epinephrine - alpha beta receptor question

[MattBiggar80]

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subject: epinephrine - alpha beta receptor question


help help
student doctor sdn

hey everybody
i have some questions regarding

i just read this paragraph:

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http://www.mc.uky.edu/pharmacology/instruction/pha824ar/pha824ar.html


Effect of Catecholamines on Vascular Smooth Muscle Epinephrine
Associated with vascular smooth muscle are a large number of alpha1 receptors relative to beta2 receptors. However, epinephrine has a higher affinity for the beta2 receptor relative to the alpha1 receptor. Activation of the beta2 receptor would produce vasodilation while activation of the alpha1 receptor would result in vasoconstriction. Recall from the lectures on drug-receptor interactions the magnitude of effect is dependent on the degree of receptor occupancy. Therefore, the effect of epinephrine on smooth muscle is dependent on its relative affinity for alpha1 and beta2 receptors and its concentration. At low doses, epinephrine can selectively stimulate beta2 receptors producing muscle relaxation and a decrease in peripheral resistance. However, once epinephrine concentrations are reached which bind to the alpha1 receptor, vasoconstriction will occur. The two effects (smooth muscle relaxation and contraction) will oppose one another. However, as the concentration of epinephrine increases, the predominant effect will be vasoconstriction.



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question:
i saw on the wiki stub that Beta receptors are more for fight or flight and alpha is more for rest and digest

If the patient is undergoing a flight or fight response, i would expect a LARGE amount of epinephrine to be released into the blood system. this would cause smooth muscle relaxation. I'm not sure if they are talking about increasing blood flow to the smooth muscles of the GI (rest and digest) or they are talking about relaxing the smooth muscle cells around the vasculature to increase the blood flow to the skeletal muscles (fight or flight).

In a nutshell, i am confused!
dont we want beta2 receptor activity to be dominant when there are High levels of epi in the blood for fight or flight?


thank you
[email protected]

 

[rootz02]

if im not mistaken, the relative concentration of each receptor is different for each tissue. for example, there will be more beta receptors associated with skeletal and cardiac muscle, allowing increased binding efficiency of epinephrine to those receptors to stimulate vasodilation in those areas; furthermore you will find more alpha receptors found in the smooth muscles of arterioles in the gut and skin, out competing the fewer neighboring beta receptors so that epinephrine can simultaneously stimulate vasoconstriction in those areas during this sympathetic N.S. response.

"Associated with vascular smooth muscle are a large number of alpha1 receptors relative to beta2 receptors"

^ I think this is somewhat misleading. In totality, alpha receptors may be more numerous, but each tissue type varies in receptor concentration. and to answer your question more directly, yes, you do want beta2 receptor activity to be dominant with high levels of epi during a fight or flight, but you also want alpha1 to be active. the key is to have them both active in the right places.. stimulate alpha1 in the gut and skin so that blood vessels constrict (divert blood away from GI tract since running from a bear isn't exactly the time to digest that pb&j sandwhich), and simultaneously stimulate beta2 in skeletal and cardiac muscles to aid in your "fighting" or "fleeing". all of this is achieved by differential receptor distribution, responding to the singular dump in epi from adrenal medulla.

 

[MattBiggar80]

Let's say there is a fight flight response.

High Epi in the blood.
With high Epi you want to activate beta 2 an open up airways and increase cardiac output.

But according to this info u have the opposite.
With high number of alpha u have vasoconstriction of blood vessels to heart and lungs.
Strange.

 

[rootz02]

i dont think my response implied that, but maybe i explained it poorly.

i'm saying you have more beta receptors than alpha receptors in the heart, lungs, and skeletal muscles. therefore the epi will bind to those beta receptors and cause dilation (favorable response given sympathetic activity). simultaneously, you have more alpha receptors than beta receptors in the gut and skin, causing vasoconstriction (also favorable, since digestion isn't needed during sympathetic dump of epi).

**unless you meant the info from the wikistub. and yeah, i don't exactly know what they mean by alpha generally out numbering beta. it might be bc the GI tract is really convoluted and coiled, and has a really high surface area to volume ratio. so if there are a ton of alpha receptors all along the GI tract, in total they might outnumber the beta receptors of the heart, lungs, and muscles.

 

[rootz02]

Beta receptors are more for fight or flight and alpha is more for rest and digest

break it down by NS: fight or flight is sympathetic response while rest and digest is parasympathetic response.

sympathetic receptors are adrenergic, including alpha and beta receptors (both 1 and 2 for each) and parasympathetic receptors are cholinergic (including muscarinic and nicotinic).

epinephrine for sympathetic response can cause both dilation (if bound to beta receptor) and constriction (if found to alpha receptor).

acetylcholine for parasympathetic can cause both dilation (if bound to m3 and coupled to NO formation, which are concentrated in GI tract) and also constriction (if bound to other M2 receptors, or when NO formation is blocked).

again - i'm only a predent, but this is my understanding of everything. hopefully one of the many colleagues who have already viewed this will take the time to answer this question.