Rabbit, a relevant model for the study of cardiac beta 3-adrenoceptors

Early nebivolol treatment is beneficial in myocardial infarction in rats partly through β3-adrenoceptor remodelling

It remains unknown whether β-blockers are useful and safe in acute myocardial infarction (MI). Owing to its pharmacological profile and vasodilating action, nebivolol (N) is useful in MI. The aim of the present study was to assess in rat whether early nebivolol treatment could be beneficial in MI. It remains unknown whether β-blockers are useful and safe in acute MI. On day (D) 0, male Sprague-Dawley rats underwent left coronary artery ligation (MI) or simple thoracotomy (SHAM). On D1 and D2, the rats were treated with either nebivolol (5 mg.kg-1 .day-1 , MI-N and Sham-N) or vehicle (V, MI-V and Sham-V). On D3, heart rate, left ventricle (LV) intrinsic contractility (PESmid) and arterial elastance were measured. Cardiac and aortic β-Adrenoceptor (AR) subtype mRNA were quantified using real time quantitative RT-qPCR. Catecholamine response was assessed on isolated heart and aortic rings with isoproterenol. PESmid was decreased in MI without worsening the decrease nebivolol. In LV, β1 - and β3 -AR mRNA were respectively decreased and increased in all MI. β3 -AR mRNA increase was partly limited by nebivolol. Ex vivo, basal contractility was less decreased in MI-N than in MI-V. Isoproterenol response was only altered in MI-V. In MI aorta, Nebi prevented β2 - and β3 -AR mRNA increases. In addition, Acetylcholine-induced relaxation was lowered in MI-V but preserved with nebivolol. We demonstrated an early modulation of cardiovascular β3 -AR transcription early MI. Despite its putative negative inotropic properties, nebivolol did not worsen cardiac function in basal conditions and preserved LV catecholamine response.

Targeting Cardiac Myocyte Na+-K+ Pump Function With β3 Adrenergic Agonist in Rabbit Model of Severe Congestive Heart Failure

BACKGROUND

Abnormally high cytosolic Na⁺ concentrations in advanced heart failure impair myocardial contractility. Stimulation of the membrane Na⁺-K⁺ pump should lower Na⁺ concentrations, and the β3 adrenoceptor (β3 AR) mediates pump stimulation in myocytes. We examined if β3 AR-selective agonists given in vivo increase myocyte Na⁺-K⁺ pump activity and reverse organ congestion in severe heart failure (HF).

METHODS

Indices for HF were lung-, heart-, and liver: body weight ratios and ascites after circumflex coronary artery ligation in rabbits. Na⁺-K⁺ pump current, I_p, was measured in voltage-clamped myocytes from noninfarct myocardium. Rabbits were treated with the β3 AR agonists CL316,243 or ASP9531, starting 2 weeks after coronary ligation.

RESULTS

Coronary ligation caused ascites in most rabbits, significantly increased lung-, heart-, and liver: body weight ratios, and decreased I_p relative to that for 10 sham-operated rabbits. Treatment with CL316,243 for 3 days significantly reduced lung-, heart-, and liver: body weight ratios and prevalence of ascites in 8 rabbits with HF relative to indices for 13 untreated rabbits with HF. It also increased I_p significantly to levels of myocytes from sham-operated rabbits. Treatment with ASP9531 for 14 days significantly reduced indices of organ congestion in 6 rabbits with HF relative to indices of 6 untreated rabbits, and it eliminated ascites. β3 AR agonists did not significantly change heart rates or blood pressures.

CONCLUSIONS

Parallel β3 AR agonists-induced reversal of Na⁺-K⁺ pump inhibition and indices of congestion suggest pump inhibition is a useful target for treatment with β3 AR agonists in congestive HF.

Three Generations of β-blockers: History, Class Differences and Clinical Applicability

BACKGROUND

Beta-adrenergic receptors are expressed in cardiomyocytes and activated by either noradrenaline released from sympathetic synapses or circulating catecholamines. Their corresponding receptors have three subtypes, namely, β1, β2 and β3, which are members of the G protein-coupled receptors (GPCRs) family. Activation of β1-adrenergic receptors causes various physiological reactions including cardiac contraction and renin secretion from juxtaglomerular cells of the kidney. Antagonists of β-adrenergic receptors, known as β-blockers, have been used effectively for over four decades and have beneficial effects in the treatment of cardiovascular diseases. There are three generations of β-blockers according to their pharmacological properties. Firstgeneration β-blockers are non-selective, blocking both β1- and β2-receptors; second-generation β- blockers are more cardioselective in that they are more selective for β1-receptors; and thirdgeneration β-blockers are highly selective drugs for β1-receptors. The latter also display vasodilator actions by blocking α1-adrenoreceptors and activating β3-adrenergic receptors. In addition, thirdgeneration β-blockers exhibit angiogenic, antioxidant, anti-proliferative, anti-hypertrophic and antiapoptotic activities among other effects that are still under investigation.

CONCLUSION

The objective of this review is to describe the evolution observed during the development of the three distinctive generations, thereby highlighting the advantages of third-generation β- blockers over the other two drug classes.

Physiology and pathophysiology of the β3-adrenergic receptor

The β₃-adrenergic receptor (β₃-AR) is an important regulator of various physiological functions, such as thermogenesis in brown adipose tissue, lipolysis in white adipose tissue, negative inotropic effect in cardiomyocyte, and relaxation in blood vessel. The activation of β₃-AR by its agonists is shown to have metabolic (antiobesity and antidiabetic) and cardiovascular effects in animal models, highlighting β₃-AR as a potential therapeutic target in the treatment of several human diseases. Moreover, a substantial number of studies performed on different populations have identified some β₃-AR polymorphic variants associated with obesity, diabetes, cardiovascular diseases, and other disorders. The clinical phenotypes and functional characteristics of these variants provide insights into potential pathophysiological roles of β₃-AR in the development of these diseases.

The Trend of β3-Adrenergic Receptor in the Development of Septic Myocardial Depression: A Lipopolysaccharide-Induced Rat Septic Shock Model

Septic shock with low cardiac output is very common in children. However, the mechanism underlying myocardial depression is unclear. The role of β3-AR in the development of myocardial depression in sepsis is unknown. In the present study, we generated an adolescent rat model of hypodynamic septic shock induced by lipopolysaccharide (LPS). Neonatal cardiomyocytes were also treated with LPS to mimic myocardial depression in sepsis, which was confirmed via an in vivo left ventricular hemodynamic study, and measurements of contractility and the Ca2+ transient in isolated adolescent and neonatal cardiomyocytes. After 16 h of LPS treatment, cultured neonatal cardiomyocytes showed a diminished Ca2+ transient amplitude associated with an increase in the β3-AR level. With the addition of a β3-AR agonist, the Ca2+ transient in LPS-treated neonatal rat cardiomyocytes gradually decreased over time; such a change was absent in cells treated with nitric oxide synthase (NOS) inhibitors prior to treatment with a β3-AR agonist. In adolescent rats with septic myocardial depression, cardiac function declined as indicated by decreased MAP, dP/dtmax, and dP/dtmix for 6 h after LPS injection; however, the β3-AR level first increased 2 h after LPS treatment and then decreased 6 h after LPS treatment in the absence of exogenous catecholamines. The results indicate that, in vitro, at the cellular level β3-AR may be involved in the development of myocardial depression (Ca2+ transient depression) in sepsis through NOS signaling pathways; however, in vivo, a complicated mechanism for modulating β3-AR may exist.

A human pluripotent stem cell model of catecholaminergic polymorphic ventricular tachycardia recapitulates patient-specific drug responses

Although β-blockers can be used to eliminate stress-induced ventricular arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), this treatment is unsuccessful in ∼25% of cases. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) generated from these patients have potential for use in investigating the phenomenon, but it remains unknown whether they can recapitulate patient-specific drug responses to β-blockers. This study assessed whether the inadequacy of β-blocker therapy in an individual can be observed in vitro using patient-derived CPVT iPSC-CMs. An individual with CPVT harboring a novel mutation in the type 2 cardiac ryanodine receptor (RyR2) was identified whose persistent ventricular arrhythmias during β-blockade with nadolol were abolished during flecainide treatment. iPSC-CMs generated from this patient and two control individuals expressed comparable levels of excitation-contraction genes, but assessment of the sarcoplasmic reticulum Ca(2+) leak and load relationship revealed intracellular Ca(2+) homeostasis was altered in the CPVT iPSC-CMs. β-adrenergic stimulation potentiated spontaneous Ca(2+) waves and unduly frequent, large and prolonged Ca(2+) sparks in CPVT compared with control iPSC-CMs, validating the disease phenotype. Pursuant to the patient's in vivo responses, nadolol treatment during β-adrenergic stimulation achieved negligible reduction of Ca(2+) wave frequency and failed to rescue Ca(2+) spark defects in CPVT iPSC-CMs. In contrast, flecainide reduced both frequency and amplitude of Ca(2+) waves and restored the frequency, width and duration of Ca(2+) sparks to baseline levels. By recapitulating the improved response of an individual with CPVT to flecainide compared with β-blocker therapy in vitro, these data provide new evidence that iPSC-CMs can capture basic components of patient-specific drug responses.

β3 Adrenergic Stimulation Restores Nitric Oxide/Redox Balance and Enhances Endothelial Function in Hyperglycemia

BACKGROUND

Perturbed balance between NO and O2 (•-). (ie, NO/redox imbalance) is central in the pathobiology of diabetes-induced vascular dysfunction. We examined whether stimulation of β3 adrenergic receptors (β3 ARs), coupled to endothelial nitric oxide synthase (eNOS) activation, would re-establish NO/redox balance, relieve oxidative inhibition of the membrane proteins eNOS and Na(+)-K(+) (NK) pump, and improve vascular function in a new animal model of hyperglycemia.

METHODS AND RESULTS

We established hyperglycemia in male White New Zealand rabbits by infusion of S961, a competitive high-affinity peptide inhibitor of the insulin receptor. Hyperglycemia impaired endothelium-dependent vasorelaxation by "uncoupling" of eNOS via glutathionylation (eNOS-GSS) that was dependent on NADPH oxidase activity. Accordingly, NO levels were lower while O2 (•-) levels were higher in hyperglycemic rabbits. Infusion of the β3 AR agonist CL316243 (CL) decreased eNOS-GSS, reduced O2 (•-), restored NO levels, and improved endothelium-dependent relaxation. CL decreased hyperglycemia-induced NADPH oxidase activation as suggested by co-immunoprecipitation experiments, and it increased eNOS co-immunoprecipitation with glutaredoxin-1, which may reflect promotion of eNOS de-glutathionylation by CL. Moreover, CL reversed hyperglycemia-induced glutathionylation of the β1 NK pump subunit that causes NK pump inhibition, and improved K(+)-induced vasorelaxation that reflects enhancement in NK pump activity. Lastly, eNOS-GSS was higher in vessels of diabetic patients and was reduced by CL, suggesting potential significance of the experimental findings in human diabetes.

CONCLUSIONS

β3 AR activation restored NO/redox balance and improved endothelial function in hyperglycemia. β3 AR agonists may confer protection against diabetes-induced vascular dysfunction.

β3-Adrenoceptor activation relieves oxidative inhibition of the cardiac Na+-K+ pump in hyperglycemia induced by insulin receptor blockade

Dysregulated nitric oxide (NO)- and superoxide (O2 (·-))-dependent signaling contributes to the pathobiology of diabetes-induced cardiovascular complications. We examined if stimulation of β3-adrenergic receptors (β3-ARs), coupled to endothelial NO synthase (eNOS) activation, relieves oxidative inhibition of eNOS and the Na(+)-K(+) pump induced by hyperglycemia. Hyperglycemia was established in male New Zealand White rabbits by infusion of the insulin receptor antagonist S961 for 7 days. Hyperglycemia increased tissue and blood indexes of oxidative stress. It induced glutathionylation of the Na(+)-K(+) pump β1-subunit in cardiac myocytes, an oxidative modification causing pump inhibition, and reduced the electrogenic pump current in voltage-clamped myocytes. Hyperglycemia also increased glutathionylation of eNOS, which causes its uncoupling, and increased coimmunoprecipitation of cytosolic p47(phox) and membranous p22(phox) NADPH oxidase subunits, consistent with NADPH oxidase activation. Blocking translocation of p47(phox) to p22(phox) with the gp91ds-tat peptide in cardiac myocytes ex vivo abolished the hyperglycemia-induced increase in glutathionylation of the Na(+)-K(+) pump β1-subunit and decrease in pump current. In vivo treatment with the β3-AR agonist CL316243 for 3 days eliminated the increase in indexes of oxidative stress, decreased coimmunoprecipitation of p22(phox) with p47(phox), abolished the hyperglycemia-induced increase in glutathionylation of eNOS and the Na(+)-K(+) pump β1-subunit, and abolished the decrease in pump current. CL316243 also increased coimmunoprecipitation of glutaredoxin-1 with the Na(+)-K(+) pump β1-subunit, which may reflect facilitation of deglutathionylation. In vivo β3-AR activation relieves oxidative inhibition of key cardiac myocyte proteins in hyperglycemia and may be effective in targeting the deleterious cardiac effects of diabetes.

β3-Adrenoceptor Impairs Mitochondrial Biogenesis and Energy Metabolism During Rapid Atrial Pacing-Induced Atrial Fibrillation

BACKGROUND

The β3-adrenoceptor (β3-AR) is implicated in cardiac remodeling. Since metabolic dysfunction due to loss of mitochondria plays an important role in heart diseases, we examined the effects of β3-AR on mitochondrial biogenesis and energy metabolism in atrial fibrillation (AF).

METHODS

Atrial fibrillation was created by rapid atrial pacing in adult rabbits. Rabbits were randomly divided into 4 groups: control, pacing (P7), β3-AR antagonist (L748337), and β3-AR agonist (BRL37344) groups. Atrial effective refractory period (AERP) and AF induction rate were measured. Atrial concentrations of adenine nucleotides and phosphocreatine were quantified through high-performance liquid chromatography. Mitochondrial DNA content was determined. Real-time polymerase chain reaction and Western blot were used to examine the expression levels of signaling intermediates related to mitochondrial biogenesis.

RESULTS

After pacing for 7 days, β3-AR was significantly upregulated, AERP was reduced, and the AF induction rate was increased. The total adenine nucleotides pool was significantly reduced due to the decrease in adenosine triphosphate (ATP). The P7 group showed decreased activity of F0F1-ATPase. Mitochondrial DNA content was decreased and mitochondrial respiratory chain subunits were downregulated after pacing. Furthermore, expression of transcription factors involved in mitochondrial biogenesis, including peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), nuclear respiratory factor 1 (NRF-1), and mitochondrial transcription factor A (Tfam), was lower in the P7 group in response to β3-AR activation. Further stimulation of β3-AR with BRL37344 exacerbated these effects, together with a significant decrease in the levels of phosphocreatine. In contrast, inhibition of β3-AR with L748337 partially restored mitochondrial biogenesis and energy metabolism of atria in the paced rabbits.

CONCLUSION

The activation of β3-AR contributes to atrial metabolic remodeling via transcriptional downregulation of PGC-1α/NRF-1/Tfam pathway that are involved in mitochondrial biogenesis, which ultimately perturbs mitochondrial function in rapid pacing-induced AF. The β3-AR is therefore a potential novel therapeutic target for the treatment or prevention of AF.

β3 -AR and the vertebrate heart: a comparative view

Recent cardiovascular research showed that, together with β1- and β2-adrenergic receptors (ARs), β3-ARs contribute to the catecholamine (CA)-dependent control of the heart. β3-ARs structure, function and ligands were investigated in mammals because of their applicative potential in human cardiovascular diseases. Only recently, the concept of a β3-AR-dependent cardiac modulation was extended to non-mammalian vertebrates, although information is still scarce and fragmentary. β3-ARs were structurally described in fish, showing a closer relationship to mammalian β1-AR than β2-AR. Functional β3-ARs are present in the cardiac tissue of teleosts and amphibians. As in mammals, activation of these receptors elicits a negative modulation of the inotropic performance through the involvement of the endothelium endocardium (EE), Gi/0 proteins and the nitric oxide (NO) signalling. This review aims to comparatively analyse data from literature on β3-ARs in mammals, with those on teleosts and amphibians. The purpose is to highlight aspects of uniformity and diversity of β3-ARs structure, ligands activity, function and signalling cascades throughout vertebrates. This may provide new perspectives aimed to clarify the biological relevance of β3-ARs in the context of the nervous and humoral control of the heart and its functional plasticity.

β3-Adrenergic receptor antagonist improves exercise performance in pacing-induced heart failure

In heart failure (HF), the impaired left ventricular (LV) arterial coupling and diastolic dysfunction present at rest are exacerbated during exercise. We have previously shown that in HF at rest stimulation of β3-adrenergic receptors by endogenous catecholamine depresses LV contraction and relaxation. β3-Adrenergic receptors are activated at higher concentrations of catecholamine. Thus exercise may cause increased stimulation of cardiac β3-adrenergic receptors and contribute to this abnormal response. We assessed the effect of L-748,337 (50 μg/kg iv), a selective β3-adrenergic receptor antagonist (β3-ANT), on LV dynamics during exercise in 12 chronically instrumented dogs with pacing-induced HF. Compared with HF at rest, exercise increased LV end-systolic pressure (PES), minimum LV pressure (LVPmin), and the time constant of LV relaxation (τ) with an upward shift of early diastolic portion of LV pressure-volume loop. LV contractility decreased and arterial elastance (EA) increased. LV arterial coupling (EES/EA) (0.40 vs. 0.51) was impaired. Compared with exercise in HF preparation, exercise after β3-ANT caused similar increases in heart rate and PES but significantly decreased τ (34.9 vs. 38.3 ms) and LVPmin with a downward shift of the early diastolic portion of LV pressure-volume loop and further augmented dV/dtmax. Both EES and EES/EA (0.68 vs. 0.40) were increased. LV mechanical efficiency improved from 0.39 to 0.53. In conclusion, after HF, β3-ANT improves LV diastolic filling; increases LV contractility, LV arterial coupling, and mechanical efficiency; and improves exercise performance.

Autoantibodies against cardiac β1-adrenoceptor do not affect the low-affinity state β1-adrenoceptor-mediated inotropy in rat cardiomyocytes

Circulating autoantibodies directed against the 2nd extracellular loop (EL-2) of β1-adrenoceptors (β1-AABs) have been detected in the serum of patients with various cardiovascular pathologies. β1-AABs induce agonistic, positive inotropic effects via β1-adrenoceptors (β1ARs). In the mammalian heart, β1-AR can exist in 2 distinct activated configurations (the so-called high- and low-affinity states). The aim of the present study was to investigate whether the action of β1-AAB is dependent on the affinity state of β1AR in isolated ventricular cardiomyocytes of adult Wistar rats. Immunoglobulin G (IgG) containing β1-AAB obtained from animals immunized with a peptide corresponding to the EL-2 of human β1-AR, caused a dose-dependent increase in cell shortening. Isoproterenol-induced inotropy was significantly reduced in cardiomyocytes that had been preincubated with IgG containing β1-AAB and in cardiomyocytes isolated from immunized rats. The negative effects of preincubation with IgG containing β1-AAB on the response to isoproterenol was inhibited in the presence of bisoprolol. CGP 12177A and pindolol-induced inotropy was not affected by IgG preincubation or immunization. No detectable inotropic effect of cell shortening was obtained with IgG containing β1-AAB in the presence of propranolol and 3-isobutyl-1-methylxanthine. The present study demonstrates that β1-AABs have no agonist/antagonist-like effects upon low-affinity state β1-ARs. This result indicates that β1-AABs recognize and stabilize the high-affinity state, but are unable to stabilize and (or) induce the low-affinity state receptor.

Beta-3 adrenoceptors as new therapeutic targets for cardiovascular pathologies

Catecholamines play a key role in the regulation of cardiovascular function, classically through ß(1/2)-adrenoreceptors (AR) activation. After ß(3)-AR cloning in the late 1980s, convincing evidence for ß(3)-AR expression and function in cardiovascular tissues recently initiated a reexamination of their involvement in the pathophysiology of cardiovascular diseases. Their upregulation in diseased cardiovascular tissues and resistance to desensitization suggest they may be attractive therapeutic targets. They may substitute for inoperant ß(1/2)-AR to mediate vasodilation in diabetic or atherosclerotic vessels. In cardiac ventricle, their contractile effects are functionally antipathetic to those of ß(1/2)-AR; in normal heart, ß(3)-ARs may mediate a moderate negative inotropic effect, but in heart failure, it may protect against adverse effects of excessive catecholamine stimulation by action on excitation-contraction coupling, electrophysiology, or remodelling. Thus, prospective studies in animals and patients at different stages of heart failure should lead to identify the best therapeutic window to use ß(3)-AR agonists and/or antagonists.

Activation of β(3)-adrenoceptor promotes rapid pacing-induced atrial electrical remodeling in rabbits

Cardiac electrophysiological function is under the regulatory control of the sympathetic nervous system. In addition to classical β-adrenoceptors (β-AR, including β(1)- and β(2)- subtypes), β(3)-AR is also expressed in human heart and shows its distinctive functions. This study is aimed to elucidate the role of β(3)-AR in the regulation of atrial fibrillation (AF), especially its role in rapid pacing-induced atrial electrical remodeling in rabbits. The rapid atrial pacing model was established by embedding electrodes in the right atrium pacing at a speed of 600 beats per minute. The protein level of β(3)-AR in the atria was found significantly upregulated by western blot. The atrial effective refractory period (AERP) and its rate adaptation were decreased after pacing which were further shortened by BRL37344, a selective β(3)-AR agonist, leading to the increase of AF inducibility and duration. Similarly, β(3)-AR activation induced time-dependent shortening of action potential duration (APD), together with decrease of L-type calcium current (I(Ca,L)) and increase of inward rectifier potassium current (I(K1)) and transient outward potassium current (I(to)) in rapid pacing atrial myocytes. Meanwhile, all the effects were abolished by specific β(3)-AR antagonist, SR59230A. In summary, our study represents that activation of β(3)-AR promotes the atrial electrical remodeling process by altering the balance of ion channels in atrial myocytes, which provides new insights into the pharmacological role of β(3)-AR in heart diseases.

β(3) adrenergic stimulation of the cardiac Na+-K+ pump by reversal of an inhibitory oxidative modification

BACKGROUND

inhibition of L-type Ca(2+) current contributes to negative inotropy of β(3) adrenergic receptor (β(3) AR) activation, but effects on other determinants of excitation-contraction coupling are not known. Of these, the Na(+)-K(+) pump is of particular interest because of adverse effects attributed to high cardiac myocyte Na(+) levels and upregulation of the β(3) AR in heart failure.

METHODS AND RESULTS

we voltage clamped rabbit ventricular myocytes and identified electrogenic Na(+)-K(+) pump current (I(p)) as the shift in holding current induced by ouabain. The synthetic β(3) AR agonists BRL37344 and CL316,243 and the natural agonist norepinephrine increased I(p). Pump stimulation was insensitive to the β(1)/β(2) AR antagonist nadolol and the protein kinase A inhibitor H-89 but sensitive to the β(3) AR antagonist L-748,337. Blockade of nitric oxide synthase abolished pump stimulation and an increase in fluorescence of myocytes loaded with a nitric oxide-sensitive dye. Exposure of myocytes to β(3) AR agonists decreased β(1) Na(+)-K(+) pump subunit glutathionylation, an oxidative modification that causes pump inhibition. The in vivo relevance of this was indicated by an increase in myocardial β(1) pump subunit glutathionylation with elimination of β(3) AR-mediated signaling in β(3) AR(-/-) mice. The in vivo effect of BRL37344 on contractility of the nonfailing and failing heart in sheep was consistent with a beneficial effect of Na(+)-K(+) pump stimulation in heart failure.

CONCLUSIONS

the β(3) AR mediates decreased β(1) subunit glutathionylation and Na(+)-K(+) pump stimulation in the heart. Upregulation of the receptor in heart failure may be a beneficial mechanism that facilitates the export of excess Na(+).

[Nebivolol: the first vasodilatory beta-blocker with a beta3-adrenergic agonist activity]

Several studies suggest that the beta(3)-adrenergic stimulation could be a new therapeutic target for the treatment of cardiovascular diseases. The vascular effects induced by beta(3)-adrenergic stimulation are able to decrease the left ventricular strain allowing to reduce after-load. In addition, the increased coronary blood flow due to vasorelaxation increases the myocardial oxygene delivery. The hypothesis about the beneficial role of beta(3)-adrenoceptors is supported by recent data about a beta-blocker of third generation, nebivolol, currently used in the treatment of heart failure and hypertension. The present review presents the beta(3)-adrenoceptors characteristics as well as its involvement in the cardiovascular effects of nebivolol.