinea pig and pig. Kd values in crude membranes from guinea pig atria and ventricles (3.3 and 3.0 nM) were higher than those in the other species (ranges, 1.5-1.8 and 1.5-1.9 nM). Bmax values were greater in atria than in ventricles in all four species, and in atria and ventricles of guinea pig (76 and 34 fmol/mg), than in the other species (ranges, 15-17 and undetectable to 12 fmol/mg). In contrast, guinea pig Kd and Bmax values for beta-adrenoceptors, which were labelled with (-)3-[125I]iodocyanopindolol, fell within the range of values for the other three mammalian species. In semipurified membrane preparations from pig, [3H]DPCPX and the agonist radioligand [125I]-N6-4-aminobenzyladenosine appeared to label a similar population of receptors and gave comparable Kd values in atria (0.73 and 0.66 nM) and in ventricles (0.57 and 0.70 nM). In semipurified preparations from pig, the agonist R-(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) displaced [3H]DPCPX in a manner consistent with the presence of both high- and low-affinity adenosine A1 receptors. The data from this study indicate that the density of adenosine A1 receptors in atria is greater than in ventricles, but similar Kd values suggest that the A1 receptor population is the same in the two cardiac tissues. Also, the data demonstrate that the [3H]DPCPX antagonist binding characteristics of guinea pig myocardium differ from those in rat, rabbit and pig.

On the high affinity binding site for [3H]-1,3-dipropyl-8-cyclopentylxanthine in frog brain membranes

1. Radioligand binding properties of the adenosine receptor ligands, [3H]-1,3-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), and [3H]-R-phenylisopropyladenosine ([3H]-R-PIA) were investigated in frog brain membranes. 2. The specific binding of the adenosine antagonist, [3H]-DPCPX to frog brain membranes showed one binding site with Kd and Bmax values of 43.8 nM and 0.238 +/- 0.016 pmol mg-1 protein, respectively. Guanosine 5'-triphosphate (GTP, 100 microM) decreased to 72 +/- 7% and Mg2+ (8 mM) increased to 121 +/- 3% [3H]-DPCPX (40 nM) binding to frog brain membranes. 3. [3H]-DPCPX saturation binding experiments performed in the presence of Mg2+ (8 mM), or in the presence of GTP showed that Mg2+ ions decreased the Kd value of [3H]-DPCPX to 14 nM, and GTP increased this value to 65.6 nM. Bmax values were not significantly (P > 0.05) modified (0.261 +/- 0.018 pmol mg-1 protein, with Mg2+, and 0.266 +/- 0.026 pmol mg-1 protein, in presence of GTP) by the presence of Mg2+ or GTP. 4. The specific binding of [3H]-R-PIA (15 nM) was decreased to 37 +/- 6% by GTP (100 microM) and increased to 123 +/- 4% by Mg2+ (8 mM). [3H]-R-PIA saturation binding experiments performed in the presence of Mg2+ (8 mM) showed one binding site with Kd and Bmax values of 0.9 nM and 0.229 +/- 0.008 pmol mg-1 of protein, respectively. 5. The concentration-inhibition curves of adenosine agonists and antagonists versus [3H]-DPCPX binding showed the following order of potencies: CPA> R-PIA~ NECA> S-PIA> > CGS 21680, for the agonists, and XAC ~-DPCPX> > XCC> PACPX, for the antagonists.6. The present results suggest that the adenosine binding site in the frog brain membranes is G-protein coupled, but that the antagonist affinities and the pharmacological profile is different from the Al or A2 adenosine receptors.

Development of an antiserum to rat-brain A1 adenosine receptor: application for immunological and structural comparison of A1 adenosine receptors from various tissues and species

An antiserum was developed in a rabbit against rat-brain A1 adenosine receptor. This antiserum recognized the denatured form of the purified rat-brain A1 adenosine receptor in immunoblot analysis and the native form of the receptor in the immunoprecipitation analysis. Immunoblot analysis of unpurified or purified adenosine receptor preparations from rat-brain membranes revealed a major immunoreactive band at a position of molecular mass of approx. 35 kDa, which corresponds to the position of purified rat-brain A1 adenosine receptor. Although A1 adenosine receptors from other rat tissues such as testis and adipocyte were also found to be immunoreactive with this antiserum by immunoblot analysis, purified human-brain A1 adenosine receptors showed a poor reactivity with this antibody. The order of the relative immunoreactivity of these A1 adenosine receptors with the antiserum was found to be brain > adipocyte > or = testis. Moreover, the immunoreactivity of these receptors significantly increased after these receptor preparations were deglycosylated by endoglycosidase F. After the deglycosylation, no significant differences in both the immunoreactivity and molecular mass among these receptor preparations were found on the immunoblot. These results suggest that the differences in the molecular mass or immunoreactivity among the A1 adenosine receptor preparations from three rat tissues were mainly due to the difference of sugar moiety present in each receptor molecule. These data are the first to provide analyses of immunological characteristics of A1 adenosine receptors from different tissues and species.

Presynaptic inhibition by adenosine A1 receptors on guinea pig small intestinal myenteric neurons

BACKGROUND

Adenosine acts at A1 receptors to inhibit the release of most neurotransmitters. This study tested the hypothesis that both exogenous adenosine (ADO) and tonic release of endogenous ADO act at presynaptic A1 receptors to suppress excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) in myenteric neurons.

METHODS

Intracellular microelectrodes were used to study actions of ADO, the agonists 2-chloro-N6-cyclopentyl ADO, its 1-deaza derivative, 5'-N-ethylcarboxamido ADO, and CGS 21680 or the antagonists 8-cyclopentyl-1,3-dimethylxanthine, its 1,3-dipropyl analog, and 1,3-dipropyl-8-p-sulfophenylxanthine on synaptic behavior in myenteric neurons.

RESULTS

Each of the agonists suppressed slow EPSPs in all 35 AH/type 2, 8 of 10 S/type 1, and 7 of 7 nonspiking neurons. ADO also decreased neuronal excitability (n = 63) in AH/type 2 neurons. Agonists suppressed fast nicotinic EPSPs in all 20 S/type 1, 10 nonspiking, and 3 AH/type 2 neurons without having any effect on postsynaptic responses to nicotinic agonists. CCPA was more potent than CGS 21680 in suppressing EPSPs. In 30% of neurons, the only action of antagonists was to block the effect of A1 or A2 agonists on EPSPs. Agonists did not inhibit IPSPs, but unmasked robust slow IPSPs by preventing slow EPSPs. Antagonists acted alone to enhance EPSPs in 70% of neurons.

CONCLUSIONS

(1) ADO acts at presynaptic A1 sites to suppress EPSPs in all neurons, (2) IPSPs are revealed by ADO, and (3) ongoing release of endogenous ADO inhibits synaptic transmission.

Adenosine receptor-blocking xanthines as inhibitors of phosphodiesterase isozymes

The pharmacological actions of methylxanthines such as theophylline and caffeine may be due to blockade of adenosine receptors and/or inhibition of phosphodiesterase (PDE) activities. In the last years, potent xanthines have been developed that display some selectivity for A1 and A2 adenosine receptors. Little is known about the PDE inhibitory potency of these xanthines. The aim of the present study was to determine the potencies of A1 and A2 receptor selective xanthines as inhibitors of several PDE isozymes, the PDE I-V subtypes. The IC50 values of 8-phenyl- and 8-cycloalkyl-1,3-dialkylxanthines for inhibition of PDE isozymes from different sources are up to 10,000-fold higher than their antagonistic potencies at adenosine receptors. However, the A1 receptor selective antagonists 1,3-diethyl-8-phenylxanthine and 1,3-dipropyl-8-cyclopentylxanthine are comparatively potent inhibitors of PDE IV activity with IC50 values in the 10 microM range and are, therefore, nearly as potent as the PDE IV selective inhibitor, rolipram. The A2 receptor selective 1,3-dipropyl-7-methylxanthine is about 10-300-fold more potent as an adenosine receptor antagonist than as a PDE inhibitor. The results indicate that some of these novel xanthines can be used as selective adenosine receptor antagonists without interference due to inhibitory effects on PDEs.

Reduced density of adenosine A1 receptors and preserved coupling of adenosine A1 receptors to G proteins in Alzheimer hippocampus: a quantitative autoradiographic study

Binding to adenosine A1 receptors and the status of their coupling to G proteins were studied in the hippocampus and parahippocampal gyrus of Alzheimer individuals and age-matched controls. The binding to A1 receptors was compared with binding to the N-methyl-D-aspartate receptor complex channel-associated sites (labeled with (+)-[3H]5-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine maleate). In vitro quantitative autoradiography demonstrated a similar anatomical distribution of A1 receptors labeled either with an agonist ((-)-[3H]phenylisopropyladenosine) or antagonist ([3H]8-cyclopentyl-1,3-dipropylxanthine) in the brains of elderly controls. In Alzheimer patients, significant decreases in the density of both agonist and antagonist binding sites were found in the molecular layer of the dentate gyrus. Decreased A1 agonist binding was also observed in the CA1 stratum oriens and outer layers of the parahippocampal gyrus, while reduced antagonist binding was found in the subiculum and CA3 region. Reduced density of the N-methyl-D-aspartate receptor channel sites was found in the CA1 region and parahippocampal gyrus. The reductions in binding to adenosine A1 and N-methyl-D-aspartate receptors were due to a decrease in the density of binding sites (Bmax), and not changes in receptor affinity (KD). In both elderly control and Alzheimer subjects, GTP substantially reduced the density of A1 agonist binding sites with a concomitant increase in the KD values, whereas antagonist binding was unaffected by GTP. The results suggest that adenosine A1 receptor agonists and antagonists recognize overlapping populations of binding sites. Reduced density of A1 receptors in the molecular layer of the dentate gyrus most probably reflects damage of the perforant path input in Alzheimer's disease, while altered binding in the CA1 and CA3 regions is probably due to loss of intrinsic neurons. Similar effects of GTP on binding to A1 receptors in control and Alzheimer individuals suggest lack of alterations in coupling of A1 receptors to G proteins in Alzheimer's disease, thus supporting the notion of normal receptor coupling to their effector systems in Alzheimer's disease.

Adenosine A1-receptor stimulated increases in intracellular calcium in the smooth muscle cell line, DDT1MF-2

1. The effect of a range of adenosine receptor agonists on intracellular free calcium concentration ([Ca2+]i) has been studied in the hamster vas deferens smooth muscle cell line DDT1MF-2. 2. Adenosine receptor agonists elicited a rapid and maintained increase in [Ca2+]i in fura-2 loaded DDT1MF-2 cells. The initial rise could be maintained in the absence of extracellular calcium, whereas the maintained or plateau phase was dependent upon the presence of extracellular calcium and appeared to be associated with calcium influx. The rank order of agonist potencies was N6-cyclopentyladenosine > 5'-N-ethylcarboxamidoadenosine > 2-chloroadenosine > adenosine. 3. The response to 2-chloroadenosine was antagonized by the antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, KD 0.14 nM) and 8-phenyltheophylline (KD 112 nM). 4. Pretreatment with the 5-lipoxygenase inhibitor AA861 (20 microM) produced only a small (14 +/- 2%) inhibition of the [Ca2+]i response elicted by N6-cyclopentyladenosine (300 nM), in nominally Ca(2+)-free buffer containing 0.1 mM EGTA. The cyclo-oxygenase inhibitor, indomethacin (2 microM) was without effect. 5. The Ca(2+)-influx associated with the plateau phase required the continued presence of agonist on the receptor. The antagonist DPCPX (100 nM) attenuated the rise in [Ca2+]i observed when extracellular Ca2+ was re-applied after the cells had been stimulated with N6-cyclopentyladenosine (CPA; 300 nM) in experiments initiated in nominally Ca(2+)-free buffer. 6. Pretreatment with pertussis toxin (200 ng ml-1 for 4 h) inhibited the CPA (100 nM) stimulated intracellular Ca2+ release and Ca2+ influx but was without effect on the response to histamine (100 microM). 7.These data suggest that adenosine A(1)-receptor activation in DDT(1)MF-2 cells stimulates release of Ca(2+) from intracellular stores and influx of extracellular Ca(2+) through Ca(2+) entry pathways in the plasma membrane which required the continued presence of agonist on the receptor.

Novel synaptic potentials in cerebellar Purkinje cells: probable mediation by metabotropic glutamate receptors

Glutamate receptors of both the ionotropic (ion channel-linked) and metabotropic (enzyme-linked) categories are abundantly expressed by Purkinje cells in the cerebellum but the functional significance of the latter receptors is unknown. We have tested the possibility that they are activated by the parallel fibre input by recording from Purkinje cells within a biplanar cerebellar slice preparation using the grease-gap technique. Under conditions where ionotropic (NMDA and non-NMDA) glutamate and GABA receptors were blocked pharmacologically, electrical stimulation of parallel fibres gave rise to two very slow potentials. The first peaked about 400 msec from the start of stimulation and was depolarising. It was not evident with single stimuli but reached maximum amplitude after 6 shocks delivered at 50 Hz. The wave was abolished when the slices were perfused with Ca(2+)-free solution or with drugs that inhibit synaptic transmission, but it was resistant to blockade of GABAB receptors, acetylcholine receptors and adrenergic receptors. Next came a slow hyperpolarising potential that peaked about 30 sec after stimulation and which was also Ca(2+)-dependent. The sequence of potentials was replicated by perfusion of an exogenous agonist acting selectively on metabotropic glutamate receptors. We conclude that parallel fibre-to-Purkinje cell synaptic transmission involves not only fast signals generated through ionotropic non-NMDA receptors but also much slower potentials that are likely to be mediated by metabotropic glutamate receptors. These potentials are likely to be significant both for shorter-term (seconds to minutes) Purkinje cell excitability as well as for the induction of longer-term synaptic plasticity.

Potential use of DPCPX as probe for in vivo localization of brain A1 adenosine receptors

The suitability of (3H)DPCPX (8-cyclopentyl-1,3-dipropylxanthine), a xanthine derivative, as an vivo probe for labelling adenosine A1 receptors was studied in rats. [3H]DPCPX (nM) penetrated largely into the brain (0.8% of the injected dose per gram of brain tissue 5 min after injection). Brain concentrations stayed at a plateau level from 5 to 15 min after the injection. The distribution in the different brain regions was heterogeneous with the highest amount of [3H]DPCPX in cerebellum and hippocampus and the lowest concentrations in hypothalamus and brain stem. Displacement (45-70% of total radioactivity) was obtained by the injection of 250 nM of cold DPCPX or cyclopentylxanthine, an analog of DPCPX. The ex vivo autoradiographic distribution of [3H]DPCPX was similar to the in vitro autoradiographic distribution of tritiated A1 adenosine receptor ligand as [3H]CHA. These results suggest the potential use of DPCPX for further in vivo investigation of A1 adenosine receptors with techniques such as positron emission tomography.

Molecular cloning and characterisation of a human brain A1 adenosine receptor cDNA

Using the sequence conservation in the G protein-coupled receptor superfamily, we have isolated an adenosine A1 receptor cDNA from a human hippocampal cDNA library by homology screening. When expressed in mammalian CHO.K1 cells, the protein encoded by this cDNA binds the A1-specific antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) with high affinity (Kd = 0.56 +/- 0.11 nM) and, functionally, is able to inhibit cAMP production upon receptor activation with the A1-specific agonist N6-cyclopentyladenosine (CPA) (> 80% inhibition at 10(-7) M CPA). The binding and functional characteristics of the expressed cDNA demonstrate that we have isolated a human brain adenosine receptor cDNA of the A1 subtype.

Stable adenine nucleotides inhibit [3H]-noradrenaline release in rabbit brain cortex slices by direct action at presynaptic adenosine A1-receptors

Effects of adenosine and nucleotides on the release of previously stored [3H]-noradrenaline were studied in rabbit brain cortex slices. The slices were stimulated twice, in most experiments by 6 electrical field pulses delivered at 100 Hz. Adenosine and the nucleotides AMP, ADP, ATP, AMPS, ADP beta S, ATP gamma S, beta,gamma-imido-ATP and beta,gamma-methylene-ATP all reduced the evoked overflow of tritiated compounds. For purines for which concentration-response curves were determined, the order of potency was adenosine greater than ATP approximately ATP gamma S approximately beta,gamma-imido-ATP approximately ADP greater than beta,gamma-methylene-ATP. AMP 30 mumol/l and AMPS 30 mumol/l were approximately equieffective with 30 mumol/l of adenosine and ATP gamma S, and ADP beta S 30 mumol/l was approximately equieffective with 30 mumol/l of ADP. alpha,beta-Methylene-ADP, 2-methylthio-ATP, UTP and GTP gamma S did not change the evoked overflow of tritium. alpha,beta-Methylene-ATP caused an increase; however, the increase was small and became significant only after 59 min of exposure to alpha,beta-methylene-ATP or when the slices were stimulated by 30 pulses, 10 Hz. Neither adenosine deaminase (100 U/l) nor the blocker of 5'-nucleotidase, alpha,beta-methylene-ADP (10 mumol/l), attenuated the inhibition caused by ATP, ATP gamma S and beta,gamma-methylene-ATP, despite the fact that adenosine deaminase abolished the effect of adenosine. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX, 10 nmol/l) shifted the concentration-response curves of adenosine, ATP gamma S, beta,gamma-imido-ATP and beta,gamma-methylene-ATP to the right by very similar degrees. 8-(p-Sulphophenyl)-theophylline (30 and 300 mumol/l) also markedly antagonized the inhibition produced by ATP gamma S. alpha,beta-Methylene-ATP (10 and 30 mumol/l) and suramin (100 mumol/l) did not modify the effects of adenosine, ATP gamma S and beta,gamma-methylene-ATP. It is concluded that nucleotides themselves can inhibit the release of noradrenaline in the rabbit brain cortex. The nucleotides and adenosine seem to act at the same site, i.e., the A1 subtype of the P1-purinoceptor. The results support the notion that metabolically stable, phosphate chain-modified nucleotides such as ATP gamma S, beta,gamma-imido-ATP and beta,gamma-methylene-ATP can be potent P1 agonists. No evidence was found for presynaptic P2x-, P2y- or P3-purinoceptors.

Adenosine receptors, cyclic AMP accumulation, and DNA-synthesis in aortic smooth muscle cell cultures of adult and neonatal rats

The effects of two adenosine analogs on cyclic AMP (cAMP) accumulation and DNA synthesis were studied in cultured smooth muscle cells (SMCs) isolated from adult and neonatal rat arteries. N-ethylcarboxamido adenosine (NECA) dose-dependently increased intracellular cAMP levels and appeared to be more potent in adult than in neonatal SMCs. R-phenylisopropyl adenosine (R-PIA), in nanomolar concentrations, counteracted the increase in cAMP evoked by 10 microM forskolin in adult but not in neonatal SMCs, indicating that the enhanced "A2" response seen in adult SMCs was not due to a lack of "A1" receptors in these cultures. Binding experiments performed using the adenosine antagonist XAC did not reveal any differences in the number or affinity of the adenosine receptors between neonatal and adult SMCs. This indicates effects presumably on the G-protein level. A high capacity to spontaneously synthesize DNA and a weak response to platelet-derived growth factor (PDGF) were seen in the neonatal SMCs. Furthermore, NECA had no effect on PDGF-induced DNA synthesis in these cells. In contrast, adult SMCs presented a low rate of spontaneous DNA synthesis and a marked proliferative response to PDGF, which was inhibited by NECA. This inhibition paralleled the increase in cAMP elicited by NECA. Our findings suggest that neonatal and adult SMCs differ both in their response to growth factors and growth inhibitors.

Bronchodilatory activity and pharmacokinetics of new xanthines in guinea-pigs

1. The in vitro biological activities and the effect of protein binding on the relaxant effects in vivo of N-3-alkylxanthine and N-3-alkyl-N-1-methylxanthine derivative were investigated in guinea-pigs. 2. A significantly positive correlation was observed among the in vitro muscle relaxant activity, the cyclic adenosine monophosphate (cAMP) phosphodiesterase (PDE) inhibitory activity and the protein-binding potency of xanthine derivatives. However, there was a weak relationship between these activities and affinity for adenosine receptors. 3. When theophylline, enprofylline and 1-methyl-3-propylxanthine (MPX) were injected intravenously in guinea-pigs, their ED50 values were 6.1, 3.3 and 1.0 mg/kg, respectively. Plasma concentrations of these drugs obtained following the intravenous injection of the ED50 approximated the theoretically effective concentration (EC50) predicted from both the relaxant effects in vitro and the protein binding parameters. A good linear correlation was observed between bodyweight in four species (rats, guinea-pigs, rabbits and humans) and certain pharmacokinetic parameters of enprofylline and theophylline. 4. The present study indicates that differences in the relaxant effects of these drugs in vitro and in vivo can be explained in part by protein binding, and that the protein binding of these xanthine bronchodilators is an important determinant for their pharmacological activity. Guinea-pigs provide a useful model for studying pharmacodynamic-pharmacokinetic relationships of new bronchodilators.

Biochemical and immunological characterization of A1 adenosine receptors purified from human brain membranes

The A1 adenosine receptor was purified approximately 13,000-fold to apparent homogeneity from human cerebral cortex membranes using a novel affinity-chromatography system developed for the purification of rat brain and rat testis A1 adenosine receptors [Nakata, H. (1989) J. Biol. Chem. 264, 16,545-16,551; Nakata, H. (1990) J. Biol. Chem. 265, 671-677]. The purified human brain receptor showed the ligand-binding specificity expected of the A1 adenosine receptor. The Bmax and Kd for the purified receptor with a specific A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-[3H]dipropylxanthine, were approximately 16 nmol/mg protein and 2 nM, respectively. SDS/PAGE of the purified receptor preparation showed one broad protein band of molecular mass of approximately 35 kDa, which is very similar to that of purified A1 adenosine receptor from rat brain membranes. Endoglycosidase F treatment of the purified receptor reduced the molecular mass to approximately 30 kDa, suggesting that the human brain A1 adenosine receptor is a glycoprotein. Comparison of the purified human and rat brain A1 adenosine receptors by peptide mapping after the proteolytic digestion showed minor differences between these receptors. Immunological comparisons of the human brain A1 adenosine receptor with rat brain A1 adenosine receptor using polyclonal antibodies against the purified rat brain A1 adenosine receptor showed that the antibodies react preferentially with the rat brain receptor and weakly with human brain receptor.

Excitatory Transmitter Amino Acid Release from the Ischemic Rat Cerebral Cortex: Effects of Adenosine Receptor Agonists and Antagonists

The effects of selective adenosine receptor agonists [N6-cyclopentyladenosine (CPA) and N-ethylcarboxamidoadenosine (NECA)] and antagonists [8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c]quinazoline-5-im ine (CGS-15943A)] on aspartate and glutamate release from the ischemic rat cerebral cortex were studied with the cortical cup technique. Cerebral ischemia (for 20 min) was elicited by four-vessel occlusion. Excitatory amino acid releases were compared from control ischemic rats and drug-treated rats. Basal levels of aspartate and glutamate release were not greatly affected by pretreatment with the adenosine receptor agonists or antagonists. However, CPA (10(-10) M) and NECA (10(-9) M) significantly inhibited the ischemia-evoked release of aspartate and glutamate into cortical superfusates. The ability to block ischemia-evoked release of excitatory amino acids was not evident at higher concentrations of CPA (10(-6) M) or NECA (10(-5) M). The selective A1 receptor antagonist DPCPX also had no effect on release when administered at a low dosage (0.01 mg/kg, i.p.) but blocked the ischemia-evoked release of aspartate and glutamate at a higher dosage (0.1 mg/kg). Evoked release was inhibited by the selective A2 receptor antagonist CGS-15943A (0.1 mg/kg, i.p.). Thus, adenosine and its analogs may suppress ischemia-evoked release of excitatory neurotransmitter amino acids via high-affinity A1 receptors, whereas coactivation of lower-affinity A2 receptors may block (or reverse) the A1-mediated response.

Adenosine A1-receptor stimulation of inositol phospholipid hydrolysis and calcium mobilisation in DDT1 MF-2 cells

1. The effect of adenosine receptor-stimulation on inositol phospholipid hydrolysis and calcium mobilization has been investigated in the hamster vas deferens smooth muscle cell line DDT1 MF-2. 2. Adenosine receptor stimulation increased the accumulation of total [3H]-inositol phosphates in DDT1 MF-2 cells prelabelled with [3H]-myo-inositol. The rank order of agonist potencies was N6-cyclopentyladenosine greater than 5'-N-ethylcarboxamidoadenosine greater than 2-chloroadenosine greater than adenosine. 3. The response to 2-chloroadenosine was antagonized by the antagonists 8-cyclopentyl-1,3-dipropylxanthine (KD 1.2 nM), PD 115,199 (KD 39 nM) and 8-phenyltheophylline (KD 31 nM). 4. The inositol phosphate response to 2-chloradenosine (10 microM) was not significantly altered when the extracellular Ca2+ ion concentration was reduced from 2.4 mM to 1.2 mM or 0.6 mM. Under calcium-free conditions, however, a reduced but still significant response to 2-chloroadenosine was evident (39 +/- 10% of the response in calcium-containing medium). 5. The 5-lipoxygenase inhibitor AA861 (10 and 100 microM) inhibited the inositol phosphate response to 2-chloroadenosine by 40 +/- 9% and 60 +/- 4% respectively. The cyclo-oxygenase inhibitor, indomethacin, however, was without significant effect at 1 microM. 6. 2-Chloroadenosine stimulated an increase in intracellular free Ca2+ ion concentration in fura-2 loaded DDT1 MF-2 cells in calcium-free medium containing 0.1 mM EGTA, which could be inhibited by the adenosine A1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM). 7. These data suggest that adenosine A1-receptor stimulation results in inositol phospholipid hydrolysis and calcium mobilization from intracellular stores in DDT1 MF-2 cells.

Evidence that adenosine receptors in the dog left atrium are not of the typical A1 or A2 adenosine receptor subtypes

The adenosine receptors from the isolated dog left atrium were characterized using the non-selective agonists 5'-N-ethyl-carboxamidoadenosine (NECA) and adenosine, the A1-selective agonist N6-R-phenylisopropyladenosine (R-PIA), and the A2 adenosine receptor agonist C2-naphthylethoxyadenosine (NEA). The potency order of the agonists in the dog left atrium was NECA greater than adenosine greater than R-PIA = NEA. This potency order was the same as that found in the guinea pig aorta (A2) but different from that in the guinea pig left atrium (A1). In the guinea pig left atrium the potency order was NECA greater than R-PIA greater than adenosine much greater than NEA. The negative inotropic responses to NECA in the dog left atrium were antagonised by the non-selective antagonist 8-phenyltheophylline (8-PT) and the A1-selective antagonists 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) and N6-endonorbornan-2-yl-9-methyladenine (N-0861), giving pKB values of 6.3, 7.3 and 5.1, respectively. These values are significantly different from those estimates determined in either the guinea pig left atrium or guinea pig aorta. The potency order of the agonists and the relatively low potencies of the A1-selective antagonists suggests that the adenosine receptors in the dog left atrium are not of the classical A1 adenosine receptor subclass and may instead be more closely related to the A2 adenosine receptor.

Penetration of adenosine antagonists into mouse brain as determined by ex vivo binding

The penetration of the adenosine antagonists 8-cyclopentyl-1,3-dimethylxanthine (CPT), 8-cyclopentyl-1,3-dipropylxanthine (CPX), 8-(p-sulfophenyltheophylline (8-PST), and 8-[4-[[[[(2-amino-ethyl)amino]carbonyl]methyl]oxy]phenyl]- 1,3-dipropylxanthine (XAC) into mouse brain was determined using ex vivo binding and locomotor studies. CPT and CPX (25 and 0.25 mg/kg, respectively) both penetrated into brain in substantial amounts: 49 and 17% of theoretical levels assuming free penetration throughout the body, 10 min after i.p. injection, respectively. Brain levels of CPT decreased rapidly, declining to undetectable levels by 30 min post-injection, whereas levels of CPX declined much more slowly. As expected, no detectable brain levels of 8-PST were found following i.p. injection of 50 mg/kg. XAC (20 mg/kg) penetrated into brain poorly: 1.6% after 10 min and 3.2% 20 min post-injection. The ability of CPT to stimulate locomotor activity paralleled the brain levels, i.e. it was similar to theophylline at short times and the effect rapidly diminished. These studies demonstrate the usefulness of ex vivo binding in determining CNS penetration of adenosine receptor ligands.

Adenosine receptors in rat basophilic leukaemia cells: transductional mechanisms and effects on 5-hydroxytryptamine release

1. The presence of adenosine receptors linked to adenylate cyclase activity and their functional role in calcium-evoked 5-hydroxytryptamine (5-HT) release was investigated in rat basophilic leukaemia (RBL) cells, a widely used model for studying the molecular mechanisms responsible for stimulus-secretion coupling. 2. In [3H]-5-HT-loaded cells triggered to release by the calcium ionophore A23187, a biphasic modulation of 5-HT secretion was induced by adenosine analogues, with inhibition of stimulated release at nM and potentiation at microM concentrations, suggesting the presence of adenosine receptor subtypes mediating opposite effects on calcium-dependent release. This was also confirmed by results obtained with other agents interfering with adenosine pharmacology, such as adenosine deaminase and the non-selective A1/A2 antagonist 8-phenyl-theophylline. 3. Similar biphasic dose-response curves were obtained with a variety of adenosine analogues on basal adenylate cyclase activity in RBL cells, with inhibition and stimulation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) production at nM and microM concentrations, respectively. The rank order of potency of adenosine analogues for inhibition and stimulation of adenylate cyclase activity and the involvement of G-proteins in modulation of cyclic AMP levels suggested the presence of cyclase-linked A1 high-affinity and A2-like low-affinity adenosine receptor subtypes. However, the atypical antagonism profile displayed by adenosine receptor xanthine antagonists on cyclase stimulation suggested that the A2-like receptor expressed by RBL cells might represent a novel cyclase-coupled A2 receptor subtype.4. Micromolar concentrations of adenosine analogues could also increase inositol phospholipid hydrolysis and inositol tris-phosphate formation in both unstimulated cells and in cells triggered to release by the calcium ionophore. The stimulation was constant, small and additive to that exerted by the calcium ionophore.5. It is concluded that RBL cells express both A1 and A2-like adenosine receptors which exert opposite effects on 5-HT release and intracellular cyclic AMP levels. However, besides modulation of cyclic AMP levels, additional transduction pathways, such as modulation of phospholipase C activity, may contribute to the release response evoked by adenosine analogues in this cell-line.

Effects of some organophosphorus compounds on the binding of a radioligand (8-cyclopentyl 1, 3-[3H]dipropylxanthine) to adenosine receptors in ovine cardiac membranes

8-Cyclopentyl-1,3-[3H] dipropylxanthine [( 3H]CPX) is a potent radioligand that specifically binds to the A1 adenosine receptors. Its high specificity makes it a suitable ligand for the characterization of A1 adenosine receptors in tissues with low receptor densities. We have demonstrated that the organophosphorus compounds soman, tabun and sarin, at relatively high concentrations, all bind to the A1 adenosine receptors in ovine cardiac membranes with Ki values of 36.7, 328 and 175 microM, respectively. The binding of soman to the receptor site was found to be totally reversible. We suggested that these organophosphorus compounds affect the mechanical responses of the heart through interaction with a potassium channel that does not seem to be closely linked to the A1 adenosine receptors.

Apparent heterogeneity of cardiac A1 adenosine receptors as revealed by radioligand binding experiments on N-ethylmaleimide-treated membranes

While G protein-coupled receptors are often studied by analyzing antagonist radioligand: "cold" agonist inhibition curves using an independent site model, it is now clear that KL and KH values determined in these analyses are not reliable estimates of the affinities of the agonists for "free" and G protein-coupled forms of the receptor. Thus, such experiments cannot be used to contrast the characteristics of a given type of receptor in different tissues, i.e., to probe for the existence of receptor subtypes. Since treatment with N-ethylmaleimide treatment blocks receptor: Gi/Go protein interactions, such analyses on N-ethylmaleimide-pretreated membranes should allow direct assessment of the affinities of competing ligands for the free receptor or for multiple receptor subtypes. As A1 adenosine receptors couple to Gi, and perhaps to Go, we have performed A1 adenosine receptor radioligand "competition" studies first on control, then on N-ethylmaleimide-pretreated bovine cardiac and cerebral cortical membranes. Results of experiments with the antagonist radioligand [3H]xanthine amine congener appeared to be confounded by ligand binding to A2 adenosine receptors present in the cardiac membrane preparations. Further experiments utilized the A1-specific radioligand [3H]1,3-dipropyl-8-cyclopentylxanthine. These experiments confirmed once more that the KL values determined by computer analysis of "competition" curves performed on control membranes are not reliable estimates of the affinities of the competing ligand for free receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine receptors and nucleoside transport sites in cardiac cells

1. Potential mechanisms responsible for the prominent depression of atrioventricular conduction by adenosine have been investigated in guinea-pig heart. 2. Adenosine A1 receptors and nucleoside transport (NT) sites were identified and enumerated in cardiac myocytes, atrioventricular conduction cells and coronary endothelial cells in 10 microns sections by autoradiographical analysis of the binding of the A1 selective antagonist 8-cyclopentyl-1,3-[3H]-dipropylxanthine ([3H]-DPCPX) and the NT ligand [3H]-nitrobenzylthioinosine ([3H]-NBMPR), respectively. 3. Atrioventricular conduction cells were identified by acetylcholinesterase histochemistry and endothelial cells by von Willebrand factor immunohistochemistry. 4. Site-specific binding of [3H]-DPCPX, when expressed as grains per cell nucleus was significantly higher (30 fold) in conduction cells than in surrounding myocytes. [3H]-DPCPX site density on endothelial cells in adjacent coronary vessels was not significantly different from myocytes. 5. In contrast, autoradiography of [3H]-NBMPR sites in these areas indicated that, relative to myocytes, conduction cells and endothelial cells were significantly enriched (2 fold and 4.5 fold, respectively) in NT sites. 6. The pronounced dromotropic effect of adenosine in guinea-pig heart is correlated with a higher density of adenosine A1 receptors in atrioventricular conduction cells than in myocytes. The NT capacity of these cells, as estimated by [3H]-NBMPR binding site density, is not increased in proportion to A1 receptors.

Solubilized rat brain adenosine receptors have two high-affinity binding sites for 1,3-dipropyl-8-cyclopentylxanthine

The specific binding of L-N6-[3H]phenylisopropyladenosine (L-[3H]PIA) to solubilized receptors from rat brain membranes was studied. The interaction of these receptors with relatively low concentrations of L-[3H]PIA (0.5-12.0 nM) in the presence of Mg2+ showed the existence of two binding sites for this agonist, with respective dissociation constant (KD) values of 0.24 and 3.56 nM and respective receptor number (Bmax) values of 0.28 +/- 0.03 and 0.66 +/- 0.05 pmol/mg of protein. In the presence of GTP, the binding of L-[3H]PIA also showed two sites with KD values of 24.7 and 811.5 nM and Bmax values of 0.27 +/- 0.09 and 0.93 +/- 0.28 pmol/mg of protein for the first and the second binding site, respectively. Inhibition of specific L-[3H]PIA binding by 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (0.1-300 nM) performed with the same preparations revealed two DPCPX binding sites with Ki values of 0.29 and 13.5 nM, respectively. [3H]DPCPX saturation binding experiments also showed two binding sites with respective KD values of 0.81 and 10.7 nM and respective Bmax values of 0.19 +/- 0.02 and 0.74 +/- 0.06 pmol/mg of protein. The results suggest that solubilized membranes from rat brain possess two adenosine receptor subtypes: one of high affinity with characteristics of the A1 subtype and another with lower affinity with characteristics of the A3 subtype of adenosine receptor.

Characterization of adenosine A1-receptor binding sites in bovine retinal membranes

Using retinas prepared from freshly dissected bovine eyes, we have characterized the binding of the A1-selective agonist, [3H]PIA (N6-R-[3H](2-phenylisopropyl)adenosine). Specific binding was linear over a range of membrane protein concentrations from 0.10 to 1.0 mg, and accounted for an average of 80-90% of the total binding. At room temperature (24 degrees C), binding reached equilibrium at 60 min, and was reversible upon addition of an excess of cold ligand. Saturation analysis and Scatchard transformation revealed two apparent populations of receptor binding sites. The higher affinity site exhibited a Kd of 0.134 +/- 0.007 nM and Bmax of 26.18 +/- 3.06 fmol-1 mg protein. The lower affinity site exhibited a Kd of 21.83 +/- 4.39 nM and Bmax of 53.94 +/- 15.80 fmol mg-1 protein. Kinetic analysis of association and dissociation rates, performed at a low concentration of [3H]PIA, yielded a calculated affinity constant for the high affinity site of 0.2 nM, in agreement with saturation studies. Competition experiments with a number of purine nucleoside agonists and antagonists were performed, using radioligand concentrations of 1 nM or less to examine binding at the high affinity site, and revealed a rank order of potency consistent with the reported pharmacology of A1 receptors. We have also assayed for adenylate cyclase activity in this same preparation and determined that PIA inhibited forskolin-activated adenylate cyclase in a dose-dependent manner. Maximum inhibition (40%) was observed with 1 nM PIA, while 10 microM 8-cyclopentyl-1,3-dipropylxanthine, an A1 selective antagonist, completely inhibited this modulation by PIA.

Adenosine: a natural modulator of L-type calcium channels in atrial myocardium?

The mechanism of action of adenosine at the level of atrial myocardium has been a matter of debate. Electrophysiological studies showed that adenosine increases K+ efflux which may reduce Ca2+ influx, indirectly, by shortening the myocardial action potential. Recently some authors proposed that adenosine also depresses Ca2+ influx by a direct action on the L calcium channel, but, this effect being lower than that on voltage-dependent K+ channels, it was considered of minor importance. The effect of adenosine and its stable analogues was studied in the presence of the dihydropyridine Bay K 8644, a highly specific L-type calcium channel agonist, on isolated guinea-pig atria. The inotropic effect of the calcium channel activator was found to be antagonized by adenosine A1-receptor agonists. Binding studies showed that the effect on Bay K 8644 was not due to the interaction between adenosine analogues and dihydropyridines at the level of a common receptor site on L-type Ca2+ channels. Inhibitors of K+ channels did not antagonize the effect of adenosine analogues against Bay K 8644. Experimental conditions aimed to unmask an effect on slow Ca2+ currents (i.e. K+ depolarized paced atria), further supported that adenosine analogues may act in atria as negative modulators on L-type Ca2+ channels. Finally, the use of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a highly specific A1-receptor antagonist, demonstrated that the antagonism of Bay K 8644 by adenosine analogues is strictly dependent on A1 receptors. The above data support the possibility of a dual signal transduction pathway to ion channels (K+ and Ca2+) linked to A1 receptors in atrial myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine A1 and non-A1 receptors: intracellular analysis of the actions of adenosine agonists and antagonists in rat hippocampal neurons

Adenosine and its agonists exert a depressant effect on neuronal activity by interacting with the adenosine A1 receptor. There is now also evidence for electrophysiological effects mediated by adenosine receptors other than the A1 type, possibly A2 receptors. A1 and A2 receptor-induced changes in the electrical properties of neuronal membranes were investigated by intracellularly recording from rat hippocampal CA1 neurons and using the adenosine agonists, 5'-N-ethylcarboxamidoadenosine (NECA) and R-phenylisopropyladenosine (PIA), and the unselective A1 and A2 receptor antagonist, theophylline and the selective A1 receptor antagonist, 8-cyclopenthyl-1,3-dipropylxanthine (DPCPX). PIA and NECA produced an inhibitory effect which was blocked by DPCPX and thus was mediated by A1 receptors. PIA produced inhibition at lower concentrations (0.1-1 mumol/l) than NECA (0.5-10 mumol/l), whereas at high concentrations (2.5 mumol/l) it exerted a dual effect, i.e. either an inhibitory or an excitatory one. During simultaneous perfusion with the A1 receptor antagonist DPCPX, PIA produced concentration-dependent excitatory effects at concentrations above 1 mumol/l. These excitatory effects were blocked by theophylline. DPCPX produced excitation that was enhanced by NECA. Forskolin caused no change in the membrane properties. It is concluded that (1) NECA and PIA affect the membrane properties not only by an action on the A1 but also on non-A1 receptors, because the excitatory effects of PIA and NECA were insensitive to DPCPX and abolished by theophylline; (2) PIA and NECA are more potent at A1 than at A2 receptors; (3) PIA is more potent than NECA at A1 and A2 receptors; (4) effects mediated by A2 receptors prevail over those mediated by A1 receptors when A2 receptors are activated; and (5) the non-A1 receptor-mediated effects are independent of an increased formation of cAMP.

Reduction of adenosine A1-receptors in the perforant pathway terminal zone in Alzheimer hippocampus

The cells of origin of the perforant pathway are destroyed in Alzheimer's disease (AD). In rat the adenosine A1-receptors are specifically localized on the perforant path terminals in the molecular layer of the dentate gyrus. In the present study the density of A1-receptors in the hippocampus of Alzheimer's disease (AD) patients (n = 9) and non-dement controls (n = 3) has been investigated autoradiographically with [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]CPDPX) as the ligand probe. In AD hippocampi binding of [3H]CPDPX was greatly reduced in the outer two thirds of the dentate gyrus molecular layer, likely due to the degeneration of the perforant path. Binding of [3H]CPDPX was not significantly altered in other parts of the AD hippocampus, e.g. the CA1 and the CA3, in spite of a pronounced cellular pathology and reduced N-methyl-D-aspartate (NMDA) receptor densities, assessed as strychnine insensitive [3H]glycine autoradiography. This contrasts with the presumed localization on dendrites of pyramidal neurons of A1 receptors within the CA1 and the CA3.

Radioligand binding to adenosine receptors and adenosine uptake sites in different brain regions of normal and narcoleptic dogs

The present study compares the characteristics of radioligand binding to adenosine receptors and adenosine uptake sites in 100- and 50-day-old normal and narcoleptic dogs. Binding to A1 receptors was quantified using a selective A1 agonist ([3H]N6-[(R)-1-methyl-2-phenylethyl] adenosine, [3H]R-PIA) and an antagonist ([3H]dipropyl-8-cyclopentyl-xanthine, [3H]CPX). Differences in the binding of [3H]R-PIA and that of [3H]5'-ethylcarboxamide adenosine ([3H]NECA), which binds to both A1 and A2 receptors with similar affinities, were used to quantify A2 receptors. Nucleoside transport sites were labeled with [3H]nitrobenzylthioinosine ([3H]NBTI), a potent inhibitor of nucleoside transport systems. The present study offered no evidence that either adenosine A1 receptors and adenosine uptake sites in the frontal cortex or adenosine A2 receptors in the putamen were altered in narcoleptic dogs. However, we found that adenosine A1 receptors in the dog exist in different affinity states and that the affinity state in which the receptor is found depends on the brain region examined. A characterization of these low- and high-affinity sites was performed and results indicated that these sites cannot be explained by a single interaction of the A1 receptor with a single G-protein population.

Adenosine receptor subtypes in the brainstem mediate distinct cardiovascular response patterns

A limited occipital craniotomy was conducted on urethane-chloralose anesthetized, spontaneously breathing rats to expose the caudal medulla in the region of the obex. Microinjections of highly selective agonists for adenosine receptor subtypes were made into the medial region of the caudal nucleus tractus solitarius (NTS) at the level of the posterior portion of the area postrema. Cardiorespiratory parameters were subsequently recorded for a 60-min test period following microinjection of drug or vehicle solutions. The following selective adenosine receptor agonists were used: the A1 agonist, N6-cyclopentyladenosine (CPA), which is 480-fold selective for A1 receptors in rat brain binding assays, and the A2 agonist, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680), which is 170-fold selective for A2 receptors in rat brain binding studies and over 1500-fold selective in functional assays. The results showed that distinct and converse cardiovascular response patterns were elicited by these selective agonists for adenosine receptor subtypes following microinjections into the caudal NTS. Specifically, CGS 21680 selectively elicited potent dose-related decreases in mean arterial blood pressure (ED50 = 0.064 nmol/kg) and dose-related decreases in pulse pressure (ED50= 0.058 nmol/kg). Conversely, CPA selectively elicited potent dose-related increases in mean arterial blood pressure (ED50 = 0.62 nmol/kg) and dose-related increases in pulse pressure (ED50 = 0.70 nmol/kg). Additionally, the overall agonist-mediated response patterns were dramatically different wherein the CGS agonist exhibited a considerably more rapid time course in eliciting its hypotensive responses whereas CPA exhibited a more delayed and substantially longer time course to exert its hypertensive responses. Additionally, these distinct and converse cardiovascular response patterns were further shown to be receptor-selective since the depressor responses elicited by the A2 receptor agonist, CGS 21680, and the pressor responses elicited by the A1 receptor agonist, CPA, were completely and selectively blocked, respectively, by the selective A2 receptor antagonist, CGS 15943A, and the selective A1 receptor antagonist, DPCPX. Taken together, these findings provide persuasive in vivo evidence showing that pharmacologic activation of adenosine receptor subtypes in the caudal NTS of rats elicits specific response patterns with selective and opposite actions on cardiorespiratory behavior. These data also indicate that separate physiologic responses are specifically mediated by A2 receptors in the intact nervous system and thereby lend additional support to the case for using in vivo models to assess the functional role of adenosine A2 receptors in brain function.

The inhibitory adenosine receptor at the neuromuscular junction and hippocampus of the rat: antagonism by 1,3,8-substituted xanthines

1. The ability of 1,3,8-substituted xanthines to antagonize the inhibitory effects of adenosine receptor agonist on the amplitude of nerve-evoked twitches of the rat phrenic-diaphragm and on the amplitude of orthodromically-evoked population spikes, recorded from the CA1 pyramidal cells of rat hippocampal slices, was investigated. 2. 1,3-Dipropyl-8-cyclopenthylxanthine (DPCPX), 1,3-dipropyl-8-(carboxymethyloxyphenyl)xanthine (XCC), 1,3-dipropyl-8-(4-[2-aminoethyl)amino)carbonylmethyloxyphenyl)x ant hine (XAC), 1,3-dipropyl-8-(2-amino-4-chlorophenyl)xanthine (PACPX), 8-phenyltheophylline (8-PT), 1,3-diethyl-8-phenylxanthine (DPX) and PD 115,199, in concentrations virtually devoid of effect on neuromuscular transmission, shifted to the right in a near parallel manner the log concentration-response curve for the inhibitory effect of 2-chloroadenosine (CADO) on nerve-evoked twitches of the phrenic-diaphragm. Linear Schild plots with slopes near to unity were obtained for all the xanthines. 3. The order of potency of the xanthines as antagonists of the effect of CADO in the phrenic-diaphragm was DPCPX (Ki = 0.54 nM) greater than XCC (Ki = 10 nM), XAC (Ki = 11 nM), PACPX (Ki = 13 nM) greater than DPX (Ki = 22 nM), 8-PT (Ki = 25 nM) greater than PD 115,199 (Ki = 57 nM). The potency of DPCPX in antagonizing the inhibitory effects of R-N6-phenylisopropyladenosine (R-PIA) and 5'-N-ethylcarboxamide adenosine (NECA) on nerve-evoked twitch response was not statistically different from its potency in antagonizing the inhibitory effect on CADO. 4. In the hippocampal slices, DPCPX, XCC and XAC, used in concentrations virtually devoid of effect on population spike amplitude, shifted to the right in a near parallel manner the log concentrationresponse curve for the inhibitory effect of CADO on the amplitude of the population spikes. The Schild plots were linear with slopes near unity. 5. The potencies of DPCPX (K, = 0.45 nM) and XAC (K, = 11 nM) in antagonizing the inhibitory adenosine receptor at the hippocampus were similar to their potencies for antagonism of the inhibitory adenosine receptor at the phrenic-diaphragm. XCC was only slightly more potent (K, = 5.4 nM) as an antagonist of the adenosine receptor in the hippocampus than in the phrenic-diaphragm. 6. The results suggest that the inhibitory adenosine receptors in the phrenic-diaphragm and in the hippocampus of the rat are similar, and that according to the antagonist potencies, these receptors belong to the A1-adenosine receptor subtype.

The anti-adrenergic effect of adenosine and its blockade by pertussis toxin: a comparative study in myocytes isolated from guinea-pig, rat and failing human hearts

1. In intact ventricular preparation, adenosine has been shown to reduce the beta-adrenoceptor-induced increase in contraction (the anti-adrenergic effect). In the present study we have investigated this effect of adenosine on isolated ventricular myocytes from failing human heart and normal guinea-pig and rat heart. 2. Adenosine in the absence of beta-adrenoceptor-mediated stimulation had no effect on contraction in human and guinea-pig myocytes but produced a variable effect in rat myocytes. 3. 8-Cyclopentyl 1,3-dipropylxanthine (CPX), a selective A1-receptor antagonist, antagonised the anti-adrenergic effect of adenosine in guinea-pig myocytes. 4. The anti-adrenergic effect of adenosine was greater in guinea-pig than rat myocytes and even more pronounced in cells isolated from failing human heart. 5. Pertussis toxin-pretreatment at 35 degrees C of guinea-pig and human myocytes abolished the anti-adrenergic effect of adenosine. Longer exposure to higher concentrations of pertussis toxin was required for complete abolition in human compared to guinea-pig cells. 6. These results support the suggestion that the adenosine receptors mediating the anti-adrenergic effect of adenosine are of the A1 subtype and are coupled to the inhibitory guanine nucleotide binding protein, Gi/Go. 7. Pertussis toxin pretreatment increased the sensitivity of guinea-pig myocytes to isoprenaline in the absence of adenosine; the EC50 value was decreased by a factor of 10. This suggests that Gi may exert a tonic inhibitory effect on the beta-adrenoceptor/adenylate cyclase interaction in normal myocardium.

Differential desensitization of A1 adenosine receptor-mediated inhibition of cardiac myocyte contractility and adenylate cyclase activity. Relation to the regulation of receptor affinity and density

Effects of chronic exposure of cultured atrial myocytes to R-N6-(2-phenylisopropyl)-adenosine (R-PIA) on the A1 adenosine receptor-mediated inhibition of adenylate cyclase activity and myocyte contractility were examined. Chronic exposure of atrial myocytes cultured from 14-day-old chick embryos to R-PIA desensitized the myocyte to the inhibitory effects of R-PIA on contractility and adenylate cyclase activity in a time- and dose-dependent manner. Desensitization of the negative inotropic response was only partial, whereas the adenosine receptor-mediated inhibition of adenylate cyclase activity was almost completely absent after 24 hours of R-PIA (1 microM) exposure. Furthermore, the contractile response to R-PIA desensitized more slowly than the desensitization of A1 adenosine receptor-mediated inhibition of adenylate cyclase (t1/2 = 11.4 +/- 0.7 hours versus 7.5 +/- 1 hours, mean +/- SEM, n = 12 and 6, respectively). Thus, the two A1 adenosine receptor-linked functional responses desensitized differently in response to chronic exposure of the myocyte to R-PIA. Binding of the antagonist radioligand [3H]-8-cyclopentyl-1,3-dipropylxanthine [( 3H]CPX) in membranes from myocytes preexposed to R-PIA demonstrated a time-dependent decrease in receptor density without any change in the affinity for the antagonist radioligand. Computer analyses of agonist competition with [3H]CPX binding in membranes from control and R-PIA-treated myocytes revealed a conversion of the high-affinity A1 adenosine receptor to a low-affinity form such that after 24 hours of 1 microM R-PIA exposure, all of the receptors were in a low-affinity form.(ABSTRACT TRUNCATED AT 250 WORDS)

Both A1 and A2a purine receptors regulate striatal acetylcholine release

The receptors responsible for the adenosine-mediated control of acetylcholine release from immunoaffinity-purified rat striatal cholinergic nerve terminals have been characterized. The relative affinities of three analogues for the inhibitory receptor were (R)-phenylisopropyladenosine greater than cyclohexyladenosine greater than N-ethylcarboxamidoadenosine (NECA), with binding being dependent of the presence of Mg2+ and inhibited by 5'-guanylylimidodiphosphate [Gpp(NH)p] and adenosine receptor antagonists. Adenosine A1 receptor agonists inhibited forskolin-stimulated cholinergic adenylate cyclase activity, with an IC50 of 0.5 nM for (R)-phenylisopropyladenosine and 500 nM for (S)-phenylisopropyladenosine. A1 agonists inhibited acetylcholine release at concentrations approximately 10% of those required to inhibit the cholinergic adenylate cyclase. High concentrations (1 microM) of adenosine A1 agonists were less effective in inhibiting both adenylate cyclase and acetylcholine release, due to the presence of a lower affinity stimulatory A2 receptor. Blockade of the A1 receptor with 8-cyclopentyl-1,3-dipropylxanthine revealed a half-maximal stimulation by NECA of the adenylate cyclase at 10 nM, and of acetylcholine release at approximately 100 nM. NECA-stimulated adenylate cyclase activity copurified with choline acetyltransferase in the preparation of the cholinergic nerve terminals, suggesting that the striatal A2 receptor is localized to cholinergic neurones. The possible role of feedback inhibitory and stimulatory receptors on cholinergic nerve terminals is discussed.