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1030 Current Pharmaceutical Design, 2015, 21, 1030-1041

Chemistry, Physiology, and Pharmacology of �-Adrenergic Mechanisms in the Heart. Why are �-Blocker Antiarrhythmics Superior?

A. József Szentmiklósi1, Norbert Szentandrássy2,3, Bence Hegyi2, Balázs Horváth2, János Magyar2,4, Tamás Bányász2 and Péter P. Nánási2,3,*

1Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Debrecen, Hungary;

2Department of Physi-

ology, Faculty of Medicine, University of Debrecen, Hungary; 3Department of Dental Physiology and Pharmacology, Faculty of Den-

tistry, University of Debrecen, Hungary; 4Division of Sport Physiology, Department of Physiology, Faculty of Medicine, University of

Debrecen, Hungary

Abstract: Stimulation of �-adrenergic receptors in the heart is the most effective endogenous way to increase the mechanical perform-ance of cardiac tissues to meet the requirements of a fight-or-flight situation or stress. On the other hand, sustained activation of cardiac �-receptors initiates maladaptive remodeling of the myocardium leading to cardiomyopathies and heart failure. Since both acute and chronic stimulation of �-adrenoceptors are arrhythmogenic, the application of �-receptor blockers exerts effective antiarrhytmic actions at both short and long time scale. Compared to other classes of antiarrhythmic agents, �-blockers are the class of antiarrhythmics that was shown to decrease mortality in postinfarct patients. Chemical, physiological, and pharmacological properties of the �-adrenoceptor re-lated signaling, the role of �-1, �-2, and �-3 receptor subtypes, consequences of acute and long term �-adrenergic stimulation and the un-derlying proarrhythmic mechanisms, including the changes in cardiac ion currents and Ca2+ handling, are reviewed in this paper together with the clinical relevance of cardioprotective �-blocking therapy.

Keywords: �-adrenergic receptors, �-receptor blockers, proarrhythmic mechanisms, antiarrhythmic drugs, cardiac ion currents, cardiac re-modeling.

1. INTRODUCTION

The sympathetic nervous system plays a central role in neuro-humoral control of the cardiovascular system and is largely in-volved in many cardiovascular diseases affecting millions of people all over the world. Catecholamines (epinephrine and norepineph-rine, the natural mediators of the sympathetic nervous system) act dominantly on �-adrenergic receptors in the heart. This receptor family is coupled to G proteins, and is involved in mediating a mul-titude of cardiac actions. The population of cardiac �-adrenergic receptors is not uniform, at least three distinct receptor subtypes in the human heart: �-1, �-2, and �-3 adrenoceptors, have been identi-fied so far. Although the chemical structure, pharmacological pro-file, and the signaling mechanism coupled to these receptor sub-types are different, the general result of �-receptor stimulation is the appearance of the five positive tropic effects: increased heart rate and conduction velocity, enhanced excitability, and increment of the magnitude and rate of contraction and relaxation. All these changes are believed to be direct consequences of activation of protein kinases (mostly protein kinase A) due to the elevated cAMP level. Unfortunately, overstimulation of the heart may result in maladaptive changes, which are highly pathological and are usually associated with various types of cardiac arrhythmias. Pharmacol-ogical blockade of �-adrenergic receptors thus has a chance to pre-vent these alterations and to diminish the incidence of arrhythmias. �-receptor blockers (i.e. drugs that suppress beta-adrenergic signal-ing by competitively inhibiting agonist binding to the receptors), therefore, have been widely used in the therapy of cardiac diseases for more than 5 decades since the first application of the �-receptor blocker, propranolol [1]. Of course, the outlines of the �-blocking strategy have changed a lot during this time with understanding more and more details of the adrenergic signal transduction in the human heart.

*Address correspondence to this author at the Department of Physiology, University of Debrecen, Nagyerdei krt. 98. H-4012 Debrecen, Hungary; Tel: +36-52-255575; Fax: +36-52-255116; E-mail: nanasi.peter@med.unideb.hu

2. CHEMISTRY OF �-ADRENERGIC AGENTS

Result of any receptor-ligand interaction depends on the affinity (ability of the receptor to bind the ligand) and the efficacy (ability of the ligand-receptor complex to initiate a biological response). Ligands are classified as agonists or antagonists depending on the presence or absence of efficacy [2]. Subtype specificity of �-receptor agonists and antagonists is strongly governed by the resi-dues present in the extracellular helical structures, while the binding affinity is determined by the conserved residues buried deep in the pocket limiting the degree of conformational and rotational free-doms to the bound ligand [3]. Considering interactions with �-adrenoceptors, the binding of an agonist or antagonist to the binding site requires hydrogen bonds, ionic interactions and pi-pi interac-tions. Therefore, the steric orientation of the hydroxyl group, the amino group and the aromatic residue are crucial for binding to the pocket [4]. These details can be well recognized in the structure of non-selective �-receptor agonists, like epinephrine or isoproterenol. Importantly, reversal of the hydroxybenzene moiety results in an antagonistic effect - i.e. loss of efficacy - as illustrated by the struc-ture of the �-receptor blocker S-propranolol in (Fig. 1A) [5]. While activation of �-1 adrenoceptors may be especially useful in emer-gency medicine to restart the heart when necessary, selective acti-vation of �-2 receptors is one of the best vasodilator and bronchodi-lator strategies. So relatively selective �-2 agonists, like albuterol, fenoterol and terbutaline, have been developed (Fig. 1B), and their action has been studied at a molecular level [6, 7]. A third group of �-receptor activators has a highest affinity to �-3 adrenoceptors. The first generation of these selective �-3 agonists (e.g. BRL 37344, N-5984, and CL 316243) are phenyletanolamine deriva-tives, having 3-chlorophenyl moiety and a carboxylic acid or an ester group within the molecule (Fig. 1C), and are effective anti-obesity and anti-diabetic agents [8-10]. In the heart �-3 agonists mediate cardioprotection based on their suppressive action on transmembrane Ca2+ entry and SR Ca2+ release, resulting in nega-tive inotropy [11, 12]. A new generation of selective �-3 agonists is represented by L 770644, LY-377604, and SB 226552 (Fig. 1D).

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�-Adrenergic Mechanisms in the Heart Current Pharmaceutical Design, 2015, Vol. 21, No. 8 1031

These drugs display potent and full agonistic activity and high se-lectivity for human �-3 adrenoceptors [13]. Recently, various struc-tures, including phenoxypropanolamines, thiourea derivatives, sub-stituted phenylthiourea derivatives, and oxadiazole benzenesul-fonamides (not shown), have been identified as selective �-3 recep-tor agonists [14, 15]. It must be mentioned, however, that most of the �-3 agonists show poor stereoselectivity, therefore they are applied usually as racemates.

3. CHEMISTRY OF �-RECEPTOR BLOCKERS

Common feature of �-blockers is the presence of an aromatic ring attached to a side alkyl chain incorporating a hydroxyl and amine functional groups. Furthermore, �-blockers must contain at

least one chiral center in their structures; one of them is a carbon atom in the alkyl chain coupled directly to a hydroxyl group [4, 16]. Due to the above mentioned chirality, the interaction of �-blockers with their receptors is highly stereoselective - the binding pocket of these receptors is often mentioned as an example for chiral recogni-tion [4, 16]. Similarly to �-receptor agonists, the subtype specificity of �-receptor antagonists depends on the steric orientation of the hydroxyl group, amino group and the aromatic residue - all being responsible for binding to the pocket of the receptor protein [4]. Indeed, aryloxy-propanolamine �-blockers, derided from 2- or 4-hydroxyphenylalkanones, with phenethyl or 3,4-dimethoxyphe-nethyl groups in the hydrophilic part of the molecule were synthe-sized and their subtype specificity was pharmacologically studied

Fig. (1). Chemical structures of �-adrenergic agents. A: Non-selective �-receptor agonist catecholamines: epinephrine and isoproterenol. In their biological active form they contain the hydroxyl group in R-configuration. Note that reversal of the hydroxybenzene moiety results in generation of the �-receptor an-tagonist propranolol, containing the hydroxyl group in S-configuration. B: Structure of preferential �-2 agonists. C and D: First generation and novel �-3 ago-nists, respectively.

epinephrine

HO

HONH

H3CCH3

OH

isoproterenol S-propranolol

A

HO

OH

NHCH3

H3CCH3

HO

B

albuterol terbutalinefenoterol

C

BRL 37344 CL 316243

SB 226552

L 770644

LY-377604

HO

HO

OH

NH

H3CCH3

CH3

HO

HONH

CH3

OH

OH

OH

O NHCH3

H3C

HO

HONHCH3

OH

O

H3C

NH

OH

Cl

OH

O N-5984

H3C

NH

OH

Cl

O

O

OOH

PO

OH

O

NHOHO

OH

HN

NH

OH

N

SO

O

NN N

N

OD

Na+

Na+O

O

O

O-

H3C

NH

OH

Cl

O O-

O

H3C

NHO

HN CH3

N NH2

OOH

1032 Current Pharmaceutical Design, 2015, Vol. 21, No. 8 Szentmiklósi et al.

by their ability to block isoproterenol-activated �-adrenoceptors. Reciprocal changes in the position of the phenoxy substituents failed to influence the selectivity of the compounds. In contrast, increasing the size of the N-substituent in the hydrophilic part of molecule resulted in a substantially higher cardioselectivity indi-cated by the greater �-1 over �-2 affinity [17]. Most �-blockers used clinically to treat cardiovascular diseases are aryloxy-propa-nolamine derivatives. From these structures the S enantiomer shows much higher �-blocking activity than the R enantiomer - this differ-ence in efficacy may be often two orders of magnitudes [18, 19].

Some of these compounds are applied in their racemic form, in spite of the fact that the �-blockade is caused practically exclusively by the S-enantiomer. This may result in accumulation of undesired side effects caused by the R-enantiomer [20]. Other drugs (e.g. timolol, levobunolol, penbutolol, and esatenolol) are used as pure S(-) enantiomers [4]. Some �-receptor blockers (such as propra-nolol, carvedilol, and labetalol) are non-selective �-receptor block-ers, since they fail to differentiate substantially between �-1 and �-2 receptors (Fig. 2A). Others, like metoprolol, betaxolol, atenolol, practolol, bisoprolol, and nebivolol are moderately selective to �-1 receptors (Fig. 2B), while a third group of �-blockers, including nadolol, alprenolol, levobunolol, penbutolol, and timolol displays higher �-2 than �-1 blocking potency (Fig. 2C). Nebivolol shows unique properties. This third generation cardioselective �-blocker, with 4 chiral centers, is a racemate of (+)-nebivolol (SRRR con-figuration) and (-)-nebivolol (RSSS configuration). Interestingly, its antihypertensive activity is associated with the R-enantiomer at the hydroxyl group, in contrast to all other �-blockers, which display antihypertensive activity in the S-enantiomer [4].

There is a further group of �-receptor blockers showing high cardioselectivity combined with ultra-short duration of action [21, 22]. This group includes esmolol type (esmolol) and morpholino type (ONO-1101) analogs (Fig. 2D). The half time of their action is 9 min, and their cardioselectivity (�-1 over �-2 affinity) is 33 and 255, respectively [23]. These agents are typically used to prevent or treat life-threatening cardiac arrhytmias in surgery patients suscep-tible to adverse �-blocker side-effects [24]. Some morpholino ana-logs (SA-113 and SA-132) contain a triple bond. Their action is also rapid (9-10 min), but their cardioselectivity index (varying between 14 and 19) is somewhat lower than that of ONO-1101, although still several times higher compared to propranolol. Simi-larly, ultra-short action and high cardioselectivity have been ob-served with flestolol and landiolol, with the latter having a cardiose-lectivity index of 225 and a half-life of 4 min [25]. The suppressive cardiac effects of these ultra-short acting �-receptor blockers have been demonstrated in various animal models, including dogs and rabbits [26-28], as well as in human subjects [29-32].

In general, the majority of �-receptor blockers have much lower affinity to �-3 than to either �-1 or �-2 receptors. In some cases this difference is not evident, i.e. non-selective �-3 blockers (bu-prandolol, CL 316243 or ICI 118551) can be considered. However, a few selective �-3 blockers have also been synthesized. These agents (such as SR 59230A, L-748,328 and L-748,337) have little therapeutic interest in cardiology - they are rather used in binding studies.

When the selectivity of clinically applied �-receptor antagonists is discussed, it must be borne in mind that their in vitro and in vivo selectivity may be quite different. For instance, �-1 over �-2 selec-tivity of �-blockers was found to be relatively poor in intact cells. Accordingly, compounds that are traditionally classified as �-1 selective drugs were shown to have higher affinity to �-2 adreno-ceptors [33]. Therefore - in spite of the large number of the cur-rently applied �-receptor blockers - the development of new, more selective compounds is still a realistic demand.

In addition to selectivity and efficacy, there is a third, extraor-dinarily important parameter of adrenergic agents - including both

agonists and antagonists - namely the hydrophobic versus hydro-phylic character of the molecule. The relative hydrophobicity index (determined using various techniques for 15 drugs) was the highest in the case of propranolol, metoprolol and alprenolol, i.e. in com-pounds carrying aryl and phenyl groups without hydrophilic moie-ties. On the other hand, the less hydrophobic (i.e. most hydrophilic) agents were albuterol, norepinephrine, epinephrine, and sotalol [34]. In the case of albuterol and the catecholamines, the hydroxyl groups may be responsible for the relative hydrophilic character, while in sotalol the methylsulphonate group. It is worthy of note that when the cardioprotective-antiarrhythmic effects of �-blockers were tested for the ability to prevent sudden cardiac death in pa-tients of high risk, only those agents were found effective which showed the highest level of hydrophobicity, i.e. propranolol and metoprolol [35]. Since the pharmacokinetic properties, including absorption, distribution and elimination, are also strongly influ-enced by the hydrophobic versus hydrophylic character of the com-pound, these features are also to be considered when developing new, more potent �-blockers.

4. CELLULAR MECHANISM MEDIATING �-

ADRENERGIC ACTIONS IN THE HEART

Stimulation of �-adrenergic receptors in the heart is the most powerful endogenous way to increase the electrical and mechanical activity of cardiac tissues including the positive chronotropic, dro-motropic, bathmotropic, inotropic, and lusitropic actions on the heart. The adaptive changes caused by �-adrenergic stimulation serve survival in the case of a fight-or-flight situation or under con-ditions of stress, and include full activation of cell metabolism, elevation of cytosolic Ca2+ concentration and Ca2+ content of the SR. However, sustained or chronic adrenergic activation initiates first physiological and later pathological electrical and mechanical remodeling of the myocardium resulting in arrhythmias, hypertro-phy, apoptosis, and necrosis - events associated with cardiomy-opathies and heart failure. As well-known, there are three distinct receptor subtypes in the human heart: �-1, �-2, and �-3 adrenocep-tors. The signaling mechanisms coupled to these subtypes are es-sentially different, however, these differences remained largely hidden for a long period of time. The most abundant cardiac sub-type is �-1. The �-1 / �-2 ratio was reported to vary between 7:3 and 8:2 in the human heart [36, 37]. Since norepinephrine is the main transmitter of sympathetic nerve fibers and norepinephrine acts mainly on �-1 receptors in the heart, the physiological regula-tion of heart rate and contractility is practically under �-1 control. However, in the case of stress, both receptor subtypes will be equally activated by large amounts of epinephrine, released from the adrenal medulla [38]. This is one important difference between the two receptor subtypes. As demonstrated in (Fig. 3), �-1 adreno-ceptors are coupled to a signal transduction cascade resulting in the downstream sequential activation of Gs, adenylate cyclase, and protein kinase A, which phosphorylates several proteins, including various enzymes, ion channels and transporters in the cell mem-brane as well as in the SR. Due to the simultaneous increase in the cytosolic Ca2+ concentration, several subsequent actions are medi-ated by the Ca2+ dependent calmodulin kinase (CaMKII). Activa-tion of �-2 adrenoceptors in healthy human heart have similar ef-fects, i.e. all the positive tropic effects are effectively induced by �-2 activation [39, 40]. The reason is that �-2 adrenoceptors are cou-pled to Gs under baseline conditions, similarly to �-1 adrenoceptors, and may be almost as effective to stimulate the heart as �-1 activa-tion itself [41, 42]. While the �-1 / Gs pathway is linear and displays universal effects throughout the myocyte, there is an alternative pathway as well for the �-2 receptors. Under some conditions �-2 uncouples from Gs and couples to Gi [43, 44]. Phosphorylation of the �-2 receptors induced by PKA and the G protein-coupled pro-tein kinase (GRK) was claimed to play a central role in this switch [45-47]. Activation of the Gi coupled pathway involves the phos-phoinositide 3 kinase / protein phosphatase 2A (PI3K/PP2A) as

�-Adrenergic Mechanisms in the Heart Current Pharmaceutical Design, 2015, Vol. 21, No. 8 1033

Fig. (2). A-C. Chemical structures of �-adrenoceptor blockers. A: Non-selective �-blockers. B: �-1 selective agents. C: �-2 selective compounds.

B

C

atenolol

metoprolol

timolol

levobunololnadolol

alprenolol

penbutolol

betaxolol

practolol

nebivololbisoprolol

A

propranolol carvedilol labetalolOH

O NHCH3

H3C

NH

O

OCH3

OH

O

HN OH2N

NHCH3

OHHO

OH

NH

H3CCH3

OO

H3C

OH2N

O NH

H3CCH3

OH

OH

NH

H3CCH3

OO

OH3C

CH3

OH

NH

H3CCH3

OO

O

F

OHNH

OH

OF

HO OH

O

OH

NHCH3

CH3H3C

OH

NH

H3CCH3

O

CH2

OH

OS N

N

N

O

NH

H3CCH3

CH3

O

O

OH

NHCH3

CH3H3C

OH

O NH

H3CCH3

CH3

H3CO

HN O NH

H3CCH3

OH

1034 Current Pharmaceutical Design, 2015, Vol. 21, No. 8 Szentmiklósi et al.

Fig. (2). D. Highly cardioselective ultra-short acting �-blockers.

Fig. (3). Mechanism of �-adrenergic action in the heart. SR: sarcoplasmic reticulum, SERCA: SR calcium pump, RyR2: ryanodine receptor, cAMP: cyclic adenosine monophosphate, cGMP: cyclic guanosine monophosphate, AA: arachidonic acid, AC: adenylate cyclase, GC: guanylate cyclase, PKA: proteinkinase A, NOS: nitric oxide synthase, PI3K: phosphoinositide 3 kinase, PP2A: protein phosphatase 2A, PLA2: phospholipase A2, CaMKII, CaMKIV: calcium-calmodulin kinases.

esmolol

ONO-1101

SA-113SA-132

landiolol

O

O

O NH

H3CCH3

OH

CH3

O

O

O NH

HNO

N

O

OH

CH3

O

O

O NH

HNO

N

O

OH

OO

H3C CH3

O

O

O NH

HNO

N

O

OH

H3C

OO

H3C CH3

O

O

O NH

HNO

N

O

OH

CH3

OH3C

D

Gs�1-AR

PKA

If

Action potential duration

cAMP

Arrhythmia Contractility

Impulse generation

Ca2+-dependent currents

ICa

INCXICl

IK(Ca)

�2-AR

Gi

CaMKII, CaMKIV

RemodelingHypertrophy

PI3K PP2A

�3-AR

Gi/o NOS NO

AC

GC cGMP ICa [Ca2+]i

RyR2

SERCA

SR

Gs

nucleus

PLA2 AA

PKA-dependent coupling

Cardio-protection

IClIKr

IKs INaICa

PKA-dependent currents

[Ca2+]i

�-Adrenergic Mechanisms in the Heart Current Pharmaceutical Design, 2015, Vol. 21, No. 8 1035

well as the phospholipase A2 / arachidonic acid (PLA2/AA) cas-cades [48-51]. Both result in a reduction in the activity of the cell, which is congruent with the observed cardioprotective effect [48]. In a recent study, however, this cardioprotective effect was ques-tioned, since phosphorylation of �-2 receptors due to upregulation of GRK was shown to result in heart failure in mice [47].

An interesting consequence of the bifurcation of the �-2 related pathway is the generalized effect of the Gs-coupled signal, which is in sharp contrast with the local effect of the Gi-coupled one. Indeed, �-2 receptors are concentrated into caveolar membranes, and as such they may govern local changes of electrical activity and ion transport, because (1) the underlying PI3K/PP2A and PLA2/AA pathways act dominantly locally available, and (2) the subcaveolar cAMP changes are localized by the existing phosphodiesterase barrier [48, 52-57]. It has also been revealed that the chemical na-ture of the applied �-2 agonist may also influence the Gs versus Gi coupling of the �-2 receptor. Gs-mediated signalization could be selectively activated by fenoterol, while salbutamol and terbutaline activated both Gs and Gi pathways [58]. Finally it was clarified that the different stereoisomers of fenoterol exert opposite effects on coupling of �-2 receptors: (R,R)-fenoterol preferentially activated Gs signaling, but the (S,R) isomer activated both Gs and Gi [59]. These results clearly indicate that the steric configuration of the applied agonist may also direct the receptor / G-protein coupling - in addition to the phosphorylation status of the receptor [60].

Another important implication of the differential activation of the �-1 and �-2 pathways is the temporal asymmetry observed in the adrenergic augmentation of Ca2+ and K+ currents. It has recently been suggested that the locally acting caveolar �-2 receptors are coupled mainly to ICa, - in contrast to the �-1 subtype which en-hances both Ca2+ and K+ currents simultaneously [61, 62]. Since activation of the local �-2 pathway seems to be faster than the more generalized �-1 related one, activation of ICa preceeds the adrener-gic activation of IKr and IKs, which is necessary for the concomitant acceleration of repolarization [62]. This is potentially proarrhyth-mic because delayed activation of K+ currents may transiently in-crease the propensity of early afterdepolarizations - as it has been demonstrated experimentally as well as in silico [61, 63].

Acute activation of �-1 and �-2 receptors leads to positive tropic effects. Chronic activation of �-1 adrenoceptors causes maladaptive remodeling, including hypertrophy, apoptosis and necrosis, domi-nantly via the CaMKII pathway [64, 65]. All these changes have been claimed to contribute to the development of chronic heart failure. In contrast, sustained stimulation of �-2 receptors is be-lieved to be cardioprotective as it was shown to result in improve-ment of heart function and myocyte viability [64, 65]. �-3 adreno-ceptors also mediate inhibitory actions in cardiac cells. Stimulation of �-3 receptors, however, activates nitric oxide synthase. The downstream NO / guanylate cyclase / cGMP pathway decreases Ca2+ entry through the cell membrane, resulting in a reduction of cytosolic Ca2+ [10]. Thus sustained activation of �-2 and �-3 recep-tors - possibly with the concomitant �-1 blockade - is believed to be cardioprotective providing a new therapeutic approach for the treat-ment of chronic heart failure [64-67].

5. PROARRHYTHMIC MANIFESTATIONS OF �-

ADRENERGIC ACTIONS

Beyond the well known 5 positive tropic effects of catechola-mines, activation of cardiac �-adrenergic receptors has important consequences regarding the arrhythmia propensity in experimental animal models as well as in humans. Considering the acute proar-rhythmic effects of �-adrenergic stimulation, most changes are as-sociated with opening of ion channels in the surface membrane of the myocytes. In nodal tissues the direct cAMP-dependent activa-tion of the pacemaker funny current, If and the PKA-dependent activation of calcium channels are the main underlying mechanisms of increased impulse generation [68-69]. Under pathological condi-

tions (remodeling) when If current is expressed also in the working myocardium, enhancement of If current may result in activation of ectopic foci. Furthermore, other important ion currents of the healthy ventricular myocardium, including ICa [70, 71], INa [72, 73], IKr [74, 75], IKs [76, 77], and ICl [78], are stimulated by the cAMP/PKA system as a consequence of sympathetic activation. Augmentation of ICa and INa is clearly proarrhythmic since it carries the risk of Ca2+ overload of the myocytes, which - in turn - results in (1) reduction of conduction velocity due to closure of gap junc-tions and (2) activation of the NCX current resulting in generation of delayed afterdepolarizations [79-81]. The increment of gap junc-tion resistance leads to reduction of conduction velocity, which is an important substrate of reentry arrhythmias. In addition, the PKA-phosphorylated Ca2+ channels have an increased tendency for re-opening during the plateau phase of the action potential, which is the primary source of early afterdepolarizations [82-84]. Both types of afterdepolarizations, mentioned also as triggered activity, are highly proarrhythmic [85, 86].

To compensate for the increased density of inward current (car-ried by ICa, INa, and INCX), outward currents including IKs, IKr, and ICl are also increased by �-adrenergic stimulation (Fig. 3). These out-ward currents tend to shorten action potential duration (APD) in contrast to the effect of the enhanced inward currents, while the plateau of the action potential is markedly elevated by the greater ICa. Thus in some mammalian species (e.g. in guinea pig), APD increases, while in others (e.g. in dog) decreases as a consequence of sympathetic stimulation [76, 87]. Action potential configuration has also strong influence on the effect of sympathetic stimulation on APD [88]. For example, in canine epicardial cells, showing a prominent spike-and-dome configuration, APD is reduced by 10 nM isoproterenol (ISO), while this reduction of APD is not evident in the endocardial cells of the same species [89]. Since in canine ventricular myocardium - which is believed to be the best human model from the electrophysiological point of view [90, 91] - ISO has negligible effect on ICl, the simultaneous activation of ICa, IKs, and IKr, has to be considered. Indeed, dose-response curves obtained for these currents are almost identical with very similar EC50 values ranging between 13 and 16 nM [89]. It is important to emphasize that IKr, which is the most prominent repolarizing current under baseline conditions, in enhanced by maximal ISO administration only to 133 % of its control value, while IKs, which has a negligible contribution to normal repolarization is increased to 420 % of con-trol [89]. Thus in case of sympathetic stimulation IKs becomes the most important outward current [76], large enough to compensate for the 340 % enhancement of ICa. This is the strongest argument against blocking IKs pharmacologically - as it has been previously suggested in order to develop a novel type class 3 antiarrhythmic action [92, 93]. In summary, - in spite of the high number of ion currents modified by sympathetic activation - there are three pri-mary changes being brutally proarrhythmic: (1) the enhanced pacemaker activity, (2) the increased tendency for reopening of calcium channels, and (3) the excessive calcium overload - two of these is associated with the large increment of calcium current dur-ing �-adrenergic activation.

In addition to the acute effects discussed above, chronic activa-tion of �-adrenergic receptors initiates a multilevel cascade of struc-tural, biochemical, electrical and mechanical alterations called pathological cardiac remodeling, which leads first to cardiac hyper-trophy and ultimately to heart failure [94-97]. These changes mod-ify cardiac function at almost all possible levels, including the en-hanced protein synthesis and increased cell size, down-regulation of �-1 with up-regulation of �-3 receptors, uncoupling of �-1 receptors from Gs while coupling them to Gi, activation of PI3 kinase and MAP kinases [38, 47, 95, 96]. The relative hypoxia of the hypertro-phied heart results in oxidative stress, which is further aggravated by the mitochondrial dysfunction caused by mitochondrial Ca2+

overload and accumulation of free radicals as a consequence of

1036 Current Pharmaceutical Design, 2015, Vol. 21, No. 8 Szentmiklósi et al.

transformation of catecholamines into aminochromes [98]. Contrac-tility is initially augmented, later strongly compromised. Regarding the most characteristic electrophysiological changes, down-regulation of various K+ channels with the concomitant reduction of the respective ion currents (IKr, IKs, IK1, and Ito) and the resultant lengthening of APD - especially at low heart rates - can be men-tioned [94]. Reorganization of calcium cycling results in an in-creased Ca2+ content of the myocyte with the concomitant en-hancement of NCX activity [94, 96]. Progressive reduction in the number and conductance of the gap junction channels leads to de-creased conduction velocity. These changes all predispose patients to development of triggered electrical activity and reentry arrhyth-mias leading to sudden death as a consequence of ventricular fibril-lation [97].

6. ANTIARRHYTHMIC ACTION OF �-BLOCKERS:

DRUGS WHICH NEVER KILL

It is absolutely not surprising that suppression of �-adrenergic stimulation exerts antiarrhythmic activity under almost all condi-tions, since acute and sustained forms of excessive �-adrenergic activation leads to a variety of cardiac arrhythmias involving multi-ple mechanisms of action. But before discussing the clinical experi-ence obtained with the antiarrhythmic actions of �-blockers, let’s take a glance at other types of antiarrhythmic agents. First we have to examine: why and how can these drugs kill patients under certain circumstances. The currently applied antiarrhythmics are usually categorized using the classic scheme of Vaughan Williams, which has been modified several times since its first publication [99-101]. According to this classification, class 1 drugs suppress action po-tential upstroke and intraventricular conduction velocity due to inhibition of fast Na channels in a use-dependent manner [102, 103]. Class 2 drugs are �-blockers. Class 3 drugs prolong APD, and consequently, the refractory period, decreasing this way the prob-ability of formation of reentrant circuits [104]. Class 4 drugs are Ca channel blockers reducing effectively Ca2+ entry into cardiac cells, which, in turn, improves their impulse conduction and prevents the development of delayed afterdepolarizations. Beyond these classic antiarrhythmic strategies there are also new approaches, including direct blockade of If current [105], pharmacological increasing of gap junction conductance [106], and manipulation of ATP-sensitive K channels [107].

Unfortunately, most of the antiarrhythmic strategies discussed above may carry proarrhythmic risks at the same time [108]. Class 1 agents - especially those having slow offset kinetics, like 1.C antiarrhythmics - may impair conduction of normal impulses [109], which is proarrhythmic [110]. Indeed, some class 1.C antiarrhyth-mics, like flecainide and encainide, significantly increased mortality in patients with acute myocardial infarction, as it was documented in the CAST study [111]. The reason for the higher risk of cardiac death was the increased incidence of reentry arrhythmias. There-fore, these drugs are used only in patients with a structurally normal heart and contraindicated in cases of structural heart disease [112]. Regarding class 3 agents, prolongation of APD increases the Ca2+ content of cardiac cells and promotes the reactivation of ICa. These mechanisms are involved in generation of late and early afterdepo-larizations, respectively [113, 114]. The increased mortality ob-served with d-sotalol in postinfarct patients of the SWORD study was clearly due to the torsadogenic action of the compound [115]. Based on the disappointing results of these and several other clini-cal trials, application of 1.C drugs are now restricted for treatment of atrial fibrillation, and development of selective IKr blockers has already been suspended. Modulation of ATP-sensitive K channels is probably the most Janus-faced intervention, since the opening of these channels have been equally declared to be antiarrhythmic and proarrhythmic - depending on the actual experimental conditions [116, 117]. Pharmacological blockade of If current results some-times in sinus bradycardia, while increasing the gap junction con-ductance may interfere with fast demarcation of the injured regions

of myocardium when it should be necessary. This ultimately in-creases the spatio-temporal electrical inhomogeneity in the myocar-dium by widening the border zone - the best substrate for reentry arrhythmias.

But what about the potential proarrhythmic effects of class 2 drugs, the �-blockers? Data from the past decades indicate that �-blockers remain among the very few pharmacologic agents that reduce the incidence of sudden cardiac death, prolong survival, and ameliorate symptoms caused by arrhythmias in patients with car-diac disease [118]. Of course, �-blockers may decrease the heart rate and increase atrioventricular conduction time, or reduce the contractile force of the heart, but - very importantly - they can in-duce these effects only when the sympathetic drive going to the heart is elevated (i.e. not under baseline conditions, not during sleep, nor at rest). Therefore, �-blockers can be considered as some kind of preventive drugs, which may protect the heart from the consequences of acute as well as sustained hyperactivity of the sympathetic nervous system. As it was shown previously, both are heavily proarrhythmic. This argumentation is also true for hearts with slightly compromised mechanical capabilities (i.e. for moder-ately failing hearts), except for terminal phase of the disease, when the cardiodepressant effects of �-blockers may cause more difficul-ties than benefits.

In addition to blocking the effects of endogenous catechola-mines on �-adrenergic receptors in the heart, �-blockers have sev-eral further effects - some of them are definitely antiarrhythmic. Many �-blockers exert class 1 antiarrhythmic action as a conse-quence of inhibition of fast Na channels [119, 120]. Propranolol is the best characterized agent from this point of view: both (R)- and (S)-propranolol were shown to block cardiac Na channels [20]. Sotalol represents a natural combination of class 2 and class 3 antiarrhythmic actions, since l-sotalol is a �-blocker, but d-sotalol is a potent inhibitor of IKr current [121, 122]. This may explain why d,l-sotalol was able to increase, while d-sotalol decreased survival when these compounds were applied to postinfarct patients [123]. The widest spectrum of targets has been reported for the third gen-eration �-blocker, carvedilol. This drug proved to be an effective antiarrhythmic agent, applied successfully in management of pa-tients with heart failure, myocardial infarction, and primary or sec-ondary atrial fibrillation [124, 125]. When studying the underlying mechanisms, an additional �-receptor blocking activity of the mole-cule together with an antioxidant effect (due to the carbazole moi-ety) were revealed, in addition to inhibition of several cardiac ion currents, including INa, ICa, IKr, and IKur [126, 127]. This ion channel blocker profile strongly resembles that of amiodarone, which is not categorized primarily as a class 2 agent, although it was shown to block the above mentioned ion currents in addition to its �-blocking activity [128-131]. Recently, carvedilol has been reported to inter-fere directly with cardiac Ca2+ handling by decreasing the magni-tude of store overload-induced Ca2+ release [132]. In summary, the most important common feature of �-blockers - in addition to the multitude of additional beneficial effects - is to prevent myocardial Ca2+ overload with all of its deteriorative consequences. From this point of view, class 2 and class 4 antiarrhythmic agents act in a similar fashion, however, �-blockers were found to be superior to Ca antagonists when being applied for secondary prevention of myocardial infarction [133].

7. CLINICAL CORRELATES OF �-BLOCKERS

�-receptor blockers are appropriate treatment for patients with hypertension, heart failure, ischemic heart disease, and obstructive cardiomyopathy. They are widely used to treat cardiac arrhythmias of various origin, and for prevention of arrhythmic episodes in pa-tients with chronic heart disease leading to pathological remodeling. Most of them differ from other antiarrhythmic agents by not di-rectly modifying ion channel function, rather they prevent the ar-rhythmogenic actions of �-adrenergic stimulation. Therefore, these

�-Adrenergic Mechanisms in the Heart Current Pharmaceutical Design, 2015, Vol. 21, No. 8 1037

agents are particularly useful in prevention of sudden death due to ventricular tachyarrhythmias associated with acute myocardial ischemia, congenital long QT syndrome, and congestive heart fail-ure. They are also valuable in controlling the ventricular rate in patients with atrial fibrillation [134]. �-blockers are useful in pa-tients having hyperkinetic circulation (palpitations, tachycardia, hypertension, and �-anxiety), migraine headache, essential tremor. But they are applied also in surgery, especially in vascular surgery, in order to reduce perioperative ischemia and cardiovascular com-plications in patients having multiple risk factors [135]. For treat-ment of hypertension �-blockers are typically used in combination with other antihypertensive drugs to achieve maximal blood pres-sure control, since the majority of investigations concludes that �-blockers should not be used as a first-line drug in treatment of hy-pertension [136-139], although others argue in support of the first-line option of the new generation �-blockers [140, 141]. Heart fail-ure is a further very important area of application of �-blockers, however, the therapeutic success strongly depends on the type of dysfunction (i.e. systolic or diastolic), the age of patient, and the �-blocker agent used. �-adrenoceptors are down-regulated in chronic heart failure due to the sustained stimulation of endogenous catecholamines, which change is correlated to the severity of the disease [38]. Cardiac �-1 and �-2 receptors respond differently: in end-stage dilated cardiomyopathy and in aortic valve disease �-1 receptors are selectively depressed, while the number of �-2 recep-tors remains nearly normal. In this case �-2 receptors can effec-tively substitute for the loss in �-1 receptors [38]. This is the reason for considering the deprivation of �-blockers in end-stage dilated cardiomyopathy. In contrast, both �-receptor subtypes are equally decreased in end-stage ischemic cardiomyopathy, mitral valve dis-ease, and tetralogy of Fallot [38, 142]. �-blockers are highly effec-tive in prevention of heart failure in postinfarct patients and in re-versing left ventricular hypertrophy in young and middle-aged hy-pertensive subjects, having efficacy comparable to that of ACE inhibitors [143]. However, the actual choice of the �-blocker is important since benefit is not a class effect. Carvedilol, bisoprolol, and metoprolol had great efficacy in decreasing the mortality in cases with systolic heart failure (carvedilol was found to be the best), while bucindolol, nebivolol, and xamoterol were inferior due to their intrinsic sympathomimetic activity [139, 144]. Indeed, carvedilol and bisoprolol were shown to improve left ventricular function and high-energy phosphate levels in patients with heart failure [145, 146]. �-adrenoceptor antagonists may have a less beneficial effect or even an adverse effect in elderly heart failure patients. In spite of the good results obtained with carvedilol in young and middle-aged people, nebivolol is believed to be more effective in elderly with chronic heart failure [147]. Unfortunately, the appropriate treatment of diastolic heart failure still remains to be determined.

8. CONCLUSION

As was shown in this brief overview, �-blocker therapy - due to its multipotent efficacy - can be applied to treat many cardiac dis-eases, including hypertension, hyperkinetic circulation syndromes, cardiac hypertrophy, ischemic heart disease, heart failure, and car-diomyopathies, and most importantly postinfarct patients with life-threatening cardiac arrhythmias, for a long time in the future. Car-dioprotective effects of �-receptor blockers can be utilized in almost all types of cardiac disorders to prevent the maladaptive changes and decrease the incidence of sudden cardiac death. This cardiopro-tective action could be best achieved by applying selective �-1 blockers with a combination of a selective �-3 agonist (when avail-able). Recent progress in the field of �-blocker therapy yielded a multitude of compounds; some of them are used because of their relatively high subtype selectivity (e.g. esmolol), while others are favored due to their additional mechanisms of actions (e.g. carve-dilol). Now, it appears that this latter group, including agents hav-ing other direct ion channel effects in addition to their subtype se-

lective or non-selective �-blocking properties, is more effective to prolong survival by diminishing the risk of sudden cardiac death. In spite of this, development of new, more selective compounds is still a realistic demand of the present days. In conclusion, since �-blockers are highly heterogeneous with respect to various pharma-cologic effects, (including the degree of intrinsic sympathomimetic activity, �-1 over �-2 selectivity, interactions with �-1 adrenergic receptors, effects on various transmembrane ion channels, effects on Ca2+-handling, antioxidant action, tissue solubility, routes of systemic elimination, potencies and duration of action, etc.) thor-ough considerations are required to select the most suitable �-blocker for treatment of individual cardiovascular syndromes [135].

CONFLICT OF INTEREST

The authors confirm that this article content has no conflict of interest.

ACKNOWLEDGEMENTS

Financial support was provided by grants from the Hungarian Research Fund (OTKA-K100151, OTKA-PD101171, OTKA-K101196, OTKA-K109736, OTKA-NK104331). Further support was obtained from TÁMOP-4.2.2.A-11/1/KONV-2012-0045 re-search project.

LIST OF ABBREVIATIONS

APD = Action potential duration

INa = Sodium current

ICa = L-type calcium current

IKr = Rapid component of delayed rectifier po-tassium current

IKs = Slow component of delayed rectifier po-tassium current

ICl = Chloride current

INCX = Na/Ca exchange current

If = “Funny” pacemaker current

SR = Sarcoplasmic reticulum

SERCA = Sarcoplasmic reticulum calcium pump

RyR2 = Cardiac ryanodine receptor

cAMP = Cyclic adenosine 3’,5’-monophosphate

cGMP = Cyclic guanosine monophosphate

ISO = Isoproterenol

AA = Arachidonic acid

AC = Adenylate cyclase

GC = Guanylate cyclase

PKA = Proteinkinase A

GRK = G protein-coupled protein kinase

NOS = Nitric oxide synthase

PI3K = Phosphoinositide 3 kinase

PP2A = Protein phosphatase 2A

PLA2 = Phospholipase A2

CaMKII, CaMKIV = Calcium-calmodulin kinases

REFERENCES

[1] Black JW, Duncan WAM, Shanks RG. Comparison of some prop-erties of pronethalol and propranolol. Br J Pharmacol 1965; 25: 577-91.

[2] Baker JG. The selectivity of �-adrenoceptor agonists at human �1-, �2- and �3- adrenoceptors. Br J Pharmacol 2010; 160: 1048-61.

[3] Roy KK, Saxena AK. Structural basis for the �-adrenergic receptor subtype selectivity of the representative agonists and antagonists. J Chem Inf Model 2011; 51: 1405-22.

1038 Current Pharmaceutical Design, 2015, Vol. 21, No. 8 Szentmiklósi et al.

[4] Siebert CD, Hänsicke A, Nagel T. Stereochemical comparison of nebivolol with other �-blockers. Chirality 2008; 20: 103-9.

[5] Siebert CD, Tampe N. Stereoisomers of compounds with symmet-ric constitutions. Arch Pharm (Weinheim) 2005; 338: 534-8.

[6] Jozwiak K, Toll L, Jimenez L, Woo AY, Xiao RP, Wainer IW. The effect of stereochemistry on the thermodynamic characteristics of the binding of fenoterol stereoisomers to the �2-adrenoceptor. Biochem Pharmacol 2010; 79: 1610-5.

[7] Jozwiak K, Plazinska A, Toll L, et al. Effect of fenoterol stereo-chemistry on the �2 adrenergic receptor system: ligand-directed chiral recognition. Chirality 2011; 23: Suppl. 1: E1-6.

[8] Yanagisawa T, Sato T, Yamada H, Sukegawa J, Nunoki K. Selec-tivity and potency of agonists for the three subtypes of cloned hu-man �-adrenoceptors expressed in Chinese hamster ovary cells. Tohoku J Exp Med 2000; 192: 181-93.

[9] Sawa M, Harada H. Recent developments in the design of orally bioavailable �3-adrenergic receptor agonists. Curr Med Chem 2006; 13: 25-37.

[10] Rozec B, Gauthier C. �3-adrenoceptors in the cardiovascular sys-tem: putative roles in human pathologies. Pharmacol Ther 2006; 111: 652-73.

[11] Gauthier C, Séze-Goismier C, Rozec B. �3-adrenoceptors in the cardiovascular system. Clin Hemorheol Microcirc 2007; 37: 193-204.

[12] Gauthier C, Rozec B, Manoury B, Balligand JL. �3-adrenoceptors as new therapeutic targets for cardiovascular pathologies. Curr Heart Fail Rep 2011; 8: 184-92.

[13] Harada H, Hirokawa Y, Suzuki K, et al. Discovery of a novel and potent human and rat �3-adrenergic receptor agonist, [3-[(2R)-[[(2R)-(3-chlorophenyl)-2-hydroxyethyl]- amino]propyl]-1H-indol-7-yloxy]acetic acid. Chem Pharm Bull (Tokyo) 2005; 53: 184-98.

[14] Biftu T, Feng DD, Liang GB, et al. Synthesis and SAR of benzyl and phenoxymethylene oxadiazole benzenesulfonamides as selec-tive �3 adrenergic receptor agonist antiobesity agents. Bioorg Med Chem Lett 2000; 10: 1431-4.

[15] Maruyama T, Seki N, Onda K, et al. Discovery of novel thiourea derivatives as potent and selective �3-adrenergic receptor agonists. Bioorg Med Chem 2009; 17: 5510-9.

[16] Mehvar R, Brocks DR. Stereospecific pharmacokinetics and phar-macodynamics of of �-adrenergic blockers in humans. J Pharm Sci 2001; 4: 185-200.

[17] Cizmáriková R, Racanská E, Hrobonova K, Lehotay J, Aghová Z, Halesová D. Synthesis, pharmacological activity and chroma-tographic separation of some novel potential �-blockers of the ary-loxyaminopropanol type. Pharmazie 2003; 58: 237-41.

[18] Adejare A, Deal SA, Day MS Jr. Syntheses and �-adrenergic bind-ing affinities of (R)- and (S)-fluoronaphthyloxypropanolamines. Chirality 1999; 11: 144-8.

[19] Gagyi L, Gyéresi Á, Kilár F. Role of chemical structure in stereose-lective recognition of �-blockers by cyclodextrins in capillary zone electrophoresis. J Biochem Biophys Methods 2008; 70: 1268-75.

[20] Stoschitzky K, Lindner W. Specific and nonspecific effects of beta receptor blockers: stereoselectively different properties exemplified by (R)- and (S)-propranolol. Wien Med Wochenschr 1990; 140: 156-62.

[21] Mokr� P, Zemanová M, Csöllei J, Racanská E, Tumová I. Synthe-sis and pharmacological evaluation of novel potential ultrashort-acting beta-blockers. Pharmazie 2003; 58: 18-21.

[22] Mio Y. New ultra-short-acting beta-blockers: landiolol and esmolol - the effects on cardiovascular system. Masui 2006; 55: 841-8.

[23] Iguchi S, Iwamura H, Nishizaki M, et al. Development of highly cardioselective ultra short-acting �-blocker, ONO-1101. Chem Pharm Bull 1992; 40: 1462-9.

[24] Greenspan AM, Spielman SR, Horowitz LN, Laddu A, Senior S. The electrophysiologic properties of esmolol, a short acting �-blocker. Int J Clin Pharmacol Ther Toxicol 1988; 26: 209-16.

[25] Hashimoto Y, Nago S, Tsunekawa M, Shibakawa K, Miyata Y. Validation of HPLC-UV methods for quantitatively determining landiolol and its major metabolite in human blood. Biol Pharm Bull 2009; 32: 121-5.

[26] Sugiyama A, Takahara A, Hashimoto K. Electrophysiologic, car-diohemodynamic and �-blocking actions of a new ultra-short-acting �-blocker, ONO-1101, assessed by the in vivo canine model in comparison with esmolol. J Cardiovasc Pharmacol 1999; 34: 70-7.

[27] Sasao J, Tarver SD, Kindscher JD, Taneyama C, Benson KT, Goto H. In rabbits, landiolol, a new ultra-short-acting �-blocker, exerts a more potent negative chronotropic effect and less effect on blood pressure than esmolol. Can J Anaesth 2001; 48: 985-9.

[28] Ikeshita K, Nishikawa K, Toriyama S, et al. Landiolol has a less potent negative inotropic effect than esmolol in isolated rabbit hearts. J Anesth 2008; 22: 361-6.

[29] Swerdlow CD, Peterson J, Turlapaty P. Clinical electrophysiology of flestolol, a potent ultra short-acting �-blocker. Am Heart J 1986; 111: 49-53.

[30] Barbier GH, Shettigar UR, Appunn DO. Clinical rationale for the use of an ultra-short acting �-blocker: esmolol. Int J Clin Pharma-col Ther 1995; 33: 212-8.

[31] Wiest D. Esmolol. A review of its therapeutic efficacy and pharma-cokinetic characteristics. Clin Pharmacokinet 1995; 28: 190-202.

[32] Saito S, Nishihara F, Akihiro T, et al. Landiolol and esmolol pre-vent tachycardia without altering cerebral blood flow. Can J An-aesth 2005; 52: 1027-34.

[33] Baker JG. The selectivity of �-adrenoceptor antagonists at the human �1, �2 and �3 adrenoceptors. Br J Pharmacol 2005; 144: 317-22.

[34] Gulyaeva N, Zaslavsky A, Lechner P, Chlenov M, Chait A, Zaslavsky B. Relative hydrophobicity and lipophilicity of �-blockers and related compounds as measured by aqueous two-phase partitioning, octanol-buffer partitioning, and HPLC. Eur J Pharm Sci 2002; 17: 81-93.

[35] Hjalmarson A. Effects of �-blockade on sudden cardiac death dur-ing acute myocardial infarction and the postinfarction period. Am J Cardiol 1997; 80: 35J-9.

[36] Bristow MR, Ginsburg R, Umans V, et al. �1- and �2-adrenergic-receptor subpopulations in nonfailing and failing human ventricular myocardium: coupling of both receptor subtypes to muscle contrac-tion and selective beta 1-receptor down-regulation in heart failure. Circ Res 1986; 59: 297-309.

[37] Kaumann AJ, Lemoine H. �2-adrenoceptor-mediated positive inotropic effect of adrenaline in human ventricular myocardium. Quantitative discrepancies with binding and adenylate cyclase stimulation. Naunyn Schmiedeberg’s Arch Pharmacol 1987; 335: 403-11.

[38] Khamssi M, Brodde OE. The role of cardiac �1- and �2-adrenoceptor stimulation in heart failure. J Cardiovasc Pharmacol 1990; 16: Suppl 5: S133-7.

[39] Kaumann A, Bartel S, Molenaar P, et al. Activation of �2-adrenergic receptors hastens relaxation and mediates phosphoryla-tion of phospholamban, troponin I, and C-protein in ventricular myocardium from patients with terminal heart failure. Circulation 1999; 99: 65-72.

[40] Molenaar P, Bartel S, Cochrane A, et al. Both �2- and �1-adrenergic receptors mediate hastened relaxation and phosphoryla-tion of phospholamban and troponin I in ventricular myocardium of Fallot infants, consistent with selective coupling of �2-adrenergic receptors to Gs-protein. Circulation 2000; 102: 1814-21.

[41] Heubach JF, Blaschke M, Harding SE, Ravens U, Kaumann AJ. Cardiostimulant and cardiodepressant effects through overex-pressed human �2-adrenoceptors in murine heart: regional differ-ences and functional role of �1-adrenoceptors. Naunyn Schmiede-berg’s Arch Pharmacol 2003; 367: 380-90.

[42] Molenaar P, Savarimuthu SM, Sarsero D, et al. (-)-Adrenaline elicits positive inotropic, lusitropic, and biochemical effects through �2-adrenoceptors in human atrial myocardium from non-failing and failing hearts, consistent with Gs coupling but not with Gi coupling. Naunyn Schmiedeberg’s Arch Pharmacol 2007; 375: 11-28.

[43] Xiao RP, Cheng H, Zhou YY, Kuschel M, Lakatta EG. Recent advances in cardiac �2-adrenergic signal transduction. Circ Res 1999; 85: 1092-100.

[44] Kuschel M, Zhou YY, Spurgeon HA, et al. �2-adrenergic cAMP signaling is uncoupled from phosphorylation of cytoplasmic pro-teins in canine heart. Circulation 1999; 99: 2458-65.

[45] Daaka Y, Luttrell LM, Lefkowitz RJ. Switching of the coupling of the beta2-adrenergic receptor to different G proteins by protein kinase A. Nature 1997; 390: 88-91.

[46] Baillie GS, Sood A, McPhee I, et al. �-arrestin-mediated PDE4 cAMP phosphodiesterase recruitment regulates �-adrenoceptor switching from Gs to Gi. Proc Natl Acad Sci USA 2003; 100: 940-5.

�-Adrenergic Mechanisms in the Heart Current Pharmaceutical Design, 2015, Vol. 21, No. 8 1039

[47] Zhu W, Petrashevskaya N, Ren S, et al. Gi-biased �2AR signaling links GRK2 upregulation to heart failure. Circ Res 2012; 110: 265-74.

[48] Xiao RP. Beta-adrenergic signaling in the heart: dual coupling of the �2-adrenergic receptor to Gs and Gi proteins. Sci STKE 2001; 104: re15.

[49] Pavoine C, Behforouz N, Gauthier C, et al. �2-Adrenergic signal-ing in human heart: shift from the cyclic AMP to the arachidonic acid pathway. Mol Pharmacol 2003; 64: 1117-25.

[50] Pavoine C, Defer N. The cardiac �2-adrenergic signalling a new role for the cPLA2. Cell Signal 2005; 17: 141-52.

[51] Ait-Mamar B, Cailleret M, Rucker-Martin C, et al. The cytosolic phospholipase A2 pathway, a safeguard of �2-adrenergic cardiac effects in rat. J Biol Chem 2005; 280: 18881-90.

[52] Jurevicius J, Fischmeister R. cAMP compartmentation is responsi-ble for a local activation of cardiac Ca2+ channels by ß-adrenergic agonists. Proc Natl Acad Sci USA 1996; 93: 295-9.

[53] Kuschel M, Zhou YY, Cheng H, et al. Gi protein-mediated func-tional compartmentalization of cardiac �2-adrenergic signaling. J Biol Chem 1999; 274: 22048-52.

[54] Steinberg SF, Brunton LL. Compartmentation of G protein-coupled signaling pathways in cardiac myocytes. Annu Rev Pharmacol Toxicol 2001; 41: 751-73.

[55] Fichmeister R, Castro LRV, Abi-Gerges A, et al. Compartmenta-tion of cyclic nucleotide signaling in the heart. The role of cyclic nucleotide phosphodiesterases. Circ Res 2006; 99: 816-828.

[56] Dodge-Kafka KL, Langeberg L, Scott JD. Compartmentation of cyclic nucleotide signaling in the heart: the role of A-kinase an-choring proteins. Circ Res 2006; 98: 993-1001.

[57] Xiang YK. Compartmentalization of �-adrenergic signals in car-diomyocytes. Circ Res 2011; 109: 231-44.

[58] Pönicke K, Gröner F, Heinroth-Hoffmann I, Brodde OE. Agonist-specific activation of the �2-adrenoceptor/Gs-protein and �2-adrenoceptor/Gi-protein pathway in adult rat ventricular cardio-myocytes. Br J Pharmacol 2006; 147: 714-9.

[59] Woo AY, Wang TB, Zeng X, et al. Stereochemistry of an agonist determines coupling preference of �2-adrenoceptor to different G proteins in cardiomyocytes. Mol Pharmacol 2009; 75: 158-65.

[60] Casella I, Ambrosio C, Gro MC, Molinari P, Costa T. Divergent agonist selectivity in activating �1- and �2-adrenoceptors for G-protein and arrestin coupling. Biochem J 2011; 438: 191-202.

[61] Heijman J, Volders PG, Westra RL, Rudy Y. Local control of �-adrenergic stimulation: Effects on ventricular myocyte electro-physiology and Ca2+-transient. J Mol Cell Cardiol 2011; 50: 863-71.

[62] Ruzsnavszky F, Hegyi B, Kistamás K, et al. Asynchronous activa-tion of calcium and potassium currents by isoproterenol in canine ventricular myocytes. Naunyn Schmiedebergs Arch Pharmacol 2014; 387: 457-67.

[63] Xie Y, Grandi E, Puglisi JL, Sato D, Bers DM. �-adrenergic stimu-lation activates early afterdepolarizations transiently via kinetic mismatch of PKA targets. J Mol Cell Cardiol 2013; 58: 153-61.

[64] Zheng M, Han QD, Xiao RP. Distinct �-adrenergic receptor sub-type signaling in the heart and their pathophysiological relevance. Sheng Li Xue Bao 2004; 56: 1-15.

[65] Woo AY, Xiao RP. �-Adrenergic receptor subtype signaling in heart: from bench to bedside. Acta Pharmacol Sin 2012; 33: 335-41.

[66] Singh K, Xiao L, Remondino A, Sawyer DB, Colucci WS. Adrenergic regulation of cardiac myocyte apoptosis. J Cell Physiol 2001; 189: 257-65.

[67] Best JM, Kamp TJ. Different subcellular populations of L-type Ca2+ channels exhibit unique regulation and functional roles in car-diomyocytes. J Mol Cell Cardiol 2012; 52: 376-87.

[68] DiFrancesco D. A study of the ionic nature of the pace-maker cur-rent in calf Purkinje fibres. J Physiol 1981; 314: 377-93.

[69] Tsien RW, Bean BP, Hess P, Lansman J, Nilius B, Nowycky MC. Mechanisms of calcium channel modulation by ß-adrenergic agents and dihydropyridine calcium agonists. J Mol Cell Cardiol 1986; 18: 691-710.

[70] Ochi R. Single-channel mechanism of ß-adrenergic enhancement of cardiac L-type calcium current. Jpn J Physiol 1993; 43: 571-84.

[71] Van der Heyden MA, Wijnhoven TJ, Opthof T. Molecular aspects of adrenergic modulation of cardiac L-type Ca2+ channels. Cardio-vasc Res 2005; 65: 28-39.

[72] Matsuda JJ, Lee H, Shibata EF. Enhancement of rabbit cardiac sodium channels by ß-adrenergic stimulation. Circ Res 1992; 70: 199-207.

[73] Ono K, Fozzard HA, Hanck DA. Mechanism of cAMP-dependent modulation of cardiac sodium channel current kinetics. Circ Res 1993; 72: 807-15.

[74] Heath BM, Terrar DA. Protein kinase C enhances the rapidly acti-vating delayed rectifier potassium current, IKr, through a reduction in C-type inactivation in guinea-pig ventricular myocytes. J Physiol 2000; 522: 391-402.

[75] Harmati G, Bányász T, Bárándi L, Szentandrássy N, Horváth B, Szabó G, Szentmiklósi JA, Szénási G, Nánási PP, Magyar J. Ef-fects of ß-adrenergic stimulation on delayed rectifier K+ currents in canine ventricular cardiomyocytes. Br J Pharmacol 2011; 162: 890-6.

[76] Volders PGA, Stengl M, van Opstal JM, et al. Probing the contri-bution of IKs to canine ventricular repolarization: Key role for �-adrenergic receptor stimulation. Circulation 2003; 107: 2753-60.

[77] Severi S, Corsi C, Rocchetti M, Zaza A. Mechanisms of ß-adrenergic modulation of IKs in the guinea-pig ventricle: insights from experimental and model-based analysis. Biophys J 2009; 96: 3862-72.

[78] Harvey RD, Clark CD, Hume JR. Cl� current in mammalian car-diac myocytes: novel mechanism for autonomic regulation of ac-tion potential duration and resting membrane potential. J Gen Physiol 1990; 95: 1077-102.

[79] Kimura J, Noma A, Irisawa H. Na+-Ca2+ exchange current in mammalian heart cells. Nature 1986; 319: 596-7.

[80] Mechmann S, Pott L. Identification of Na+-Ca2+ exchange current in single cardiac myocytes. Nature 1986; 319: 597-9.

[81] Janvier NC, Boyett MR. The role of Na+-Ca2+ exchange current in the cardiac action potential. Cardiovasc Res 1996; 32: 69-84.

[82] Striessnig J. Pharmacology, structure and function of cardiac L-type Ca2+ channels. Cell Physiol Biochem 1999; 9: 242-69.

[83] Catterall WA. Structure and regulation of voltage-gated Ca2+ chan-nels. Annu Rev Dev Biol 2000; 16: 521-55.

[84] Kamp TJ, Hell JW. Regulation of cardiac L-type calcium channels by protein kinase A and protein kinase C. Circ Res 2000; 87: 1095-102.

[85] January CT, Fozzard HA. Delayed afterdepolarizations in heart muscle: mechanism and relevance. Pharmacol Rev 1988; 40: 219-27.

[86] January CT, Riddle MJ. Early afterdepolarizations: Mechanism of induction and block, a role for L-type Ca2+ current. Circ Res 1990; 64: 977-90.

[87] Rocchetti M, Freli V, Perego V, Altomare C, Mostacciuolo G, Zaza A. Rate dependency of beta-adrenergic modulation of repolarizing currents in the guinea-pig ventricle. J Physiol 2006; 574: 183-193.

[88] Malfatto G, Rocchetti M, Zaza A. The role of the autonomic sys-tem in rate-dependent repolarization changes. Heart Rhythm 2010; 7: 1700-3.

[89] Szentandrássy N, Farkas V, Bárándi L, et al. Role of action potential configuration and the contribution of Ca2+ and K+ currents to isoprenaline-induced changes in canine ventricular cells. Br J Pharmacol 2012; 167: 599-611.

[90] Szabó G, Szentandrássy N, Bíró T, et al. Asymmetrical distribution of ion channels in canine and human left ventricular wall: epicar-dium versus midmyocardium. Pflügers Arch 2005; 450: 307-16.

[91] Szentandrássy N, Bányász T, Bíró T, et al. Apico-basal inhomoge-neity in distribution of ion channels in canine and human ventricu-lar myocardium. Cardiovasc Res 2005; 65: 851-60.

[92] Bauer A, Becker R, Freigang KD, et al. Electrophysiologic effects of the new IKs-blocking agent chromanol 293b in the postinfarction canine heart. Preserved positive use-dependence and preferential prolongation of refractoriness in the infarct zone. Basic Res Cardiol 2000; 95: 324-32.

[93] Salata JJ, Selnick HG, Lynch JJ. Pharmacological modulation of IKs: Potential for antiarrhythmic therapy. Curr Med Chem 2004; 11: 29-44.

[94] Janse MJ. Electrophysiological changes in heart failure and their relationship to arrhythmogenesis. Cardiovasc Res 2004; 61: 208-17.

[95] Li F, Wang X, Yi XP, Gerdes AM. Structural basis of ventricular remodeling: role of the myocyte. Curr Heart Fail Rep 2004; 1: 5-8.

1040 Current Pharmaceutical Design, 2015, Vol. 21, No. 8 Szentmiklósi et al.

[96] Osadchii OE. Cardiac hypertrophy induced by sustained �-adrenoreceptor activation: pathophysiological aspects. Heart Fail Rev 2007; 12: 66-86.

[97] Jin H, Lyon AR, Akar FG. Arrhythmia mechanisms in the failing heart. Pacing Clin Electrophysiol 2008; 31: 1048-456.

[98] Liaudet L, Calderari B, Pacher P. Pathophysiological mechanisms of catecholamine and cocaine-mediated cardiotoxicity. Heart Fail Rev 2014; Ahead of print. doi: 10.1007/s.10741-014-9418-y.

[99] Vaughan Williams EM. Classification of antidysrhythmic drugs. Pharmacol Ther B 1975; 1: 115-38.

[100] Vaughan Williams EM. Classification of antiarrhythmic actions re-assessed after a decade of new drugs. J Clin Pharmacol 1984; 24: 129-47.

[101] Vaughan Williams EM. Significance of classifying antiarrhythmic actions since the cardiac arrhythmia suppression trial. J Clin Phar-macol 1991; 31: 123-35.

[102] Hondeghem LM, Katzung BG. Time and voltage-dependent inter-actions of antiarrhythmic drugs with cardiac sodium channels. Bio-chim Biophys Acta 1977; 472: 373-98.

[103] Hondeghem LM, Katzung BG. Antiarrhythmic agents: the modu-lated receptor mechanism of action of sodium and calcium channel-blocking drugs. Annu Rev Pharmacol Toxicol 1984; 24: 387-423.

[104] Nair LA, Grant AO. Emerging class III antiarrhythmic agents: mechanism of action and proarrhythmic potential. Cardiovasc Drugs Ther 1997; 11: 149-67.

[105] DiFrancesco D, Camm JA. Heart rate lowering by specific and selective If current inhibition with ivabradine: a new therapeutic perspective in cardiovascular disease. Drugs 2004; 64: 1757-65.

[106] Kjølbye AL, Dikshteyn M, Eloff BC, Deschênes I, Rosenbaum DS. Maintenance of intercellular coupling by the antiarrhythmic peptide rotigaptide suppresses arrhythmogenic discordant alternans. Am J Physiol 2008; 294: H41-9.

[107] Miura T, Miki T. ATP-sensitive K+ channel openers: old drugs with new clinical benefits for the heart. Curr Vasc Pharmacol 2003; 1: 251-8.

[108] Bigger JT, Sahar DI. Clinical types of proarrhythmic response to antiarrhythmic drugs. Am J Cardiol 1987; 59: E2-9.

[109] Ikeda N, Singh BN, Davis LD, Hauswirth O. Effects of flecainide on the electrophysiological properties of isolated canine and rabbit myocardial fibers. J Am Coll Cardiol 1985; 5: 303-10.

[110] Gintant GA, Cohen IS. Advances in cardiac cellular electrophysi-ology: implications for automaticy and therapeutics. Annu Rev Pharmacol Toxicol 1988; 28: 61-81.

[111] Investigators of the Cardiac Arrhythmia Suppression Trial. Pre-liminary report. Effect of encainide and flecainide on mortality in randomized trial of arrhthmia supression after myocardial infarc-tion. N Eng J Med 1989; 321: 406-12.

[112] Malhotra S, Das MK. Delayed and indirect effects of antiarrhyth-mic drugs in reducing sudden cardiac death. Future Cardiol 2011; 7: 203-17.

[113] Hoffman BF, Dangman KH. Mechanism of cardiac arrhythmias. Experientia 1987; 43: 1049-1056.

[114] Kass RS, Lederer WJ, Tsien RW, Weingart R. Role of calcium ions in transient inward currents and aftercontraction induced by stro-phantidin in cardiac Purkinje fibres. J Physiol 1978; 281: 187-208.

[115] Waldo AL, Camm AJ, deRuyter H, et al. Survival with oral d-sotalol in patients with left ventricular dysfunction after myocardial infarction: rationale, design, and methods (The SWORD Trial) Am J Cardiol 1995; 75: 1023-7.

[116] Lathrop DA, Nánási PP, Varró A. In vitro cardiac models of dog Purkinje fibre triggered and spontaneous electrical activity: effects of nicorandil. Br J Pharmacol 1990; 99: 119-23.

[117] Le Grand B, Hatem S, Le Heuzey JY, Deroubaix E, Benitah JP, Coraboeuf E. Pro-arrhythmic effect of nicorandil in isolated rabbit atria and its suppression by tolbutamide and quinidine. Eur J Phar-macol 1992; 229: 91-6.

[118] Singh BN. �-adrenergic blockers as antiarrhythmic and antifibrilla-tory compounds: an overview. J Cardiovasc Pharmacol Ther 2005; 10: Suppl. 1: S3-14.

[119] Sada H, Ban T. Effects of various structurally related �-adrenoceptor blocking agents on maximum upstroke velocity of ac-tion potential in guinea-pig papillary muscles. Naunyn Schmiede-berg’s Arch Pharmacol 1981; 317: 245-51.

[120] Bankston JR, Kass RS. Molecular determinants of local anesthetic action of �-blocking drugs: Implications for therapeutic manage-

ment of long QT syndrome variant 3. J Mol Cell Cardiol 2010; 48: 246-53.

[121] Varró A, Nánási PP, Lathrop DA. Effect of sotalol on transmem-brane ionic currents responsible for repolarization in cardiac ven-tricular myocytes from rabbit and guinea pig. Life Sci 1991; 49: PL7-12.

[122] Lathrop DA, Varró A, Nánási PP, Bódi I, Takyi E, Pankucsi C. Differences in the effects of d- and dl-sotalol on isolated human ventricular muscle electromechanical activity after �-adrenoceptor stimulation. J Cardiovasc Pharmacol Ther 1996; 1: 65-74.

[123] Lazzara R. From first class to third class: recent upheaval in antiar-rhythmic therapy - lessons from clinical trials. Am J Cardiol 1996; 78: 28-33.

[124] Kowey PR. A review of carvedilol arrhythmia data in clinical trials. J Cardiovasc Pharmacol Ther 2005; 10: Suppl 1: S59-68.

[125] El-Sherif N, Turitto G. Electrophysiologic effects of carvedilol: is carvedilol an antiarrhythmic agent? Pacing Clin Electrophysiol 2005; 28: 985-90.

[126] Naccarelli GV, Lukas MA. Carvedilol's antiarrhythmic properties: therapeutic implications in patients with left ventricular dysfunc-tion. Clin Cardiol 2005; 28: 165-73.

[127] Jeong I, Choi BH, Yoon SH, Hahn SJ. Carvedilol blocks the cloned cardiac Kv1.5 channels in a �-adrenergic receptor-independent manner. Biochem Pharmacol 2012; 83: 497-505.

[128] Lalevée N, Nargeot J, Barrére-Lemaire S, Gautier P, Richard S. Effects of amiodarone and dronedarone on voltage-dependent so-dium current in human cardiomyocytes. J Cardiovasc Electro-physiol 2003; 14: 885-90.

[129] Ding S, Chen F, Klitzner TS, Wetzel GT. Inhibition of L-type Ca2+ channel current in Xenopus oocytes by amiodarone. J Invest Med 2001; 49: 346-52.

[130] Waldhauser KM, Brecht K, Hebeisen S, et al. Interaction with the hERG channel and cytotoxicity of amiodarone and amiodarone analogues. Br J Pharmacol 2008; 155: 585-595.

[131] Chatelain P, Meysmans L, Mattéazzi JR, Beaufort P, Clinet M. Interaction of the antiarrhythmic agents SR 33589 and amiodarone with the beta-adrenoceptor and adenylate cyclase in rat heart. Br J Pharmacol 1995; 116: 1949-56.

[132] Zhou Q, Xiao J, Jiang D, et al. Carvedilol and its new analogs suppress arrhythmogenic store overload-induced Ca2+ release. Nat Med 2011; 17: 1003-9.

[133] Singh BN. Advantages of �-blockers versus antiarrhythmic agents and calcium antagonists in secondary prevention after myocardial infarction. Am J Cardiol 1990; 66: 9C-20.

[134] Zicha S, Tsuji Y, Shiroshita-Takeshita A, Nattel S. �-blockers as antiarrhythmic agents. Handb Exp Pharmacol 2006; 171: 235-66.

[135] Frishman WH, Saunders E. �-adrenergic blockers. J Clin Hypertens (Greenwich) 2011; 13: 649-53.

[136] Bradley HA, Wiysonge CS, Volmink JA, Mayosi BM, Opie LH. How strong is the evidence for use of �-blockers as first-line ther-apy for hypertension? Systematic review and meta-analysis. J Hy-pertens 2006; 24: 2131-41.

[137] Wu A. Should �-blockers still be used as initial antihypertensive agents in uncomplicated hypertension? Ann Acad Med Singapore 2007; 36: 962-4.

[138] Wiysonge CS, Bradley H, Mayosi BM, Maroney R, Mbewu A, Opie LH, Volmink J. �-blockers for hypertension. Cohrane Data-base Syst Rev 2007; 24: CD002003.

[139] De Caterina AR, Leone AM. Why �-blockers should not be used as first choice in uncomplicated hypertension. Am J Cardiol 2010; 105: 1433-8.

[140] Prichard BN, Cruickshank JM, Graham BR. �-adrenergic blocking drugs in the treatment of hypertension. Blood Press 2001; 10: 366-86.

[141] Lardizabal JA, Deedwania PC. The current state of �-blockers in hypertension therapy. Indian Heart J 2010; 62: 111-117.

[142] Brodde OE, Hillemann S, Kunde K, Vogelsang M, Zerkowski HR. Receptor systems affecting force of contraction in the human heart and their alterations in chronic heart failure. J Heart Lung Trans-plant 1992; 11: S164-74.

[143] Cruickshank JM. �-blockers and heart failure. Indian Heart J 2010; 62: 101-10.

[144] Ong HT, Kow FP. �-blockers for heart failure: Why you should use them more. J Family Practice 2011; 60: 472-7.

�-Adrenergic Mechanisms in the Heart Current Pharmaceutical Design, 2015, Vol. 21, No. 8 1041

[145] Hashemzadeh M, Movahed MR, Russu WA, Soroush L, Hill DN. Novel design and synthesis of modified structure of carvedilol. Recent Pat Cardiovasc Drug Discov 2011; 6: 175-9.

[146] Spoladore R, Fragasso G, Perseghin G, et al. Beneficial effects of �-blockers on left ventricular function and cellular energy reserve

in patients with heart failure. Fundam Clin Pharmacol 2013; 27: 455-64.

[147] Coats AJ. �-adrenoceptor antagonists in elderly patients with chronic heart failure: therapeutic potential of third-generation agents. Drugs Aging 2006; 23: 93-9.

Received: July 2, 2014 Accepted: October 24, 2014