epinephrine receptor specificity

[unoriginal]

i know epinephrine binds a1,a2,b1,b2, with preference toward b receports... but does it prefer b2 over b1 (or vice versa). A qbank questions states that if you give phentolamine (alpha blocker), then epinephrine, HR will decrease becase b2 action causes vasodilation... if this is true, then the b2 activity must be greater than the b1 on the heart... is this correct?

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[fakin' the funk]

A qbank questions states that if you give phentolamine (alpha blocker), then epinephrine, HR will decrease becase b2 action causes vasodilation... if this is true, then the b2 activity must be greater than the b1 on the heart... is this correct?

USMLE loves questions about the "epinephrine reversal" phenomenon so you gotta learn this one well. The text bolded above is incorrect.

With an alpha-blocker present like phentolamine, epinephrine has no pressor effects at alpha receptors and so acts like a pure beta agonist. Beta-2's cause vasodilation mainly in skeletal smooth muscle and this causes the BP drop; you could reason that this would cause reflex tach. Beta-1's cause tach, increased contractility etc, which will also raise the HR.