Beta-adrenergic and muscarinic cholinergic receptor densities in the human sinoatrial node: identification of a high beta 2-adrenergic receptor density

An overview of the efficacy and safety of β2-adrenoceptor antagonists for the treatment of chronic obstructive pulmonary disease

INTRODUCTION

The safety of β2-AR antagonists in the treatment of patients with COPD continues to be a topic of research and discussion within the medical community. Emerging evidence suggests potentially benefits in the management of this complex respiratory condition. However, antagonists that display a preference for β2-AR over β1-AR present a complex therapeutic challenge in COPD management, necessitating an understanding of differences in their pharmacological profiles and clinical implications.

AREAS COVERED

An overview of the mechanisms of action of β2-AR antagonists and their potential impact on respiratory function, their pharmacological interactions, clinical implications, and future perspectives in COPD.

EXPERT OPINION

β-Blockers have the potential to become a versatile class of therapeutic agents with benefits beyond their original cardiovascular use. However, the one-size-fits-all approach of prescribing β-blockers regardless of their receptor selectivity to COPD patients with concomitant heart disease may not be appropriate. Instead, it is advisable to develop an individualized treatment strategy based on a thorough assessment of the patient's overall health. The use of non selective β2-AR antagonists, functioning as inverse agonists at β2-ARs, has garnered interest and debate, but further research efforts should focus on elucidating the optimal use of β-AR antagonists in COPD, balancing cardiovascular benefits with potential respiratory risks to enhance outcomes and quality of life for individuals living with this debilitating respiratory condition.

Vibegron shows high selectivity and potent agonist activity for β3-adrenoceptors, irrespective of receptor density

β3-Adrenoceptor (AR) agonists are used to treat patients with an overactive bladder (OAB). Clinical proof-of-concept data have been obtained for the β3-AR agonists vibegron, mirabegron, solabegron, and ritobegron; however, the selectivities of these agents have not been compared directly under the same experimental conditions. Moreover, the bladders of some patients express lower β3-AR densities than those of healthy individuals, and the β3-AR density might be expected to affect agonist activity. This study assessed the β3-AR selectivities of four β3-AR agonists and examined the effects of β-AR density on their pharmacological profiles. Functional cellular assays were performed using Chinese hamster ovary-K1 cells expressing three human β-AR subtypes transfected with different amounts of plasmid DNA (0.1, 0.05, 0.025 μg/well). The half-maximal effective concentration values, intrinsic activities (IAs), and β3-AR selectivities of vibegron, mirabegron, solabegron, and ritobegron were calculated to assess their pharmacological profiles. The β3-AR selectivities of vibegron, mirabegron, solabegron, and ritobegron were >7937-, 517-, 21.3-, and >124-fold higher than for β1-ARs, and >7937-, 496-, >362- and 28.1-fold higher than for β2-ARs, respectively, under the same experimental conditions. The IAs of mirabegron, solabegron, and ritobegron decreased in line with decreasing receptor density, while the IA of vibegron was maintained at the same level as that of the full agonist isoproterenol at various β3-AR densities. Vibegron has high β3-AR selectivity and exhibits full agonist activity, regardless of the β3-AR density. These results suggest that vibegron is a highly effective and safe drug for treating OAB.

Inhaled Short-Acting Beta Agonist Treatment-Associated Supraventricular Tachycardia in Children: Still a Matter of Concern in Pediatric Emergency Departments?

Inhaled selective short-acting β-2 agonists (SABA), such as salbutamol, are the rescue treatment of choice for the relief of symptoms of acute asthma exacerbations: one of the leading causes of pediatric emergency department admission and hospitalization. Cardiovascular events, including supraventricular arrhythmias, are the most frequent side effects reported with inhaled SABA in children with asthma and are the main reason for a continuing debate about their safety, despite their widespread use. Although supraventricular tachycardia (SVT) is the most common potentially serious dysrhythmia in children, the incidence and risk factor of SVT after SABA administration is currently unknown. We here reported three cases and conducted a review of the literature in an attempt to gain insight into this issue.

Regulation of heart rate following genetic deletion of the ß1 adrenergic receptor in larval zebrafish

Aim

Although zebrafish are gaining popularity as biomedical models of cardiovascular disease, our understanding of their cardiac control mechanisms is fragmentary. Our goal was to clarify the controversial role of the ß1‐adrenergic receptor (AR) in the regulation of heart rate in zebrafish.

Methods

CRISPR‐Cas9 was used to delete the adrb1 gene in zebrafish allowing us to generate a stable adrb1^−/− line. Larval heart rates were measured during pharmacological protocols and with exposure to hypercapnia. Expression of the five zebrafish adrb genes were measured in larval zebrafish hearts using qPCR.

Results

Compared with genetically matched wild‐types (adrb1^+/+), adrb1^−/− larvae exhibited ~20 beats min⁻¹ lower heart rate, measured from 2 to 21 days post‐fertilization (dpf). Nevertheless, adrb1^−/− larvae exhibited preserved positive chronotropic responses to pharmacological treatment with AR agonists (adrenaline, noradrenaline, isoproterenol), which were blocked by propranolol (general ß‐AR antagonist). Regardless of genotype, larvae exhibited similar increases in heart rate in response to hypercapnia (1% CO₂) at 5 dpf, but tachycardia was blunted in adrb1^−/− larvae at 6 dpf. adrb1 gene expression was abolished in the hearts of adrb1^−/− larvae, confirming successful knockout. While gene expression of adrb2a and adrb3a was unchanged, adrb2b and adrb3b mRNA levels increased in adrb1^−/− larval hearts.

Conclusion

Despite adrb1 contributing to the setting of resting heart rate in larvae, it is not strictly essential for zebrafish, as we generated a viable and breeding adrb1^−/− line. The chronotropic effects of adrenergic stimulation persist in adrb1^−/− zebrafish, likely due to the upregulation of other ß‐AR subtypes.

Aging Alters the Formation and Functionality of Signaling Microdomains Between L-type Calcium Channels and β2-Adrenergic Receptors in Cardiac Pacemaker Cells

Heart rate is accelerated to match physiological demands through the action of noradrenaline on the cardiac pacemaker. Noradrenaline is released from sympathetic terminals and activates β1-and β2-adrenergic receptors (ΑRs) located at the plasma membrane of pacemaker cells. L-type calcium channels are one of the main downstream targets potentiated by the activation of β-ARs. For this signaling to occur, L-type calcium channels need to be located in close proximity to β-ARs inside caveolae. Although it is known that aging causes a slowdown of the pacemaker rate and a reduction in the response of pacemaker cells to noradrenaline, there is a lack of in-depth mechanistic insights into these age-associated changes. Here, we show that aging affects the formation and function of adrenergic signaling microdomains inside caveolae. By evaluating the β1 and β2 components of the adrenergic regulation of the L-type calcium current, we show that aging does not alter the regulation mediated by β1-ARs but drastically impairs that mediated by β2-ARs. We studied the integrity of the signaling microdomains formed between L-type calcium channels and β-ARs by combining high-resolution microscopy and proximity ligation assays. We show that consistent with the electrophysiological data, aging decreases the physical association between β2-ARs and L-type calcium channels. Interestingly, this reduction is associated with a decrease in the association of L-type calcium channels with the scaffolding protein AKAP150. Old pacemaker cells also have a reduction in caveolae density and in the association of L-type calcium channels with caveolin-3. Together the age-dependent alterations in caveolar formation and the nano-organization of β2-ARs and L-type calcium channels result in a reduced sensitivity of the channels to β2 adrenergic modulation. Our results highlight the importance of these signaling microdomains in maintaining the chronotropic modulation of the heart and also pinpoint the direct impact that aging has on their function.

Emerging Technologies in Cardiac Pacing From Leadless Pacers to Stem Cells

Modern pacemakers can sense and pace multiple chambers of the heart. These pacemakers have different modes and features to optimize atrioventricular synchrony and promote intrinsic conduction. Despite recent advancements, current pacemakers have several drawbacks that limit their feasibility. In this review article, we discuss several of these limitations and detail several emerging technologies in cardiac pacing aimed to solve some of these limitations. We present several technological advancements in cardiac pacing, including the use of leadless pacemakers, physiologic pacing, battery improvements, and bioartificial pacemakers. More research still needs to be done in testing the safety and efficacy of these new developments.

Neurohumoral Control of Sinoatrial Node Activity and Heart Rate: Insight From Experimental Models and Findings From Humans

The sinoatrial node is perhaps one of the most important tissues in the entire body: it is the natural pacemaker of the heart, making it responsible for initiating each-and-every normal heartbeat. As such, its activity is heavily controlled, allowing heart rate to rapidly adapt to changes in physiological demand. Control of sinoatrial node activity, however, is complex, occurring through the autonomic nervous system and various circulating and locally released factors. In this review we discuss the coupled-clock pacemaker system and how its manipulation by neurohumoral signaling alters heart rate, considering the multitude of canonical and non-canonical agents that are known to modulate sinoatrial node activity. For each, we discuss the principal receptors involved and known intracellular signaling and protein targets, highlighting gaps in our knowledge and understanding from experimental models and human studies that represent areas for future research.

Hypotensive Effect of Hydroalcoholic Ziziphus jujuba Extract on Normotensive Rats

Objective:

Ziziphus jujuba (Z.J) is a medicinal plant with several properties such as antioxidant, antibacterial, hypnotic and wound healing activity. It also has lowering blood pressure effect and people use it to treat hypertension. In the present study, the effect of hydroalcoholic extract of Z.J on cardiovascular parameters in the normotensive rats was investigated.

Methods:

Animal groups include: 1) Sham, only cannulation of artery 2) saline, received saline for four weeks 3-5) Z.J groups (100, 200 and 400 mg/kg) that treated with Z.J via gavage for four weeks. In the experiment day (28th Day) femoral artery of all rats cannulated and Systolic Blood Pressure (SBP), Mean Atrial Pressure (MAP) and Heart Rate (HR) recorded by a power lab system. The weight of animal in all groups also was recorded in the beginning and the last day of experiment.

Results:

There was no significant difference in HR, SBP and MAP parameters between sham and control groups. The SBP and MAP in higher doses of Z.J (200 and 400 mg) significantly decreased compared to control group. The HR only in dose 200 mg of Z.J significantly decreased than the control group.

Conclusion:

The results of present study show that hydroalcoholic extract of Z.J has an inhibitory effect on basal cardiovascular parameters and its best effects have been shown in dose 200 mg/kg.

Sinus node-like pacemaker mechanisms regulate ectopic pacemaker activity in the adult rat atrioventricular ring

In adult mammalian hearts, atrioventricular rings (AVRs) surround the atrial orifices of atrioventricular valves and are hotbed of ectopic activity in patients with focal atrial tachycardia. Experimental data offering mechanistic insights into initiation and maintenance of ectopic foci is lacking. We aimed to characterise AVRs in structurally normal rat hearts, identify arrhythmia predisposition and investigate mechanisms underlying arrhythmogenicity. Extracellular potential mapping and intracellular action potential recording techniques were used for electrophysiology, qPCR for gene and, Western blot and immunohistochemistry for protein expression. Conditions favouring ectopic foci were assessed by simulations. In right atrial preparations, sinus node (SN) was dominant and AVRs displayed 1:1 impulse conduction. Detaching SN unmasked ectopic pacemaking in AVRs and pacemaker action potentials were SN-like. Blocking pacemaker current I_f, and disrupting intracellular Ca²⁺ release, prolonged spontaneous cycle length in AVRs, indicating a role for SN-like pacemaker mechanisms. AVRs labelled positive for HCN4, and SERCA2a was comparable to SN. Pacemaking was potentiated by isoproterenol and abolished with carbachol and AVRs had abundant sympathetic nerve endings. β₂-adrenergic and M₂-muscarinic receptor mRNA and β₂-receptor protein were comparable to SN. In computer simulations of a sick SN, ectopic foci in AVR were unmasked, causing transient suppression of SN pacemaking.

Gene Therapy Approaches to Biological Pacemakers

Bradycardia arising from pacemaker dysfunction can be debilitating and life threatening. Electronic pacemakers serve as effective treatment options for pacemaker dysfunction. They however present their own limitations and complications. This has motivated research into discovering more effective and innovative ways to treat pacemaker dysfunction. Gene therapy is being explored for its potential to treat various cardiac conditions including cardiac arrhythmias. Gene transfer vectors with increasing transduction efficiency and biosafety have been developed and trialed for cardiovascular disease treatment. With an improved understanding of the molecular mechanisms driving pacemaker development, several gene therapy targets have been identified to generate the phenotypic changes required to correct pacemaker dysfunction. This review will discuss the gene therapy vectors in use today along with methods for their delivery. Furthermore, it will evaluate several gene therapy strategies attempting to restore biological pacing, having the potential to emerge as viable therapies for pacemaker dysfunction.

Biased G Protein-Coupled Receptor Signaling: New Player in Modulating Physiology and Pathology

G protein-coupled receptors (GPCRs) are a family of cell-surface proteins that play critical roles in regulating a variety of pathophysiological processes and thus are targeted by almost a third of currently available therapeutics. It was originally thought that GPCRs convert extracellular stimuli into intracellular signals through activating G proteins, whereas β-arrestins have important roles in internalization and desensitization of the receptor. Over the past decade, several novel functional aspects of β-arrestins in regulating GPCR signaling have been discovered. These previously unanticipated roles of β-arrestins to act as signal transducers and mediators of G protein-independent signaling have led to the concept of biased agonism. Biased GPCR ligands are able to engage with their target receptors in a manner that preferentially activates only G protein- or β-arrestin-mediated downstream signaling. This offers the potential for next generation drugs with high selectivity to therapeutically relevant GPCR signaling pathways. In this review, we provide a summary of the recent studies highlighting G protein- or β-arrestin-biased GPCR signaling and the effects of biased ligands on disease pathogenesis and regulation.

Albuterol Improves Alveolar-Capillary Membrane Conductance in Healthy Humans

BACKGROUND

Beta-2 adrenergic receptors (β₂ARs) are located throughout the body including airway and alveolar cells. The β₂ARs regulate lung fluid clearance through a variety of mechanisms including ion transport on alveolar cells and relaxation of the pulmonary lymphatics. We examined the effect of an inhaled β₂-agonist (albuterol) on alveolar-capillary membrane conductance (DM) and pulmonary capillary blood volume (V_C) in healthy humans.

METHODS

We assessed the diffusing capacity of the lungs for carbon monoxide (DLCO) and nitric oxide (DLNO) at baseline, 30 minutes, and 60 minutes following nebulized albuterol (2.5 mg, diluted in 3 mL normal saline) in 45 healthy subjects. Seventeen subjects repeated these measures following nebulized normal saline (age = 27 ± 9 years, height = 165 ± 21 cm, weight = 68 ± 12 kg, BMI = 26 ± 9 kg/m²). Cardiac output (Q), heart rate, systemic vascular resistance (SVR), blood pressure, oxygen saturation, forced expiratory volume at one-second (FEV₁), and forced expiratory flow at 50% of forced vital capacity (FEF₅₀) were assessed at baseline, 30 minutes, and 60 minutes following the administration of albuterol or saline.

RESULTS

Albuterol resulted in a decrease in SVR, and an increase in Q, FEV₁, and FEF₅₀ compared to saline controls. Albuterol also resulted in a decrease in V_C at 60 minutes post albuterol. Both albuterol and normal saline resulted in no change in DLCO or DM when assessed alone, but a significant increase was observed in DM when accounting for changes in V_C.

CONCLUSION

These data suggest that nebulized albuterol improves pulmonary function in healthy humans, while nebulization of both albuterol and saline results in an increase in DM/V_C.

Activating autoantibodies to the β1/2-adrenergic and M2 muscarinic receptors associate with atrial tachyarrhythmias in patients with hyperthyroidism

We have previously demonstrated that activating autoantibodies to β1-adrenergic receptor (β1AR) and M2 muscarinic receptor (M2R) facilitate atrial fibrillation (AF) in patients with Graves' disease (GD). The objectives of this expanded study were to examine the prevalence of β1AR, β2AR, and M2R autoantibodies in hyperthyroidism subjects. Sera from 81 patients including 31 with GD and AF, 36 with GD and sinus rhythm, 9 with toxic multinodular goiter, 5 with subacute thyroiditis, and 10 control subjects were examined for these autoantibodies by ELISA. Sera from 20 ELISA-positive GD subjects, 10 with AF and 10 with sinus rhythm, were assayed for autoantibody bioactivity using cell-based bioassays. In patients with GD and AF, 45, 65, and 77 % were ELISA positive for β1AR, M2R, and β2AR autoantibodies, respectively. In patients with GD and sinus rhythm, 17, 39, and 75 % were ELISA positive for β1AR, M2R, and β2AR autoantibodies, respectively. β1AR and M2R autoantibodies were co-present in 39 % of patients with GD and AF compared to 14 % in GD with sinus rhythm (p = 0.026). Patients with toxic multinodular goiter or subacute thyroiditis had a low prevalence of autoantibodies. The mean β1AR and M2R autoantibody activity was elevated in both GD groups but higher in those with AF than those with sinus rhythm. β2AR autoantibody activity was also increased in both groups. In conclusion, β1AR, β2AR, and M2R autoantibodies were elevated in GD. β1AR and M2R autoantibodies appear to be related to concurrent AF, while β2AR autoantibodies were equally prevalent in those with a sinus tachycardia and those with AF.

Vascular Dilation, Tachycardia, and Increased Inotropy Occur Sequentially with Increasing Epinephrine Dose Rate, Plasma and Myocardial Concentrations, and cAMP

BACKGROUND

While epinephrine infusion is widely used in critical care for inotropic support, there is no direct method to detect the onset and measure the magnitude of this response. We hypothesised that surrogate measurements, such as heart rate and vascular tone, may indicate if the plasma and tissue concentrations of epinephrine and cAMP are in a range sufficient to increase myocardial contractility.

METHODS

Cardiovascular responses to epinephrine infusion (0.05-0.5 mcgkg(-1)min(-1)) were measured in rats using arterial and left ventricular catheters. Epinephrine and cAMP levels were measured using ELISA techniques.

RESULTS

The lowest dose of epinephrine infusion (0.05 mcgkg(-1)min(-1)) did not raise plasma epinephrine levels and did not lead to cardiovascular response. Incremental increase in epinephrine infusion (0.1 mcgkg(-1)min(-1)) elevated plasma but not myocardial epinephrine levels, providing vascular, but not cardiac effects. Further increase in the infusion rate (0.2 mcgkg(-1)min(-1)) raised myocardial tissue epinephrine levels sufficient to increase heart rate but not contractility. Inotropic and lusitropic effects were significant at the infusion rate of 0.3 mcgkg(-1)min(-1). Correlation of plasma epinephrine to haemodynamic parameters suggest that as plasma concentration increases, systemic vascular resistance falls (EC50=47 pg/ml), then HR increases (ED50=168 pg/ml), followed by a rise in contractility and lusitropy (ED50=346 pg/ml and ED50=324 pg/ml accordingly).

CONCLUSIONS

The dose response of epinephrine is distinct for vascular tone, HR and contractility. The need for higher doses to see cardiac effects is likely due to the threshold for drug accumulation in tissue. Successful inotropic support with epinephrine cannot be achieved unless the infusion is sufficient to raise the heart rate.

Biased β2-adrenoceptor signalling in heart failure: pathophysiology and drug discovery

The body is constantly faced with a dynamic requirement for blood flow. The heart is able to respond to these changing needs by adjusting cardiac output based on cues emitted by circulating catecholamine levels. Cardiac β-adrenoceptors transduce the signal produced by catecholamine stimulation via Gs proteins to their downstream effectors to increase heart contractility. During heart failure, cardiac output is insufficient to meet the needs of the body; catecholamine levels are high and β-adrenoceptors become hyperstimulated. The hyperstimulated β1-adrenoceptors induce a cardiotoxic effect, which could be counteracted by the cardioprotective effect of β2-adrenoceptor-mediated Gi signalling. However, β2-adrenoceptor-Gi signalling negates the stimulatory effect of the Gs signalling on cardiomyocyte contraction and further exacerbates cardiodepression. Here, further to the localization of β1- and β2-adrenoceptors and β2-adrenoceptor-mediated β-arrestin signalling in cardiomyocytes, we discuss features of the dysregulation of β-adrenoceptor subtype signalling in the failing heart, and conclude that Gi-biased β2-adrenoceptor signalling is a pathogenic pathway in heart failure that plays a crucial role in cardiac remodelling. In contrast, β2-adrenoceptor-Gs signalling increases cardiomyocyte contractility without causing cardiotoxicity. Finally, we discuss a novel therapeutic approach for heart failure using a Gs-biased β2-adrenoceptor agonist and a β1-adrenoceptor antagonist in combination. This combination treatment normalizes the β-adrenoceptor subtype signalling in the failing heart and produces therapeutic effects that outperform traditional heart failure therapies in animal models. The present review illustrates how the concept of biased signalling can be applied to increase our understanding of the pathophysiology of diseases and in the development of novel therapies.

Atrial tachyarrhythmias induced by the combined effects of β1/2-adrenergic autoantibodies and thyroid hormone in the rabbit

Activating autoantibodies (AAb) to β-adrenergic receptors (βAR) are associated with atrial fibrillation in patients with Graves' disease. In the present study, we examined the interaction of thyroid hormone with β1/2AR-AAb in inducing atrial tachyarrhythmias in the rabbit. Immunization of rabbits with a β1AR or β2AR second extracellular loop peptide produced high titers of β1AR-AAb or β2AR-AAb. Thyroid hormone in combination with β1AR-AAb or β2AR-AAb induced a significant number of sustained sinus tachycardia and atrial tachycardia, respectively. Both combinations resulted in significantly increased inductions of sustained arrhythmias compared to AAb alone. Thyroid hormone alone induced sustained sinus and junctional tachycardia. Sera from immunized rabbits specifically bound to and activated β1AR or β2AR in transfected cells in vitro. This study demonstrates thyroid hormone qualitatively accentuates the specific arrhythmogenic action of these AAb and quantitatively enhances their rate. Our data support a dual role of AAb and thyroid hormone in Graves'-associated tachyarrhythmias.

Inducible cardiac arrhythmias caused by enhanced β1-adrenergic autoantibody expression in the rabbit

Previous studies demonstrated burst pacing and intravenous infusion of ACh induced sustained atrial tachycardia when rabbits were immunized to produce β2-adrenergic receptor (β2AR)-activating autoantibodies. The objective of this study was to examine the arrhythmogenic effect of β1-adrenergic receptor (β1AR)-activating autoantibodies in the rabbit. Eight New Zealand white rabbits were immunized with a β1AR second extracellular loop peptide to raise β1AR antibody titers. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization. Arrhythmia occurrence was determined in response to burst pacing before and after ACh infusion in incremental concentrations of 10 μM, 100 μM, and 1 mM. The baseline sinus heart rate before and after immunization averaged 149 ± 17 per min and 169 ± 16 per min, respectively (P < 0.05). In the preimmune studies, there were five sustained (≥10 s) arrhythmias in 32 induction attempts, which occurred in only four of eight rabbits. In the postimmune studies, there were 22 sustained arrhythmias in 32 induction attempts, which occurred in all eight rabbits (P < 0.0001 for the independent effect of immunization). Of the 22 sustained arrhythmias postimmunization, 15 were sinus tachycardia compared with only two before immunization (P < 0.01 for the independent effect of immunization). Postimmune (but not preimmune) rabbit sera demonstrated specific binding to β1AR and induced significant β1AR activation in transfected cells in vitro. No cross-reactivity with β2AR was observed. In conclusion, in contrast with rabbits with β2AR-activating autoantibodies that demonstrate predominantly atrial tachycardias, enhanced autoantibody activation of β1AR in the rabbit leads to tachyarrhythmias mainly in the form of sustained sinus tachycardia.

Atrial tachycardia provoked in the presence of activating autoantibodies to β2-adrenergic receptor in the rabbit

BACKGROUND

A recent clinical study of patients with inappropriate sinus tachycardia reported that autoantibodies to β-adrenergic receptors (β2ARs) could act as agonists to induce atrial arrhythmias.

OBJECTIVE

To test the hypothesis that activating autoantibodies to the β2AR in the rabbit atrium are arrhythmogenic.

METHODS

Five New Zealand white rabbits were immunized with a β2AR second extracellular loop peptide to raise β2AR antibody titers. A catheter-based electrophysiologic study was performed on anesthetized rabbits before and after immunization. Arrhythmia occurrence was determined in response to burst pacing before and after the infusion of acetylcholine in incremental concentrations of 10 μM, 100 μM, and 1 mM at 1 mL/min.

RESULTS

In the preimmune studies when β2AR antibody titers were undetectable, of a total of 20 events, only 3 episodes of nonsustained (<10 seconds) atrial arrhythmias were induced. In the postimmune studies when β2AR antibody titers ranged from 1:160,000 to 1:1.28 million, burst pacing induced 10 episodes of nonsustained or sustained (≥10 seconds) arrhythmias in 20 events (P = .04 vs preimmune; χ(2) and Fisher exact test). Taking into account only the sustained arrhythmias, there were 6 episodes in 20 events in the postimmune studies compared with 0 episodes in 20 events in the preimmune studies (P = .02). Immunized rabbits demonstrated immunoglobulin G deposition in the atria, and their sera induced significant activation of β2AR in transfected cells in vitro compared to the preimmune sera.

CONCLUSIONS

Enhanced autoantibody activation of β2AR in the rabbit atrium leads to atrial arrhythmias mainly in the form of sustained atrial tachycardia.

Phosphodiesterases reduce spontaneous sinoatrial beating but not the 'fight or flight' tachycardia elicited by agonists through Gs-protein-coupled receptors

Cyclic AMP (cAMP) steers the generation of basal heart beat in the sinoatrial node. It also induces sinoatrial tachycardia and increased cardiac force, elicited through activation of Gs-protein-coupled receptors (GsPCRs). Phosphodiesterases (PDEs) hydrolyse cAMP. In the heart mainly PDE3 and PDE4 would be expected to limit those functions, and the PDE isoenzymes do indeed reduce basal sinoatrial beating rate and blunt the positive inotropic effects of agonists, mediated by GsPCRs. By contrast, recent evidence shows that GsPCR-mediated sinoatrial tachycardia is not controlled by PDE1-5. A PDE-resistant cAMP pool in sinoatrial cells, generated through activation of GsPCRs, including β(1)- and β(2)-adrenoceptors, appears to guarantee unrestrained tachycardia during fight or flight stress.

Pacemaker activity of the human sinoatrial node: role of the hyperpolarization-activated current, I(f)

The mechanism of primary, spontaneous cardiac pacemaker activity of the sinoatrial node (SAN) has extensively been studied in several animal species, but is virtually unexplored in man. Understanding the mechanisms of human SAN pacemaker activity is important for developing new therapeutic approaches for controlling the heart rate in the sick sinus syndrome and in diseased myocardium. Here we review the functional role of the hyperpolarization-activated 'funny' current, I(f), in human SAN pacemaker activity. Despite the many animal studies performed over the years, the contribution of I(f) to pacemaker activity is still controversial and not fully established. However, recent clinical data on mutations in the I(f) encoding HCN4 gene, which is thought to be the most abundant isoform of the HCN gene family in SAN, suggest a functional role of I(f) in human pacemaker activity. These clinical findings are supported by recent experimental data from single isolated human SAN cells that provide direct evidence that I(f) contributes to human SAN pacemaker activity. Therefore, controlling heart rate in clinical practice via I(f) blockers offers a valuable approach to lowering heart rate and provides an attractive alternative to conventional treatment for a wide range of patients with confirmed stable angina, while upregulation or artificial expression of I(f) may relieve disease-causing bradycardias.

Control of cardiac rate by "funny" channels in health and disease

Activation of the "funny" (pacemaker, I f) current during the diastolic depolarization phase of an action potential is the main mechanism underlying spontaneous, rhythmic activity of cardiac pacemaker cells. In the past three decades, a wealth of evidence elucidating the function of the funny current in the generation and modulation of cardiac pacemaker activity has been gathered. The slope of early diastolic depolarization, and thus the heart rate, is controlled precisely by the degree of I f activation during diastole. I f is also accurately and rapidly modulated by changes of the cytosolic concentration of the second messenger cAMP, operated by the autonomous nervous system through beta-adrenergic, mainly beta2, and in the opposite way by muscarinic receptor, stimulation. Recently, novel in vivo data, both in animal models and humans, have been collected that confirm the key role of I f in pacemaking. In particular, an inheritable point mutation in the cyclic nucleotide-binding domain of human HCN4, the main hyperpolarization-activated cyclic nucleotide (HCN) isoform contributing to native funny channels of the sinoatrial node, was shown to be associated with sinus bradycardia in a large family. Because of their properties, funny channels have long been a major target of classical pharmacological research and are now target of innovative gene/cell-based therapeutic approaches aimed to exploit their function in cardiac rate control.

Localization of f-channels to caveolae mediates specific β2-adrenergic receptor modulation of rate in sinoatrial myocytes

beta(1)- and beta(2)-adrenergic receptors (ARs) coexist in different regions of the heart. The beta(2)/beta(1) expression ratio is higher in the sinoatrial node (SAN) than in atria and ventricles, but the specific contribution of either type of receptor to rate modulation is still not well established. We have recently demonstrated that pacemaker ("funny") f-channels are located in lipid rafts of the rabbit SAN. Since in ventricular myocytes beta(2)-, but not beta(1)-ARs, localize to caveolae, we hypothesized that modulation of f-channels and of pacemaker activity in SAN myocytes is controlled mainly by beta(2)-AR activation. To address this point, we investigated the caveolar localization of proteins by co-immunoprecipitation and immunocytochemistry, and found that f-channels interact with caveolin 3. We also recorded I(f) current and spontaneous activity from SAN myocytes, and found that beta-AR activation by the non-selective agonists isoproterenol and fenoterol shifted the I(f) activation curve similarly (by 6.3 and 5.3 mV) and increased similarly spontaneous rate (by 23.1% and 21.6%, respectively). Specific beta(2) stimulation had similar effects (4.9 mV shift of the activation curve and 16.9% rate increase), but specific beta(1) stimulation was less effective (1.7 mV shift and 7.2% rate increase). However, after caveolar disorganization by MbetaCD (2%), stimulation of beta(1)-ARs was as effective as non-specific beta-AR stimulation. These data show that specific stimulation of beta(2)-ARs is the main mechanism by which heart rate is modulated through a positive shift of the I(f) activation curve and that this mechanism requires specific membrane compartmentation.

The role of adrenergic receptor polymorphisms in heart failure

The main function of the cardiac adrenergic system is to regulate cardiac work both in physiologic and pathologic states. A better understanding of this system has permitted the elucidation of its role in the development and progression of heart failure. Regardless of the initial insult, depressed cardiac output results in sympathetic activation. Adrenergic receptors provide a limiting step to this activation and their sustained recruitment in chronic heart failure has proven to be deleterious to the failing heart. This concept has been confirmed by examining the effect of beta-blockers on the progression of heart failure. Studies of adrenergic receptor polymorphisms have recently focused on their impact on the adrenergic system regarding its adaptive mechanisms, susceptibilities and pharmacological responses. In this article, we review the function of the adrenergic system and its maladaptive responses in heart failure. Next, we discuss major adrenergic receptor polymorphisms and their consequences for heart failure risk, progression and prognosis. Finally, we discuss possible therapeutic implications resulting from the understanding of polymorphisms and the identification of individual genetic characteristics.

Inhaled beta2-adrenoceptor agonists: cardiovascular safety in patients with obstructive lung disease

Although large surveys have documented the favourable safety profile of beta(2)-adrenoceptor agonists (beta(2)-agonists) and, above all, that of the long-acting agents, the presence in the literature of reports of adverse cardiovascular events in patients with obstructive airway disease must induce physicians to consider this eventuality. The coexistence of beta(1)- and beta(2)-adrenoceptors in the heart clearly indicates that beta(2)-agonists do have some effect on the heart, even when they are highly selective. It should also be taken into account that the beta(2)-agonists utilised in clinical practice have differing selectivities and potencies. beta(2)-agonist use has, in effect, been associated with an increased risk of myocardial infarction, congestive heart failure, cardiac arrest and sudden cardiac death. Moreover, patients who have either asthma or chronic obstructive pulmonary disease may be at increased risk of cardiovascular complications because these diseases amplify the impact of these agents on the heart and, unfortunately, are a confounding factor when the impact of beta(2)-agonists on the heart is evaluated. Whatever the case may be, this effect is of particular concern for those patients with underlying cardiac conditions. Therefore, beta(2)-agonists must always be used with caution in patients with cardiopathies because these agents may precipitate the concomitant cardiac disease.

Arg16Gly polymorphism of the beta2-adrenergic receptor is associated with differences in cardiovascular function at rest and during exercise in humans

In humans, subjects homozygous for arginine (ArgArg) at codon 16 of the beta2-adrenergic receptor (beta2AR) have been shown to have greater agonist-mediated desensitization than subjects homozygous for glycine (GlyGly). We sought to determine if this substitution differentially influenced cardiovascular function during short duration (9 min) low and high intensity exercise (40 and 75% of peak work). Healthy Caucasian ArgArg (n = 16), GlyGly (n = 31) and ArgGly (n = 17) subjects matched for age, sex and peak oxygen uptake were studied. There were no differences in adrenaline (ADR) at rest or with heavy exercise, but the ArgArg group had lower ADR with light exercise (P = 0.04). Resting heart rate (HR) was higher in ArgArg (P < 0.01), while cardiac output (Q), stroke volume (SV), and mean arterial pressure (MAP) were lower than the other groups (HR = 86+/-2, 78+/-2, 80+/-1 beats min(-1); Q = 5.7+/-0.81, 6.1+/-0.18, 6.7+/-0.22 l min(-1); SV = 68+/-3, 82+/-3, 89+/-4 ml beat(-1); MAP = 92+/-1, 103+/-2, 98+/-1 mmHg-- for ArgArg, ArgGly and GlyGly, respectively, means +/-s.e.m., P < 0.01), however, no differences were observed in systemic vascular resistance (SVR). With low intensity exercise and high intensity exercise the ArgArg group continued to have a lower , SV and MAP compared to the other groups (P < 0.05), with no differences observed in SVR. During recovery, the ArgArg subjects continued to have a lower MAP but there were no differences in HR, , or SVR. These data suggest that subjects homozygous for Arg at codon 16 of the beta2AR have reduced and MAP at rest that persist during exercise with no evidence for differential changes over the course of exercise despite large changes in catecholamines. This may suggest possible genotype-related differences in baseline receptor function or density which causes phenotypic differences at rest that are sustained during short-term exercise.

Acute electrophysiologic effects of inhaled salbutamol in humans

STUDY OBJECTIVES

Although inhaled beta2-agonists are in widespread use, several reports question their potential arrhythmogenic effects. The purpose of this study was to evaluate the cardiac electrophysiologic effects of a single, regular dose of an inhaled beta2-agonist in humans.

DESIGN

Prospective study.

SETTING

Tertiary referral center.

PATIENTS

Six patients with bronchial asthma and 12 patients with mild COPD.

INTERVENTIONS

All patients underwent an electrophysiologic study before and after the administration of salbutamol solution (5 mg in a single dose).

MEASUREMENTS AND RESULTS

Sinus cycle length, sinus node recovery time (SNRT), interval from the earliest reproducible rapid deflection of the atrial electrogram in the His bundle recording to the onset of the His deflection (AH), interval from the His deflection to the onset of ventricular depolarization (HV), Wenckebach cycle length (WCL), atrial effective refractory period (AERP), and ventricular effective refractory period (VERP) were evaluated just before and 30 min after the scheduled intervention. Salbutamol, a selective beta2-agonist, administered by nebulizer had significant electrophysiologic effects on the atrium, nodes, and ventricle. The AH length decreased from 86.1 +/- 19.5 ms at baseline to 78.8 +/- 18.4 ms (p < 0.001), and the WCL decreased from 354.4 +/- 44.2 to 336.6 +/- 41.7 ms (p = 0.001). Salbutamol significantly decreased the AERP and VERP too while leaving the HV unchanged. Additionally, inhaled salbutamol increased heart rate (from 75.5 +/- 12.8 beats/min at baseline to 93.1 +/- 16 beats/min, p < 0.001) and shortened the SNRT (from 1,073.5 +/- 178.7 to 925.2 +/- 204.9 ms, p = 0.001).

CONCLUSION

Inhaled salbutamol results in significant changes of cardiac electrophysiologic properties. Salbutamol enhances atrioventricular (AV) nodal conduction and decreases AV nodal, atrial, and ventricular refractoriness in addition to its positive chronotropic effects. These alterations could contribute to the generation of spontaneous arrhythmias.

[Sinus node and ionic currents: physiology and regulation]

The physiology of the sinus node is complex. Several currents are involved in spontaneous diastolic depolarisation, the genesis of automatism: the slow and transient calcium currents, the delayed potassium (K) current, and the If current or pacemaker current. In the cellular organisation of the sinus node, there are dominant and latent cells. An intrasinusal pacemaker shift of the dominant pacemaker may be observed under the effect of many extrinsic factors. Positive chronotropic stimuli induce a shift towards the cephalic part of the sinus node, while the negative chronotropic stimuli induce a shift towards the caudal part of the sinus node. All the channels involved are possible pharmacological targets. Sinus node activity may also be extrinsically modified by many pharmacological agents at the unicellular level or at the level of intercellular relationships, combining automatism and conduction.

Localization of pacemaker channels in lipid rafts regulates channel kinetics

Lipid rafts are discrete membrane subdomains rich in sphingolipids and cholesterol. In ventricular myocytes a function of caveolae, a type of lipid rafts, is to concentrate in close proximity several proteins of the beta-adrenergic transduction pathway. We have investigated the subcellular localization of HCN4 channels expressed in HEK cells and studied the effects of such localization on the properties of pacemaker channels in HEK and rabbit sinoatrial (SAN) cells. We used a discontinuous sucrose gradient and Western blot analysis to detect HCN4 proteins in HEK and in SAN cells, and found that HCN4 proteins localize to low-density membrane fractions together with flotillin (HEK) or caveolin-3 (SAN), structural proteins of caveolae. Lipid raft disruption by cell incubation with methyl-beta-cyclodextrin (MbetaCD) impaired specific HCN4 localization. It also shifted the midpoint of activation of the HCN4 current in HEK cells and of I(f) in SAN cells to the positive direction by 11.9 and 10.4 mV, respectively. These latter effects were not due to elevation of basal cyclic nucleotide levels because the cholesterol-depletion treatment did not alter the current response to cyclic nucleotides. In accordance with an increased I(f), MbetaCD-treated SAN cells showed large increases of diastolic depolarization slope (87%) and rate (58%). We also found that the kinetics of HCN4- and native f-channel deactivation were slower after lipid raft disorganization. In conclusion, our work indicates that pacemaker channels localize to lipid rafts and that disruption of lipid rafts causes channels to redistribute within the membrane and modifies their kinetic properties.

Electrophysiologic Effects of Salbutamol, a β 2 ‐Selective Agonist

INTRODUCTION

A positive chronotropic effect of beta2 stimulation is well known. Case reports of ventricular arrhythmias during beta2-inhalation therapy have been published. The aim of this study was to asses the overall electrophysiologic effects of the beta2-agonist salbutamol.

METHODS AND RESULTS

Electrophysiologic and hemodynamic variables were measured in 10 healthy volunteers during atrial pacing at baseline and during infusion of salbutamol at two different rates (0.1 and 0.2 microg/kg/min). To characterize beta2-agonist effects, a comparison was made with the beta1-selective agonist dobutamine. Salbutamol infusion produced significant changes in electrophysiologic properties in both myocardial and nodal tissues, with significantly greater effects on nodal properties. The proportional decreases in AV nodal parameters were more pronounced than in the sinus node (P < 0.001). An interesting result was a significant increase in the duration of the QS interval, which in the presence of an unchanged His-Purkinje conduction (HV) represents slower depolarization of the ventricle. QT dispersion also increased.

CONCLUSION

Infusion of salbutamol results in significant electrophysiologic effects on most heart structures, proportionally most pronounced in the AV node. Discordant effects on ventricular conduction, which slowed, and the refractoriness of the ventricular myocardium, which shortened, were seen. QT dispersion was increased.

Decreased expression of beta1- and beta2-adrenoceptors in human diabetic atrial appendage

BACKGROUND

Using the streptozotocin-induced diabetic rat model, we have recently showed that the expression and function of beta1-adrenoreceptor were decreased in the diabetic rat heart. However, the effect of diabetes on expression of beta-adrenoreceptors in human cardiac tissue remains undefined. Therefore, the focus of the present study was to investigate the effect of diabetes on mRNA encoding beta1- and beta2-ARs in human atrial tissues.

METHODS

Right atrial appendages from five diabetic (mean age 65 +/- 4.5; 4 female, 1 male) and five nondiabetic patients (mean age 56.2 +/- 2.8; 4 male, 1 female) undergoing coronary artery bypass grafting were collected and assayed using reverse transcriptase-polymerase chain reaction (RT-PCR) for their mRNA content. No patient from these two groups suffered from acute myocardial infarction and/or failure. All diabetic patients received insulin for at least two years and had been diagnosed as diabetics for at least five years.

RESULTS

When compared with levels in nondiabetics, steady state levels of mRNA encoding beta1-adrenoreceptor decreased by 69.2 +/- 7.6% in diabetic patients while beta2-adrenoreceptor mRNA decreased by 32.2 +/- 5.5% (p < 0.001).

CONCLUSIONS

Our findings show a decreased expression of beta1- and beta2-adrenoreceptors in human diabetic atrial appendage.

Heterogeneous distribution of beta-adrenoceptors and muscarinic receptors in the sinoatrial node and right atrium of the dog

1. The pacemaker site is known to shift away from the sinoatrial (SA) node in response to autonomic stimuli. 2. To test whether the shifting of the impulse-initiation site that occurs during autonomic nerve stimulation can be explained by the spatial distributions of beta-adrenoceptor and muscarinic receptors, we obtained densities (B(max)) and dissociation constants (K((d)) for these receptors in three regions along the sulcus terminalis (the SA node itself and regions superior and inferior to it) and a region of the right atrial appendage in the dog. 3. The Bmax values for [(125)I]-iodocyanopindolol binding to beta-adrenoceptors were not different among the four regions. The Bmax value for [(3)H]-quinuclidinyl benzilate binding to muscarinic receptors was significantly higher in the SA node area than in any other region (P < 0.01), although there was no difference among the other three regions. 4. For each receptor, K(d) values were similar among the four regions. 5. These results suggest that spatial variations in the densities of beta-adrenoceptors and muscarinic receptors are not important for shifting the impulse-initiation site and that other factors, such as non-uniform innervation by autonomic nerve fibres, may be mainly responsible for the pacemaker shift induced by autonomic nerve stimulation.

Antisense inhibition of beta(1)-adrenergic receptor mRNA in a single dose produces a profound and prolonged reduction in high blood pressure in spontaneously hypertensive rats

BACKGROUND

beta-Blockers are the first line of therapy for hypertension. However, they are associated with side effects because of central nervous system (CNS) effects and beta(2)-adrenergic antagonism. To overcome these problems and provide a long-term beta(1)-blockade, antisense oligonucleotides against rat beta(1)-adrenergic receptor (beta(1)-AR) mRNA (beta(1)-AS-ODN) were designed and tested for the ability to inhibit cardiac beta(1)-ARs as well as lower blood pressure in spontaneously hypertensive rats (SHRs).

METHODS AND RESULTS

Radioligand binding assay showed that a single intravenous injection of beta(1)-AS-ODN delivered in cationic liposomes significantly decreased cardiac beta(1)-AR density by 30% to 50% for 18 days (P<0.01), with no effect on beta(2)-ARs. This was accompanied by marked attenuation of beta(1)-AR-mediated positive inotropic response in isolated perfused hearts in vitro (P<0.02) and in conscious SHRs monitored by telemetry in vivo (P<0.02). Furthermore, the blood pressure of SHRs was reduced for 20 days, with a 38 mm Hg maximum drop. Heart rate was not significantly decreased. Quantitative autoradiography was performed to assess beta(1)-AS-ODN effects on the CNS, which demonstrated no changes in beta(1)-ARs in brain, in contrast to a significant reduction in heart and kidney (P<0.05). For comparison with beta-blockers, the effects of atenolol on cardiovascular hemodynamics were examined, which lowered blood pressure for only 10 hours and elicited appreciable bradycardia in SHRs.

CONCLUSIONS

These results indicate that beta(1)-AS-ODN, a novel approach to specific beta(1)-blockade, has advantages over currently used beta-blockers in providing a profound and prolonged reduction in blood pressure without affecting heart rate, beta(2)-ARs, and the CNS. Diminished cardiac contractility resulting from less beta(1)-AR expression contributes to the antihypertensive effect.

Impaired atrial M(2)-cholinoceptor function in obesity-related hypertension

The aim of this study was to investigate the activity of the parasympathetic limb of the baroreflex arch in a canine model of obesity-related hypertension. Twelve male beagle dogs were randomized into 2 groups. Six dogs were fed with normal canine food and 6 were submitted to a 10-week high-fat diet (HFD). We have evaluated the consequences of HFD on heart rate (HR) and blood pressure (BP) circadian cycles and methylscopolamine dose-response curves. Binding of [(3)H]-AF-DX 384 and adenylyl cyclase activity were investigated to determine the density and functionality of M(2)-cholinoceptors on right atrial membranes from control and HFD dogs. HFD induced a significant increase in body weight (15+/-1 vs 12+/-1 kg), systolic BP (161+/-5 vs 145+/-4 mm Hg), diastolic BP (92+/-3 vs 79+/-2 mm Hg), and HR (96+/-4 vs 81+/-3 bpm). Circadian rhythms of HR and BP observed in the baseline period were abolished after 9 weeks of HFD. After propranolol (1 mg/kg) pretreatment, the dose of methylscopolamine able to induce 50% maximum tachycardia was significantly increased after 9 weeks of HFD (7.4+/-0.3 vs 4.7+/-0.1 microg/kg). In the control group, the experimental period failed to modify these parameters. The numbers of M(2)-cholinoceptors measured in right atrial membranes were significantly lower in HFD than in control groups (54+/-6 vs 27+/-6 fmol/mg protein). The ability of carbachol to inhibit isoproterenol-stimulated adenylyl cyclase activity was significantly lower in HFD than in control groups (IC(50)=47+/-12 vs 6.4+/-1.4 micromol/L). However, the basal activity of adenylyl cyclase was unchanged by HFD. HFD decreases M(2)-cholinoceptor number and function in cardiomyocytes. This could explain the abolition of circadian rhythm of HR and the changes in chronotropic effect brought about by methylscopolamine.

Beta-adrenergic receptors and receptor signaling in heart failure

Cardiac beta-adrenergic receptors, which respond to neuronally released and circulating catecholamines, are important regulators of cardiac function. Congestive heart failure, a common clinical condition, is associated with a number of alterations in the activation and deactivation of beta-adrenergic receptor pathways. Studies with failing hearts from humans and animals indicate that such alterations include changes in the expression or function of beta-adrenergic receptors, G-proteins, adenylyl cyclases, and G-protein receptor kinases. The net effect of these alterations is the substantial blunting of beta-adrenergic receptor-mediated cardiac response. An important unanswered question is whether the loss of cardiac beta-adrenergic receptor responsiveness is a contributing cause, or a result, of ventricular dysfunction. Even though this question remains unanswered, the concept of targeting the beta-adrenergic pathway in the failing heart is becoming increasing popular and several new therapeutic strategies are in development.

Beta-adrenergic and muscarinic receptor mRNA accumulation in the sinoatrial node area of adult and senescent rat hearts

The sinoatrial (SA) node is the cardiac pacemaker and changes in its adrenergic-muscarinic phenotype have been postulated as a determinant of age-associated modifications in heart rate variability. To address this question, right atria were microdissected, the SA node area was identified by acetylcholinesterase staining, and, using a RT-PCR method, the accumulation of mRNA molecules encoding beta1- and beta2-adrenergic (beta1- and beta2-AR) and muscarinic (M2-R) receptor was quantified to define the proportion between beta-AR and M2-R mRNAs within the sinoatrial area of adult (3 months) and senescent (24 months) individual rat hearts. In adult hearts, the highest M2-R/beta-AR mRNA ratio was observed within the sinoatrial area compared with adjacent atrial myocardium, while in the senescent hearts, no difference was observed between sinoatrial and adjacent areas. This change was specific of the sinoatrial area since adult and senescent whole atrial or ventricular myocardium did not differ in their M2-R/beta-AR mRNA ratio, and was associated with a fragmentation of acetylcholinesterase staining of the senescent SA node. Quantitative changes in the expression of genes encoding proteins involved in heart rate regulation specifically affect the sinoatrial area of the senescent heart.