Adrenergic Nervous System in Heart Failure

Introduction

Heart failure is a clinical syndrome that occurs over time after a cardiac injury or chronic stress causing a decline in contractile function of this pumping efficiency of the heart. It is marked by a dynamic interplay between underlying myocyte dysfunction and compensatory mechanisms that are neurohumoral in nature as these signals attempt to stimulate the pump to maintain cardiac output. Among these neurohormonal mechanisms, elevated activities of the adrenergic (or sympathetic) nervous system (ANS), of the renin–angiotensin–aldosterone system (RAAS), and of several cytokines play central roles.^1,2 These systems are initially activated to compensate for the reduced cardiac function and to maintain cardiac output. However, upon long-term presence of the initial insult to the heart muscle, cardiac function ultimately succumbs to their deleterious effects on myocytes and other cells of the heart, leading to cardiac decompensation and progressively worsening function and the inability to sustain daily life activities. The present review discusses the role of the ANS in HF pathophysiology and therapeutics, starting with a discussion of its role in normal cardiac function.

ANS and Cardiac Function

The ANS exerts a plethora of cardiovascular effects, including heart rate increase (positive chronotropy, predisposing to arrhythmias), increase in cardiac contractility (positive inotropy), accelerated cardiac relaxation (positive lusitropy), decrease in venous capacitance, and constriction of resistance and cutaneous arteries (Figure 1). All of these effects aim to increase cardiac performance to prepare for and enable the body’s so-called fight-or-flight response. Conversely, the mirror branch of the autonomic nervous system, the parasympathetic (cholinergic) nervous system, slows the heart rate (bradycardia) through vagal nerve impulses, with minimal or no effect on cardiac contractility. This is because the cardiac ventricles, whose contraction is responsible for pumping the blood into the systemic and pulmonary circulations, receive almost exclusively adrenergic fiber innervations, whereas the cholinergic fibers follow the vagus nerve under the endocardium, primarily reaching the atria with limited ventricular connections.^3,4 Therefore, whereas heart rate can be controlled (in opposing fashion) by both autonomic branches, cardiac contraction/relaxation is controlled almost solely by the ANS (Figure 1).

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The ventricular ANS innervation presents as gradient pattern from base to apex.⁵ Afferent, efferent, and interconnecting neurons comprise the neuronal system of the heart.6 Several reflex responses regulate ANS outflow to the heart and to the peripheral circulation, such as the carotid baroreceptors (mediating ANS inhibition), cardiorespiratory baroreceptors (eg, the Bezold–Jarisch reflex, also mediating ANS inhibition), and peripheral chemoreceptors (mediating ANS activation).^4,7

ANS activation in the cardiovascular system translates into release of the 2 catecholamines that mediate its effects, that is, norepinephrine (or noradrenaline) and epinephrine (or adrenaline), and this can occur via the following mechanisms (Figure 2): (1) norepinephrine released by cardiac sympathetic nerve terminals located in the sinus and atrioventricular nodes (resulting in tachycardia and acceleration of atrioventricular conduction) and in the left ventricle (resulting in an increased force of contraction); (2) epinephrine (and to a much lesser extent norepinephrine) released into the circulation by the adrenal medulla, modulating both myocardium and peripheral vessels; and (3) local release of norepinephrine and epinephrine by various peripheral ANS’s that can synthesize and release these catecholamines in an autocrine/paracrine manner and are located in blood vessels and in cardiac myocytes themselves.^8,9

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Adrenergic Receptors in the Cardiovascular System

Cardiac Adrenergic Receptor Signaling and Regulation

The ANS neurotransmitters norepinephrine and epinephrine mediate their effects in cells and tissues by binding to specific cell surface adrenergic receptors (ARs), which belong to the superfamily of G-protein-coupled receptors (GPCRs) or 7 transmembrane-spanning receptors or heptahelical receptors. Approximately 80% of norepinephrine put forth by ANS nerve terminals is returned by the norepinephrine transporter type 1, whereas the remainder spills over to become circulatory in nature.¹⁰ The receptors for both ANS catecholamines are divided into 3 types and 9 total different subtypes, as follows: 3 α₁AR subtypes (α_1A, α_1B, α_1D); 3 α₂AR subtypes (α_2A, α_2B, α_2C); and 3 βAR subtypes (β₁, β₂, β₃).¹¹ All ARs primarily signal through heterotrimeric G proteins. The human heart contains all 3 βAR subtypes.¹² β₁AR is the predominant subtype in the (normal, healthy) myocardium, representing 75% to 80% of total βAR density, followed by β₂AR, which comprises ≈15% to 18% of total cardiomyocyte βARs, and the remaining 2% to 3% is β₃ARs (under normal conditions).¹³ The principal role of βARs in the heart is the regulation of cardiac rate and contractility in response to norepinephrine and epinephrine. Stimulation of β₁ARs (mainly) and stimulation of β₂ARs (to a lesser extent) increases cardiac contractility (inotropy), frequency (chronotropy), rate of relaxation (lusitropy), acceleration of impulse conduction through the atrioventricular node (dromotropy) as well as increased pacemaker activity from the sinoatrial node.14 β3ARs are largely silent during normal cardiac physiology;15 however, when they are activated they can produce negative inotropy, which opposes effects from activated β1ARs and β2ARs, and this involves the nitric oxide synthase pathway.16 Thus, the β3AR can act as a “fuse” against cardiac adrenergic overstimulation.17 Agonist-occupied and activated βARs generally cause the exchange of guanosine triphosphate for GDP on the G_α subunit of heterotrimeric G proteins, resulting in the dissociation of the heterotrimer into active G_α and free G_βγ subunits (always associated together, a heterodimer that functions essentially as a monomer), which can transduce intracellular signals independently of each other.¹⁸ The most powerful physiological mechanism to increase cardiac performance is activation of cardiomyocyte β₁ARs and β₂ARs, which, in turn, activate G_s proteins (stimulatory G proteins). G_s-protein signaling stimulates the effector adenylate cyclase, which converts ATP to adenosine 3′,5′-monophosphate or cAMP, which as a second messenger causes the activation of the cAMP-dependent protein kinase (protein kinase A [PKA]). PKA is the major effector of cAMP (there is also Epac, exchange protein directly activated by cAMP, which can be activated by cAMP independently of PKA), and by phosphorylating a variety of substrates, it ultimately results in significant increase in free intracellular Ca²⁺ concentration, which is the master regulator of cardiac muscle contraction (Figure 3).¹⁹ Among the main targets of PKA phosphorylation in the cardiac myocyte are the following: the cell membrane–located L-type calcium channels and the sarcoplasmic reticulum–located ryanodine receptors, and these 2 molecules both lead to increased cellular Ca2+;¹⁹ phospholamban, a negative modulator of sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase whose phosphorylation by PKA deinhibits sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase, thus accelerating Ca²⁺ reuptake by the sarcoplasmic reticulum after contraction and increasing sarcoplasmic reticulum Ca²⁺ stores available for the next contraction;¹⁹ hyperpolarization-activated cyclic nucleotide-gated channels, which generate the hyperpolarization-activated cation inward current (If), that can alter and modulate cardiac pacemaker cells;²⁰ troponin I and myosin binding protein-C, which regulate myofilament Ca2+ sensitivity and relaxation;²¹ and phospholemman, closely associated with Na^+/K⁺-ATPase and inhibiting its function, whose phosphorylation by PKA relieves this inhibition and stimulates the sodium pump, thereby accelerating cardiac muscle repolarization and relaxation (Figure 3).²² Moreover, PKA can phosphorylate the βARs themselves (and other GPCRs) in the heart, causing functional desensitization and uncoupling of the receptor, which is known as heterologous or agonist-independent desensitization.²³

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Of note, β₂AR also mediates the effects of catecholamines in the heart, but in a qualitatively different manner from β₁AR, because it can also couple to the adenylate cyclase inhibitory G-protein (G_i). In fact, this switching of β₂AR signaling from G_s to G_i proteins is postulated to be induced by the phosphorylation of the β₂AR by PKA.²³ Nonetheless, it is now generally accepted that, in the heart, β₂AR signals and functions in a substantially different way compared with β₁AR.^24–26 Importantly, whereas β₁AR activation enhances cardiomyocyte apoptosis, β₂AR exerts antiapoptotic effects in the heart.^24–27 This essential difference between the 2 receptor subtypes is ascribed to the signal of β₂AR through G_i proteins.²⁵ Studies using transgenic mice, β₂AR-selective stimulation, and adenoviral-mediated β₂AR overexpression have demonstrated the protective effects of β₂AR signaling in the myocardium, including improved cardiac function and decreased apoptosis. Conversely, hyperstimulation or overexpression of β₁AR has detrimental effects in the heart.^27,28

Both α₂ARs and βARs, like the majority of GPCRs, are subject to agonist-promoted (homologous) desensitization and downregulation, a regulatory process that diminishes receptor response to continuous or repeated agonist stimulation.^29,30 At the molecular level, this process is initiated by receptor phosphorylation by a family of kinases, termed GPCR kinases (GRKs), followed by binding of β-arrestins (β-arrs) to the GRK-phosphorylated receptor. The β-arrs then uncouple the receptor from its cognate G proteins, sterically hinder its further binding to them (functional desensitization), and subsequently target the receptor for internalization.^29,30 Across all mammalian species, GRK2 and GRK5 are the most physiologically important members of the GRK family because they are expressed ubiquitously and regulate the majority of GPCRs. They are particularly abundant in neuronal tissues and in the heart.^31,32

Importantly, the differences between the 2 predominant cardiac βARs, that is, β₁AR and β₂AR, in terms of their signaling properties might take quite a different shape and have a much larger bearing on pathophysiologic implications in the setting of human HF; for instance, and as discussed in more detail in subsequent sections, β₁AR is selectively downregulated (ie, functional receptor number reduced) in human HF, thus shifting the aforementioned stoichiometry of β₁AR:β₂AR toward 50:50 in the failing heart from ≈75%:20% in the normal, healthy heart.^33,34 However, β₂AR is also nonfunctional and does not signal properly in the failing heart.^32–34 In addition, emerging evidence suggests that β₂AR signaling in the failing heart is quite different from that in the normal heart (ie, is more diffuse and noncompartmentalized and resembles the proapoptotic diffuse cAMP signaling pattern of the β₁AR).³⁵ Therefore, this stoichiometric shift in favor of the supposedly good β₂AR in HF seems unable to help the heart improve its structure and function.

The human heart also expresses α_1AARs and α_1BARs, albeit at much lower levels than βARs (≈20% of total βARs).³⁶ How important a role cardiac α₁ARs play in cardiac physiology is still a matter of debate. In contrast, their role in regulation of blood flow by inducing constriction in the smooth muscle wall of major arteries (eg, aorta, pulmonary arteries, mesenteric vessels, coronary arteries) is well-known and indisputable.³⁷ The α₁ARs couple to the G_q/11 family of heterotrimeric G proteins, thereby activating phospholipase C-β. Phospholipase C-β generates the second messengers, inositol [1,4,5]-trisphosphate (IP₃) and 2-diacylglycerol (DAG), from the cell membrane component, phospholipid phosphatidylinositol (4,5)-bisphosphate. IP₃ binds specific receptors in the sarcoplasmic reticulum membrane, which causes release of Ca²⁺ from intracellular stores, whereas DAG activates PKC and transient receptor potential channels (Figure 3). The end result is again raised intracellular [Ca²⁺], which leads to contraction (vasoconstriction; Figure 3).

Finally, regarding α₂AR subtypes, α_2BARs are known to be present in vascular smooth muscle, causing constriction of certain vascular beds, whereas centrally located α_2AARs can inhibit sympathetic outflow (presynaptic inhibitory autoreceptors) and thus lower systemic blood pressure.^38,39 The release of norepinephrine from cardiac sympathetic nerve terminals is controlled by both presynaptic α_2AARs and α_2CARs,⁴⁰ and genetic deletion of both of these α₂AR subtypes leads to cardiac hypertrophy and HF because of chronically enhanced cardiac norepinephrine release, as well as enhanced norepinephrine and epinephrine secretion from the adrenal medulla.^8,41,42

AR Polymorphisms and Cardiac Function

There are some important genetic polymorphisms in human βAR and αAR genes, which have been associated with HF phenotypes and interaction with β-blocker therapy, a mainstay of HF standard of care, and can significantly influence cardiac function. Thus, it would be useful to briefly discuss these polymorphisms and their functional consequences for the heart. The Arg389Gly human β₁AR gene polymorphism is probably the best-studied AR gene polymorphism to date; it is associated with significantly elevated adenylate cyclase/PKA activities and, hence, β₁AR-stimulated cardiac contractility in the Arg389 variant carriers (compared with Gly389 carriers).⁴³ Another human β₁AR polymorphism is the Ser49Gly variation, which is associated with increased agonist-promoted receptor downregulation for the Gly49 over the Ser49 variant.⁴⁴ A lower prevalence of ventricular arrhythmias has been associated with the Gly389 allele.⁴⁵ Of note, African American HF patients with an enhanced activity mutation (Leu41) of GRK5, the second (after GRK2) major cardiac GRK, demonstrate improved survival (explained by this GRK5 mutant acting as a genetic β-blocker).⁴⁶ Additionally, the Gly49 allele of the human β₁AR gene has been shown to confer enhanced survival benefit in response to β-blocker treatment⁴⁷ and significant reduction in left ventricular end-diastolic diameter⁴⁸ compared with Ser49 homozygous carriers. Ameliorated cardiac adverse remodeling has been reported for Arg389 homozygous HF patients treated with the β-blocker carvedilol⁴⁹ and for Arg389Gly heterozygous carriers.⁵⁰ There are also negative studies, however, reporting no associations of these polymorphisms with any cardiac outcome.^51–53 Further adding to the confusion, a dependency of whether the Ser49Gly polymorphism has any bearing on dilated cardiomyopathy outcomes on the dose of the β-blocker has been reported, according to which the Gly49 allele associates with worse outcomes than Ser49 allele homozygosity at low β-blocker doses; however, at higher doses, genotype apparently has no effect.⁵⁴ On the other hand, HF patients carrying the Arg389 genotype had a larger agonist-dependent ventricular contractility and better survival (adjusted for age, sex, and race) than Gly389 carriers in a β-Blocker Evaluation of Survival Trial subcohort.⁵⁵

The human β₂AR gene is known to display 2 variations that are in linkage disequilibrium (and thus constitute a haplotype), Gly16Arg and Gln27Glu, which affect receptor downregulation, and a third polymorphism, Thr164Ile, which confers impaired receptor–G-protein coupling and reduced adenylate cyclase–mediated signaling.⁵⁶ Finally, a 4-amino-acid deletion in the human α_2CAR gene (Del322–325) leads to increased norepinephrine release from ANS cardiac nerve terminals,⁵⁷ and this polymorphism, in combination with the β₁AR Arg389Gly polymorphism, has been used for patient stratification based on the clinical response to the β-blocker bucindolol into “very favorable,” “favorable,” and “unfavorable” response genotypes.⁵⁸ The latter polymorphism (β₁AR Arg389Gly) also belongs to a set of 3 genetic polymorphisms that were recently shown to serve as predictors of appropriate implantable cardioverter-defibrillator shock therapies in HF patients.⁵⁹ In conclusion, human AR genetic polymorphisms may prove to be useful tools in guiding the individual tailoring (personalization) of HF therapy in the future.^60–62

Assessment of ANS Activity

Plasma norepinephrine measurements represent the usual crude method to assess ANS function/activity levels, because the latter depends on the rate of immediate norepinephrine reuptake as well as norepinephrine clearance from the circulation.⁶³ The two standard techniques to assess human ANS activity are radiotracer measurements of regional norepinephrine spillover and microneurography (microelectrode measurements of postganglionic nerve activity). These techniques can discern central from peripheral contributions of increased plasma norepinephrine levels and facilitate precise assessment of the regional (eg, cardiac) sympathetic nerve function, both under physiological and pathological conditions. Cardiac neuronal function is visualized with γ-cameras and positron emission tomography scanners using radiolabeled norepinephrine analogs.⁶⁴ ANS activity or its pharmacological inhibition can also be measured with [¹²³I]-labeled metaiodobenzylguanidine (MIBG), an analogue of norepinephrine.⁶⁵ [¹²³I]-labeled MIBG cardiac imaging has also been shown to carry independent prognostic information for risk stratification of HF patients in a complementary manner to ejection fraction and B-type natriuretic peptide.⁶⁶

Regulation of ANS Outflow and Activity in Health and in Chronic HF

Cardiac Sympathetic Efferent Nerves

There are several mechanisms by which the ANS controls cardiac function. The first one to be documented historically is through the aortic arch and carotid sinus (high pressure) and cardiopulmonary (low pressure) baroreceptor reflexes.⁶⁷ Aside from these baroreceptor inputs, additional factors that act within the central nervous system play a role in regulation of cardiac ANS activity. In particular, suprabulbar subcortical monoaminergic neurons and brain stem angiotensin II have attracted interest for their ability to regulate ANS outflow in HF (Figure 2). Norepinephrine turnover in subcortical regions in HF is significantly higher than that in the cortex and higher than that in healthy subjects.⁶⁸ Moreover, the rate of subcortical norepinephrine release correlates well with global ANS activity, as measured by total body norepinephrine plasma spillover. Angiotensin II–dependent ANS activation plays an important role in adverse hemodynamic and left ventricular remodeling responses to myocardial infarction, possibly through superoxide formation.^69,70 Thus, part of the benefit of RAAS modulators in HF might derive from centrally mediated suppression of ANS activity.

As the heart becomes progressively unresponsive to the stimulatory effects of catecholamines, chronic stimulation of cardiac ANS nerve terminals leads to chronically elevated norepinephrine release from the heart (increased norepinephrine spillover). Presynaptic α₂ARs present on cardiac ANS nerve terminals and acting as norepinephrine release–inhibiting autoreceptors play a crucial role in regulation of cardiac norepinephrine release from sympathetic nerves.^38,39 Knockout mice lacking either the α_2AAR or the α_2CAR subtype show significantly enhanced cardiac ANS activity and circulating catecholamine levels, as well as significantly worse heart function and clinical indices, during the course of surgical pressure overload by means of transverse aortic constriction–induced HF compared with age-matched wild-type HF mice.^40,41 Moreover, double α_2A/α_2CAR knockout mice exhibit even worse cardiac phenotypes than single α_2AAR knockout mice; by 4 months of age, they spontaneously develop cardiomyopathy (without stress or any specific insult).⁷¹ In HF patients, the expected inhibitory effects of α₂AR stimulation on norepinephrine spillover are markedly blunted, thereby contributing to the increase in cardiac norepinephrine spillover observed in chronic HF.⁷² In addition, the human α_2CAR Del322–325 variant that displays impaired signaling and sympathoinhibitory function is alone associated with increased risk of HF in homozygous African American carriers, especially when cocarried with the hyperfunctional cardiac β₁AR Arg389 variant, with the most probable mechanism being attenuated autoinhibitory feedback of, and thus enhanced norepinephrine release from, the cardiac sympathetic nerves.⁵⁷ In fact, even in healthy humans, the α_2CAR Del322–325 variant is associated with increased sympathetic nervous and adrenomedullary hormonal activities during both supine rest and pharmacologically evoked catecholamine release.⁷³ Thus, presynaptic inhibitory α₂-adrenergic autoreceptors crucially regulate ANS cardiac nerve activity and norepinephrine release into the heart, and any dysfunction of these receptors attributable to either genetic polymorphisms or enhanced desensitization/downregulation translate into increased morbidity and mortality in chronic HF (Figure 2). Perhaps the crucial role of presynaptic α₂ARs in regulating norepinephrine release from cardiac ANS nerves stems from the fact that they are the only presynaptic ARs that can inhibit norepinephrine release; presynaptic βARs (of the β₂AR subtype, mainly) are facilitatory autoreceptors enhancing norepinephrine release at sympathetic nerve terminals,⁷⁴ a phenomenon whose inhibition may contribute to the therapeutic benefit of β-blockers in HF (Figure 2).

Adrenal Glands

Circulating epinephrine and norepinephrine derive from 2 major sources in the body: the cardiac sympathetic nerve endings, which release norepinephrine directly onto the cardiac muscle, and the adrenal medulla, whose chromaffin cells synthesize, store, and release epinephrine (mainly) and norepinephrine on acetylcholine stimulation of the nicotinic cholinergic receptors present on their cell membranes (Figure 2).⁸ Epinephrine represents ≈80% of the total adrenal catecholamine secretion under normal conditions, with norepinephrine representing the other ≈20%.⁷⁵ However, these percentages vary widely depending on the physiological condition of the adrenal gland and of the whole body. Thus, all of the epinephrine in the body and a significant amount of circulating norepinephrine derive from the adrenal medulla, and the total amount of catecholamines presented to cardiac ARs at any given time is composed of these circulating norepinephrine and epinephrine plus norepinephrine released locally from sympathetic nerve terminals within the heart.⁸ The secretion of catecholamines from the adrenal glands is regulated in a complex manner by a variety of cell membrane receptors present in chromaffin cells. Many of these receptors are GPCRs, including, similarly to cardiac ANS nerve endings, α₂ARs that inhibit secretion (inhibitory presynaptic autoreceptors) and βARs that enhance it (facilitatory presynaptic autoreceptors; Figure 2).^8,38,41,42 Of note, although various presynaptic autoreceptors and heteroreceptors facilitate (increase) adrenal catecholamine secretion, for example, βARs, muscarinic cholinergic receptors, angiotensin II-ergic, histaminergic, and adrenomedullin receptors, the α₂ARs are the only receptor type reported to date to inhibit adrenal catecholamine secretion.^8,42,76

An increase in GRK2 expression and activity has been documented in several cardiovascular diseases, including increased cardiac expression in HF^77–79 and increased expression in some vascular beds in hypertension.⁸⁰ A few years ago, we reported that GRK2 expression and activity are increased also in the adrenal medulla during HF.⁸¹ Specifically, our studies over the past few years have established that adrenal GRK2 upregulation is responsible for severe adrenal α₂AR dysfunction in chronic HF, which causes a loss of the sympathoinhibitory function of these receptors in the adrenal gland, and catecholamine secretion is thus chronically elevated (Figure 2).^81–85 This emerging crucial role for adrenal GRK2 in HF is underlined by the fact that its specific inhibition, via adenoviral-mediated βAR kinase carboxyl terminal adrenal gene delivery, leads to a significant reduction in circulating catecholamine levels, restoring not only adrenal but also cardiac function in HF.⁸¹ Additional evidence for the crucial role of adrenal GRK2–regulated α₂ARs in regulating adrenal ANS tone in HF comes from the phenylethanolamine-N-methyl transferase–driven GRK2 knockout mice.⁸² These mice, which do not express GRK2 in their adrenal medullae from birth, display decreased ANS outflow and circulating catecholamines in response to myocardial infarction, which translates into preserved cardiac function and morphology over the course of the ensuing HF.⁸² Of note, elevated GRK2–dependent α₂AR dysfunction during HF might also occur in other peripheral sympathetic nerve terminals of the heart (Figure 2) and of other organs, thus contributing to the increased norepinephrine release and spillover, as well as to the presynaptic α₂AR dysfunction in ANS neurons observed in chronic HF.^72,86

Effects of ANS Overactivity in Chronic HF

Myocardial systolic dysfunction is associated with neurohormonal hyperactivity as a compensatory mechanism to maintain cardiac output in the face of declining cardiac function. The neuronal part of this response is represented by ANS cardiac nerve terminals, whereas the hormonal (or humoral) part is represented by increased secretion and elevated circulating levels of certain hormones, the most prominent being epinephrine and norepinephrine, along with the RAAS hormones (ie, angiotensin II and aldosterone).⁸⁷ ANS hyperactivity is evidenced by increased plasma norepinephrine and epinephrine levels, elevated (central) sympathetic outflow, and heightened NE spillover.⁸⁸ Cardiac norepinephrine spillover in untreated HF patients can reach ≤50-fold higher levels than those of healthy individuals under maximal exercise conditions.⁸⁹ The information on chronic ANS activation in HF with preserved left ventricular ejection fraction (ie, diastolic HF) is very limited but certainly can participate in increased HF risk.⁹⁰ In systolic HF, patients may actually have decreased ANS neuronal density and function, resulting in decreased norepinephrine concentration within the heart, in addition to decreased postsynaptic βAR density, due to depletion of cardiac ANS neuronal norepinephrine stores and decreased norepinephrine presynaptic reuptake secondary to norepinephrine transporter downregulation.^91,92

Effects on Cardiac ARs

The elevated ANS outflow and norepinephrine and epinephrine levels in chronic HF lead to chronically elevated stimulation of the cardiac βAR system, which has detrimental repercussions for the failing heart. Extensive investigations over the past 3 decades have helped delineate the molecular alterations afflicting the cardiac βAR system that occur during HF, and it is now well-known that, in chronic human HF, cardiomyocyte βAR signaling and function are significantly deranged and the adrenergic reserve of the heart is diminished.^33,34,93 Cardiac βAR dysfunction in human HF is characterized at the molecular level by selective reduction of β₁AR density at the plasma membrane (downregulation) and by uncoupling of the remaining membrane β₁ARs and β₂ARs from G proteins (functional desensitization).³³ Importantly, myocardial levels and activities of the most important, versatile, and ubiquitous GRKs, GRK2 and GRK5, are elevated both in humans and in animal models of HF.^{32,77–79,94} The current consensus is that in chronic HF the excessive amount of ANS-derived catecholamines hitting cardiac βARs extracellularly triggers the GRK2 upregulation inside the cardiomyocytes, thus leading to a reduction in cardiac βAR density and responsiveness and resulting in cardiac inotropic reserve depletion.^95,96 This GRK2 elevation possibly serves as a homeostatic protective mechanism aimed at defending the heart against excessive catecholaminergic toxicity. However, several studies soon refuted this assumption, demonstrating that GRK2 upregulation is detrimental for the heart and causes the functional uncoupling of βARs in vivo.⁹⁴ This finding prompted investigations of the role GRK2 plays in cardiac function, which revealed that cardiac GRK2 is an absolutely critical regulator of cardiac βAR–dependent contractility and function. Specifically, cardiomyocyte-restricted overexpression of GRK2 to the same level of upregulation found in human HF (ie, 3-fold to 4-fold) markedly attenuated βAR signaling and contractile reserve, showing that GRK2 is the main culprit for the functional desensitization of cardiac βARs in HF (Figure 3).⁹⁷ The proof for this was provided by studies of the in vivo inhibition of cardiomyocyte GRK2, which were enabled by the development of the βAR kinase carboxyl terminal mini-gene, which blocks cell membrane translocation and hence activation of GRK2, and its cardiomyocyte-specific expression in vivo in transgenic mice by virtue of the α-myosin heavy chain gene promoter.⁹⁷ GRK2 inhibition in vivo in the heart with βAR kinase carboxyl terminal (or its partial genetic deletion) enhances cardiac contractility both at baseline and after adrenergic stimulation, reverses the contractile and βAR dysfunctions, and preserves or even augments cardiac function and survival in HF.^97–102 On the down side, loss of cardiac GRK2 can predispose the heart to catecholamine toxicity, exactly because it works, in essence, as a positive inotropic therapy.¹⁰³ In summary, elevated ANS activity in chronic HF causes enhanced GRK2–mediated cardiac β₁AR and β₂AR desensitization and β₁AR downregulation, which leads to the progressive loss of the adrenergic and inotropic reserves of the heart, the hallmark molecular abnormality of this disease (Figure 3).¹⁰⁴

With regard to the other major AR type expressed in the heart, α₁ARs in HF may function compensatively to preserve cardiac contractility, but their involvement in cardiac pathophysiology appears limited to situations of cardiac hypertrophy that ultimately lead to HF.¹⁰⁵ For instance, in the presence of pressure overload, cardiac α_1AARs get activated and promote cardiomyocyte survival (ie, block apoptosis), protecting against adverse remodeling and decompensation to HF.^106,107

ANS Cardiotoxicity

ANS cardiac toxicity is well-documented. For instance, intravenous infusion of isoproterenol (a standard nonsubtype selective βAR full agonist) or norepinephrine results in acute contraction band lesions attributed to relative hypoxia, calcium overload, elevation of cAMP, activation of α- and βARs, and formation of reactive oxygen species.^108,109 Chronic catecholaminergic stimulation has been shown in animals to cause cardiac fibrosis, to reduce adrenergic and inotropic reserves, and to induce cardiac apoptosis and dysfunction via left ventricular dilatation.^110,111 Moreover, norepinephrine induces cardiac apoptosis via both β₁AR-mediated and reactive oxygen species/tumor necrosis factor/caspase–mediated signaling pathways.^112,113 A perfect example of catecholamine-induced cardiac toxicity leading to cardiac dysfunction is found in stress (Takotsubo) cardiomyopathy; high circulating levels of epinephrine trigger a form of myocardial stunning, which includes the signaling switch of the cardiac β₂AR from G_s to G_i proteins, especially in the region of the apical myocardium, where βAR density is highest,¹¹⁴ thus negatively affecting inotropy.¹¹⁵

ANS Therapeutics in HF

AR Blockers

β-Blockers

β-blockers (Table) comprise first generation agents (nonsubtype selective, competitive blockers of both the β1- and β2ARs, eg, propranolol); second generation agents (much higher affinity for the β1- than for the β2AR, eg, atenolol, metoprolol); and third generation agents (subtype-selective, eg, celiprolol, nebivolol, or nonselective, eg, bucindolol, carvedilol, labetalol). The latter ones can also block α₁ARs, thereby causing peripheral vasodilation (bucindolol, carvedilol, labetalol). Celiprolol possesses β₂AR agonist properties, whereas nebivolol can also induce nitric oxide synthesis.¹¹⁶ Cardioselectivity, that is, selectivity for the β₁AR over the β₂AR subtype, is dose-dependent and decreases with increasing dosage. Both subtype-selective and subtype-nonselective agents have negative chronotropic and inotropic effects. β₁AR-selective agents have a lesser inhibitory effect on the β₂AR and thus are less likely to cause peripheral vasoconstriction (and bronchoconstriction).¹¹⁶ β1AR-selective agents carry less risk for limiting exercise performance, since they spare the β2AR which increases skeletal muscle blood flow (via vasodilation) in response to exercise. Finally, some β-blockers are mixed βAR agonists/antagonists (or partial agonists); in other words, at low concentrations they antagonize the receptors, but at high concentrations they actually activate βARs (act as agonists), causing cardiac stimulation. These β-blockers possess (the so-called) intrinsic sympathomimetic activity, for example, pindolol, alprenolol, and oxprenolol, and inhibit the effects of catecholamines through the high-affinity binding state of the myocardial β₁AR while mimicking catecholamines when binding to the low-affinity state of the cardiomyocyte β₁AR.¹¹⁷ The β-blockers with intrinsic sympathomimetic activity have a high propensity for arrhythmias and should not be used for chronic HF treatment. The majority of β-blockers are partially or completely metabolized by CYP2D6, a gene with considerable genetic variability.¹¹⁸ All β-blockers are approved for chronic HF treatment (Table).¹¹⁹ Conversely, they are all contraindicated in acute HF (because of the acute decrease in cardiac output they cause).¹¹⁹ Chronic β-blocker therapy reverses cardiac adverse remodeling (reverse remodeling), reduces hospitalization rates, improves patient survival, reduces risk of arrhythmias (sudden cardiac death), improves coronary blood flow to the heart (relieves angina), and protects the heart against cardiotoxic overstimulation by the catecholamines. All of these effects result in a decrease in the oxygen/energy and metabolic demands of the heart (cardiac workload is decreased) and in an increase in its oxygen/energy supply, thereby improving, in the long-term, left ventricular function and performance. Various molecular mechanisms underlying these effects have been postulated as follows: (1) direct antagonism of catecholaminergic cardiotoxic effects; (2) cardiac βAR upregulation and restoration of their signaling and function, that is, increase in adrenergic and inotropic reserves of the heart, partly via cardiac GRK2 downregulation;¹²⁰ (3) suppression of the elevated cardiotoxic, adverse remodeling–promoting, and proapoptotic neurohormonal systems (RAAS, endothelin); (4) coronary blood flow enhancement (as a result of diastolic prolongation); and (5) restoration of the reflex controls on the heart and the circulation.¹²¹ In addition, restoration of adrenal GRK2–α₂AR–catecholamine secretion axis and suppression of norepinephrine release from cardiac ANS endings might contribute to the beneficial effects of β-blockers in chronic HF, as well.^74,85

α-Blockers

In human HF, the α1-blocker prazosin led to worse outcomes than vasodilators combination of hydralazine and isosorbide dinitrate (BiDil), possibly due to the fact that prazosin causes feedback sympathetic activation, which may diminish any potential benefit reaped from vascular smooth muscle α₁AR inhibition-induced vasodilation (Table).^122,123 Adding to the inappropriateness of α₁AR blockers for HF therapy, the doxazosin arm in the antihypertensive and lipid-lowering treatment to prevent heart attack trial (ALLHAT) was terminated early because of higher rate of HF-related events (Table).¹²⁴

Centrally Acting Sympatholytic Agents

Central nervous system-localized α²ARs inhibit ANS outflow via an autocrine negative feedback mechanism.³⁹ Clonidine is a centrally acting α²AR agonist that can attenuate cardiac and renal sympathetic tone in HF (sympatholytic). It exerts its sympathoinhibitory effects without apparent negative clinical outcomes (Table).¹²⁵ However, clinical trials are needed to fully evaluate whether it can have a place in the HF therapeutic armamentarium. Another centrally acting sympatholytic agent, moxonidine, also an imidazoline derivative like clonidine, has been used in clinical trials for HF. Moxonidine is an α₂AR agonist as well as an agonist at the putative imidazoline receptors.^126,127 It causes marked reductions in plasma norepinephrine,¹²⁸ and it failed in clinical trials because it was found to increase HF-related mortality (Table).¹²⁹ As a possible explanation for this, excessive sympatholysis to a point that was incompatible with life was postulated. However, another explanation might have been the reported α₂AR desensitization and downregulation that accompanies HF,⁷² which renders α₂ARs dysfunctional, increasing sympathetic outflow in HF and limiting efficacy of α₂AR sympatholytic agonists (Table).

RAAS-Modulating Drugs

Hyperactivation of the RAAS is another neurohormonal hallmark of chronic HF, and the degree of its activation correlates with prognosis. Angiotensin II enhances the release and inhibits the reuptake of norepinephrine at ANS nerve endings (Figure 2).^130,131 Angiotensin-converting enzyme inhibitors, by decreasing angiotensin II and aldosterone levels, increase plasma renin activity and also decrease circulating catecholamines and vasopressin because of the hemodynamic improvements they bring about.¹³² Plasma aldosterone levels may be elevated 20-fold in some patients with HF, and this heightened serum level is due to increased production by the adrenal glands, which is a consequence of high plasma angiotensin II.^133–135 Our laboratory recently has identified another mechanism for the enhanced cardiotoxic aldosterone production by the adrenal cortex in HF: enhanced activity of adrenal β-arrestin1, a cofactor of GRKs in receptor desensitization, at the AT₁R angiotensin II receptor.^136,137 In addition to its electrolyte, hemodynamic, and metabolic effects, aldosterone has several direct detrimental effects on the myocardium, promoting cardiac adverse remodeling and HF progression, and also mediates several of the cardiotoxic effects of angiotensin II in the cardiac muscle (eg, myocardial fibrosis, increased oxidative stress, inflammation).^133–135 With regard to the ANS in HF, aldosterone, like angiotensin II, can decrease norepinephrine reuptake from ANS presynaptic neurons, thereby contributing to the enhanced ANS outflow in chronic HF (Figure 2).^131,133 Some of the beneficial effects of aldosterone antagonists, such as spironolactone and eplerenone, in human HF thus may derive from suppression of this effect of aldosterone on the ANS (ie, partial sympatholysis; Table).^138,139

Sympathomimetics (AR Agonists)

Dopamine, dobutamine, and milrinone (its congener amrinone has been withdrawn from the market) represent the most commonly used sympathomimetic drugs (adrenergic agonists) used as positive inotropes for acute HF (Table). All positive inotropes lead to cAMP accumulation inside cardiomyocytes, which increases contractility via elevation of intracellular free Ca²⁺ concentration (Figure 3). Dopamine and dobutamine achieve that by binding to and activating cardiac β₁ARs, whereas milrinone blocks phosphodiesterase type 3 (cAMP-specific phosphodiesterase), thereby preventing cAMP degradation.¹⁴⁰ Of course, the elevation of intracellular free Ca²⁺ inside cardioymocytes predisposes to arrhythmias (major adverse effect of positive inotropes). All 3 inotropes produce a vasodilatory effect and can cause a reduction in blood pressure; this is especially the case for milrinone, because there are no β₁ARs in vascular smooth muscle (β₂AR is the βAR subtype there; Table).¹⁴¹ Finally, the effects of dobutamine and dopamine are blunted when the patient is already using β-blocker therapy.¹⁴² In that case, non–βAR-related inotropes are preferred, such as milrinone or glucagon (which can also increase cAMP in cardiomyocytes through its own G_s-protein-coupled receptor). Despite the straightforward rationale for using positive inotropes in HF (decrease in cardiac output, hence administration of an agent that will directly increase it), clinical trials have clearly demonstrated that sympathomimetics significantly increase mortality in chronic HF.¹⁴³ Therefore, they are used only in acute HF management, such as in cardiac hypoperfusion or shock (Table).

Another therapeutic strategy involving sympathomimetics in HF is combined β₁AR blockade with simultaneous β₂AR stimulation with clenbuterol (a β₂AR-selective agonist), which has been shown to help reverse severe HF in selected patients requiring left ventricular assist devices.¹⁴⁴ The rationale for this approach was based on studies demonstrating that clenbuterol is able to improve left ventricular function by affecting cardiac cell morphology, excitation-contraction coupling, and calcium sensitivity.¹⁴⁵ However, a recent trial demonstrated that clenbuterol leads to increases in lean mass, in lean mass-to-fat ratio, and in muscle strength, resulting in increased exercise capacity of chronic HF patients (presumably via enhanced vascular smooth muscle β₂AR–dependent vasodilation, which increases skeletal blood flow), and it is, in fact, used in sports medicine as a performance-enhancing drug.¹⁴⁶ Therefore, determination of the ultimate role of clenbuterol in HF therapy requires further investigations in larger prospective trials.

Nondrug Therapeutic Modalities Affecting ANS in HF

Exercise Training

Intolerance to exercise is a major symptom of chronic HF, and skeletal myopathies can certainly contribute to the reduced functional capacity in HF.¹⁴⁷ ANS activation serves as a coordinator of cardiac and vascular muscle to maintain adequate blood pressure during exercise.¹⁴⁸ However, enhanced ANS activity can also lead to peripheral myopathies that are often seen in HF, and this can be due, in part, to ANS-mediated vasoconstriction that limits blood flow to skeletal muscle both at rest and during exercise, leading to hypoperfusion/ischemia, release of ROS, and chronic inflammation.¹⁴⁹ On the other hand, exercise training has been shown to improve centrally mediated hemodynamic parameters and skeletal muscle performance that can lead to a reflex reduction in ANS activity in HF patients, alone or in conjunction with β-blocker therapy, resulting in reductions of all-cause and cardiovascular mortalities (Table).^150,151 The postulated mechanisms for these positive results after exercise training in HF include improvements in reflex controls in arteries, decreased and abnormal central ANS outflow, increases in blood flow peripherally, and reduction in circulating proinflammatory cytokines.¹⁵² Additionally, studies from our laboratory and others in experimental HF animals have shown that exercise training improves cardiac βAR signaling and function, increases adrenergic and inotropic reserves of the heart ameliorating cardiac contractility and function, and helps restore normal ANS activity/outflow and circulating catecholamine levels, partly via normalization of GRK2 activity and restoration of sympathoinhibitory α₂AR function in the adrenal gland (Table).^81,153,154

Cholinergic System Modulation

Vagus nerve afferent activation from the periphery can modulate efferent adrenergic and cholinergic neurons centrally and at the baroreceptors. Moreover, cholinergic neurons exert tonic inhibition of adrenergic neuron activation and of norepinephrine release from presynaptic ANS nerve terminals. The well-known cardiovascular effects of the parasympathetic nervous system, that is, heart rate reduction (bradycardia, indirectly via inhibition of the ANS and directly by hyperpolarization and pacemaker activity suppression of the sinoatrial node) and vasorelaxation (indirectly via nitric oxide synthesis),¹⁵⁵ are significantly attenuated in chronic HF, which leads to, among other consequences, lifting of the harness of ANS activation that the cholinergic system normally imposes and, thus, (indirect) enhancement of ANS outflow (Table).¹⁵⁶ Clinical and experimental data suggest that β-blockade augments reflex vagal nerve control of heart rate in HF via suppression of the cardiac sympathetic presynaptic β₂AR–facilitated norepinephrine release.¹⁵⁷ Additionally, muscarinic cholinergic M₂ receptors (M₂ muscarinic cholinergic receptors) are upregulated in the left ventricular free wall, resulting in reduced heart rate variability.^158,159 Finally, vagus nerve stimulation therapy, combined with chronic β-blocker therapy, has been shown to further improve left ventricular function and reverse remodeling beyond what is achieved with β-blockers alone (Table).^160,161

Conclusions

A vast number of studies over the past few decades have established the crucial role of activated ANS in the compensatory response of the circulation to retain its hemodynamic stability in the face of a cardiac insult and, when this fails, its excessive activation that accelerates HF progression and poses severe toxicity on the chronically failing heart. Additionally, the benefits of β-blockers and other therapeutic modalities that mitigate or protect the heart against this ANS hyperactivity are also well-documented. Several recent developments in the basic cardiovascular research field that are at various stages of preclinical testing to ultimately reach the bedside in HF therapeutics also aim at reducing the activity and the detrimental effects of the ANS on the failing heart. Among these are sympatholytic agents (α₂AR agonists), polymorphic variants of cardiac ARs that confer better prognosis in HF or better responses to current HF treatments, new sympathomimetics that seek to augment the function of the seemingly cardioprotective β₂AR while simultaneously blocking the cardiotoxic β₁AR (eg, clenbuterol), activation of the cardiac parasympathetic nervous system, and, last but not least, augmentation of cardiac βAR–dependent function without the accompanying elevation of ANS activity/outflow. The latter is pursued with the promising GRK2 inhibition therapeutic approach, which poses to improve cardiac adrenergic and inotropic reserves by restoring cardiac βAR signaling and function (ie, to provide positive inotropy) while keeping the ANS outflow at bay by restoring or augmenting central, cardiac, and adrenal sympathoinhibitory α₂AR function. Further understanding of the mechanisms of ANS activation and of the repercussions this has on regulation of cardiac function and structure in chronic HF is most certainly bound to provide the clinicians of the future with some desperately needed, newer, and better weapons in the battle against this devastating disease.

Footnotes