Cellular basis for the negative dromotropic effect of adenosine on rabbit single atrioventricular nodal cells

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Medicinal Chemistry and Therapeutic Potential of Agonists, Antagonists and Allosteric Modulators of A1 Adenosine Receptor: Current Status and Perspectives

Adenosine is a purine nucleoside, responsible for the regulation of a wide range of physiological and pathophysiological conditions by binding with four G-protein-coupled receptors (GPCRs), namely A1, A2A, A2B and A3 adenosine receptors (ARs). In particular, A1 AR is ubiquitously present, mediating a variety of physiological processes throughout the body, thus represents a promising drug target for the management of various pathological conditions. Agonists of A1 AR are found to be useful for the treatment of atrial arrhythmia, angina, type-2 diabetes, glaucoma, neuropathic pain, epilepsy, depression and Huntington's disease, whereas antagonists are being investigated for the treatment of diuresis, congestive heart failure, asthma, COPD, anxiety and dementia. However, treatment with full A1 AR agonists has been associated with numerous challenges like cardiovascular side effects, off-target activation as well as desensitization of A1 AR leading to tachyphylaxis. In this regard, partial agonists of A1 AR have been found to be beneficial in enhancing insulin sensitivity and subsequently reducing blood glucose level, while avoiding severe CVS side effects and tachyphylaxis. Allosteric enhancer of A1 AR is found to be potent for the treatment of neuropathic pain, culminating the side effects related to off-target tissue activation of A1 AR. This review provides an overview of the medicinal chemistry and therapeutic potential of various agonists/partial agonists, antagonists and allosteric modulators of A1 AR, with a particular emphasis on their current status and future perspectives in clinical settings.

Adenosine and the Cardiovascular System

Adenosine is an endogenous nucleoside with a short half-life that regulates many physiological functions involving the heart and cardiovascular system. Among the cardioprotective properties of adenosine are its ability to improve cholesterol homeostasis, impact platelet aggregation and inhibit the inflammatory response. Through modulation of forward and reverse cholesterol transport pathways, adenosine can improve cholesterol balance and thereby protect macrophages from lipid overload and foam cell transformation. The function of adenosine is controlled through four G-protein coupled receptors: A₁, A_2A, A_2B and A₃. Of these four, it is the A_2A receptor that is in a large part responsible for the anti-inflammatory effects of adenosine as well as defense against excess cholesterol accumulation. A_2A receptor agonists are the focus of efforts by the pharmaceutical industry to develop new cardiovascular therapies, and pharmacological actions of the atheroprotective and anti-inflammatory drug methotrexate are mediated via release of adenosine and activation of the A_2A receptor. Also relevant are anti-platelet agents that decrease platelet activation and adhesion and reduce thrombotic occlusion of atherosclerotic arteries by antagonizing adenosine diphosphate-mediated effects on the P2Y12 receptor. The purpose of this review is to discuss the effects of adenosine on cell types found in the arterial wall that are involved in atherosclerosis, to describe use of adenosine and its receptor ligands to limit excess cholesterol accumulation and to explore clinically applied anti-platelet effects. Its impact on electrophysiology and use as a clinical treatment for myocardial preservation during infarct will also be covered. Results of cell culture studies, animal experiments and human clinical trials are presented. Finally, we highlight future directions of research in the application of adenosine as an approach to improving outcomes in persons with cardiovascular disease.

In Silico Assessment of Efficacy and Safety of IKur Inhibitors in Chronic Atrial Fibrillation: Role of Kinetics and State-Dependence of Drug Binding

Current pharmacological therapy against atrial fibrillation (AF), the most common cardiac arrhythmia, is limited by moderate efficacy and adverse side effects including ventricular proarrhythmia and organ toxicity. One way to circumvent the former is to target ion channels that are predominantly expressed in atria vs. ventricles, such as K_V1.5, carrying the ultra-rapid delayed-rectifier K⁺ current (I_Kur). Recently, we used an in silico strategy to define optimal K_V1.5-targeting drug characteristics, including kinetics and state-dependent binding, that maximize AF-selectivity in human atrial cardiomyocytes in normal sinus rhythm (nSR). However, because of evidence for I_Kur being strongly diminished in long-standing persistent (chronic) AF (cAF), the therapeutic potential of drugs targeting I_Kur may be limited in cAF patients. Here, we sought to simulate the efficacy (and safety) of I_Kur inhibitors in cAF conditions. To this end, we utilized sensitivity analysis of our human atrial cardiomyocyte model to assess the importance of I_Kur for atrial cardiomyocyte electrophysiological properties, simulated hundreds of theoretical drugs to reveal those exhibiting anti-AF selectivity, and compared the results obtained in cAF with those in nSR. We found that despite being downregulated, I_Kur contributes more prominently to action potential (AP) and effective refractory period (ERP) duration in cAF vs. nSR, with ideal drugs improving atrial electrophysiology (e.g., ERP prolongation) more in cAF than in nSR. Notably, the trajectory of the AP during cAF is such that more I_Kur is available during the more depolarized plateau potential. Furthermore, I_Kur block in cAF has less cardiotoxic effects (e.g., AP duration not exceeding nSR values) and can increase Ca²⁺ transient amplitude thereby enhancing atrial contractility. We propose that in silico strategies such as that presented here should be combined with in vitro and in vivo assays to validate model predictions and facilitate the ongoing search for novel agents against AF.

Revealing kinetics and state-dependent binding properties of IKur-targeting drugs that maximize atrial fibrillation selectivity

The K_V1.5 potassium channel, which underlies the ultra-rapid delayed-rectifier current (I_Kur) and is predominantly expressed in atria vs. ventricles, has emerged as a promising target to treat atrial fibrillation (AF). However, while numerous K_V1.5-selective compounds have been screened, characterized, and tested in various animal models of AF, evidence of antiarrhythmic efficacy in humans is still lacking. Moreover, current guidelines for pre-clinical assessment of candidate drugs heavily rely on steady-state concentration-response curves or IC₅₀ values, which can overlook adverse cardiotoxic effects. We sought to investigate the effects of kinetics and state-dependent binding of I_Kur-targeting drugs on atrial electrophysiology in silico and reveal the ideal properties of I_Kur blockers that maximize anti-AF efficacy and minimize pro-arrhythmic risk. To this aim, we developed a new Markov model of I_Kur that describes K_V1.5 gating based on experimental voltage-clamp data in atrial myocytes from patient right-atrial samples in normal sinus rhythm. We extended the I_Kur formulation to account for state-specificity and kinetics of K_V1.5-drug interactions and incorporated it into our human atrial cell model. We simulated 1- and 3-Hz pacing protocols in drug-free conditions and with a [drug] equal to the IC₅₀ value. The effects of binding and unbinding kinetics were determined by examining permutations of the forward (k_on) and reverse (k_off) binding rates to the closed, open, and inactivated states of the K_V1.5 channel. We identified a subset of ideal drugs exhibiting anti-AF electrophysiological parameter changes at fast pacing rates (effective refractory period prolongation), while having little effect on normal sinus rhythm (limited action potential prolongation). Our results highlight that accurately accounting for channel interactions with drugs, including kinetics and state-dependent binding, is critical for developing safer and more effective pharmacological anti-AF options.

[Mapping and ablation of a mechanically blocked concealed accessory pathway under repeated adenosine bolus infusions]

During mapping and catheter ablation of an accessory pathway, a mechanically induced conduction block can occur. Adenosine is used to detect dormant conduction of incomplete ablation lesions. Presented in this article is the case of a patient with a left-sided accessory pathway, which was mechanically blocked during the mapping procedure and could only be successfully ablated after repeated adenosine bolus infusions, which resulted in intermittent restitution of conduction via the accessory pathway.

INO-8875, a highly selective A1 adenosine receptor agonist: evaluation of chronotropic, dromotropic, and hemodynamic effects in rats

Selective pharmacological activation of the adenosine 1 receptor (A(1)R) is a promising new approach to achieve a potent block of atrioventricular (A-V)-nodal conduction without significant cardiovascular side effects. The purpose of the present study was to evaluate the cardiovascular profile of INO-8875, a highly selective A(1)R agonist, and to compare its properties with N-[3(R)-tetrahydrofuranyl]-6-aminopurine riboside (CVT-510), which has already been shown to induce negative dromotropic effects with minimal cardiovascular side effects in animals and in clinical studies. Dose-response experiments in the isolated hearts of rats were used to evaluate the functional selectivity of INO-8875 for the slowing of A-V-nodal conduction. Ventilated adult rats were used to study the effects of INO-8875, in vivo, on arterial blood pressure as well as on supraventricular electrophysiology. Ex vivo, INO-8875 (100 nM to 3 μM) progressively prolonged A-V-nodal conduction without reducing left ventricular function or coronary resistance. In vivo, INO-8875 up to a dose of 50 μg/kg did not reduce the carotid arterial blood pressure (n = 4). INO-8875 (1-50 μg/kg) and CVT-510 (20 and 50 μg/kg) both induced a dose-dependent decrease in heart rate and atrial refractoriness, as well as slowing of A-V-nodal conduction. However, compared with CVT-510, the activity of INO-8875 was more pronounced in A-V-nodal function. INO-8875 exhibited a greater duration of action, lasting up to 2.5 hours post dosing, whereas the effects of CVT-510 dissipated over 1 hour. INO-8875 demonstrates functional properties of a highly selective A(1)R agonist. INO-8875 exhibits an increased dromotropic effect and greater duration of action compared with CVT-510.

Adenosine A1 receptor activation is arrhythmogenic in the developing heart through NADPH oxidase/ERK- and PLC/PKC-dependent mechanisms

Whether adenosine, a crucial regulator of the developing cardiovascular system, can provoke arrhythmias in the embryonic/fetal heart remains controversial. Here, we aimed to establish a mechanistic basis of how an adenosinergic stimulation alters function of the developing heart. Spontaneously beating hearts or dissected atria and ventricle obtained from 4-day-old chick embryos were exposed to adenosine or specific agonists of the receptors A(1)AR (CCPA), A(2A)AR (CGS-21680) and A(3)AR (IB-MECA). Expression of the receptors was determined by quantitative PCR. The functional consequences of blockade of NADPH oxidase, extracellular signal-regulated kinase (ERK), phospholipase C (PLC), protein kinase C (PKC) and L-type calcium channel (LCC) in combination with adenosine or CCPA, were investigated in vitro by electrocardiography. Furthermore, the time-course of ERK phosphorylation was determined by western blotting. Expression of A(1)AR, A(2A)AR and A(2B)AR was higher in atria than in ventricle while A(3)AR was equally expressed. Adenosine (100μM) triggered transient atrial ectopy and second degree atrio-ventricular blocks (AVB) whereas CCPA induced mainly Mobitz type I AVB. Atrial rhythm and atrio-ventricular propagation fully recovered after 60min. These arrhythmias were prevented by the specific A(1)AR antagonist DPCPX. Adenosine and CCPA transiently increased ERK phosphorylation and induced arrhythmias in isolated atria but not in ventricle. By contrast, A(2A)AR and A(3)AR agonists had no effect. Interestingly, the proarrhythmic effect of A(1)AR stimulation was markedly reduced by inhibition of NADPH oxidase, ERK, PLC, PKC or LCC. Moreover, NADPH oxidase inhibition or antioxidant MPG prevented both A(1)AR-mediated arrhythmias and ERK phosphorylation. These results suggest that pacemaking and conduction disturbances are induced via A(1)AR through concomitant stimulation of NADPH oxidase and PLC, followed by downstream activation of ERK and PKC with LCC as possible target.

A1 adenosine receptor antagonists, agonists, and allosteric enhancers

Intense efforts of many pharmaceutical companies and academicians in the A(1) adenosine receptor (AR) field have led to the discovery of clinical candidates that are antagonists, agonists, and allosteric enhancers. The A(1)AR antagonists currently in clinical development are KW3902, BG9928, and SLV320. All three have high affinity for the human (h) A(1)AR subtype (hA(1) K (i) < 10 nM), > 200-fold selectivity over the hA(2A) subtype, and demonstrate renal protective effects in multiple animal models of disease and pharmacologic effects in human subjects. In the A(1)AR agonist area, clinical candidates have been discovered for the following conditions: atrial arrhythmias (tecadenoson, selodenoson and PJ-875); Type II diabetes and insulin sensitizing agents (GR79236, ARA, RPR-749, and CVT-3619); and angina (BAY 68-4986). The challenges associated with the development of any A(1)AR agonist are to obtain tissue-specific effects but avoid off-target tissue side effects and A(1)AR desensitization leading to tachyphylaxis. For the IV antiarrhythmic agents that act as ventricular rate control agents, a selective response can be accomplished by careful IV dosing paradigms. The treatment of type II diabetes using A(1)AR agonists in the clinic has met with limited success due to cardiovascular side effects and a well-defined desensitization of full agonists in human trials (GR79236, ARA, and RPR 749). However, new partial A(1)AR agonists are in development, including CVT-3619 hA(1) AR K(i) = 55nM, hA(2A:hA2B:hA(3))1,000:20, CV Therapeutics), which have the potential to provide enhanced insulin sensitivity without cardiovascular side effects and tachyphylaxis. The nonnucleosidic A(1)AR agonist BAY 68-4986 (capadenoson) represents a novel approach to angina wherein both animal studies and early human studies are promising. T-62 is an A(1)AR allosteric enhancer that is currently being evaluated in clinical trials as a potential treatment for neuropathic pain. The challenges associated with developing A(1)AR antagonists, agonists, or allosteric enhancers for therapeutic intervention are now well defined in humans. Significant progress has been made in identifying A(1)AR antagonists for the treatment of edema associated with congestive heart failure (CHF), A(1)AR agonists for the treatment of atrial arrhythmias, type II diabetes and angina, and A(1)AR allosteric enhancers for the treatment of neuropathic pain.

Sinus arrest during adenosine stress testing in liver transplant recipients with graft failure: three case reports and a review of the literature

BACKGROUND

Nuclear stress imaging is used frequently to evaluate patients with end-stage liver disease who are being considered for orthotopic liver transplantation.

METHODS AND RESULTS

We present three patients who, following graft failure, developed sinus arrest during adenosine stress testing performed in anticipation of repeat liver transplantation. All had undergone uneventful adenosine stress imaging prior to initial transplantation. The mechanisms of action, pharmacokinetics, and pharmacodynamics of adenosine are reviewed, and possible reasons for this phenomenon are discussed. Finally, cautions regarding the use of adenosine and treatment of adenosine-induced sinoatrial and atrioventricular block are reviewed.

CONCLUSION

Adenosine should be used with caution in patients following orthotopic liver transplantation due to an increased risk of sinus arrest. Should sinus arrest or atrioventricular block occur, it appears to respond readily to cessation of adenosine infusion and intravenous aminophylline with no significant sequelae.

Complement activation-related cardiac anaphylaxis in pigs: role of C5a anaphylatoxin and adenosine in liposome-induced abnormalities in ECG and heart function

Cardiac anaphylaxis is a severe, life-threatening manifestation of acute hypersensitivity reactions to allergens and drugs. Earlier studies highlighted an amplifying effect of locally applied C5a on the process; however, the role of systemic complement (C) activation with C5a liberation in blood has not been explored to date. In the present study, we used the porcine liposome-induced cardiopulmonary distress model for 1) characterizing and quantifying peripheral C activation-related cardiac dysfunction; 2) exploring the role of C5a in cardiac abnormalities and therapeutic potential of C blockage by soluble C receptor type 1 (sCR1) and an anti-C5a antibody (GS1); and 3) elucidating the role of adenosine and adenosine receptors in paradoxical bradycardia, one of the symptoms observed in this model. Pigs were injected intravenously with different liposomes [Doxil and multilamellar vesicles (MLV)], zymosan, recombinant human (rhu) C5a, and adenosine, and the ensuing hemodynamic and cardiac changes (hypotension, tachy- or bradycardia, arrhythmias, ST-T changes, ventricular fibrillation, and arrest) were quantified by ranking on an arbitrary scale [cardiac abnormality score (CAS)]. There was significant correlation between CAS and C5a production by liposomes in vitro, and the liposome-induced cardiac abnormalities were partially or fully reproduced with zymosan, rhuC5a, adenosine, and the selective adenosine A1 receptor agonist cyclopentyl-adenosine. The use of C nonactivator liposomes or pretreatment of pigs with sCR1 or GS1 attenuated the abnormalities. The selective A1 blocker cyclopentyl-xanthine inhibited bradycardia without influencing hypotension, whereas the A(2) blocker 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-24135) had no such effect. These data suggest that 1) systemic C activation can underlie cardiac anaphylaxis, 2) C5a plays a causal role in the reaction, 3) adenosine action via A1 receptors may explain paradoxical bradycardia, and 4) inhibition of C5a formation or action or of A1-receptor function may alleviate the acute cardiotoxicity of liposomal drugs and other intravenous agents that activate C.

Affinity and intrinsic efficacy (IE) of 5′-carbamoyl adenosine analogues for the A1 adenosine receptor—efforts towards the discovery of a chronic ventricular rate control agent for the treatment of atrial fibrillation (AF)

The SAR for the affinity to the A(1) adenosine receptor and relative intrinsic efficacy (IE, [(35)S]-GTPgammaS binding) of a series of 5'-carbamate and 5'-thionocarbamate derivatives of tecadenoson is described. Based on this SAR, selected compounds were evaluated in guinea pig isolated hearts to determine whether they were partial or full agonists with respect to their negative dromotropism, an A(1) AdoR mediated effect. Progress towards obtaining a partial A(1) AdoR agonist to potentially control ventricular rate during atrial fibrillation has been made with the discovery of several potent partial A(1) AdoR agonists (compounds 13, 14, and 17).

Anti-adrenergic effect of adenosine on Na(+)-Ca(2+) exchange current recorded from guinea-pig ventricular myocytes

The Na(+)-Ca(2+) exchanger is a protein present in the cell membrane of many cell types. In heart it plays important roles in Ca homeostasis and ionic current generation. Recently, it has been reported that the beta-adrenergic agonist isoprenaline (ISO) can increase directly Na(+)-Ca(2+) exchanger activity in guinea-pig ventricular myocytes. Adenosine (ADO) exerts anti-adrenergic properties that make it effective against some arrhythmias and the aim of the present study was to determine whether or not ADO can antagonize the direct modulatory effect of ISO on the exchanger.Whole-cell patch clamp measurements of Na(+)-Ca(2+) exchanger current (I(NaCa)) were made from guinea-pig ventricular myocytes, with major interfering currents inhibited. I(NaCa) was measured at 378 degrees C as current sensitive to external nickel (Ni(2+), 10 mM) during an applied descending voltage ramp. ISO (1 microM) significantly increased both inward and outward I(NaCa). This effect was abolished in the presence of ADO (200 microM). ADO alone did not significantly alter the amplitude of I(NaCa). The effect of ADO on the response of I(NaCa) to ISO was mimicked by the A(1)ADO receptor agonist N(6)-cyclopentyladenosine (CPA, 10 microM), whereas the effect of ADO on the response of I(NaCa) to ISO was inhibited by the A(1)ADO receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 2 microM). These data suggest that the A(1)ADO receptor mediated the response. The anti-adrenergic effects on I(NaCa) of ADO were not affected by the protein kinase C (PKC) inhibitor, chelerythrine (CLT, 1 microM), nor by the nitric oxide (NO) synthase inhibitor, N (G)-nitro-L-arginine methyl ester((L)-NAME, 0.5 mM). Moreover, in the presence of PKC activator phorbol 12-myristate 13-acetate (PMA, 1 microM) or exogenous NO donor sodium nitroprusside (SNP, 100 microM), ISO preserved its stimulatory effect on I(NaCa). However, prior incubation of myocytes with pertussis toxin (PTX, 5 microg ml(-1) did prevent the effect of ADO. The anti-adrenergic effect of ADO on I(NaCa) was mimicked by externally applied carbachol (CCh, 10 microM), a muscarinic receptor agonist. We conclude that ADO antagonized the effect of beta-adrenergic stimulation of I(NaCa) by directly activating inhibitory G-protein (G(i))-linked A(1) receptors in guinea-pig ventricular myocytes. These findings may suggest a novel mechanism by which adenosine exerts some of its antiarrhythmic effects.

Antagonism of the positive dromotropic effect of isoproterenol by adenosine: role of nitric oxide, cGMP-dependent cAMP-phosphodiesterase and protein kinase G

We hypothesized that nitric oxide (NO) plays an important role in mediating the anti-adrenergic effect of adenosine on atrioventricular (AV) nodal conduction. In guinea-pig hearts instrumented for measurement of AV nodal conduction time (atrium-to-His bundle, A-H, interval), the NO synthase (NOS) inhibitor, l-NMMA (100 microm), reversibly inhibited 80% (P=0.009, n=6) of adenosine's anti-adrenergic action on the positive dromotropic effect of isoproterenol (0.01 microm). In parallel studies carried out in rabbit AV nodal myocytes, intracellular mechanisms whereby NO mediates the inhibitory effect of adenosine on isoproterenol-induced A-H interval shortening were studied. Adenosine (3 microm) inhibited isoproterenol-stimulated (0.1 microm) I(Ca,L)(beta -I(Ca,L)) by 46+/-6% (P<0.001, n=17). Consistent with isolated heart data, the NOS inhibitors, l -NMMA (100 microm) and L-NNA (500 microm) attenuated the effect of adenosine on beta -I(Ca,L)by 69+/-8% (P<0.001, n=16) and 69+/-7% (P<0.001, n=10), respectively. An inhibitor of NO-stimulated guanylyl cyclase LY83538 (40 microm) reduced the inhibitory effect of adenosine on beta -I(Ca,L)by 97+/-6% (P=0.004, n=15). Similarly, the non-specific inhibitor of cAMP-phosphodiesterases IBMX (50 microm) decreased the anti-adrenergic effect of adenosine by 60% (P=0.02, n=6), whereas the extracellular application of the non-hydrolyzeable cAMP analog 8-Br-cAMP (500 microm) prevented this action of adenosine. Activation of cGMP-dependent protein kinase (PKG) by CPT-cGMP (300 microm) diminished beta -I(Ca,L), but to a significantly smaller degree (16+/-4%, P=0.025, n=12) than that caused by adenosine. NO mediates the anti-adrenergic effect of adenosine on AV nodal conduction by a mechanism predominately involving activation of cGMP-dependent cAMP-phosphodiesterase and to a lesser extent activation of PKG.

Rate-dependency of action potential duration and refractoriness in isolated myocytes from the rabbit AV node and atrium

During atrial fibrillation, ventricular rate is determined by atrioventricular nodal (AVN) conduction, which in part is dependent upon the refractoriness of single AVN cells. The aims of this study were to investigate the rate-dependency of the action potential duration (APD) and effective refractory period (ERP) in single myocytes isolated from the AV node and atrium of rabbit hearts, using whole cell patch clamping, and to determine the contribution of the 4-aminopyridine (4-AP)-sensitive current, I(TO1)to these relationships in the two cell types. AVN cells had a more positive maximum diastolic potential (-60+/-1 v-71+/-2 mV), lower V(max)(8+/-2 v 144+/-17 V/s) and higher input resistance [420+/-46 v 65+/-7 MOmega (mean+/-s.eP<0.05 n=9-33)], respectively, than atrial myocytes. Stepwise increases in rate from 75 beats/min caused activation failure and Wenckebach periodicity in AVN cells (at around 400 beats/min), but 1:1 activation in atrial cells (at up to 600 beats/min). Rate reduction from 300 to 75 beats/min shortened the ERP in both cell types (from 155+/-7 to 135+/-11 ms in AVN cells [P<0.05, n=6] and from 130+/-8 to 106+/-7 ms in atrial cells [P<0.05, n=10]). Rate increase from 300 to 480 and 600 beats/min shortened ERP in atrial cells, by 12+/-4% (n=8) and 26+/-7% (n=7), respectively (P<0.05). By contrast, AVN ERP did not shorten at rates >300 beats/min. In atrial cells, rate reduction to 75 beats/min caused marked shortening of APD(50)(from 51+/-6 to 29+/-6 ms, P<0. 05). 4-AP (1 m m) significantly prolonged atrial APD(50)at 75 beats/min (P<0.05, n=7), but not at 300 or 400 beats/min. In AVN cells, in contrast, there was less effect of rate change on APD, and 4-AP did not alter APD(50)at any rate. 4-AP also did not affect APD(90)or ERP in either cell type. In conclusion, a lack of ERP-shortening at high rates in rabbit single AVN cells may contribute to ventricular rate control. I(TO1)contributed to the APD(50)rate relation in atrial, but not AVN cells and did not contribute to the ERP rate relation in either cell type.

A unique xanthine derivative KMCP-98 with activation of adenosine receptor subtypes

KMCP-98 is a newly synthesized adenosine receptor agonist by alkylation at the 7-position of the xanthines nucleus. We first investigated the pharmacological activities of KMCP-98 under in vivo and in vitro conditions. Acute intravenous injection of KMCP-98 (1.0, 2.0 and 3.0 mg/kg) produced a temporary fall in blood pressure and heart rate, followed by a sustained fall in heart rate in pentobarbital-anesthetized Wistar rats. The hypotensive and bradycardiac responses were inhibited by pretreatment with an A(1) adenosine receptor antagonist 8-phenyltheophylline (8-PT, 0.5 mg/kg). Both KMCP-98 and adenosine (0.3-100 microM) produced negative inotropic activity in isolated guinea pig left atria. The negative inotropic activity of KMCP-98 was significantly blocked by pretreatment with A(1) receptor antagonists 8-PT (10 microM) and xanthine amine congener (XAC, 10 microM), a nonselective adenosine antagonist theophylline (10 microM), a K(+) channel blocker tetraethylammonium (TEA, 10 mM) and a K(ATP) channel blocker glibenclamide (1 microM). KMCP-98 (0.03-30 microM) produced concentration-dependent relaxations in carbachol (1 microM) precontracted guinea pig tracheal smooth muscle. The trachea relaxant response of KMCP-98 was markedly inhibited by A(2), A(2a) and A(2b) adenosine receptor antagonists 3,7-dimethyl-1-propargylxanthine (DMPX, 10 microM), 8-(3-chlorostyryl)caffeine (CSC, 10 microM) and alloxazine (10 microM), respectively, the nitric oxide synthase (NOS) inhibitor L-NAME (100 microM) and also by TEA and glibenclamide. In addition, KMCP-98 (0.03-30 microM) elicited relaxant response in norepinephrine (3 microM) precontracted rat thoracic aorta in a concentration-dependent manner. The thoracic aorta relaxant response of KMCP-98 was also significantly inhibited by DMPX, CSC, alloxazine, L-NAME, TEA and glibenclamide. Furthermore, the binding characteristics of KMCP-98, adenosine and 5'-N-ethylcarboxaminoadenosine (NECA) were evaluated in [(3)H]DPCPX and [(3)H]CGS 21680 binding to rat cortex and striatum, respectively. The K(i) values of KMCP-98 for predominate A(1) and A(2) adenosine receptor sites were 3908+/-952 and 158+/-10 nM, respectively. In conclusion, KMCP-98 was found to be a xanthine-based adenosine receptor agonist associated cardiac depression, tracheal and aortic smooth muscle relaxations.

Adenosine with cold blood cardioplegia during coronary revascularization

OBJECTIVE

To investigate whether adenosine in association with blood cardioplegia results in more rapid cardiac arrest or improved myocardial protection.

DESIGN

A prospective, randomized, placebo-controlled double-blind clinical study.

SETTING

Operative and intensive care units in a university hospital, Finland.

PARTICIPANTS

Forty patients undergoing primary, elective coronary revascularization.

INTERVENTION

Adenosine as a bolus dose, 12 mg intravenously, was given immediately before the induction of blood cardioplegia.

MEASUREMENTS AND MAIN RESULTS

There were nonsignificantly higher serial serum values of CK (MB) (p = 0.33), troponin-T (p = 0.23), and troponin-I (p = 0.10) in the adenosine group. There were no differences between the groups in arrest time, blood pressure decrease, or lactate extraction.

CONCLUSIONS

The adenosine regimen used in this study did not cause more rapid arrest with blood cardioplegia. The effect on cardioprotection was insignificant.

Imaging of adenosine bolus transit following intravenous administration: insights into antiarrhythmic efficacy

OBJECTIVE

To study the effects of the site of intravenous injection of adenosine and to assess the site of action of adenosine in the heart by correlating cardiac effects with bolus transit.

METHODS

Ten patients undergoing routine technetium (Tc-99m) gated blood pool ventriculography consented to the coadministration of intravenous adenosine. The dose of adenosine required to produce heart block during sinus rhythm was determined following antecubital vein administration. This dose (6-18 mg) was mixed with Tc-99m and given first into the same antecubital vein (proximal injection) and then repeated into a hand vein (distal injection). The ECG was recorded and the transit of the bolus was imaged using a gamma camera.

RESULTS

Heart block occurred in all 10 patients (second degree in seven, first degree in three) at (mean (SEM)) 17.5 (1.0) seconds after the proximal injection of adenosine. Distal injection produced heart block in six patients (second degree in two, first degree in four) at 21.9 (4.4) seconds (p < 0.01). In eight of 10 patients the electrophysiological effects were less with distal injection. The onset of heart block was close to the time of peak bolus Tc-99m activity in the left ventricle. Peak bolus activity was delayed (by about three seconds) and the duration of bolus activity in the left ventricle was increased with distal injection compared with proximal injection, at 17.2 (4.2) v 9.2 (3.1) seconds, p < 0.01.

CONCLUSIONS

The lesser electrophysiological effects of adenosine following distal intravenous injections were associated with delay in transit time and dispersion of the bolus. The correlation of adenosine induced heart block with bolus activity in the left heart indicated dependence on coronary arterial delivery of adenosine to the atrioventricular node.

Effects of a selective A1-adenosine receptor agonist on heart rate and heart rate variability during permanent atrial fibrillation

BACKGROUND

Mean heart rate and irregularity of the rate, i.e., heart rate variability (HRV), are two aspects of heart rate during atrial fibrillation (AF). An important goal of AF therapy is to control mean heart rate during exercise; the determinants of HRV during AF remain poorly known although its prognostic value has been established.

OBJECTIVES

To investigate the effects of a stable, long-acting, selective A1-adenosine receptor agonist, SDZ WAG994, on heart rate during exercise and on HRV.

METHODS

In a multicenter, double-blind, randomized, placebo-controlled, parallel group study, patients with permanent AF performed a symptom-limited exercise test and underwent 24-hour ECG monitoring on day 1 during treatment with placebo, and on day 2 during treatment with either placebo or 2 mg SDZWAG994 orally. Changes in mean heart rate during exercise and changes in HRV indices between day 1 and day 2 were compared between the two groups.

RESULTS

Thirty-two patients (64 +/- 8 years; 81% male; 25% in NYHA Class II; 38% with no structural heart disease) were included in the study. During active treatments, heart rate remained unchanged at rest and increased significantly during exercise. A significant daytime increase in short-term HRV indices (DpNN50 = 4.5% P = 0.01; DrMSSD = 6% P = 0.03; DSDNN Index = 6% P = 0.02) occurred during active treatment.

CONCLUSIONS

Selective A1-adenosine receptor agonism with SDZ/WAG994 limits the increase in mean heart rate during exercise in patients with AF. In addition, this agonist selectively increases short-term HRV indices, suggesting that pNN50, rMSSD, and SDNN reflect vagal influences during AF.

Mediation by nitric oxide of the indirect effects of adenosine on calcium current in rabbit heart pacemaker cells

1. Adenosine (ADO) is a potent negative chronotropic agent in the mammalian myocardium. We have used single myocytes from rabbit sino-atrial node (SAN) to examine whether nitric oxide (NO) is a significant mediator of the effects of ADO on the pacemaker activity, or the underlying Ca2+ and K+ currents. 2. SAN pacemaker cells were isolated from rabbit hearts by enzymatic dispersion, and Ca2+ and K+ currents were recorded by the nystatin-perforated patch voltage clamp method. ADO was applied in the presence of the beta-adrenoceptor agonist, isopremaline (Iso) to mimic the adrenergic tone which the SAN is subjected to in vivo. 3. Control experiments confirmed that isolated SAN cells responded to ADO (10-100 microM) with the expected (i) small increase in background inwardly rectifying K+ current, IK-ADOi and (ii) pronounced decrease in L-type Ca2+ current, ICa-L. These effects were mimicked by a selective A1 purinoceptor agonist, N6-cyclopentyladenosine (CPA, 10 microM); and were inhibited following bath application of the antagonist, DPCPX (10 microM), which selectively blocks A1 purinoceptors. DMPX (10 microM), a blocker of A2 purinoceptor, had no effect on the actions of ADO. 4. A nitric oxide synthase inhibitor, L-NMMA (100 microM), abolished the inhibitory effect of ADO on ICa-L but did not alter activation of IK-ADO. After L-NMMA washoff, it was possible to obtain the normal response (inhibition) of ICa-L to ADO in the same cell. 5. To evaluate whether the observed effect of nitric oxide (NO) on ICa-L was mediated by an increase in guanylyl cyclase (GC) activity and cyclic GMP formation, the guanylyl cyclase inhibitor, LY 83583 (40 microM) was applied prior to ADO. Under these conditions, the inhibitory effect of ADO on ICa-L was abolished, but the activation of IK-ADO was still observed. 6. In combination, these findings strongly suggest that in mammalian primary pacemaker tissue which is under adrenergic tone, the effects of ADO on ICa-L are mediated by NO.