[3H]xanthine amine congener of 1,3-dipropyl-8-phenylxanthine: an antagonist radioligand for adenosine receptors

Fluorescent A2A and A3 adenosine receptor antagonists as flow cytometry probes

Adenosine receptor (AR) ligands are being developed for metabolic, cardiovascular, neurological, and inflammatory diseases and cancer. The ease of drug discovery is contingent on the availability of pharmacological tools. Fluorescent antagonist ligands for the human A2A and A3ARs were synthesized using two validated pharmacophores, 1,3-dipropyl-8-phenylxanthine and triazolo[1,5-c]quinazolin-5-yl)amine, which were coupled to eight reporter fluorophores: AlexaFluor, JaneliaFluor (JF), cyanine, and near infrared (NIR) dyes. The conjugates were first screened using radioligand binding in HEK293 cells expressing one of the three AR subtypes. The highest affinities at A2AAR were Ki 144-316 nM for 10, 12, and 19, and at A3AR affinity of Ki 21.6 nM for 19. Specific binding of JF646 conjugate MRS7774 12 to the HEK293 cell surface A2AAR was imaged using confocal microscopy. Compound 19 MRS7535, a triazolo[1,5-c]quinazolin-5-yl)amine containing a Sulfo-Cy7 NIR dye, was suitable for A3AR characterization in whole cells by flow cytometry (Kd 11.8 nM), and its bitopic interaction mode with an A3AR homology model was predicted. Given its affinity and selectivity (11-fold vs. A2AAR, ~ 50-fold vs. A1AR and A2BAR) and a good specific-to-nonspecific binding ratio, 19 could be useful for live cell or potentially a diagnostic in vivo NIR imaging tool and/or therapy targeting the A3AR.

A2A Adenosine Receptor Antagonists in Neurodegenerative Diseases

BACKGROUND

Alzheimer's disease (AD) is the most common form of dementia worldwide, with approximately 6 million cases reported in America in 2020. The clinical signs of AD include cognitive dysfunction, apathy, anxiety and neuropsychiatric signs, and pathogenetic mechanisms that involve amyloid peptide-β extracellular accumulation and tau hyperphosphorylation. Unfortunately, current drugs to treat AD can provide only symptomatic relief but are not disease-modifying molecules able to revert AD progression. The endogenous modulator adenosine, through A_2A receptor activation, plays a role in synaptic loss and neuroinflammation, which are crucial for cognitive impairment and memory damage.

OBJECTIVE

In this review, recent advances covering A_2A adenosine receptor antagonists will be extensively reviewed, providing a basis for the rational design of future A_2A inhibitors.

METHODS

Herein, the literature on A_2A adenosine receptors and their role in synaptic plasticity and neuroinflammation, as well as the effects of A_2A antagonism in animal models of AD and in humans, are reviewed. Furthermore, current chemical and structure-based strategies are presented.

RESULTS

Caffeine, the most widely consumed natural product stimulant and an A_2A antagonist, improves human memory. Similarly, synthetic A_2A receptor antagonists, as described in this review, may provide a means to fight AD.

CONCLUSION

This review highlights the clinical potential of A_2A adenosine receptor antagonists as a novel approach to treat patients with AD.

The Medicinal Chemistry of Caffeine

The purine alkaloid caffeine is the most widely consumed psychostimulant drug in the world and has multiple beneficial pharmacological activities, for example, in neurodegenerative diseases. However, despite being an extensively studied bioactive natural product, the mechanistic understanding of caffeine's pharmacological effects is incomplete. While several molecular targets of caffeine such as adenosine receptors and phosphodiesterases have been known for decades and inspired numerous medicinal chemistry programs, new protein interactions of the xanthine are continuously discovered providing potentially improved pharmacological understanding and a molecular basis for future medicinal chemistry. In this Perspective, we gather knowledge on the confirmed protein interactions, structure activity relationship, and chemical biology of caffeine on well-known and upcoming targets. The diversity of caffeine's molecular activities on receptors and enzymes, many of which are abundant in the CNS, indicates a complex interplay of several mechanisms contributing to neuroprotective effects and highlights new targets as attractive subjects for drug discovery.

Programmable nanoscaffolds that control ligand display to a G-protein-coupled receptor in membranes to allow dissection of multivalent effects

A programmable ligand display system can be used to dissect the multivalent effects of ligand binding to a membrane receptor. An antagonist of the A_2A adenosine receptor, a G-protein-coupled receptor that is a drug target for neurodegenerative conditions, was displayed in 35 different multivalent configurations, and binding to A_2A was determined. A theoretical model based on statistical mechanics was developed to interpret the binding data, suggesting the importance of receptor dimers. Using this model, extended multivalent arrangements of ligands were constructed with progressive improvements in binding to A_2A. The results highlight the ability to use a highly controllable multivalent approach to determine optimal ligand valency and spacing that can be subsequently optimized for binding to a membrane receptor. Models explaining the multivalent binding data are also presented.

[(3)H]XAC (xanthine amine congener) is a radioligand for A(2)-adenosine receptors in rabbit striatum

The intrinsic affinity of 8-phenylxanthine analogs at striatal A(2)-adenosine receptors is highly species dependent. [(3)H]XAC (8-[2-aminoethyl[amino[carbonyl[methyl[oxyphenyl]]]]]-1,3-dipropylxanthine), although A(1)-selective in the rat brain, binds to A(2) receptors in rabbit striatal membranes with sufficiently high affinity to serve as a radioligand. In the presence of 50 nM CPX (8-cyclopentyl-1,3-dipropylxanthine), an A(1)-selective antagonist added to eliminate binding to A(1) receptors, [(3)H]XAC exhibits saturable, specific binding (70% of total) to A(2) sites with a K(d) of 3.8 nM and a B(max) of 1.23 pmol/mg protein. At 24 degrees C, the association and dissociation rate constants were 0.13 min(?1) nM(?1) and 0.36 min(?1), respectively. Binding was performed for 1 h, with non-specific binding defined in the presence of 100 ?M NECA (N-ethylcarboxamidoadenosine). The potency order for antagonists against 1 nM [(3)H]XAC at rabbit A(2)-receptors was XAC ? N(?)-Me-XAC ? CPX = XCC > 1,3-dipropyl-8-p-sulfophenylxanthine > PSPT. The relative potency order for agonists was CGS ? NECA > APEC [= 2-(aminoethylaminocarbonyl-ethylphenylethylamino)-NECA] > PAPA-APEC > ADAC > R-PIA (N(6)-phenylisopropyladenosine) > S-PIA. The ability to characterize central A(2)-adenosine receptors using an antagonist ligand that is chemically functionalized offers the possibility to design affinity labeling probes for this receptor subtype in the brain, similar to those antagonist probes already developed for A(1)-receptors. The results also suggest that affinity columns containing chemically immobilized XAC may be used for isolating central A(2)-adenosine receptors from rabbit striatum.

Activation of G-protein-coupled receptors in cell-derived plasma membranes supported on porous beads

G-protein-coupled receptors (GPCRs) are ubiquitous mediators of signal transduction across cell membranes and constitute a very important class of therapeutic targets. In order to study the complex biochemical signaling network coupling to the intracellular side of GPCRs, it is necessary to engineer and control the downstream signaling components, which is difficult to realize in living cells. We have developed a bioanalytical platform enabling the study of GPCRs in their native membrane transferred inside-out from live cells to lectin-coated beads, with both membrane sides of the receptor being accessible for molecular interactions. Using heterologously expressed adenosine A(2A) receptor carrying a yellow fluorescent protein, we showed that the tethered membranes comprised fully functional receptors in terms of ligand and G protein binding. The interactions between the different signaling partners during the formation and subsequent dissociation of the ternary signaling complex on single beads could be observed in real time using multicolor fluorescence microscopy. This approach of tethering inside-out native membranes accessible from both sides is straightforward and readily applied to other transmembrane proteins. It represents a generic platform suitable for ensemble as well as single-molecule measurements to investigate signaling processes at plasma membranes.

T-cell specific defect in expression of the NTPDase CD39 as a biomarker for lupus

Regulatory T cells (T(regs)) are critical for maintenance of peripheral tolerance via suppression of T-cell responses, and absence of T(regs) results in autoimmunity. The role of aberrations in the T(reg) pool for the development of systemic lupus erythematosus (SLE, lupus) remains uncertain. T(reg)-mediated generation of adenosine, dependent on the ectonucleotidase CD39, is an important mechanism for suppression of T-cell responses. We tested whether decreases in numbers of T(regs), and specifically CD39-expressing T(regs), are associated with human lupus. We studied 15 SLE patients, six patients with rheumatoid arthritis (RA) and 24 healthy controls. T(reg) phenotypic markers, including CD39 expression, were studied by flow cytometry. Varying numbers of sorted T(regs) cells were co-cultured with responder T (T(resp)) cells, with proliferation assessed by (3)H-thymidine incorporation. The proportion of T(regs) as defined by Foxp3(+) CD25(+high) CD127(-/low) was similar in lupus and control populations. CD39-expressing T(regs) comprised 37±13% of the T(reg) population in healthy controls and 36±21% in lupus subjects using nonsteroidal immunosuppressants to control active disease, but was nearly absent in five of six lupus subjects with minimally active disease. In contrast to healthy controls and lupus subjects without the CD39 defect, in SLE subjects with the CD39 defect, adenosine-dependent T(reg)-mediated suppression was nearly absent. These results suggest that functional defects in T(regs), rather than reduced T(reg) numbers, are important for the loss of peripheral tolerance in lupus. Presentation of this defect may serve as a biomarker for untreated disease.

Biophysical mapping of the adenosine A2A receptor

A new approach to generating information on ligand receptor interactions within the binding pocket of G protein-coupled receptors has been developed, called Biophysical Mapping (BPM). Starting from a stabilized receptor (StaR), minimally engineered for thermostability, additional single mutations are then added at positions that could be involved in small molecule interactions. The StaR and a panel of binding site mutants are captured onto Biacore chips to enable characterization of the binding of small molecule ligands using surface plasmon resonance (SPR) measurement. A matrix of binding data for a set of ligands versus each active site mutation is then generated, providing specific affinity and kinetic information (K(D), k(on), and k(off)) of receptor-ligand interactions. This data set, in combination with molecular modeling and docking, is used to map the small molecule binding site for each class of compounds. Taken together, the many constraints provided by these data identify key protein-ligand interactions and allow the shape of the site to be refined to produce a high quality three-dimensional picture of ligand binding, thereby facilitating structure based drug design. Results of biophysical mapping of the adenosine A(2A) receptor are presented.

Selective Bronchodilators from 1-(5′-Oxohexyl)xanthines

A series of twenty one 1-(5′-oxohexyl)xanthines substituted with alkyl chains at the N 3 and N 7 positions of the xanthine nucleus were prepared and their relaxant activity in guinea-pig isolated tracheal muscle and positive chronotropic activity in isolated right atrium of guinea-pig were compared. The tracheal relaxant activities were markedly increased with alkyl chain length at the N 3 position, but decreased by the N 7 alkylation. The positive chronotropic activities in the right atrium were increased by introduction of an n-propyl group at the N 3 position but decreased by substitution of longer alkyl chains, and the action on the heart was diminished by N 7 substitution. The activities of compounds on cAMP-phosphodiesterase (PDE) and binding of [3H]8-cyclopentyl-1,3-dipropylxanthine were measured in the homogenate of tracheal muscle and the membrane preparation of cerebral cortex, respectively. No relationship among tracheal muscle relaxant activity, cAMP-PDE inhibitory activity and adenosine antagonism of these xanthines was observed, and other action mechanisms should be considered for their relaxant activities. This study indicated that N 3 alkylation is important for the selectivity for tracheal muscle, while the introduction of long alkyl chains such as n-butyl and n-pentyl groups at the N 3 and N 7 positions diminished the potency for the right atrium in guinea-pigs. 3-n-Pentyl- and 7-methy]-3-n-pentyl-1-(5′-oxohexyl)xanthines showed much higher bronchoselectivity than oxpentifylline and theophylline.

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

Adenosine Receptors: The Contributions by John W. Daly

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

Recent developments in adenosine A2A receptor ligands

The development of potent and selective agonists and antagonists of adenosine receptors (ARs) has been a target of medicinal chemistry research for several decades, and recently the US Food and Drug Administration has approved Lexiscan, an adenosine derivative substituted at the 2 position, for use as a pharmacologic stress agent in radionuclide myocardial perfusion imaging. Currently, some other adenosine A(2A) receptor (A(2A)AR) agonists and antagonists are undergoing preclinical testing and clinical trials. While agonists are potent antiinflammatory agents also showing hypotensive effects, antagonists are being developed for the treatment of Parkinson's disease.However, since there are still major problems in this field, including side effects, low brain penetration (for the targeting of CNS diseases), short half-life, or lack of in vivo effects, the design and development of new AR ligands is a hot research topic.This review presents an update on the medicinal chemistry of A(2A)AR agonists and antagonists, and stresses the strong need for more selective ligands at the human A(2A)AR subtype, in particular in the case of agonists.

Search for new antagonist ligands for adenosine receptors from QSAR point of view. How close are we?

In view of the large libraries of nucleoside analogues that are now being handled in organic synthesis, the identification of drug biological activity is advisable prior to synthesis and this can be achieved by employing predictive biological property methods. In this sense, Quantitative Structure-Activity Relationships (QSAR) or docking approaches have emerged as promising tools. Although a large number of in silico approaches have been described in the literature for the prediction of different biological activities, the use of QSAR applications to develop adenosine receptor (AR) antagonists is not common as for the case of the antibiotics and anticancer compounds for instance. The intention of this review is to summarize the present knowledge concerning computational predictions of new molecules as adenosine receptor antagonists.

Probing the binding site of the A1 adenosine receptor reengineered for orthogonal recognition by tailored nucleosides

His272 (7.43) in the seventh transmembrane domain (TM7) of the human A3 adenosine receptor (AR) interacts with the 3' position of nucleosides, based on selective affinity enhancement at a H272E mutant A3 AR (neoceptor) of 3'-ureido, but not 3'-OH, adenosine analogues. Here, mutation of the analogous H278 of the human A1 AR to Ala, Asp, Glu, or Leu enhanced the affinity of novel 2'- and 3'-ureido adenosine analogues, such as 10 (N6-cyclopentyl-3'-ureido-3'-deoxyadenosine), by >100-fold, while decreasing the affinity or potency of adenosine and other 3'-OH adenosine analogues. His278 mutant receptors produced a similar enhancement regardless of the charge character of the substituted residue, implicating steric rather than electrostatic factors in the gain of function, a hypothesis supported by rhodopsin-based molecular modeling. It was also demonstrated that this interaction was orientationally specific; i.e., mutations at the neighboring Thr277 did not enhance the affinity for a series of 2'- and 3'-ureido nucleosides. Additionally, H-bonding groups placed on substituents at the N6 or 5' position demonstrated no enhancement in the mutant receptors. These reengineered human A1 ARs revealed orthogonality similar to that of the A3 but not the A2A AR, in which mutation of the corresponding residue, His278, to Asp did not enhance nucleoside affinity. Functionally, the H278D A1 AR was detectable only in a measure of membrane potential and not in calcium mobilization. This neoceptor approach should be useful for the validation of molecular modeling and the dissection of promiscuous GPCR signaling.

The receptor mechanism mediating the contractile response to adenosine on lung parenchymal strips from actively sensitised, allergen-challenged Brown Norway rats

Parenchymal strips prepared from lungs removed from actively sensitised Brown Norway rats challenged with allergen show hyperresponsiveness to adenosine. The response is mast cell mediated and a preliminary pharmacological analysis suggested the involvement of a receptor (or receptors) that could not be classified as any of the known adenosine receptor subtypes. We present a further analysis of the response. Male Brown Norway (BN) rats, actively sensitised to ovalbumin (OA), were challenged intratracheally with OA and killed 3 h later to provide parenchymal strip preparations. The augmented contractile responses to adenosine were partially blocked by the 5-HT receptor antagonist, methysergide, or the A(1) receptor antagonist, DPCPX, and abolished in the presence of both antagonists. Responses to high concentrations of the A(1) receptor agonist, CPA were, like those to adenosine, augmented on tissues from allergen-challenged animals and blocked by a combination of methysergide and DPCPX. The A(3) receptor agonist, Cl-IB-MECA, did not contract the tissue, but partially blocked the response to adenosine. A combination of Cl-IB-MECA and methysergide induced a similar degree of blockade to that seen with either drug given alone. Combination of Cl-IB-MECA and/or methysergide with DPCPX abolished the response to adenosine. The effects of the A(3) receptor agonist, inosine, were augmented on tissues from allergen-challenged animals and markedly inhibited by disodium cromoglycate, methysergide or Cl-IB-MECA. Responses to adenosine were abolished when parenchymal strips were taken from rats pretreated 48 h previously with pertussis toxin. 8-SPT, CGS 15943, XAC, MRS 1754, DPCPX and theophylline, at concentrations which inhibit the A(1) A(2A) and/or A(2B) receptors but have negligible affinity for the rat A(3) receptor, inhibited responses to adenosine, but high concentrations were required and blockade was incomplete. MRS 1523 and MRS 1191, which are antagonists at the rat A(3) receptor, had no effect on the response to adenosine. The present results support and clarify our earlier conclusion that an atypical receptor mechanism mediates contraction of the parenchymal strip prepared from the lungs of actively sensitised BN rats challenged with allergen to adenosine. The response arises from a combined effect of adenosine on the A(1) receptor and a receptor with similarities to the A(3) receptor, but where Cl-IB-MECA behaves as an antagonist and MRS 1523 and MRS 1191 are inactive at concentrations that substantially exceed their affinities for the rat A(3) receptor.

International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors

Four adenosine receptors have been cloned and characterized from several mammalian species. The receptors are named adenosine A(1), A(2A), A(2B), and A(3). The A(2A) and A(2B) receptors preferably interact with members of the G(s) family of G proteins and the A(1) and A(3) receptors with G(i/o) proteins. However, other G protein interactions have also been described. Adenosine is the preferred endogenous agonist at all these receptors, but inosine can also activate the A(3) receptor. The levels of adenosine seen under basal conditions are sufficient to cause some activation of all the receptors, at least where they are abundantly expressed. Adenosine levels during, e.g., ischemia can activate all receptors even when expressed in low abundance. Accordingly, experiments with receptor antagonists and mice with targeted disruption of adenosine A(1), A(2A), and A(3) expression reveal roles for these receptors under physiological and particularly pathophysiological conditions. There are pharmacological tools that can be used to classify A(1), A(2A), and A(3) receptors but few drugs that interact selectively with A(2B) receptors. Testable models of the interaction of these drugs with their receptors have been generated by site-directed mutagenesis and homology-based modelling. Both agonists and antagonists are being developed as potential drugs.

Alkylxanthines as research tools

(1) The methylxanthine caffeine has many pharmacological effects, most of which can be linked to blockade of adenosine receptors, inhibition of phosphodiesterases, and augmentation of calcium-dependent release of calcium from intracellular stores. (2) A variety of xanthines have been developed as potent and/or selective antagonists for adenosine receptors. (3) Several xanthines have been developed that are more potent and more selective inhibitors of cyclic nucleotide phosphodiesterase than caffeine or theophylline. (4) Caffeine remains the xanthine of choice for activation of intracellular calcium-sensitive calcium release channels although millimolar concentrations are required, which can have effects on other aspects of calcium regulation.

Aza-analogs of 8-styrylxanthines as A2A-adenosine receptor antagonists

In the present study we synthesized aza-analogs of 8-styrylxanthines, in which the ethenyl bridge is replaced by an imine, amide, or azo function, in order to investigate structure-activity relationships of the 8-substituent of A2A-selective xanthine derivatives. Thus, various 8-substituents were combined with theophylline or caffeine, respectively, and affinities of the novel compounds for adenosine A1- and A2a-receptors were determined and compared with those of analogous 8-styrylxanthine derivatives. 8-(Benzylideneamino)caffeine derivatives exhibited high affinity and selectivity for A2A-adenosine receptors, but were unstable in aqueous buffer solution at physiological pH values. 8-(Phenylazo)caffeine derivatives were less potent than corresponding 8-styrylcaffeine derivatives at adenosine receptors. The most potent azo compound of the present series was 8-(m-chlorophenylazo)caffeine (14b) exhibiting a Ki value of 400 nM at A2A-adenosine receptors and 20-fold selectivity versus A1-receptors. Due to the facile synthetic access to 8-(phenylazo)xanthine derivatives, which are obtained by coupling of 8-unsubstituted xanthines with phenyldiazonium salts, 14b may be an interesting new lead compound for the development of more potent and selective A2A-antagonists with azo structure.

KW-3902, a selective high affinity antagonist for adenosine A1 receptors

1. We demonstrate that 8-(noradamantan-3-yl)-1,3-dipropylxanthine (KW-3902) is a very potent and selective adenosine A1 receptor antagonist, assessed by radioligand binding and cyclic AMP response in cells. 2. In rat forebrain adenosine A1 receptors labelled with [3H]-cyclohexyladenosine (CHA), KW-3902 had a Ki value of 0.19 nM, whereas it showed a Ki value of 170 nM in rat striatal A2A receptors labelled with [3H]-2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoad enosine (CGS21680), indicating 890 fold A1 receptor selectivity versus the A2A receptor. KW-3902 at 10 microM showed no effect on recombinant rat A3 receptors expressed on CHO cells. 3. Saturation studies with [3H]-KW-3902 revealed that it bound with high affinity (Kd = 77 pM) and limited capacity (Bmax = 470 fmol mg-1 of protein) to a single class of recognition sites. A high positive correlation was observed between the pharmacological profile of adenosine ligands inhibiting the binding of [3H]-KW-3902 and that of [3H]-CHA. 4. KW-3902 showed potent A1 antagonism against the inhibition of forskolin-induced cyclic AMP accumulation in DDT1 MF-2 cells by the A1-selective agonist, cyclopentyladenosine with a dissociation constant (KB value) of 0.34 nM. KW-3902 antagonized 5'-N-ethylcarboxamidoadenosine-elicited cyclic AMP accumulation via A2B receptors with a KB value of 52 nM. 5. KW-3902 exhibited marked species-dependent differences in the binding affinities. The highest affinity was for the rat A1 receptor (ki = 0.19 nM) and these values for guinea-pig and dog A1 receptors were 1.3 and 10 nM, respectively.

Structure-activity relationships of alkylxanthines: alkyl chain elongation at the N1- or N7-position decreases cardiotonic activity in the isolated guinea pig heart

Relationships between the alkyl substitutions (C1-C6) and cardiac inotropic activities of xanthine derivatives were studied in isolated guinea pig heart muscles. Most of the alkylxanthines exhibited positive inotropic activity on the left atrium, which was increased with an elongation of alkyl chain at the N3-position but decreased by substitution of a long alkyl group at the N1- or N7-position of the xanthine skeleton. Although positive inotropic activity in the right ventricular papillary muscle was also increased by longer alkyl groups at the N3-position, the inotropic activity became negative with an increment in alkyl chain length at the N1- or N7-position. The positive inotropic activity of alkylxanthines was correlated with their inhibitory activity on the phosphodiesterase (PDE) III isoenzyme. Adenosine A1 antagonism and PDE IV inhibitory activity were also partly associated with the inotropic activity because H-89, an inhibitor of cyclic AMP-dependent protein kinase, diminished the positive inotropic action and potentiated the negative inotropic action. These results indicate that the positive inotropic activity of alkylxanthines becomes weak with elongation of alkyl chains at the N1- and N7-positions; In particular, xanthines having two long alkyl chains show a negative inotropic activity on the right ventricular papillary muscle, an effect that could not be elucidated from their cyclic AMP-dependent action.

Brain distribution characteristics of xanthine derivatives and relation to their locomotor activity in mice

The relationship between the brain distribution and motor activity in mice of the xanthines, theophylline, enprofylline, 1-methyl-3-propylxanthine (MPX) and oxpentifylline was investigated. Their plasma protein binding and hydrophobicity were also examined. When these xanthines were administered orally, enprofylline and oxpentifylline had no effect on motor activity. While theophylline increased motor activity over 10 mg kg-1, MPX caused a decrease in such activity over 10 mg kg-1. The protein-binding behaviour varied among these xanthines and was closely related to their hydrophobicity, which is represented as a logarithmic partition coefficient (log PC). MPX had the highest hydrophobicity, while oxpentifylline had the lowest. Brain distribution characteristics varied among these xanthines, with the rank order of their brain penetration ratio, calculated as the ratio of brain to unbound plasma concentrations, being theophylline > oxpentifylline > MPX > enprofylline. The inhibition constants (Ki) for adenosine A1 receptors and cyclic 3',5'-adenosine monophosphate (cAMP)-phosphodiesterase (PDE) of these xanthines were 44.6 and 134, > 1000 and 112, 26.4 and 49, and > 1000 and 111 microM for theophylline, enprofylline, MPX, and oxpentifylline, respectively. These findings suggest that the lack of effects of enprofylline and oxpentifylline on motor activity is probably due to their low brain penetration ratio or low adenosine A1 affinity in comparison with theophylline. The decrease in the motor activity by MPX may be, in part, mediated by cAMP or 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)

Frog secretions and hunting magic in the upper Amazon: identification of a peptide that interacts with an adenosine receptor

A frog used for "hunting magic" by several groups of Panoan-speaking Indians in the borderline between Brazil and Peru is identified as Phyllomedusa bicolor. This frog's skin secretion, which the Indians introduce into the body through fresh burns, is rich in peptides. These include vasoactive peptides, opioid peptides, and a peptide that we have named adenoregulin, with the sequence GLWSKIKEVGKEAAKAAAKAAGKAALGAVSEAV as determined from mass spectrometry and Edman degradation. The natural peptide may contain a D amino acid residue, since it is not identical in chromatographic properties to the synthetic peptide. Adenoregulin enhances binding of agonists to A1 adenosine receptors; it is accompanied in the skin secretion by peptides that inhibit binding. The vasoactive peptide sauvagine, the opioid peptides, and adenoregulin and related peptides affect behavior in mice and presumably contribute to the behavioral sequelae observed in humans.

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.

Agonist activity of 2- and 5'-substituted adenosine analogs and their N6-cycloalkyl derivatives at A1- and A2-adenosine receptors coupled to adenylate cyclase

The activity of N6-cycloalkyl derivatives of adenosine, 2-chloroadenosine, 5'-chloroadenosine and N-ethylcarboximidoadenosine (NECA) and of 2-fluoroadenosine and 5-methylthioadenosines were compared at the A1-adenosine receptor inhibitory to adenylate cyclase in rat fat cell membranes and at the A2A-adenosine receptors stimulatory to adenylate cyclase in rat PC12 cell membranes. The N6-cycloalkyl derivatives in all cases were more potent (4- to 23-fold) than the parent compound at the A1 receptor, and were less potent (1.6- to 11-fold) than the parent compound at the A2A receptor. N6-Cyclopentyl-5'-chloroadenosine was the most selective agonist (900-fold) for the A1 receptor, while 2-fluoroadenosine was the only agonist with some selectivity (4.8-fold) for the A2A receptor. 5'-Methylthioadenosine was a weak agonist at both adenosine receptors. A 2-fluoro derivative of 5'-methylthioadenosine was somewhat more potent. Affinities of these analogs for inhibition of binding of radioligands to rat brain A1 and A2A receptors are presented.

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.

Characterization of human striatal A2-adenosine receptors using radioligand binding and photoaffinity labeling

The adenosine agonist [3H]CGS21680 (2-[4-[[2-carboxyethyl]phenyl]ethylamino]-5'- N-ethylcarboxamidoadenosine) bound to A2 receptors in human striatal membranes with a Kd of 17.8 +/- 1.1 nM and a Bmax of 313 +/- 10 fmol/mg protein. The addition of 100 microM GTP diminished both the affinity of agonist radioligand for A2 adenosine binding sites and the total binding, resulting in Kd and Bmax values of 28.6 +/- 1.0 nM and 185 +/- 22 fmol/mg of protein. Adenosine ligands competed for [3H]CGS21680 with the expected potency order. The adenosine antagonist [3H]XAC (8-[4-[[[[(2-aminoethyl)-amino]carbonyl]methyl] oxy]phenyl]-1,3-dipropylxanthine), although A1-selective in the rat, binds to human striatal A2 receptors with high affinity. 25 nM CPX (8-cyclopentyl-1,3-dipropylxanthine), an A1-selective antagonist, was added to the incubation medium and effectively eliminated 91% of [3H]XAC (1 nM) binding to human A1 receptors, yet preserved 90% of binding to A2 receptors. [3H]XAC exhibited saturable, specific binding (50% of total) to A2 sites with a Kd of 2.98 +/- 0.54 nM and a Bmax of 0.71 +/- 0.23 pmol/mg protein (25 degrees C, non-specific binding defined with 100 microM NECA). The potency order for antagonists against 1 nM [3H]XAC was CGS15943A greater than XAC approximately PD115,119 greater than PAPA-XAC greater than CPX greater than HTQZ approximately XCC approximately CP-66,713 greater than theophylline approximately caffeine, indicative of an A2-type binding site. A2a-receptors were found to be present in the human cortex, albeit at a much lower density than in the striatum. Photoaffinity labeling using 125I-PAPA-APEC revealed a molecular weight of 45K, but proteolytic cleavage was observed, resulting in fragments of MW 43K and 37K. In the absence of proteolytic inhibitors the 37K fragment, which still bound 125I-PAPA-APEC, was predominant.

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)

Trifunctional agents as a design strategy for tailoring ligand properties: irreversible inhibitors of A1 adenosine receptors

The 1,3-phenylene diisothiocyanate conjugate of XAC (8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]- oxy]phenyl]-1,3-dipropylxanthine, a potent A1 selective adenosine antagonist) has been characterized as an irreversible inhibitor of A1 adenosine receptors. To further extend this work, a series of analogues were prepared containing a third substituent in the phenyl isothiocyanate ring, incorporated to modify the physiochemical or spectroscopic properties of the conjugate. Symmetrical trifunctional cross-linking reagents bearing two isothiocyanate groups were prepared as general intermediates for cross-linking functionalized congeners and receptors. Xanthine isothiocyanate derivatives containing hydrophilic, fluorescent, or reactive substituents, linked via an amide, thiourea, or methylene group in the 5-position, were synthesized and found to be irreversible inhibitors of A1 adenosine receptors. The effects of the 5-substituent on water solubility and on the A1/A2 selectivity ratio derived from binding assays in rat brain membranes were examined. Inhibition of binding of [3H]-N6-(2-phenylisopropyl)-adenosine and [3H] CGS21680 (2-[2-[4-carboxyethyl)phenyl]ethyl]amino] adenosine-5'-N-ethylcarboxamide) at central A1 and A2 adenosine receptors, respectively, was measured. A conjugate of XAC and 1,3,5-triisothiocyanatobenzene was 894-fold selective for A1 receptors. Reporter groups, such as fluorescent dyes and a spin-label, were included as chain substituents in the irreversible binding analogues, which were designed for spectroscopic assays, histochemical characterization, and biochemical characterization of the receptor protein.

Adenosine receptors and signaling in the kidney

It is now generally accepted that adenosine is capable of regulating a wide range of physiological functions. Nowhere is the diversity of this action better illustrated than in the kidney. When adenosine binds to plasma membrane receptors on a variety of cell types in the kidney, it stimulates functional responses that span the entire spectrum of renal physiology, including alterations in hemodynamics, hormone and neurotransmitter release, and tubular reabsorption. These responses to adenosine appear to represent a means by which the organ and its constituent cell types can regulate their metabolic demand such that it is maintained at an appropriate level for the prevailing metabolic supply. Extracellular adenosine, produced from the hydrolysis of adenosine 5'-monophosphate and stimulated by increased substrate availability and enzyme induction, acts on at least two types of cell surface receptors to stimulate or inhibit the production of cyclic adenosine-3',5'-monophosphate and also acts in some renal cells to stimulate the production of inositol phosphates and elevation of cytosolic calcium concentration. To understand when and why this complicated system becomes activated, how it interacts with other known extracellular effector systems, and ultimately how to manipulate the system to therapeutic advantage by selective agonists or antagonists, requires a detailed knowledge of renal adenosine receptors and their signaling mechanisms. The following discussion attempts to highlight our knowledge in this area, to present a modified hypothesis for adenosine as a feedback regulator of renal function, and to identify some important questions regarding the specific cellular mechanisms of adenosine in renal cell types.

Comparison of A1 adenosine receptors in brain from different species by radioligand binding and photoaffinity labelling

Radioligand binding to A1 adenosine receptors at brain membranes from seven species was investigated. The antagonist 8-cyclopentyl-1,3-[3H]dipropylxanthine ([3H]DPCPX) bound with affinities between 0.17 nM in sheep brain and 2.1 nM in guinea pig brain. Competition of several antagonists for [3H]DPCPX binding showed that the most potent compounds were DPCPX with Ki values of 0.05 nM in bovine brain and 1.1 nM in guinea pig brain and xanthine amine congener (XAC) with Ki values of 0.03 nM in bovine brain and 5.5 nM in guinea pig brain. The differences in affinity of the agonist radioligand 2-chloro-N6-[3H]cyclopentyl-adenosine ([3H]CCPA) were less pronounced, ranging from a KD value of 0.12 nM (hamster brain) to 0.42 nM (guinea pig brain). Agonist competition for [3H]DPCPX binding of photoaffinity labelling, however, exhibited marked species differences. N-Ethylcarboxamidoadenosine (NECA) and S-N6-phenylisopropyladenosine (S-PIA) showed 20 to 25-fold different KD values in different species. NECA had a particularly high affinity in guinea pig brain and was only two-fold less potent than R-PIA. Thus, the difference from the "classical" A1 receptor profile (R-PIA greater than -NECA greater than S-PIA) is not sufficient to speculate that A1 receptor subtypes may exist that are coupled to different effector systems. Our data show that these difference can easily be explained by species differences.

Possible involvement of pertussis toxin-sensitive G proteins and D2 dopamine receptors in the A1 adenosine receptor-adenylate cyclase system in rat cerebral cortex

To identify the involvement of dopamine receptors in the transmembrane signaling of the adenosine receptor-G protein-adenylate cyclase system in the CNS, we examined the effects of pertussis toxin (islet-activating protein, IAP) and apomorphine on A1 adenosine agonist (-)N6-R-[3H]phenylisopropyladenosine ([3H]PIA) and antagonist [3H]xanthine amine congener ([3H]XAC) binding activity and adenylate cyclase activity in cerebral cortex membranes of the rat brain. Specific binding to a single class of sites for [3H]XAC with a dissociation constant (KD) of 6.0 +/- 1.3 nM was observed. The number of maximal binding sites (Bmax) was 1.21 +/- 0.13 pmol/mg protein. Studies of the inhibition of [3H]XAC binding by PIA revealed the presence of two classes of PIA binding states, a high-affinity state (KD = 2.30 +/- 1.16 nM) and a low-affinity state (KD = 1.220 +/- 230 nM). Guanosine 5'-(3-O-thio)triphosphate or IAP treatment reduced the number of the high-affinity state binding sites without altering the KD for PIA. Apomorphine (100 microM) increased the KD value 10-fold and decreased Bmax by approximately 20% for [3H]PIA. The effect of apomorphine on the KD value increase was irreversible and due to a conversion from high-affinity to low-affinity states for PIA. The effect was dose dependent and was mediated via D2 dopamine receptors, since the D2 antagonist sulpiride blocked the phenomenon. The inhibitory effect of PIA on adenylate cyclase activity was abolished by apomorphine treatment. There was no effect of apomorphine on displacement of [3H]quinuclidinyl benzilate (muscarinic ligand) binding by carbachol. These data suggest that A1 adenosine receptor binding and function are selectively modified by D2 dopaminergic agents.

Agonists and antagonists recognize different but overlapping populations of A1 adenosine receptors: modulation of receptor number by MgCl2, solubilization, and guanine nucleotides

A1 selective agonist and antagonist radioligands bind to the same A1 adenosine receptor binding subunit, as documented by photoaffinity labelling and partial peptide maps. In this study we document that although these radioligands recognize the same A1 adenosine receptor (A1AR), they recognize different numbers of A1ARs in bovine brain membranes, with agonist number being greater than antagonist number. Neither addition of guanine nucleotides nor removal of Mg2+ ions enhanced antagonist binding in membranes. On solubilization, agonists still recognized a greater number of A1ARs but addition of guanine nucleotides or removal of Mg2+ substantially increased the number of receptors detected with antagonist radioligands. The effects of Mg2+ and guanine nucleotides were not additive, suggesting that formation of a "low agonist-receptor-G protein state" by either modulating agent was sufficient to alter the receptor conformation such that it could be recognized by antagonist. These studies suggest that a proportion of the "precoupled A1AR-G protein complex" in membranes are in a conformation that cannot be recognized by antagonists and that membrane constraints are such that ions or guanine nucleotides cannot sufficiently modulate the conformation to allow it to recognize antagonists. On removal of membrane structure by solubilization, these constraints are removed.

Analysis of agonist-antagonist interactions at A1 adenosine receptors

Previous work from our laboratory using sucrose gradient centrifugation and the antagonist radioligand [3H]xanthine amine congener led us to propose that A1 adenosine receptors are coupled to a GTP-binding protein (G protein) in the absence of an agonist and that adenosine receptor antagonists bind to free uncoupled receptors with high affinity and coupled receptors with low affinity and cause a destabilization of receptor-G protein complexes [Mol. Pharmacol. 36:412-419 (1989)]. Because agonists form high affinity ternary complexes composed of the agonist, receptor, and G protein, this hypothesis would imply that interactions between adenosine receptor agonists and antagonists, while competitive, would appear to be "noncompetitive" in nature. Interactions between unlabeled and radiolabeled A1 receptor agonist and antagonist ligands have been investigated using bovine cerebral cortical membranes to further probe this point. The availability of both 3H- and 125I-radioligands allowed us to use both single- and dual-isotope experimental designs. Radioligand antagonist-agonist competition curves along with saturation analyses using filtration and centrifugation to isolate bound radioligand suggested that agonists bind to two sites or receptor states with high affinity and to one site with low affinity. Agonist radioligand saturation curves with or without unlabeled antagonist suggested that antagonists do not bind to all states of the receptor with equal affinity. The computer program EQUIL was used to define models capable of simultaneously fitting all parts of complex experiments in which 125I-N6-aminobenzyladenosine saturation isotherms with or without 8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX) and a saturation isotherm of [3H]CPX were performed. The data were not compatible with two-independent site models or with ternary complex models involving one receptor and one G protein. The data were fit by a model involving one receptor and two G proteins and by a model involving two receptors and one G protein. Both models suggest that 1) a high percentage of the receptor(s) is coupled to a G protein in the absence of an agonist and 2) agonists stabilize whereas antagonists destabilize precoupled receptor-G protein complexes. Because of this, competitive interactions between A1 agonists and antagonists appear noncompetitive in nature.

Guanine nucleotide effects on 8-cyclopentyl-1,3-[3H]dipropylxanthine binding to membrane-bound and solubilized A1 adenosine receptors of rat brain

The effects of guanine nucleotides on binding of 8-cyclopentyl-1,3-[3H]dipropylxanthine ([3H]DPCPX), a highly selective A1 adenosine receptor antagonist, have been investigated in rat brain membranes and solubilized A1 receptors. GTP, which induces uncoupling of receptors from guanine nucleotide binding proteins, increased binding of [3H]DPCPX in a concentration-dependent manner. The rank order of potency for different guanine nucleotides for increasing [3H]DPCPX binding was the same as for guanine nucleotide-induced inhibition of agonist binding. Therefore, a role for a guanine nucleotide binding protein, e.g., Gi, in the regulation of antagonist binding is suggested. This was confirmed by inactivation of Gi by N-ethylmaleimide (NEM) treatment of membranes, which resulted in an increase in [3H]DPCPX binding similar to that seen with addition of GTP. Kinetic and equilibrium binding studies showed that the GTP- or NEM-induced increase in antagonist binding was not caused by an affinity change of A1 receptors for [3H]DPCPX but by an increased Bmax value. Guanine nucleotides had similar effects on membrane-bound and solubilized receptors, with the effects in the solubilized system being more pronounced. In the absence of GTP, when most receptors are in a high-affinity state for agonists, only a few receptors are labeled by [3H]DPCPX. It is suggested that [3H]DPCPX binding is inhibited when receptors are coupled to Gi. Therefore, uncoupling of A1 receptors from Gi by guanine nucleotides or by inactivation of Gi with NEM results in an increased antagonist binding.

Demonstration of both A1 and A2 adenosine receptors in DDT1 MF-2 smooth muscle cells

Adenosine receptors of the A1 and A2 subtypes were characterized in membranes from DDT1 MF-2 smooth muscle cells. These cells possess a high density of A1 adenosine receptors (Bmax = 0.8-0.9 pmol/mg of protein), as measured by both agonist and antagonist radioligands. Agonists compete for [125I]N6-[2-(4-amino-3-iodophenyl)ethyl]-adenosine (A1 receptor-selective radioligand) binding with the following potency series: (R)-phenylisopropyladenosine [(R)-PIA] greater than 5'-N-ethylcarboxamide adenosine (NECA) greater than (S)-PIA, indicative of their interaction with A1 adenosine receptors. Agonist competition for [3H]8-(4-[[[(2-aminoethyl)amino]carbonyl)methyl)oxy]phenyl)-1, 3-dipropylxanthine [( 3H]XAC) (an antagonist radioligand for the A1 adenosine receptor) was described by a two-state model of 1.3 nM (high affinity state, KK) and 370 nM (low affinity state, KL), with 70% of the receptors in the high affinity state (RH). Addition of guanosine 5'-[beta, alpha-imido]triphosphate (100 microM) shifted the (R)-PIA competition curves to the right to lower affinities. Photoaffinity labeling with the agonist photoprobe [125I]N6-[2-(4-amino-3-iodophenyl) ethyl]adenosine indicates that the A1 adenosine receptor binding subunit is a Mr 38,000 protein. Adenosine receptor agonists [(R)-PIA, NECA, and (S)-PIA] inhibited isoproterenol-stimulated adenylate cyclase activity in DDT1 MF-2 cell membranes with IC50 values of 62, 538, and 750 nM, respectively. Inhibition of adenylate cyclase by (R)-PIA was attenuated by the A1 receptor antagonist XAC and following inactivation of Gi with pertussis toxin (100 ng/ml). Using a recently developed A2 adenosine receptor agonist radioligand 2-[4-(2-[( 4-aminophenyl]methylcarbonyl)ethyl) phenyl]ethylamino-5'-N-ethylcarboxamido adenosine (125I-PAPA-APEC), we have demonstrated the presence of A2 adenosine receptors in this cell line. Saturation curves with 125I-PAPA-APEC indicated the Bmax and Kd values to be 0.21 pmol/mg of protein and 4.0 nM, respectively. In competition experiments, NECA was more potent at inhibiting 125I-PAPA-APEC binding than (R)-PIA, with their respective IC50 values being 5.6 and 351 nM. The photolabeled A2 adenosine receptor migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an Mr of 42,000. Finally, adenosine receptor agonists stimulated adenylate cyclase activity by approximately 2-3 fold with the following potency series: PAPA-APEC greater than or equal to NECA greater than (R)-PIA, indicative of their interaction at A2 receptors. These data represent the first demonstration of the presence of both A1 and A2 receptors in a single cell line, DDT1 MF-2 smooth muscle cells.

Affinity chromatography of the bovine cerebral cortex A1 adenosine receptor

An approximate 140-fold purification of the A1 adenosine receptor of bovine cerebral cortex has been obtained via affinity chromatography. The affinity column consists of Affi-Gel 10 coupled through an amide linkage to XAC, a high-affinity A1 adenosine receptor antagonist. As assessed by [3H]XAC binding, bovine brain membranes solubilized with the detergent CHAPS had a specific binding activity of 1.1 pmol/mg protein. Interaction of solubilized A1 adenosine receptors with the XAC-Affi-Gel was biospecific and 30% of the receptor activity was bound by the gel. Demonstration of [3H]XAC binding in the material eluted from the column with R-PIA required insertion of receptor into phospholipid vesicles. The specific activity of the affinity column purified receptor was 146 +/- 22 pmol/mg protein with typically 5-15% of the bound receptor recovered. The purified receptor displayed high-affinity antagonist binding and bound agonists with the potency order expected of the bovine brain A1 adenosine receptor: R-PIA greater than S-PIA greater than NECA. In purified preparations, the photoaffinity probe [125I]PAPAXAC-SANPAH specifically labelled a protein of molecular mass 38,000 which has previously been shown to be the A1 adenosine receptor binding subunit.

Identification of the A2 adenosine receptor binding subunit by photoaffinity crosslinking

A high-affinity iodinated agonist radioligand for the A2 adenosine receptor has been synthesized to facilitate studies of the A2 adenosine receptor binding subunit. The radioligand 125I-labeled PAPA-APEC (125I-labeled 2-[4-(2-[2-[(4- aminophenyl)methylcarbonylamino]ethylaminocarbonyl]- ethyl)phenyl]ethylamino-5'-N-ethylcarboxamidoadenosine) was synthesized and found to bind to the A2 adenosine receptor in bovine striatal membranes with high affinity (Kd = 1.5 nM) and A2 receptor selectivity. Competitive binding studies reveal the appropriate A2 receptor pharmacologic potency order with 5'-N-ethylcarboxamidoadenosine (NECA) greater than (-)-N6-[(R)-1-methyl- 2-phenylethyl]adenosine (R-PIA) greater than (+)-N6-[(S)-1-methyl-2- phenylethyl]adenosine (S-PIA). Adenylate cyclase assays, in human platelet membranes, demonstrate a dose-dependent stimulation of cAMP production. PAPA-APEC (1 microM) produces a 43% increase in cAMP production, which is essentially the same degree of increase produced by 5'-N- ethylcarboxamidoadenosine (the prototypic A2 receptor agonist). These findings combined with the observed guanine nucleotide-mediated decrease in binding suggest that PAPA-APEC is a full A2 agonist. The A2 receptor binding subunit was identified by photoaffinity-crosslinking studies using 125I-labeled PAPA-APEC and the heterobifunctional crosslinking agent N-succinimidyl 6-(4'-azido-2'-nitrophenylamino)hexanoate (SANPAH). After covalent incorporation, a single specifically radiolabeled protein with an apparent molecular mass of 45 kDa was observed on NaDodSO4/PAGE/autoradiography. Incorporation of 125I-labeled PAPA-APEC into this polypeptide is blocked by agonists and antagonists with the expected potency for A2 receptors (see above) and is decreased in the presence of 10(-4) M guanosine 5'-[beta, gamma-imido]triphosphate. Photoaffinity crosslinking of the A1 adenosine receptor binding subunit with 125I-labeled 8-[4-[2-(4- aminophenylacetylamino)ethyl]carbonylmethyloxyphenyl]-1,3-di propylxanthine (PAPAXAC) (an A1 selective photoaffinity probe) in the same tissue reveals a 38-kDa peptide that exhibits the appropriate A1 receptor pharmacology. 125I-labeled PAPA-APEC, therefore, has identified the A2 receptor binding subunit as a 45-kDa protein that is unique and distinct from the A1 binding subunit.

Sulfur-containing 1,3-dialkylxanthine derivatives as selective antagonists at A1-adenosine receptors

Sulfur-containing analogues of 8-substituted xanthines were prepared in an effort to increase selectivity or potency as antagonists at adenosine receptors. Either cyclopentyl or various aryl substituents were utilized at the 8-position, because of the association of these groups with high potency at A1-adenosine receptors. Sulfur was incorporated on the purine ring at positions 2 and/or 6, in the 8-position substituent in the form of 2- or 3-thienyl groups, or via thienyl groups separated from an 8-aryl substituent through an amide-containing chain. The feasibility of using the thienyl group as a prosthetic group for selective iodination via its Hg2+ derivative was explored. Receptor selectivity was determined in binding assays using membrane homogenates from rat cortex [( 3H]-N6-(phenylisopropyl)adenosine as radioligand] or striatum [3H]-5'-(N-ethylcarbamoyl)adenosine as radioligand] for A1- and A2-adenosine receptors, respectively. Generally, 2-thio-8-cycloalkylxanthines were at least as A1 selective as the corresponding oxygen analogue. 2-Thio-8-aryl derivatives tended to be more potent at A2 receptors than the oxygen analogue. 8-[4-[(Carboxy-methyl)oxyl] phenyl]-1,3-dipropyl-2-thioxanthine ethyl ester was greater than 740-fold A1 selective.

The effect of 5'-N-ethylcarboxamideadenosine on the isolated hemidiaphragm of the rat during direct and indirect electrical stimulation

Similar to adenosine and some of its derivatives, 5'-N-ethylcarboxamideadenosine (NECA) produced a biphasic effect on the isolated hemidiaphragm of the rat: a potentiation of the isometric contraction during direct electrical stimulation (in the presence of dipyridamole), and a depression during indirect electrical stimulation, and particularly so in a partially curarized preparation. The potentiating effect is presumed to be due to activation of the cAMP system, probably through A2 receptor sites, although a differentiation of the receptor subtype could not be made with aminophylline and XAC. Inhibition is most probably due to a depressant action of NECA on the acetylcholine release from the motor nerve terminals. These results accord with our previous findings, suggesting that adenosine may be part of a buffer system which participates in balancing the excitatory and inhibitory influences on skeletal muscle contraction.

Electrophilic derivatives of purines as irreversible inhibitors of A1 adenosine receptors

Functionalized congeners derived from 1,3-dipropyl-8-phenylxanthine and from N6-phenyladenosine were derivatized to contain electrophilic groups (isothiocyanate, N-hydroxysuccinimide ester, maleimide, sulfonyl chloride, or alpha-haloacyl group) capable of reaction with nucleophiles on biopolymers. The goal was to inhibit chemically the A1 adenosine receptor by using reactive agonist and antagonist ligands. Some of the electrophilic derivatives were synthesized through acylation of amine-functionalized congeners using hetero- or homobifunctional reagents available for protein cross-linking. The affinity for A1 adenosine receptors was evaluated in competitive binding assays by using rat and bovine brain membranes. Several xanthine and adenosine thiourea derivatives prepared from 1,3- and 1,4-phenylene diisothiocyanate (DITC) were potent irreversible inhibitors of adenosine receptors. Derivatives of m-DITC, at concentrations between 10 and 500 nM, irreversibly eliminated binding at 90% of the A1-receptor sites. Receptor affinity of both xanthine and adenosine derivatives containing distal phenylthiourea substituents was diminished by electron-donating groups on the ring.

High affinity acylating antagonists for the A1 adenosine receptor: identification of binding subunit

Two isomeric isothiocyanate derivatives of the A1 adenosine receptor antagonist xanthine amine cogener (XAC) have been synthesized and found to be potent affinity labels (irreversibly bound ligands) for A1 adenosine receptors. The interaction of m- and p-isomers of 1,3-dipropyl-8-isothiocyanatophenyl(aminothiocarbonyl (2-aminoethylaminocarbonyl(4-methyloxy(phenyl]])-xanthine (DITC-XAC) with rat brain A1 receptors is of high affinity (EC50 = 27 and 52 nM, respectively) as determined by radioligand competition curves. These compounds reduced the number of A1 receptors (greater than 90% at 500 nM m-DITC-XAC) in brain membranes, without any change in the affinity of the remaining receptors for [125I]N6-2-(4-aminophenyl)ethyladenosine. Prior reaction of the isothiocyanate moiety with ethylenediamine did not alter the affinity of the XAC derivative for the A1 receptor but eliminated its ability to covalently incorporate into the receptor. Incubation of brain membranes with radiolabeled p- and m-DITC-XAC results in the specific labeling of a Mr 38,000 peptide. This labeling can be blocked with both an A1 adenosine receptor-specific agonist and an antagonist. This specific protein has the same molecular weight as the protein labeled with A1-selective photoaffinity probes. The much higher efficiency of incorporation of these affinity probes compared with photoaffinity probes should make them extremely useful for structural studies of A1 adenosine receptors.

A1 adenosine receptor-G protein coupling in bovine brain membranes: effects of guanine nucleotides, salt, and solubilization

The effects of guanine nucleotides, NaCl, and solubilization on the interaction of antagonists and agonists with the A1 adenosine receptor of bovine brain membranes were studied using the high-affinity antagonist radioligand [3H]xanthine amine congener ([3H]XAC). In membranes, guanine nucleotides and NaCl had no effect on [3H]XAC saturation curves. Using agonist (R)-phenylisopropyladenosine (R-PIA) competition curves versus [3H]XAC, it was demonstrated that agonists could differentiate two affinity states having high and low affinity for agonist and that guanine nucleotides shifted the equilibrium to an all-low-affinity state that was indistinguishable from the low-affinity state in the absence of guanine nucleotides. In contrast, NaCl decreased agonist affinity by a distinctly different mechanism characterized by a parallel rightward shifted agonist curve such that R-PIA still recognized two affinity states albeit of lower affinity than in the absence of salt. R-PIA competition curves in the presence of both guanine nucleotides and salt were still shallow but were shifted far to the right, and two very low affinity states were discerned. On solubilization, guanine nucleotides in a reversible, concentration-dependent manner increased antagonist ([3H]XAC) but not agonist (R-N6-[3H]phenylisopropyladenosine) binding. This was consequent to a change in maximal binding capacity. R-PIA competition curves (versus [3H]XAC) in solubilized preparations demonstrated that agonist could still differentiate two agonist specific affinity states which were modulated by guanine nucleotides. In the presence of guanine nucleotides all the receptors were shifted to a uniform low-affinity state. In contrast, NaCl had no effect on agonist affinity as determined by agonist competition curves in a solubilized receptor preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

8-Substituted xanthines as antagonists at A1- and A2-adenosine receptors

Two classes of 8-substituted analogs of theophylline (1,3-dialkylxanthines), having 8-cycloalkyl, 8-cycloalkenyl or 8-(para-substituted aryl) groups, were shown to be potent and, in some cases, receptor subtype selective antagonists at A1- and A2-adenosine receptors. New analogs based on a functionalized cogener approach and on classical medicinal chemical approaches were prepared. Affinity at A1-adenosine receptors was evaluated by inhibition of binding of [3H]N6-phenylisopropyladenosine to rat brain membranes. Activity at A2-adenosine receptors was measured by the reversal of 5'-N-ethylcarboxamidoadenosine (NECA)-stimulated production of cyclic AMP in membranes from rat pheochromocytoma PC12 cells. Cycloalkenyl analogs containing rigid olefinic bonds differed greatly in potency from the saturated analogs. The selectivity of phenylsulfonamide analogs depended on distal structural features. Novel xanthine analogs include diamino-, thiol-, aldehyde, and halogen-substituted derivatives, peptide conjugates of 8-[4-[2-aminoethylaminocarbonylmethyloxy]phenyl]1,3-dipropylxan thi ne (XAC), and a hydroxyethylamide analog of XAC.

Multiple components of the A1 adenosine receptor-adenylate cyclase system are regulated in rat cerebral cortex by chronic caffeine ingestion

The effects of chronic caffeine on the A1 adenosine receptor-adenylate cyclase system of rat cerebral cortical membranes were studied. Caffeine treatment significantly increased the number of A1 adenosine receptors as determined with the A1 adenosine receptor antagonist radioligand [3H]xanthine amine congener (XAC). R-PIA (agonist) competition curves constructed with [3H]XAC were most appropriately described by a two affinity state model in control membranes with a KH of 2.1 +/- 0.8 and a KL of 404 +/- 330 nM with 50 +/- 4% of receptors in the high affinity state (%RH). In contrast, in membranes from treated animals, there was a marked shift towards the high affinity state. In three of seven animals all of the receptors were shifted to a unique high affinity state which was indistinguishable from the KH observed in membranes from control animals. In four of seven animals the %RH increased from 50 to 69% with KH and KL indistinguishable from the control values. Thus, the agonist specific high affinity form of the receptor was enhanced following caffeine treatment. Maximal inhibition of adenylate cyclase activity in cerebral cortical membranes by R-PIA (1 microM) was significantly increased by 28% following caffeine treatment, consistent with an increased coupling of receptor-Gi protein with adenylate cyclase. Importantly, the quantity of Gi (alpha i) in rat cerebral cortex, determined by pertussis toxin-mediated labeling, was also increased to 133% of control values by this treatment. Thus, multiple components and interactions of the A1 adenosine receptor-adenylate cyclase complex are regulated by caffeine. These changes are likely compensatory measures to offset blockade of A1 receptors in vivo by caffeine and lead to a sensitization of this inhibitory receptor system.