ARs are members of the A-class of rhodopsin-like receptors, which bind to adenosine to produce their downstream actions. Based on the order of discovered and the distinct reactions upon activation, ARs are divided into four subtypes: A₁AR, A₃AR, A_2aAR, and A_2bAR, (Haskó et al., 2018; Sheth et al., 2014) (Table 1). A₁AR and A_2aAR are active in the nanomolar range, A_2bAR and A₃AR are active in micromolar concentrations, and A_2bAR has the lowest affinity for adenosine (Sheth et al., 2014; Jacobson and Gao, 2006). AC catalyzes the conversion of ATP to cAMP. A₁AR and A₃AR couple to Gα_i to inhibit AC activation, while A_2aAR and A_2bAR coupled to Gα_s to activate AC (Zhou et al., 2020). A_2bAR and A₃AR also mediate the activation of phospholipase C through activation of Gα_q, which increases diacylglycerol, inositol trisphosphate, and intracellular Ca²⁺. In addition, A₁AR can be coupled to the pertussis toxin-sensitive Gα_o protein coupled with the activation of ion channels. Through their modulation of cAMP and Ca²⁺ levels, ARs contribution to the cardiovascular system. Protein kinase A (PKA), exchange protein, and cyclic nucleotide-gated ion channels are the three main targets that cAMP activates (Borea et al., 2018). The phosphatidyl signaling pathway is a double messenger system that generates inositol triphosphate and diacylglycerol, which trigger a cascade reaction within the cell by releasing and interacting with calcium ions (Borea et al., 2018).

The mitogen-activated protein kinase (MAPK) family, which includes extracellular signal-regulated kinases, p38, and Jun NH₂ terminal kinases, can include the A₁AR in its intracellular phosphorylation cascade response process (Schulte and Fredholm, 2000; Schulte and Fredholm, 2003). Glycogen synthase kinase 3 is increased when the A₃AR is active because it lowers cAMP. β-catenin, cyclin D1, and c-myc will all be downregulated at the same time, and nuclear factor κB’s capacity to bind DNA will correspondingly decline. Additionally, the MAPK, nuclear factor-κB (NF-κB), and phosphatidylinositol-3-kinase-protein kinase B (PI3K-PKB/Akt) signaling pathways are all regulated by the A₃AR (Borea et al., 2015). The most prevalent effector in the event of A_2aAR activation is cAMP-dependent PKA, and the A_2aAR also regulates the MAPK signal. Notably, the A_2aAR may also be dependent on its lengthy COOH terminus. It interacts with auxiliary proteins such as α-actin, ubiquitin-specific protease, d₂-dopamine receptor, ADP-ribosylation factor nucleotide site opener, and Translin-related protein X (Baraldi et al., 2008; Chen et al., 2013). Likewise, the A_2bAR receptor has the ability to phosphorylate PKA and trigger the phosphorylation of c-Jun n-terminal kinase 1/2, extracellular signal-regulated kinase 1/2, and mitogen-activated protein kinase p38 (Merighi et al., 2017).

Atrial fibrillation (AF) is the most common arrhythmia, involving 1%–4% of the population, and prolonged AF can cause a series of adverse clinical outcomes such as stroke and heart failure (HF), with stroke being the most important cause of death and disability in AF (Andrade et al., 2020). ARs are therapeutic and diagnostic targets for AF.

In high-risk patients, AF can be induced by adenosine (Ip et al., 2013). The right atria of high-risk patients have shown significant expression of G-protein-coupled inwardly rectifying potassium channels and heterogeneous expression of A1R, indicating that localized reentry in the right atria is the cause of adenosine-induced AF (Li et al., 2016). In a canine model, adenosine-induced anomalies in atrioventricular node conduction and shortening of atrial repolarization were linked to elevated A1AR expression in the atrioventricular node and atrial cardiomyocytes, which raised the risk of atrial fibrillation (Lou et al., 2014). The isolated hearts of rats treated with [1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (PSB36) and [2-chloro-N ⁶-cyclopentyladenosine (CCPA) shortened and prolonged action potential duration at 90% of repolarization and effective refractory period, respectively, suggesting that antagonizing A₁AR with PSB36 prevented AF event and shortened AF duration (Soattin et al., 2020). A_2aAR and A_2bAR expression was significantly upregulated in the left atrium after AF surgery (Maille et al., 2021). A_2aAR was pro-arrhythmic by promoting spontaneous calcium release, whereas activation of A₃AR reduced A_2aAR-mediated spontaneous calcium release in human atrial myocytes (Tarifa et al., 2023). In addition, A_2bAR may be associated with the regulation of fibrosis in AF (Maille et al., 2022). The two-sided regulatory role of ARs is well illustrated by the fact that different subtypes of ARs exert or promote or inhibit the development of AF. In addition, the adenosinergic system has a diagnostic value for AF.

Pulmonary vein ectopic pulsation has been identified as a key AF trigger and maintenance mechanism, laying the theoretical foundation for pulmonary vein ablation for AF, but the rate of postoperative recurrence remains high (Haïssaguerre et al., 1998). Most of the recurrent cases were attributed to the recovery of concealed conduction between the pulmonary vein and the left atrium, which led to failure of pulmonary vein isolation. Following pulmonary vein isolation, the adenosine test provides a strong predictive value for AF recurrence. Arentz et al. were the first to report a possible association between adenosine-induced occult conduction in the pulmonary veins and AF recurrence (Arentz et al., 2004). In recent years, the adenosine test has been used clinically to screen for the restoration of pulmonary vein conduction, with the aim of guiding ablation strategies and improving success rates. Large and small-scale studies have shown that the adenosine testing can improve the success rate of ablation, but there are still many limitations in the use of the adenosine test in the post-ablation period: discrepancies between the evaluation of ablation styles and indices in different centers, the adenosine dosage and the time of administration have not yet been standardized as well as the drug’s adverse effects and the time-consuming operation, etc.,. (McLellan et al., 2017; Macle et al., 2015). Further high-quality studies are needed to clarify the specific clinical use and possible benefits of the adenosine tests.

Atherosclerosis (AS) is a progressive, long-term condition of large and medium-sized arteries marked by the formation of atherosclerotic plaques and is a major cause of cardiovascular disease (Libby, 2021). With the formation, enlargement, and accumulation of foam cells, hypoxia and inflammation are exacerbated, adenosine levels are elevated, and they are involved in AS. Based on the multiple effects of ARs, the treatment of AS requires inhibition of AS progression by antagonizing or activating different subtypes.

When DPCPX, an A1AR antagonist, was administered to apolipoprotein E knockout (ApoE^−/−) mice, atherosclerotic lesions decreased along with the levels of plasma IL-5, IL-6, and IL-13. This could be connected to A1AR’s pro-inflammatory and pro-mitotic characteristics. (Teng et al., 2014). A_2aAR inactivation similarly protect ApoE−/−mice from AS. AS was suppressed and macrophage numbers were reduced in double knockout mice produced by crossing ApoE−/−mice with A_2aAR knockout mice compared to ApoE−/−mice (Wang et al., 2009). This is due to macrophage and foam cell death resulting from increased p38 mitogen-activated protein kinase activity in A_2aAR knockout mice. Endothelium-specific A_2aAR deficiency or blockade of A_2aAR with Istradefylline inhibited/attenuated AS in ApoE−/−mice (Cai et al., 2024). In contrast to antagonizing the protective role of A₁AR and A_2aAR in AS, A_2bAR deficiency led to more pronounced AS in ApoE−/−mice, and in vivo administration of the A_2bAR agonist BAY 60-6,853 reduced lipid levels to inhibit AS (Koupenova et al., 2012).

The hardening of coronary blood arteries is a hallmark of CAD, thereby leading to myocardial ischemia, hypoxia, or necrosis. Since CAD is the world’s leading cause of death and disability, early detection and treatment are crucial (Ralapanawa and Sivakanesan, 2021).

In CAD, an imbalance between oxygen supply and demand leads to adenosine accumulation during ischemia. The regulatory role of the adenosinergic system in CAD was first considered and investigated at the beginning of the 21st century, and the development of highly potent and selective A_2aAR agonists has been the subject of chemical research in the ensuing 30 years (Klotz, 2000; Cristalli et al., 2003; Paganelli et al., 2021). Adenosine, as a sensitive marker of myocardial ischemia, is a useful tool for CAD patients during myocardial imaging (Lassen et al., 2022). Adenosine-loaded myocardial perfusion imaging is based on imaging abnormalities in myocardial cell perfusion or metabolism. Intravenous adenosine binding to the A₂AR rapidly dilates coronary arteries and increases myocardial blood flow reserve, whereas failure of diseased coronary arteries to dilate accordingly results in increased perfusion contrast between normal and stenotic coronary regions of the heart (Paganelli et al., 2021; Garcia-Dorado et al., 2014). In pharmacologic stress tests, dipyridamole, a vasodilator, increased coronary vasodilation and coronary blood flow via A_2aAR while inhibiting adenosine uptake and metabolism (Gaudry et al., 2020). Regadenoson, a selective A_2aAR agonist, was approved by the U.S. Food and Drug Administration in 2008 for use in myocardial perfusion imaging (MPI) and exercise contraindications. It is more frequently employed in clinical practice because of its quick start of action, short duration that is enough to produce a congestive response, efficacy that is comparable to that of adenosine, and lack of negative reactions and side effects (Al Jaroudi and Iskandrian, 2009).

ARs are mainly used to treat CAD through antiplatelet activation. A_2aAR expressed on platelets plays a significant role in the inhibition of platelet aggregation (Wolska and Rozalski, 2019). 5′-N-ethylcarboxamidoadenosine, CGS21680, and ATL-146e both exerted antiplatelet aggregation effects by binding to A_2aAR (Clark et al., 2019; Linden et al., 2008; Fuentes et al., 2014). By altering the activation of the AC-cAMP-PKA pathway, 1.8-cineole (A_2aAR agonist) prevented the expression, release, and platelet aggregation of platelet activation indicators (Petry et al., 2024). Dipyridamole is anti-platelet aggregation by inhibiting platelet PDE, leading to the accumulation of cAMP and guanine cyclic phosphate (Kim and Liao, 2008). However, due to the lack of sufficient evidence to support its use for various indications and with the emergence of newer antiplatelet agents, the use of dipyridamole in CAD has been out of practice (Ye et al., 2010). It is clear from the above that lower A_2aAR levels favor platelet aggregation in acute coronary syndromes. Conversely, drugs that increase plasma adenosine levels can inhibit platelets. According to the results of an investigation, AR agonists (regadenoson, NECA, LUF5835) and P2Y₁₂ antagonists (cangreor or prasugrel metabolite) work together to limit platelet activity more effectively than P2Y₁₂ antagonists alone (Wolska et al., 2019). Moreover, ARs agonists have a more important role in people who react badly to P2Y₁₂ inhibitors. This finding offers a useful foundation for the clinical management of platelet aggregation with combination medications.

ARs may also play a cardioprotective role in ischemia and reperfusion through IPC and IPO, and a large number of reports on the cardioprotective aspects of AR-mediated cardio protection have appeared since the 1990s. Although the research results on whether administering adenosine in the coronary artery before direct PCI can reduce the size of myocardial infarction are inconsistent, adenosine treatment for patients with shorter ischemic times undergoing direct percutaneous coronary intervention can salvage more myocardium and is advantageous for improving left ventricular ejection fraction (Polimeni et al., 2016). All four ARs can reduced infarct size and protected the heart through IPC and IPO (Zhang et al., 2024; Ruan et al., 2023; Wan et al., 2019; Williams-Pritchard et al., 2011). However, when cardioprotective therapies in humans, a number of comorbidities, including obesity, diabetes, and hypercholesterolemia, are prevalent in comparison to healthy laboratory animals. Given this circumstance, additional information is required to design medicine (Lasley, 2018).

HF is a clinical syndrome that results in elevated intracardiac pressure and decreased cardiac output both under stress and at rest. Using classic diuretics to treat HF usually induces or exacerbates kidney damage. Additionally, heart failure with preserved ejection fraction (HFpEF) also lacks effective treatment (McMurray et al., 2012; Ponikowski et al., 2016). Drugs that antagonize A₁AR to exert a diuretic effect and activate A₁AR to provide cardio protection have been developed to address these problems.

A₁AR antagonists, such as naxifylline and rolofylline, have been used in clinical phase II and III trials. Antagonism of A₁AR improves glomerular filtration by dilating afferent small arterioles and blocking interglomerular feedback. Clinical trial studies of naxifylline and rolofylline have demonstrated that combining them with diuretics reduces the collateral diuretic dosage (Gottlieb et al., 2002). In a large phase III study, rolofylline had a modest diuretic effect in patients with acute HF (AHF) with mild to moderate renal insufficiency but did not prevent sustained deterioration of renal function in patients with AHF with volume overload and renal insufficiency and had a higher incidence of seizures and stroke (Voors et al., 2011). Further evaluation of its efficacy and safety in phase IV clinical trials is needed to provide more options for diuretic therapy in HF patients. Neladenoson bialanate, a drug targeting A₁AR for the treatment of HFpEF, is currently in clinical phase II trials. Without the negative side effects of full A1AR agonists or A1AR antagonists, Neladenoson bialanate, a partial adenosine A1AR agonist, has been demonstrated in preclinical models to better mitochondrial function, perform energy substrate utilization, increase serca2a activity, reverse ventricular remodeling, and supply anti-ischemic cardio protection. No significant dose-response relationship was detected for nelladenoson bialanate in HFpEF patients in terms of changes in exercise capacity from baseline to 20 weeks. In light of these findings, if Neladenoson bialanate is further developed for the treatment of patients with HFpEF, new approaches will be required (Shah et al., 2019).

Persistent systemic arterial hypertension is the hallmark of hypertension and is the most important modifiable factor in cardiovascular morbidity and mortality worldwide (GBD 2019 Risk Factors Collaborators, 2020). The pathophysiology of hypertension involves ARs, and various AR subtypes have distinct effects on blood pressure regulation.

A_2aAR is abundantly expressed in multiple cell types and regulates cardiovascular responses. A_2aAR activation reduced hypertension by promoting vasodilation, controlling adipokine secretion, preventing immune cells from releasing inflammatory substances, and promoting cardiovascular homeostasis (Quast et al., 2017; Ruan et al., 2018). In Dahl salt-sensitive hypertensive animals, deletion of A_2aAR led to salt-induced elevation of blood pressure (Jackson et al., 1979). Pre-eclampsia also led to elevated blood pressure in mothers. High levels of adenosine have been found in the fetal-placental circulation in preeclamptic pregnancies, which may be related to A_2aAR-NO signaling (Escudero et al., 2013). According to recent research, a pathogenic etiology of hypertension may be sodium and fluid imbalance brought on by defective lymphangiogenesis and lymphatic dysfunction. Activation of A_2aAR using CGS21680 reduced blood pressure by increasing lymphatic capillary density in mice (Zhuang et al., 2023). Activation of A₁AR in proximal tubules led to excess Na⁺ reabsorption, resulting in Na⁺ retention, which induced hypertension (Lee et al., 2012). In one study, adenosine alone versus a combination of adenosine and caffeine resulted in no change in renin secretion, presumably due to the counteracting effects of both A₁AR and A_2aAR, but the inference needs to be confirmed by further studies (Wierema et al., 2005). In the aorta of hypertensive mice with high A₁AR levels, the A₁AR agonist CCPA induced more pronounced vasoconstriction (Yadav et al., 2019). In addition, the protective effects of A₁AR and A_2bAR against hypertension were different in different sexes of hypertensive model rats. A recent experiment showed that deletion of A₁AR and A_2bAR was protective against salt-induced hypertension in female rats and that this protective effect was overcome when the salt diet reached extremely high levels (Jackson et al., 1979). Perhaps when the salt diet reaches a certain level, the renal ARs in pathophysiology as well as their excretion function as well as the degree of inflammatory activation may counteract the antihypertensive effect of the A₁AR deletion, leading to treatment failure. The different effects of ARs in therapy described above demonstrate the complexity of ARs and suggest that future therapeutic strategies need to take into account the specificity of AR subtypes as well as their changing roles in different pathologic states.