The affinity of adenosine for the high- and low-affinity states of the human adenosine A1 receptor

The Brain-Heart Network of Syncope

Observed and recorded in various forms since ancient times, 'syncope' is often popularly called 'fainting', such that the two terms are used synonymously. Syncope/fainting can be caused by a variety of conditions, including but not limited to head injuries, vertigo, and oxygen deficiency. Here, we draw on a large body of literature on syncope, including the role of a recently discovered set of specialized mammalian neurons. Although the etiology of syncope still remains a mystery, we have attempted to provide a comprehensive account of what is known and what still needs to be performed. Much of our understanding of syncope is owing to studies in the laboratory mouse, whereas evidence from human patients remains scarce. Interestingly, the cardioinhibitory Bezold-Jarisch reflex, recognized in the early 1900s, has an intriguing similarity to-and forms the basis of-syncope. In this review, we have integrated this minimal model into the modern view of the brain-neuron-heart signaling loop of syncope, to which several signaling events contribute. Molecular signaling is our major focus here, presented in terms of a normal heart, and thus, syncope due to abnormal or weak heart activity is not discussed in detail. In addition, we have offered possible directions for clinical intervention based on this model. Overall, this article is expected to generate interest in chronic vertigo and syncope/fainting, an enigmatic condition that affects most humans at some point in life; it is also hoped that this may lead to a mechanism-based clinical intervention in the future.

Adenosinergic System and Neuroendocrine Syncope: What Is the Link?

Although very common, the precise mechanisms that explain the symptomatology of neuroendocrine syncope (NES) remain poorly understood. This disease, which can be very incapacitating, manifests itself as a drop in blood pressure secondary to vasodilation and/or extreme slowing of heart rate. As studies continue, the involvement of the adenosinergic system is becoming increasingly evident. Adenosine, which is an ATP derivative, may be involved in a large number of cases. Adenosine acts on G protein-coupled receptors with seven transmembrane domains. A1 and A2A adenosine receptor dysfunction seem to be particularly implicated since the activation leads to severe bradycardia or vasodilation, respectively, two cardinal symptoms of NES. This mini-review aims to shed light on the links between dysfunction of the adenosinergic system and NHS. In particular, signal transduction pathways through the modulation of cAMP production and ion channels in relation to effects on the cardiovascular system are addressed. A better understanding of these mechanisms could guide the pharmacological development of new therapeutic approaches.

Adenosine, Adenosine Receptors and Neurohumoral Syncope: From Molecular Basis to Personalized Treatment

Adenosine is a ubiquitous nucleoside that is implicated in the occurrence of clinical manifestations of neuro-humoral syncope (NHS). NHS is characterized by a drop in blood pressure due to vasodepression together with cardio inhibition. These manifestations are often preceded by prodromes such as headaches, abdominal pain, feeling of discomfort or sweating. There is evidence that adenosine is implicated in NHS. Adenosine acts via four subtypes of receptors, named A1 (A1R), A2A (A2AR), A2B (A2BR) and A3 (A3R) receptors, with all subtypes belonging to G protein membrane receptors. The main effects of adenosine on the cardiovascular system occurs via the modulation of potassium ion channels (IK Ado, K ATP), voltage-gate calcium channels and via cAMP production inhibition (A1R and A3R) or, conversely, through the increased production of cAMP (A2A/BR) in target cells. However, it turns out that adenosine, via the activation of A1R, leads to bradycardia, sinus arrest or atrioventricular block, while the activation of A2AR leads to vasodilation; these same manifestations are found during episodes of syncope. The use of adenosine receptor antagonists, such as theophylline or caffeine, should be useful in the treatment of some forms of NHS. The aim of this review was to summarize the main data regarding the link between the adenosinergic system and NHS and the possible consequences on NHS treatment by means of adenosine receptor antagonists.

Blood Adenosine Increase During Apnea in Spearfishermen Reinforces the Efficiency of the Cardiovascular Component of the Diving Reflex

The physiopathology consequences of hypoxia during breath-hold diving are a matter of debate. Adenosine (AD), an ATP derivative, is suspected to be implicated in the adaptive cardiovascular response to apnea, because of its vasodilating and bradycardic properties, two clinical manifestations observed during voluntary apnea. The aim of this study was to evaluate the adenosine response to apnea-induced hypoxia in trained spearfishermen (SFM) who are used to perform repetitive dives for 4-5 h. Twelve SFM (11 men and 1 woman, mean age 41 ± 3 years, apnea experience: 18 ± 9 years) and 10 control (CTL) subjects (age 44 ± 7 years) were enrolled in the study. Subjects were asked to main a dry static apnea and stopped it when they began the struggle phase (average duration: SFM 120 ± 78 s, CTL 78 ± 12 s). Capillary blood samples were collected at baseline and immediately after the apnea and analyzed for standard parameters and adenosine blood concentration ([AD]b). Heart rate (HR), systolic (SBP), and diastolic (DBP) blood pressures were also recorded continuously during the apnea. During the apnea, an increase in SBP and DBP and a decrease in HR were observed in both SFM and CTL. At baseline, [AD]b was higher in SFM compared with CTL (1.05 ± 0.2 vs. 0.73 ± 0.11 μM, p < 0.01). [AD]b increased significantly at the end of the apnea in both groups, but the increase was significantly greater in SFM than in controls (+90.4 vs. +12%, p < 0.01). Importantly, in SFM, we also observed significant correlations between [AD]b and HR (R = -0.8, p = 0.02), SpO₂ (R = -0.69, p = 0.01), SBP (R = -0.89, p = 0.02), and DBP (R = -0.68, p = 0.03). Such associations were absent in CTL. The adenosine release during apnea was associated with blood O₂ saturation and cardiovascular parameters in trained divers but not in controls. These data therefore suggest that adenosine may play a major role in the adaptive cardiovascular response to apnea and could reflect the level of training.

Adenosine and the Cardiovascular System: The Good and the Bad

Adenosine is a nucleoside that impacts the cardiovascular system via the activation of its membrane receptors, named A₁R, A_2AR, A_2BR and A₃R. Adenosine is released during hypoxia, ischemia, beta-adrenergic stimulation or inflammation and impacts heart rhythm and produces strong vasodilation in the systemic, coronary or pulmonary vascular system. This review summarizes the main role of adenosine on the cardiovascular system in several diseases and conditions. Adenosine release participates directly in the pathophysiology of atrial fibrillation and neurohumoral syncope. Adenosine has a key role in the adaptive response in pulmonary hypertension and heart failure, with the most relevant effects being slowing of heart rhythm, coronary vasodilation and decreasing blood pressure. In other conditions, such as altitude or apnea-induced hypoxia, obstructive sleep apnea, or systemic hypertension, the adenosinergic system activation appears in a context of an adaptive response. Due to its short half-life, adenosine allows very rapid adaptation of the cardiovascular system. Finally, the effects of adenosine on the cardiovascular system are sometimes beneficial and other times harmful. Future research should aim to develop modulating agents of adenosine receptors to slow down or conversely amplify the adenosinergic response according to the occurrence of different pathologic conditions.

Adenosine A1 receptor-dependent antinociception induced by inosine in mice: pharmacological, genetic and biochemical aspects

Inosine is an endogenous nucleoside that has anti-inflammatory and antinociceptive properties. Inosine is a metabolite of adenosine, and some of its actions suggest the involvement of adenosine A1 receptors (A1Rs). The purpose of this study was to better understand mechanisms of inosine-induced antinociception by investigating the role of A1Rs and purine metabolism inhibitors. Inosine antinociception was evaluated using the formalin test in mice. An A1R-selective antagonist (DPCPX), A1R knockout mice (gene deletion) and mice with A1R reduced expression (antisense oligonucleotides) were used to assess the role of A1Rs in the antinociceptive action of inosine. Binding assays were performed to compare the affinity of inosine and adenosine for A1Rs. Finally, the role of adenosine and inosine breakdown was assessed using deoxycoformycin (DCF) and forodesine (FDS) as enzymatic inhibitors of adenosine deaminase and purine nucleoside phosphorylase, respectively. Inosine induced antinociception in the formalin test when given by systemic, spinal and peripheral routes. Systemically, inosine exhibited a potency similar to adenosine, and its effects were inhibited by DPCPX. Inosine did not induce antinociception in A1R knockout mice or in mice with reduced A1R expression. In binding studies, inosine bound to A1Rs with an affinity similar to adenosine. DCF had no effect on inosine actions. FDS augmented the antinociceptive effect of a low systemic dose of inosine and, at a higher dose, induced antinociception by itself. Collectively, these data indicate that inosine is an agonist for A1Rs with antinociceptive properties and a potency similar to adenosine and can be considered another endogenous ligand for this receptor.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

The effect of allosteric modulators on the kinetics of agonist-G protein-coupled receptor interactions in single living cells

Allosteric binding sites on adenosine -A(1) and -A(3) receptors represent attractive therapeutic targets for amplifying, in a spatially and temporally selective manner, the tissue protective actions of endogenous adenosine. This study has directly quantified the kinetics of agonist/G protein-coupled receptor interactions at the single-cell level, reflecting the physiological situation in which intracellular signaling proteins can exert major allosteric effects on agonist-receptor interactions. The association and dissociation rate constants at both A(1) and A(3) receptors, and therefore the affinity of the fluorescent adenosine derivative ABA-X-BY630 (structure appears in J Med Chem 50:782-793, 2007), were concentration-independent. The equilibrium dissociation constants of ABA-X-BY630 at A(1) and A(3) receptors were approximately 50 and 10 nM, respectively, suggesting that, even in live cells, low agonist concentrations predominantly detect high-affinity receptor states. At A(1) receptors, the dissociation of ABA-X-BY630 (30 nM) was significantly faster in the absence (k(off) = 1.95 +/- 0.09 min(-1)) compared with the presence of the allosteric enhancer (2-amino-4,5-dimethyl-3-thienyl)(3-(trifluoromethyl)phenyl)-methanone (PD81,723; 10 microM; k(off) = 0.80 +/- 0.03 min(-1)) and allosteric inhibitor 4-methoxy-N-(7-methyl-3-(2-pyridinyl)-1-isoquinolinyl)benzamide (VUF5455; 1 microM; k(off) = 1.48 +/- 0.16 min(-1)). In contrast, ABA-X-BY630 dissociation from A(3) receptors was significantly slower in the absence (k(off) = 0.78 +/- 0.18 min(-1)) than in the presence of the allosteric inhibitors VUF5455 (1 microM; k(off) = 3.15 +/- 0.12 min(-1)) and PD81,723 (10 microM; k(off) = 2.46 +/- 0.18 min(-1)). An allosteric mechanism of action has previously not been identified for PD81,723 at the A(3) receptor or VUF5455 at the A(1) receptor. Furthermore, the marked enhancement in fluorescent agonist dissociation by VUF5455 in living cells contrasts previous observations from broken cell preparations and emphasizes the need to study the allosteric regulation of agonist binding in living cells.

Exploring the mechanism of agonist efficacy: a relationship between efficacy and agonist dissociation rate at the muscarinic M3 receptor

Although there are several empirical approaches that enable the comparison of relative agonist efficacy, the molecular basis that underlies differences in the ability of G protein-coupled receptor agonists to elicit a response is still largely unexplained. Several models have been described that incorporate the kinetics of receptor-mediated initiation of the G protein cycle, but these have not directly addressed the influence of agonist-binding kinetics. To test this, we investigated the relationship between the efficacy of seven M(3) muscarinic receptor agonists and their rate of dissociation (k(off)) from the M(3) receptor. The association and dissociation rate constants of the agonists were determined using a l-[N-methyl]-[(3)H]scopolamine methyl chloride competition binding assay in the presence of GTP. The agonists displayed a range of association and dissociation rates. Relative agonist efficacy was measured at two points after M(3) receptor activation: the stimulation of guanosine 5'-O-(3-[(35)S]thio)triphosphate binding to G alpha subunits, and the subsequent increase in intracellular calcium levels. These experiments revealed a range of intrinsic efficacy, from the low-efficacy pilocarpine and oxotremorine to high-efficacy acetylcholine. There was no relationship between agonist efficacy and the equilibrium binding affinity of each agonist (K(d)). When efficacy was compared with the dissociation rate constant, however, the two were highly correlated, suggesting a relationship between the duration of agonist binding at the receptor and the intrinsic efficacy. These data suggest that kinetic models incorporating the mean lifetime of specific complexes will be required to fully explain the nature of agonist efficacy.

Effects of urea pretreatment on the binding properties of adenosine A1 receptors

The effect of denaturation and/or extraction of nonintegral membrane proteins by 7 M urea on the binding of the antagonist [3H]cyclopentyl-1,3-dipropylxanthine 8 dipropyl-2,3 ([3H]DPCPX), and the agonists adenosine, (-)-N6-(2-phenylisopropyl)-adenosine (R-PIA) and N6-cyclohexyladenosine (CHA), was investigated at human A1 adenosine receptors stably expressed in CHO cells. Pretreatment with urea caused a 56% reduction in membrane proteins. Compared to controls, the use of adenosine deaminase (ADA), 100 microM 5'-guanylylimidodiphosphate (Gpp(NH)p) or urea each caused equivalent increases in specific [3H]DPCPX binding. Neither the binding kinetics nor the affinity of [3H]DPCPX were significantly different in urea-pretreated compared to ADA-pretreated membranes. At 25 degrees C in ADA-pretreated membranes, the competition isotherms for R-PIA and CHA were characterized by two affinity states. Gpp(NH)p (100 microM) reduced, but did not abolish, the value of the high-affinity dissociation constant. Similar results were obtained after treatment with urea for R-PIA, whereas the high-affinity state for CHA was abolished. At 37 degrees C, urea pretreatment, but not 100 microM Gpp(NH)p, abolished high-affinity agonist competition binding. There was no significant effect of any of the treatments on the low-affinity agonist binding state. In urea-pretreated membranes, exogenously added adenosine competed according to a simple mass-action model with a pK(L) of 5.66+/-0.05 (n=3). Compared to the more common approaches of ADA treatment and/or use of guanine nucleotides, our findings suggest that urea pretreatment represents an inexpensive and useful approach for investigating the binding properties of adenosine A1 ligands (including adenosine) to the G protein-uncoupled form of the receptor.

Modulating effect of adenosine deaminase on function of adenosine A1 receptors

AIM

To study the modulating effect of adenosine deaminase (ADA) on the adenosine A1 receptor (A1R) in HEK293 cells stably expressing the human A1R.

METHODS

cDNA was amplified by RT-PCR using total RNA from human embryo brain tissue as the template. The PCR products were subcloned into the plasmid pcDNA3 and cloned into the plasmid pcDNA3.1. The cloned A1R cDNA was sequenced and stably expressed in HEK293 cells. The modulating effect of adenosine deaminase on A1R was studied by using [3H]DPCPX binding assay and an intracellular calcium assay.

RESULTS

HEK293 cells stably expressing human A1R were obtained. Saturation studies showed that the K(D) value and B(max) value of [3H]DPCPX were 1.6+/-0.2 nmol/L and 1.819+/-0.215 nmol/g of protein respectively, in the absence of ecto-ADA respectively, and 1.3+/-0.2 nmol/L and 1.992+/-0.130 nmol/g of protein in the presence of ecto-ADA respectively, suggesting that the K(D) value and B(max) value of [3H]DPCPX were unaffected by ecto-ADA. In the case of [3H]DPCPX competition curves obtained from intact cells or membranes, A1R agonist CCPA/[3H]DPCPX competition curve could be fitted well to a one-site model in the absence of ecto-ADA and a two-site model in the presence of ecto-ADA with a K(H) value of 0.74 (0.11+/-4.8) nmol/L (intact cells) or 1.8 (0.25+/-10) nmol/L (membrane) and a K(L) value of 0.94 (0.62+/-1.41) micromol/L (intact cells) or 0.77 (0.29+/-0.99) micromol/L (membrane). The K(L) value is not significantly different from the IC50 value of 0.84(0.57+/-1.23) micromol/L (intact cells) or 0.84 (0.63+/-1.12) micromol/L (membrane) obtained in the absence of ecto-ADA. Similar results were obtained from the CPA/[3H]DPCPX competition curve in the absence or presence of ecto-ADA on intact cells or membranes. Intracellular calcium assay demonstrated that the EC50 value of CPA were 10 (5+/-29) nmol/L and 94 (38+/-229) nmol/L in the presence or absence of ecto-ADA, respectively.

CONCLUSION

A1R stably expressed in the HEK293 cells display a low affinity for agonists in the absence of ADA and high and low affinities for agonists in the presence of ADA. The presence of ADA may promote the signaling through the adenosine A1 receptor in HEK293 cells.

Interactions among adenosine deaminase, adenosine A(1) receptors and dopamine D(1) receptors in stably cotransfected fibroblast cells and neurons

The role of adenosine deaminase in the interactions between adenosine A(1) and dopamine D(1) receptors was studied in a mouse fibroblast cell line stably cotransfected with human D(1) receptor and A(1) receptor cDNAs (A(1)D(1) cells). Confocal laser microscopy analysis showed a high degree of adenosine deaminase immunoreactivity on the membrane of the A(1)D(1) cells but not of the D(1) cells (only cotransfected with human D(1) receptor cDNAs). In double immunolabelling experiments in A(1)D(1) cells and cortical neurons a marked overlap in the distribution of the A(1) receptor and adenosine deaminase immunoreactivities and of the D(1) receptor and adenosine deaminase immunoreactivities was found. Quantitative analysis of A(1)D(1) cells showed that adenosine deaminase immunoreactivity to a large extent colocalizes with A(1) and D(1) receptor immunoreactivity, respectively. The A(1) receptor agonist caused in A(1)D(1) cells and in cortical neurons coaggregation of A(1) receptors and adenosine deaminase, and of D(1) receptors and adenosine deaminase. The A(1) receptor agonist-induced aggregation was blocked by R-deoxycoformycin, an irreversible adenosine deaminase inhibitor. The competitive binding experiments with the D(1) receptor antagonist [(3)H]SCH-23390 showed that the D(1) receptors had a better fit for two binding sites for dopamine, and treatment with the A(1) receptor agonist produced a disappearance of the high-affinity site for dopamine at the D(1) receptor. R-Deoxycoformycin treatment, which has previously been shown to block the interaction between adenosine deaminase and A(1) receptors, and which is crucial for the high-affinity state of the A(1) receptor, also blocked the A(1) receptor agonist-induced loss of high-affinity D(1) receptor binding. The conclusion of the present studies is that the high-affinity state of the A(1) receptor is essential for the A(1) receptor-mediated antagonistic modulation of D(1) receptors and for the A(1) receptor-induced coaggregates of A(1) and adenosine deaminase, and of D(1) and adenosine deaminase. Thus, the confocal experiments indicate that both A(1) and D(1) receptors form agonist-regulated clusters with adenosine deaminase, where the presence of a structurally intact adenosine deaminase bound to A(1) receptors is important for the A(1)-D(1) receptor-receptor interaction at the level of the D(1) receptor recognition.

Comparison of the potency of adenosine as an agonist at human adenosine receptors expressed in Chinese hamster ovary cells

The potency of adenosine and inosine as agonists at human adenosine receptors was examined in a functional assay using changes in cyclic AMP (cAMP) formation in intact Chinese hamster ovary (CHO) cells stably transfected with the human A1, A2A, A2B, and A3 receptors. Adenosine increased cAMP formation in cells expressing the A2A (EC(50): 0.7 microM) and A2B (EC(50): 24 microM) receptors and inhibited forskolin (0.3-3 microM)-stimulated cAMP formation in cells expressing the A1 (EC(50): 0.31 microM) and A3 receptors (EC(50): 0.29 microM). The potency of adenosine at the A2A and A2B receptors was not altered by the presence of the uptake inhibitor nitrobenzylthioinosine (NBMPR), whereas it was increased about 6-fold by NBMPR at the A1 and A3 receptors. In the presence of NBMPR, inosine was a potent agonist (EC(50): 7 and 0.08 microM at the A1 and A3 receptors, respectively), but with low efficacy especially at the A3 receptors. No effect of inosine was seen at the A(2) receptors. Caffeine, theophylline, and paraxanthine shifted the dose-response curve for adenosine at the A1, A2A, and A2B receptors. These results indicate that adenosine is the endogenous agonist at all human adenosine receptors and that physiological levels of this nucleoside can activate A1, A2A, and A3 receptors on cells where they are abundantly expressed, whereas pathophysiological conditions are required to stimulate A2B receptors to produce cyclic AMP.