From a classic pharmacological point of view, beta-blockers (or β-blockers) are antagonists of β-adrenergic receptors (ARs), which play an important role in the control of physiological processes such as blood pressure, heart rate and airway strength or reactivity, as well as other metabolic and central nervous system processes.1–4 After their discovery by Nobel prize-winner, Sir Henry H. Dale in 19065 (Figure 1), ARs became key targets in cardiovascular diseases, such as hypertension and heart failure (HF), in respiratory diseases such as asthma, and other no less important diseases, such as benign prostatic hypertrophy, nasal congestion, obesity, and pain, among many others.1–4

Figure 1.

Historical perspective of β-AR and β-blockers. AR, adrenegic receptor. Photographs were acquired from Wikimedia Commons repository under the Creative Commons Attribution 2.0 Generic and 4.0 international licenses.

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However, it was not until 1948 that Raymond P. Ahlquist observed 2 differentiated pathways inducing pharmacological responses depending on the organ in which the drugs were studied. Based on these experiments, Ahlquist divided ARs into 2 types, the α-ARs (associated with most “excitatory” functions such as vasoconstriction) and β-ARs (associated with most “inhibitory” functions, including vasodilation, and 1 “excitatory” effect, stimulation of the myocardium).6 Later on, in 1958, Sir James Black introduced the first β-blocker in the search for a treatment able to reduce oxygen consumption in the event of an angina attack, and corroborated Ahlquist's theory. This invention, considered one of the most important achievements in medicine in the 20th century, gained Black and the world of ARs a second Nobel prize in 19887 (Figure 1).

In 1967, Alonzo M. Lands and his collaborators proposed the division of β-ARs into 2 different subtypes: β1-ARs, mostly present in heart, and β2-ARs, responsible for vascular and airway relaxation.8 This classification was supported by the subsequent discovery of selective antagonists for β1-ARs.9 Very soon a third subtype with as many similarities as differences, insensitive to the most commonly used drugs, was identified in the cells of brown adipose tissue from rats and named β3-AR.10,11

The latest milestone was achieved by Robert J. Lefkowitz and Brian K. Kobilka, who helped to identify the interaction of β-ARs with cell structures, their dynamic regulation and desensitization and finally to solve the β2-AR 3-dimensional crystalline structure in 2007 (Figure 1). This work led to Lefkowitz and Kobilka being awarded the third Nobel prize for work on ARs in 2012.12

A better knowledge of the complex signaling networks triggered downstream of β-AR stimulation and of their alterations in pathological conditions is key for understanding the effects of β-blockers and for the design of therapeutic strategies. The 3 subtypes of β-ARs (β1-AR, β2-AR, β3-AR) are present in cardiac tissue. While all β-ARs belong to the G protein-coupled receptor (GPCR) superfamily of membrane receptors and share several structural and functional features, the 3 subtypes display different affinities for given ligands, specific cellular expression and subcellular localization patterns, differential coupling to signaling cascades, and distinct regulatory mechanisms2,3,37 (Figure 2).

Figure 2.

Intracellular pathway mediated by β-adrenergic receptors (β-ARs). 1: Main pathway: catecholamines bind to β-adrenoceptors inducing coupling to the heterotrimeric Gs protein; 2: Dissociation of the Gαs-GTP subunit and activation of adenylyl cyclase (AC); 3: Synthesis of cyclic adenosine monophosphate (cAMP); 4: protein kinase A (PKA) activation; 5: Coordinated phosphorylation of various targets by PKA, including the plasma membrane L-type calcium channel (LTCC) or the RyR2 calcium channel in the sarcoplasmic reticulum, results in increased cytosolic Ca2+ concentration available for contraction of cardiac muscle. 6: Continuous stimulation (as described in chronic heart failure) of β1-AR induces apoptosis via Ca2+/calmodulin-dependent protein kinase II (CaMKII) leading to apoptosis and heart damage. 7: Continued stimulation of β2 (increased when using selective β1-blockers) induces coupling to Gi protein; 8: AC is inhibited by the Gα-GTP subunit of Gi; 9: The Gβγ subunit of Gi induces both the inhibition of apoptosis (via stimulation of mitogen-activated protein kinases [MAPK] and the phosphatidylinositol 3-kinase [PI3K]-protein kinsase B [AKT] pathway) and of Gs-mediated deleterious effects (10), leading to cardioprotection. 11: In heart failure, stimulation of the β -adrenoceptor might lead to cardioprotection and a reduction in cardiac remodeling via nitric oxide synthase (NOS) activation. Adapted with permission from Watson et al.37.

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Upon agonist binding, GPCRs couple to heterotrimeric G proteins, thus facilitating exchange of GDP by GTP in the Gα subunits, which subsequently dissociate from the βγ dimers. Free Gα and βγ subunits transiently interact with effectors (such as adenylyl cyclases or phospholipases, among others) to trigger signal transduction cascades.4 In addition, agonist-activated GPCRs are specifically phosphorylated in the third cytoplasmic loop and/or the C-terminal tail by GPCR kinases (GRKs), a family of 7 serine/threonine protein kinases.38,39 Subsequently, cytosolic protein β-arrestins are recruited to the phosphorylated receptor, leading to uncoupling from G proteins, a process termed GPCR desensitization. In addition, β-arrestins can act as a scaffold for proteins of the endocytic machinery and for many other signal transduction partners, thus triggering clathrin-mediated receptor internalization and recycling and a second wave of G protein-independent transduction cascades.40 Therefore, the overall cellular effects of GPCR stimulation would result from the balance between the G protein-dependent and GRKs/β-arrestin-dependent branches of GPCR signaling.

Affinity is the ability of a drug to bind the receptor, and efficacy is the ability to induce a response. Drugs are classified as agonists or antagonists depending on whether or not they have efficacy.

All β-blockers share a common mechanism, which is their affinity for binding to β-ARs but, in contrast to β-AR agonists, they are not efficacious in evoking physiological responses. β-blockers compete with agonists for the binding site at β-AR, and the consequence is the inhibition of agonist activity. For this reason, they have been classically considered as competitive antagonists and their effects can be overcome by increasing the concentration of the agonist.15

Despite this common mechanism, in clinical studies, β-blockers do not behave as a single class of drugs. For example, bisoprolol, carvedilol, metoprolol and nebivolol have been proved to be helpful in HF treatment, bucindolol had no benefit, and xamoterol increased mortality.15 Therefore, a more rigorous analysis of the mechanism of action is needed to understand the clinical usefulness of this group.

There are some aspects that make the difference:

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  Selective affinity for β-AR subtypes. β-AR subtypes are not interchangeable entities and β-blockers exhibit a different affinity for each β-AR subtype, resulting in a particular pharmacological profile.

The functional consequences of β1-AR blockade in the heart are bradycardia and improved diastolic coronary filling time, reduced oxygen requirements, and a reduction in renin; all these effects are beneficial in HF and myocardial ischemia.68 However, the consequences of the blockade of β2 or β3-ARs are not positive since it avoids the bronchodilatation mediated by the β2 subtype, as well as the cardioprotective and vasodilatory mechanisms triggered by both subtypes. In fact, in vessels, they are present in vascular smooth muscle cells as well as endothelium, where they couple to the eNOS/NO-cGMP/PKG pathway, promoting vasodilation.69

The first group of the clinically available β-blockers exhibited higher affinity for the β1 and β2 subtypes than for the β3 subtype (Table 1), so at clinical doses, their therapeutic activity would be mainly related to β1- and β2-AR blockade (Table 2).

The “cardioselective” β-blockers have a higher affinity for the β1 subtype than for the β2 and β3 subtypes. When used at low doses, they inhibit cardiac β1-ARs but not β2–AR-mediated vasodilatation or bronchodilatation. However, β1-AR selectivity is relative (Table 1) and is lost with higher doses, and therefore the use of β1-selective blockers should be considered with caution in patients with airway diseases.

Of note, among the β-blockers that have been approved for the treatment of HF, bisoprolol and nebivolol are the most β1 selective, metoprolol exhibits moderate β1 selectivity, and carvedilol has slight β2-selectivity (Table 1). Therefore it is not possible to determine whether β1 selectivity is essential for maximal beneficial outcomes in HF.

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  Inverse agonism. Traditional theory about drug-receptor interaction is based on a quiescent population of receptors that only act when they bind a ligand that possesses efficacy (agonist). However, we know that β-ARs, in the absence of agonists, can spontaneously adopt active conformations capable of regulating signaling systems70 and coupling to different transducing mechanisms.3 Therefore, the simplistic interpretation that β-blockers are drugs without efficacy to activate the receptor must be revised.

The evidence of this “constitutive activity” of β-ARs in the absence of agonists led to the discovery of drugs that could reduce it. Since their effects were opposite to those of agonists, these drugs were considered as “inverse agonists”,71 ie, rather than just occupying the binding site and thus blocking the actions of agonists, they stabilize the conformations of the receptor that are not coupled to G proteins, and prevent the constitutively activated signaling pathways. Although this idea was originally met with skepticism, it is now accepted that all receptors can signal in the absence of agonists and most β-blockers previously characterized as antagonists are now recognized as inverse agonists.16,21,23 What is the relevance of this observation? In a system with measurable constitutive activity, an inverse agonist will reduce receptor response whereas an antagonist does not, but both prevent the agonist activity.

Moreover, constitutive receptor activity results in activation of desensitization mechanisms that cause downregulation of receptors.70 Treatment with an inverse agonist stops this receptor downregulation, resulting in increased receptor expression and enhanced responsiveness to agonist stimulation.71 Sustained exposure of human β2-AR to inverse agonists resulted in (approximately) a doubling in membrane levels of the receptor, whereas equivalent treatment with an antagonist was unable to produce this effect.72

Studies in humans and animal models show upregulation of β1 and β2-ARs in the heart or β2-ARs in lymphocytes with chronic propranolol treatment, which is the reason for the observed β-AR supersensitivity after abrupt propranolol withdrawal.15 Moreover, β1-selective blockers such as atenolol, metoprolol and bisoprolol increase β1 but not β2-AR density.15 Because a general feature of HF patients is a decrease in cardiac β1-AR density,52,73 upregulation would be helpful in restoring maximal contractile responses. However, carvedilol did not upregulate cardiac β-ARs in HF patients, but was as effective as metoprolol and bisoprolol in improving cardiac performance.15 Therefore, it is under discussion whether upregulation of β-AR by β-blockers could be a beneficial property.

The data on the inverse agonist activity of β-blockers are summarized in Table 1.

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  Partial agonism. Traditionally, some β-blockers have been considered to have intrinsic sympathomimetic activity. This activity appears if the drug has antagonist activity at the β1-AR subtype, but behaves as an agonist at another/others, or if the drug has the ability to promote a partial response of 1, 2 or all 3 subtypes (partial agonist). The consequence of partial activation of β-ARs is blockade of the stimulatory activity of high-efficacy agonists, such as catecholamines, but the stimulation of a low level of β-AR response in the absence of an agonist. This combined action could be beneficial since it manifests itself only when the sympathetic system is activated.74 However, β-blockers with partial agonist activity at β1-ARs appear to be less advantageous in the treatment of HF.28 On the other hand, an antagonist activity at β1-AR together with an agonist activity at β2- or β3-ARs produces vasodilatation and a cardioprotective effect that could represent an additional benefit.75

Older studies with β-blockers that detect intrinsic sympathomimetic activity do not differentiate between these mechanisms or the subtype involved. More recently, partial agonist activity on each β-AR subtype has been extensively studied at the cellular and tissue levels.17,74 Studies with human β-AR subtypes show differences depending on the β-blockers and the subtype studied: oxprenolol, carazolol, pindolol and nadolol have very evident partial agonist effects on β1 and β3-AR but no significant intrinsic activity on β2-ARs.17 Celiprolol has been described as an antagonist of the β1-subtype but as a partial agonist on β2 and β3-ARs.76

Table 1 summarizes some of the data available and shows conflicting results in some cases. Nebivolol does not promote cAMP accumulation in cells expressing the human β-AR subtypes17,18,28 and does not relax the rat urinary bladder, a prototypical β3–AR-mediated response,18 so it does not behave as a partial or total agonist in these conditions. However, nebivolol, through β3-AR activation, induces NO-mediated vasodilatation77–79 with a negative inotropic effect,80 and protects against myocardial infarction injury.81 Moreover, it reduces pulmonary vascular resistance and improves right ventricular performance in a porcine model of chronic pulmonary hypertension.82 These controversial results could be reconciled if we suppose that, depending on the cell type where the β3-AR is expressed, different signaling pathways were activated. This hypothesis links to the following section in which we address the concept of “biased agonism”.

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  Biased agonism. A single β-AR can couple not only to 1 but to different G proteins, leading to complex signaling profiles including cAMP accumulation and mitogen-activated protein kinase activation.2 Additionally, for the β1 and β2 subtypes, G-protein-independent signaling has also been reported primarily through β-arrestins, which are responsible for desensitization/endocytosis machinery and noncanonical signaling via intracellular pathways such as the ERK1/2 mediated pathway.2,28

Ligands have been identified that bind β-ARs and activate distinct and specific subsets of these signaling pathways. This phenomenon has been referred to as “ligand-directed stimulus trafficking”, “functional selectivity”, and “biased agonism”.70,83 Particularly striking are studies reporting that some β-blockers can have opposite efficacies toward 2 different signaling pathways, suggesting that efficacy is a more complex parameter than was originally thought. In fact, multiple efficacy combinations are theoretically possible. Compounds could be agonist for the 2 pathways, inverse agonist for the 2 pathways, or have opposite efficacies on each of the pathways. For example, propranolol, which acts as an inverse agonist on the β2-AR toward Gs/AC/cAMP/PKA pathway, was shown to be partial a agonist when tested on ERK activity.24

More interestingly, among an extensive group of β-blockers, only carvedilol22 and nebivolol32 induced β2-AR internalization and G protein-independent but β-arrestin-dependent activation of ERK1/2. Similar results have been described for carvedilol, alprenolol20 and nebivolol32 on β1-AR- β-arrestin-mediated EGFR transactivation. Given that β–arrestin-mediated β1-AR transactivation of EGFR may confer cardioprotection,49 β-blockers activating this pathway might possess superior efficacy in treating cardiovascular disorders.20 Additionally, carvedilol selectively promotes the recruitment and activation of Gαi to the β1-AR subtype triggering β-arrestin-mediated signaling.29

However, caution is advised because the cell or the physiological state may result in different results and interpretations of the signaling systems. Thus, other investigators84 failed to find evidence of β-arrestin recruitment by these β-blockers acting on β2-ARs (Table 1).

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  Additional mechanisms. Individual properties of certain β-blockers are independent of their β-blocking properties but contribute to their therapeutic efficacy. They are summarized in Table 1 and include:

K+-channel blockade, as exerted by sotalol. This characteristic confers sotalol an additional antiarrhythmic activity characterized by slowing repolarization and prolonged action potential in cardiac tissues.27

α1-AR antagonist activity exerted by carvedilol30 and labetalol.31 This additional α1-adrenergic-blocking action leads to vasodilatation with a reduction in peripheral vascular resistance that acts to maintain higher levels of cardiac output. In contrast, nonvasodilating β-blockers tend to raise peripheral vascular resistance and reduce cardiac output and left ventricular function.

NO-releasing activity, which involves an additional vasodilator effect. This property was exhibited by nebivolol and could be mediated by a partial agonist activity mainly on β3-AR, although other not well determined mechanisms cannot be excluded.85 The increased NO release accompanied by decreased oxidative stress leads to an increase in NO bioavailability19 that participates in the antihypertensive activity of nebivolol. In the same way, carvedilol significantly increases plasma NO levels by stimulation of NOS86 and improves NO availability derived from its antioxidant properties. However, these actions do not appear to be mediated by a partial agonist activity on β3-AR.85

Since their invention more than 50 years ago, β-blockers are still one of the most useful groups of drugs in clinical practice. They continue to be used for their original purpose to treat ischemic heart disease but, paradoxically, they are also effective in congestive HF. In addition, β-blockers are also used as antihypertensive drugs and in the treatment of cardiac arrhythmias, esophageal variceal bleeding, and pulmonary hypertension. Furthermore, β-blockers have additional applications such as management of glaucoma, tremor, migraine, anxiety, and hyperthyroidism. The more that is known about their specific intracellular mechanisms of action, the greater the number of therapeutic applications. Emerging avenues of research should focus on the detailed study of unexplored, cell type-specific mechanism of β-blockers by considering them as individual molecules rather than as a homogeneous group of drugs. Half a century later, β-blockers will keep surprising the research community with new therapeutic applications undreamed by James Black.

F. Mayor is supported by Ministerio de Economía, Industria y Competitividad (MINECO) of Spain (grant SAF2017-84125-R), CIBERCV-Instituto de Salud Carlos III, Spain (grant CB16/11/00278, co-funded with European FEDER contribution), and Programa de Actividades en Biomedicina de la Comunidad de Madrid-B2017/BMD-3671-INFLAMUNE. E. Oliver is recipient of funds from Programa de Atracción de Talento (2017-T1/BMD-5185) of Comunidad de Madrid. The Spanish Center for Cardiovascular Research (CNIC) is supported by the Ministerio de Ciencia, Innovación y Universidades and the Pro CNIC Foundation and is a Severo Ochoa Center of Excellence (SEV-2015-0505). We also acknowledge institutional support to the Centro de Biología Molecular ‘Severo Ochoa’ from Fundación Ramón Areces.

None declared.