Originally Posted by Hope2Pass
1) A non-competitive antagonist works at a different receptor binding sites and alter the configuration of the actual receptors for the agonist. Therefore, it reduces the number of receptors available for the agonist and that reduces the effect (efficacy). Non-competitive antagonist DO NOT compete with the agonist for the receptors. That is why the Potency remains the same because both drugs (the agonist and the antagonist) work on different receptors. The same dose of the agonist can still cause the effect but it is a reduced effect due to lack of receptors.
2) In competitive antagonist the potency decreases because the agonist and antagonist compete for the SAME receptors. The effect does not change because the same number of receptors are available for both. That is primarily why, if you increase the dose of the agonist, it will overcome the effects of competitive antagonist. Basically, you need a bigger dose (Potency) to get the same effect because with increased dose/concentration, it will bind more to the receptors than the competitive antagonist.
I'm actually wondering about noncompetitive antagonist and the "potency" being the same. I wonder if this is just a matter of terminology. Noncompetitive antagonists, when added, should result in a decreased Vmax and an unchanged Km. While I believe it is valid to equate (to some degree) potency with Km, saying that the potency doesn't change here seems incorrect to me.
Potency, according to first aid, is the "AMOUNT of drug needed for a given effect", in other words, the dosage you need to give in order to produce that same effect (that same rate of V). The Vmax is already decreased with the addition of a noncompetitive antagonist, which should mean that to produce the same effect of rate of V, the same absolute rate (V), not a PERCENTAGE of Vmax, should require more substrate. There is a distinction to be made between the absolute, numerical rate (V) as opposed to the "percent of maximum effect."
I hope this makes sense...