Non-selective Adenosine Receptor Agonist Research Articles

Regadenoson Stress Testing: A Comprehensive Review With a Focused Update.

Regadenoson is a pharmacological stress agent that has been widely used since its approval by the Food and Drug Administration (FDA) in 2008. For many years, dipyridamole and adenosine, which are non-selective adenosine receptor agonists, were more popular. However, these agents are less preferred now due to their undesirable adverse effects as compared to regadenoson. In the ADVANCE (ADenoscan Versus regAdenosoN Comparative Evaluation) phase 3 clinical trial, regadenoson demonstrated non-inferiority to adenosine for detecting reversible myocardial ischemia. This review summarizes the clinical utilities of regadenoson as the most widely used pharmacological stress agent. Moreover, the use of regadenoson has been documented in specific patient populations. Although regadenoson has established safety and efficacy in most patients with chronic diseases, there are equivocal results in the literature for other chronic diseases. It is warranted to highlight that the use of regadenoson has not been studied in patients of low socioeconomic class; it is a condition that carries a significant burden on the cardiovascular system.

The antioxidant resveratrol acts as a non-selective adenosine receptor agonist

Resveratrol (RSV) is a natural polyphenolic antioxidant with a proven protective role in several human diseases involving oxidative stress, although the molecular mechanism underlying this effect remains unclear. The present work tried to elucidate the molecular mechanism of RSV's role on signal transduction modulation. Our biochemical analysis, including radioligand binding, real time PCR, western blotting and adenylyl cyclase activity, and computational studies provide insights into the RSV binding pathway, kinetics and the most favored binding pose involving adenosine receptors, mainly A2A subtype. In this study, we show that RSV target adenosine receptors (AdoRs), affecting gene expression, receptor levels, and the downstream adenylyl cyclase (AC)/PKA pathway. Our data demonstrate that RSV activates AdoRs. Moreover, RSV activate A2A receptors by directly binding to the classical orthosteric binding site. Intriguingly, RSV-induced receptor activation can stimulate or inhibit AC activity depending on concentration and exposure time. Such subtle and multifaceted regulation of the AdoRs/AC/PKA pathway might contribute to the protective role of RSV. Our findings suggest that RSV molecular action is mediated, at least in part, by activation of adenosine receptors and create the opportunity to interrogate the therapeutic use of RSV in pathological conditions involving AdoRs, such as Alzheimer.

Adenosine receptor agonists deepen the inhibition of platelet aggregation by P2Y12 antagonists

Several adenosine receptor (AR) agonists have been shown in the past to possess anti-platelet potential; however, the adjunctive role of AR agonists in anti-platelet therapy with the use of P2Y12 receptor inhibitors has not been elucidated so far.This in vitro aggregation-based study investigates whether the inhibition of platelet function mediated by cangrelor or prasugrel metabolite can be potentiated by AR agonists. It evaluates the effect of non-selective (2-chloroadenosine), A2A-selective (UK 432097, MRE 0094, PSB 0777) and A2B-selective AR agonists (BAY 60-6583) on platelet function in relation to their toxicity, specificity towards adenosine receptor subtypes, structure and solubility.UK 432097, 2-chloroadenosine, MRE 0094 and PSB 0777 were found to be more or less potent inhibitors of ADP-induced platelet aggregation when acting alone, and that they remained non-cytotoxic to the cells. These AR agonists were also effective in the potentiation of the effects exerted by P2Y12 antagonists. Considering the estimated IC50 value, UK 432097, showing a relatively high binding affinity to the A2A adenosine receptor, has been identified as the most potent anti-aggregatory agent. This compound diminished platelet aggregation at nanomolar concentrations and further augmented platelet inhibition by P2Y12 antagonists by approx. 60% (P < .01).Our results indicate the importance of adenosine receptors as therapeutic targets and point out challenges and potential benefits of therapeutic use of a combined therapy of P2Y12 antagonist and AR agonist in cardioprotection. Our comparative analysis of the effects of AR agonists on platelet response in plasma and whole blood may indirectly suggest that other blood morphology elements contribute little to the inhibition of platelet function by AR agonists.

Capadenoson, a clinically trialed partial adenosine A1 receptor agonist, can stimulate adenosine A2B receptor biased agonism

The adenosine A2B receptor (A2BAR) has been identified as an important therapeutic target in cardiovascular disease, however in vitro and in vivo targeting has been limited by the paucity of pharmacological tools, particularly potent agonists. Interestingly, 2-((6-amino-3,5-dicyano-4-(4-(cyclopropylmethoxy)phenyl)-2-pyridinyl)thio)acetamide (BAY60-6583), a potent and subtype-selective A2BAR agonist, has the same core structure as 2-amino-6-[[2-(4-chlorophenyl)-1,3-thiazol-4-yl]methylsulfanyl]-4-[4-(2-hydroxyethoxy)phenyl]pyridine-3,5-dicarbonitril (capadenoson). Capadenoson, currently classified as an adenosine A1 receptor (A1AR) partial agonist, has undergone two Phase IIa clinical trials, initially in patients with atrial fibrillation and subsequently in patients with stable angina. Capadenoson has also been shown to decrease cardiac remodeling in an animal model of advanced heart failure and a capadenoson derivative, neladenoson bialanate, recently entered clinical development for the treatment of chronic heart failure. The therapeutic effects of capadenoson are currently thought to be mediated through the A1AR. However, the ability of capadenoson to stimulate additional adenosine receptor subtypes, in particular the A2BAR, has not been rigorously assessed. In this study, we demonstrate that capadenoson does indeed have significant A2BAR activity in physiologically relevant cells, cardiac fibroblasts and cardiomyocytes, which endogenously express the A2BAR. Relative to the non-selective adenosine receptor agonist NECA, capadenoson was a biased A2BAR agonist with a preference for cAMP signal transduction over other downstream mediators in cells with recombinant and endogenous A2BAR expression. These findings suggest the reclassification of capadenoson as a dual A1AR/A2BAR agonist. Furthermore, a potential A2BAR contribution should be an important consideration for the future clinical development of capadenoson-like therapeutics, as the A2BAR can promote cardioprotection and modulate cardiac fibrosis in heart disease.

Role of A1 and A2A adenosine receptor agonists in adipose tissue inflammation induced by obesity in mice

Adenosine receptors are expressed in adipose tissue and control physiological and pathological events such as lipolysis and inflammation. The aim of this study was to evaluate the activity of N6-cyclopentyladenosine (CPA), a potent and selective A1 adenosine receptor agonist; 2-p-(2-carboxyethyl)phenethylamino-5′-N-ethylcarboxyamidoadenosine hydrochloride (CGS-21680), an A2A adenosine receptor agonist; and 5′-N-ethylcarboxamidoadenosine (NECA), a potent non-selective adenosine receptor agonist on adipose tissue inflammatory alterations induced by obesity in mice. Swiss mice were fed with a high-fat diet for 12 weeks and agonists were administered in the last two weeks. Body weight, adiposity and glucose homeostasis were evaluated. Inflammation in adipose tissue was assessed by evaluation of adipokine production and macrophage infiltration. Adenosine receptor signaling in adipose tissue was also evaluated. Mice that received CGS21680 presented an improvement in glucose homeostasis in association with systemically reduced inflammatory markers (TNF-α, PAI-1) and in the visceral adipose tissue (TNF-α, MCP-1, macrophage infiltration). Activation of p38 signaling was found in adipose tissue of this group of mice. NECA-treated mice presented some improvements in glucose homeostasis associated with an observed weight loss. Mice that received CPA presented only a reduction in the ex vivo basal lipolysis rate measured within visceral adipose tissue. In conclusion, administration of the A2A receptor agonist to obese mice resulted in improvements in glucose homeostasis and adipose tissue inflammation, corroborating the idea that new therapeutics to treat obesity could emerge from these compounds.

Roles of Endoplasmic Reticulum Stress in NECA-Induced Cardioprotection against Ischemia/Reperfusion Injury.

Objective This study aimed to investigate whether the nonselective A2 adenosine receptor agonist NECA induces cardioprotection against myocardial ischemia/reperfusion (I/R) injury via glycogen synthase kinase 3β (GSK-3β) and the mitochondrial permeability transition pore (mPTP) through inhibition of endoplasmic reticulum stress (ERS). Methods and Results H9c2 cells were exposed to H2O2 for 20 minutes. NECA significantly prevented H2O2-induced TMRE fluorescence reduction, indicating that NECA inhibited the mPTP opening. NECA blocked H2O2-induced GSK-3β phosphorylation and GRP94 expression. NECA increased GSK-3β phosphorylation and decreased GRP94 expression, which were prevented by both ERS inductor 2-DG and PKG inhibitor KT5823, suggesting that NECA may induce cardioprotection through GSK-3β and cGMP/PKG via ERS. In isolated rat hearts, both NECA and the ERS inhibitor TUDCA decreased myocardial infarction, increased GSK-3β phosphorylation, and reversed GRP94 expression at reperfusion, suggesting that NECA protected the heart by inhibiting GSK-3β and ERS. Transmission electron microscopy showed that NECA and TUDCA reduced mitochondrial swelling and endoplasmic reticulum expansion, further supporting that NECA protected the heart by preventing the mPTP opening and ERS. Conclusion These data suggest that NECA prevents the mPTP opening through inactivation of GSK-3β via ERS inhibition. The cGMP/PKG signaling pathway is responsible for GSK-3β inactivation by NECA.

Administration of caffeine inhibited adenosine receptor agonist-induced decreases in motor performance, thermoregulation, and brain neurotransmitter release in exercising rats

We examined the effects of an adenosine receptor agonist on caffeine-induced changes in thermoregulation, neurotransmitter release in the preoptic area and anterior hypothalamus, and endurance exercise performance in rats. One hour before the start of exercise, rats were intraperitoneally injected with either saline alone (SAL), 10mgkg−1 caffeine and saline (CAF), a non-selective adenosine receptor agonist (5ʹ-N-ethylcarboxamidoadenosine [NECA]: 0.5mgkg−1) and saline (NECA), or the combination of caffeine and NECA (CAF+NECA). Rats ran until fatigue on the treadmill with a 5% grade at a speed of 18mmin−1 at 23°C. Compared to the SAL group, the run time to fatigue (RTTF) was significantly increased by 52% following caffeine administration and significantly decreased by 65% following NECA injection (SAL: 91±14.1min; CAF: 137±25.8min; NECA: 31±13.7min; CAF+NECA: 85±11.8min; p<0.05). NECA decreased the core body temperature (Tcore), oxygen consumption, which is an index of heat production, tail skin temperature, which is an index of heat loss, and extracellular dopamine (DA) release at rest and during exercise. Furthermore, caffeine injection inhibited the NECA-induced decreases in the RTTF, Tcore, heat production, heat loss, and extracellular DA release. Neither caffeine nor NECA affected extracellular noradrenaline or serotonin release. These results support the findings of previous studies showing improved endurance performance and overrides in body limitations after caffeine administration, and imply that the ergogenic effects of caffeine may be associated with the adenosine receptor blockade-induced increases in brain DA release.

Angiotensin II stimulation alters vasomotor response to adenosine in mouse mesenteric artery: role for A1 and A2B adenosine receptors.

Stimulation of the A1 adenosine receptor and angiotensin II receptor type-1 (AT1 receptor) causes vasoconstriction through activation of cytochrome P450 4A (CYP4A) and ERK1/2. Thus, we hypothesized that acute angiotensin II activation alters the vasomotor response induced by the non-selective adenosine receptor agonist, NECA, in mouse mesenteric arteries (MAs). We used a Danish Myo Technology wire myograph to measure muscle tension in isolated MAs from wild type (WT), A1 receptor and A2B receptor knockout (KO) mice. Western blots were performed to determine the expression of AT1 receptors and CYP4A. Acute exposure (15 min) to angiotensin II attenuated the NECA-dependent vasodilatation and enhanced vasoconstriction. This vasoconstrictor effect of angiotensin II in NECA-treated MAs was abolished in A1 receptor KO mice and in WT mice treated with the A1 receptor antagonist DPCPX, CYP4A inhibitor HET0016 and ERK1/2 inhibitor PD98059. In MAs from A2B receptor KO mice, the vasoconstrictor effect of angiotensin II on the NECA-induced response was shown to be dependent on A1 receptors. Furthermore, in A2B receptor KO mice, the expression of AT1 receptors and CYP4A was increased and the angiotensin II-induced vasoconstriction enhanced. In addition, inhibition of KATP channels with glibenclamide significantly reduced NECA-induced vasodilatation in WT mice. Acute angiotensin II stimulation enhanced A1 receptor-dependent vasoconstriction and inhibited A2B receptor-dependent vasodilatation, leading to a net vasoconstriction and altered vasomotor response to NECA in MAs. This interaction may be important in the regulation of BP.

A2B adenosine receptor activation switches differentiation of bone marrow cells to a CD11c(+)Gr-1(+) dendritic cell subset that promotes the Th17 response.

Adenosine is one of the major molecules associated with inflammation. We have previously reported that an adenosine receptor (AR) agonist has an enhancing effect on Th17 autoimmune responses, even though it suppressed Th1 responses. To determine the mechanism involved, we have examined the effect of AR agonists on mouse bone marrow dendritic cell (BMDC) differentiation and function. We show that mouse bone marrow cells (BMCs) differentiated into CD11c(+)Gr-1(+) dentritic cells (DCs) when cultured in granulocyte macrophage colony-stimulating factor (GM-CSF)-containing medium containing an AR agonist. The non-selective AR agonist NECA and an A2BR-specific agonist had a similar effect, and the effect of NECA could be blocked by an A2BR-specific antagonist. Unlike CD11c(+)Gr-1(-) BMDCs, which have a greater stimulatory effect on Th1 T cells than Th17 cells, CD11c(+)Gr-1(+) BMDCs had a greater stimulatory effect on Th17 autoreactive T cells than on Th1 autoreactive T cells and this effect depended on γδ T cell activation.

Adenosine A2B receptor stimulates angiogenesis by inducing VEGF and eNOS in human microvascular endothelial cells.

Angiogenesis is critical to wound repair due to its role in providing oxygen and nutrients that are required to support the growth and function of reparative cells in damaged tissues. Adenosine receptors are claimed to be of paramount importance in driving wound angiogenesis by inducing VEGF. However, the underlying mechanisms for the regulation of adenosine receptors in VEGF as well as eNOS remain poorly understood. In the present study, we found that adenosine and the non-selective adenosine receptor agonists (NECA) induced tube formation in HMEC-1 in a dose-dependent manner. Adenosine or NECA (10 µmol/L) significantly augmented the number and length of the segments in comparison with the control. Simultaneously, VEGF and eNOS were significantly upregulated following the administration of 10 µmol/L NECA, while they were suppressed after A2B AR genetic silencing and pharmacological inhibition by MRS1754. In addition, VEGF expression and eNOS bioavailability elimination significantly reduced the formation of capillary-like structures. Furthermore, the activation of A2B AR by NECA significantly increased the intracellular cAMP levels and concomitant CREB phosphorylation, eventually leading to the production of VEGF in HMEC-1. However, the activated PKA-CREB pathway seemed to be invalidated in the induction of eNOS. Moreover, we found that the elicited PI3K/AKT signaling in response to the induction of NECA assisted in regulating eNOS but failed to impact on VEGF generation. In conclusion, the A2B AR activation-driven angiogenesis via cAMP-PKA-CREB mediated VEGF production and PI3K/AKT-dependent upregulation of eNOS in HMEC-1.

Creatine, similarly to ketamine, affords antidepressant-like effects in the tail suspension test via adenosine A₁ and A2A receptor activation.

The benefits of creatine supplementation have been reported in a broad range of central nervous systems diseases, including depression. A previous study from our group demonstrated that creatine produces an antidepressant-like effect in the tail suspension test (TST), a predictive model of antidepressant activity. Since depression is associated with a dysfunction of the adenosinergic system, we investigated the involvement of adenosine A1 and A2A receptors in the antidepressant-like effect of creatine in the TST. The anti-immobility effect of creatine (1 mg/kg, po) or ketamine (a fast-acting antidepressant, 1 mg/kg, ip) in the TST was prevented by pretreatment of mice with caffeine (3 mg/kg, ip, nonselective adenosine receptor antagonist), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (2 mg/kg, ip, selective adenosine A1 receptor antagonist), and 4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo-{2,3-a}{1,3,5}triazin-5-yl-amino]ethyl)-phenol (ZM241385) (1 mg/kg, ip, selective adenosine A2A receptor antagonist). In addition, the combined administration of subeffective doses of creatine and adenosine (0.1 mg/kg, ip, nonselective adenosine receptor agonist) or inosine (0.1 mg/kg, ip, nucleoside formed by the breakdown of adenosine) reduced immobility time in the TST. Moreover, the administration of subeffective doses of creatine or ketamine combined with N-6-cyclohexyladenosine (CHA) (0.05 mg/kg, ip, selective adenosine A1 receptor agonist), N-6-[2-(3,5-dimethoxyphenyl)-2-(methylphenyl)ethyl]adenosine (DPMA) (0.1 mg/kg, ip, selective adenosine A2A receptor agonist), or dipyridamole (0.1 μg/mouse, icv, adenosine transporter inhibitor) produced a synergistic antidepressant-like effect in the TST. These results indicate that creatine, similarly to ketamine, exhibits antidepressant-like effect in the TST probably mediated by the activation of both adenosine A1 and A2A receptors, further reinforcing the potential of targeting the purinergic system to the management of mood disorders.

Photomodulation of G protein-coupled adenosine receptors by a novel light-switchable ligand.

The adenosinergic system operates through G protein-coupled adenosine receptors, which have become promising therapeutic targets for a wide range of pathological conditions. However, the ubiquity of adenosine receptors and the eventual lack of selectivity of adenosine-based drugs have frequently diminished their therapeutic potential. Accordingly, here we aimed to develop a new generation of light-switchable adenosine receptor ligands that change their intrinsic activity upon irradiation, thus allowing the spatiotemporal control of receptor functioning (i.e., receptor activation/inactivation dependent on location and timing). Therefore, we synthesized an orthosteric, photoisomerizable, and nonselective adenosine receptor agonist, nucleoside derivative MRS5543 containing an aryl diazo linkage on the N6 substituent, which in the dark (relaxed isomer) behaved as a full adenosine A3 receptor (A3R) and partial adenosine A2A receptor (A2AR) agonist. Conversely, upon photoisomerization with blue light (460 nm), it remained a full A3R agonist but became an A2AR antagonist. Interestingly, molecular modeling suggested that structural differences encountered within the third extracellular loop of each receptor could modulate the intrinsic, receptor subtype-dependent, activity. Overall, the development of adenosine receptor ligands with photoswitchable activity expands the pharmacological toolbox in support of research and possibly opens new pharmacotherapeutic opportunities.

A2B-mediated endothelial cell proliferation is associated with decoupling of the NO and VEGF signaling pathway during pre-eclampsia.

A2B-mediated endothelial cell proliferation is associated with decoupling of the NO and VEGF signaling pathway during pre-eclampsia.

Pharmacological postconditioning of the rabbit heart with non-selective, A1 , A2A and A3 adenosine receptor agonists.

We investigated the effects of novel selective and non-selective adenosine receptor agonists (ARs) on cardioprotection. Male rabbits divided into six groups were subjected to 30-min heart ischaemia and 3-h reperfusion: (1) control group, (2) postconditioning (PostC) group, (3) group A: treated with the non-selective agonist (S)-PHPNECA, (4) group B: treated with the A1 agonist CCPA, (5) group C: treated with the A2A agonist VT 7 and (6) group D: treated with the A3 agonist AR 170. The infarcted (I) and the areas at risk (R) were estimated as %I/R. In additional rabbits of all groups, heart samples were taken for determination of Akt, eNOS and STAT 3 at the 10th reperfusion minute. (S)-PHPNECA and CCPA reduced the infarct size (17.2 ± 2.9% and 17.9 ± 2.0% vs 46.8 ± 1.9% in control, P < 0.05), conferring a benefit similar to PostC (26.4 ± 0.3%). Selective A2A and A3 receptor agonists did not reduce the infarct size (39.5 ± 0.8% and 38.7 ± 3.5%, P = NS vs control). Akt, eNOS and STAT 3 were significantly activated after non-selective A1 ARs and PostC. Non-selective and A1 but not A2A and A3 ARs agonists are essential for triggering cardioprotection. The molecular mechanism involves both RISK and the JAK/STAT pathways.

Increased basal and adenosine‐mediated coronary flow in ex vivo hearts from type I diabetic mice (1051.16)

Diabetes is known to cause morphological and functional changes in the cardiovascular system, resulting in abnormal responses to various stimuli such as adenosine. Our aim was to examine the functional changes in coronary flow (CF) to adenosine receptor (AR) agonists resulting from type I diabetes. Diabetes was induced in mice using streptozotocin (STZ, 10mg/Kg; 8 weeks) and concentration response curves to the non‐selective AR agonist NECA and to the A2A AR selective agonist (CGS‐21680; CGS) were performed in isolated perfused hearts from STZ and vehicle (C) group. Diabetes resulted in a decrease in heart weight (0.093g ± 0.002 in STZ vs. 0.117g ± 0.004 in C; p&lt;0.05), an increase in blood glucose level (510.8mg/dL ± 22.2 in STZ vs. 140.9mg/dL ± 9.4 in C; p&lt;0.05) and an increase in the basal coronary flow (21.6 ± 1.5 in STZ vs. 16.1 ± 0.6 in C; p&lt;0.05). Both NECA and CGS induced a concentration‐dependent increase in CF (ml.min‐1.mg‐1) which was significantly increased in STZ compared to C (NECA: Emax 39.8 ± 1.8 in STZ vs. 28.2 ± 0.7 in C; p&lt;0.05) (CGS; Emax 38.8 ± 1 in STZ vs. 28.3 ± 1.8 in C; p&lt;0.05). No significant differences were observed between the groups for changes in both heart rate and left ventricular diastolic pressure to both agonists. Our data suggest that STZ‐induced diabetes resulted in an increase in basal CF as well as increased vasodilatory response to adenosine agonists, probably by upregulating A2A receptor signaling.Grant Funding Source: Supported by HL 094447 and HL 027339

The Adenosine Receptor Agonist 5’-&lt;i&gt;N&lt;/i&gt;-Ethylcarboxamide-Adenosine Increases Glucose 6-Phosphatase Expression and Gluconeogenesis

Intraperitoneal administration of the non-selective adenosine receptor agonist 5’-N-ethylcarboxamide-adenosine (NECA) (0.1 or 0.3 mg/kg) increased fasting serum glucose levels in mice. To clarify the mechanism responsible for this, the expression of liver glucose 6-phosphatase (G6Pase: a gluconeogenic enzyme) was analyzed, and it was found that G6Pase mRNA was increased by NECA treatment. Administration of 0.3 mg/kg NECA resulted in elevated serum glucose levels at 1 h and were further elevated at 6 h. Administration of 0.1 mg/kg NECA increased serum glucose levels at 1 h and had returned to control levels by 6 h. The increase in fasting serum glucose levels induced by NECA are thought to be caused, in part, by elevated G6Pase expression.

Adenosine increases LPS-induced nuclear factor kappa B activation in smooth muscle cells via an intracellular mechanism and modulates it via actions on adenosine receptors

In inflamed and damaged cardiovascular tissues, local extracellular adenosine concentrations increase coincidentally with activation of the transcription factor nuclear factor kappa B (NFκB). To investigate whether adenosine influences NFκB activation in vascular smooth muscle cells (VSMCs) and, if so, to examine the role of its receptors. VSMCs were isolated from NFκB-luciferase reporter mice, cultured and then treated by lipopolysaccharide (LPS) to activate NFκB signalling. Adenosine, adenosine receptor agonists and antagonists, adenosine deaminase and uptake inhibitors were used together with LPS to evaluate the role of adenosine and its receptors on NFκB activation, which was assessed by luciferase activity and NFκB target gene expression. Adenosine potentiated LPS-induced NFκB activation. This was dependent on adenosine uptake and enhanced by an adenosine deaminase inhibitor, suggesting that intracellular adenosine plays an important role. Non-selective adenosine receptor agonists (2Cl-Ado and NECA) inhibited NFκB activation induced by LPS. Selective A1 or A2A antagonist given alone could not completely antagonize the NECA effect, indicating that the inhibitory effect was due to multiple adenosine receptors. The activation of the A3 receptor further increased LPS-induced NFκB activation. Adenosine increases LPS-induced nuclear factor kappa B activation in smooth muscle cells via an intracellular mechanism and decreases it via actions on A1 and A2A receptors. These results provide novel insights into the role of adenosine as a regulator of inflammation-induced NFκB activation.

Acute Myocardial Infarction During Regadenoson Myocardial Perfusion Imaging

Pharmacologic stress testing uses vasodilators to provide objective evidence of myocardial ischemia. Adenosine and dipyridamole are nonselective adenosine receptor agonists that have been associated with myocardial infarction (MI) during intravenous infusion. Mechanisms postulated for this effect include coronary steal, transmural steal, global hypotension, and direct vasoconstriction. Regadenoson, a direct A2A agonist, was approved for use in stress testing in 2008. We describe a 68-year-old man who presented to our institution with typical angina, relieved by nitroglycerin. He did not have electrocardiogram (ECG) changes suggestive of myocardial pathology, and laboratory testing did not reveal a significant rise in troponin-I levels. To further assess the etiology of his symptoms, he underwent a pharmacologic stress test with regadenoson followed by technetium 99 m sestamibi. Six minutes after regadenoson infusion, the patient developed severe retrosternal chest pain accompanied by ST elevations on ECG. Sublingual nitroglycerin was administered that resolved both the pain and ECG changes. The patient subsequently underwent urgent coronary angiography and was found to have a 95% critical stenosis involving the left anterior descending artery. We conclude this case represents a MI secondary to coronary steal phenomenon induced by regadenoson infusion. Clinicians should be aware this adverse effect can occur despite the improved side-effect profile of regadenoson. Continuous monitoring of vital signs and the ECG with regular assessment of symptoms is imperative to identify this rare but potentially devastating adverse event.

Impaired A2A adenosine receptor/nitric oxide/VEGF signaling pathway in fetal endothelium during late- and early-onset preeclampsia

To investigate whether fetal endothelial cell proliferation and migration are modulated by the A2A adenosine receptor (A2AAR), nitric oxide (NO) and the vascular endothelial growth factor (VEGF) signaling pathway, we isolated human umbilical vein endothelial cells from normal pregnancy (n = 23), preterm delivery (n = 4), and late-onset (LOPE, n = 10) and early-onset preeclampsia (EOPE, n = 8). We used the non-selective adenosine receptor agonist (NECA) and the selective agonist (CGS-21680) and/or selective antagonist (ZM-241385) for A2AAR. Also, the nitric oxide synthase (NOS) inhibitor, L-NAME, was used in co-incubation with CGS-21680. Compared to normal pregnancy, EOPE exhibited low cell proliferation and migration associated with reduced expressions of A2AAR and VEGF and NO synthesis (i.e., total and phosphorylated serine(1177) endothelial NOS and nitrite formation). In contrast, LOPE exhibited the opposite behavior in all these markers compared to normal pregnancy or EOPE. Cell proliferation and migration were increased by CGS-21680 (or NECA) in all analyzed groups (EOPE>LOPE>normal pregnancy) compared to their respective basal conditions, an effect that was associated with high NO and VEGF synthesis and blocked by ZM-241385 with significantly different IC50 for each group (EOPE>LOPE>normal pregnancy). The differences seem independent of gestational age. L-NAME blocked the CGS-21680-mediated cell proliferation and migration in normal pregnancy and LOPE (IC50 = 36.2 ± 2.5 and 8.6 ± 2.2 nM, respectively) as well as the VEGF expression in normal pregnancy. Therefore, the A2AAR/NO/VEGF signaling pathway exhibits a pro-angiogenic effect in normal pregnancies and LOPE, whereas impairment in this pathway seems related to the reduced angiogenic capacity of the fetal endothelium in EOPE.

Impact and benefit of A2B-adenosine receptor agonists for the respiratory tract: mucociliary clearance, ciliary beat frequency, trachea muscle tonus and cytokine release

Adenosine is known to induce a bronchospasm in asthma- and COPD patients. The role of A(2B) receptors was investigated with respect to several parameters of the respiratory tract: tonus of smooth muscle, ciliary beat frequency as measured by high-speed video camera connected to a microscope (both in rats) and mucociliary clearance (MCC; transport of a fluorescent dye using a microdialysis procedure) in mice. NECA (5'-N-ethylcarboxamidoadenosine) (a non-selective adenosine receptor agonist) was able to acutely induce a contraction, which was reversed to a relaxation after repeated dosing. This relaxation was completely abolished by PSB-1115, an A(2B) receptor antagonist. IL-13 (cytokine) was not involved mediating acute contractility effects. MCC was increased by BAY 60-6583 (A(2B) receptor agonist) and NECA (counteracted by the A(2B) receptor antagonist PSB-1115). Activation of A(2B) adenosine receptors by BAY 60-6583 induced an increase of the ciliary beat frequency, which could be reduced by administration of PSB-1115. Several cytokines were increased by NECA although only some are relevant because they are not blocked by A(2B) receptor antagonism. The A(2B) receptors are involved in airway relaxation, MCC improvement and ciliary beat frequency. A(2B) receptor agonists may be of therapeutic value and should be developed.