Beta 2-adrenergic receptors contribute to catecholamine-stimulated shortening of action potential duration in dog atrial muscle

Incremental Benefit of Stepwise Nonpulmonary Vein Trigger Provocation During Catheter Ablation of Atrial Fibrillation

BACKGROUND

The importance of nonpulmonary vein (PV) triggers for the initiation/recurrence of atrial fibrillation (AF) is well established.

OBJECTIVES

This study sought to assess the incremental benefit of provocative maneuvers for identifying non-PV triggers.

METHODS

We included consecutive patients undergoing first-time AF ablation between 2020 and 2022. The provocation protocol included step 1, identification of spontaneous non-PV triggers after cardioversion of AF and/or during sinus rhythm; step 2, isoproterenol infusion (3, 6, 12, and 20-30 μg/min); and step 3, atrial burst pacing to induce AF followed by cardioversion during residual or low-dose isoproterenol infusion or induce focal atrial tachycardia. Non-PV triggers were defined as non-PV ectopic beats triggering AF or sustained focal atrial tachycardia.

RESULTS

Of 1,372 patients included, 883 (64.4%) underwent the complete stepwise provocation protocol with isoproterenol infusion and burst pacing, 334 (24.3%) isoproterenol infusion only, 77 (5.6%) burst pacing only, and 78 (5.7%) no provocative maneuvers (only step 1). Overall, 161 non-PV triggers were found in 135 (9.8%) patients. Of these, 51 (31.7%) non-PV triggers occurred spontaneously, and the remaining 110 (68.3%) required provocative maneuvers for induction. Among those receiving the complete stepwise provocation protocol, there was a 2.2-fold increase in the number of patients with non-PV triggers after isoproterenol infusion, and the addition of burst pacing after isoproterenol infusion led to a total increase of 3.6-fold with the complete stepwise provocation protocol.

CONCLUSIONS

The majority of non-PV triggers require provocative maneuvers for induction. A stepwise provocation protocol consisting of isoproterenol infusion followed by burst pacing identifies a 3.6-fold higher number of patients with non-PV triggers.

Atrial arrhythmogenesis in a rabbit model of chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) increases the risk of atrial fibrillation (AF), however, its arrhythmogenic mechanisms are unclear. This study investigated the effects of COPD on AF triggers (pulmonary veins, PVs) and substrates (atria), and their potential underlying mechanisms. Electrocardiographic, echocardiographic, and biochemical studies were conducted in control rabbits and rabbits with human leukocyte elastase (0.3 unit/kg)-induced COPD. Conventional microelectrode, Western blotting, and histological examinations were performed on PV, left atrium (LA), right atrium, and sinoatrial node (SAN) preparations from control rabbits and those with COPD. The rabbits with COPD had a higher incidence of atrial premature complexes, PV burst firing and delayed afterdepolarizations, higher sympathetic activity, larger LA, and faster PV spontaneous activity than did the control rabbits; but they exhibited a slower SAN beating rate. The LA of the rabbits with COPD had a shorter action potential duration and longer tachyarrhythmia induced by tachypacing (20 Hz) and isoproterenol (1 μM). Additionally, the rabbits with COPD had higher fibrosis in the PVs, LA, and SAN. H89 (10 μM), KN93 (1 μM), and KB-R7943 (10 μM) significantly suppressed burst firing and delayed afterdepolarizations in the PVs of the rabbits with COPD. Moreover, compared with the control rabbits, those with COPD had lower expression levels of the β1 adrenergic receptor, Cav 1.2, and Na⁺/Ca²⁺ exchanger in the PVs; Cav 1.2 in the LA; and hyperpolarization-activated cyclic nucleotide-gated K⁺ channel 4 in the SAN. COPD increases atrial arrhythmogenesis by modulating the distinctive electrophysiological characteristics of the PVs, LA, and SAN.

Pharmacological Tests in Atrial Fibrillation Ablation

The invasive management of atrial fibrillation (AF) has been considerably changed by the identification of major sites of AF initiation and/or maintenance within the pulmonary vein antra. Percutaneous catheter ablation of these targets has become the standard of care for sustained maintenance of sinus rhythm. Long-term failure of ablation is related to an inability to create a durable transmural lesion or to identify all of the non-pulmonary vein arrhythmia triggers. Pharmacological challenges during catheter ablation have been suggested to improve outcomes in both paroxysmal and persistent AF. Herein we review the mechanism and evidence for the use of pharmacological adjuncts during the catheter ablation of AF.

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.

Renal sympathetic denervation provides ventricular rate control but does not prevent atrial electrical remodeling during atrial fibrillation

Renal denervation (RDN) reduces renal efferent and afferent sympathetic activity thereby lowering blood pressure in resistant hypertension. The effect of modulation of the autonomic nervous system by RDN on atrial electrophysiology and ventricular rate control during atrial fibrillation (AF) is unknown. Here we report a reduction of ventricular heart rate in a patient with permanent AF undergoing RDN. Subsequently, we investigated the effect of RDN on AF-induced shortening of atrial effective refractory period, AF inducibility, and ventricular rate control during AF maintained by rapid atrial pacing in 12 pigs undergoing RDN (n=7) or sham procedure (n=5). During sinus rhythm, RDN reduced heart rate (RR-interval, 708±12 versus 577±19 ms; P=0.0021) and increased atrioventricular node conduction time (PQ-interval, 112±12 versus 88±9 ms; P=0.0001). Atrial tachypacing for 30 minutes increased AF inducibility and decreased AF cycle length. This was not influenced by RDN. RDN reduced ventricular rate during AF episodes by ≈24% (119±9 versus 158±19 bpm; P=0.0001). AF episodes were shorter after RDN compared with sham (12±3 versus 34±4 s; P=0.0091), but atrial effective refractory period was not modified by RDN. RDN reduced heart rate and reduced atrioventricular node conduction time during sinus rhythm and provided rate control during AF. AF-induced atrial electrical remodeling, AF inducibility, and AF cycle length were not modified, but duration of AF episodes was shorter after RDN. Modulation of the autonomic nervous system by RDN might provide rate control and reduce susceptibility to AF. Whether RDN may provide rate control in a larger number of patients with AF deserves further clinical studies.

Renal sympathetic denervation suppresses postapneic blood pressure rises and atrial fibrillation in a model for sleep apnea

The aim of this study was to identify the relative impact of adrenergic and cholinergic activity on atrial fibrillation (AF) inducibility and blood pressure (BP) in a model for obstructive sleep apnea. Obstructive sleep apnea is associated with sympathovagal disbalance, AF, and postapneic BP rises. Renal denervation (RDN) reduces renal efferent and possibly also afferent sympathetic activity and BP in resistant hypertension. The effects of RDN compared with β-blockade by atenolol on atrial electrophysiological changes, AF inducibility, and BP during obstructive events and on shortening of atrial effective refractory period (AERP) induced by high-frequency stimulation of ganglionated plexi were investigated in 20 anesthetized pigs. Tracheal occlusion with applied negative tracheal pressure (NTP; at -80 mbar) induced pronounced AERP shortening and increased AF inducibility in all of the pigs. RDN but not atenolol reduced NTP-induced AF-inducibility (20% versus 100% at baseline; P=0.0001) and attenuated NTP-induced AERP shortening more than atenolol (27±5 versus 43±3 ms after atenolol; P=0.0272). Administration of atropine after RDN or atenolol completely inhibited NTP-induced AERP shortening. AERP shortening induced by high-frequency stimulation of ganglionated plexi was not influenced by RDN, suggesting that changes in sensitivity of ganglionated plexi do not play a role in the antiarrhythmic effect of RDN. Postapneic BP rise was inhibited by RDN and not modified by atenolol. We showed that vagally mediated NTP-induced AERP shortening is modulated by RDN or atenolol, which emphasizes the importance of autonomic disbalance in obstructive sleep apnea-associated AF. Renal denervation displays antiarrhythmic effects by reducing NTP-induced AERP shortening and inhibits postapneic BP rises associated with obstructive events.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Cardiac adrenergic control and atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and it causes substantial mortality. The autonomic nervous system, and particularly the adrenergic/cholinergic balance, has a profound influence on the occurrence of AF. Adrenergic stimulation from catecholamines can cause AF in patients. In human atrium, catecholamines can affect each of the electrophysiological mechanisms of AF initiation and/or maintenance. Catecholamines may produce membrane potential oscillations characteristic of afterdepolarisations, by increasing Ca(2+) current, [Ca(2+)](i) and consequent Na(+)-Ca(2+) exchange, and may also enhance automaticity. Catecholamines might affect reentry, by altering excitability or conduction, rather than action potential terminal repolarisation or refractory period. However, which arrhythmia mechanisms predominate is unclear, and likely depends on cardiac pathology and adrenergic tone. Heart failure (HF), a major cause of AF, causes adrenergic activation and adaptational changes, remodelling, of atrial electrophysiology, Ca(2+) homeostasis, and adrenergic responses. Chronic AF also remodels these, but differently to HF. Myocardial infarction and AF cause neural remodelling that also may promote AF. beta-Adrenoceptor antagonists (beta-blockers) are used in the treatment of AF, mainly to control the ventricular rate, by slowing atrioventricular conduction. beta-Blockers also reduce the incidence of AF, particularly in HF or after cardiac surgery, when adrenergic tone is high. Furthermore, the chronic treatment of patients with beta-blockers remodels the atria, with a potentially antiarrhythmic increase in the refractory period. Therefore, the suppression of AF by beta-blocker treatment may involve an attenuation of arrhythmic activity that is caused by increased [Ca(2+)](i), coupled with effects of adaptation to the treatment. An improved understanding of the involvement of the adrenergic system and its control in basic mechanisms of AF under differing cardiac pathologies might lead to better treatments.

Pharmacological evaluation of plasma membrane beta-adrenoceptors in rat hearts using the tissue segment binding method

This study evaluates beta-adrenoceptors in rat atria and ventricle using the tissue segment binding method and compares the results with those obtained using conventional homogenate binding assays. In studies with tissue segment binding, the hydrophilic radioligand [(3)H]-CGP12177 selectively bound to plasma membrane beta-adrenoceptors, and the B(max) levels were significantly higher than those obtained with homogenate binding. However, both binding approaches revealed similar proportions of beta(1)- and beta(2)-adrenoceptors. The regional distribution of plasma membrane beta(1)- and beta(2)-adrenoceptors in rat hearts were also determined using tissue segment binding. Abundance of beta-adrenoceptors and proportion of beta(1)-adrenoceptors were higher in atria than in ventricle, but there was no significant difference between right and left atria or within ventricle (right and left ventricle free walls, apex, and interventricular septum). To establish the ability of the tissue segment binding method to study beta-adrenoceptor regulation such as the internalization of receptors, the effect of prolonged exposure of rat ventricle to (-)-isoprenaline was also investigated by using tissue segments and homogenate binding. Incubation with (-)-isoprenaline for 1 h in vitro caused a concentration-dependent decrease in the density of beta-adrenoceptors, predominantly beta(2)-adrenoceptors, when assessed with tissue segment binding method. In contrast, the subtype-specific change after treatment with (-)-isoprenaline was not detected using homogenate binding. In summary, the tissue segment binding method with [(3)H]-CGP12177 enables a more precise quantitation of plasma membrane beta(1)- and beta(2)-adrenoceptors in rat hearts and is suitable for studying their regulation.

β2 Adrenergic receptors mediate important electrophysiological effects in human ventricular myocardium

OBJECTIVE

To define the effects of β2 adrenergic receptor stimulation on ventricular repolarisation in vivo.

DESIGN

Prospective study.

SETTING

Tertiary referral centre.

PATIENTS

85 patients with coronary artery disease and 22 normal controls.

INTERVENTIONS

Intravenous and intracoronary salbutamol (a β2 adrenergic receptor selective agonist; 10–30 μg/min and 1–10 μg/min), and intravenous isoprenaline (a mixed β1/β2adrenergic receptor agonist; 1–5 μg/min), infused during fixed atrial pacing.

MAIN OUTCOME MEASURES

QT intervals, QT dispersion, monophasic action potential duration.

RESULTS

In patients with coronary artery disease, salbutamol decreased QTonset and QTpeak but increased QTend duration; QTonset–QTpeak and QTpeak–QTend intervals increased, resulting in T wave prolongation (mean (SEM): 201 (2) ms to 233 (2) ms; p < 0.01). There was a large increase in dispersion of QTonset, QTpeak, and QTend which was more pronounced in patients with coronary artery disease—for example, QTend dispersion: 50 (2) ms baselinev 98 (4) ms salbutamol (controls), and 70 (1) ms baseline v 108 (3) ms salbutamol (coronary artery disease); p < 0.001. Similar responses were obtained with isoprenaline. Monophasic action potential duration at 90% repolarisation shortened during intracoronary infusion of salbutamol, from 278 (4.1) ms to 257 (3.8) ms (p < 0.05).

CONCLUSIONS

β2adrenergic receptors mediate important electrophysiological effects in human ventricular myocardium. The increase in dispersion of repolarisation provides a mechanism whereby catecholamines acting through this receptor subtype may trigger ventricular arrhythmias.

Beta(2) adrenergic receptors mediate important electrophysiological effects in human ventricular myocardium

OBJECTIVE

To define the effects of beta(2) adrenergic receptor stimulation on ventricular repolarisation in vivo.

DESIGN

Prospective study.

SETTING

Tertiary referral centre.

PATIENTS

85 patients with coronary artery disease and 22 normal controls.

INTERVENTIONS

Intravenous and intracoronary salbutamol (a beta(2) adrenergic receptor selective agonist; 10-30 microg/min and 1-10 microg/min), and intravenous isoprenaline (a mixed beta(1)/beta(2) adrenergic receptor agonist; 1-5 microg/min), infused during fixed atrial pacing.

MAIN OUTCOME MEASURES

QT intervals, QT dispersion, monophasic action potential duration.

RESULTS

In patients with coronary artery disease, salbutamol decreased QT(onset) and QT(peak) but increased QT(end) duration; QT(onset)-QT(peak) and QT(peak)-QT(end) intervals increased, resulting in T wave prolongation (mean (SEM): 201 (2) ms to 233 (2) ms; p < 0.01). There was a large increase in dispersion of QT(onset), QT(peak), and QT(end) which was more pronounced in patients with coronary artery disease-for example, QT(end) dispersion: 50 (2) ms baseline v 98 (4) ms salbutamol (controls), and 70 (1) ms baseline v 108 (3) ms salbutamol (coronary artery disease); p < 0.001. Similar responses were obtained with isoprenaline. Monophasic action potential duration at 90% repolarisation shortened during intracoronary infusion of salbutamol, from 278 (4.1) ms to 257 (3.8) ms (p < 0.05).

CONCLUSIONS

beta(2) adrenergic receptors mediate important electrophysiological effects in human ventricular myocardium. The increase in dispersion of repolarisation provides a mechanism whereby catecholamines acting through this receptor subtype may trigger ventricular arrhythmias.

Action potential shortening through the putative beta4-adrenoceptor in ferret ventricle: comparison with beta1- and beta2-adrenoceptor-mediated effects

The electrophysiological responses to (-)-CGP 12177 ((-)-4-(3-tertiarybutylamino-2-hydroxypropoxy) benzimidazol-2-one), an agonist for the putative beta4-adrenoceptor, were investigated on isolated perfused ferret hearts paced at 100 min(-1) and compared to those of (-)-noradrenaline and (-)-adrenaline, mediated through beta1- and beta2-adrenoceptors respectively. The three agonists decreased ventricular monophasic action potential duration but prolonged the action potential plateau; beta3-adrenoceptor-selective agonists had no effect. (-)-CGP 12177 was the most potent, but (-)-noradrenaline the most efficacious; both agonists caused ventricular extra-systoles. Because only (-)-noradrenaline but not (-)-CGP 12177 elicited shortening of the refractory period, the mechanism of arrhythmias mediated through beta1- and putative beta4-adrenoceptors may be different.

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

The objective of this study was to measure autonomic receptor densities in the human sinoatrial node and adjacent atrial myocardium to gain further insights into autonomic regulation of sinoatrial node function in the human heart. Sinoatrial nodes (n = 9) were acquired from human donors. Quantitative light microscopic autoradiography of radioligand binding sites in tissue sections was used to compare beta-adrenergic and muscarinic cholinergic receptor densities within specific tissue compartments of the sinoatrial node and adjacent myocardium. Total beta-adrenergic receptors were measured with the nonsubtype selective radioligand [125I]iodocyanopindolol. beta 2-Adrenergic receptors were determined by measuring the amount of radioactivity bound to sections incubated with radioligand in the presence of the highly beta 1-selective antagonist CGP-20712A. Specific autoradiographic grain densities were normalized to myocyte area/unit tissue area. Myocytes in the sinoatrial node occupied 47.7% +/- 0.1% of the total tissue area compared with 92.8% +/- 0.1% in myocardium (P < 0.001). Total specific beta-adrenergic receptor density per unit myocyte area was 3.5 +/- 0.9 times greater in the sinoatrial node than in myocardium (P < 0.001). The relative densities of beta 1-(4.2, P < 0.002), beta 2-(2.6, P < 0.002), and muscarinic (3.3, P < 0.001) receptors were significantly greater in the sinoatrial node than in the atrium. Thus, total beta-adrenergic and muscarinic cholinergic receptor densities are > 3-fold higher in the sinoatrial node than adjacent atrial myocardium, reflecting their specialized roles in regulating cardiac rate and rhythm. The beta 1-subtype is predominant in both regions. The beta 2-subtype, however, is > 2.5-fold more abundant in the sinoatrial node than in atrial myocardium. The relatively high beta 2-receptor density in the human sinoatrial node is consistent with physiologic studies that implicate this receptor in regulating cardiac chronotropism.

Beta 2-adrenergic receptor actions in neonatal and adult rat ventricular myocytes

The physiological function of beta 2-adrenergic receptors in the neonatal and adult heart is incompletely understood, and possible age-dependent differences in beta 2-receptor actions have not been considered. We used isoproterenol (mixed beta 1- and beta 2-receptor agonist) and zinterol (beta 2-selective agonist) to compare beta-receptor subtype actions in neonatal and adult rat ventricular myocytes. When delivered as a bolus at a final concentration of 10(-7) mol/L, both isoproterenol and zinterol increased the amplitude and hastened the kinetics of the calcium and cell-shortening transients in neonatal myocytes. Under identical experimental conditions, isoproterenol increased the amplitude and accelerated the kinetics of the calcium transient and the twitch in adult myocytes, whereas zinterol did not. In the presence of CGP 20712A (beta 1-receptor blocker), a 100-fold higher concentration of zinterol increased the amplitude but prolonged the duration of the twitch in adult myocytes. To probe the mechanism for this age-dependent difference in beta 2-receptor responsiveness, we compared beta-receptor expression and stimulation of cAMP accumulation in neonatal and adult myocytes. beta-Receptor density was 44,339 +/- 5178 sites per cell in neonatal myocytes and 186,346 +/- 13,356 sites per cell in adult myocytes; the relative proportion of beta 2-receptors was comparable in each (16.7 +/- 2.3% and 16.9 +/- 0.9%, respectively). Isoproterenol induced a large increase in cAMP accumulation in neonatal and adult myocytes (20.0 +/- 1.0- and 20.6 +/- 2.6-fold over basal). In contrast, zinterol evoked a substantial increase in cAMP accumulation in neonatal myocytes but only a minor increase in adult myocytes. These studies provide evidence that at low agonist concentrations, beta 2-receptor activation contributes to the positive inotropic response by increasing cAMP and increasing the amplitude and hastening the kinetics of the twitch in neonatal, but not adult, myocytes. Moreover, these results suggest that age-dependent differences in beta 2-receptor coupling to more distal elements in the signaling cascade can influence myocyte beta 2-receptor responsiveness.

Autoradiographic characterization of beta-adrenergic receptor subtype in the canine conduction system

It has been hypothesized, based on physiological evidence, that there is a greater proportion of beta 2-adrenergic receptors on the myocytes of the conduction system when compared with the working myocardium. The purpose of these studies was to examine beta-adrenergic receptor subtype in the conduction system of the dog by using the technique of coverslip autoradiography. Scintillation studies of [125I]pindolol binding to ventricular sections demonstrated that binding was saturable (dissociation constant of 116 pM), had the correct order of potency for a beta-receptor, and was stereoselective. Both betaxolol (beta 1-selective) and ICI-118,551 (beta 2-selective) competition curves fit a two-site model in nonlinear curve-fitting analyses (78% beta 1-receptors). Autoradiographic studies determined that the myocytes of the sinoatrial node had approximately twice as many autoradiographic grains as the surrounding atrial myocytes. The myocytes of the atrioventricular bundle had a number of grains similar to the number in surrounding septal myocytes. Autoradiographic inhibition curves with betaxolol or ICI-118,551 demonstrated that both the sinoatrial node and the atrioventricular bundle had inhibition profiles similar to the surrounding myocytes (predominantly beta 1) but unlike the inhibition profiles of arterioles (predominantly beta 2). Calculations using the dissociation constants derived from the nonlinear curve-fitting analysis and the percent specific binding in the presence of 4 x 10(-7) M betaxolol or ICI-118,551 determined that the proportion of beta 1- to beta 2-receptors was the same (70-80% beta 1) when comparing the sinoatrial node and the surrounding atrial myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic receptor-G protein-adenylate cyclase complex in experimental canine congestive heart failure produced by rapid ventricular pacing

Changes in the beta-adrenergic receptor-G protein-adenylate cyclase complex were investigated in an experimental canine model of low-output heart failure produced by chronic rapid ventricular pacing. The contractile response occurring after exposure to the beta-adrenergic agonist dobutamine, measured as peak left ventricular + dP/dt, was decreased after 3 weeks of pacing. To further characterize the diminished functional responsiveness to beta-adrenergic receptor stimulation, beta-adrenergic receptor-adenylate cyclase coupling was investigated using membranes prepared from both control and paced animals. The density of beta-adrenergic receptors was decreased by 40% with a selective downregulation of the beta 1-subtype. The affinity of the receptor for the antagonist radioligand [125I]iodocyanopindolol remained unchanged. A defect in coupling was suggested by a decreased ability of isoproterenol, fluoride, and forskolin to stimulate adenylate cyclase in membranes prepared from failing hearts. Determination of the levels of Gi alpha (the alpha-subunit of Gi) by immunoblotting and pertussis toxin labeling revealed modest increases of approximately 30%. Furthermore, Mn2+ and purified Gs failed to stimulate adenylate cyclase in membranes prepared from failing hearts, indicating an impairment in the catalytic moiety of adenylate cyclase itself or in the ability of adenylate cyclase to couple to Gs. In contrast, complementation assay did not reveal differences in the functional activity of Gs alpha (the alpha-subunit of Gs). Taken together, these data demonstrate a selective decrease in the beta 1-subtype of adrenergic receptors and an increase in a 40-kd G1-like protein in the failing heart. Similar changes have been described in human idiopathic dilated cardiomyopathy. In addition to these changes, we identified a possible defect at the level of the catalytic subunit of adenylate cyclase.