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Discussion Starter · #1 ·
Q. Why competitive antagonist brings no change in efficacy and no effect on Vmax? My understanding is- if antagonist will bind to the receptor site where agonist normally binds, the effect (efficacy) of the agonist should be decreased,isn't it? .Next, about Vmax, when competitive antagonist goes and blocks the the active site of the enzyme where substrate/agonist would normally bind, wouldn't it decrease the reaction rate (Vmax) since enzyme is not working normally in this case due to binding of competitive antagonist?

I know I am missing some concepts here.Can anybody pls help me out to understand this.Thanks. (from:FA, enzyme kinetics & pharmacodynamics)
 

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Discussion Starter · #2 ·
Wanted to add one example to my above question.

If flumazenil (competitive antagonist of diazepam) doesn't decreases the efficacy (effect) of the diazepam, why then it used as an antidote of diazepam overdose? But, again it is said that use of competitive antagonist has no change in efficacy and Vmax. My understanding is in contrast to what is mentioned in book. Need sb's assistance.
 

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Well yeah it's called Vmax... Max....

It means the max that can be achieved....

With competitive antagonists we actually block the receptors with weak bonds...the bonds can be easily broken if we increase the conc of the agonist itself!

So although it decreases or hampers the activities of the agonist it doesn't do it permanently...ie it's competitive! We add more agonist and the receptor will do its job at its max potential!

But in case of non competitive antagonist they bind and destroy a receptor for good...so if we are destroying receptors we are decreasing the tissues maximum ability to receive signals and act...

But in the previous case we only tried to mess up the receptors for a temporary time being! So no effect on Vmax!
 

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Discussion Starter · #4 ·
Thanks cleverfox.That means, only when we increase the concentration of agonist, we can reverse the efficacy and Vmax (not altering the efficacy/Vmax). But, if we do not increase the concentration of agonist (keep the concentration of agonist constant), then efficacy and Vmax will remain decreased,right?So, I don't think it is 100% accurate to say -"Competitive antagonist doesn't change efficacy & Vmax".Maybe it is more accurate to say,competitive antagonist decreases efficacy/vmax until agonist are added;after addition of competitive antagonist , the efficacy/Vmax will be back to its former value.What do u say?Do u agree with me?
 

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Like I said it's v max.... MAX is the word....

And you should remember that we are dealing with the receptor action and not the concentration of the drug when using the term max....

So when we are talking Vmax we mean unlimited agonists stimulating the functional receptors.... So you can assume to keep conc constant
 
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