spare receptors

[Miracoli]

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Drug X is an agonist in a system that contains spare receptors. Drug Y does not produce any
action by itself but decreases the effect of drug X by binding to a different site. Which of the following graphs represent the effect of drug Y, in which the first curve is drug X alone and the other 2 curves are drug X plus increasing doses of drug Y?

A)A
B)B
C)C
D)none of the graphs

 

[sanj238]

Drug X is an agonist in a system that contains spare receptors. Drug Y does not produce any
action by itself but decreases the effect of drug X by binding to a different site. Which of the following graphs represent the effect of drug Y, in which the first curve is drug X alone and the other 2 curves are drug X plus increasing doses of drug Y?

A)A
B)B
C)C
D)none of the graphs

Should be B since the concentrations arent affected...havent looked at this in a while but I dont know why the concentrations are changing for it....the affinity isnt being affected, merely its rate of action or ymax so if B is not a mistake on their end then its D

 

[ChessMaster3000]

Should be B since the concentrations arent affected...havent looked at this in a while but I dont know why the concentrations are changing for it....the affinity isnt being affected, merely its rate of action or ymax so if B is not a mistake on their end then its D

I don't understand the whole spare vs no spare receptors thing, but if there are enough spare receptors, then why not C?

 

[sanj238]

I don't understand the whole spare vs no spare receptors thing, but if there are enough spare receptors, then why not C?

I mean I could be wrong so this whole exercise may be futile but Ill try and make sense of it. So there are a few ideas that you should get a hold of. First, if a ligand or drug is binding elsewhere and not the receptor then it has NO effect on affinity- thus it does not affect concentration. concentration refers to the ligand that normally binds to the receptor to cause an action. The amount of ligand needed for the reaction is dependent on affinity. The greater the affinity the lower the concentration needed. This is Km. If the drug binds elsewhere to the receptor it is a NON competitive drug because it is not competing with the ligand for the receptor - the ligand can bind to the receptor just as much as it has before the drug was there. However, if the durg is a competitive blocker then it actively is getting in the way of the ligand- so the affinity of the receptor to the ligand is decreasing. Think of it this way. If you're shooting a basketball into a hoop in a game of horse someone can block your shot by shooting another bball in the way- thats what a competitive inhibitor does. The affinity of the hoop for your basketball has now dropped. so MORE concentration of your balls are needed to increase the probability that one of them will go into the hoop and avoid the other person's basketball...

Anyway, in this case we have a NON compeittive inhibitor that acts on a site elsewhere to the receptor- so the (switching to metaphor now)- proverbial basketball can go through the hoop with no interference from another b-ball. The concentration should not be affected.

As you can see in C- the concentration is increasing with each curve to maintain teh same activity- which should not take place

 

[Miracoli]

check the answer and explanation here
http://www.usmleforum.com/files/forum/2013/1/762619-1.php
http://www.usmleforum.com/files/forum/2013/1/762597.php

 

[Miracoli]

check the answer and explanation here
www. usmleforum. com/files/forum/2013/1/762619-1.php
www. usmleforum. com/files/forum/2013/1/762597.php

 

[sanj238]

while I thank you for the effort, the answer you have is so long winded its hard to even read it. Its just tl dr.

check the answer and explanation here
www. usmleforum. com/files/forum/2013/1/762619-1.php
www. usmleforum. com/files/forum/2013/1/762597.php

 

[Miracoli]

It is a complex topic🙂

 

[Fatalis]

Explain how B is not correct

 

[NMDO]

Here is a graph I think helps with this from the links that someone posted above, just in case you didn't want to go digging through those forums.

http://i.imgur.com/F2N1a85.png

 

[Miracoli]

It is all there🙂
The start point of all 3 curves should be at same point in B in order to be Noncompetitive antagonist with no spare receptors
and
The start point of the of the 2 red curves should be at same point in C in order to be Noncompetitive antagonist with spare receptors.

Check these graphs for
http://i.imgur.com/F2N1a85.png
A) for Competitive antagonist
B) for Noncompetitive antagonist with NO spare receptors
C) for Noncompetitive antagonist with spare receptors