Pharmacology
Summary
Alpha antagonist drugs block alpha-1 and alpha-2 receptors, inhibiting their activation by catecholamines and related agonists. Alpha antagonists include phentolamine and phenoxybenzamine. Phentolamine, an alpha-1 and alpha-2 receptor blocker, can be used in cases of cocaine-induced hypertension. Phenoxybenzamine, an irreversible antagonist, is used to prevent the pressor effects of sympathomimetic agents and in the treatment of pheochromocytoma. Alpha-1 selective antagonists, such as prazosin, terazosin, doxazosin, and tamsulosin are particularly useful in patients with hypertension and benign prostatic hyperplasia (BPH). Prazosin also treats some symptoms of post-traumatic stress disorder (PTSD). Lastly, mirtazapine is an atypical antidepressant and alpha-2 antagonist.
Alpha-2 agonists, like clonidine, act centrally on alpha-2 receptors, reducing sympathetic outflow at the brain stem level. Clonidine is used as a sympatholytic in the treatment of hypertension, hypertensive urgency, and attention deficit hyperactivity disorder (ADHD). It also serves as the first-line therapy for chronic multiple tics or Tourette's syndrome. Other alpha-2 agonists include alpha-methyldopa and tizanidine. Alpha-methyldopa is a centrally-acting sympatholytic used for hypertension, particularly in pregnancy, but it can cause a lupus-like syndrome as a rare side effect. Tizanidine, on the other hand, is a muscle relaxant.
Lesson Outline
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FAQs
Clonidine, an alpha-2 agonist, has multiple medical applications. It is extensively used to treat hypertension due to its ability to decrease sympathetic tone, thus reducing blood pressure. Furthermore, it can be utilized in the management of hypertensive urgency. Clonidine is also beneficial in treating conditions such as Attention Deficit Hyperactivity Disorder (ADHD) and Tourette syndrome.
L-methyldopa functions as an alpha-2 receptor agonist, leading to a reduction in sympathetic tone and subsequently decreased blood pressure. This makes it an effective drug in treating hypertension. L-methyldopa is primarily used to manage gestational hypertension. However, it's worth noting that this drug can cause a lupus-like syndrome.
Tizanidine, an alpha-2 agonist, acts as an effective muscle relaxant. On the other hand, Phentolamine is a reversible alpha-1 and alpha-2 receptor antagonist. It causes vasodilation due to its alpha-1 antagonist effect, which is useful in treating cocaine toxicity. It can also aid in managing the symptoms of pheochromocytoma by controlling blood pressure preoperatively. However, it may also cause orthostatic hypotension, leading to reflex tachycardia.
Alpha-1 antagonists, indicated by the "-osin" suffix (such as prazosin, terazosin, doxazosin, tamsulosin), are known to relax smooth muscle in the urethra and prostate, thus treating symptoms of Benign Prostatic Hyperplasia (BPH). Prazosin, uniquely, can also be used to treat Post Traumatic Stress Disorder (PTSD).
Mirtazapine works as an atypical antidepressant with antagonist effects at alpha-2 and other receptors. It is unique in that it enhances the release of serotonin, which can significantly aid in treating depression.
