Both beta(2)- and beta(1)-adrenergic receptors mediate hastened relaxation and phosphorylation of phospholamban and troponin I in ventricular myocardium of Fallot infants, consistent with selective coupling of beta(2)-adrenergic receptors to G(s)-protein

Proteomic and cardiac dysregulation by representative perfluoroalkyl acids of different chemical speciation during early embryogenesis of zebrafish

Perfluoroalkyl acids (PFAAs) of different chemical speciation were previously found to cause diverse toxicity. However, the toxicological mechanisms depending on chemical speciation are still largely unknown. In this follow-up study, zebrafish embryos were acutely exposed to only one concentration at 4.67 μM of the acid and salt of representative PFAAs, including perfluorooctanoic acid (PFOA), perfluorobutane carboxylic acid (PFBA), and perfluorobutanesulfonic acid (PFBS), till 96 h post-fertilization (hpf), aiming to gain more mechanistic insights. High-throughput proteomics found that PFAA acid and salt exerted discriminative effects on protein expression pattern. Bioinformatic analyses based on differentially expressed proteins underlined the developmental cardiotoxicity of PFOA acid with regard to cardiac muscle contraction, vascular smooth muscle contraction, adrenergic signaling in cardiomyocytes, and multiple terms related to myocardial contraction. PFOA salt and PFBS acid merely disrupted the cardiac muscle contraction pathway, while cardiac muscle cell differentiation was significantly enriched in PFBA acid-exposed zebrafish larvae. Consistently, under PFAA exposure, especially PFOA and PFBS acid forms, transcriptional levels of key genes for cardiogenesis and the concentrations of troponin and epinephrine associated with myocardial contraction were significantly dysregulated. Moreover, a transgenic line Tg (my17: GFP) expressing green fluorescent protein in myocardial cells was employed to visualize the histopathology of developing heart. PFOA acid concurrently caused multiple deficits in heart morphogenesis and function, which were characterized by the significant increase in sinus venosus and bulbus arteriosus distance (SV-BA distance), the induction of pericardial edema, and the decrease in heart rate, further confirming the stronger toxicity of PFOA acid than the salt counterpart on heart development. Overall, this study highlighted the developmental cardiotoxicity of PFAAs, with potency ranking PFOA > PFBS > PFBA. The acid forms of PFAAs induced stronger cardiac toxicity than their salt counterparts, providing an additional insight into the structure-toxicity relationship.

Pathological changes in GPCR signal organisation: Opportunities for targeted therapies for triple negative breast cancer

Triple negative breast cancer (TNBC) has the poorest prognosis compared to other breast cancer subtypes, due to a historical lack of targeted therapies and high rates of relapse. Greater insight into the components of signalling pathways in TNBC tumour cells has led to the clinical evaluation, and in some cases approval, of targeted therapies. In the last decade, G protein-coupled receptors, such as the β₂-adrenoceptor, have emerged as potential new therapeutic targets. Here, we describe how the β₂-adrenoceptor accelerates TNBC progression in response to stress, and the unique signalling pathway activated by the β₂-adrenoceptor to drive the invasion of an aggressive TNBC tumour cell. We highlight evidence that supports an altered organisation of GPCRs in tumour cells, and suggests that activation of the same GPCR in a different cellular location can control unique cell responses. Finally, we speculate how the relocation of GPCRs to the "wrong" place in tumour cells presents opportunities to develop targeted anti-cancer GPCR drugs with greater efficacy and minimal adverse effects.

Selective activation of adrenoceptors potentiates IKs current in pulmonary vein cardiomyocytes through the protein kinase A and C signaling pathways

Delayed rectifier K⁺ current (I_Ks) is a key contributor to repolarization of action potentials. This study investigated the mechanisms underlying the adrenoceptor-induced potentiation of I_Ks in pulmonary vein cardiomyocytes (PVC). PVC were isolated from guinea pig pulmonary vein. The action potentials and I_Ks current were recorded using perforated and conventional whole-cell patch-clamp techniques. The expression of I_Ks was examined using immunocytochemistry and Western blotting. KCNQ1, a I_Ks pore-forming protein was detected as a signal band approximately 100 kDa in size, and its immunofluorescence signal was found to be mainly localized on the cell membrane. The I_Ks current in PVC was markedly enhanced by both β₁- and β₂-adrenoceptor stimulation with a negative voltage shift in the current activation, although the potentiation was more effectively induced by β₂-adrenoceptor stimulation than β₁-adrenoceptor stimulation. Both β-adrenoceptor-mediated increases in I_Ks were attenuated by treatment with the adenylyl cyclase (AC) inhibitor or protein kinase A (PKA) inhibitor. Furthermore, the I_Ks current was increased by α₁-adrenoceptor agonist but attenuated by the protein kinase C (PKC) inhibitor. PVC exhibited action potentials in normal Tyrode solution which was slightly reduced by HMR-1556 a selective I_Ks blocker. However, HMR-1556 markedly reduced the β-adrenoceptor-potentiated firing rate. The stimulatory effects of β- and α₁-adrenoceptor on I_Ks in PVC are mediated via the PKA and PKC signal pathways. HMR-1556 effectively reduced the firing rate under β-adrenoceptor activation, suggesting that the functional role of I_Ks might increase during sympathetic excitation under in vivo conditions.

Effects of omecamtiv mecarbil on failing human ventricular trabeculae and interaction with (-)-noradrenaline

Omecamtiv mecarbil (OM) is a novel medicine for systolic heart failure, targeting myosin to enhance cardiomyocyte performance. To assist translation to clinical practice we investigated OMs effect on explanted human failing hearts, specifically; contractile dynamics, interaction with the β₁–adrenoceptor (AR) agonist (−)‐noradrenaline and spontaneous contractions. Left and right ventricular trabeculae from 13 explanted failing hearts, and trabeculae from 58 right atrial appendages of non‐failing hearts, were incubated with or without a single concentration of OM for 120 min. Time to peak force (TPF) and 50% relaxation (t_50%) were recorded. In other experiments, trabeculae were observed for spontaneous contractions and cumulative concentration‐effect curves were established to (−)‐noradrenaline at β₁‐ARs in the absence or presence of OM. OM prolonged TPF and t_50% in ventricular trabeculae (600 nM, 2 µM, p < .001). OM had no significant inotropic effect but reduced time dependent deterioration in contractile strength compared to control (p < .001). OM did not affect the generation of spontaneous contractions. The potency of (−)‐noradrenaline (pEC₅₀ 6.05 ± 0.10), for inotropic effect, was unchanged in the presence of OM 600 nM or 2 µM. Co‐incubation with (−)‐noradrenaline reduced TPF and t_50%, reversing the negative diastolic effects of OM. OM, at both 600 nM and 2 µM, preserved contractile force in left ventricular trabeculae, but imparted negative diastolic effects in trabeculae from human failing heart. (−)‐Noradrenaline reversed the negative diastolic effects, co‐administration may limit the titration of inotropes by reducing the threshold for ischemic side effects.

Human iPSC-engineered cardiac tissue platform faithfully models important cardiac physiology

Cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CM) may provide an important bridge between animal models and the intact human myocardium. Fulfilling this potential is hampered by their relative immaturity, leading to poor physiological responsiveness. hiPSC-CMs grown in traditional two-dimensional (2D) culture lack a t-tubular system, have only rudimentary intracellular calcium-handling systems, express predominantly embryonic sarcomeric protein isoforms, and preferentially use glucose as an energy substrate. Culturing hiPSC-CM in a variety of three-dimensional (3D) environments and the addition of nutritional, pharmacological, and electromechanical stimuli have proven, to various degrees, to be beneficial for maturation. We present a detailed assessment of a novel model in which hiPSC-CMs and hiPSC-derived cardiac fibroblasts are cocultured in a 3D fibrin matrix to form engineered cardiac tissue constructs (hiPSC-ECTs). The hiPSC-ECTs are responsive to physiological stimuli, including stretch, frequency, and β-adrenergic stimulation, develop a t-tubular system, and demonstrate calcium-handling and contractile kinetics that compare favorably with ventricular human myocardium. Furthermore, transcript levels of various genes involved in calcium-handling and contraction are increased. These markers of maturation become more robust over a relatively short period of time in culture (6 wk vs. 2 wk in hiPSC-ECTs). A comparison of the hiPSC-ECT molecular and performance variables with those of human cardiac tissue and other available engineered tissue platforms is provided to aid selection of the most appropriate platform for the research question at hand. Important and noteworthy aspects of this human cardiac model system are its reliance on "off-the-shelf" equipment, ability to provide detailed physiological performance data, and the ability to achieve a relatively mature cardiac physiology without additional nutritional, pharmacological, and electromechanical stimuli that may elicit unintended effects on function.NEW & NOTEWORTHY This study seeks to provide an in-depth assessment of contractile performance of human iPSC-derived cardiomyocytes cultured together with fibroblasts in a 3-dimensional-engineered tissue and compares performance both over time as cells mature, and with corresponding measures found in the literature using alternative 3D culture configurations. The suitability of 3D-engineered human cardiac tissues to model cardiac function is emphasized, and data provided to assist in the selection of the most appropriate configuration based on the target application.

Understanding How Phosphorylation and Redox Modifications Regulate Cardiac Ryanodine Receptor Type 2 Activity to Produce an Arrhythmogenic Phenotype in Advanced Heart Failure

Heart failure (HF) is a global pandemic with significant mortality and morbidity. Despite current medications, 50% of individuals die within 5 years of diagnosis. Of these deaths, 30-50% will be a result of sudden cardiac death from ventricular arrhythmias. This review discusses two stress-induced mechanisms, phosphorylation from chronic β-adrenoceptor (β-AR) stimulation and thiol modifications from oxidative stress, and how they modulate the cardiac ryanodine receptor type 2 (RyR2) and foster an arrhythmogenic phenotype. Calcium (Ca²⁺) is the ubiquitous secondary messenger of excitation-contraction coupling and provides a common pathway for contractile dysfunction and arrhythmia genesis. In a healthy heart, Ca²⁺ is released from the sarcoplasmic reticulum (SR) by RyR2. The open probability of RyR2 is under the dynamic influence of co-proteins, ions, and kinases that are in strict balance to ensure normal physiological functioning. In HF, chronic β-AR activity and production of reactive oxygen species and reactive nitrogen species provide two stress-induced mechanisms uncoupling RyR2 control, resulting in pathological diastolic SR Ca²⁺ leak. This increased cytosolic [Ca²⁺] promotes Ca²⁺ extrusion via the local Na⁺/Ca²⁺ exchanger, resulting in net sarcolemmal depolarization, delayed after depolarization and ventricular arrhythmia. Experimental models researching oxidative stress and phosphorylation have aimed to identify how post-translational modifications to the RyR2 macromolecular complex, and the associated Na⁺/Ca²⁺ cycling proteins, result in pathological Ca²⁺ handling and diastolic leak. However, the causative molecular changes remain controversial and undefined. Through understanding the molecular mechanisms that produce an arrhythmic phenotype, novel therapeutic targets to treat HF and prevent its malignant course can be identified.

Sarcoplasmic reticulum calcium mishandling: central tenet in heart failure?

Excitation-contraction coupling links excitation of the sarcolemmal surface membrane to mechanical contraction. In the heart this link is established via a Ca²⁺-induced Ca²⁺ release process, which, following sarcolemmal depolarisation, prompts Ca²⁺ release from the sarcoplasmic reticulum (SR) though the ryanodine receptor (RyR2). This substantially raises the cytoplasmic Ca²⁺ concentration to trigger systole. In diastole, Ca²⁺ is removed from the cytoplasm, primarily via the sarcoplasmic-endoplasmic reticulum Ca²⁺-dependent ATPase (SERCA) pump on the SR membrane, returning Ca²⁺ to the SR store. Ca²⁺ movement across the SR is thus fundamental to the systole/diastole cycle and plays an essential role in maintaining cardiac contractile function. Altered SR Ca²⁺ homeostasis (due to disrupted Ca²⁺ release, storage, and reuptake pathways) is a central tenet of heart failure and contributes to depressed contractility, impaired relaxation, and propensity to arrhythmia. This review will focus on the molecular mechanisms that underlie asynchronous Ca²⁺ cycling around the SR in the failing heart. Further, this review will illustrate that the combined effects of expression changes and disruptions to RyR2 and SERCA2a regulatory pathways are critical to the pathogenesis of heart failure.

Cardiac β-adrenergic receptor activation mediates distinct and cell type-dependent changes in the expression and distribution of connexin 43

Activation of the sympatho-β-adrenergic receptors (β-ARs) system is a hallmark of heart failure, leading to fibrosis and arrhythmias. Connexin 43 (Cx43) is the most abundant gap junctional protein in the myocardium. Current knowledge is limited regarding Cx43 remodelling in diverse cell types in the diseased myocardium and the underlying mechanism. We studied cell type-dependent changes in Cx43 remodelling due to β-AR overactivation and molecular mechanisms involved. Mouse models of isoproterenol stimulation or transgenic cardiomyocyte overexpression of β₂ -AR were used, which exhibited cardiac fibrosis and up-regulated total Cx43 abundance. In both models, whereas Cx43 expression in cardiomyocytes was reduced and more laterally distributed, fibroblasts exhibited elevated Cx43 expression and enhanced gap junction communication. Mechanistically, activation of β₂ -AR in fibroblasts in vitro elevated Cx43 expression, which was abolished by the β₂ -antagonist ICI-118551 or protein kinase A inhibitor H-89, but simulated by the adenylyl cyclase activator forskolin. Our in vitro and in vivo data showed that β-AR activation-induced production of IL-18 sequentially stimulated Cx43 expression in fibroblasts in a paracrine fashion. In summary, our findings demonstrate a pivotal role of β-AR in mediating distinct and cell type-dependent changes in the expression and distribution of Cx43, leading to pathological gap junction remodelling in the myocardium.

Phenytoin Reduces Activity of Cardiac Ryanodine Receptor 2; A Potential Mechanism for Its Cardioprotective Action

Phenytoin is a hydantoin derivative that is used clinically for the treatment of epilepsy and has been reported to have antiarrhythmic actions on the heart. In a failing heart, the elevated diastolic Ca²⁺ leak from the sarcoplasmic reticulum can be normalized by the cardiac ryanodine receptor 2 (RyR2) inhibitor, dantrolene, without inhibiting Ca²⁺ release during systole or affecting Ca²⁺ release in normal healthy hearts. Unfortunately, dantrolene is hepatotoxic and unsuitable for chronic long-term administration. Because phenytoin and dantrolene belong to the hydantoin class of compounds, we test the hypothesis that dantrolene and phenytoin have similar inhibitory effects on RyR2 using a single-channel recording of RyR2 activity in artificial lipid bilayers. Phenytoin produced a reversible inhibition of RyR2 channels from sheep and human failing hearts. It followed a hyperbolic dose response with maximal inhibition of ∼50%, Hill coefficient ∼1, and IC₅₀ ranging from 10 to 20 µM. It caused inhibition at diastolic cytoplasmic [Ca²⁺] but not at Ca²⁺ levels in the dyadic cleft during systole. Notably, phenytoin inhibits RyR2 from failing human heart but not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans. We conclude that phenytoin could effectively inhibit RyR2-mediated release of Ca²⁺ in a manner paralleling that of dantrolene. Moreover, the IC₅₀ of phenytoin in RyR2 is at least threefold lower than for other ion channels and clinically used serum levels, pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrythmias. SIGNIFICANCE STATEMENT: We show that phenytoin, a Na channel blocker used clinically for treatment of epilepsy, is a diastolic inhibitor of cardiac calcium release channels [cardiac ryanodine receptor 2 (RyR2)] at doses threefold lower than its current therapeutic levels. Phenytoin inhibits RyR2 from failing human heart and not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans and pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrhythmias.

Enhancement of human iPSC-derived cardiomyocyte maturation by chemical conditioning in a 3D environment

Recent advances in the understanding and use of pluripotent stem cells have produced major changes in approaches to the diagnosis and treatment of human disease. An obstacle to the use of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for regenerative medicine, disease modeling and drug discovery is their immature state relative to adult myocardium. We show the effects of a combination of biochemical factors, thyroid hormone, dexamethasone, and insulin-like growth factor-1 (TDI) on the maturation of hiPSC-CMs in 3D cardiac microtissues (CMTs) that recapitulate aspects of the native myocardium. Based on a comparison of the gene expression profiles and the structural, ultrastructural, and electrophysiological properties of hiPSC-CMs in monolayers and CMTs, and measurements of the mechanical and pharmacological properties of CMTs, we find that TDI treatment in a 3D tissue context yields a higher fidelity adult cardiac phenotype, including sarcoplasmic reticulum function and contractile properties consistent with promotion of the maturation of hiPSC derived cardiomyocytes.

Calmodulin inhibition of human RyR2 channels requires phosphorylation of RyR2-S2808 or RyR2-S2814

Calmodulin (CaM) is a Ca-binding protein that binds to, and can directly inhibit cardiac ryanodine receptor calcium release channels (RyR2). Animal studies have shown that RyR2 hyperphosphorylation reduces CaM binding to RyR2 in failing hearts, but data are lacking on how CaM regulates human RyR2 and how this regulation is affected by RyR2 phosphorylation. Physiological concentrations of CaM (100 nM) inhibited the diastolic activity of RyR2 isolated from failing human hearts by ~50% but had no effect on RyR2 from healthy human hearts. Using FRET between donor-FKBP12.6 and acceptor-CaM bound to RyR2, we determined that CaM binds to RyR2 from healthy human heart with a K_d = 121 ± 14 nM. Ex-vivo phosphorylation/dephosphorylation experiments suggested that the divergent CaM regulation of healthy and failing human RyR2 was caused by differences in RyR2 phosphorylation by protein kinase A and Ca-CaM-dependent kinase II. Ca²⁺-spark measurements in murine cardiomyocytes harbouring RyR2 phosphomimetic or phosphoablated mutants at S2814 and S2808 suggest that phosphorylation of residues corresponding to either human RyR2-S2808 or S2814 is both necessary and sufficient for RyR2 regulation by CaM. Our results challenge the current concept that CaM universally functions as a canonical inhibitor of RyR2 across species. Rather, CaM's biological action on human RyR2 appears to be more nuanced, with inhibitory activity only on phosphorylated RyR2 channels, which occurs during exercise or in patients with heart failure.

Different Responses to Drug Safety Screening Targets between Human Neonatal and Infantile Heart Tissue and Cardiac Bodies Derived from Human-Induced Pluripotent Stem Cells

Aims

Induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) have become a promising tool in cardiovascular safety pharmacology. Immaturity of iPS-CMs remains an ongoing concern. We compared electrophysiological and contractile features of cardiac bodies (hiPS-CBs) derived from human-induced pluripotent stem cells and human neonatal and infantile myocardial slices relevant for drug screening.

Methods and Results

Myocardial tissue slices were prepared from biopsies obtained from patients undergoing surgery for hypoplastic left heart syndrome (HLHS) and tetralogy of Fallot (TOF). Electrophysiological features and response to I_k,r blockade as well as contractile properties were investigated using microelectrodes and isometric force measurements and were compared to hiPS-CBs. Both native myocardial tissue slices as well as hiPS-CBs showed action potential prolongation after I_k,r blockade, but early afterdepolarisations could be observed in native myocardial tissue slices only. The force-frequency relationship (FFR) varied at lower frequencies and was negative throughout at higher frequencies in hiPS-CBs. In contrast, native myocardial tissue slices exhibited positive, negative, and biphasic FFRs. In contrast to native myocardial tissue slices, hiPS-CBs failed to show an inotropic response to ß-adrenergic stimulation. Although all groups showed ß-adrenergic induced positive lusitropy, the effect was more pronounced in myocardial tissue slices.

Conclusion

hiPS-CBs were able to reproduce AP prolongation after I_k,r blockade, but to a lesser extent compared to human neonatal and infantile myocardial tissue slices. Early afterdepolarisations could not be induced in hiPS-CBs. Contractile force was differently regulated by β-adrenergic stimulation in hiPS-CBs and the native myocardium. If used for cardiotoxicity screening, caution is warranted as hiPS-CBs might be less sensitive to pharmacologic targets compared to the native myocardium of neonates and infants.

Understanding Key Mechanisms of Exercise-Induced Cardiac Protection to Mitigate Disease: Current Knowledge and Emerging Concepts

The benefits of exercise on the heart are well recognized, and clinical studies have demonstrated that exercise is an intervention that can improve cardiac function in heart failure patients. This has led to significant research into understanding the key mechanisms responsible for exercise-induced cardiac protection. Here, we summarize molecular mechanisms that regulate exercise-induced cardiac myocyte growth and proliferation. We discuss in detail the effects of exercise on other cardiac cells, organelles, and systems that have received less or little attention and require further investigation. This includes cardiac excitation and contraction, mitochondrial adaptations, cellular stress responses to promote survival (heat shock response, ubiquitin-proteasome system, autophagy-lysosomal system, endoplasmic reticulum unfolded protein response, DNA damage response), extracellular matrix, inflammatory response, and organ-to-organ crosstalk. We summarize therapeutic strategies targeting known regulators of exercise-induced protection and the challenges translating findings from bench to bedside. We conclude that technological advancements that allow for in-depth profiling of the genome, transcriptome, proteome and metabolome, combined with animal and human studies, provide new opportunities for comprehensively defining the signaling and regulatory aspects of cell/organelle functions that underpin the protective properties of exercise. This is likely to lead to the identification of novel biomarkers and therapeutic targets for heart disease.

Effects of hypoxia-ischemia and inotropes on expression of cardiac adrenoceptors in the preterm fetal sheep

Preterm infants frequently suffer cardiovascular compromise, with hypotension and/or low systemic blood flow, leading to tissue hypoxia-ischemia (HI). Many preterm infants respond inadequately to inotropic treatments using adrenergic agonists such as dobutamine (DB) or dopamine (DA). This may be because of altered cardiac adrenoceptor expression because of tissue HI or prolonged exposure to adrenergic agonists. We assessed the effects of severe HI with and without DB/DA treatment on cardiac adrenoceptor expression in preterm fetal sheep. Fetal sheep (93-95 days) exposed to sham surgery or severe HI induced by umbilical cord occlusion received intravenous DB or saline for 74 h (HI + DB, HI, Sham + DB, Sham). The HI groups were also compared with fetal sheep exposed to HI and DA. Fetal hearts were collected to determine β-adrenoceptor numbers using [¹²⁵I]-cyanopindolol binding and mRNA expression of β₁-, β₂-, α_1A-, α_2A-, or α_2B-adrenoceptors. The HI group had increased β-adrenoceptor numbers compared with all other groups in all four heart chambers ( P < 0.05). This increase in β-adrenoceptor numbers in the HI group was significantly reduced by DB infusion in all four heart chambers, but DA infusion in the HI group only reduced β-adrenoceptor numbers in the left atria and ventricle. DB alone did not affect β-adrenoceptor numbers in the sham animals. Changes in β₁-adrenoceptor mRNA levels trended to parallel the binding results. We conclude that HI upregulates preterm fetal cardiac β-adrenoceptors, but prolonged exposure to adrenergic agonists downregulates adrenoceptors in the preterm heart exposed to HI and may underpin the frequent failure of inotropic therapy in preterm infants. NEW & NOTEWORTHY This is the first study, to our knowledge, on the effects of hypoxia-ischemia and adrenergic agonists on adrenoceptors in the preterm heart. In fetal sheep, we demonstrate that hypoxia-ischemia increases cardiac β-adrenoceptor numbers. However, exposure to both hypoxia-ischemia and adrenergic agonists (dobutamine or dopamine) reduces the increase in β-adrenoceptor numbers, which may underpin the inadequate response in human preterm infants to inotropic therapy using adrenergic agonists. Dobutamine alone does not affect the cardiac adrenoceptors in the sham animals.

Blinded Contractility Analysis in hiPSC-Cardiomyocytes in Engineered Heart Tissue Format: Comparison With Human Atrial Trabeculae

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) may serve as a new assay for drug testing in a human context, but their validity particularly for the evaluation of inotropic drug effects remains unclear. In this blinded analysis, we compared the effects of 10 indicator compounds with known inotropic effects in electrically stimulated (1.5 Hz) hiPSC-CM-derived 3-dimensional engineered heart tissue (EHT) and human atrial trabeculae (hAT). Human EHTs were prepared from iCell hiPSC-CM, hAT obtained at routine heart surgery. Mean intra-batch variation coefficient in baseline force measurement was 17% for EHT and 49% for hAT. The PDE-inhibitor milrinone did not affect EHT contraction force, but increased force in hAT. Citalopram (selective serotonin reuptake inhibitor), nifedipine (LTCC-blocker) and lidocaine (Na+ channel-blocker) had negative inotropic effects on EHT and hAT. Formoterol (beta-2 agonist) had positive lusitropic but no inotropic effect in EHT, and positive clinotropic, lusitropic, and inotropic effects in hAT. Tacrolimus (calcineurin-inhibitor) had a negative inotropic effect in EHTs, but no effect in hAT. Digoxin (Na+-K+-ATPase-inhibitor) showed a positive inotropic effect only in EHTs, but no effect in hAT probably due to short incubation time. Ryanodine (ryanodine receptor-inhibitor) reduced contraction force in both models. Rolipram and acetylsalicylic acid showed noninterpretable results in hAT. Contraction amplitude and kinetics were more stable over time and less variable in hiPSC-EHTs than hAT. HiPSC-EHT faithfully detected cAMP-dependent and -independent positive and negative inotropic effects, but limited beta-2 adrenergic or PDE3 effects, compatible with an immature CM phenotype.

Defined Engineered Human Myocardium With Advanced Maturation for Applications in Heart Failure Modeling and Repair

BACKGROUND

Advancing structural and functional maturation of stem cell-derived cardiomyocytes remains a key challenge for applications in disease modeling, drug screening, and heart repair. Here, we sought to advance cardiomyocyte maturation in engineered human myocardium (EHM) toward an adult phenotype under defined conditions.

METHODS

We systematically investigated cell composition, matrix, and media conditions to generate EHM from embryonic and induced pluripotent stem cell-derived cardiomyocytes and fibroblasts with organotypic functionality under serum-free conditions. We used morphological, functional, and transcriptome analyses to benchmark maturation of EHM.

RESULTS

EHM demonstrated important structural and functional properties of postnatal myocardium, including: (1) rod-shaped cardiomyocytes with M bands assembled as a functional syncytium; (2) systolic twitch forces at a similar level as observed in bona fide postnatal myocardium; (3) a positive force-frequency response; (4) inotropic responses to β-adrenergic stimulation mediated via canonical β₁- and β₂-adrenoceptor signaling pathways; and (5) evidence for advanced molecular maturation by transcriptome profiling. EHM responded to chronic catecholamine toxicity with contractile dysfunction, cardiomyocyte hypertrophy, cardiomyocyte death, and N-terminal pro B-type natriuretic peptide release; all are classical hallmarks of heart failure. In addition, we demonstrate the scalability of EHM according to anticipated clinical demands for cardiac repair.

CONCLUSIONS

We provide proof-of-concept for a universally applicable technology for the engineering of macroscale human myocardium for disease modeling and heart repair from embryonic and induced pluripotent stem cell-derived cardiomyocytes under defined, serum-free conditions.

Molecular Mechanisms of Right Ventricular Failure

An abundance of data has provided insight into the mechanisms underlying the development of left ventricular (LV) hypertrophy and its progression to LV failure. In contrast, there is minimal data on the adaptation of the right ventricle (RV) to pressure and volume overload and the transition to RV failure. This is a critical clinical question, because the RV is uniquely at risk in many patients with repaired or palliated congenital heart disease and in those with pulmonary hypertension. Standard heart failure therapies have failed to improve function or survival in these patients, suggesting a divergence in the molecular mechanisms of RV versus LV failure. Although, on the cellular level, the remodeling responses of the RV and LV to pressure overload are largely similar, there are several key differences: the stressed RV is more susceptible to oxidative stress, has a reduced angiogenic response, and is more likely to activate cell death pathways than the stressed LV. Together, these differences could explain the more rapid progression of the RV to failure versus the LV. This review will highlight known molecular differences between the RV and LV responses to hemodynamic stress, the unique stressors on the RV associated with congenital heart disease, and the need to better understand these molecular mechanisms if we are to develop RV-specific heart failure therapeutics.

Adrenergic receptor genotype influences heart failure severity and β-blocker response in children with dilated cardiomyopathy

BACKGROUND

Adrenergic receptor (ADR) genotypes are associated with heart failure (HF) and β-blocker response in adults. We assessed the influence of ADR genotypes in children with dilated cardiomyopathy (DCM).

METHODS

Ninety-one children with advanced DCM and 44 with stable DCM were genotyped for three ADR genotypes associated with HF risk in adults: α2cdel322-325, β1Arg389, and β2Arg16. Data were analyzed by genotype and β-blocker use. Mean age at enrollment was 8.5 y.

RESULTS

One-year event-free survival was 51% in advanced and 80% in stable DCM. High-risk genotypes were associated with higher left ventricular (LV) filling pressures, higher systemic and pulmonary vascular resistance, greater decline in LV ejection fraction (P < 0.05), and a higher frequency of mechanical circulatory support while awaiting transplant (P = 0.05). While β-blockers did not reduce HF severity in the overall cohort, in the subset with multiple high-risk genotypes, those receiving β-blockers showed better preservation of cardiac function and hemodynamics compared with those not receiving β-blockers (interaction P < 0.05).

CONCLUSION

Our study identifies genetic risk markers that may help in the identification of patients at risk for developing decompensated HF and who may benefit from early institution of β-blocker therapy before progression to decompensated HF.

G protein-coupled receptors in cardiac biology: old and new receptors

G protein-coupled receptors (GPCRs) are seven-transmembrane-spanning proteins that mediate cellular and physiological responses. They are critical for cardiovascular function and are targeted for the treatment of hypertension and heart failure. Nevertheless, current therapies only target a small fraction of the cardiac GPCR repertoire, indicating that there are many opportunities to investigate unappreciated aspects of heart biology. Here, we offer an update on the contemporary view of GPCRs and the complexities of their signalling, and review the roles of the 'classical' GPCRs in cardiovascular physiology and disease. We then provide insights into other GPCRs that have been less extensively studied in the heart, including orphan, odorant and taste receptors. We contend that these novel cardiac GPCRs contribute to heart function in health and disease and thereby offer exciting opportunities to therapeutically modulate heart function.

Carvedilol induces greater control of β2- than β 1-adrenoceptor-mediated inotropic and lusitropic effects by PDE3, while PDE4 has no effect in human failing myocardium

The β-blockers carvedilol and metoprolol provide important therapeutic strategies for heart failure treatment. Therapy with metoprolol facilitates the control by phosphodiesterase PDE3, but not PDE4, of inotropic effects of catecholamines in human failing ventricle. However, it is not known whether carvedilol has the same effect. We investigated whether the PDE3-selective inhibitor cilostamide (0.3 μM) or PDE4-selective inhibitor rolipram (1 μM) modified the positive inotropic and lusitropic effects of catecholamines in ventricular myocardium of heart failure patients treated with carvedilol. Right ventricular trabeculae from explanted hearts of nine carvedilol-treated patients with terminal heart failure were paced to contract at 1 Hz. The effects of (-)-noradrenaline, mediated through β1-adrenoceptors (β2-adrenoceptors blocked with ICI118551), and (-)-adrenaline, mediated through β2-adrenoceptors (β1-adrenoceptors blocked with CGP20712A), were assessed in the absence and presence of the PDE inhibitors. The inotropic potency, estimated from -logEC50s, was unchanged for (-)-noradrenaline but decreased 16-fold for (-)-adrenaline in carvedilol-treated compared to non-β-blocker-treated patients, consistent with the previously reported β2-adrenoceptor-selectivity of carvedilol. Cilostamide caused 2- to 3-fold and 10- to 35-fold potentiations of the inotropic and lusitropic effects of (-)-noradrenaline and (-)-adrenaline, respectively, in trabeculae from carvedilol-treated patients. Rolipram did not affect the inotropic and lusitropic potencies of (-)-noradrenaline or (-)-adrenaline. Treatment of heart failure patients with carvedilol induces PDE3 to selectively control the positive inotropic and lusitropic effects mediated through ventricular β2-adrenoceptors compared to β1-adrenoceptors. The β2-adrenoceptor-selectivity of carvedilol may provide protection against β2-adrenoceptor-mediated ventricular overstimulation in PDE3 inhibitor-treated patients. PDE4 does not control β1- and β2-adrenoceptor-mediated inotropic and lusitropic effects in carvedilol-treated patients.

Recent structural advances of β1 and β2 adrenoceptors yield keys for ligand recognition and drug design

Because they represent attractive drug targets, adrenoceptors have been widely studied. Recent progress in structural data of β-adrenoceptors allows us to understand and predict key interactions in ligand recognition and receptor activation. Nevertheless, an important aspect of this process has only begun to be explored: the stabilization of a conformational state of these receptors upon contact with a ligand and the capacity of a ligand to influence receptor conformation through allosteric modulation, biased signaling, and selectivity. The aim of the present Perspective is to identify the well-defined orthosteric binding site and possible allosteric sites and to analyze the importance of the ligand-receptor interaction in the stabilization of certain receptor conformations. For this purpose, we have reviewed recent advances made through the use of X-ray data from ligand-β-adrenoceptor (including ADRB1 and ADRB2) crystal structures. Most importantly, implications in the medicinal chemistry field are explored in relation to drug design.

Spatial control of the βAR system in heart failure: the transverse tubule and beyond

The beta1-adrenoceptors (β(1)AR) and beta-2 (β(2)AR) adrenoceptors represent the predominant pathway for sympathetic control of myocardial function. Diverse mechanisms have evolved to translate signalling via these two molecules into differential effects on physiology. In this review, we discuss how the functions of the βAR are organized from the level of secondary messengers to the whole heart to achieve this. Using novel microscopy and bio-imaging methods researchers have uncovered subtle organization of the control of cyclic adenosine monophosphate (cAMP), the predominant positively inotropic pathway for the βAR. The β(2)AR in particular is demonstrated to give rise to highly compartmentalized, spatially confined cAMP signals. Organization of β(2)AR within the T-tubule and caveolae of cardiomyocytes concentrates this receptor with molecules which buffer and shape its cAMP signal to give fine control. This situation is undermined in various forms of heart failure. Human and animal models of heart failure demonstrate disruption of cellular micro-architecture which contributes to the change in response to cardiac βARs. Loss of cellular structure has proved key to the observed loss of confined β(2)AR signalling. Some pharmacological and genetic treatments have been successful in returning failing cells to a more structured phenotype. Within these cells it has been possible to observe the partial restoration of normal β(2)AR signalling. At the level of the organ, the expression of the two βAR subtypes varies between regions with the β(2)AR forming a greater proportion of the βAR population at the apex. This distribution may contribute to regional wall motion abnormalities in Takotsubo cardiomyopathy, a syndrome of high sympathetic activity, where the phosphorylated β(2)AR can signal via Gi protein to produce negatively inotropic effects.

Cardiovascular catecholamine receptors in children: their significance in cardiac disease

Adrenoceptors and dopamine receptors are grouped together under the name 'catecholamine receptors.' Catecholamines and catecholaminergic drugs act on catecholamine receptors located on or near the cardiovascular system. The physiological effects of catecholamine receptor stimulation are only partly understood. The catecholaminergic drugs used in critical care medicine today are not selective, or are, at best, in part selective for the various catecholamine receptor subtypes. Many patients, however, depend on them. A variety of animal models has been developed to unravel catecholamine distribution and function. However, the identification of species heterogeneity makes it imperative to determine catecholamine receptor distribution and function in humans. In addition, age-related alterations in catecholamine receptor distribution and function have been identified in human adults. This might have implications for our understanding of the effect of catecholamines in pediatric patients. This article will focus on the pediatric population and will review currently available in vitro data on the distribution and the function of catecholamine receptors in the cardiovascular system of fetuses and children. Also discussed are relevant young animal models and in vivo hemodynamic effects of cardiotonic drugs acting on the catecholamine receptor in children requiring major cardiac surgery. A better understanding of these topics might provide clues for new, receptor subtype-selective, therapeutic approaches in newborns and children with cardiac disease.

Calcitonin gene-related peptide restores disrupted excitation-contraction coupling in myotubes expressing central core disease mutations in RyR1

Central core disease (CCD) is a congenital human myopathy associated with mutations in the gene encoding the skeletal muscle ryanodine receptor (RyR1), resulting in skeletal muscle weakness and lower limb deformities. The muscle weakness can be at least partially explained by a reduced magnitude of voltage-gated Ca(2+) release (VGCR). To date, only a few studies have focused on identifying potential therapeutic agents for CCD. Therefore, in this work we investigated the potential use of the calcitonin gene related peptide (CGRP) to restore VGCR in myotubes expressing CCD RyR1 mutants. We also examined the influence of CCD mutants on Ca(2+)-dependent processes involved in myogenesis (myoblast fusion and sarcoendoplasmic reticulum Ca(2+)-ATPase isoform 2 (SERCA2) gene expression). C2C12 cells were transfected with cDNAs encoding either wild-type RyR1 or CCD mutants, and then exposed to CGRP (100 nm, 1-4 h). Expression of the I4897T mutant significantly inhibited SERCA2 gene expression and myoblast fusion, whereas the Y523S mutant exerted the opposite effect. Interestingly, both mutants clearly inhibited VGCR (50%), due to a reduction in SR Ca(2+) content. However, no major changes due to CGRP or CCD mutants were observed in I(CaL). Our data suggest that the Y523S mutant results in store depletion via decompensated SR Ca(2+) leak, while the I4897T mutant inhibits SERCA2 gene expression. Remarkably, in both cases CGRP restored VGCR, likely to have been by enhancing phospholamban (PLB) phosphorylation, SERCA activity and SR Ca(2+) content. Taken together, our data show that in the C2C12 model system, changes in excitation-contraction coupling induced by the expression of RyR1 channels bearing CCD mutations Y523S or I4897T can be reversed by CGRP.

Differential regulation of β2 -adrenoceptor-mediated inotropic and lusitropic response by PDE3 and PDE4 in failing and non-failing rat cardiac ventricle

BACKGROUND AND PURPOSE

β-Adrenoceptors play a major role in regulating myocardial function through cAMP-dependent pathways. Different phosphodiesterases (PDEs) regulate intracellular cAMP-pools and thereby contribute to the compartmentalization of cAMP-dependent effects. We explored the involvement of PDEs in limiting the β(2) adrenoceptor-mediated positive inotropic (PIR) and lusitropic (LR) responses in sham-operated (Sham) and failing rat hearts.

EXPERIMENTAL APPROACH

Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats. Rats developing heart failure were studied 6 weeks after surgery. Contractility was measured in left ventricular strips from failing and Sham hearts. cAMP was quantified by RIA.

KEY RESULTS

In ventricular strips, stimulation of β(2) -adrenoceptors with (-)-adrenaline (300 nM CGP20712A present) exerted a small PIR and LR. In Sham hearts, β(2) -adrenoceptor-mediated as well as β(1) -adrenoceptor-mediated PIR and LR were increased by selective inhibition of either PDE3 (1 µM cilostamide) or PDE4 (10 µM rolipram). In failing rat hearts, PDE3 inhibition enhanced PIR and LR to both β(1) - and β(2) -adrenoceptor stimulation while PDE4 inhibition had no effect on these responses despite a significant increase in cAMP levels. Combined PDE3/4 inhibition further enhanced the PIR and LR of β(2) - and β(1) -adrenoceptor activation both in Sham and failing hearts, compared with PDE3 inhibition alone. PDE4 enzyme activity was reduced in failing hearts.

CONCLUSIONS AND IMPLICATIONS

Both PDE3 and PDE4 attenuated β(2) - and β(1) -adrenoceptor-mediated contractile responses in Sham hearts. In failing hearts, these responses are attenuated solely by PDE3 and thus even selective PDE3 inhibitors may provide a profound enhancement of β-adrenoceptor-mediated responses in heart failure.

Force frequency relationship of the human ventricle increases during early postnatal development

Understanding developmental changes in contractility is critical to improving therapies for young cardiac patients. Isometric developed force was measured in human ventricular muscle strips from two age groups: newborns (<2 wk) and infants (3-14 mo) undergoing repair for congenital heart defects. Muscle strips were paced at several cycle lengths (CLs) to determine the force frequency response (FFR). Changes in Na/Ca exchanger (NCX), sarcoplasmic reticulum Ca-ATPase (SERCA), and phospholamban (PLB) were characterized. At CL 2000 ms, developed force was similar in the two groups. Decreasing CL increased developed force in the infant group to 131 +/- 8% (CL 1000 ms) and 157 +/- 18% (CL 500 ms) demonstrating a positive FFR. The FFR in the newborn group was flat. NCX mRNA and protein levels were significantly larger in the newborn than infant group whereas SERCA levels were unchanged. PLB mRNA levels and PLB/SERCA ratio increased with age. Immunostaining for NCX in isolated newborn cells showed peripheral staining. In infant cells, NCX was also found in T-tubules. SERCA staining was regular and striated in both groups. This study shows for the first time that the newborn human ventricle has a flat FFR, which increases with age and may be caused by developmental changes in calcium handling.

Dopamine increases L-type calcium current more in newborn than adult rabbit cardiomyocytes via D1 and β2 receptors

Dopamine is used to treat heart failure, particularly after cardiac surgery in infants, but the mechanisms of action are unclear. We investigated differences in the effect of dopamine on L-type calcium current ( ICa) between newborn (NB, 1–4 days) and adult (AD, 3–4 mo) rabbit ventricular myocytes. Myocytes were enzymatically dissociated from NB and AD rabbit hearts. ICa was recorded by using the whole cell patch-clamp technique. mRNA levels of cardiac dopamine receptor type 1 (D1), type 2 (D2), and β-adrenergic receptors (β-ARs) were measured by real-time RT-PCR. Dopamine (100 μM) increased ICa more in NB (Emax 87 ± 10%) than in AD ventricular cells (Emax 21 ± 3%). Further investigation of this difference showed that mRNA levels of the D1 receptor were significantly higher in NB, and, with β-AR blockade, dopamine increased ICa more in NB than AD cells. Additionally, SKF-38393 (selective D1 receptor agonist) significantly increased ICa by 55 ± 4% in NB ( P < 0.05, n = 4) and by 11 ± 1% in AD ( P < 0.05, n = 6). Dopamine in the presence of SCH-23390 (D1 receptor antagonist) increased ICa in NB cells by 67 ± 5% and by 22 ± 2% in AD cells, suggesting a role for β-AR stimulation. Selective blockade of β1- or β2-receptors (with block of D1 receptors) showed that the β-AR action of dopamine in the NB was largely mediated via β2-AR activation. Dopamine produces a larger increase in ICa in NB cardiomyocytes compared with ADs. The mechanism of action is not only through β2-ARs but also due to higher expression of cardiac D1 receptor in NB.

Chronic beta2-adrenoceptor stimulation impairs cardiac relaxation via reduced SR Ca2+-ATPase protein and activity

We determined the cardiovascular effects of chronic beta2-adrenoceptor (beta2-AR) stimulation in vivo and examined the mechanism for the previously observed prolonged diastolic relaxation. Rats (3 mo old; n = 6), instrumented with implantable radiotelemeters, received the selective beta2-AR agonist formoterol (25 microg.kg(-1).day(-1) ip) for 4 wk, with selected cardiovascular parameters measured daily throughout this period, and for a further 7 days after cessation of treatment. Chronic beta2-AR stimulation was associated with an increase in heart rate (HR) of 17% (days 1-14) and 5% (days 15-28); a 11% (days 1-14) and 6% (days 15-28) decrease in mean arterial blood pressure; and a 24% (days 1-14) increase in the rate of cardiac relaxation (-dP/dt) compared with initial values (P < 0.05). Cessation of beta2-AR stimulation resulted in an 8% decrease in HR and a 7% decrease in -dP/dt, compared with initial values (P < 0.05). The prolonged cardiac relaxation with chronic beta2-AR stimulation was associated with a 30% decrease in the maximal rate (Vmax) of sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) activity, likely attributed to a 50% decrease in SERCA2a protein (P < 0.05). glycogen synthase kinase-3beta (GSK-3beta) has been implicated as a negative regulator of SERCA2 gene transcription, and we observed a approximately 60% decrease (P < 0.05) in phosphorylated GSK-3beta protein after chronic beta2-AR stimulation. Finally, we found a 40% decrease (P < 0.05) in the mRNA expression of the novel A kinase anchoring protein AKAP18, also implicated in beta2-AR-mediated cardiac relaxation. These findings highlight some detrimental cardiovascular effects of chronic beta2-AR agonist administration and identify concerns for their current and future use for treating asthma or for conditions where muscle wasting and weakness are indicated.

beta(1)- and beta(2)-adrenoceptor responses in cardiomyocytes derived from human embryonic stem cells: comparison with failing and non-failing adult human heart

BACKGROUND AND PURPOSE

Characterization of human embryonic stem cell-derived cardiomyocytes (hESC-CM) in relation to adult myocytes is essential for their future use in transplantation or as a model system. The beta-adrenoceptor pathways, which are known to be effective early in hESC-CM development, are of major importance because of their control of rate and force of beating, arrhythmia generation and apoptosis/necrosis. We have therefore performed detailed pharmacological analysis of the beta-adrenoceptor responses in developing hESC-CM.

EXPERIMENTAL APPROACH

hESC-CMs were differentiated from H7 ESCs and studied up to 79 days of differentiation. Rate of beating and time course of contraction and relaxation were measured in superfused preparations.

KEY RESULTS

Responses to the mixed beta(1)- and beta(2)-adrenoceptor agonist isoprenaline were evident from day 10 to day 79. Stability of the responses during an application, for repeated applications on the same experimental day and over the time of development, was determined. Concentrations for half-maximal response (12.9 nM) were similar to those from adult human heart, but closer to those obtained from failing rather than normal ventricle. Acceleration of both contraction and relaxation was quantitatively similar to that in adult ventricular myocytes, as was sensitivity to muscarinic inhibition. Use of specific antagonists showed that both beta(1)- and beta(2)-adrenoceptors contributed to contractile responses, as seen with adult myocytes.

CONCLUSIONS AND IMPLICATIONS

These data show the compatibility of hESC-CM with adult human myocardium in terms of beta-adrenoceptor response. The experiments described here also confirm the utility of the hESC-CM preparation for detailed pharmacological analysis.

Phosphodiesterase-4 blunts inotropism and arrhythmias but not sinoatrial tachycardia of (-)-adrenaline mediated through mouse cardiac beta(1)-adrenoceptors

BACKGROUND AND PURPOSE

beta(1) and beta(2)-adrenoceptors coexist in murine heart but beta(2)-adrenoceptor-mediated effects have not been detected in atrial and ventricular tissues, possibly due to marked phosphodiesterase (PDE) activity. We investigated the influence of the PDE3 inhibitor cilostamide and PDE4 inhibitor rolipram on the effects of (-)-adrenaline in three regions of murine heart.

EXPERIMENTAL APPROACH

(-)-Adrenaline-evoked cardiostimulation was compared on sinoatrial beating rate, left atrial and right ventricular contractile force in isolated tissues from 129SvxC57B1/6 cross mice. Ventricular arrhythmic contractions were also assessed.

KEY RESULTS

Both rolipram (1 microM) and cilostamide (300 nM) caused transient sinoatrial tachycardia but neither enhanced the chronotropic potency of (-)-adrenaline. Rolipram potentiated 19-fold (left atrium) and 7-fold (right ventricle) the inotropic effects of (-)-adrenaline. (-)-Adrenaline elicited concentration-dependent ventricular arrhythmias that were potentiated by rolipram. All effects of (-)-adrenaline were antagonized by the beta(1)-adrenoceptor-selective antagonist CGP20712A (300 nM). Cilostamide (300 nM) did not increase the chronotropic and inotropic potencies of (-)-adrenaline, but administered jointly with rolipram in the presence of CGP20712A, uncovered left atrial inotropic effects of (-)-adrenaline that were prevented by the beta(2)-adrenoceptor-selective antagonist ICI118551.

CONCLUSIONS AND IMPLICATIONS

PDE4 blunts the beta(1)-adrenoceptor-mediated effects of (-)-adrenaline in left atrium and right ventricle but not in sinoatrial node. Both PDE3 and PDE4 reduce basal sinoatrial rate in a compartment distinct from the beta(1)-adrenoceptor compartment. PDE3 and PDE4, acting in concert, prevent left atrial beta(2)-adrenoceptor-mediated inotropy. PDE4 partially protects the right ventricle against (-)-adrenaline-evoked arrhythmias.

HUMAN HEART ?-ADRENOCEPTORS: ?1-ADRENOCEPTOR DIVERSIFICATION THROUGH ?AFFINITY STATES? AND POLYMORPHISM

1. In atrium and ventricle from failing and non-failing human hearts, activation of beta(1)- or beta(2)-adrenoceptors causes increases in contractile force, hastening of relaxation, protein kinase A-catalysed phosphorylation of proteins implicated in the hastening of relaxation, phospholamban, troponin I and C-protein, consistent with coupling of both beta(1)- and beta(2)-adrenoceptors to stimulatory G(salpha)-protein but not inhibitory G(ialpha)-protein. 2. Two 'affinity states', namely beta(1H) and beta(1L), of the beta(1)-adrenoceptor exist. In human heart, noradrenaline elicits powerful increases in contractile force and hastening of relaxation. These effects are blocked with high affinity by beta-adenoceptor antagonists, including propranolol, (-)-pindolol, (-)-CGP 12177 and carvedilol. Some beta-blockers, typified by (-)-pindolol and (-)-CGP 12177, not only block the receptor, but also activate it, albeit at much higher concentrations (approximately 2 log units) than those required to antagonize the effects of catecholamines. In human heart, both (-)-CGP 12177 and (-)-pindolol increase contractile force and hasten relaxation. However, the involvement of the beta(1)-adrenoceptor was not immediately obvious because (-)-pindolol- and (-)-CGP 12177-evoked responses were relatively resistant to blockade by (-)-propranolol. Abrogation of cardiostimulant effects of (-)-CGP 12177 in beta(1)-/beta(2)-adrenoceptor double-knockout mice, but not beta(2)-adrenoceptor-knockout mice, revealed an obligatory role of the beta(1)-adrenoceptor. On the basis of these results, two 'affinity states' have been designated, the beta(1H)- and beta(1L)-adrenoceptor, where the beta(1H)-adrenoceptor is activated by noradrenaline and blocked with high affinity by beta-blockers and the beta(1L)-adrenoceptor is activated by drugs such as (-)-CGP 12177 and (-)-pindolol and blocked with low affinity by beta-blockers such as (-)-propranolol. The beta(1H)- and beta(1L)-adrenoceptor states are consistent with high- and low-affinity binding sites for (-)-[(3)H]-CGP 12177 radioligand binding found in cardiac muscle and recombinant beta(1)-adrenoceptors. 3. There are two common polymorphic locations of the beta(1)-adrenoceptor, at amino acids 49 (Ser/Gly) and 389 (Arg/Gly). Their existence has raised several questions, including their role in determining the effectiveness of heart failure treatment with beta-blockers. We have investigated the effect of long-term maximally tolerated carvedilol administration (> 1 year) on left ventricular ejection fraction (LVEF) in patients with non-ischaemic cardiomyopathy (mean left ventricular ejection fraction 23 +/- 7%; n = 135 patients). The administration of carvedilol improved LVEF to 37 +/- 13% (P < 0.005); however, the improvement was variable, with 32% of patients showing pound 5% improvement. Upon segregation of patients into Arg389Gly-beta(1)-adrenoceptors, it was found that carvedilol caused a greater increase in left ventricular ejection faction in patients carrying the Arg389 allele with Arg389Arg > Arg389Gly > Gly389Gly.

Arginine 16 Glycine β2-Adrenoceptor Polymorphism and Cardiovascular Structure and Function in Patients with Heart Failure

The beta2/beta1 adrenoceptor ratio increases in congestive heart failure (CHF), making the heart relatively more dependent on inotropic, lusitropic, and chronotropic stimulation by the beta2-adrenergic receptor (ADRB2). In healthy human beings, those who are homozygous for arginine (Arg) at amino acid 16 of the ADRB2 have reduced receptor function when compared with individuals homozygous for glycine (Gly) at this position. The cardiovascular effects of the Arg16Gly polymorphism of the ADRB2 in CHF are not well understood. The aim of this study was to examine the influence of common polymorphisms of the ADRB2 on cardiovascular structure and function in patients with CHF. Echocardiography, neurohormonal assays, and exercise tests were performed in 68 healthy individuals and 95 patients with CHF. All of the patients with CHF were stable, New York Heart Association class II to III, of ischemic or nonischemic cause, with an ejection fraction of 40% or less. Of the patients with CHF, 16 were Arg/Arg, 36 were Arg/Gly, and 43 were Gly/Gly at amino acid 16. In those without CHF, the Arg16Gly polymorphism of the ADRB2 had no effect on cardiovascular function. In contrast, in CHF, Arg/Arg homozygotes had higher plasma norepinephrine and atrial natriuretic peptide levels, greater left atrial diastolic dimension, higher peak velocity of early/late diastolic filling ratio, and shorter deceleration time compared with Gly16 homozygotes. Furthermore, Arg16 homozygotes had reduced exercise tolerance compared with Gly16 homozygotes (evidenced by shorter exercise duration and lower peak oxygen consumption per unit time), and a lesser chronotropic response to exercise. In patients with CHF, but not in demographically matched healthy persons, the Arg16Gly polymorphism of the ADRB2 exerts important effects on cardiovascular structure and function, neurohormonal activation, and exercise tolerance.

(-)-Adrenaline elicits positive inotropic, lusitropic, and biochemical effects through beta2 -adrenoceptors in human atrial myocardium from nonfailing and failing hearts, consistent with Gs coupling but not with Gi coupling

Activation of either coexisting beta1- or beta2 -adrenoceptors with noradrenaline or adrenaline, respectively, causes maximum increases of contractility of human atrial myocardium. Previous biochemical work with the beta2 -selective agonist zinterol is consistent with activation of the cascade beta2 -adrenoceptors-->Gsalpha-protein-->adenylyl cyclase-->cAMP-->protein kinase (PKA)-->phosphorylation of phospholamban, troponin I, and C-protein-->hastened relaxation of human atria from nonfailing hearts. However, in feline and rodent myocardium, catecholamines and zinterol usually do not hasten relaxation through activation of beta2 -adrenoceptors, presumably because of coupling of the receptors to Gi protein. It is unknown whether the endogenously occurring beta2 -adrenoceptor agonist adrenaline acts through the above cascade in human atrium and whether its mode of action could be changed in heart failure. We assessed the effects of (-)-adrenaline, mediated through beta2 -adrenoceptors (in the presence of CGP 20712A 300 nM to block beta1 -adrenoceptors), on contractility and relaxation of right atrial trabecula obtained from nonfailing and failing human hearts. Cyclic AMP levels were measured as well as phosphorylation of phospholamban, troponin I, and protein C with Western blots and the back-phosphorylation procedure. For comparison, beta1 -adrenoceptor-mediated effects of (-)-noradrenaline were investigated in the presence of ICI 118,551 (50 nM to block beta2 -adrenoceptors). The positive inotropic effects of both (-)-noradrenaline and (-)-adrenaline were accompanied by reductions in time to peak force and time to reach 50% relaxation. (-)-Adrenaline caused similar positive inotropic and lusitropic effects in atrial trabeculae from failing hearts. However, the inotropic potency, but not the lusitropic potency, of (-)-noradrenaline was reduced fourfold in atrial trabeculae from heart failure patients. Both (-)-adrenaline and (-)-noradrenaline enhanced cyclic AMP levels and produced phosphorylation of phospholamban, troponin I, and C-protein to a similar extent in atrial trabeculae from nonfailing hearts. The hastening of relaxation caused by (-)-adrenaline together with the PKA-catalyzed phosphorylation of the three proteins involved in relaxation, indicate coupling of beta2 -adrenoceptors to Gs protein. The phosphorylation of phospholamban at serine16 and threonine17 evoked by (-)-adrenaline through beta2 -adrenoceptors and by (-)-noradrenaline through beta1 -adrenoceptors was not different in atria from nonfailing and failing hearts. Activation of beta2 -adrenoceptors caused an increase in phosphorylase a activity in atrium from failing hearts further emphasizing the presence of the beta2 -adrenoceptor-Gsalpha-protein pathway in human heart. The positive inotropic and lusitropic potencies of (-)-adrenaline were conserved across Arg16Gly- and Gln27Glu-beta2 -adrenoceptor polymorphisms in the right atrium from patients undergoing coronary artery bypass surgery, chronically treated with beta1 -selective blockers. The persistent relaxant and biochemical effects of (-)-adrenaline through beta2 -adrenoceptors and of (-)-noradrenaline through beta1 -adrenoceptors in heart failure are inconsistent with an important role of coupling of beta2 -adrenoceptors with Gialpha-protein in human atrial myocardium.

Cardiac implications for the use of beta2-adrenoceptor agonists for the management of muscle wasting

There are proposals for the implementation of beta(2)-adrenoceptor agonists for the management of muscle wasting diseases. The idea has been initiated by studies in animal models which show that beta(2)-adrenoceptor agonists cause hypertrophy of skeletal muscle. Their use in clinical practice will also need an understanding of possible effects of activation of human heart beta(2)-adrenoceptors. Consequences could include an increased probability of arrhythmias in susceptible patients.

Stress and cardiac beta adrenoceptors

Most modern theories about stress recognize that although stress is not a disease, it may be the trigger for the majority of diseases when allostatic overload has been generated. During stress, the glucocorticoids and catecholamines play a key role in the regulation of physiological parameters and homeostasis during stress. In the heart, positive chronotropic, inotropic, and lusitropic responses to catecholamines are mediated by various subtypes of adrenergic receptors (beta-ARs), mainly beta1- and beta2-adrenergic receptors. beta-ARs also control cardiomyocyte growth and death, thus contributing to cardiac remodelling. The structural basis of each beta-AR subtype, as well as their signalling pathways, and adaptive responses to stress are discussed. The participation of beta3- and putative beta4-ARs in the control of cardiac function is also discussed, with emphasis on low affinity beta-AR isoforms and the role they play in the response to the catecholamines under stress. The changes in beta-AR signalling under pathogenic conditions as well as under stress are reviewed.

Cardiac adrenoceptors: physiological and pathophysiological relevance

At present, nine adrenoceptor (AR) subtypes have been identified: alpha(1A)-, alpha(1B)-, alpha(1D)-, alpha(2A)-, alpha(2B)-, alpha(2C)-, beta(1)-, beta(2)-, and beta(3)AR. In the human heart, beta(1)- and beta(2)AR are the most powerful physiologic mechanism to acutely increase cardiac performance. Changes in betaAR play an important role in chronic heart failure (CHF). Thus, due to increased sympathetic activity in CHF, betaAR are chronically (over)stimulated, and that results in beta(1)AR desensitization and alterations of down-stream mechanisms. However, several questions remain open: What is the role of beta(2)AR in CHF? What is the role of increases in cardiac G(i)-protein in CHF? Do increases in G-protein-coupled receptor kinase (GRK)s play a role in CHF? Does betaAR-blocker treatment cause its beneficial effects in CHF, at least partly, by reducing GRK-activity? In this review these aspects of cardiac AR pharmacology in CHF are discussed. In addition, new insights into the functional importance of beta(1)- and beta(2)AR gene polymorphisms are discussed. At present it seems that for cardiovascular diseases, betaAR polymorphisms do not play a role as disease-causing genes; however, they might be risk factors, might modify disease, and/or might influence progression of disease. Furthermore, betaAR polymorphisms might influence drug responses. Thus, evidence has accumulated that a beta(1)AR polymorphism (the Arg389Gly beta(1)AR) may affect the response to betaAR-blocker treatment.

Activation of calcineurin in human failing heart ventricle by endothelin-1, angiotensin II and urotensin II

1 The calcineurin (CaN) enzyme-transcriptional pathway is critically involved in hypertrophy of heart muscle in some animal models. Currently there is no information concerning the regulation of CaN activation by endogenous agonists in human heart. 2 Human right ventricular trabeculae from explanted human (14 male/2 female) failing hearts were set up in a tissue bath and electrically paced at 1 Hz and incubated with or without 100 nM endothelin-1 (ET-1), 10 M, angiotensin-II (Ang II) or 20 nM human urotensin-II (hUII) for 30 min. Tissues from four patients were incubated with 200 nM tacrolimus (FK506) for 30 min and then incubated in the presence or absence of ET-1 for a further 30 min. 3 ET-1 increased contractile force in all 13 patients (P<0.001). Ang II and hUII increased contractile force in three out of eight and four out of 10 patients but overall nonsignificantly (P>0.1). FK506 had no effect on contractile force (P=0.12). 4 ET-1, Ang II and hUII increased calcineurin activity by 32, 71 and 15%, respectively, while FK506 reduced activity by 34%. ET-1 in the presence of FK506 did not restore calcineurin activity (P=0.1). 5 There was no relationship between basal CaN activity and expression levels in the right ventricle. Increased levels of free phosphate were detected in ventricular homogenates that were incubated with PKC(epsilon) compared to samples incubated without PKC(epsilon). 6 Endogenous cardiostimulants which activate G(alpha)q-coupled receptors increase the activity of calcineurin in human heart following acute (30 min) exposure. PKC may contribute to this effect by increasing levels of phosphorylated calcineurin substrate.

The cardiac beta2-adrenergic signalling a new role for the cPLA2

The cardiac actions of catecholamines have long been attributed to the predominant beta(1)-AR subtype that couples to the classical Gs/AC/cAMP pathway. Recent research clearly indicates that cardiac beta(2)-ARs play a functional role in healthy heart and assume increasing importance in failing and aged heart. beta(2)-ARs are primarily coupled to an atypical compartmentalized cAMP pathway, regulated by phosphorylation and/or oligomerization of beta(2)-ARs, and under the control of additional beta(2)-AR/Gi-coupled lipidic pathways, the impact of which seems to vary depending on the animal species, the developmental and pathophysiological state. This review focuses, more especially, on one of the last identified beta(2)-AR/Gi pathway, namely the cPLA(2).

Epinephrine activates both Gs and Gi pathways, but norepinephrine activates only the Gs pathway through human beta2-adrenoceptors overexpressed in mouse heart

Isoproterenol increases and decreases contractile force at low and high concentrations, respectively, through beta(2)-adrenoceptors overexpressed in transgenic mouse heart (TG4), consistent with activation of both G(s) and G(i) proteins. Using TG4 hearts, we demonstrated that epinephrine behaves like isoproterenol, but norepinephrine does not. Epinephrine both increased (-log EC(50)M = 9.4) and decreased (-log EC(50)M = 6.5) left atrial force. Pertussis toxin (PTX) abolished the negative inotropic effects of epinephrine, consistent with mediation through G(i) protein. Norepinephrine only increased contractile force (-log EC(50)M = 7.5). Norepinephrine (10-100 microM) prevented the positive inotropic effects but hardly affected the negative inotropic effects of epinephrine. Cardiodepressive epinephrine concentrations (1-10 microM) antagonized the positive inotropic effects of norepinephrine. In the free wall of TG4 right ventricle, norepinephrine and low epinephrine concentrations caused positive inotropic effects, and high epinephrine concentrations caused PTX-sensitive negative inotropic effects, as observed in the left atrium. Epinephrine (10 nM), a concentration causing maximum increase in contractile force, and norepinephrine (1 and 100 microM) increased cAMP-dependent protein kinase activity in TG4 left ventricle. Cardiodepressive concentrations of epinephrine (1 and 100 microM) did not increase cAMP-dependent protein kinase activity. The inotropic results were simulated with a model of two beta(2)-adrenoceptor sites. For one site involved in receptor coupling to G(s), both epinephrine and norepinephrine compete. The other site, recognized by epinephrine but not by norepinephrine, leads to receptor G(i) coupling.

Age Increases Expression and Receptor-Mediated Activation of Gαi in Human Atria

Recently, we demonstrated that beta2AR and several other Galphas-coupled receptors in human atria also couple to Galphai, a G protein that inhibits adenylyl cyclase (AC). The present study was undertaken to determine whether age increases expression of Galphai in human atrium, and more specifically whether it results in an increase in receptor-mediated activation of Galphai. Right atrial appendages were obtained from 14 mature adult (40-55 years) and 14 elderly (71-79 years) patients undergoing cardiac surgery. Immunoblotting of atrial membranes indicates that elderly atria have 82 +/- 18% more Galphai2 than atria from mature adults (P < 0.002); this increase in Galphai with age is confirmed by pertussis toxin-catalyzed ADP-ribosylation as well as by photoaffinity labeling with [32P]azidoanilido-GTP. We also find that receptor-mediated activation of Galphai is greater in elderly atria and that both basal and receptor-mediated AC activities decrease in elderly atria. These decreases in AC activity can be reversed by disabling Galphai with pertussis toxin, indicating that the age-dependent increases in Galphai expression and activation have functional consequences. Because beta2ARs in human atria mediate contractility through cAMP-mediated phosphorylation of phospholamban, we conclude that an age-induced increase in Galphai may have a role in depressing cardiac function in aged human atria.

Role of cyclic nucleotide phosphodiesterase isoforms in cAMP compartmentation following beta2-adrenergic stimulation of ICa,L in frog ventricular myocytes

The role of cyclic nucleotide phosphodiesterase (PDE) isoforms in the beta2-adrenergic stimulation of the L-type Ca2+ current (ICa,L) was investigated in frog ventricular myocytes using double patch-clamp and double-barrelled microperfusion techniques. Isoprenaline (ISO, 1 nM to 10 microM) was applied on one half of the cell, either alone or in the presence of PDE inhibitors, and the local and distant responses of ICa,L were used to determine the gradient of local vs. distant cAMP concentration (alpha). IBMX (100 microM), a non-selective PDE inhibitor, reduced alpha from 40 to 4.4 indicating a 9-fold reduction in intracellular cAMP compartmentation when all PDE activity was blocked. While PDE1 and PDE2 inhibition had no effect, PDE3 inhibition by milrinone (3 microM) or PDE4 inhibition by Ro 20-1724 (3 microM) reduced alpha by 6- and 4-fold, respectively. A simultaneous application of milrinone and Ro 20-1724 produced a similar effect to IBMX, showing that PDE3 and PDE4 were the major PDEs accounting for cAMP compartmentation. Okadaic acid (3 microM), a non-selective phosphatase inhibitor, or H89 (1 microM), an inhibitor of cAMP-dependent protein kinase (PKA), had no effect on the distant response of ICa,L to ISO indicating that PDE activation by PKA played a minor role in cAMP compartmentation. Our results demonstrate that PDE activity determines the degree of cAMP compartmentation in frog ventricular cells upon beta2-adrenergic stimulation. PDE3 and PDE4 subtypes play a major role in this process, and contribute equally to ensure a functional coupling of beta2-adrenergic receptors with nearby Ca2+ channels via local elevations of cAMP.

Ambulatory blood pressure and left ventricular structure and function in relation to the G-protein beta3-subunit polymorphism C825T in White Europeans

The 825T allele of the G-protein beta(3)-subunit is associated with increased intracellular signalling. Its association with hypertension is inconsistent. We, therefore, studied the C825T polymorphism in relation to ambulatory blood pressure as well as left ventricular structure and function in two European populations. We genotyped 248 parents and 318 offspring, enrolled in the European Project on Genes in Hypertension in Cracow, Poland (n=286) and in Novosibirsk, Russian Federation (n=280). The 24-h ambulatory blood pressure was recorded using oscillometric SpaceLabs 90207 monitors. Within each centre, a single observer performed two-dimensionally guided M-mode echocardiography and Doppler sonography to measure left ventricular structure (American Society of Echocardiography conventions) and diastolic function: early (E) and late (A) peak diastolic inflow velocities. We used analysis of covariance and generalized estimating equations to allow for covariables and nonindependence among related subjects. Genotype frequencies were similar (P=0.25) in Cracow and Novosibirsk and amounted to 44.7% for CC, 47.2% for CT, and 8.1% for TT. Among parents (mean age: 51.3 years)-but not among offspring (mean age 25.1 years)-24-h, daytime and night time systolic blood pressures were 5-6 mmHg higher in TT homozygotes than in C allele carriers. In TT homozygous parents (-8.2 cm/sec, P=0.004) as well as in TT homozygous offspring (-7.5 cm/sec, P=0.02), the E-wave was significantly reduced, which in offspring also resulted in a lower E/A ratio (-0.25, P=0.002). Neither in parents nor in offspring, left ventricular mass index was associated with the C825T polymorphism. In conclusion, in TT homozygotes of both generations, early left ventricular relaxation was reduced. In TT homozygous parents, the latter observation might be because of the higher systolic pressure associated with the TT genotype.

Age increases cardiac Galpha(i2) expression, resulting in enhanced coupling to G protein-coupled receptors

Cardiac G protein-coupled receptors that function through stimulatory G protein Galpha(s), such as beta(1)- and beta(2)-adrenergic receptors (beta(1)ARs and beta(2)ARs), play a key role in cardiac contractility. Recent data indicate that several Galpha(s)-coupled receptors in heart also activate Galpha(i), including beta(2)ARs (but not beta(1)ARs). Coupling of cardiac beta(2)ARs to Galpha(i) inhibits adenylyl cyclase and opposes beta(1)AR-mediated apoptosis. Dual coupling of beta(2)AR to both Galpha(s) and Galpha(i) is likely to alter beta(2)AR function in disease, such as congestive heart failure in which Galpha(i) levels are increased. Indeed, heart failure is characterized by reduced responsiveness of betaARs. Cardiac betaAR-responsiveness is also decreased with aging. However, whether age increases cardiac Galpha(i) has been controversial, with some studies reporting an increase and others reporting no change. The present study examines Galpha(i) in left ventricular membranes from young and old Fisher 344 rats by employing a comprehensive battery of biochemical assays. Immunoblotting reveals significant increases with age in left ventricular Galpha(i2), but no changes in Galpha(i3), Galpha(o), Galpha(s), Gbeta(1), or Gbeta(2). Aging also increases ADP-ribosylation of pertussis toxin-sensitive G proteins. Consistent with these results, basal as well as receptor-mediated incorporation of photoaffinity label [(32)P]azidoanilido-GTP indicates higher amounts of Galpha(i2) in older left ventricular membranes. Moreover, both basal and receptor-mediated adenylyl cyclase activities are lower in left ventricular membranes from older rats, and disabling of Galpha(i) with pertussis toxin increases both basal and receptor-stimulated adenylyl cyclase activity. Finally, age produces small but significant increases in muscarinic potency for the inhibition of both beta(1)AR- and beta(2)AR-stimulated adenylyl cyclase activity. The present study establishes that Galpha(i2) increases with age and provides data indicating that this increase dampens adenylyl cyclase activity.

Physiological antagonism between ventricular beta 1-adrenoceptors and alpha 1-adrenoceptors but no evidence for beta 2- and beta 3-adrenoceptor function in murine heart

1. Murine left atrium lacks inotropic beta(2)-adrenoceptor function. We investigated whether beta(2)-adrenoceptors are involved in the cardiostimulant effects of (-)-adrenaline on spontaneously beating right atria and paced right ventricular myocardium of C57BL6 mice. We also studied a negative inotropic effect of (-)-adrenaline. 2. Sinoatrial tachycardia, evoked by (-)-adrenaline was resistant to blockade by beta(2)-selective ICI 118,551 (50 nM) but antagonized by beta(1)-selective CGP 20712A (300 nM). This pattern was unaffected by pretreatment with pertussis toxin (PTX, 600 microg kg(-1) i.p. 24 h) which reversed carbachol-evoked bradycardia to tachycardia. 3. Increases of ventricular force by (-)-adrenaline and (-)-noradrenaline were not blocked by ICI 118,551 but antagonized by CGP 20712A. 4. Under blockade of beta-adrenoceptors, (-)-adrenaline and (-)-noradrenaline depressed ventricular force (-logIC(50)M=7.7 and 6.9). The cardiodepressant effects of (-)-adrenaline were antagonized by phentolamine (1 microM) and prazosin (1 microM) but not by (-)-bupranolol (1 microM). Prazosin potentiated the positive inotropic effects of (-)-adrenaline (in the absence of beta-blockers) from -logEC(50)M=6.2 - 6.8. 5. PTX-treatment reduced carbachol-evoked depression of ventricular force in the presence of high catecholamine concentrations. Inhibition of ventricular function of G(i) protein was verified by 82% reduction of in vitro ADP-ribosylation. PTX-treatment tended to increase the positive inotropic potency of (-)-adrenaline under all conditions investigated, including the presence of ICI 118,551. 6. (-)-Adrenaline causes murine cardiostimulation through beta(1)-adrenoceptors but not through beta(2)-adrenoceptors. The negative inotropic effects of (-)-adrenaline are mediated through ventricular alpha(1)-adrenoceptors but not through beta(3)-adrenoceptors. Both G(i) protein and alpha(1)-adrenoceptors restrain (-)-adrenaline-evoked increases in right ventricular force mediated through beta(1)-adrenoceptors.

Comparison of the affinity of beta-blockers for two states of the beta 1-adrenoceptor in ferret ventricular myocardium

We compared the potency of 11 clinically available beta-blockers as antagonists of the positive inotropic effects of (-)-isoprenaline and CGP12177 on ferret ventricular myocardium. (-)-CGP12177, (-)-pindolol and (-)-alprenolol were non-conventional partial agonists with intrinsic activity of 0.7, 0.2 and 0.1 respectively. All beta-blockers antagonized in a concentration-dependent and surmountable manner the positive inotropic effects of both (-)-isoprenaline and CGP12177. The potency of each beta-blocker was consistently higher against (-)-isoprenaline than against CGP12177. Two groups of beta-blockers were identified. In one group the difference between the pK(B) values of blockade against (-)-isoprenaline and CGP12177 was 1.1 - 1.6 log units ((-)-alprenolol, (-)-pindolol, (-)-bupranolol, nadolol and carvedilol). In the other group the pK(B) difference was of 2.1 - 3.0 log units ((-)-atenolol, metoprolol, bisoprolol, sotalol, (-)-propranolol and (-)-timolol). The beta-blockers competed with (-)-[(125)I]-cyanopindolol for binding to ventricular beta(1)-adrenoceptors. The binding affinities correlated with the corresponding blocking potencies against (-)-isoprenaline. On average the pK(i) values were 0.5 log units smaller than the pK(B) values against (-)-isoprenaline but 1.6 log units greater than the pK(B) values against CGP12177. In ferret ventricle the effects of (-)-isoprenaline appear to be antagonized by beta-blockers through the state of the beta(1)-adrenoceptor for which (-)-[(125)I]-cyanopindolol and beta-blockers have high affinity. The cardiostimulant effects of CGP12177 appear to be mediated through a low-affinity state of the beta(1)-adrenoceptor for which beta-blockers have low affinity.