Alpha-1 Adrenoreceptors

                                    Overview

             Where are alpha-1 receptors located?

Blood Vessels

1. Vasoconstriction: Activation of Alpha-1 receptors in the smooth muscle of blood vessels results in vasoconstriction thereby increasing blood pressure and cardiac output, which is useful in patients with hypotension. 
• Hypotension due to shock is treated with phenylephrine, norepinephrine, or epinephrine.
  
• Midodrine is a prodrug that is enzymatically hydrolyzed to desglymidodrine, a selective alpha-1-receptor agonist used to increase peripheral resistance due to impaired autonomic nervous system function seen in orthostatic hypotension. 
  
2. Epistaxis: 
•  Epinephrine is usually applied topically in nasal packs to reduce regional blood flow .
  
3. Nasal decongestion: decreased blood flow through the nasal mucosa results in reduced mucus membrane congestion.
•  phenylephrine or the longer-acting oxymetazoline are often used in over-the-counter nasal decongestant to reduce the discomfort of allergic rhinitis or the common cold
  
4. Increase duration of local anesthesia 
•  lidocaine with epinephrine is used to reduce diffusion of local anesthetics away from the site of administration 

Iris 

• The iris dilator is composed of radially arranged muscle fibers, which upon alpha-1 activation results in pupillary dilation (mydriasis). 
• Mydriatics such as phenylephrine (mydfrin) are used in eye drops to dilate pupils to facilitate examination of the retina. 
• Sympathomimetics administered as ophthalmic drops are also useful in localizing the lesion in Horner’s syndrome

Liver

• Stimulates liver to release glucose into blood
  
• For example, catecholamines such as epinephrine and norepinephrine induce glycogenolysis within the liver.

Digestive Tract 

• causes decreased motility by relaxation of gastrointestinal smooth muscle

Genitourinary Organs 

• Urinary Continence: the smooth muscle of the bladder wall base, urethral sphincter, and prostate contain alpha-1A receptors that mediate contraction and therefore promote urinary continence. 
• Alpha-1a antagonists are used in the management of symptoms of urinary flow obstruction.

Salivary Gland

• contain adrenoceptors that regulate secretion of amylase and water. 
• dry mouth is usually seen as a side effect when centrally acting sympathomimetic drugs, eg, clonidine, produce symptoms of dry mouth
  

           What medications are adrenoreceptor agonists?