Reduced alpha 1- and beta 2-adrenoceptor-mediated positive inotropic effects in human end-stage heart failure

Clinical pharmacology of cardiac cyclic AMP in human heart failure: too much or too little?

INTRODUCTION

Cyclic 3', 5'-adenosine monophosphate (cAMP) is a major signaling hub in cardiac physiology. Although cAMP signaling has been extensively studied in cardiac cells and animal models of heart failure (HF), not much is known about its actual amount present inside human failing or non-failing cardiomyocytes. Since many drugs used in HF work via cAMP, it is crucial to determine the status of its intracellular levels in failing vs. normal human hearts.

AREAS COVERED

Only studies performed on explanted/excised cardiac tissues from patients were examined. Studies that contained no data from human hearts or no data on cAMP levels per se were excluded from this perspective's analysis.

EXPERT OPINION

Currently, there is no consensus on the status of cAMP levels in human failing vs. non-failing hearts. Several studies on animal models may suggest maladaptive (e.g. pro-apoptotic) effects of cAMP on HF, advocating for cAMP lowering for therapy, but human studies almost universally indicate that myocardial cAMP levels are deficient in human failing hearts. It is the expert opinion of this perspective that intracellular cAMP levels are too low in human failing hearts, contributing to the disease. Strategies to increase (restore), not decrease, these levels should be pursued in human HF.

Dimerization of β2-adrenergic receptor is responsible for the constitutive activity subjected to inverse agonism

Dimerization of beta 2-adrenergic receptor (β₂-AR) has been observed across various physiologies. However, the function of dimeric β₂-AR is still elusive. Here, we revealed that dimerization of β₂-AR is responsible for the constitutive activity of β₂-AR generating inverse agonism. Using a co-immunoimmobilization assay, we found that transient β₂-AR dimers exist in a resting state, and the dimer was disrupted by the inverse agonists. A Gα_s preferentially interacts with dimeric β₂-AR, but not monomeric β₂-AR, in a resting state, resulting in the production of a resting cAMP level. The formation of β₂-AR dimers requires cholesterol on the plasma membrane. The cholesterol did not interfere with the agonist-induced activation of monomeric β₂-AR, unlike the inverse agonists, implying that the cholesterol is a specific factor regulating the dimerization of β₂-AR. Our model not only shows the function of dimeric β₂-AR but also provides a molecular insight into the mechanism of the inverse agonism of β₂-AR.

Cardiac and Vascular α1-Adrenoceptors in Congestive Heart Failure: A Systematic Review

As heart failure (HF) is a devastating health problem worldwide, a better understanding and the development of more effective therapeutic approaches are required. HF is characterized by sympathetic system activation which stimulates α- and β-adrenoceptors (ARs). The exposure of the cardiovascular system to the increased locally released and circulating levels of catecholamines leads to a well-described downregulation and desensitization of β-ARs. However, information on the role of α-AR is limited. We have performed a systematic literature review examining the role of both cardiac and vascular α1-ARs in HF using 5 databases for our search. All three α1-AR subtypes (α1A, α1B and α1D) are expressed in human and animal hearts and blood vessels in a tissue-dependent manner. We summarize the changes observed in HF regarding the density, signaling and responses of α1-ARs. Conflicting findings arise from different studies concerning the influence that HF has on α1-AR expression and function; in contrast to β-ARs there is no consistent evidence for down-regulation or desensitization of cardiac or vascular α1-ARs. Whether α1-ARs are a therapeutic target in HF remains a matter of debate.

Analysis of the Indacaterol-Regulated Transcriptome in Human Airway Epithelial Cells Implicates Gene Expression Changes in the Adverse and Therapeutic Effects of β2-Adrenoceptor Agonists

The contribution of gene expression changes to the adverse and therapeutic effects of β₂-adrenoceptor agonists in asthma was investigated using human airway epithelial cells as a therapeutically relevant target. Operational model-fitting established that the long-acting β₂-adrenoceptor agonists (LABA) indacaterol, salmeterol, formoterol, and picumeterol were full agonists on BEAS-2B cells transfected with a cAMP-response element reporter but differed in efficacy (indacaterol ≥ formoterol > salmeterol ≥ picumeterol). The transcriptomic signature of indacaterol in BEAS-2B cells identified 180, 368, 252, and 10 genes that were differentially expressed (>1.5- to <0.67-fold) after 1-, 2-, 6-, and 18-hour of exposure, respectively. Many upregulated genes (e.g., AREG, BDNF, CCL20, CXCL2, EDN1, IL6, IL15, IL20) encode proteins with proinflammatory activity and are annotated by several, enriched gene ontology (GO) terms, including cellular response to interleukin-1, cytokine activity, and positive regulation of neutrophil chemotaxis The general enriched GO term extracellular space was also associated with indacaterol-induced genes, and many of those, including CRISPLD2, DMBT1, GAS1, and SOCS3, have putative anti-inflammatory, antibacterial, and/or antiviral activity. Numerous indacaterol-regulated genes were also induced or repressed in BEAS-2B cells and human primary bronchial epithelial cells by the low efficacy LABA salmeterol, indicating that this genomic effect was neither unique to indacaterol nor restricted to the BEAS-2B airway epithelial cell line. Collectively, these data suggest that the consequences of inhaling a β₂-adrenoceptor agonist may be complex and involve widespread changes in gene expression. We propose that this genomic effect represents a generally unappreciated mechanism that may contribute to the adverse and therapeutic actions of β₂-adrenoceptor agonists in asthma.

AkrinorTM, a Cafedrine/ Theodrenaline Mixture (20:1), Increases Force of Contraction of Human Atrial Myocardium But Does Not Constrict Internal Mammary Artery In Vitro

Background: Intraoperative hypotension is a common problem and direct or indirect sympathomimetic drugs are frequently needed to stabilize blood pressure. Akrinor^TM consists of the direct and the indirect sympathomimetic noradrenaline and norephedrine. Both substances are covalently bound to the phosphodiesterase (PDE) inhibitor theophylline, yielding theodrenaline and cafedrine, respectively. We investigated pharmacodynamic effects of Akrinor^TM and its constituents on contractile force and tension in human atrial trabeculae and internal A. mammaria rings.

Methods: Isometric contractions were measured in human atrial trabeculae at 1 Hz and 37°C. CGP 20712A and ICI 118,551 were used to elaborate β₁- and β₂-adrenoceptor (AR) subtypes involved and phenoxybenzamine to estimate indirect sympathomimetic action. PDE-inhibition was measured as a potentiation of force increase upon direct activation of adenylyl cyclase by forskolin. Human A. mammaria preparations were used to estimate intrinsic vasoconstriction and impact on the noradrenaline-induced vasoconstriction.

Results: Clinically relevant concentrations of Akrinor^TM (4.2–420 mg/l) robustly increased force in human atrial trabeculae (EC₅₀ 41 ± 3 mg/l). This direct sympathomimetic action was mediated via β₁-AR and the effect size was as large as with high concentrations of calcium. Only the highest and clinically irrelevant concentration of Akrinor^TM increased the potency of forskolin to a minor extent. Norephedrine has lost its indirect sympathomimetic effect when bound to theophylline. Increasing concentrations of Akrinor^TM (4.2–168 mg/l) alone did not affect the tension of human A. mammaria interna rings, but shifted the noradrenaline curve rightward from -logEC₅₀ 6.18 ± 0.08 to 5.23 ± 0.05 M.

Conclusion: Akrinor^TM increased cardiac contractile force by direct sympathomimetic actions and PDE inhibition, did not constrict A. mammaria preparations, but shifted the concentration-response curve to the right, compatible with an α-AR antagonistic effect or PDE inhibition. The pharmacodynamic profile and potency of Akrinor^TM differs from noradrenaline and norephedrine in vitro. We anticipate metabolism of theodrenaline and cafedrine resulting in a different pharmacodynamic profile of Akrinor^TMin vivo.

Pathophysiology and prevention of sudden cardiac death

Sudden cardiac death (SCD) is known to occur in individuals with diverse diseases. Each disease state has a specific etiology and pathophysiology, and is diagnosed and treated differently. Etiologies for SCD include cardiac arrhythmias, coronary artery disease, congenital coronary artery anomalies, hypertrophic cardiomyopathy, arrhythmogenic right ventricular dysplasia, dilated cardiomyopathy, and aortic valve stenosis. A potential unifying mechanism of SCD in these diseases involves a massive stimulation of the sympathetic nervous system’s stress response and the subsequent elevation of circulating catecholamines. The diagnosis of cardiac diseases that contribute to an increased risk for SCD is accomplished by a combination of different techniques including electrocardiography, echocardiography, magnetic resonance imaging, and invasive cardiac catheterization. Several therapies including anti-arrhythmic drugs, β-blockers, and antiplatelet agents may be used as medical treatment in patients for the prevention of SCD. Invasive therapies including percutaneous angioplasty, coronary artery bypass surgery, and implantable cardioverter-defibrillators are also used in the clinical management of SCD.

Alpha-1-adrenergic receptors in heart failure: the adaptive arm of the cardiac response to chronic catecholamine stimulation

Alpha-1-adrenergic receptors (ARs) are G protein-coupled receptors activated by catecholamines. The alpha-1A and alpha-1B subtypes are expressed in mouse and human myocardium, whereas the alpha-1D protein is found only in coronary arteries. There are far fewer alpha-1-ARs than beta-ARs in the nonfailing heart, but their abundance is maintained or increased in the setting of heart failure, which is characterized by pronounced chronic elevation of catecholamines and beta-AR dysfunction. Decades of evidence from gain and loss-of-function studies in isolated cardiac myocytes and numerous animal models demonstrate important adaptive functions for cardiac alpha-1-ARs to include physiological hypertrophy, positive inotropy, ischemic preconditioning, and protection from cell death. Clinical trial data indicate that blocking alpha-1-ARs is associated with incident heart failure in patients with hypertension. Collectively, these findings suggest that alpha-1-AR activation might mitigate the well-recognized toxic effects of beta-ARs in the hyperadrenergic setting of chronic heart failure. Thus, exogenous cardioselective activation of alpha-1-ARs might represent a novel and viable approach to the treatment of heart failure.

Cardiac alpha1-adrenergic receptors: novel aspects of expression, signaling mechanisms, physiologic function, and clinical importance

Adrenergic receptors (AR) are G-protein-coupled receptors (GPCRs) that have a crucial role in cardiac physiology in health and disease. Alpha1-ARs signal through Gαq, and signaling through Gq, for example, by endothelin and angiotensin receptors, is thought to be detrimental to the heart. In contrast, cardiac alpha1-ARs mediate important protective and adaptive functions in the heart, although alpha1-ARs are only a minor fraction of total cardiac ARs. Cardiac alpha1-ARs activate pleiotropic downstream signaling to prevent pathologic remodeling in heart failure. Mechanisms defined in animal and cell models include activation of adaptive hypertrophy, prevention of cardiac myocyte death, augmentation of contractility, and induction of ischemic preconditioning. Surprisingly, at the molecular level, alpha1-ARs localize to and signal at the nucleus in cardiac myocytes, and, unlike most GPCRs, activate "inside-out" signaling to cause cardioprotection. Contrary to past opinion, human cardiac alpha1-AR expression is similar to that in the mouse, where alpha1-AR effects are seen most convincingly in knockout models. Human clinical studies show that alpha1-blockade worsens heart failure in hypertension and does not improve outcomes in heart failure, implying a cardioprotective role for human alpha1-ARs. In summary, these findings identify novel functional and mechanistic aspects of cardiac alpha1-AR function and suggest that activation of cardiac alpha1-AR might be a viable therapeutic strategy in heart failure.

Stoichiometric analysis of oligomerization of membrane proteins on living cells using coiled-coil labeling and spectral imaging

Many membrane proteins are proposed to work as oligomers; however, the conclusion is sometimes controversial, as for β2-adorenergic receptor (β2AR), which is one of the best-studied family A G-protein-coupled receptors. This is due to the lack of methods for easy and precise detection of the oligomeric state of membrane proteins on living cells. Here, we show that a combination of the coiled-coil tag-probe labeling method and spectral imaging enable a stoichiometric analysis of the oligomeric state of membrane proteins on living cells using monomeric, dimeric, and tetrameric standard membrane proteins. Using this method, we found that β2ARs do not form constitutive homooligomers, while they exhibit their functions such as the cyclic adenosine 5'-monophosphate (cAMP) signaling and internalization upon agonist stimulation.

Cardiac and neuroprotection regulated by α(1)-adrenergic receptor subtypes

Sympathetic nervous system regulation by the α(1)-adrenergic receptor (AR) subtypes (α(1A), α(1B), α(1D)) is complex, whereby chronic activity can be either detrimental or protective for both heart and brain function. This review will summarize the evidence that this dual regulation can be mediated through the different α(1)-AR subtypes in the context of cardiac hypertrophy, heart failure, apoptosis, ischemic preconditioning, neurogenesis, locomotion, neurodegeneration, cognition, neuroplasticity, depression, anxiety, epilepsy, and mental illness.

Conserved expression and functions of PDE4 in rodent and human heart

PDE4 isoenzymes are critical in the control of cAMP signaling in rodent cardiac myocytes. Ablation of PDE4 affects multiple key players in excitation–contraction coupling and predisposes mice to the development of heart failure. As little is known about PDE4 in human heart, we explored to what extent cardiac expression and functions of PDE4 are conserved between rodents and humans. We find considerable similarities including comparable amounts of PDE4 activity expressed, expression of the same PDE4 subtypes and splicing variants, anchoring of PDE4 to the same subcellular compartments and macromolecular signaling complexes, and downregulation of PDE4 activity and protein in heart failure. The major difference between the species is a fivefold higher amount of non-PDE4 activity in human hearts compared to rodents. As a consequence, the effect of PDE4 inactivation is different in rodents and humans. PDE4 inhibition leads to increased phosphorylation of virtually all PKA substrates in mouse cardiomyocytes, but increased phosphorylation of only a restricted number of proteins in human cardiomyocytes. Our findings suggest that PDE4s have a similar role in the local regulation of cAMP signaling in rodent and human heart. However, inhibition of PDE4 has ‘global’ effects on cAMP signaling only in rodent hearts, as PDE4 comprises a large fraction of the total cardiac PDE activity in rodents but not in humans. These differences may explain the distinct pharmacological effects of PDE4 inhibition in rodent and human hearts.

Alpha-1-adrenergic receptors: targets for agonist drugs to treat heart failure

Evidence from cell, animal, and human studies demonstrates that α1-adrenergic receptors mediate adaptive and protective effects in the heart. These effects may be particularly important in chronic heart failure, when catecholamine levels are elevated and β-adrenergic receptors are down-regulated and dysfunctional. This review summarizes these data and proposes that selectively activating α1-adrenergic receptors in the heart might represent a novel and effective way to treat heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Use of afterload hemodynamic stress as a practical method for assessing cardiac performance in rats with heart failure

After myocardial infarction, the hemodynamics under basal conditions might appear to be unaltered, which makes it difficult to identify cardiac dysfunction by the usual approaches. Thus, we tested the response to sudden afterload stress in infarcted rats with apparently normal ejection function. Control (CT) and infarcted (MI) Wistar rats with various MI sizes were submitted to echocardiography 30 days after coronary occlusion, followed by assessment of hemodynamics under basal conditions and during a pharmacologically induced sudden pressure overload (phenylephrine 15-25 microg/kg, i.v.). Coronary occlusion resulted in cardiac remodeling proportional to MI size, although several functional parameters such as systolic pressure (SP), stroke volume (SV), and stroke work (SW) of all MI rats were similar to those of CT rats. However, the afterload stress that was produced led to a relative preservation of SV and an increase of SW in CT rats; MI rats exhibited a significant reduction in SV and SW generation, although global cardiac function was normal under basal conditions, as indicated by regular echocardiography and hemodynamics assessment. Thus, we propose the use of sudden pharmacologically induced afterload stress as a practical and efficient procedure for identifying impaired performance of the heart in anesthetized rats, providing an additional physiological variable to be evaluated in experimental therapeutic studies.

Inotropic effects of L-lysine in the mammalian heart

We studied the effects of L-lysine in cardiac preparations of mice and men. Of note, L-lysine increased force of contraction in a concentration- and time-dependent manner in isolated electrically paced left atrium of mouse and in human right atrium. It further increased heart rate and left ventricular pressure in the isolated perfused mouse heart. In isolated adult mouse cardiomyocytes, the contractility as assessed by edge detection was increased as well as the Ca(2+) transients after electrically pacing by field stimulation. However, using the patch clamp technique, no effect of L-lysine on action potential duration from a constant holding potential or on current through L-type calcium channels could be observed. However, L-lysine led to a depolarization of unclamped cells. Furthermore, effects of L-lysine were stereospecific, as they were not elicited by D-lysine. The inotropic effects of L-lysine were not abrogated by additionally applied L-ornithine or L-arginine (known inhibitors of lysine transport). However, L-lysine (5 mM) shifted the concentration-response curve for a positive inotropic effect of 5-hydroxytryptamine (5-HT; serotonin) in atrium of transgenic mice (with cardiac specific overexpression of 5-HT(4) receptors) to higher concentrations. In summary, we describe a novel positive inotropic effect of an essential amino acid, L-lysine, in the mammalian heart. One might speculate that L-lysine treatment under certain conditions could sustain cardiac performance. Moreover, L-lysine is able to block, at least in part, cardiac 5-HT(4) receptors.

Cardiac dilatation and pump dysfunction without intrinsic myocardial systolic failure following chronic β-adrenoreceptor activation

There is no direct evidence to indicate that pump dysfunction in a dilated chamber reflects the impact of chamber dilatation rather than the degree of intrinsic systolic failure resulting from myocardial damage. In the present study, we explored the relative roles of intrinsic myocardial systolic dysfunction and chamber dilatation as mediators of left ventricular (LV) pump dysfunction. Administration of isoproterenol, a β-adrenoreceptor agonist, for 3 mo to rats (0.1 mg·kg−1·day−1) resulted in LV pump dysfunction as evidenced by a reduced LV endocardial fractional shortening (echocardiography) and a decrease in the slope of the LV systolic pressure-volume relation (isolated heart preparations). Although chronic β-adrenoreceptor activation induced cardiomyocyte damage (deoxynucleotidyl transferase-mediated dUTP nick-end labeling) as well as β1- and β2-adrenoreceptor inotropic downregulation (attenuated contractile responses to dobutamine and salbutamol), these changes failed to translate into alterations in intrinsic myocardial contractility. Indeed, LV midwall fractional shortening (echocardiography) and the slope of the LV systolic stress-strain relation (isolated heart preparations) were unchanged. A normal intrinsic myocardial systolic function, despite the presence of cardiomyocyte damage and β-adrenoreceptor inotropic downregulation, was ascribed to marked increases in myocardial norepinephrine release, to upregulation of α-adrenoreceptor-mediated contractile effects as determined by phenylephrine responsiveness, and to compensatory LV hypertrophy. LV pump failure was attributed to LV dilatation, as evidenced by increased LV internal dimensions (echocardiography), and a right shift and increased volume intercept of the LV diastolic pressure-volume relation. In conclusion, chronic sympathetic stimulation, despite reducing β-adrenoreceptor-mediated inotropic responses and promoting myocyte apoptosis, may nevertheless induce pump dysfunction primarily through LV dilatation, rather than intrinsic myocardial systolic failure.

Myocardial phosphodiesterases and regulation of cardiac contractility in health and cardiac disease

Phosphodiesterase (PDE) inhibitors are potent cardiotonic agents used for parenteral inotropic support in heart failure. Contractile effects of these agents are mediated through cAMP-protein kinase A-induced stimulation of I (Ca2+) which ultimately results in increased Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. A number of additional effects such as increases in sarcoplasmic reticulum Ca(2+) stores, stimulation of reverse mode Na(+)-Ca(2+) exchange, direct or cAMP-mediated effects on sarcoplasmic reticulum ryanodine receptor, stimulation of the voltage-sensitive sarcoplasmic reticulum Ca(2+) release mechanism, as well as A(1) adenosine receptor blockade could contribute to positive inotropic responses to PDE inhibitors. Moreover, some PDE inhibitors exhibit Ca(2+) sensitizer properties as they could increase the affinity of troponin C Ca(2+)-binding sites as well as reduce Ca(2+) threshold for thin myofilament sliding and facilitate cross-bridge cycling. Inotropic responses to PDE inhibitors are significantly reduced in cardiac disease, an effect largely attributed to downregulation of cAMP-mediated signalling due to sustained sympathetic activation. Four PDE isoenzymes (PDE1, PDE2, PDE3 and PDE4) are present in myocardial tissue of various mammalian species, of which PDE3 and PDE4 are particularly involved in regulation of cardiac myocyte contraction. PDE cAMP-hydrolysing activity is preserved in compensated cardiac hypertrophy but significantly reduced in animal models of heart failure. However, clinical studies have not revealed any changes in distribution profile as well as kinetic and regulatory properties of myocardial PDEs in failing human hearts. A reduction of PDE inhibitors-induced contractile responses in heart failure has therefore been ascribed to reduced cAMP synthesis due to uncoupling of adenylyl cyclase from beta-adrenoreceptor. In cardiac myocytes, PDEs are targeted to distinct subcellular compartments by scaffolding proteins such as myomegalin, mAKAP and beta-arrestins. Over subcellular microdomains, cAMP hydrolysis by PDE3 and PDE4 allows to control the activity of local pools of protein kinase A and therefore the extent of protein kinase A-mediated phosphorylation of cellular proteins.

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.

Regulators of G-protein signalling: multifunctional proteins with impact on signalling in the cardiovascular system

Regulator of G-protein signalling (RGS) proteins form a superfamily of at least 25 proteins, which are highly diverse in structure, expression patterns, and function. They share a 120 amino acid homology domain (RGS domain), which exhibits GTPase accelerating activity for alpha-subunits of heterotrimeric G-proteins, and thus, are negative regulators of G-protein-mediated signalling. Based on the organisation of the Rgs genes, structural similarities, and differences in functions, they can be divided into at least six subfamilies of RGS proteins and three more families of RGS-like proteins. Many of these proteins regulate signalling processes within cells, not only via interaction with G-protein alpha-subunits, but are G-protein-regulated effectors, Gbetagamma scavenger, or scaffolding proteins in signal transduction complexes as well. The expression of at least 16 different RGS proteins in the mammalian or human myocardium have been described. A subgroup of at least eight was detected in a single atrial myocyte. The exact functions of these proteins remain mostly elusive, but RGS proteins such as RGS4 are involved in the regulation of G(i)-protein betagamma-subunit-gated K(+) channels. An up-regulation of RGS4 expression has been consistently found in human heart failure and some animal models. Evidence is increasing that the enhanced RGS4 expression counter-regulates the G(q/11)-induced signalling caused by hypertrophic stimuli. In the vascular system, RGS5 seems to be an important signalling regulator. It is expressed in vascular endothelial cells, but not in cultured smooth muscle cells. Its down-regulation, both in a model of capillary morphogenesis and in an animal model of stroke, render it a candidate gene, which may be involved in the regulation of capillary growth, angiogenesis, and in the pathophysiology of stroke.

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.

Altered coronary and cardiac adrenergic response in the failing hamster heart: role of cyclooxygenase derivatives

Although the influence of the adrenergic system has been studied in the presence of heart failure, controversies still exist. Since cyclooxygenase derivatives appear to modulate coronary and cardiac adaptation in the failing heart, we hypothesized that cyclooxygenase derivatives may participate in the altered adrenergic responses in this situation. Isolated hearts from cardiomyopathic (UM-X7.1 subline) and normal hamsters, aged > 240 days, were utilized. Coronary and cardiac response to alpha1-, beta1-, and beta2-adrenergic stimulations was observed before and after pretreatment with indomethacin, a cyclooxygenase inhibitor. Reduction of coronary flow elicited by alpha1-adrenergic stimulation was unchanged in the presence of heart failure, while beta1- and beta2-induced vasodilatations were reduced. Inotropic response to alpha1 and beta1 stimulations were also reduced in failing hearts, while beta2-adrenergic action was unchanged. Pretreatment with indomethacin exacerbated coronary flow reduction observed with alpha1 stimulation in failing hearts only. Beta2-induced coronary vasodilatation and inotropic response to alpha1 and beta2 stimulations were impaired similarly in the presence of indomethacin in normal and failing hearts. The results suggest a complex interaction between adrenergic and cyclooxygenase activation.

Mechanisms of desensitization to a PDE inhibitor (milrinone) in conscious dogs with heart failure

The goal of this study was to determine the extent to which the effects of milrinone were desensitized in heart failure (HF) and to determine the mechanisms, i.e., whether these effects could be ascribed to changes in cAMP or phosphodiesterase (PDE) activity in HF. Accordingly, we examined the effects of milrinone in seven conscious dogs before and after HF was induced by rapid ventricular pacing at 240 beats/min. The dogs were chronically instrumented for measurements of left ventricular (LV) pressure and first derivative of LV pressure (dP/dt), arterial pressure, LV internal diameter, and wall thickness. Milrinone (10 micrograms . kg-1. min-1 iv) increased LV dP/dt by 1,854 +/- 157 from 2,701 +/- 105 mmHg/s (P < 0.05) before HF. After HF the increase in LV dP/dt in response to milrinone was attenuated significantly (P < 0.05); it increased by 615 +/- 67 from 1,550 +/- 107 mmHg/s, indicating marked desensitization. In the presence of ganglionic blockade the increases in LV dP/dt (+445 +/- 65 mmHg/s) in response to milrinone were markedly less (P < 0.01), and milrinone increased LV dP/dt even less in HF (+240 +/- 65 mmHg/s). cAMP and PDE activity were measured in endocardial and epicardial layers in normal and failing myocardium. cAMP was decreased significantly (P < 0.05) in LV endocardium (-26%) but not significantly in LV epicardium (-14%). PDE activity was also decreased significantly (P < 0.05) in LV endocardium (-18%) but not in LV epicardium (-4%). Thus significant desensitization to milrinone was observed in conscious dogs with HF. The major effect was autonomically mediated. The biochemical mechanism appears to be due in part to the modest reductions in PDE activity in failing myocardium, which, in turn, may be a compensatory mechanism to maintain cAMP levels in HF. Reductions in cAMP and PDE levels were restricted to the subendocardium, suggesting that the increased wall stress and reduced coronary reserve play a role in mediating these changes.

Dissociation of the effects of forskolin and dibutyryl cAMP on force of contraction and phospholamban phosphorylation in human heart failure

Forskolin and dibutyryl cyclic adenosine monophosphate (cAMP) stimulate force of contraction independent of beta-adrenoceptor stimulation. We studied their effects on force of contraction and phosphorylation of regulatory proteins in isolated electrically driven trabeculae carneae from failing human ventricles. The phosphorylation state of the regulatory protein phospholamban was studied because its phosphorylation usually faithfully follows contractility. For comparison, the phosphorylation state of the inhibitory subunit of troponin was studied. The phosphorylation state was inferred from in vitro phosphorylation of homogenates with cAMP-dependent protein kinase in the presence of radioactive gamma[32P]ATP Proteins were separated by electrophoresis, and radioactivity in the proteins of interest was quantified. The maximal positive inotropic effects occurred at 30 microM forskolin and were attenuated in comparison with the maximal effects to dibutyryl cAMP (1 mM). Both forskolin and dibutyryl cAMP enhanced phospholamban phosphorylation. However, phospholamban phosphorylation in intact trabeculae treated with 30 microM forskolin and 1 mM dibutyryl cAMP was comparable. It is suggested that phospholamban phosphorylation can be dissociated from inotropy at least in isolated trabeculae from failing human hearts.

The myocardial beta-adrenergic system in spontaneously hypertensive heart failure (SHHF) rats

-Responsiveness to beta-adrenergic stimulation is reduced in the failing human myocardium. This results principally from reduced beta-adrenergic receptor (betaAR) density, elevated beta-adrenergic receptor kinase 1 (betaARK1) levels, and functional uncoupling of remaining receptors. The temporal nature of changes in the human myocardial beta-adrenergic system relative to onset of symptomatic heart failure (HF) has been difficult to discern. A relatively new model of HF, the spontaneously hypertensive heart failure (SHHF) rat spontaneously and reproducibly develops left ventricular hypertrophy (LVH) and progresses to HF, thus enabling longitudinal studies to examine the cellular and molecular bases for hypertension-induced cardiac hypertrophy and subsequent HF. The purpose of this study was to examine age-dependent changes in the betaAR system in this model. Lean male SHHF rats at 3, 7, 14, and 20 months were compared with age-matched Sprague-Dawley (SD) control rats ([C]; 4 animals/group). At all ages the SHHF rats had elevated blood pressures and left ventricular end-diastolic pressure relative to the SD control rats (P<0.05). Compared with age-matched SD control rats, LVH was evident by 3 months in SHHF rats; 20-month-old SHHF rats had significantly greater LVH compared with the other SHHF rat groups. beta-adrenergic responsiveness (maximal heart rate to isoproterenol) was reduced only in 20-month-old SHHF rats. betaARK1 protein levels and activity were elevated at 14 months (162+/-10% and 195+/-20% C, respectively), and betaARK1 protein remained elevated at 20 months (140+/-14% C). In contrast, G protein-coupled receptor kinase 5, a second receptor kinase in the heart, remained unchanged at all ages. betaAR density did not change with age in the SD control rats and was similar in the SHHF rats until 20 months of age when the receptor number was reduced (30+/-1%). These data indicate that cardiac dysfunction is coincident with reduced betaAR density. Importantly, cardiac dysfunction was preceded by elevated betaARK1 levels and activity, thus suggesting that betaARK1 may be a precipitating factor in the transition from hypertension-induced compensatory cardiac hypertrophy to HF. Furthermore, these results indicate that the SHHF rat is a powerful model for use in examination of the mechanisms involved in alterations of beta-adrenergic signaling that occur in human HF.

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

In end-stage heart failure, cardiac beta-adrenoceptors are decreased and cardiac Gi protein is increased. We assessed beta-adrenoceptors, G proteins, and effects of several beta-adrenoceptor agonists, histamine, and 5-HT on adenylyl cyclase activity in right and left atria and left ventricles and on left ventricular contractility in six potential heart transplant donors (nonfailing hearts; NFHs) and in nine patients with end-stage dilated cardiomyopathy (DCM) and 11 patients with end-stage ischemic cardiomyopathy (ICM) to establish whether the functional responsiveness of all cardiac Gs-coupled receptors is reduced. Beta-adrenoceptors were reduced in all three tissues; in DCM, beta1-adrenoceptors were more markedly downregulated; in ICM, both beta1- and beta2-adrenoceptors were diminished. In all three tissues, isoprenaline-, terbutaline-, histamine- and 5-HT-induced adenylyl cyclase activation was reduced similarly in DCM and ICM. Moreover, in DCM and ICM, guanosine triphosphate (GTP)- (involving Gs and Gi) activated adenylyl cyclase was significantly diminished, whereas NaF-activated (involving only Gs) and Mn2+-activated (acting at the catalytic unit of the enzyme) adenylyl cyclase was unaltered. Left ventricular positive inotropic responses to beta1- (noradrenaline, dopamine, and dobutamine), beta2- (terbutaline), and beta1- and beta2-adrenoceptors (isoprenaline, adrenaline, and epinine), as well as H2-receptor (histamine) stimulation were significantly reduced. The extent of reduction was not different for each agonist in ICM and DCM. We conclude that in DCM and ICM, functional responsiveness of all cardiac Gs-coupled receptors is similarly reduced.

Preserved Frank-Starling mechanism in human end stage heart failure

OBJECTIVE

The goal of the present study was to examine the ability of failing myocardium to respond to enhanced preload with an increase in force development.

METHODS

The effect of various preload conditions (2.5-15 mN) on force development was studied in right ventricular trabeculae carneae from explanted human failing hearts with ischemic cardiomyopathy (ICM, n = 5, 42 preparations) or idiopathic dilated cardiomyopathy (DCM, n = 9, 77 preparations). To determine the severity of cardiac impairment we measured the positive inotropic effect of beta-adrenoceptor stimulation and calcium (ISO/Ca2+ ratio) and the expression of atrial natriuretic peptide (ANP) mRNA in all hearts.

RESULTS

(1) Force of contraction increased with stepwise augmentation of preload (length at 2.5 mN preload to length of maximal force development) from 3.7 +/- 0.5 (ICM) and 2.7 +/- 0.4 (DCM) to 8.3 +/- 0.9 and 6.5 +/- 0.8 mN/mm2, respectively (p < 0.05). (2) The ISO/Ca2+ ratio was 0.40 +/- 0.04 (ICM) and 0.35 +/- 0.03 (DCM), respectively. (3) ANP mRNA was expressed in all preparations, albeit at greatly varying levels (ICM 22.5 +/- 6.1 and DCM 18.7 +/- 4.7 normalized arbitrary units). (4) Contraction experiments performed in left ventricular tissue (n = 3, 32 preparations) essentially confirmed the results.

CONCLUSION

The Frank-Starling mechanism is preserved in terminally failing human hearts irrespective of the underlying etiology. We found no relation between the severity of cardiac impairment as assessed by either ANP expression or the ISO/Ca2+ ratio and the ability of failing human myocardium to respond to enhanced preload with an increase in force development.

Alpha-adrenergic signal transduction in renin transgenic rats

The alpha1-adrenoceptor-G protein-phosphoinositide-specific phospholipase C (PLC) signal transduction pathway is assumed to play an important role in the regulation of contractile force and in the pathophysiology of myocardial hypertrophy. In the present study, the components of this pathway were investigated in left ventricles of hearts from hypertensive transgenic rats overexpressing the mouse renin gene [TG(mREN2)27] in comparison to age- and weight-matched Sprague-Dawley control rats. Contractile force was assessed in isolated electrically driven left ventricular papillary muscle strips. Alpha1-adrenoceptor density was measured by radioligand binding using [3H]prazosin, steady state levels of alpha q/11, and G protein beta-subunits by Western blotting. PLC activity was determined by a cell-free assay using exogenous phospholipid vesicles containing [3H]phosphatidylinositol (4,5)-bisphosphate as a substrate. Alpha1-adrenoceptor density was significantly increased (by 80%) in transgenic rats compared with control rats, while the positive inotropic response to the alpha1-adrenoceptor agonist phenylephrine was significantly reduced, suggesting a postreceptor defect in TG(mREN2)27. The expression of alpha q and alpha11 was verified by reverse transcription-polymerase chain reaction, and alpha q/11 steady state protein levels were shown to be similar in transgenic and control rats. Western blotting using a beta-common antibody revealed two bands at approximately 35 and 36 kD. The quantities of both were similar in TG(mREN2)27 compared with those in control rats. In contrast, PLC activity was significantly reduced (by 32%) in transgenic rats. In conclusion, our findings are consistent with a desensitization of the alpha1-adrenergic signal transduction pathway at the level of the effector.

Myocardial alpha1-adrenoceptor: inotropic effect and physiologic and pathologic implications

Alpha1-adrenergic receptors have been found in myocardium of all mammalian species. Although the exact underlying mechanisms have not been conclusively determined, it would appear that the myocardial effects of alpha1-adrenoceptors may vary in importance according to the pathophysiologic process involved. In physiological conditions, this receptor system plays a role in cardiac growth, cardiac contraction, and has both an antiarrhythmic function as well as a role in cardiac adaptation to various situations. This system is also involved in some pathological processes such as ischemia/reperfusion, ischemic preconditioning, and cardiac hypertrophy. The role of alpha1-adrenoceptors in heart failure is somewhat controversial. Experimental evidence suggests that myocardial alpha1-adrenoceptors can have either beneficial or deleterious effects on the heart. It thus seems possible that the development of agents specific to certain subtypes of alpha1-adrenoceptor and a better understanding of their role in pathophysiologic states could be clinically relevant.

Negative chronotropic and inotropic effects exerted by diadenosine hexaphosphate (AP6A) via A1-adenosine receptors

1. Diadenosine hexaphosphate (AP6A) exerts vasoconstrictive effects. The purpose of this study was to investigate whether AP6A has any effect on cardiac function. 2. The effects of AP6A (0.1-100 microM) on cardiac contractility and frequency were studied in guinea-pig and human isolated cardiac preparations. Furthermore, the effects of AP6A on the amplitude of the L-type calcium current, on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content and on the phosphorylation of regulatory phosphoproteins, i.e. phospholamban and troponin inhibitor, were investigated in guinea-pig isolated ventricular myocytes. 3. In isolated spontaneously beating right atria of the guinea-pig AP6A exerted a negative chronotropic effect and reduced the rate of contraction maximally by 35% (IC20 = 35 microM). 4. In isolated electrically driven left atria of the guinea-pig AP6A exerted a negative inotropic effect and reduced force of contraction maximally by 23% (IC20 = 70 microM). 5. In isolated electrically driven papillary muscles of the guinea-pig AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction maximally by 23% (IC20 = 60 microM). Furthermore, AP6A attenuated the relaxant effect of isoprenaline. 6. In human isolated electrically driven ventricular preparations AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction by maximally 42% (IC20 = 18 microM). Moreover, AP6A attenuated the relaxant effect of isoprenaline. 7. All these effects of AP6A were abolished by the selective A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX, 0.3 microM), whereas the M-cholinoceptor antagonist atropine (10 microM) and the P2-purinoceptor antagonist suramin (300 microM) failed to abolish the effects of AP6A. 8. AP6A 100 microM had no effect on the amplitude of the L-type calcium current, but attenuated isoprenaline-stimulated L-type calcium current. The maximum of the current-voltage relationship (I-V curve) was shifted to the left by isoprenaline and additional application of AP6A shifted the I-V curve back to the right to the control value. The phosphorylation state of phospholamban and the troponin inhibitor was unchanged by AP6A alone, but was markedly attenuated by AP6A in the presence of isoprenaline. Cyclic AMP levels remained unchanged by AP6A, even after stimulation with isoprenaline. 9. In summary, AP6A exerts negative chronotropic and inotropic effects in guinea-pig and human cardiac preparations. These effects are mediated via A1-adenosine receptors as all effects were sensitive to the selective A1-adenosine receptor antagonist DPCPX. Furthermore, the effects of AP6A on cyclic AMP levels, protein phosphorylation and the L-type calcium current are in accordance with stimulation of A1-adenosine receptors.

Effects of cantharidin on force of contraction and phosphatase activity in nonfailing and failing human hearts

1. The effect of the phosphatase inhibitor, cantharidin (3-300 microM) on force of contraction was studied in isolated electrically driven right ventricular trabeculae carneae from human myocardium. 2. The positive inotropic effect of cantharidin started at a concentration of 100 microM with a positive inotropic effect to 199% and to 276% of the predrug value in nonfailing and failing human hearts, respectively. 3. Under basal conditions the contraction time parameters were prolonged in human heart failure vs. nonfailing preparations. However, the positive inotropic effect of cantharidin did not affect contraction time parameters. Thus, time to peak tension, time of relaxation and total contraction time were not shortened by cantharidin in nonfailing and failing preparations. 4. The phosphatase activity was unchanged in preparations from failing hearts compared to nonfailing hearts. 5. Cantharidin inhibited phosphatase activity in a concentration-dependent manner. The IC50 value of cantharidin was about 3 microM in both nonfailing and failing human myocardium. 6. The positive inotropic effect of cantharidin was similar in nonfailing and failing human hearts, accompanied by a similar inhibitory effect of cantharidin on the phosphatase activity. The positive inotropic effect of cantharidin in failing hearts was as strong as the effect of isoprenaline in nonfailing hearts. 7. It is concluded that the treatment with a phosphatase inhibitor may offer a new positive inotropic modality for the treatment of human heart failure.

Alpha 1-adrenergic receptor coupling with Gh in the failing human heart

BACKGROUND

We recently demonstrated that Gh, which transfers the signal from the alpha 1-adrenergic receptor to the 69-kD phospholipase C, is the previously identified tissue-type transglutaminase (TGase II). The alpha 1-adrenergic receptor mediates actions of the sympathetic nervous system, including cardiac, arteriolar, and smooth muscle contractions. In human cardiac tissue, the expression of the alpha 1-adrenergic receptor is increased under pathophysiological conditions, but changes in the physiological response are small. Therefore, it has been suggested that the other components involved in the alpha 1-adrenergic receptor-mediated signaling pathway are probably altered.

METHODS AND RESULTS

Immunological and biochemical studies with nonfailling and failing human heart tissues revealed that the GTP-binding and TGase activities of human heart TGase II (hhG alpha n) are downregulated in both ischemic and dilated cardiomyopathic human heart. In ischemic cardiomyopathy, the alpha 1-adrenergic receptor number increased twofold (27.0 fmol/mg) compared with the nonfailing (12.8 fmol/mg) and the dilated cardiomyopathic (15.6 fmol/mg) heart tissues, but the coupling of hhG alpha h with the alpha 1-adrenergic receptor did not increase. The intrinsic activity of hhG alpha h, was greatly decreased in membrane fractions, whereas the cytosolic TGase activity was not changed. In the dilated cardiomyopathic human heart, these intrinsic enzyme activities of hhG alpha h were also downregulated in the membrane fraction, whereas the amount of hhG alpha h protein was greatly increased (2.8-fold) compared with the nonfailing heart.

CONCLUSIONS

The results of the present study clearly demonstrate that the alpha 1-adrenergic receptor in human heart couples with Gh (TGase II) and indicate that downregulation of hhG alpha h activity is associated with human cardiac failure but that the mechanism differs between ischemic and dilated cardiomyopathies.

Beta-adrenergic receptors in failing human myocardium

In the human heart the beta-adrenergic receptor-G-protein-adenylyl cyclase system is the most powerful physiologic mechanism to acutely increase contractility and/or heart rate. In the failing human myocardium beta 1-adrenergic receptor number is decreased, and this is accompanied by a reduced beta 1-adrenergic receptor mediated positive inotropic effect. Cardiac beta 2-adrenergic receptor number may or may not decrease; however, beta 2- adrenergic receptor mediated positive inotropic effects are also reduced, possibly because the functional activity of myocardial Gi is increased, thereby inhibiting cyclic AMP formation. The aging human heart shows some similarities with the failing human heart: in both settings, of chronic heart failure and age, beta-adrenergic receptor mediated effects and all other cyclic AMP dependent effects are depressed and Gi-protein is increased.

Altered inotropism in the failing human myocardium

Beta-adrenoreceptor-cAMP-dependent inotropic interventions lose their effectiveness depending on the degree of myocardial failure. This blunted effect of beta-adrenoreceptor-dependent stimulation might be due to a downregulation of beta-adrenoreceptors and an increase of inhibitory G-proteins leading to decreased intracellular cAMP-concentrations. However, the maximal positive inotropic effect elicited by elevation of the extracellular [Ca2+] does not differ between failing and nonfailing human myocardium, indicating that terminally failing human myocardium is effective to increase force of contraction to the same degree as nonfailing tissue. Agents which increase force of contraction primarily via increasing the intracellular [Na+], e.g., cardiac glycosides and the Na(+)-channel activator BDF 9148, exert a higher potency in failing myocardium than in nonfailing tissue to increase force of contraction. This could result from an enhanced protein expression of the Na+/Ca(2+)-exchanger observed in diseased human hearts. Alterations in the intracellular Ca(2+)-homeostasis reported in failing myocardium lead to a negative force-frequency-relationship and a prolonged relaxation. As the protein expression of SERCA IIa and phospholamban seems to be similar in NYHAIV and nonfailing tissue, the reduced Ca(2+)- uptake may result from an altered regulation of these proteins, e.g., reduced phosphorylation of phospholamban or the SERCA IIa. After inhibition of the Ca(2+)-ATPase of the sarcoplasmic reticulum with the high specific inhibitor cyclopiazonic acid the former positive force-frequency-relationship became significantly less positive even in the nonfailing tissue and twitch course became similar to diseased hearts. These findings may be indicative for the importance of the Ca(2+)-reuptake mechanism into the sarcoplasmic reticulum in addition to the regulatory control at the site of the contractile apparatus for the regulation of contraction and relaxation in human myocardium.

Inotropic and chronotropic effects of 4-(4'-n-butylaniline)-7,8- dimethoxy-5H-pyrimido[5,4-b]indole in guinea-pig atria

Cardiotonic effect of 4-(4'-n-butylaniline)-7,8-dimethoxy- 5H-pyrimido[5,4-b]indole (B11) was investigated in isolated cardiac tissue preparations. The action of this agent on force of contraction, beating frequency and cyclic nucleotide phosphodiesterase (PDE) activity was studied. Amrinone was used for comparison. B11 produced concentration-dependent (5 x 10(6)-1 x 10(-4)M) positive inotropic and positive chronotropic responses in guinea-pig atrial tissues. The potency of B11 was greater than that of amrinone. The cardiotonic effects of B11 were not modified by beta-adrenoceptor blockade. Carbachol inhibited the positive inotropic effect of B11. The activity of B11 was increased in desensitized left atrial tissues. B11 inhibited the activities of PDE isoenzymes (type I, II, IV and V) from dog heart ventricle and PDE type IV from guinea-pig heart ventricle nonselectively. It is concluded that B11 possesses potent positive inotropic activity in guinea-pig atria, and the effect is probably mediated by a non-selective inhibition of PDE activity.

Alterations of beta-adrenoceptor-G-protein-regulated adenylyl cyclase in heart failure

Alterations of receptor-G-protein-regulated adenylyl cyclase activity have been suggested to represent an important alteration leading to contractile dysfunction in the failing human heart. Recent experiments suggest that the beta 1-adrenoceptor (beta 1 AR) density and mRNA levels are reduced, while beta 2-adrenoceptors and stimulatory G-proteins are unchanged (mRNA and protein level). Functional assays demonstrated that the catalyst of the adenylyl cyclase is not different between failing and nonfailing myocardium. Inhibitory G-proteins are increased (pertussis toxin substrates, protein and mRNA) and correlate to the reduced inotropic effects of beta-adrenoceptor agonists and of cAMP-PDE inhibitors. Gi alpha-coupled m-cholinoceptors and A1-adrenergic receptors are unchanged in density and affinity. Stimulation of these receptors resulted in an unchanged antiadrenergic effect on force of contraction. In conclusion, a downregulation of beta 1 AR and an increase of Gi alpha have been observed as signal transduction alteration in failing human myocardium. These alterations are due to alterations of gene expression in the failing heart and are related to a defective regulation of force of contraction in heart failure.

Alterations of cardiac alpha- and beta-adrenoceptors and inotropic responsiveness in hypertensive transgenic rats harbouring the mouse renin gene (TGR(mREN2)27)

In the present study, we investigated the alpha- and beta-adrenoceptor-mediated effects on myocardial force of contraction (using phenylephrine in the presence of propranolol and isoprenaline) and receptor densities (binding studies using [3H]-prazosin and [125I]-iodocyanopindolol) in hypertensive transgenic rats (TGR(mREN2)27) and age-matched Sprague-Dawley rats (SP) as controls. In TGR(mREN2)27 the positive inotropic effects of isoprenaline and phenylephrine were reduced, while the effect of Ca2+ was unchanged. The EC50-values did not differ in both groups. A down-regulation of the beta-adrenoceptors was observed in the hypertrophied left ventricles of transgenic rats, which is postulated to be involved in the reduced beta-adrenoceptor-mediated positive inotropic effect. The alpha-adrenoceptor density was increased, which could represent a compensatory mechanism for the impaired effectiveness of the beta-adrenergic pathway. However, since the effect of alpha-adrenoceptor agonist is not enhanced but even reduced, an uncoupling of alpha-adrenoceptors from post receptor events could play a role in the observed effects.

Hypertrophic cardiomyopathy: a desensitized cardiac beta-adrenergic system in the presence of normal plasma catecholamine concentrations

Only few data are available concerning the biochemical and functional state of the beta-adrenergic system in hypertrophied human myocardium. The present study was to investigate the myocardial beta-adrenergic signal transduction system in hypertrophic obstructive cardiomyopathy (HOCM). Thin myocardial strips were prepared from surgically excised, septal myocardium from 7 patients with HOCM and their force of contraction was measured in vitro. The positive inotropic effects of calcium and dihydro-ouabain, both acting independently of beta-adrenoceptors and cAMP, were similar in these preparations to those, previously published, seen with nonfailing myocardium. In contrast, the beta-adrenoceptor agonist isoprenaline and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) had reduced positive inotropic effects. Their EC50-values were about 10 fold higher than the respective EC50-values published for nonfailing myocardium. The positive inotropic potencies of isoprenaline and IBMX were reduced in HOCM by as much as they were in the additionally investigated myocardium from 6 patients with severe mitral regurgitation (MR, NYHA III). In order to clarify whether the functional alterations are related to changes in the beta-adrenoceptors, beta-adrenoceptor density and beta 1: beta 2-adrenoceptor subtype distribution were determined in the same myocardium using 125I-Iodocyanopindolol saturation binding. Myocardial beta-adrenoceptor density was reduced to 68% in HOCM and to 56% in MR compared to nonfailing myocardium controls (NF: 64.8 +/- 6.5 fmol/mg protein). In HOCM, this reduction was due to a selective down regulation of beta 1-adrenoceptors (24.9 +/- 3.7 fmol/mg protein vs NF: 46.4 +/- 6.8 fmol/mg protein, P < 0.05), whereas beta 2-adrenoceptor density was unchanged (19.0 +/- 1.9 fmol/mg protein vs NF: 18.4 +/- 3.3 fmol/mg protein, n.s.). In MR both beta-adrenoceptor subtypes were reduced (beta 1: 26.9 +/- 1.4 fmol/mg protein, beta 2: 9.6 +/- 1.7 fmol/mg protein; both P < 0.05 vs NF). Electrochemically determined plasma catecholamine levels were elevated in MR. However, plasma catecholamine levels were normal or slightly below normal in HOCM. In summary, myocardial beta-adrenoceptors are downregulated and their function is impaired in HOCM. This desensitization is not caused by a negative feedback regulation due to increased plasma catecholamines. The present results show that the desensitizations of the beta-adrenergic system associated with HOCM has characteristics that indicate a major deviation in its development from that of the beta-adrenergic desensitization previously described to occur in congestive heart failure.

Adrenergic Modulation of Potassium Currents in Isolated Human Atrial Myocytes

The adrenergic modulation of inwardly rectifying and depolarization-activated outward potassium currents was studied in single cardiac myocytes obtained from the human atrium. Membrane currents were recorded in enzymatically dissociated cells using the whole-cell voltage-clamp technique. It was observed that, in the presence or absence of atenolol (or 1 &mgr;M propranolol), 30 &mgr;M phenylephrine attenuated inwardly rectifying and depolarization-activated outward potassium currents including both transient and late-activated current. This suppressant effect of phenylephrine could be prevented by pretreatment with an alpha-adrenoceptor antagonist. Isoproterenol (30 &mgr;M) increased the late outward potassium current and net transient outward current. It is concluded that, in human atrial myocytes, alpha-adrenergic activation reduces depolarization-activated transient and late outward potassium current and inwardly rectifying background potassium current. beta-Adrenergic activation resulted in an increase in the depolarization-activated transient and late outward potassium current. Copyright 1994 S. Karger AG, Basel

Spontaneous and renal hypertensive rats: responsiveness of isolated right and left atria to noradrenaline, isoprenaline and methoxamine

The adrenergic responsiveness of right and left atria isolated from spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR) was studied. Right atria isolated from SHR showed subsensitivity to the chronotropic effect of noradrenaline (21.3-fold at the EC50 level, P < 0.05) and isoprenaline (12.0-fold, P < 0.05). However, atria isolated from RHR did not exhibit any significant alteration (P > 0.05) in sensitivity to the chronotropic effect of noradrenaline or isoprenaline. Chronotropic responsiveness to theophylline was not altered in right atria isolated from SHR or RHR. alpha 1-Mediated inotropic responses to noradrenaline and methoxamine were blunted in left atria isolated from SHR. Left atria isolated from RHR showed supersensitivity to the inotropic effect of noradrenaline (5.4-fold at the EC50 level, P < 0.05) and subsensitivity to the inotropic effect of methoxamine (6.0-fold, P < 0.05). It is concluded that the present results could, at least partially, explain the reduced cardiac output observed during established hypertension in SHR and the increased cardiac output observed in the initial phase of renal hypertension in RHR.

Modulation by pertussis toxin of salbutamol- and arecoline-induced effects in the isolated heart and aorta of the rat

The modulatory effects of pertussis toxin pretreatment on responses mediated via beta-adrenoceptors and muscarinic acetylcholine receptors were investigated in isolated rat hearts and aortic rings 4 days after in vivo administration of pertussis toxin. In isolated hearts, pertussis toxin increased heart weight and baseline coronary flow values but did not effect baseline left ventricular pressure values. In unpaced hearts, pertussis toxin inhibited the arecoline-induced cardiac standstill, while in paced hearts, the beta 2-adrenoceptor agonist salbutamol produced a dose-dependent vasodilation with similar characteristics in pertussis toxin and control preparations. Pertussis toxin had no effect on myocardial or aortic cyclic nucleotide levels and the myocardial beta-adrenoceptor density (Bmax) and dissociation constant (Kd). In precontracted aortic rings, pertussis toxin had no effect on the salbutamol or arecoline induced vasorelaxation. In summary, we demonstrated a reduced cholinergic responsiveness in isolated hearts but an intact beta 2-adrenoceptor pathway in isolated hearts as well as in isolated aortic rings after pertussis toxin pretreatment. In aortic rings no change in muscarinic acetylcholine receptor responsiveness occurred.

Beta-adrenoceptors in cardiac disease

The human heart contains both beta 1 and beta 2-adrenoceptors; both mediate positive inotropic and chronotropic effects. In chronic heart failure, beta-adrenoceptor number is reduced, presumably, by down-regulation by endogenous noradrenaline which is elevated due to increased sympathetic activity. Since the human heart contains only a few spare receptors for beta-adrenoceptor-mediated positive inotropic effects and the amount of spare receptors declines in chronic heart failure, it is not surprising that the reduced beta-adrenoceptor number is accompanied by decreased contractile responses to beta-adrenoceptor agonists (including endogenous catecholamines), and the extent of decrease in maximal inotropic response is more pronounced as the disease becomes more advanced. Moreover, in chronic heart failure myocardial G(i)-protein, which inhibits cAMP formation, is increased, which might further contribute to the reduction in beta-adrenoceptor-mediated effects. It appears that, at present, the best therapy for severe heart failure is a successful heart transplant, since in the transplanted heart beta-adrenoceptor number and function seems to be normalized. Moreover, the data currently available do not suggest any development of super- or subsensitivity of postsynaptic cardiac beta-adrenoceptors in the transplanted human heart.

Changes in cardiac beta-adrenoceptors in human heart diseases: relationship to the degree of heart failure and further evidence for etiology-related regulation of beta 1 and beta 2 subtypes

Radioligand binding studies were performed to investigate total beta-adrenoceptor density (Bmax) and beta 1 and beta 2 subtype distribution in left ventricular biopsies obtained from 8 prospective transplant donors serving as controls and from 143 patients with different degrees of heart failure (NYHA class II to IV) undergoing aortic or mitral valve surgery due to aortic or mitral stenosis, aortic or mitral regurgitation, as well as combined aortic or mitral valve lesions (stenosis and regurgitation). In 13 other patients, heart failure was due to hypertrophic obstructive cardiomyopathy (N = 6, NYHA III), tetralogy of Fallot (N = 4, NYHA III), or Becker's muscular dystrophy (N = 3, NYHA IV). Bmax was assessed by (-)-(125I)-iodocyanopindolol used as radioligand. Competition experiments with the highly selective beta 1-adrenoceptor antagonist CGP 20712A were performed for determination of beta 1- and beta 2-adrenoceptor subtypes. In biopsies taken from transplant donors, the Bmax was found to be 70.1 +/- 5.8 fmol/mg protein. In all groups investigated the extent of total beta-adrenoceptor downregulation was related to the degree of heart failure. The decrease in Bmax was found to be about 20% (NYHA II), 45% (NYHA III), and 60% (NYHA IV) when compared with controls. There was no significant difference in the reduction of total beta-adrenoceptor density between isolated aortic or mitral valve diseases and combined valve lesions. Independent of the degree of heart failure, selective downregulation of the beta 1 subtype was found in patients with isolated or combined aortic valve diseases, hypertrophic obstructive cardiomyopathy, and Becker's muscular dystrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

Incomplete reversal of beta-adrenoceptor desensitization in human and guinea-pig cardiomyocytes by cyclic nucleotide phosphodiesterase inhibitors

1. The decreased response to beta-adrenoceptor stimulation seen in heart failure may be related to a defect in cyclic AMP production. The inotropic effects of the selective phosphodiesterase (PDE) III inhibitors, SK&F 94120 and SK&F94836, and the non-selective PDE inhibitor, 3-isobutyl-l-methylxanthine (IBMX), alone and when combined synergistically with isoprenaline, were studied in control and beta-adrenoceptor-desensitized ventricular myocytes. 2. Myocytes isolated from noradrenaline-treated guinea-pigs had a reduced maximum response to isoprenaline compared with control animals (60.0 +/- 2.5%, n = 42 vs 79.5 +/- 1.7% maximum calcium: n = 46, P < 0.001). Together with an approximately 20 fold increase in the isoprenaline EC50, this is indicative of beta-adrenoceptor desensitization as a result with chronic infusion with noradrenaline. 3. The maximum inotropic response of IBMX was depressed following noradrenaline treatment, from 74.9 +/- 4.6% (n = 7) in control, to 61.7 +/- 2.70% (n = 6), as a percentage of maximum calcium in noradrenaline-treated guinea-pig ventricular myocytes (P < 0.02). The pD2 value for IBMX was also reduced (P < 0.02). No significant differences in the inotropic effects of SK&F94120 and SK&F94836 were seen between control and beta-adrenoceptor desensitized myocytes. 4. Threshold inotropic concentrations of SK&F94120 and SK&F94836 caused a five fold decrease in the EC50 of control myocytes for isoprenaline, and an 11 fold decrease in the noradrenaline-treated guinea-pig ventricular myocytes. 5. The maximum response to isoprenaline in myocytes isolated from normal guinea-pigs was unaffected by PDE inhibition; either at threshold or maximum inotropic concentrations, or by CPT cyclic AMP, an analogue of cyclic AMP.6. A significant potentiation of the maximum isoprenaline response by threshold inotropic concentrations was observed with SK&F 94120 (P<0.05), but not with IBMX or SK&F 94836, in myocytes isolated from noradrenaline-treated guinea-pig hearts. This potentiation, however, did not completely restore the response to levels seen in control myocytes.7. The extent of potentiation of the maximum isoprenaline response by maximum inotropic concentrations of either IBMX or CPT cyclic AMP, was no greater than that by threshold concentrations of IBMX, in myocytes isolated from noradrenaline-treated guinea-pig hearts.8. In cardiac myocytes isolated from the explanted hearts of 16 patients with heart failure, threshold concentrations of IBMX and SK&F 94120 decreased the isoprenaline EC50 by a factor of four and six,respectively, but potentiation of the maximum isoprenaline response occurred only with SK&F 94120.The attenuated isoprenaline response was increased from 60.3 +/- 4.5% to 74.3 +/- 4.2% as a % maximum calcium (P<0.05, n = 6), but remained substantially lower than the 116 +/- 7% (P<0.001, n = 6) seen in myocytes isolated from non-failing hearts.9. We conclude that the reduced maximum contraction amplitude with isoprenaline in cardiac myocytes from either patients in end-stage failure, or noradrenaline-treated guinea-pigs, is partly but not solely due to insufficient cyclic AMP levels, since inhibition of cyclic AMP degradation does not result incomplete reversal of the beta-adrenoceptor desensitization.

Comparison of the positive inotropic effects of serotonin, histamine, angiotensin II, endothelin and isoprenaline in the isolated human right atrium

The receptor systems through which serotonin (5-HT), histamine, angiotensin II and endothelin increase the force of contraction were studied in isolated right atria from patients without apparent heart failure. All agonists increased the atrial force of contraction in a concentration-dependent manner; maximal effects, however, were significantly less than those evoked by isoprenaline or Ca2+. 5-HT and histamine, but not angiotensin II and endothelin, activated adenylate cyclase, whereas endothelin and angiotensin II stimulated inositol phosphate generation. Experiments with subtype-selective antagonists revealed that histamine effects were mediated by H2-receptors (sensitive to ranitidine), 5-HT-effects by 5-HT4-receptors (sensitive to SDZ 205-557) and angiotensin II effects by AT1-receptors (sensitive to losartan). We conclude that in human right atria the force of contraction can be increased by cyclic AMP-dependent (histamine, 5-HT) and -independent (angiotensin II, endothelin) pathways. Compared to beta-adrenoceptors, however, all other receptor systems increase the force of contraction only submaximally indicating that the beta-adrenoceptor pathway is the most important physiological mechanism to regulate force of contraction and/or heart rate in the human heart.

Down-regulated beta-adrenoceptors in severely failing human ventricles: uniform regional distribution, but no increased internalization

In chronic heart failure cardiac beta-adrenoceptors are decreased. In this study we investigated whether a) in severely failing human ventricles beta-adrenoceptors are uniformly decreased or regional variations exist, and b) the beta-adrenoceptor decrease is caused by increased internalization or is a real loss in beta-adrenoceptors. For this purpose we assessed beta-adrenoceptor number and subtype distribution in a particulate fraction (mainly sarcolemmal plasma membranes) and a light vesicle fraction of right and left ventricular segments (obtained by cutting transversal rings of 2 cm from the midventricular regions) of explanted hearts from 2 patients with end-stage congestive dilated cardiomyopathy (DCM) and one patient with end-stage ischemic cardiomyopathy (ICM). In all three hearts ventricular beta-adrenoceptor number was very low (7.5-10 and 21-26 fmol/mg protein in DCM, 15-22 fmol/mg protein in ICM compared to 68-74 fmol/mg protein in non-failing ventricles). beta-Adrenoceptors were uniformly decreased over the whole ventricular region and no considerable regional variations existed. The same held true for beta 1- and beta 2-adrenoceptors. In ICM decrease in beta-adrenoceptors was due to a concomitant reduction in beta 1- and beta 2-adrenoceptors, in DCM it was mainly caused by beta 1-adrenoceptor down-regulation. In all ventricular segments investigated light vesicle beta-adrenoceptors amounted to about 5-7% of total ventricular beta-adrenoceptors, and this was not significantly different from non-failing left ventricles. We conclude that a) in severely failing human ventricles beta-adrenoceptors are evenly down-regulated and no regional variations exist, and b) the decrease in beta-adrenoceptors is not due to enhanced internalization but is a real loss of beta-adrenoceptors.

Differential response of hypertrophied rat hearts to various alpha 1-adrenoceptor agonists

With respect to the heart, the prolonged existence of hypertension, both in man and in experimental animals is predominantly characterized by an increase in left ventricular myocardial mass. In this process, the autonomic nervous system plays an important role. Although endogenous catecholamine stimulation of the heart is mainly exerted via the beta-adrenoceptors, in several mammalian species, the stimulation of cardiac alpha 1-adrenoceptors also mediates positive inotropic actions. We investigated the functional responses of isolated hypertrophied hearts taken from spontaneously hypertensive rats (SHR) and rats with an induced aortic stenosis (ASR) to various alpha 1-adrenoceptor agonists and compared them with those from age matched Wistar Kyoto (WKY) and "sham" operated controls. Accordingly, we studied the functional response to: methoxamine (alpha 1), cirazoline (alpha 1) and phenylephrine (alpha 1 > beta 1). The inotropic response to the alpha 1-adrenoceptor agonists cirazoline and methoxamine proved to be significantly weaker in hypertrophied hearts from SHR and ASR than in non-hypertrophied hearts from WHY and "sham" operated controls (p < 0.05). The inotropic response to phenylephrine remained intact in hypertrophied myocardial tissue. However, it was significantly reduced when the hearts were pre-treated with the intracellular Ca(2+)-antagonists ryanodine and TMB-8. These findings show that the mechanism of sarcolemmal Ca2+ release, activated by phenylephrine, is still intact in the hypertrophied myocardial cell. In conclusion, these data show that cardiac hypertrophy, be it of genetical or mechanical origin, leads to a reduced response of the isolated heart to alpha 1-adrenoceptor stimulation.