Step I Epinephrine reversal cause?

[cpufreak3]

Why does an alpha blocker cause reversal of epinephrine?

Epi = a1, a2, b1, b2 agonist

If you block alpha only, you get Epi acting as a beta-agonist.

I understand b2 causes vasodilation, but wouldn't heart rate also increase due to b1 stimulation, thereby negating the effect?

 

[Jamiu22]

Why does an alpha blocker cause reversal of epinephrine?

Epi = a1, a2, b1, b2 agonist

If you block alpha only, you get Epi acting as a beta-agonist.

I understand b2 causes vasodilation, but wouldn't heart rate also increase due to b1 stimulation, thereby negating the effect?

What you say definitely makes sense, but the questions I've seen regarding this very topic deal only with BP and it seems the vasodilation effects of B2 is dominant enough to overcome the adrenagic effects of B1/B2. As you know, increased heart rate from B1 won't increase BP, but rather help the heart function better if preload is lower...

...although the ionotropic effects (increased contractile force) from B1/B2 could raise the BP... I feel that maybe the fact that you are vasodilated means you would get a lower blood return/preload to the heart, thus reducing contractile force.... makes sense in my head.. but don't know if its accurate.

 

[addo]

In the presence of an alpha blocker, the net amount of vasodilatation increases due to the unopposed effect of b2.
Net contractility is not going to change, because the same amount of b1 receptors are going to be stimulated with or without an alpha blocker. However, there may be a reflex increase in cardiac output due to sympathetic outflow due to decreased tension on the carotid sinus... but this is not the same as saying that the alpha blocker directly increased the cardiac output.