Jaanu
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Can someone please explain this to me? Thanks in advance

Why does an alpha receptor antagonist drug reduce hypertension but cause reflex tachycardia?
And why is a beta 2 receptor agonist used for the treatment of asthma?
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thegodofgod
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(Original post by Jaanu)
Can someone please explain this to me? Thanks in advance

Why does an alpha receptor antagonist drug reduce hypertension but cause reflex tachycardia?
And why is a beta 2 receptor agonist used for the treatment of asthma?
All adrenergic (alpha and beta) receptors are G-protein-coupled receptors, and are located in various tissues. Alpha1 receptors are located on smooth muscle tissue in walls of veins, and upon activation through Gq and IP3 as the second messenger, release intracellular Ca2+, causing smooth muscle contraction. This causes the lumen of the veins to constrict, increasing blood pressure. Therefore, if you gave an alpha1 receptor antagonist, e.g. doxazosin, it would cause hypotension. This reduction in blood pressure is sensed by the cardiorespiratory centre in the medulla oblongata, which causes activation of the sympathetic nervous system, sending more action potentials down towards the sinoatrial node in the heart, causing the heart to beat faster (tachycardia). This is a reflex in order to increase blood pressure.

Bear in mind not to confuse alpha1 and alpha2 receptors, as they have opposite effects with regards to blood pressure (alpha2 receptor agonists, e.g. clonidine, will reduce blood pressure).

Some beta2 receptors are located on the smooth muscle cells of bronchioles in the lungs. Upon activation through Gs, intracellular cAMP concentration is increased. cAMP inhibits an enzyme called myosin light chain kinase, and activates an enzyme called myosin light chain phosphatase. Both of these actions causes smooth muscle relaxation, causing bronchodilation, which relieves symptoms of an asthma attack, e.g. shortness of breath. Therefore, beta2 receptor agonists, e.g. salbutamol (short-acting) and salmeterol (long-acting) can be used in an acute asthma attack and in asthma maintenance therapy, respectively.

Hope this helps!
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