Diminished responsiveness of Gs-coupled receptors in severely failing human hearts: no difference in dilated versus ischemic cardiomyopathy

Role of Dopamine in the Heart in Health and Disease

Dopamine has effects on the mammalian heart. These effects can include an increase in the force of contraction, and an elevation of the beating rate and the constriction of coronary arteries. Depending on the species studied, positive inotropic effects were strong, very modest, or absent, or even negative inotropic effects occurred. We can discern five dopamine receptors. In addition, the signal transduction by dopamine receptors and the regulation of the expression of cardiac dopamine receptors will be of interest to us, because this might be a tempting area of drug development. Dopamine acts in a species-dependent fashion on these cardiac dopamine receptors, but also on cardiac adrenergic receptors. We will discuss the utility of drugs that are currently available as tools to understand cardiac dopamine receptors. The molecule dopamine itself is present in the mammalian heart. Therefore, cardiac dopamine might act as an autocrine or paracrine compound in the mammalian heart. Dopamine itself might cause cardiac diseases. Moreover, the cardiac function of dopamine and the expression of dopamine receptors in the heart can be altered in diseases such as sepsis. Various drugs for cardiac and non-cardiac diseases are currently in the clinic that are, at least in part, agonists or antagonists at dopamine receptors. We define the research needs in order to understand dopamine receptors in the heart better. All in all, an update on the role of dopamine receptors in the human heart appears to be clinically relevant, and is thus presented here.

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Hemodynamic Effects of Glucagon: A Literature Review

CONTEXT

Glucagon's effects on hemodynamic parameters, most notably heart rate and cardiac contractility, are often overlooked. The glucagon receptor is a central target in novel and anticipated type 2 diabetes therapies, and hemodynamic consequences of glucagon signaling have therefore become increasingly important. In this review, we summarize and evaluate published studies on glucagon pharmacology with a focus on clinical hemodynamic effects in humans.

EVIDENCE ACQUISITION

PubMed, Embase, and the Cochrane Library were searched for clinical studies concerning hemodynamic effects of glucagon (no year restriction). Papers reporting effects of a defined glucagon dose on any hemodynamic parameter were included. Reference searches were conducted in retrieved articles.

EVIDENCE SYNTHESIS

Hemodynamic effects of glucagon have been investigated mainly in cohort studies of patients suffering from heart failure receiving large glucagon bolus injections. The identified studies had shortcomings related to restricted patient groups, lack of a control group, randomization, or blinding. We identified no properly conducted randomized clinical trials. The majority of human studies report stimulating effects of pharmacological glucagon doses on heart rate, cardiac contractility, and blood pressure. The effects were characterized by short duration, interindividual variation, and rapid desensitization. Some studies reported no measurable effects of glucagon.

CONCLUSIONS

The level of evidence regarding hemodynamic effects of glucagon is low, and observations in published studies are inconsistent. Actual effects, interindividual variation, dose-response relationships, and possible long-term effects of supraphysiological glucagon levels warrant further investigation.

Critical Roles of STAT3 in β-Adrenergic Functions in the Heart

BACKGROUND

β-Adrenergic receptors (βARs) play paradoxical roles in the heart. On one hand, βARs augment cardiac performance to fulfill the physiological demands, but on the other hand, prolonged activations of βARs exert deleterious effects that result in heart failure. The signal transducer and activator of transcription 3 (STAT3) plays a dynamic role in integrating multiple cytokine signaling pathways in a number of tissues. Altered activation of STAT3 has been observed in failing hearts in both human patients and animal models. Our objective is to determine the potential regulatory roles of STAT3 in cardiac βAR-mediated signaling and function.

METHODS AND RESULTS

We observed that STAT3 can be directly activated in cardiomyocytes by β-adrenergic agonists. To follow up this finding, we analyzed βAR function in cardiomyocyte-restricted STAT3 knockouts and discovered that the conditional loss of STAT3 in cardiomyocytes markedly reduced the cardiac contractile response to acute βAR stimulation, and caused disengagement of calcium coupling and muscle contraction. Under chronic β-adrenergic stimulation, Stat3cKO hearts exhibited pronounced cardiomyocyte hypertrophy, cell death, and subsequent cardiac fibrosis. Biochemical and genetic data supported that Gαs and Src kinases are required for βAR-mediated activation of STAT3. Finally, we demonstrated that STAT3 transcriptionally regulates several key components of βAR pathway, including β1AR, protein kinase A, and T-type Ca(2+) channels.

CONCLUSIONS

Our data demonstrate for the first time that STAT3 has a fundamental role in βAR signaling and functions in the heart. STAT3 serves as a critical transcriptional regulator for βAR-mediated cardiac stress adaption, pathological remodeling, and heart failure.

Disruption of histamine H2 receptor slows heart failure progression through reducing myocardial apoptosis and fibrosis

Histamine H2 receptor (H2R) blockade has been reported to be beneficial for patients with chronic heart failure (CHF), but the mechanisms involved are not entirely clear. In the present study, we assessed the influences of H2R disruption on left ventricular (LV) dysfunction and the mechanisms involved in mitochondrial dysfunction and calcineurin-mediated myocardial fibrosis. H2R-knockout mice and their wild-type littermates were subjected to transverse aortic constriction (TAC) or sham surgery. The influences of H2R activation or inactivation on mitochondrial function, apoptosis and fibrosis were evaluated in cultured neonatal rat cardiomyocytes and fibroblasts as well as in murine hearts. After 4 weeks, H2R-knockout mice had higher echocardiographic LV fractional shortening, a larger contractility index, a significantly lower LV end-diastolic pressure, and more importantly, markedly lower pulmonary congestion compared with the wild-type mice. Similar results were obtained in wild-type TAC mice treated with H2R blocker famotidine. Histological examinations showed a lower degree of cardiac fibrosis and apoptosis in H2R-knockout mice. H2R activation increased mitochondrial permeability and induced cell apoptosis in cultured cardiomyocytes, and also enhanced the protein expression of calcineurin, nuclear factor of activated T-cell and fibronectin in fibroblasts rather than in cardiomyocytes. These findings indicate that a lack of H2R generates resistance towards heart failure and the process is associated with the inhibition of cardiac fibrosis and apoptosis, adding to the rationale for using H2R blockers to treat patients with CHF.

Plant polyphenols in the treatment of age-associated diseases: revealing the pleiotropic effects of icariin by network analysis

Polyphenols are a broad class of compounds. Some are ingested in substantial quantities from nutritional sources, more are produced by medicinal plants, and some of them are taken as drugs. It is becoming clear, that a single polyphenol is impacting several cellular pathways. Thus, a network approach is becoming feasible, describing the interaction of a single polyphenol with cellular networks. Here we have selected icariin to draw a prototypic network of icariin activities. Icariin appears to be a promising drug to treat major age-related diseases, like neurodegeneration, memory and depressive disorders, chronic inflammation, diabetes, and osteoporosis. It interacts with several relevant pathways, like PDE, TGF-ß, MAPK, PPAR, NOS, IGF, Sirtuin, and others. Such networks will be useful to future comparative studies of complex effects of polyphenols.

Regulation of cyclic adenosine monophosphate release by selective β2-adrenergic receptor stimulation in human terminal failing myocardium before and after ventricular assist device support

BACKGROUND

Response to catecholamines is blunted in terminal heart failure due to β-receptor downregulation and uncoupling from adenylyl cyclase (AC). Improved myocardial responsiveness to catecholamines after ventricular assist device (VAD) support is associated with upregulation of β1-adrenergic receptors (β1-ARs). Little is known about the regulation of AC and β2-AR coupling after VAD; moreover β2-AR stimulation during VAD was claimed to induce myocardial recovery.

METHODS

We analyzed in VAD-supported human myocardium the regulation of AC activity upon β1-AR and selective β2-AR stimulation in 8 non-failing hearts (NF) and 17 paired samples of VAD patients. AC messenger RNA was measured by TaqMan. AC was stimulated via β2-AR using clenbuterol (β2-AR agonist) and bisoprolol (β1-AR blocker). Organ bath experiments were done with trabeculae from both ventricles. Samples were stratified according to chronic or acute heart failure history.

RESULTS

Isoprenaline-induced AC activity was downregulated (p < 0.001) pre-VAD and increased significantly (p < 0.05) after unloading (mean ± standard deviation pmole/mg/min) in NF (47.9 ± 14.9), pre-VAD (24.35 ± 13.3), and post-VAD (50.04 ± 50.25). Forskolin stimulation revealed significant (p < 0.05) upregulation of AC activity during VAD, especially in acutely failing hearts (NF, 192.1 ± 68.7; pre-VAD, 191.1 ± 60.4; post-VAD, 281.5 ± 133). However, forskolin stimulation relative to isoprenaline-induced inotropy remained reduced before and after VAD compared with NF. The selective stimulation of β2-AR did not reveal influence of VAD support on β2-AR-AC coupling. Stimulation of ventricular trabeculae by > 100 μmole/liter clenbuterol revealed negative inotropic responses.

CONCLUSIONS

VAD does not influence β2-AR coupling to AC stimulation. Elevated response to catecholamines after VAD support is influenced by β1-AR upregulation and modulation of AC activity. Restoration of β-adrenergic responsiveness was restricted to acutely failing hearts.

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.

Selective upregulation of β1-adrenergic receptors and dephosphorylation of troponin I in end-stage heart failure patients supported by ventricular assist devices

In terminal failing hearts, adrenergic receptors are downregulated and intracellular adrenergic signal transduction is inhibited. Mechanical circulatory support by ventricular assist devices (VAD) is used to bridge patients to heart transplantation. Mechanical unloading by VAD may induce reverse remodeling in heart transplantation (HTx) candidates. However, little is known on beta-adrenergic receptor subtype regulation and adrenergic signal transduction under VAD-support. We investigated paired myocardial samples from 16 VAD-supported patients and 9 non-failing donor hearts. We analyzed beta-adrenergic receptor subtype regulation by real-time PCR and radioligand binding and cardiac troponin I phosphorylation (by phospho-cTnI-specific antibodies). We found that the beta1-adrenergic receptor (beta1AR) is downregulated at VAD-implantation on mRNA and protein levels whereas the beta2-adrenergic receptor (beta2AR) was not. After VAD-support, beta1AR protein but not its mRNA was upregulated, whereas the degree of cTnI-phosphorylation was reduced. Upregulation of beta1AR was enhanced by beta blocking medication during VAD-support. However, in 9 out of 15 patients, beta1AR-density remained below the 0.25 percentile of donor hearts. VAD-support is associated with partial normalization of the betaAR-signal transduction pathways. This beneficial effect is related to a posttranscriptional increase in beta1AR-density.

Effects of autologous bone marrow stem cell transplantation on beta-adrenoceptor density and electrical activation pattern in a rabbit model of non-ischemic heart failure

Background

Since only little is known on stem cell therapy in non-ischemic heart failure we wanted to know whether a long-term improvement of cardiac function in non-ischemic heart failure can be achieved by stem cell transplantation.

Methods

White male New Zealand rabbits were treated with doxorubicine (3 mg/kg/week; 6 weeks) to induce dilative non-ischemic cardiomyopathy. Thereafter, we obtained autologous bone marrow stem cells (BMSC) and injected 1.5–2.0 Mio cells in 1 ml medium by infiltrating the myocardium via a left anterolateral thoracotomy in comparison to sham-operated rabbits. 4 weeks later intracardiac contractility was determined in-vivo using a Millar catheter. Thereafter, the heart was excised and processed for radioligand binding assays to detect β₁- and β₂-adrenoceptor density. In addition, catecholamine plasma levels were determined via HPLC. In a subgroup we investigated cardiac electrophysiology by use of 256 channel mapping.

Results

In doxorubicine-treated animals β-adrenoceptor density was significantly down-regulated in left ventricle and septum, but not in right ventricle, thereby indicating a typical left ventricular heart failure. Sham-operated rabbits exhibited the same down-regulation. In contrast, BMSC transplantation led to significantly less β-adrenoceptor down-regulation in septum and left ventricle. Cardiac contractility was significantly decreased in heart failure and sham-operated rabbits, but was significantly higher in BMSC-transplanted hearts. Norepinephrine and epinephrine plasma levels were enhanced in heart failure and sham-operated animals, while these were not different from normal in BMSC-transplanted animals. Electrophysiological mapping revealed unaltered electrophysiology and did not show signs of arrhythmogeneity.

Conclusion

BMSC transplantation improves sympathoadrenal dysregualtion in non-ischemic heart failure.

Interaction between simvastatin and metoprolol with respect to cardiac beta-adrenoceptor density, catecholamine levels and perioperative catecholamine requirements in cardiac surgery patients

Beta-blockade is a standard cardiovascular therapy known to induce the up-regulation of beta-adrenoceptor density. Upon ligand-binding, beta-adrenoceptors are normally internalised via the arrestin pathway, and after dissociation they are re-inserted into the membrane. This means that at high catecholamine levels the adrenoceptor density is low and under beta-blockade it is high. The insertion of receptors into the membrane is often dependent on farnesylation processes that can be inhibited by statins. We carried out a prospective, controlled, observational study to determine whether beta-blockade-induced up-regulation of beta-adrenoceptor density is attenuated by statin therapy and whether this would subsequently affect catecholamine consumption during surgery. We obtained pre-operative blood samples and intra-operative biopsies of the right atrial appendage from 39 patients (age: 65+/-5 years; BMI: 28+/-1) undergoing coronary bypass surgery with or without simvastatin (20 mg/day) therapy and with or without concomitant metoprolol therapy (50 mg/day). The atrial tissue was used for radioligand-binding studies with (-)-[125I]-iodocyanopindolol (ICYP) and for assessment of the beta-adrenoceptor subtype distribution following standard protocols. In the blood samples, plasma adrenaline and noradrenaline concentrations were determined using HPLC. In all tissue samples, we found a total beta-adrenoceptor density of 38+/-4 fmol/mg protein in untreated controls; this which was up-regulated to 55+/-5 fmol/mg protein in patients receiving metoprolol. This increase in receptor number was nearly prevented completely by simvastatin therapy (42+/-5 fmol/mg protein). The up-regulation could be attributed to increases in the beta1-adrenoceptor subtype. In contrast, simvastatin alone had no effect on beta-adrenoceptor density. Pre-operative adrenaline levels were slightly reduced in all drug therapy groups (nonsignificant differences), while the levels noradrenaline were not significantly different among the groups. With respect to the perioperative catecholamine requirements, patients on metoprolol needed significantly less dopamine than control patients, while patients undergoing simvastatin/metoprolol therapy needed as much as the controls. The post-operative total catecholamine requirements were not different among the four groups of patients. There were no differences in plasma metoprolol concentration between patients receiving metoprolol alone and those receiving a combination of metoprolol and simvastatin.

IN CONCLUSION

Simvastatin therapy seems to counter-regulate the up-regulation of beta-adrenoceptor density. In the up-regulated state induced by metoprolol therapy, the patients seemed to need less catecholamines during cardiac surgery, which may be due to the higher number of beta-adrenoceptors. Additional simvastatin therapy did not reduce post-operative catecholamine consumption.

Agonist-independent alteration in beta-adrenoceptor-G-protein-adenylate cyclase system in an equine model of recurrent airway obstruction

We examined the inhibitory sympathetic beta-adrenergic mechanisms in peripheral lung, bronchi and trachea of an equine model of recurrent airway obstruction (RAO), to support the hypothesis that the beta-adrenergic receptor dysfunction is not only restricted to cell surface receptor density but rather encompasses a mechanistic defect apart from the receptor, to the intracellular signaling components. The non-asthmatic lung possessed 3.2-fold more beta-adrenergic receptors than bronchi (496 +/- 19.4 vs. 155.1+/- 19.6 fmol/mg protein; P < 0.01) and 6.2-fold higher than in the trachea (79.8 +/- 12.6 fmol/mg protein; P < 0.001) (assessed by radioligand binding assays using (-)-[(125)I]-iodocyanopindolol, ICYP) and in all tissues a greater proportion of the beta(2)- than the beta(1)-subtype (75-80%). The receptor density (B(max)) in lung parenchyma and bronchial membranes was 33 and 42%, respectively, lower (P < 0.001) in RAO than in control animals, attributable to a decrease in the beta(2)-subtype. This receptor down-regulation was accompanied with an attenuated coupling efficiency of the receptor to the stimulatory G(S)-protein (P < 0.05 vs. control). Concomitantly, activation of adenylate cyclase evoked by isoproterenol was significantly reduced in lung and bronchial membranes of animals with RAO, whereas effects of 10 microM GTP, 10mM NaF, 10 microM forskolin and 10 mM Mn(2+) were not altered. There was no difference in beta-adrenergic receptor density, G(S)-protein or adenylate cyclase coupling in the trachea between asthmatic and control animals. In conclusion, in stable asthma the pulmonary beta-adrenergic receptor-G(S)-protein-adenylate cyclase system is impaired, thus the pathologic process involves all signaling components, and due to its close similarity, this animal model seems to serve as a suitable model, at least partly, of chronic asthmatic patients.

Autonomic receptor systems in the failing and aging human heart: similarities and differences

Changes in autonomic receptor systems (alpha- and beta-adrenoceptors and muscarinic receptors) were compared in the aging and failing human heart. In both settings responsiveness of beta-adrenoceptors and all other receptor systems that evoke their effects via cyclic AMP accumulation was diminished. Muscarinic receptor function, on the other hand, was decreased in the aging, but unchanged in the failing heart; in contrast, G protein-coupled receptor kinase activity was increased in the failing, but unchanged in the aging heart. alpha-Adrenoceptor function was unchanged or slightly decreased in the failing heart. However, nothing is known on alpha-adrenoceptor changes in the aging heart. These results indicate that in the failing human heart all autonomic receptor systems appear to be altered in the direction to attenuate beta-adrenoceptor responses to sympathetic (over)stimulation while in the aging human heart autonomic receptor systems appear to be altered in a direction that protects the heart against too pronounced reduction in beta-adrenoceptor responsiveness.

Differential functional expression of human myocardial G protein receptor kinases in left ventricular cardiac diseases

The relationship between myocardial G protein receptor kinase (GRK) expression and beta-adrenoceptor signalling in human left heart diseases has not been fully elucidated yet. In this study, we characterized and compared the GRK2-7 expression in patients with left ventricular volume overload disorders and dilated cardiomyopathic hearts, and evaluated the relationship of this expression with alterations in myocardial beta-adrenoceptor signalling in volume overload, in order to test the notion that GRK functional expression is influenced in a disease-specific and selective fashion. We established that GRK2, GRK3, and GRK5 are well expressed, while GRK4, GRK6, and GRK7 are only scarcely detectable in the healthy human heart. Compared to control hearts (n=8), GRK2 mRNA expression was elevated by 71% (P<0.005) in the left ventricle, 110% (P<0.05) in the right ventricle, 130% (P<0.05) in the left atrium, and 1300% (P<0.005) in the right atrium (RA) of the dilated cardiomyopathy hearts (n=6). In the volume overload group (n=10), it was increased by approximately 40% (P<0.05) in the left ventricle, 38% in the right ventricle, 81% (P<0.05) in the left atrium, and 850% (P<0.005) in the right atrium. On the other hand, GRK5 was significantly elevated only in the left ventricle by 68% (P<0.05) in the dilated cardiomyopathy hearts and by 48% (P<0.01) in volume overload patients, while in contrast, GRK3 remained unchanged in dilated cardiomyopathy, but was slightly elevated by 36% (P=0.05) in the right ventricle of the volume overload patients. The alterations in GRK expression were accompanied with a decrease in myocardial beta(1)-adrenoceptor mRNA in all four chambers, and these trends in gene expression were paralleled with those of their immunodetectable protein levels. Furthermore, these changes were in association with a decrease in downstream receptor-stimulated, adenylyl cyclase-mediated functional expression and an increase in ventricular protein kinase A activity. The results point to differences in which myocardial GRKs are regulated in cardiac disease, whereby changes in GRK2 expression may be related to the global effects of the disease on myocardial adrenoceptor function and those in GRK5 may be localized to the ventricles, depending on the nature of the myocardial load.

Unchanged G-protein-coupled receptor kinase activity in the aging human heart

OBJECTIVES

We sought to find out whether G-protein-coupled receptor kinase (GRK) activity is also increased in the aging human heart.

BACKGROUND

In the aging and failing human heart, cardiac beta-adrenoceptors (beta-AR) are desensitized. In heart failure (HF), an increase in cardiac GRK activity considerably contributes to this beta-AR desensitization.

METHODS

We assessed GRK activity (by in vitro rhodopsin phosphorylation) in the right atria (RA) from 16 children (mean age 9 +/- 2 years) and 17 elderly patients (mean age 67 +/- 2 years) without apparent HF and in the RA from four patients with end-stage HF.Cytosolic and membranous GRK activities in the RA from children were not significantly different from those in elderly patients; in contrast, cytosolic and membranous GRK activities in the RA from patients with end-stage HF were significantly increased.

CONCLUSIONS

In contrast to the failing human heart, in the aging human heart, GRK activity is not increased. Thus, GRK activity appears to not play an important role in beta-AR desensitization in the aging human heart.

Effect of a beta 2-adrenoceptor stimulation on hyperglycemia-induced endothelial dysfunction

To investigate whether beta(2)-adrenoceptors exist on endothelial cells and whether a beta(2)-adrenoceptor stimulation might prevent the development of hyperglycemia-induced endothelial dysfunction, porcine aortic endothelial cells (PAECs) were cultured and chronically exposed to either 5 mM D-glucose ("normoglycemia") or 20 mM D-glucose ("hyperglycemia"), with or without 100 nM salbutamol in absence or presence of beta(2)-adrenoceptor antagonist ICI 118,551 [1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxyl]-3-[(1-methylethyl)-amino]-2-butanol] or beta(1)-antagonist metoprolol. For osmotic control, PAECs were exposed to 15 mM L-glucose. We measured nitric oxide release using the met-hemoglobin assay and assessed beta-adrenoceptor density and subtypes by radioligand binding. Furthermore, we determined intracellular NADH and NADPH using high-performance liquid chromatography. High D-glucose concentrations but not L-glucose led to significantly reduced basal and stimulated nitric oxide release. Chronic salbutamol treatment significantly antagonized the impairment of the nitric oxide response, which was inhibited by ICI 118,551 but not by metoprolol. The number of giant cells was significantly increased in hyperglycemia, which could be prevented by salbutamol. Binding of the radioligand (-)-[(125)I]iodocyanopindolol revealed a total beta-adrenoceptor density of 29.8 +/- 3.7 (normoglycemic) and 30.3 +/- 3.6 (hyperglycemic) fmol/mg protein. Displacement by ICI 118,551 revealed beta-adrenoceptor subtype distribution with 30.3 +/- 4.4 (normoglycemic) and 29.1 +/- 3.8% beta(2)-adrenoceptors. NADH production increased in hyperglycemia, which was completely prevented by salbutamol. We conclude that hyperglycemia in PAEC induces endothelial dysfunction with impaired nitric oxide release and that this can be prevented by beta(2)-adrenoceptor stimulation.

Effects of different beta adrenoceptor ligands in mice with permanent occlusion of the left anterior descending coronary artery

1. We have studied the effects of three betaAR ligands (carvedilol, alprenolol, and ICI-118551) with different pharmacological profiles and negative efficacy at the beta2AR on cardiac in vivo, in vitro, biochemical and gene expression parameters in mice with permanent occlusion of the left anterior descending coronary artery. 2. Cardiac in vivo parameters were determined using Doppler studies. Mitral-wave E peak velocity (EPV) and aortic peak velocity (AoPV) decreased in the first 2 weeks postocclusion. After 3 weeks of drug treatment, EPV was improved in the carvedilol group to preocclusion values; however, a further reduction in EPV in the alprenolol and control permanent occlusion group was measured and there was no change after ICI-118551 treatment. AoPV was unchanged between weeks 2 and 5 in all groups. 3. The left atria were isolated to record isometric tension responses to isoprenaline. Permanent occlusion significantly reduced the maximum isoprenaline response to 30% of control and carvedilol increased the maximum response to isoprenaline significantly to 60%. 4. The biochemical and gene expression studies revealed different effects of the three betaAR ligands. Most notably, carvedilol reduced gene expression of myosin heavy chain beta. 5. These results indicate that chronic treatment with carvedilol is beneficial in a mouse model of myocardial damage resulting from ischaemia. We hypothesise that these beneficial effects of the drug may be because of the negative efficacy at the beta2AR, combined with beta1AR antagonism.

Overexpression of wild-type Galpha(i)-2 suppresses beta-adrenergic signaling in cardiac myocytes

The role of Galpha(i)-2 overexpression in desensitization of beta-adrenergic signaling in heart failure is controversial. An adenovirus-based approach was used to investigate whether overexpression of Galpha(i)-2 impairs beta-adrenergic stimulation of adenylyl cyclase (AC) activity and cAMP levels in neonatal rat cardiac myocytes (NRCM) and cell shortening of adult rat ventricular myocytes (ARVM). Infection of NRCM with Ad5Galpha(i)-2 increased Galpha(i)-2 by 50-600% in a virus dose-dependent manner. Overexpression was paralleled by suppression of GTP- and isoprenaline-stimulated AC by 10-72% (P<0.001) in a PTX-sensitive manner. Isoprenaline-stimulated shortening of Ad5Galpha(i)-2-infected ARVM was attenuated by 34% (P<0.01). Ad5Galpha(i)-2/GFP (Galpha(i)-2, green fluorescent protein; bicistronic) was constructed to monitor transfection homogeneity and target Galpha(i)-2 overexpression to levels found in heart failure. At Galpha(i)-2 levels of 93% above control, isoprenaline-stimulated AC activity and cAMP levels were reduced by 17% and 40% (P<0.02), respectively. Beta1- and beta2-adrenergic stimulation was reduced similarly. Our results suggest that (a) the Galpha(i)-2 system exhibits tonic inhibition of stimulated AC in cardiac myocytes, (b) Galpha(i)-2-mediated inhibition is concentration-dependent and occurs at Galpha(i)-2 levels seen in heart failure, and (c) Galpha(i)-2-mediated inhibition affects both beta1- and beta2-adrenergic stimulation of AC. The data argue for an important, independent role of the Galpha(i)-2 increase in heart failure.

Effects of the AT1-receptor antagonist eprosartan on the progression of left ventricular dysfunction in dogs with heart failure

1. We examined the effects of eprosartan, an AT(1) receptor antagonist, on the progression of left ventricular (LV) dysfunction and remodelling in dogs with heart failure (HF) produced by intracoronary microembolizations (LV ejection fraction, EF 30 to 40%). 2. Dogs were randomized to 3 months of oral therapy with low-dose eprosartan (600 mg once daily, n=8), high-dose eprosartan (1200 mg once daily, n=8), or placebo (n=8). 3. In the placebo group, LV end-diastolic (EDV) and end-systolic (ESV) volumes increased after 3 months (68+/-7 vs 82+/-9 ml, P<0.004, 43+/-1 vs 58+/-7 ml, P<0.003, respectively), and EF decreased (37+/-1 vs 29+/-1%, P<0.001). In dogs treated with low-dose eprosartan, EF, EDV, and ESV remained unchanged over the course of therapy, whereas in dogs treated with high-dose eprosartan, EF increased (38+/-1 vs 42+/-1%, P<0.004) and ESV decreased (41+/-1 vs 37+/-1 ml, P<0.006), Eprosartan also decreased interstitial fibrosis and cardiomyocyte hypertrophy. 4. We conclude that eprosartan prevents progressive LV dysfunction and attenuates progressive LV remodelling in dogs with moderate HF and may be useful in treating patients with chronic HF.

Ventricular hypertrophy plus neurohumoral activation is necessary to alter the cardiac beta-adrenoceptor system in experimental heart failure

Treatment of rats with monocrotaline (MCT) leads to pulmonary hypertension, right ventricular (RV) hypertrophy, and finally to RV heart failure. This is associated with characteristic changes in right ventricular beta-adrenoceptors (beta-AR), neuronal noradrenaline transporter (NAT) density and activity (uptake1), and G protein-coupled receptor kinase (GRK) activity. This study aimed to find out factors that determine beta-AR, uptake1, and GRK changes. Thus, 6-week-old rats were treated with 50 mg/kg MCT subcutaneous or 0.9% saline. Within 13 to 19 days after MCT application (group A), RV weight (222+/-6 versus 147+/-5 mg) and RV/left ventricular (LV) weight ratio (0.42+/-0.01 versus 0.29+/-0.01) were significantly increased, whereas plasma noradrenaline, RV beta-AR density, RV NAT density and activity, and RV GRK activity were not significantly altered. Twenty-one to twenty-eight days after MCT (group B), however, not only RV weight (316+/-4 versus 148+/-2 mg) and RV/LV weight ratio (0.61+/-0.01 versus 0.3+/-0.01) were markedly increased but also plasma noradrenaline (645+/-63 versus 278+/-18 pg/mL); now, RV beta-AR density (13.4+/-1.3 versus 26.5+/-1.1 fmol/mg protein), RV NAT density (50.9+/-11.3 versus 79.6+/-2.9 fmol/mg protein), and RV NAT activity (65.4+/-7.4 versus 111.8+/-15.9 pmol [3H]-NA/mg tissue slices/15 min) were significantly decreased and RV-membrane GRK activity (100+/-15 versus 67+/-6 [32P]-rhodopsin in cpm) significantly increased. LV parameters of MCT-treated rats were only marginally different from control LV. We conclude that in MCT-treated rats ventricular hypertrophy per se is not sufficient to cause characteristic alterations in the myocardial beta-AR system often seen in heart failure; only if ventricular hypertrophy is associated with neurohumoral activation beta-ARs are downregulated and GRK activity is increased.

Quantitative Assessment of β1- and β2-Adrenergic Receptor Homo- and Heterodimerization by Bioluminescence Resonance Energy Transfer

Quantitative bioluminescence resonance energy transfer (BRET) analysis was applied to the study of beta(1)- and beta(2)-adrenergic receptor homo- and heterodimerization. To assess the relative affinity between each of the protomers, BRET saturation experiments were carried out in HEK-293T cells. beta(1)- and beta(2)-adrenergic receptors were found to have similar propensity to engage in homo- and heterotropic interactions suggesting that, at equivalent expression levels of the two receptor subtypes, an equal proportion of homo- and heterodimers would form. Analysis of the data also revealed that, at equimolar expression levels of energy donor and acceptor, more than 80% of the receptor molecules exist as dimers and that this high incidence of receptor dimerization is insensitive to receptor density for expression levels varying between 1.4 and 26.9 pmol of receptor/mg of membrane protein. Taken together, these results indicate that most of the receptors expressed in cells exist as constitutive dimers and that, at least in undifferentiated fibroblasts, the proportion of homo- and heterodimers between the closely related beta(1)- and beta(2)-adrenergic receptors is determined by their relative levels of expression.

Age-dependent changes of cardiac neuronal noradrenaline reuptake transporter (uptake1) in the human heart

OBJECTIVES

The purpose of this study was to elucidate whether the neuronal noradrenaline reuptake transporter (uptake1) undergoes age-dependent regulation in the human heart.

BACKGROUND

Aging is associated with various alterations in cardiovascular function.

METHODS

We determined uptake1 density (by [3H]-nisoxetine binding to membranes) and activity (by accumulation of [3H]-noradrenaline into tissue slices) in the right atria (RA) of 42 patients (age range 3 months to 76 years) undergoing open-heart surgery without apparent heart failure. Moreover, the effects of 1 micromol/l desipramine on the noradrenaline-induced positive inotropic effect were assessed in the isolated, electrically driven RA trabeculae of these patients.

RESULTS

There was a significant negative correlation between RA uptake1 density and age; moreover, RA uptake1 activity was significantly reduced in elderly patients. Desipramine (1 micromol/l) significantly shifted noradrenaline concentration-response curves to the left; this shift was significantly more pronounced in younger patients than in older patients.

CONCLUSIONS

With increasing age, human myocardial uptake1 activity decreases, possibly because of age-dependent downregulation of uptake1 density.

Chronic infusion of beta-adrenoceptor antagonist and inverse agonists decreases elevated protein kinase A activity in transgenic mice with cardiac-specific overexpression of human beta 2-adrenoceptor

Neutral antagonists and inverse agonists can produce different cellular responses in some systems. The effects of chronic (14-day) infusion of three ligands, ICI-118,551, carvedilol, and alprenolol were examined in cardiac tissue from wild-type and transgenic mice with cardiac-specific overexpression of the human beta2-adrenoceptor. These ligands vary in their negative efficacy at the human beta2-adrenoceptor, with two (ICI-118,551 and carvedilol) behaving as inverse agonists and one (alprenolol) behaving as a neutral antagonist. Cardiac tissue from the transgenic mice exhibited elevated levels of protein kinase A activity and G protein receptor kinase-2. Fourteen-day infusions of the three ligands lowered the elevated levels of protein kinase A activity of the transgenic hearts to control levels. Alprenolol and carvedilol also decreased G protein receptor kinase-2 amounts to control levels. The left atria from transgenic mice exhibited an impaired inotropic response to histamine relative to responses of wild-type mice atria. Infusions of the inverse agonists and a neutral antagonist at the beta2-adrenoceptor significantly restored the impaired histamine response. Restoration of protein kinase A activity and the impaired histamine responses in the atria from transgenic mice can be observed following 14-day infusions of both a neutral antagonist and inverse agonists. The reversal of the effects of the transgene by both inverse agonists and a neutral antagonist suggests that agonist occupancy, and not spontaneous activity, of the beta2-adrenoceptor is producing the elevated protein kinase A activity and the impaired histamine response.

Cytoplasmic signaling pathways that regulate cardiac hypertrophy

This review discusses the rapidly progressing field of cardiomyocyte signal transduction and the regulation of the hypertrophic response. When stimulated by a wide array of neurohumoral factors or when faced with an increase in ventricular-wall tension, individual cardiomyocytes undergo hypertrophic growth as an adaptive response. However, sustained cardiac hypertrophy is a leading predictor of future heart failure. A growing number of intracellular signaling pathways have been characterized as important transducers of the hypertrophic response, including specific G protein isoforms, low-molecular-weight GTPases (Ras, RhoA, and Rac), mitogen-activated protein kinase cascades, protein kinase C, calcineurin, gp130-signal transducer and activator of transcription, insulin-like growth factor I receptor pathway, fibroblast growth factor and transforming growth factor beta receptor pathways, and many others. Each of these signaling pathways has been implicated as a hypertrophic transducer, which collectively suggests an emerging paradigm whereby multiple pathways operate in concert to orchestrate a hypertrophic response

The cardiac beta-adrenoceptor-G-protein(s)-adenylyl cyclase system in monocrotaline-treated rats

In rats, injection of the alkaloid monocrotaline (MCT) causes right ventricular hypertrophy and cardiac failure. In order to study whether, in MCT-treated rats, changes in the cardiac beta -adrenoceptor-G-protein(s)-adenylyl cyclase system might be comparable to those found in human primary pulmonary hypertension, we assessed in right and left ventricles from MCT-treated rats the components of the beta -adrenoceptor system: the receptor number and subtype distribution (by (-)-[(125)I]iodocyanopindolol binding), the G-proteins (by quantitative Western blotting), and the activity of adenylyl cyclase. A single injection of 60 mg/kg i.p. MCT caused in rats right ventricular hypertrophy (RVH); part of the rats developed cardiac failure (RVF). In these rats the cardiac beta -adrenoceptor-G-protein(s)-adenylyl cyclase system was markedly changed beta -adrenoceptors were desensitized due to a decrease in receptor number, an uncoupling of the receptor from the G(s)-adenylyl cyclase system, a decrease in G(s)and a decrease in the activity of the catalytic unit of adenylyl cyclase. In general, these changes were more pronounced in right ventricles v left ventricles, and in rats with RVF v rats with RVH. On the other hand, cardiac muscarinic receptors and G(i)appeared not to be altered. We conclude that in MCT-treated rats changes in the cardiac beta -adrenoceptor-G-protein(s)-adenylyl cyclase system occur that resemble those observed in human primary pulmonary hypertension. Thus, MCT-treated rat appears to be a suitable animal model to study in more detail the pathophysiology of the development of right heart failure, and to identify new therapeutic possibilities.

Beta(2)-adrenergic and several other G protein-coupled receptors in human atrial membranes activate both G(s) and G(i)

Cardiac G protein-coupled receptors that couple to Galpha(s) and stimulate cAMP formation (eg, beta-adrenergic, histamine, serotonin, and glucagon receptors) play a key role in cardiac inotropy. Recent studies in rodent cardiac myocytes and transfected cells have revealed that one of these receptors, the beta(2)-adrenergic receptor (AR), also couples to the inhibitory G protein Galpha(i) (activation of which inhibits cAMP formation). If beta(2)ARs could be shown to couple to Galpha(i) in the human heart, it would have important ramifications, because levels of Galpha(i) increase with age and in failing human heart. Therefore, we investigated whether beta(2)ARs in the human heart activate Galpha(i). By photoaffinity labeling human atrial membranes with [(32)P]azidoanilido-GTP, followed by immunoprecipitation with antibodies specific for Galpha(i), we found that Galpha(i) is activated by stimulation of beta(2)ARs but not of beta(1)ARs. In addition, we found that other Galpha(s)-coupled receptors also couple to Galpha(i), including histamine, serotonin, and glucagon. When coupling of these receptors to Galpha(i) is disrupted by pertussis toxin, their ability to stimulate adenylyl cyclase is enhanced. These data provide the first evidence that beta(2)AR and many other Galpha(s)-coupled receptors in human atrium also couple to Galpha(i) and that abolishing the coupling of these receptors to Galpha(i) increases the receptor-mediated adenylyl cyclase activity.

Age- and sex-associated changes in cardiac beta(1)-adrenoceptors from the muscular dystrophy (mdx) mouse

Variations in beta(1)-adrenoceptor function due to age or sex were examined in the mouse model of Duchenne muscular dystrophy. Positive chronotropic and inotropic responses to (-)-isoprenaline and antagonist effects of CGP20712A were determined in isolated right and left atria from young (12 week) and old (12 month) male and old (12 month) female mdx mice and their age- and sex-matched C57BL/10ScSn (C57) mice. There was no difference in efficacy to (-)-isoprenaline when expressed as an increase in the rate of contraction or force of contraction as a percentage of Ca(2+)-induced increase respectively in right or left atria from age- and sex-matched mdx and C57. Old mdx males showed a decreased sensitivity to (-)-isoprenaline (P<0.05) and a reduced affinity to CGP 20712A (P<0.05) in both right and left atria compared with old C57 males. These same changes were also observed in left atria between old and young mdx males. A reduced efficacy to (-)-isoprenaline was also evident when young and old mdx males were compared. In contrast, in old females, mdx showed an increased affinity to CGP20712A in left and right atria (P<0.05), and an enhanced sensitivity to (-)-isoprenaline in right atria. Finally, in left atria, the maximum Ca(2+)-induced increase in force of contraction was lower in all mdx compared to their age- and sex-matched C57 (P<0.05). In conclusion, age- and sex-associated changes in beta(1)-adrenoceptor function and responses to calcium were demonstrated in cardiac muscle from mdx mice, with a marked deterioration in beta(1)-adrenoceptor function occurring with aging in male mdx being particularly evident.

Cardiac beta-adrenoceptors in chronic uremia: studies in humans and rats

OBJECTIVES

The purpose of this study was to elucidate whether cardiac beta-adrenergic effects may be blunted in patients on maintenance hemodialysis (HD) and may help to explain autonomic dysfunction.

BACKGROUND

Patients on HD often suffer from autonomic dysfunction.

METHODS

We investigated the cardiovascular response of five HD patients (age: 46.1+/-7.9 years) and six healthy volunteers (age: 48.2+/-7.5 years) to isoprenaline, pirenzepine and phenylephrine. For analysis of underlying mechanisms of beta-adrenoceptor hyporesponsiveness, six-week-old male Wistar rats were rendered uremic by 5/6-nephrectomy (n = 9; SNX) and were killed for removal of the heart after six to seven weeks. Sham-operated rats (n = 15) served as controls.

RESULTS

In the patient study, isoprenaline (3.5, 7, 17, 35 ng/kg/min, i.v.) led to an increase in heart rate, and shortening of the heart rate corrected duration of the electromechanical systole (QS2c), both of which were significantly reduced in HD patients. Baroreflex sensitivity was significantly reduced in HD patients. The response to low parasympathomimetic doses of pirenzepine was unchanged. In the rat study, left ventricular strips were placed in an organ bath, electrically driven and exposed to isoprenaline (10(-11) to 10(-6) mol/liter). While pD2 values were unchanged, maximum effect at the highest concentration was significantly reduced in SNX rats. The response to carbachol was not altered, nor was the M2-cholinoceptor density. There was no difference in beta-adrenoceptor density, or in immunodetectable amount of Gs and Gi protein. Activation of adenylyl cyclase evoked by isoprenaline was significantly reduced in left ventricular membranes of SNX rats, whereas effects of 10 micromol/liter GTP, 10 mmol/liter NaF, 10 micromol/liter forskolin and 10 mmol/liter Mn2+ were not altered.

CONCLUSIONS

Cardiac beta-adrenergic responses are blunted in chronic uremia due to reduced isoprenaline-dependent activation of adenylyl cyclase. This might be caused by an "uncoupling" of the receptor or by an inhibition of the receptor by uremic toxins.

Muscarinic receptors in the failing human heart

In the human heart, as in the heart of several other species, muscarinic receptors are predominantly of the M2-subtype that couple via a pertussis toxin-sensitive Gi-protein to inhibit adenylyl cyclase. However, it is not clear whether an additional muscarinic receptor subtype exists in the human heart. In human right atrium, stimulation of muscarinic M2 receptors causes direct negative inotropic and chronotropic effects; in human ventricular myocardium, however, the negative inotropic effect can be only achieved when basal force of contraction has been pre-stimulated by cyclic AMP-elevating agents such as beta-adrenoceptor agonists, forskolin or phosphodiesterase inhibitors (indirect effect); this has been shown in various in vitro and in vivo studies. Evidence has accumulated that in chronic heart failure vagal activity is decreased. Cardiac muscarinic M2 receptor density and functional responsiveness (inhibition of adenylyl cyclase activity and negative inotropic effects), however, are not considerably changed when compared with non-failing hearts although cardiac Gi-activity is increased.

Expression, activity and functional significance of inducible nitric oxide synthase in the failing human heart

OBJECTIVES

The study was designed to evaluate the functional impact of nitric oxide (NO) generation within the myocardium on cardiac contraction in the failing human heart.

BACKGROUND

Heart failure is associated with activation of cytokines and expression of inducible nitric oxide synthase (NOS II), which generates NO from L-arginine. Nitric oxide has been shown to modulate myocardial performance, raising the possibility that cardiac generation of NO by NOS II modulates cardiac contraction in the failing human heart.

METHODS

Left ventricular (LV) tissue of 24 patients with end-stage heart failure was obtained during cardiac transplantation. Gene expression of NOS II and endothelial NO-synthase (NOS III) was quantified by competitive reverse transcription-polymerase chain reaction and compared to tissues of five nonfailing donor hearts. Nitric oxide synthase II activity was determined by citrulline assay and related to changes in force of contraction induced by the beta-adrenergic agonist isoproterenol, NO-donors and/or N-mono-methyl-L-arginine (L-NMMA), an inhibitor of NOS.

RESULTS

While NOS III mRNA was reduced in failing hearts, NOS II mRNA was increased in failing LV tissue and correlated with NOS II activity. High NOS II activity was associated with early relaxation and impaired responsiveness to beta-adrenergic stimulation, that is, the inotropic response to isoproterenol in failing hearts was inversely related to NOS II activity (r=0.61, p < 0.005). Nitric oxide donors or L-NMMA did not affect myocardial performance in failing hearts at baseline. However, L-NMMA enhanced the positive inotropic response to beta-adrenergic stimulation in failing hearts with high NOS II activity. Nitric oxide donors attenuated the isoproterenol-induced increase in force of contraction of failing hearts.

CONCLUSIONS

Cardiac production of NO by NOS II attenuates the positive inotropic effects of beta-adrenergic stimulation and hastens relaxation in failing human hearts.