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Saturday, March 9, 2013

How do respiratory medicines work?

This post is meant to be a througough review of how beta agonist medicine works.  This includes medicine such as epinephrine (adrenaline), albuterol (proventil, ventolin), and lebalbuterol (xopenex).  From the moment the medicine is entered into the body, what happens?

To answer this question we have to first understand how medicine works.  To get an understanding of how medicines work on the body we have to have an understanding of some basic anatomy.  So I'm going to start right from the beginning. I will try to make this simple.

There are two nervous systems. 
  1. Autonomic Nervous System:  It controls the many body functions that you do not have control over, such as your heart, vessels, stomach, intestines.  
  2. Somatic Nervous System:  It allows you to control various parts of your body, such as your arms, legs, and breathing.  
For the case of this post, we are only concerned with the sympathetic nervous system.  I will discuss this in a later post, or you can read about it here.

Sympathetic Nervous System:  It has two divisions that both effect heart, smooth muscles, iris of the eye, salivary glands, and urinary bladder.
  1. Symmpathetic Nervous System (SNS): Also called flight or fright.  It prepares the body to handle stress, either real or perceived. The stress could be trauma, or it could be someone holding a gun to your head.  It could be that you just heard about a family member dying, or your boss screaming at you.  When any sort of stress occurs, your sympathetic response will be to cause vasodilation that increases your blood pressure and heart rate, and relaxes involuntary smooth muscles to dilate air passages, bladder, gastrointestinal tract, etc. Various medicines can mimic all or any of the sympathetic responses, and are called sympathomimetic or adrenergic agonists.  
  2. Parasympathetic Nervous System (PNS): It generally does the opposite of the sympathetic and causes vasodilation and smooth muscle constriction to decrease heart rate and blood pressure, empty the bowel and bladder, etc. It attempts to put things back the way they were before the sympathetic responded to stress. Medicines that mimic this response are called parasympathomimetic or cholineric agonists. Medicines that block this are called anticholinergic. 
Receptors:  Along all the muscles and vessels inside your body are receptor sites.  Many of these are attached along a nerve, and are at the receiving end of an impulse.  When certain hormones are sent along the nerve and received by that receptor, a series of chemical reactions occur that causes a response by the muscle or vessel (either dilation or contraction). 

The main organ that makes the hormone that we are concerned with is the adrenal gland, which sits on either side of the kidney.  When you become excited or stressed, this gland secretes adrenaline that is sent down neurons to the various receptor sites.  Adrenalin extracts were discovered and named just prior to the turn of the 20th century, and isolated in 1901. It was learned that these extracts (later learned to be the hormone adrenaline) mimic the sympathetic response, and worked great for asthma and hay fever.  It is for this reason that receptor sites for this system are called adrenergic receptors.  In Britain the term adrenaline continues to be used, although in the United States the name epinephrine is used.  So this should explain some of the wording used here.  

Alpha Receptor sites:  Hormones released by the SNS system become attached to the following receptors to cause the following response:
  1. Beta 1 (B1):  Located on heart muscle.  Causes increased rate and force of heart  to increased cardiac output)  You are causing a narrow tube, so blood will flow faster with a stronger force.  A sign that this is happening is you will notice a full or bounding pulse, or, better yet, an increase in blood pressure.  Blood pressure is directly correlated with cardiac output.  It is for this effect that epinephrine is used during cardiac arrest. It is a strong vasopressor (increases blood pressure)
  2. Beta 2 (B2):  Located in lungs.  Cause smooth muscle that wrap around the lungs to relax and this causes bronchodilation.  This is easy to remember because you have 2 lungs.
  3. Alpha 1 (A1):  Located in peripheral blood vessels.  Causes vasoconstriction to increase heart rate and force of contraction (increased blood pressure)
  4. Alpha 2 (A2): Located by the nerve synapse.  Causes vasodilation to lower blood pressure. These act like a thermostat and once the heart rate and force are to high, it shuts turns them down.  
Adrenaline (epinephrine):  This hormone regulates the SNS response and readies the body for flight or fight.  Adrenaline is released and attaches to B2, A1, or A2 receptors. Albuterol and Lebalbuterol are refined versions of adrenaline, and they have a strong effect on B2 receptors to cause bronchodilation and have a very weak effect on A1 and A2 receptors to minimize side effects.  You can easily remember the effects in the following manner:
  1. B2: You have 2 lungs
  2. B1:  You have one heart
  3. A1: You have one heart
Noradrenaline (norepinephrine):  Attach to B2 and A1 to act as vasopressors. 

Dopamine:  It is also created by the adrenal gland, and drugs that mimic it attach to A1 and A2 receptors to cause vasoconstriction and increased rate and force of heart and increased blood pressure. When attached to receptor sites, it stimulates the release of norepinephrine to generate a better blood pressure (vasopressor)

Dobutamine:  Effects B1 receptor sits and causes increased heart rate and strength of cardiac contraction (increased blood pressure).  It is generally used for heart failure (CHF) to make the heart a stronger muscles.  It increases cardiac output and blood pressure without much increase in heart rate. 

Beta blockers:  These are drugs that block the beta receptors.  The effect is mainly to try to control blood pressure, although a major side effect may be to cause narrowed air passages.  It is for this reason Beta blockers should be used with caution on patients with asthma or similar lung diseases. 

Albuterol:  It is a refined version of epinephrine without the side effects.  It has a strong affinity to B2 receptors and only slight affinity to B1 and A1.  It is very safe and the best selling asthma medicine of all time. 

Levalbuterol:  It is a refined version of Albuterol.  Debate is still out if it has fewer side effects or a stronger B2 effect. Initial studies showed it did, recent studies show it does not. 

Adrenal Gland:  It makes the hormones that effect upon the adrenergic receptor sites.  It makes the neurotransmitter dopamine, which goes through a series of chemical changes to become the neurotransmitter norepinephrine and then the neurotransmitter epinephrine. It also makes the neurotransmitter acetylcholinen which acts upon the PNS receptor sites, which are referred to as Cholinergic Receptor Sites. 

Cholinergic Receptor Sites:  Receptor sites used for the PNS are called cholinergic receptors.  The main neurotransmitter here is acetylcholine, hence the name cholinergic. It is used to cause bronchoconstriction and vasodilation, or to return things back to normal.  It basically has the opposite effect as the SNS.  The two types of receptors are:
  1. Nicotonic:   Found in central nervous system, autonomic ganglia, and striated muscle. 
  2. Muscarine:  Found in cardiac and smooth muscle, exocrine glands and brain
Atropine:  It competes with aceylcholine for muscarine receptors, and therefore blocks the effects of the PNS.  This results in an increase heart rate and bronchodilation.  It is used for bradycardia and asystole (flatline, non beating heart).   Herbs that contained this chemical were used for asthma-like symptoms going all the way back to ancient Egypt.  So when you read the history of asthma, you will probably hear about asthma cigarettes, incense, and other inhaled methods.  The active ingredient was always atropine, and the herbs it was contained in were strammonium and belladonna. 

Atrovent:  This is a refined version of atropine without the side effects.  It is recommended as a preventative medicine for COPD and hardluck asthma.  It needs to be taken four times a day for effect.

Spiriva:  This is a refined version of Atrovent that lasts 12 hours and only needs to be taken twice a day. 

  1. Guy, Jeffrey, "Pharmacology for the prehospital setting," 2007, U.S., Jones and Bartlett Learning, 

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