epinephrine, norepinephrine and insulin release

[Zzmed]

FA states that epinephrine (but not norepinephrine) activates adenylate cyclase---> increases intracellular Ca ---> induces insulin release.

That's incorrect. Epinephrine acts on a2 adrenergic receptors which are coupled to Gi proteins that inhibit adenylate cyclase---> decreases intracellular Ca---> inhibits insulin release.
To reinforce that, BRS phys states that both norepi and epi inhibit insulin release.

What do you think?

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[MCAT guy]

I think its more helpful to think in terms of receptors and then know what NE and epi do after that. My FA copy says beta agonists increase secretion and alpha 2 decreases.

Now if you've taken pharm you know that NE and API work on multi receptors and concentration matters.

My FA copy 2012 is correct.

 

[Ycut]

Epinephrine acts on B2 receptors as well and increases insulin release whereas norepinephrine has no effect on B2

 

[Phloston]

This is why I didn't really care for BRS Physiology. On the USMLE, it would be 100% wrong to ever say that epinephrine inhibits insulin release. They get all technical about the alpha-2 crap, but it's not relevant.

Epinephrine acts most sensitively on beta-2, so it causes insulin release via G-alpha-s.

Low-dose epinephrine = beta-2 > alpha-1/2
High-dose epinephrine = alpha-1/2 > beta-2

However, in both cases, insulin is still released because the beta-cells have beta-2 receptors that respond directly to the epinephrine, whereas the alpha-2 receptors (as far as I'm aware) act indirectly (like giving exogenous guanethidine), so even with high-dose epinephrine, the beta-2 effects on the beta-cells still predominate over the alpha-2 effects.

The only time you have to remember the low- vs high-dose epinephrine rule is for systemic vasoconstriction vs dilatation.

 

[Charles_Carmichael]

The way I think about it, if you're in a fight-or-flight situation, you've got big release of epinephrine into your bloodstream. You're also increasing plasma levels of glucose, so that your body has lots of energy it can utilize in this situation. However, since most of the tissues in your body require insulin in order to take in glucose, you also want to increase insulin secretion -- it's a waste if you raise your plasma glucose but your muscles have no way of taking it in.

From there, it's just a matter of remembering that a2 receptors decrease insulin secretion while b2 receptors increase insulin secretion. I'm pretty sure (but not entirely) that even at high doses of epinephrine, where it acts on alphas as well, the b2 effect predominates in terms of insulin release.

Hope that helps.