vasodilatory mechanisms of beta receptor blockade

ANTIHYPERTENSIVE AGENTS: MECHANISMS OF DRUG ACTION (HM SIRAGYAND B WAEBER, SECTION EDITORS)

Vasodilatory Mechanisms of Beta Receptor Blockade

Géraldine Rath & Jean-Luc Balligand & Dessy Chantal

Published online: 26 May 2012# Springer Science+Business Media, LLC 2012

Abstract Beta-blockers are widely prescribed for the treat-ment of a variety of cardiovascular pathologies. Comparedto traditional beta-adrenergic antagonists, beta-blockers ofthe new generation exhibit ancillary properties such as va-sodilation through different mechanisms. This translatesinto a more favorable hemodynamic profile. The relativeaffinities of beta-adrenoreceptor antagonists towards the threebeta-adrenoreceptor isotypes matter for predicting their func-tional impact on vasomotor control. This review will focus onthe mechanisms underlying beta-blocker-evoked vasorelaxa-tion with a specific emphasis on agonist properties of beta3-adrenergic receptors.

Keywords Hypertension . Beta-blockers . Beta1-blocker .

Vasodilation . Beta3 agonism . Cardiovascular disease .

Arteries

Introduction

Beta-blockers are widely prescribed for the treatment of avariety of cardiovascular pathologies including hyperten-sion, heart failure, primary treatment of myocardial infarc-tion, secondary prevention of ischemic cardiac events aswell as for other non-cardiovascular diseases. Consistentwith so many different beneficial effects, a large variety ofbeta-blockers exist, which differ in their receptor selectivity,their pharmacokinetic and pharmacodynamic properties.The first generation beta-blockers with propranolol are non-selective and block both beta1- and beta2-adrenoceptors. Thesecond generation includes agents like metoprolol and ateno-lol, which are called cardioselective and preferentially blockbeta1-adrenoreceptors. More recently, beta-blockers with ad-ditional ancillary properties have been developed, amongwhich are Nebivolol and Carvedilol. Compared to classicalbeta-antagonists, they promote a vasodilation through differ-ent mechanisms, which translate into a more favorable hemo-dynamic profile compared to non-vasodilating beta-blockers.This review will focus on the mechanism(s) behind this beta-blocker-evoked vasorelaxation.

Underlying Mechanisms of Vasodilation

Endothelium-Dependent Relaxation

The endothelium largely contributes to vascular homeostasisthrough the synthesis and release of several contracting andrelaxing factors. Nitric oxide (NO) is the main vasodilatorreleased from endothelial cells (ECs); the others are prostacy-clin (PGI2) and endothelium-derived hyperpolarizing factor(EDHF).

In the vasculature, the predominant NOS isoform iseNOS, which is responsible for most of the NO production

G. Rath : J.-L. Balligand :D. Chantal (*)Pole de Pharmacologie et Thérapeutique (FATH),Institut de Recherche Expérimentale et Clinique (IREC),Université catholique de Louvain,B01.5309, Avenue Mounier 52,1200 Brussels, Belgiume-mail: [email protected]

G. Rathe-mail: [email protected]

J.-L. Balligande-mail: [email protected]

J.-L. BalligandCliniques Universitaires Saint-Luc,Université catholique de Louvain,B01.5309, Avenue Mounier 52,1200 Brussels, Belgium

Curr Hypertens Rep (2012) 14:310–317DOI 10.1007/s11906-012-0278-3

[1]. Like other NOS isoforms, eNOS generates NO throughthe conversion of L-arginine to L-citrulline, and its activa-tion relies on intracellular calcium concentration as well ason the presence of tetrahydrobiopterin (BH4) and NADPH[2]. NO synthesis is stimulated in response to shear stress orcalcium-mobilizing agonists such as acetylcholine anddilates blood vessels by inducing the formation of cGMPfrom GTP in the underlying smooth muscle cells [3, 4]. Inthe context of this review, the beta-blocker, Nebivolol, wasshown to activate both NO and cGMP production in differ-ent vascular beds (see Fig. 1) [5, 6]. The observation that, in

a rat model of hypertension, carvedilol-induced decreases inblood pressure are associatedwith increased NO plasma levelsalso suggests that Carvedilol relaxation properties partly relateto the NO pathway [7]. Whether this relates to an improvedNO production or a reduced oxidative stress-dependent NOdegradation remains to be resolved. In addition to its vaso-dilating effect, NO inhibits platelet aggregation and adhesion,leukocyte activation and smooth muscle proliferation. Thus,NO is an important actor in the endogenous defense againstvascular injury, inflammation and thrombosis [8]; these latterproperties should be taken into account when considering the

Fig. 1 Schematic overview of adrenoreceptor distribution in endothelialand smooth muscle cells illustrating the potential target pathways induc-ing vasomotion. Relevant interrelationships (→0activators, 0 inhib-itors) between adrenoreceptors and some of the vasodilator substancesproduced by the endothelial cell (EC) and second messenger vasodilatorpathways in the vascular smoothmuscle cell (VSMC) are represented.AAarachidonic acid, α-AR alpha-adrenoreceptors, Akt protein kinase B, ATPadenosine triphosphate, β-AR beta-adrenoreceptors, Ca2+ calcium fromboth voltage- and agonist-dependent channels, CAMKK calmodulin-dependent protein kinase kinase, cAMP adenosine 3′,5′-cyclic mono-phosphate, cGMP cyclic guanosine 3′,5′-monophosphate, COX

cyclooxygenase, DAG diacylglycerol, EDH(F) endothelium-derived hy-perpolarization (or hyperpolarizing factor), Gi inhibitory regulative G-protein, Gq heterotrimeric G protein subunit q, Gs stimulative regulativeG-protein, GTP guanosine triphosphate, IP3 inositol 1,4,5-triphosphate,K+ potassium from both voltage- and agonist-dependent channels, L-argL-arginine, NADPH nicotinamide adenine dinucleotide phosphate, NOnitric oxide, NOS nitric oxide synthase, PGH2 prostaglandin H2, PGI2prostacyclin, PI3K phosphoinositide-3-kinase, PIP2 phosphatidylinositolbisphosphate, PKA protein kinase-A, PKG protein kinase-G, PL (C, D,A2) phospholipase (C, D, A2), ROS reactive oxygen species, S.R. 0sarcoplasmic reticulum

Curr Hypertens Rep (2012) 14:310–317 311

potential beneficial effects of drugs (beta-blockers in this case)that promote the NO pathway.

Prostacyclin is a product of cyclooxygenase (COX)formed from arachidonic acid in ECs and causes relaxationvia activation of an IP receptor (prostaglandin I2 receptor)[9] stimulating adenylate cyclase and increasing the intra-cellular cAMP level [10]. Despite the fact that its contributionto endothelium-dependent relaxation is not preeminent, pros-tacyclin, like NO, has antiplatelet and antithrombotic activity.They act synergistically, and both effects are tightly relatedsince PGI2 potentiates NO release and NO potentiates PGI2'seffect on vascular smooth muscle cells (VSMCs) [11]. Thelink between beta-adrenoceptors and prostacyclin productionin the endothelium is ambiguous as beta-adrenergic stimula-tion promotes prostacyclin synthesis, which is countered by acAMP-dependent inhibition of phospholipase D activity [12].

A third pathway that is independent of the two previouslycited is termed “endothelium-derived hyperpolarizing fac-tor” (EDHF). The name may be confusing since both NOand/or PGI2 are able to hyperpolarize the underlying smoothmuscle cells [13, 14], and the phenomenon is increasinglydescribed as simply “endothelium-dependent hyperpolariza-tion” (EDH) [15]. What is meant with EDH(F) is the hyper-polarization of both ECs and VSMCs, which requires anintracellular Ca2+ increase in ECs, a subsequent opening ofsmall and intermediate calcium-activated potassium chan-nels (SKCa and IKCa) and the spread of hyperpolarizationthroughout the media via myoendothelial gap junctionstoward VSMCs [16]. Despite remaining controversies re-garding the mediators involved, an increasing amount ofexperimental evidence suggests that EDH(F) contributes tocontrol blood flow, especially in resistance arteries, andsystemic blood pressure [17•]. EDH(F) is a potential, stillunder-explored, pharmaceutical target that participates inthe beta3-adrenoceptor-mediated vasodilation (see belowfor details and Fig. 1).

Endothelium-Independent Relaxation

In addition to endothelium-dependent relaxation, vasodila-tion also occurs through a direct effect on smooth musclecells. VSMC tone is highly dependent on the membranepotential, which is essentially determined by K+ channelactivity. The general working of these channels is as fol-lows: once activated, the resultant increased K+ effluxcauses membrane potential hyperpolarization, which closesvoltage-dependent Ca2+ channels and therefore decreasesCa2+ entry, leading to VSMC relaxation. Among thesechannels, voltage-gated potassium channels (Kv), high con-ductance Ca2+-activated channels (BKCa) as well as ATP-sensitive K+ (KATP) channels contribute to the majority ofthe K+ current [18]. In the context of the present review thelatter are of highest interest as several reports implicated

KATP channel activation in beta-adrenoreceptor-mediatedvasodilation. Indeed, using isoprenaline together with theKATP channel inhibitor glibenclamide on rat mesentericarteries, Randall and McCulloch showed that the vasodilatorpotency of this beta-adrenoreceptor agonist is coupled to theopening of KATP channels. They additionally demonstratedthat both beta1- and beta2-adrenoreceptors are implicatedsince dobutamine and terbutaline (beta1 and beta2-agonists,respectively) gave similar results [19]. Notably, the under-lying pathway was described by Wellman, Quayle andStanden. They showed that KATP channel activation bybeta-adrenoreceptors occurs via stimulation of PKA, result-ing from adenylyl cyclase-mediated cAMP generation [20,21] (see Fig. 1). In opposition to the endothelium-independent process, a very recent study demonstrated thatfollowing focal beta-adrenoceptor stimulation, a KATP andendothelium-related hyperpolarization underlies the abilityof vasodilatation to spread along the artery wall [22]. Thislatter mechanism might also be of physiological and phar-macological relevance when considering vasorelaxing prop-erties of beta-adrenoceptor modulators. Of note, it has beenshown in cat atrial myocytes that beta1-adrenoreceptor ex-clusively couples via Gs-protein to adenylate cyclase to stim-ulate cAMP synthesis, while beta2-adrenoreceptor alsocouples, via Gi-protein and PI3K/Akt signaling, to NOproduction and cGMP activation [23]. This finding is ofparticular interest as it suggests that the vasodilation providedby beta-adrenoreceptor stimulation may involve bothendothelium-independent (cAMP pathway) and -dependent(eNOS stimulation and cGMP pathway activation) mecha-nisms. This point will be developed later on.

Αdrenoreceptor Distribution in Heart and Vessels

Adrenoreceptors belong to the family of G-protein coupledreceptors stimulated by catecholamines and are found innearly all peripheral tissue membranes. When after theirdiscovery divergent pharmacological characteristics andmolecular differences were rapidly observed, adrenorecep-tors were first divided into two major types, namely alphaand beta [24]. Subsequently, both types were subdividedinto alpha1, alpha2, beta1 and beta2 subtypes and furtherdivided into at least three groups [25]. Finally, the beta-adrenoceptor family was enriched with a third membercloned in 1989 [26]: the beta3-adrenoreceptor, which is ofparticular interest in the present review as it has been linkedto beta-associated vasodilation.

The Alpha-Adrenoreceptors

As the common and most described function of alpha-adrenoreceptor is the vasoconstriction of arteries and veins,

312 Curr Hypertens Rep (2012) 14:310–317

they will only briefly be considered here. It has been widelydemonstrated in animals and humans that both alpha1- andalpha2-adrenoceptors contribute to coronary vasoconstric-tion. Using large and small canine coronary arteries, Heuschand coworkers interestingly showed that alpha1-adrenore-ceptor rather induces vasoconstriction of conduit vessels,whereas alpha2-adrenoreceptor is predominant in mediatingmicrovascular contraction of resistance vessels [27]. Addi-tionally, both alpha-adrenoreceptor subtypes have been ob-served in endothelial and smooth muscle cells in variousvascular beds with perhaps a preferential location of alpha1-adrenoreceptors in smooth muscle cells, whereas alpha2-adrenoreceptors may be more expressed in the endothelium[28, 29]. Importantly, both subtypes can interact, as illus-trated in rat tail arteries by Xiao and Rand, who showed thatalpha2-adrenoreceptor enhances the agonist response to al-pha1-adrenoreceptor through a Ca2+-dependent mechanism[30].

Perhaps more relevant is the fact that despite extensivedescription of their contracting properties, alpha-adrenoreceptors are also proposed as indirect vasodilators.Although the underlying mechanisms may still be somewhatdisputed [31, 32], endothelial alpha2-adrenoreceptors wereclearly shown to couple to NOS activation [33]. The aboveproperties help interpreting the effect of combined alpha1and beta-antagonism of Carvedilol (and labetalol to a lesserextent). Although it cannot fully explain all carvedilol an-cillary properties, alpha1 blockade could partially accountfor carvedilol-induced vasorelaxation [7] and reduced vas-cular resistance.

The Beta-Adrenoreceptors

The technical advances provided by, first, autoradiographyand, later, fluorescent labeling and molecular cloning,yielded definitive identification of beta-adrenoceptor identi-ty and distribution at the cellular and tissue level. Indeed,Lipe and Summers could demonstrate, using autoradiogra-phy, that although both beta1- and beta2-adrenoreceptorswere represented in most vascular beads, beta2-adrenorecep-tors were predominant in the dog splenic artery and vein[34]. With the same method, they examined the distributionand quantity of beta1 and beta2-adrenoreceptors in canineconductance coronary arteries. They showed that both re-ceptor subtypes were present in a 85:15 % ratio in thesmooth muscle cells and that beta2-adrenoreceptors wereadditionally located in nerve tissue and adventitia [35].Nevertheless, the same group evidenced that in humaninternal mammary artery, beta2-adrenoreceptors had a ratherendothelial location with a low density in the smooth musclecells, whereas in the saphenous vein the same receptorswere found only on the outer smooth muscle [36]. Morerecently, Chruscinski et al performed relaxation assays on

several vessel types from mice lacking beta1-adrenorecep-tor, beta2-adrenoreceptor or both receptors. They surprising-ly showed that in mice, beta1-adrenoreceptors predominatein most vessel types including the femoral, pulmonary andsuperior mesenteric arteries, as well as in femoral and jug-ular veins. However, in large conduit arteries and in theportal vein, both adrenoreceptors mediated adrenergicvasodilation [37].

Based on the functional evidence that beta1- and beta2-adrenoreceptors display a distinct reaction to inhibitory orstimulatory modulation, the potential compartmentalizationof beta-adrenoreceptor subtypes to membrane subdomainssuch as caveolae has also been investigated. Rybin et al. forinstance identified the caveolae as a site to assemble func-tionally active beta-adrenoreceptor complexes in rat cardio-myocytes. Using subcellular fractioning they evidenced thatbeta2-adrenoreceptors are confined to the caveolae, whereasbeta1-adrenoreceptors are detected across the sucrose gradi-ent, thus also located in non-caveolar plasma membranefractions [38]. These results were further confirmed usingfilipin, a caveolar-disrupting agent that selectively affectedthe functional properties of the beta2-adrenoreceptor in car-diac myocytes [39].

Concerning the location of the beta3-adrenoreceptor on avascular level, many studies concur to propose its endothe-lial location as the removal of the endothelium stronglyimpairs beta3-adrenoreceptor activation. This was shown inseveral models, such as in rat thoracic aorta [40], in humancoronary microarteries [41, 42] and in human mammaryarteries [43]. Endothelial expression of the beta3 isotypewas also confirmed by transcription-polymerase chain reac-tion assay in endothelial cells isolated from human cardiactissue by laser capture [41]. Conversely, Viard and col-leagues, using similar transcription-polymerase chain reac-tion assays, showed that beta3-adrenoreceptors wereexpressed in rat portal vein myocytes together with beta1-and beta2-adrenoreceptors [44]. The presence of functionalbeta3-adrenoceptors has also been demonstrated in the ratretinal vascular bed. As in other vascular beds, their stimu-lation mediates a vasodilation; however, their specific loca-tion (endothelium versus smooth muscle cells) was notcompletely characterized [45].

In the heart, the anatomical location of beta3-adrenore-ceptor has been little studied so far; beta3-adrenoreceptorsare clearly expressed in cardiac myocytes, including inhumans [46, 47] and several other species [47]; althoughwe did not find any difference of beta3 expression amongthe different layers of the human ventricular muscle, theabundance of beta3-adrenoreceptor strickingly increases inseveral forms of cardiomyopathies (compared with non-diseased hearts) [48].

A summary of the beta-adrenoreceptor location in thevascular compartments is proposed in Fig. 1, which helps


to interpret the effects of integrated molecular and pharmaco-logical events resulting from the diverse abundance and loca-tions of beta-adrenoreceptor in normal and disease states.

Pharmacology of Beta-Blockers and Incidenceon Vasodilatation

As outlined above, the relative affinities of beta-adrenoreceptorantagonists towards the three beta-adrenoreceptor isotypesmatter for predicting their functional impact on vasomotorcontrol. This in turn will influence both the efficacy of anyspecific beta-blocker on cardiovascular disease and the inci-dence of side effects, particularly on tissue perfusion.

Since the early development of beta-adrenoreceptorantagonists following the pioneering work of Sir J. Black[49], the therapeutic armamentum has been enriched fromnon-specific pan-beta-adrenoreceptor antagonists to beta-adrenoreceptor-specific blockers that mostly target the be-ta1-adrenoreceptor. This is indeed the isotype that has main-ly been involved in the anti-anginal and anti-hypertensiveeffect of beta-blockers by opposing the inotropic effect ofcatecholamines on cardiac muscle. The same effect wassubsequently demonstrated to confer beneficial effects inthe setting of heart failure, probably by preventing fromadverse myocardial remodeling, protecting failing cardiacmyocytes from the toxicity of neurohormonal overstimula-tion, as well as by restoring beta-adrenoreceptor expressionand density, resulting in restoration of heart rate variability[50].

The use of the first beta1-adrenoreceptor “preferential”antagonists, however, has exposed patients to common sideeffects due both to the expected beta1-adrenoreceptor block-ade (e.g., bradycardia), but also to off-target effects attrib-utable to targeting of the other isotypes [51]. Accordingly,blockade of beta2-adrenoreceptor (and possibly beta3-adre-noreceptor) is classically associated with bronchospasm aswell as peripheral vasoconstriction due to loss of beta2 (and,possibly beta3)-adrenoreceptor-mediated relaxation of smoothmuscles [51].

Further drug development led to the introduction of beta-blockers with either better selectivity for the beta1-adrenor-eceptor, ancillary vasodilating properties or both. Amongthe first, drugs such as metoprolol have proven their betterbeta1-adrenoreceptor selectivity with lower vascular orbronchial side effects, without compromising efficacy [51].Other less (or non-) selective beta-blockers, such as carve-dilol, are endowed with additional properties that compen-sate for the lack of selectivity, e.g., through alpha-adrenoceptorblockade and possibly antioxidant properties, both ofwhich may preserve vasodilatation (on top of antagonizingadverse effects of adrenergic tone on oxidant stress and tissueremodeling) [7].

In this context, drugs that improve endothelial functionhave received recent renewed attention, especially in thecontext of evidence demonstrating the prognostic value ofendothelial dysfunction and its pathophysiologic role in thedevelopment of cardiovascular disease, e.g., atherosclerosis,ischemic cardiovascular disease and heart failure. Some“third-generation” beta-adrenoreceptor blockers were in-deed shown to influence endothelial function through eithertargeting of specific beta-adrenoreceptor isotypes or otherreceptors on the endothelial cells.

Celiprolol, a beta1-specific antagonist, was shown toexert agonist properties on beta2-adrenoreceptors, whichhave been implicated in enhanced endothelial NO synthesis,vascular anti-oxidant effects and preserved vasodilatation[52]. Whether this translates into better protection fromcardiovascular morbidity/mortality in patients remains anunanswered question.

Endothelial function involves more than NO synthesisand production (as outlined in the previous sections). In thisregard, another recent beta-blocker, Nebivolol, may presentthe most pleiotropic vasculoprotective properties. It is high-ly selective for beta1-adrenoreceptor blockade (>200-foldover beta2) [53]; in addition, in several animal but alsohuman vessels, it activates beta3-adrenoreceptor [42], which(as outlined above) is expected to activate not only NOproduction from eNOS, but also to recruit additional com-ponents of endothelium-derived vasodilatation, such as“EDH(F)(s).” Importantly, these vasodilatory propertieswere demonstrated in human coronary microvessels, butalso in human mammary arteries from diseased patients[41–43], emphasizing their potential therapeutic relevance.Finally, as beta3-adrenoreceptors were also shown to beexpressed in cardiac myocytes from human ventricles [47],their activation of myocardial NO production would pro-mote NO-cGMP-mediated relaxation as well as protectionagainst excessive catecholamine stimulation that, togetherwith coronary vasodilatation, would combine beneficialeffects on the myocardial oxygen supply and demand topreserve ventricular function [54•].

Again, whether this translates to superior protectionfrom morbidity/mortality in patients needs to be demon-strated with a “head-to-head” comparison of Nebivololwith a pure beta1-adrenoreceptor blocker in a prospec-tive, randomized trial. So far, the SENIORS trial in elderlypatients with moderate heart failure only compared Nebiovo-lol with placebo, and showed an improvement in clinicalsymptoms [55] (as previously demonstrated with other beta-blockers in HF trials). This suggests that if beta3 agonistproperties are operative in vivo, they at least do not negatethe expected beneficial effects of beta1-adrenoreceptorblockade.

Moreover, the comparative efficacy of Nebivolol and the“pure” beta1-adrenoreceptor blocker, Metoprolol, has been


tested on the protection against adverse remodeling andmortality in a mouse model of myocardial infarction. Thisshowed the superior efficacy of Nebivolol against hypertro-phic remodeling of the remaining viable myocardium, betterpreserved endothelium-dependent relaxation ex-vivo andendothelial progenitor cells function [56]. Importantly, theuse of a mouse model allowed the direct comparison ofNebivolol protection in wild-type mice and mice with ge-netic deletion of eNOS; Nebivolol’s effect was lost in thelatter, showing an obligatory role of eNOS, at least in themouse [57]. This could be correlated with Nebibolol’s prop-erties to prevent oxidative stress in isolated endothelial cellsthat were lost upon full beta1-2–3 blockade [58•].

In the next section, we will outline the experimentalevidence for these ancillary properties of Nebivolol (as wellas other specific beta3-adrenoreceptor agonists) on endothe-lial function in animal and human vessels.

Beta3-Adrenoceptor and Vasorelaxation

The observation that Nebivolol dose-dependently relaxedpreconstricted rodent coronary resistance microarteries butfailed to do so in microarteries isolated from beta3-adrenoreceptor-deficient mice demonstrates a beta3 agonismfor Nebivolol [42]. Additional pharmacologic evidence sup-ports this hypothesis as Nebivolol-induced relaxation ofhuman coronary microvessels was insensitive to beta(1–2)-blockade [42]. Similarly, Nebivolol-induced relaxation ofthe rat thoracic aorta is resistant to beta2 blockade, butinhibited by S-(−)- Cyanopindolol or by L-748,337, a selec-tive beta3-adrenoceptor antagonist [59, 60 and 61•]. Asdescribed earlier, the beta3-adrenoreceptor-subtype is main-ly expressed on the vascular endothelium, consistent withfunctional evidence. Indeed, several studies described theloss of beta3-dependent vasodilation in endothelium-denuded vessels [42, 60]. However, BRL 37344, a selectivebeta3-adrenoceptor agonist, was shown to evoke a relaxationof endothelium-denuded human internal mammary arteries,suggesting that in some vascular beds, an endothelium-independent pathway may exist [62]. NOS inhibitionstrongly reduces the relaxation of human coronary micro-arteries and rat aorta to Nebivolol or BRL37344 [42, 60];likewise L-NAME attenuated vasorelaxing responses toboth the beta3-adrenoceptor agonist BRL 37344 and non-specific isoprenaline in rat carotid arteries [63]. In culturedendothelial cells, beta3-adrenoceptor stimulation with Nebi-volol increased NO release as measured by electron para-magnetic resonance spin trapping. In parallel, fura-2calcium fluorescence was increased and endothelial NOSwas dephosphorylated on threonine(495/497) [42]. Similar-ly, epinephrine-dependent eNOS activation through beta(3)-adrenoreceptor signaling is accompanied by an increase in

phosphorylation of eNOS at Ser(1179) and with decreasedeNOS phosphorylation at the inhibitory phosphorylationsites Ser(116) and Thr(497) [64], confirming that beta3adrenoceptor vasodilation is associated the endothelial syn-thesis/release role of nitric oxide. In the context of NO-mediated response to Nebivolol, it is also worth mentioningthat Nebivolol promotes angiogenesis in a NOS/beta3-dependent manner, which could be of particular interestin the course of treatment of ischemia-associated dis-eases. Besides, in human coronary resistance arteries, inthe presence of NOS and Cox inhibition, an endothelium-dependent relaxation to beta3-adrenoceptor stimulation per-sists. BRL37344-mediated hyperpolarization of smoothmuscle cell membrane and the inhibitory effect of the com-bination of apamin and charybotoxin concur to suggestthat beta3-adrenoceptor stimulation with BRL37344-evokedrelaxation is partly mediated via an endothelium-derived hy-perpolarization (EDH(F)). This could be a real advantagein pathological situations associated with reduced NObioavalailability.

Of note, Nebivolol is used as a racemate mixture of thetwo enantiomers D-Nebivolol (+SRRR Nebivolol) and L-Nebivolol (−RSSS nebivolol), which present different phar-macological properties. Both Nebivolol enantiomers pro-duced a vasorelaxation through activation of beta3-adrenoceptors. However, D-Nebivolol-produced vasorelaxa-tion is also mediated by activation of beta2-adrenoceptorsand antagonism of alpha1-adrenoceptors [61•]. This couldbroaden the clinical interest of Nebivolol as beta2 agonismcould alleviate the contraindication of beta-blockade in ob-structive respiratory disease.

Conclusion

In conclusion, beta-blockers are widely used for the treat-ment of hypertension. The latest generation of beta-blockersexhibits ancillary properties, such as Nebivolol, which dem-onstrates additive pleiotropic vasculoprotective effects incomparison to traditional beta-antagonists. In particular,Nebivolol exerts agonist activity on beta3 adrenoceptors,which mediates endothelium-dependent modulation of vas-cular tone. As nitric oxide release promotes vasodilation andangiogenesis, this suggests that beta3-adrenoceptors couldrepresent a new therapeutic target to improve the hemody-namic profile in hypertension.

Acknowledgments This work was supported by an Action de Re-cherche Concertée (06/11-338), Pôle d’Attraction Interuniversitaire(IUAP P6/30) of the Politique Scientifique Fédérale, Waleo, from theRegion Wallonne, and the Fonds National de la Recherche Scientifi-que. CD is Senior Research Associate of the FNRS.


Disclosure Drs. Rath, Balligand and Dessy reported no potentialconflicts of interest relevant to this article.

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