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Alpha & Beta Receptors Discussion


AnthonyM83

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Alpha blockers tend to be centrally acting for HTN or BPH

http://www.mayoclinic.com/health/alpha-blockers/HI00055

They seem to be pretty popular in the local ER for new onset HTN. They can also be used off-label for behavioral disorders.

When dealing with beta blockers, one should remember that the newer agents are a bit more cardio-specific. Propranolol was the prototype agent, and caused many respiratory problems. The more beta-1 specific agents also tend to lose their specificity at higher doses.

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http://www.mayoclinic.com/health/beta-blockers/HI00059

Above page says beta blockers help blood vessels relax and open, improving blood flow. Which beta receptor function would that be countering? Beta receptors don't cause vasoconstriction (rather dilation in coronary and skeletal arteries)...

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You are absolutly right... I just spent 1/2hr researching this and cannot find any info to support thier claim at all.

I even looked at Beta 3 (not important don't confuse yourself), and studied individual B Blockers for a vasodilation effect. None.

Who knows where they got thier info from.

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I'm willing to speculate on the direction they are going with that statement.

Because a beta blocker can help to reduce the release of sympathetic agonists, it stands to reason that having less NE/Epi circulating will allow for some vasodilation. Also, if the beta blocker in question is beta-1 specific, the beta-2 receptor should remain unaffected by it's effect allowing for smooth muscle relaxation.

The Mayo page seems to be targeting the lay public with very general information, so it may well be simplifying the way they work.

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If the parasympathetic system uses cholinergic neurotransmittors what affect would an anticholinergic drug such as Atrovent do?

It would act as an antagonist for muscarinic cholinergic receptors (mAchR), such as those on the heart and lungs. It binds to the receptors and stops the neurotransmitter (Ach) from getting to the receptor. Atrovent is usually used for chronic lung diseases, but atropine, a very similar drug, for temorarily negating the effects of the parasympathetic nervous system on the heart (i.e. get the heart to beat faster).

I believe Atrovent is not specific enough to the beta receptors in the lung not to be able to cause some other beta antagonist effects, so don't be surprised if it increases heart rate, cardiac output and even blood pressure.

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I believe Atrovent is not specific enough to the beta receptors in the lung not to be able to cause some other beta antagonist effects, so don't be surprised if it increases heart rate, cardiac output and even blood pressure.

Atrovent does not directly act on sympathetic receptors. It will merely antagonize the effects of a receptor that will reduce the effects that you see. To use an earlier analogy, more brake, same amount of gas.

Also remember that different tissue locations will have different concentrations of individual receptors. Smaller bronchial smooth muscle has more sympathetic, larger conductive airways have more parasympathetic, muscarinic in this case.

This also accounts for the primary way epinephrine works during cardiac arrest. There are more beta-2 receptors in the central circulation (smooth muscle relaxation), and more alpha-1 receptors in the peripheral vasculature (vasoconstriction). The combined effect allows less theoretically oxygenated blood from leaving the vital organs.

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Atrovent does not directly act on sympathetic receptors. It will merely antagonize the effects of a receptor that will reduce the effects that you see. To use an earlier analogy, more brake, same amount of gas.

Ehemm - my mistake, I have no idea how the word "beta" got in there. :shock: It is, as you said, an anticholinergic drug, an antagonist for muscarinic, cholinergic receptors of the parasympathetic nervous system. The word "beta" doesn't even make sense in my post, especially given the rest of the sentence about increased heart rate (how would a beta antagonist raise heart rate? ;) ).

My post was in response to a question from mobey on anticholinergic drugs, so I wasn't really thinking of the sympathetic nervous system, although 5% of the sympathetic nervous system does use acetylcholine as a neurotransmitter (post-ganglionic).

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I can contribute some memory aids:

You have one heart (B1) and two lungs (B2).

ABEAM = common B1 selective beta blockers

Atenolol

Bextaxolol

Ethambutol

Acebutolol

Metoprolol

The name ------olol, as you can see above, is pretty useful for recognizing a B-blocker.

"Point and Shoot" = Parasympathetic controls errection, Sympathetic controls ejaculation.

Regarding some of the things mentioned but not discussed above:

1.Nicotinic acetylcholine receptors - these are actually used quite commonly, although we often don't think of it as such. The drugs that act here are the paralytics, vecuronium for example. They cause paralysis by blocking nicotinic cholinergic receptors in the nerves that go to the muscles (which are neither sympathetic nor parasympathetic.

2. Alpha 1 blockers are actually quite common drugs, used for the indication of "BPH" Benign Prostatic Hypertrophy, basically difficulty peeing due to large prostate. This of course makes sense if you think back to "fight or flight"....no time to pee when you are fighting or running away. Flomax (tamulosin) is an example, although admittedly it is specific to a subtype of alpha receptors and should not cause a lot of orthostatic hypotension (low BP with standing up). This fact also makes terazosin (a less specific alpha blocker) a good choice for those with both hypertension and BPH.

3. It might intrest you to know that there are not only chemical differences between the parasympathetic and sympathetic systems (ie catecholamines (like epi) vs acetylcholine) but the systems are also anatomically seperate. The parasympathetic nerves come only from the cranial nerves and sacral levels of the spine; "craniosacral in origin". The sympathetic nerves only come from the thoracic and lumbar levels of the spine; "thoracolumbar in origin". Thus the parasympathetic to the heart arrives via a small nerve ( the vagus) that comes dirrectly off of the brain and travels down to the heart (and beyond...). The spinal cord is completely uninvolved.

I thought of a drug that hasnt been discussed here yet:

Can anyone think of an alpha agonist that can be used to treat hypertension (or withdrawl :lol: )? Seems counter-intuitive. How does that work?

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