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Medical What is the role of alpha 2 receptors?

  1. Sep 18, 2012 #1
    Hello everyone,

    Ok my textbook says inhibition of transmitter release, vasoconstriction and more things. Now my question is what does inhibition of transmitter release mean. Since alpha 1 receptor is present on post synaptic membrane, and alpha 2 is present in pre synaptic membrane, does this mean alpha 2 actually controls the action of alpha 1 receptors by inhibiting neurotransmitter release. If that is the case why are there actions similar, shouldn't alpha 2 have oppposite actions of alpha 1 and actually decrease sympathetic activity. Also if you give a non selective alpha agonist which acts on both alpha 1 and alpha 2 receptors does vasodilation occur, how? Thank you :smile:
  2. jcsd
  3. Sep 18, 2012 #2
    It really depends on where the receptors are, but in general in the blood vessels alpha 2 does the negative feedback job, which is inhibitory. The agonist will constrict blood vessels, antagonist will dilate. The antihypertensive drugs for alpha blocking are usually type 1 at low doses, while both types at high doses, a part of its chemistry, cant be changed.
  4. Sep 19, 2012 #3
    Hey thanks for the help, but since I'm not very familiar with this area I'm bit confused. What I want to know is this. Ok Alpha 1 receptor generally causes vasocontriction. Now alpha 2 receptor can also cause general vasoconstriction. Now my question is alpha 2 also inhibits norepinephrine release, so shouldn't alpha 2 cause parasympathetic stuff, why is it doing sympathetic stuff if it is inhibiting release of nor epinephrine. Thanks :smile:
  5. Sep 19, 2012 #4
    Alpha 2 leads to vasodilation in general arterial circulation. It's a negative feedback mechanism. The Alpha 1 is the constrictor and dominates.
  6. Sep 20, 2012 #5
    Thanks again :smile: Ok my textbook says both vasoconstricts, but I think that is a mistake since, when I look at other sources main actions of alpha 2 are.

    1.Inhibition of norepinephrine release
    2.Inhibition of insulin release
    3. Decreases cAMP

    It doesn't say vasodilation, but I think what u mean is since alpha 2 is on presynpatic nerves, it gives negative feedback for release of norepinephrine from post synapse causing negative feedback of vasoconstriction which is vasodilation. Am I correct?

    Now my question is why is alpha 2 considered an adrenergic receptor, or a sympathetic system receptor, when it does all parasympathetic stuff. Shouldn't it be part of parasympathetic system.
  7. Sep 20, 2012 #6
    I think adrenergic receptor just refers to its binding with epi and norepi.
  8. Aug 27, 2013 #7
    alpha 2 receptor

    All alpha receptor are excitatory except alpha 2 receptor which is inhipitory:
    As alpha 2 receptor has no direct effect on the receptor ( alpha 1) , but it affect on the neurotransemiter like epi nephrin and other ,
    And so;
    If you block alpha 2 by any alpha 2 blocker( like brimonidine ) it will release more epinephrine and norepi. which have excitatory effect on alpha 1 receptor which cause vasoconstriction if present on blood vessels....the epi. and norepi. released dont have any effect on alpha 2 receptor ( which is preganglionic) because it is blocked by the antagonist.
    You can observe that alpha 2 receptor is only regulatory

    I hope it become clear now
  9. Aug 27, 2013 #8
    In addition to inhibiting epi. and norepi., alpha 2 receptors also inhibit acetylcholine release.
  10. Feb 10, 2016 #9
    No one has commented on the fact that alpha 2, when on the postsynaptic cell (in the vasculature of the skin), causes vasoconstriction.
  11. Feb 10, 2016 #10
    This thread is coming up on being three years old.
  12. Feb 10, 2016 #11
    Yes, thank you. It's over 3 years old. Let's try and stay on topic. Do you know the answer aroc91?
    I have found about 6 papers so far explaining the role of a2 receptors in the periphery vs CNS. In short, in the CNS they are inhibitory, and in the PNS there are subtypes that cause vasoconstriction. However, I do not have access to most of these articles - just the abstracts. The articles that are free to view do not give a mechanism of action for these vasoconstriction alpha 2 receptors. I have also looked through 3 medical school textbooks and not found an exact mechanism - only allusions to the fact that this occurs.

    I hope someone will contribute to this question in a positive way: Alpha 2 receptors are coupled to Gi proteins, which reduce cAMP. How do alpha 2 receptors cause vasoconstriction?
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