DrVirgo
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The following changes in heart rate are noted giving Drug Q alone and in presence of other drugs:
Drug Q given alone: decrease HR
Drug Q in presence of Hexamethonium: increase HR
Drug Q in presence of Atropine: increase HR
Drug Q in presence of Phenoxybenzamine: increase HR
What is Drug Q?
A. Ach
B. Hydralazine
C. Norepinephrine
D. Isoproterenol
E. Edrophonium
Please EXPLAIN your answer!
Thanks. 
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| doc179
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I think the ans is B..hydralazine cos hexamethonium is increasing HR and phenoxybenzamine is also increasing HR and given alone it dcreases HR
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| doc179
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can not be A as hexamethonium is increasing HR in the Q
can not be C cos phenoxybenzamine is increasing HR in the Q
can not be D as given alone its decreasing HR in the Q
can not be E as it is increasing HR with hexamethonium
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| msyamp
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hmmm. hi virgo. the answer is Norepinephrine. the decrease in heart rate is due to the reflex.
now with hexamethonium the reflex is gone so increase heat rate.
now with atropine again the reflex is lost and also atropine it self causes high HR.
phenoxy bezamine an a blocker causes vasodilation and only action of norepi is now b1 and so inc HR
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| malinda
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My guess is Ach.It causes vasodilation and bradycardia.
hexamethonium acts on ganglions blocking it...so increasing HR.
phenoxy benzamine block alpha and cause reflex tachycardia
atropine blocks M receptors ie post ganglionic PANS causing tachycardia.
now norepinephrine makes sense too
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| malinda
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msyamp just one q...NE acts on both apha 1,2 and beta 1.If it is having a reflex bradycardia,then it is also causing increased HR thru beta 1..why is it that reflex is dominant?
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| DrVirgo
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msyamp is right. It is Norepi.
Here is the explanation according to me.
(I understood the q and a, except for one q i have which is in bold bleow)
First Know:
Norepi has alpha 1, alpha 2, and beta 1 activity.
alpha 1 efx through a reflex can decrease HR
beta 1 efx acting directly on the heart can increase HR
-So Norepi given alone: I understand that an alpha 1 stimulation causes vasoconstriction which increases TPR, which then increases MAP, so there is a reflex bradycardia to compensate.
-My question is: Why did we see the efx of alpha 1 causing bradycardia and NOT the effects of beta 1 which causes tachycardia? (does it have to do with dose?)... As far as I know Norepi is typically known to increase heart rate.
-Next: Norepi given with Hexamethonium: Increase HR. This is because Hexamethonium blocked the alpha 1 reflex bradycardia, and the reason we see an INCREASE in HR instead of no change is because Norepi also has beta 1 activity, which increases heart rate.
-Norepi given with Atropine: Produces an Muscarinic block on the M2 receptors of the heart, so if the M2 receptors are blocked, reflex tachy can no longer occur. so if there the reflex is lost like msyamp said, then the beta 1 effects can predominate, and heart rate increases.
-Norepi with Phenoxybenzamine: Increases HR. Again its a block of the alpha 1 efx since phenoxybenzamine is an alpha blocker. Now beta 1 effects can predominate and the heart rate increases.
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| DrVirgo
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Ok, this q was tricky, but here is my explanation from another post:
So the net effects of Norepi are:
-Beta 1: Increase HR, increase Pulse pressure
-alpha 1: Increase TPR (VC) ---> increase diastolic pressure ---> Increase MAP (BP) ---> Baroreceptor reflex ---> Reflex bradycardia.
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