What is the role of alpha 2 receptors?

  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. 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. 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. Alpha 2 leads to vasodilation in general arterial circulation. It's a negative feedback mechanism. The Alpha 1 is the constrictor and dominates.
  6. 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. I think adrenergic receptor just refers to its binding with epi and norepi.
  8. 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. In addition to inhibiting epi. and norepi., alpha 2 receptors also inhibit acetylcholine release.
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