Safety and tolerability of β-blockers: importance of cardioselectivity

Cardioselective β-blockade is generally well tolerated in practice and contraindications to this therapy are uncommon. β-blockers are a diverse therapeutic class, and their individual tolerability profiles are influenced strongly by their pharmacodynamic effects across different adrenergic receptors. Bisoprolol, probably the β-blocker with the highest selectivity for blockade of β1- vs. β2-adrenoceptors, does not block β2-adrenoceptors to an appreciable extent at doses in therapeutic use. Side-effects often attributed to β-blockers, such as erectile dysfunction and adverse metabolic effects are uncommon with bisoprolol and other β-blockers used at doses which only block β1-adrenoceptors. Cautious use of a cardioselective β-blocker is not contraindicated in people with chronic obstructive pulmonary disease or asthma and the outcomes benefits of β-blockers in patients with coronary heart disease or heart failure are also apparent in patients with concurrent COPD. Starting with a low dose and titrating upwards carefully is important for optimising the tolerability of a β-blocker. Most people with hypertension will receive combination antihypertensive therapy in practice, and the low-dose combination therapy approach provides a useful strategy for optimising the efficacy and tolerability of a regimen that includes a β-blocker, compared with up-titrating an existing monotherapy.

Three Days of Chronic Intermittent Hypoxia Induce β1-Adrenoceptor Dependent Increases in Left Ventricular Contractility

Sleep apnea is characterized by bouts of chronic intermittent hypoxia (CIH) that elicit sympathetic hyperactivity resulting in residual hypertension. We previously demonstrated that exposure to CIH increases cardiac output and sought to determine if enhanced cardiac contractility manifests prior to hypertension.Male Wistar rats were exposed to cyclical bouts of hypoxia (FiO₂ = 0.05 nadir; 90 s) and normoxia (FiO₂ = 0.21; 210 s) 8 h/day for 3 days (CIH; n = 6). Control animals (n = 7) were exposed to room air. Data are presented as mean ± SD and were analyzed using unpaired Student t-tests.Three-day exposure to CIH did not elicit changes in heart rate and blood pressure (p > 0.05). However, baseline left ventricular contractility (dP/dt_MAX) was significantly increased in CIH-exposed animals compared with control (15300 ± 2002 vs. 12320 ± 2725 mmHg/s; p = 0.025), despite no difference in catecholamine concentrations. Acute β₁-adrenoceptor inhibition reduced contractility in CIH-exposed animals (-7604 ± 1298 vs. -4747 ± 2080 mmHg/s; p = 0.014), to levels equivalent to control, while preserving cardiovascular parameters. Sympathetic ganglion blockade (hexamethonium 25 mg/kg; i.v.) produced equivalent cardiovascular responses suggesting similar global sympathetic activity between groups. Interestingly, gene expression of the β₁-adrenoceptor pathway in cardiac tissue was unchanged.Our results suggest that CIH increases cardiac contractility via β₁-adrenoceptor dependent mechanisms prior to development of global sympathetic hyperactivity suggesting that positive cardiac inotropy contributes to the development of hypertension in CIH-exposed rats.

O-GlcNAc transferase affects the signal transduction of β1 adrenoceptor in adult rat cardiomyocytes by increasing the O-GlcNAcylation of β1 adrenoceptor

O-GlcNAcylation was first found by Torres and Hart in monocytes. It is a dynamic and reversible post-translational modification catalyzed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation is increased in diabetic cardiomyopathy (DCM) patients and it has been reported that OGT plays an important role in the regulation of cardiac gene transcription, cell cycle and calcium homeostasis. The purpose of this study is to investigate the effects of OGT on signal transduction and function of β1-adrenoceptor (β1AR) in adult rat cardiomyocytes. We found that after overexpressing OGT by adenovirus vector in adult rat cardiomyocytes, cAMP formation and phosphorylation of phospholamban (PLB) at Ser16 (p16-PLB) were decreased under isoprenaline (ISO) stimulation. Over expression of OGT increased the intracellular [Ca²⁺]i and deteriorated the death of cardiomyocytes induced by prolonged stimulation with ISO. β1-adrenoceptor was overexpressed using a plasmid vector and then co-immunoprecipitation (co-IP) followed by Western blot was employed to define the O-GlcNAcylation of β1-adrenoceptor. The results showed that O-GlcNAcylation of β1-adrenoceptor was increased in OGT overexpressed cells, and there was no significant change in the formation of cAMP and phosphorylation of PLB after β1-adrenoceptor was blocked by CGP20712A. Given that OGT affects the signal transduction of β1-adrenoceptor in adult rat cardiomyocytes by increasing the O-GlcNAcylation of β1-adrenoceptor, the mechanism revealed in this study indicates that OGT and β1AR may be therapeutic targets in patients undergoing diabetic cardiomyopathy.

Proliferation of the human urothelium is induced by atypical β1 -adrenoceptors

We wanted to assess whether β-adrenoceptors mediate proliferation in the normal and malignant urothelial cell lines UROtsa and T24, respectively. Urothelial cells were cultured for 24 h in the presence of the β-adrenoceptor agonists isoprenaline (β1/2/3 ), dobutamine (β1 ), salbutamol (β2 ), BRL 37344 (β3 ), CGP 12177 (a partial β-agonist) or β-adrenoceptor antagonists (metoprolol; β1 , propranolol; β1/2 ). Phosphorylation of kinases was screened with a Human Phospho-Kinase Array Kit (R&D systems). Intracellular pathways activated by proliferation of urothelial cells were characterized by incubating cells with the MEK1/2 inhibitor PD 98,059, the p38 kinase inhibitor losmapimod or with the Akt 1/2 kinase inhibitor. Proliferation was assessed with the MTT proliferation assay (ATCC). Western blot and immunocytochemistry were used for detection of the β1 -adrenoceptor. Isoprenaline and dobutamine induced proliferation, while salbutamol and BRL 37344 did not. Dobutamine-induced proliferation was not affected by metoprolol or propranolol but was instead antagonized by CGP 12177 in T24 but not in UROtsa. In response to stimulation with dobutamine, Akt1/2/3 was phosphorylated in UROtsa, while ERK1/2 and p38 were phosphorylated in T24. MEK1/2 inhibition blocked basal and dobutamine-induced proliferation in T24 but only basal proliferation in UROtsa. Losmapimod slightly inhibited basal proliferation in T24 but not dobutamine-induced proliferation. Akt 1/2 inhibitor blocked basal and dobutamine-induced proliferation in UROtsa. Immunocytochemistry and Western blot revealed expression of β1 -adrenoceptors in both urothelial cell lines. The present data show that the urothelium expresses atypical β1-adrenoceptors that activate intracellular kinases inducing urothelial proliferation.

β1 -Adrenoceptor autoantibodies increase the susceptibility to ventricular arrhythmias involving abnormal repolarization in guinea-pigs

NEW FINDINGS

What is the central question of this study? High titres of autoantibodies against the second extracellular loop of the β₁ -adrenergic receptor (β₁ -AAs) can be detected in the sera of patients with ventricular arrhythmias, but a causal relationship between β₁ -AAs and ventricular arrhythmias has not been established. What is the main finding and its importance? Monoclonal β₁ -AAs (β₁ -AR mAbs) were used in the experiments. We showed that β₁ -AR mAbs increased susceptibility to ventricular arrhythmias and induced repolarization abnormalities. Antibody adsorption of β₁ -AAs will be a potential new therapeutic strategy for ventricular arrhythmias in patients with high titres of β₁ -AAs. High titres of autoantibodies against the second extracellular loop of the β₁ -adrenergic receptor (β₁ -AAs) can be detected in sera from patients with ventricular arrhythmias, but a causal relationship between β₁ -AAs and ventricular arrhythmias has not been established. In this work, ECGs of guinea-pigs and isolated guinea-pig hearts were recorded. Ventricular tachycardia (VT) and ventricular fibrillation (VF) were evoked by programmed electrical stimulation of the left ventricular epicardium of isolated guinea-pig hearts. The monophasic action potential and effective refractory period of the left ventricle were recorded in paced isolated guinea-pig hearts. Furthermore, to increase the specificity, monoclonal autoantibodies against the second extracellular loop of the β₁ -adrenergic receptor (β₁ -AR mAbs) were used in all experiments. The results showed that β₁ -AR mAbs induced premature ventricular contractions in guinea-pigs and isolated guinea-pig hearts. In addition, β₁ -AR mAbs decreased the threshold of VT/VF and prolonged the duration of VT/VF. Furthermore, β₁ -AR mAbs shortened the corrected QT interval and effective refractory period, and prolonged late-phase repolarization of the monophasic action potential (MAPD_90-30 ). These changes in electrophysiological parameters might be attributed, at least in part, to the arrhythmogenicity of β₁ -AR mAbs.

Activation of β1-adrenoceptor triggers oxidative stress mediated myocardial membrane destabilization in isoproterenol induced myocardial infarcted rats: 7-hydroxycoumarin and its counter action

Activation of β1-adrenoceptor stimulates myocardial membrane destabilization in isoproterenol induced rats. Male albino Wistar rats were pre and co-treated with 7-hydroxycoumarin (16mg/kg body weight) daily for 8 days. Myocardial infarction was induced into rats by the subcutaneous administration of isoproterenol (100mg/kg body weight) at an interval of 24h daily for a period of two days (7th and 8th day). The levels/activities of serum cardiac troponin-T, lactate dehydrogenase and the concentrations of heart lipid peroxidation products were significantly increased and the antioxidant status was significantly decreased in isoproterenol induced rats. Furthermore, the activity of sodium/potassium-dependent adenosine triphosphatase was significantly decreased and the activities of calcium and magnesium-dependent adenosine triphosphatases were significantly increased in the heart of isoproterenol induced myocardial infarcted rats. Isoproterenol induced rats also revealed increased concentrations of sodium and calcium and decreased concentrations of potassium in the heart. 7-hydroxycoumarin pre- and co-treatment showed considerable impact on all biochemical parameters assessed. Also, 7-HC greatly reduced the infarct size of the myocardium. The in vitro study confirmed its potent free radical scavenging activity. Thus, the present study revealed that 7-HC attenuates myocardial membrane destabilization by reinstating the activities/levels of adenosine triphosphatases and minerals in isoproterenol induced rats by inhibiting oxidative stress. These effects are attributed to the membrane stabilizing and free radical scavenging properties of 7-hydroxycoumarin.

Antagonists of the Vasopressin V1 Receptor and of the β(1)-Adrenoceptor Inhibit Cytotoxic Brain Edema in Stroke by Effects on Astrocytes - but the Mechanisms Differ

Brain edema is a serious complication in ischemic stroke because even relatively small changes in brain volume can compromise cerebral blood flow or result in compression of vital brain structures on account of the fixed volume of the rigid skull. Literature data indicate that administration of either antagonists of the V1 vasopressin (AVP) receptor or the β1-adrenergic receptor are able to reduce edema or infarct size when administered after the onset of ischemia, a key advantage for possible clinical use. The present review discusses possible mechanisms, focusing on the role of NKCC1, an astrocytic cotransporter of Na(+), K(+), 2Cl(-) and water and its activation by highly increased extracellular K(+) concentrations in the development of cytotoxic cell swelling. However, it also mentions that due to a 3/2 ratio between Na(+) release and K(+) uptake by the Na(+),K(+)-ATPase driving NKCC1 brain extracellular fluid can become hypertonic, which may facilitate water entry across the blood-brain barrier, essential for development of edema. It shows that brain edema does not develop until during reperfusion, which can be explained by lack of metabolic energy during ischemia. V1 antagonists are likely to protect against cytotoxic edema formation by inhibiting AVP enhancement of NKCC1-mediated uptake of ions and water, whereas β1-adrenergic antagonists prevent edema formation because β1-adrenergic stimulation alone is responsible for stimulation of the Na(+),K(+)-ATPase driving NKCC1, first and foremost due to decrease in extracellular Ca(2+) concentration. Inhibition of NKCC1 also has adverse effects, e.g. on memory and the treatment should probably be of shortest possible duration.

Expression of β1- and β2-adrenoceptors in different subtypes of interneurons in the medial prefrontal cortex of mice

Noradrenaline acting via β-adrenoceptors (β-ARs) in the CNS plays an important role in learning/memory and cognitive functions. β-ARs have been shown to be expressed in cortical pyramidal and subcortical principal cells. However, little is known about β-AR expression in different subtypes of GABAergic neurons. Here, we report that both β1- and β2-ARs are expressed in a majority of GABAergic interneurons in the medial prefrontal cortex of mice, including parvalbumin (PV)-, calretinin (CR)-, calbindin D-28k (CB)-, somatostatin (SST)- and Reelin-immunoreactive (ir) interneurons. Relative to PV-, CB-, SST- and Reelin-ir interneurons, CR-ir interneurons are less likely to express β1- and β2-ARs. SST-ir interneurons are more likely to express β2-AR compared with the other subtypes of interneurons. The present results are of significance for understanding the role of β-ARs in prefrontal cortical functions.