Vasodilatory Mechanisms of Beta Receptor Blockade Géraldine Rath

Vasodilatory Mechanisms of Beta Receptor Blockade

Géraldine Rath, Jean-Luc Balligand & Dessy Chantal

ISSN 1522-6417 Volume 14 Number 4

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

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

ANTIHYPERTENSIVE AGENTS: MECHANISMS OF DRUG ACTION (HM SIRAGY AND 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- Introduction ment of a variety of cardiovascular pathologies. Compared to traditional beta-adrenergic antagonists, beta-blockers of Beta-blockers are widely prescribed for the treatment of a the new generation exhibit ancillary properties such as va- variety of cardiovascular pathologies including hyperten- sodilation through different mechanisms. This translates sion, heart failure, primary treatment of myocardial infarc- into a more favorable hemodynamic profile. The relative tion, secondary prevention of ischemic cardiac events as affinities of beta-adrenoreceptor antagonists towards the three well as for other non-cardiovascular diseases. Consistent beta-adrenoreceptor isotypes matter for predicting their func- with so many different beneficial effects, a large variety of tional impact on vasomotor control. This review will focus on beta-blockers exist, which differ in their receptor selectivity, the mechanisms underlying beta-blocker-evoked vasorelaxa- their pharmacokinetic and pharmacodynamic properties. tion with a specific emphasis on agonist properties of beta3- The first generation beta-blockers with propranolol are non- adrenergic receptors. selective and block both beta1- and beta2-adrenoceptors. The second generation includes agents like metoprolol and ateno-

Keywords Hypertension . Beta-blockers . Beta1-blocker . lol, which are called cardioselective and preferentially block Vasodilation . Beta3 agonism . Cardiovascular disease . beta1-adrenoreceptors. More recently, beta-blockers with ad- Arteries ditional ancillary properties have been developed, among which are Nebivolol and Carvedilol. Compared to classical beta-antagonists, they promote a vasodilation through different mechanisms, which translate into a more favorable hemodynamic profile compared to non-vasodilating beta-blockers. This review will focus on the mechanism(s) behind this beta- blocker-evoked vasorelaxation. G. Rath : J.-L. Balligand : D. Chantal (*) Pole de Pharmacologie et Thérapeutique (FATH), Institut de Recherche Expérimentale et Clinique (IREC), Université catholique de Louvain, Underlying Mechanisms of Vasodilation B01.5309, Avenue Mounier 52, 1200 Brussels, Belgium e-mail: [email protected] Endothelium-Dependent Relaxation G. Rath e-mail: [email protected] The endothelium largely contributes to vascular homeostasis through the synthesis and release of several contracting and J.-L. Balligand e-mail: [email protected] relaxing factors. Nitric oxide (NO) is the main vasodilator released from endothelial cells (ECs); the others are prostacy- J.-L. Balligand clin (PGI2) and endothelium-derived hyperpolarizing factor Cliniques Universitaires Saint-Luc, (EDHF). Université catholique de Louvain, B01.5309, Avenue Mounier 52, In the vasculature, the predominant NOS isoform is 1200 Brussels, Belgium eNOS, which is responsible for most of the NO production Author's personal copy

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[1]. Like other NOS isoforms, eNOS generates NO through a rat model of hypertension, carvedilol-induced decreases in the conversion of L-arginine to L-citrulline, and its activa- blood pressure are associated with increased NO plasma levels tion relies on intracellular calcium concentration as well as also suggests that Carvedilol relaxation properties partly relate on the presence of tetrahydrobiopterin (BH4) and NADPH to the NO pathway [7]. Whether this relates to an improved [2]. NO synthesis is stimulated in response to shear stress or NO production or a reduced oxidative stress-dependent NO calcium-mobilizing agonists such as acetylcholine and degradation remains to be resolved. In addition to its vaso- dilates blood vessels by inducing the formation of cGMP dilating effect, NO inhibits platelet aggregation and adhesion, from GTP in the underlying smooth muscle cells [3, 4]. In leukocyte activation and smooth muscle proliferation. Thus, the context of this review, the beta-blocker, Nebivolol, was NO is an important actor in the endogenous defense against shown to activate both NO and cGMP production in differ- vascular injury, inflammation and thrombosis [8]; these latter ent vascular beds (see Fig. 1)[5, 6]. The observation that, in properties should be taken into account when considering the

Fig. 1 Schematic overview of adrenoreceptor distribution in endothelial cyclooxygenase, DAG diacylglycerol, EDH(F) endothelium-derived hy- and smooth muscle cells illustrating the potential target pathways induc- perpolarization (or hyperpolarizing factor), Gi inhibitory regulative G- ing vasomotion. Relevant interrelationships (→0activators, 0 inhib- protein, Gq heterotrimeric G protein subunit q, Gs stimulative regulative itors) between adrenoreceptors and some of the vasodilator substances G-protein, GTP guanosine triphosphate, IP3 inositol 1,4,5-triphosphate, produced by the endothelial cell (EC) and second messenger vasodilator K+ potassium from both voltage- and agonist-dependent channels, L-arg pathways in the vascular smooth muscle cell (VSMC) are represented. AA L-arginine, NADPH nicotinamide adenine dinucleotide phosphate, NO arachidonic acid, α-AR alpha-adrenoreceptors, Akt protein kinase B, ATP nitric oxide, NOS nitric oxide synthase, PGH2 prostaglandin H2, PGI2 adenosine triphosphate, β-AR beta-adrenoreceptors, Ca2+ calcium from prostacyclin, PI3K phosphoinositide-3-kinase, PIP2 phosphatidylinositol both voltage- and agonist-dependent channels, CAMKK calmodulin- bisphosphate, PKA protein kinase-A, PKG protein kinase-G, PL (C, D, dependent protein kinase kinase, cAMP adenosine 3′,5′-cyclic mono- A2) phospholipase (C, D, A2), ROS reactive oxygen species, S.R. 0 phosphate, cGMP cyclic guanosine 3′,5′-monophosphate, COX sarcoplasmic reticulum Author's personal copy

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potential beneficial effects of drugs (beta-blockers in this case) KATP channel activation in beta-adrenoreceptor-mediated that promote the NO pathway. vasodilation. Indeed, using isoprenaline together with the

Prostacyclin is a product of cyclooxygenase (COX) KATP channel inhibitor glibenclamide on rat mesenteric formed from arachidonic acid in ECs and causes relaxation arteries, Randall and McCulloch showed that the vasodilator via activation of an IP receptor (prostaglandin I2 receptor) potency of this beta-adrenoreceptor agonist is coupled to the

[9] stimulating adenylate cyclase and increasing the intra- opening of KATP channels. They additionally demonstrated cellular cAMP level [10]. Despite the fact that its contribution that both beta1- and beta2-adrenoreceptors are implicated to endothelium-dependent relaxation is not preeminent, pros- since dobutamine and terbutaline (beta1 and beta2-agonists, tacyclin, like NO, has antiplatelet and antithrombotic activity. respectively) gave similar results [19]. Notably, the under- They act synergistically, and both effects are tightly related lying pathway was described by Wellman, Quayle and since PGI2 potentiates NO release and NO potentiates PGI2's Standen. They showed that KATP channel activation by effect on vascular smooth muscle cells (VSMCs) [11]. The beta-adrenoreceptors occurs via stimulation of PKA, result- link between beta-adrenoceptors and prostacyclin production ing from adenylyl cyclase-mediated cAMP generation [20, in the endothelium is ambiguous as beta-adrenergic stimula- 21](seeFig.1). In opposition to the endothelium- tion promotes prostacyclin synthesis, which is countered by a independent process, a very recent study demonstrated that cAMP-dependent inhibition of phospholipase D activity [12]. following focal beta-adrenoceptor stimulation, a KATP and A third pathway that is independent of the two previously endothelium-related hyperpolarization underlies the ability cited is termed “endothelium-derived hyperpolarizing fac- of vasodilatation to spread along the artery wall [22]. This tor” (EDHF). The name may be confusing since both NO latter mechanism might also be of physiological and phar- and/or PGI2 are able to hyperpolarize the underlying smooth macological relevance when considering vasorelaxing prop- muscle cells [13, 14], and the phenomenon is increasingly erties of beta-adrenoceptor modulators. Of note, it has been described as simply “endothelium-dependent hyperpolariza- shown in cat atrial myocytes that beta1-adrenoreceptor ex- tion” (EDH) [15]. What is meant with EDH(F) is the hyper- clusively couples via Gs-protein to adenylate cyclase to stim- polarization of both ECs and VSMCs, which requires an ulate cAMP synthesis, while beta2-adrenoreceptor also intracellular Ca2+ increase in ECs, a subsequent opening of couples, via Gi-protein and PI3K/Akt signaling, to NO small and intermediate calcium-activated potassium chan- production and cGMP activation [23]. This finding is of nels (SKCa and IKCa) and the spread of hyperpolarization particular interest as it suggests that the vasodilation provided throughout the media via myoendothelial gap junctions by beta-adrenoreceptor stimulation may involve both toward VSMCs [16]. Despite remaining controversies re- endothelium-independent (cAMP pathway) and -dependent garding the mediators involved, an increasing amount of (eNOS stimulation and cGMP pathway activation) mecha- experimental evidence suggests that EDH(F) contributes to nisms. This point will be developed later on. control blood flow, especially in resistance arteries, and systemic blood pressure [17•]. EDH(F) is a potential, still under-explored, pharmaceutical target that participates in Αdrenoreceptor Distribution in Heart and Vessels the beta3-adrenoceptor-mediated vasodilation (see below for details and Fig. 1). Adrenoreceptors belong to the family of G-protein coupled receptors stimulated by catecholamines and are found in Endothelium-Independent Relaxation nearly all peripheral tissue membranes. When after their discovery divergent pharmacological characteristics and In addition to endothelium-dependent relaxation, vasodila- molecular differences were rapidly observed, adrenorecep- tion also occurs through a direct effect on smooth muscle tors were first divided into two major types, namely alpha cells. VSMC tone is highly dependent on the membrane and beta [24]. Subsequently, both types were subdivided + potential, which is essentially determined by K channel into alpha1, alpha2, beta1 and beta2 subtypes and further activity. The general working of these channels is as fol- divided into at least three groups [25]. Finally, the beta- lows: once activated, the resultant increased K+ efflux adrenoceptor family was enriched with a third member causes membrane potential hyperpolarization, which closes cloned in 1989 [26]: the beta3-adrenoreceptor, which is of voltage-dependent Ca2+ channels and therefore decreases particular interest in the present review as it has been linked Ca2+ entry, leading to VSMC relaxation. Among these to beta-associated vasodilation. channels, voltage-gated potassium channels (Kv), high con- 2+ ductance Ca -activated channels (BKCa) as well as ATP- The Alpha-Adrenoreceptors + sensitive K (KATP) channels contribute to the majority of the K+ current [18]. In the context of the present review the As the common and most described function of alpha- latter are of highest interest as several reports implicated adrenoreceptor is the vasoconstriction of arteries and veins, Author's personal copy

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they will only briefly be considered here. It has been widely several vessel types from mice lacking beta1-adrenorecep- demonstrated in animals and humans that both alpha1- and tor, beta2-adrenoreceptor or both receptors. They surprising- alpha2-adrenoceptors contribute to coronary vasoconstric- ly showed that in mice, beta1-adrenoreceptors predominate tion. Using large and small canine coronary arteries, Heusch in most vessel types including the femoral, pulmonary and and coworkers interestingly showed that alpha1-adrenore- superior mesenteric arteries, as well as in femoral and jug- ceptor rather induces vasoconstriction of conduit vessels, ular veins. However, in large conduit arteries and in the whereas alpha2-adrenoreceptor is predominant in mediating portal vein, both adrenoreceptors mediated adrenergic microvascular contraction of resistance vessels [27]. Addi- vasodilation [37]. tionally, both alpha-adrenoreceptor subtypes have been ob- Based on the functional evidence that beta1- and beta2- served in endothelial and smooth muscle cells in various adrenoreceptors display a distinct reaction to inhibitory or vascular beds with perhaps a preferential location of alpha1- stimulatory modulation, the potential compartmentalization adrenoreceptors in smooth muscle cells, whereas alpha2- of beta-adrenoreceptor subtypes to membrane subdomains adrenoreceptors may be more expressed in the endothelium such as caveolae has also been investigated. Rybin et al. for [28, 29]. Importantly, both subtypes can interact, as illus- instance identified the caveolae as a site to assemble func- trated in rat tail arteries by Xiao and Rand, who showed that tionally active beta-adrenoreceptor complexes in rat cardio- alpha2-adrenoreceptor enhances the agonist response to al- myocytes. Using subcellular fractioning they evidenced that 2+ pha1-adrenoreceptor through a Ca -dependent mechanism beta2-adrenoreceptors are confined to the caveolae, whereas [30]. beta1-adrenoreceptors are detected across the sucrose gradi- Perhaps more relevant is the fact that despite extensive ent, thus also located in non-caveolar plasma membrane description of their contracting properties, alpha- fractions [38]. These results were further confirmed using adrenoreceptors are also proposed as indirect vasodilators. filipin, a caveolar-disrupting agent that selectively affected

Although the underlying mechanisms may still be somewhat the functional properties of the beta2-adrenoreceptor in car- disputed [31, 32], endothelial alpha2-adrenoreceptors were diac myocytes [39]. clearly shown to couple to NOS activation [33]. The above Concerning the location of the beta3-adrenoreceptor on a properties help interpreting the effect of combined alpha1 vascular level, many studies concur to propose its endothe- and beta-antagonism of Carvedilol (and labetalol to a lesser lial location as the removal of the endothelium strongly extent). Although it cannot fully explain all carvedilol an- impairs beta3-adrenoreceptor activation. This was shown in cillary properties, alpha1 blockade could partially account several models, such as in rat thoracic aorta [40], in human for carvedilol-induced vasorelaxation [7] and reduced vas- coronary microarteries [41, 42] and in human mammary cular resistance. arteries [43]. Endothelial expression of the beta3 isotype was also confirmed by transcription-polymerase chain reac- The Beta-Adrenoreceptors tion assay in endothelial cells isolated from human cardiac tissue by laser capture [41]. Conversely, Viard and col- The technical advances provided by, first, autoradiography leagues, using similar transcription-polymerase chain reac- and, later, fluorescent labeling and molecular cloning, tion assays, showed that beta3-adrenoreceptors were yielded definitive identification of beta-adrenoceptor identi- expressed in rat portal vein myocytes together with beta1- ty and distribution at the cellular and tissue level. Indeed, and beta2-adrenoreceptors [44]. The presence of functional Lipe and Summers could demonstrate, using autoradiogra- beta3-adrenoceptors has also been demonstrated in the rat phy, that although both beta1- and beta2-adrenoreceptors retinal vascular bed. As in other vascular beds, their stimu- were represented in most vascular beads, beta2-adrenorecep- lation mediates a vasodilation; however, their specific loca- tors were predominant in the dog splenic artery and vein tion (endothelium versus smooth muscle cells) was not [34]. With the same method, they examined the distribution completely characterized [45]. and quantity of beta1 and beta2-adrenoreceptors in canine In the heart, the anatomical location of beta3-adrenore- conductance coronary arteries. They showed that both receptor has been little studied so far; beta3-adrenoreceptors ceptor subtypes were present in a 85:15 % ratio in the are clearly expressed in cardiac myocytes, including in smooth muscle cells and that beta2-adrenoreceptors were humans [46, 47] and several other species [47]; although additionally located in nerve tissue and adventitia [35]. we did not find any difference of beta3 expression among Nevertheless, the same group evidenced that in human the different layers of the human ventricular muscle, the internal mammary artery, beta2-adrenoreceptors had a rather abundance of beta3-adrenoreceptor strickingly increases in endothelial location with a low density in the smooth muscle several forms of cardiomyopathies (compared with non- cells, whereas in the saphenous vein the same receptors diseased hearts) [48]. were found only on the outer smooth muscle [36]. More A summary of the beta-adrenoreceptor location in the recently, Chruscinski et al performed relaxation assays on vascular compartments is proposed in Fig. 1, which helps Author's personal copy

314 Curr Hypertens Rep (2012) 14:310–317 to interpret the effects of integrated molecular and pharmaco- In this context, drugs that improve endothelial function logical events resulting from the diverse abundance and loca- have received recent renewed attention, especially in the tions of beta-adrenoreceptor in normal and disease states. context of evidence demonstrating the prognostic value of endothelial dysfunction and its pathophysiologic role in the development of cardiovascular disease, e.g., atherosclerosis, Pharmacology of Beta-Blockers and Incidence ischemic cardiovascular disease and heart failure. Some on Vasodilatation “third-generation” beta-adrenoreceptor blockers were indeed shown to influence endothelial function through either As outlined above, the relative affinities of beta-adrenoreceptor targeting of specific beta-adrenoreceptor isotypes or other antagonists towards the three beta-adrenoreceptor isotypes receptors on the endothelial cells. matter for predicting their functional impact on vasomotor Celiprolol, a beta1-specific antagonist, was shown to control. This in turn will influence both the efficacy of any exert agonist properties on beta2-adrenoreceptors, which specific beta-blocker on cardiovascular disease and the inci- have been implicated in enhanced endothelial NO synthesis, dence of side effects, particularly on tissue perfusion. vascular anti-oxidant effects and preserved vasodilatation Since the early development of beta-adrenoreceptor [52]. Whether this translates into better protection from antagonists following the pioneering work of Sir J. Black cardiovascular morbidity/mortality in patients remains an [49], the therapeutic armamentum has been enriched from unanswered question. non-specific pan-beta-adrenoreceptor antagonists to beta- Endothelial function involves more than NO synthesis adrenoreceptor-specific blockers that mostly target the be- and production (as outlined in the previous sections). In this ta1-adrenoreceptor. This is indeed the isotype that has main- regard, another recent beta-blocker, Nebivolol, may present ly been involved in the anti-anginal and anti-hypertensive the most pleiotropic vasculoprotective properties. It is high- effect of beta-blockers by opposing the inotropic effect of ly selective for beta1-adrenoreceptor blockade (>200-fold catecholamines on cardiac muscle. The same effect was over beta2)[53]; in addition, in several animal but also subsequently demonstrated to confer beneficial effects in human vessels, it activates beta3-adrenoreceptor [42], which the setting of heart failure, probably by preventing from (as outlined above) is expected to activate not only NO adverse myocardial remodeling, protecting failing cardiac production from eNOS, but also to recruit additional com- myocytes from the toxicity of neurohormonal overstimula- ponents of endothelium-derived vasodilatation, such as tion, as well as by restoring beta-adrenoreceptor expression “EDH(F)(s).” Importantly, these vasodilatory properties and density, resulting in restoration of heart rate variability were demonstrated in human coronary microvessels, but [50]. also in human mammary arteries from diseased patients

The use of the first beta1-adrenoreceptor “preferential” [41–43], emphasizing their potential therapeutic relevance. antagonists, however, has exposed patients to common side Finally, as beta3-adrenoreceptors were also shown to be effects due both to the expected beta1-adrenoreceptor block- expressed in cardiac myocytes from human ventricles [47], ade (e.g., bradycardia), but also to off-target effects attrib- their activation of myocardial NO production would pro- utable to targeting of the other isotypes [51]. Accordingly, mote NO-cGMP-mediated relaxation as well as protection blockade of beta2-adrenoreceptor (and possibly beta3-adre- against excessive catecholamine stimulation that, together noreceptor) is classically associated with bronchospasm as with coronary vasodilatation, would combine beneficial well as peripheral vasoconstriction due to loss of beta2 (and, effects on the myocardial oxygen supply and demand to possibly beta3)-adrenoreceptor-mediated relaxation of smooth preserve ventricular function [54•]. muscles [51]. Again, whether this translates to superior protection Further drug development led to the introduction of beta- from morbidity/mortality in patients needs to be demon- blockers with either better selectivity for the beta1-adrenor- strated with a “head-to-head” comparison of Nebivolol eceptor, ancillary vasodilating properties or both. Among with a pure beta1-adrenoreceptor blocker in a prospec- the first, drugs such as metoprolol have proven their better tive, randomized trial. So far, the SENIORS trial in elderly beta1-adrenoreceptor selectivity with lower vascular or patients with moderate heart failure only compared Nebiovo- bronchial side effects, without compromising efficacy [51]. lol with placebo, and showed an improvement in clinical Other less (or non-) selective beta-blockers, such as carve- symptoms [55] (as previously demonstrated with other beta- dilol, are endowed with additional properties that compen- blockers in HF trials). This suggests that if beta3 agonist sate for the lack of selectivity, e.g., through alpha-adrenoceptor properties are operative in vivo, they at least do not negate blockade and possibly antioxidant properties, both of the expected beneficial effects of beta1-adrenoreceptor which may preserve vasodilatation (on top of antagonizing blockade. adverse effects of adrenergic tone on oxidant stress and tissue Moreover, the comparative efficacy of Nebivolol and the remodeling) [7]. “pure” beta1-adrenoreceptor blocker, Metoprolol, has been Author's personal copy

Curr Hypertens Rep (2012) 14:310–317 315 tested on the protection against adverse remodeling and phosphorylation of eNOS at Ser(1179) and with decreased mortality in a mouse model of myocardial infarction. This eNOS phosphorylation at the inhibitory phosphorylation showed the superior efficacy of Nebivolol against hypertro- sites Ser(116) and Thr(497) [64], confirming that beta3 phic remodeling of the remaining viable myocardium, better adrenoceptor vasodilation is associated the endothelial syn- preserved endothelium-dependent relaxation ex-vivo and thesis/release role of nitric oxide. In the context of NO- endothelial progenitor cells function [56]. Importantly, the mediated response to Nebivolol, it is also worth mentioning use of a mouse model allowed the direct comparison of that Nebivolol promotes angiogenesis in a NOS/beta3- Nebivolol protection in wild-type mice and mice with ge- dependent manner, which could be of particular interest netic deletion of eNOS; Nebivolol’s effect was lost in the in the course of treatment of ischemia-associated dis- latter, showing an obligatory role of eNOS, at least in the eases. Besides, in human coronary resistance arteries, in mouse [57]. This could be correlated with Nebibolol’s prop- the presence of NOS and Cox inhibition, an endothelium- erties to prevent oxidative stress in isolated endothelial cells dependent relaxation to beta3-adrenoceptor stimulation per- that were lost upon full beta1-2–3 blockade [58•]. sists. BRL37344-mediated hyperpolarization of smooth In the next section, we will outline the experimental muscle cell membrane and the inhibitory effect of the com- evidence for these ancillary properties of Nebivolol (as well bination of apamin and charybotoxin concur to suggest as other specific beta3-adrenoreceptor agonists) on endothe- that beta3-adrenoceptor stimulation with BRL37344-evoked lial function in animal and human vessels. relaxation is partly mediated via an endothelium-derived hyperpolarization (EDH(F)). This could be a real advantage in pathological situations associated with reduced NO

Beta3-Adrenoceptor and Vasorelaxation bioavalailability. Of note, Nebivolol is used as a racemate mixture of the The observation that Nebivolol dose-dependently relaxed two enantiomers D-Nebivolol (+SRRR Nebivolol) and L- preconstricted rodent coronary resistance microarteries but Nebivolol (−RSSS nebivolol), which present different phar- failed to do so in microarteries isolated from beta3- macological properties. Both Nebivolol enantiomers pro- adrenoreceptor-deficient mice demonstrates a beta3 agonism duced a vasorelaxation through activation of beta3- for Nebivolol [42]. Additional pharmacologic evidence sup- adrenoceptors. However, D-Nebivolol-produced vasorelaxa- ports this hypothesis as Nebivolol-induced relaxation of tion is also mediated by activation of beta2-adrenoceptors human coronary microvessels was insensitive to beta(1–2)- and antagonism of alpha1-adrenoceptors [61•]. This could blockade [42]. Similarly, Nebivolol-induced relaxation of broaden the clinical interest of Nebivolol as beta2 agonism the rat thoracic aorta is resistant to beta2 blockade, but could alleviate the contraindication of beta-blockade in ob- inhibited by S-(−)- Cyanopindolol or by L-748,337, a selec- structive respiratory disease. tive beta3-adrenoceptor antagonist [59, 60 and 61•]. As described earlier, the beta3-adrenoreceptor-subtype is main- ly expressed on the vascular endothelium, consistent with functional evidence. Indeed, several studies described the Conclusion loss of beta3-dependent vasodilation in endothelium- denuded vessels [42, 60]. However, BRL 37344, a selective In conclusion, beta-blockers are widely used for the treat- beta3-adrenoceptor agonist, was shown to evoke a relaxation ment of hypertension. The latest generation of beta-blockers of endothelium-denuded human internal mammary arteries, exhibits ancillary properties, such as Nebivolol, which dem- suggesting that in some vascular beds, an endothelium- onstrates additive pleiotropic vasculoprotective effects in independent pathway may exist [62]. NOS inhibition comparison to traditional beta-antagonists. In particular, strongly reduces the relaxation of human coronary micro- Nebivolol exerts agonist activity on beta3 adrenoceptors, arteries and rat aorta to Nebivolol or BRL37344 [42, 60]; which mediates endothelium-dependent modulation of vas- likewise L-NAME attenuated vasorelaxing responses to cular tone. As nitric oxide release promotes vasodilation and both the beta3-adrenoceptor agonist BRL 37344 and non- angiogenesis, this suggests that beta3-adrenoceptors could specific isoprenaline in rat carotid arteries [63]. In cultured represent a new therapeutic target to improve the hemody- endothelial cells, beta3-adrenoceptor stimulation with Nebi- namic profile in hypertension. volol increased NO release as measured by electron para- magnetic resonance spin trapping. In parallel, fura-2 calcium fluorescence was increased and endothelial NOS Acknowledgments This work was supported by an Action de Re- ’ was dephosphorylated on threonine(495/497) [42]. Similar- cherche Concertée (06/11-338), Pôle d Attraction Interuniversitaire (IUAP P6/30) of the Politique Scientifique Fédérale, Waleo, from the ly, epinephrine-dependent eNOS activation through beta(3)- Region Wallonne, and the Fonds National de la Recherche Scientifi- adrenoreceptor signaling is accompanied by an increase in que. CD is Senior Research Associate of the FNRS. Author's personal copy

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