Pre- and postganglionic stimulation-induced noradrenaline overflow is markedly facilitated by a prejunctional beta 2-adrenoceptor-mediated control mechanism in the pithed rat

Effects of β-adrenoceptor activation on haemodynamics during hypoxic stress in rats

NEW FINDINGS

What is the central question of this study? The acute hypoxic compensatory reaction is based on haemodynamic changes, and β-adrenoceptors are involved in haemodynamic regulation. What is the role of β-adrenoceptors in haemodynamics during hypoxic exposure? What is the main finding and its importance? Activation of β₂ -adrenoceptors attenuates the increase in pulmonary artery pressure during hypoxic exposure. This compensatory reaction activated by β₂ -adrenoceptors during hypoxic stress is very important to maintain the activities of normal life.

ABSTRACT

The acute hypoxic compensatory reaction is accompanied by haemodynamic changes. We monitored the haemodynamic changes in rats undergoing acute hypoxic stress and applied antagonists of β-adrenoceptor (β-ARs) subtypes to reveal the regulatory role of β-ARs on haemodynamics. Sprague-Dawley rats were randomly divided into control, atenolol (β₁ -AR antagonist), ICI 118,551 (β₂ -AR antagonist) and propranolol (non-selective β-AR antagonist) groups. Rats were continuously recorded for changes in haemodynamic indexes for 10 min after administration. Then, a hypoxic ventilation experiment [15% O₂ , 2200 m a.sl., 582 mmHg (0.765 Pa), P O 2 87.3 mmHg; Xining, China] was conducted, and the indexes were monitored for 5 min after induction of hypoxia. Plasma catecholamine concentrations were also measured. We found that, during normoxia, the mean arterial pressure, heart rate, ascending aortic blood flow and pulmonary artery pressure were reduced in the propranolol and atenolol groups. Catecholamine concentrations were increased significantly in the atenolol group compared with the control group. During hypoxia, mean arterial pressure and total peripheral resistance were decreased in the control, propranolol and ICI 118,551 groups. Pulmonary arterial pressure and pulmonary vascular resistance were increased in the propranolol and ICI 118,551 groups. During hypoxia, catecholamine concentrations were increased significantly in the control group, but decreased in β-AR antagonist groups. In conclusion, the β₂ -AR is involved in regulation of pulmonary haemodynamics in the acute hypoxic compensatory reaction, and the activation of β₂ -ARs attenuates the increase in pulmonary arterial pressure during hypoxic stress. This compensatory reaction activated by β₂ -ARs during hypoxic stress is very important to maintain activities of normal life.

Neurotransmitter Switching Coupled to β-Adrenergic Signaling in Sympathetic Neurons in Prehypertensive States

Single or combinatorial administration of β-blockers is a mainstay treatment strategy for conditions caused by sympathetic overactivity. Conventional wisdom suggests that the main beneficial effect of β-blockers includes resensitization and restoration of β1-adrenergic signaling pathways in the myocardium, improvements in cardiomyocyte contractility, and reversal of ventricular sensitization. However, emerging evidence indicates that another beneficial effect of β-blockers in disease may reside in sympathetic neurons. We investigated whether β-adrenoceptors are present on postganglionic sympathetic neurons and facilitate neurotransmission in a feed-forward manner. Using a combination of immunocytochemistry, RNA sequencing, Förster resonance energy transfer, and intracellular Ca2+ imaging, we demonstrate the presence of β-adrenoceptors on presynaptic sympathetic neurons in both human and rat stellate ganglia. In diseased neurons from the prehypertensive rat, there was enhanced β-adrenoceptor-mediated signaling predominantly via β2-adrenoceptor activation. Moreover, in human and rat neurons, we identified the presence of the epinephrine-synthesizing enzyme PNMT (phenylethanolamine-N-methyltransferase). Using high-pressure liquid chromatography with electrochemical detection, we measured greater epinephrine content and evoked release from the prehypertensive rat cardiac-stellate ganglia. We conclude that neurotransmitter switching resulting in enhanced epinephrine release, may provide presynaptic positive feedback on β-adrenoceptors to promote further release, that leads to greater postsynaptic excitability in disease, before increases in arterial blood pressure. Targeting neuronal β-adrenoceptor downstream signaling could provide therapeutic opportunity to minimize end-organ damage caused by sympathetic overactivity.

Dose-dependent apoptotic and necrotic myocyte death induced by the beta2-adrenergic receptor agonist, clenbuterol

We have investigated the dose- and time-dependency of myocyte apoptosis and necrosis induced by the beta2-adrenergic receptor agonist, clenbuterol, with the aim of determining whether myocyte apoptosis and necrosis are two separate processes or a continuum of events. Male Wistar rats were administered subcutaneous injections of clenbuterol, and immunohistochemistry was used to detect myocyte-specific apoptosis and necrosis. Myocyte apoptosis peaked 4 h after, and necrosis 12 h after, clenbuterol administration. In the soleus, peak apoptosis (5.8 +/- 2.0%; P < 0.05) was induced by 10 mug and peak necrosis (7.4 +/- 1.7%; P < 0.05) by 5 mg x kg(-1) clenbuterol. Twelve hours after clenbuterol administration, 73% of damaged myocytes labeled as necrotic, 27% as apoptotic and necrotic, and 0% as purely apoptotic. Administrations of clenbuterol (10 microg x kg(-1)) at 48-h intervals induced cumulative myocyte death over 8 days. These data show that the phenotype of myocyte death is dependent on the magnitude of the insult and the time at which it is investigated. Only very low doses induced apoptosis alone; in most cases apoptotic myocytes lysed and became necrotic and the magnitude of necrosis was greater than that of apoptosis. Thus, it is important to investigate both apoptotic and necrotic myocyte death, contrary to the current trend of only investigating apoptotic cell death.