Step I Beta-blocker question

[Phloston]

Could someone please explain why a beta-1-selective blocker would cause the findings in the bottom graph?

Cheers,

 

[thehundredthone]

Erm, because HR is B1 mediated, and bronchodilation is B2 mediated.

 

[usmle270]

Epinephrine > B1 + B2 agonist

B1 > increase in heart rate
B2 > bronchodliation ( graph1 )

Antagonist by themselves have NO ACTION ( graph 2 )

Epinephrine + B1 selective > Will block b1 action but b2 action is unaffected ( graph 3 )

 

[busy08]

If it was b2# then epi +b2# shd have zero resist
B1# at high doses also block some B2 recep so graph shows some bronchoconstriction

 

[Phloston]

If it was b2# then epi +b2# shd have zero resist
B1# at high doses also block some B2 recep so graph shows some bronchoconstriction

So that's the reason why the bronchodilation isn't as substantial, because large doses of beta-1-selective blockers also block beta-2?

The answer to that question was metoprolol for drug-X, but it didn't make sense to me why bronchoconstriction would be affected.

 

[usmle270]

If it was b2# then epi +b2# shd have zero resist
B1# at high doses also block some B2 recep so graph shows some bronchoconstriction

b1 selective has some b2 blocking action .

 

[Phloston]

Assuming the same dose of metoprolol is used for the "drug-X alone" graph, if in fact it has beta-2 blocking action, why wouldn't the bar be positive (implying bronchoconstriction) then? It's almost as though the beta-1-selective blocker, at high doses, doesn't cause bronchoconstriction on its own via blocking beta-2, but instead merely prevents optimal bronchodilation in the presence of a beta-2 agonist.

 

[thehundredthone]

http://www.rxlist.com/lopressor-drug/clinical-pharmacology.htm

This preferential effect is not absolute, however, and at higher doses, Lopressor (metoprolol tartrate) also inhibits beta2 adrenoreceptors, chiefly located in the bronchial and vascular musculature.

Although it goes on to say this too

Relative beta1 selectivity has been confirmed by the following: (1) In normal subjects, Lopressor (metoprolol tartrate) is unable to reverse the beta2-mediated vasodilating effects of epinephrine.

 

[thehundredthone]

Assuming the same dose of metoprolol is used for the "drug-X alone" graph, if in fact it has beta-2 blocking action, why wouldn't the bar be positive (implying bronchoconstriction) then? It's almost as though the beta-1-selective blocker, at high doses, doesn't cause bronchoconstriction on its own via blocking beta-2, but instead merely prevents optimal bronchodilation in the presence of a beta-2 agonist.

No B2 blocker will cause bronchoconstriction of its own accord (i.e. in a normal individual). It is a risk for those already predisposed to bronchoconstriction.

 

[usmle270]

Assuming the same dose of metoprolol is used for the "drug-X alone" graph, if in fact it has beta-2 blocking action, why wouldn't the bar be positive (implying bronchoconstriction) then? It's almost as though the beta-1-selective blocker, at high doses, doesn't cause bronchoconstriction on its own via blocking beta-2, but instead merely prevents optimal bronchodilation in the presence of a beta-2 agonist.

Antagonist have no action of their own.Even if this was hypothetically a b2 blocker it would never CAUSE bronchocontriction.It would just now allow vasodilation .The graph would show no change then.

 

[busy08]

B2# is antagonist so it cant cause bronchoconstriction on its own