Competitive antagonists increase the ED50 while the Emax remains the same. Irreversible antagonists do not change the ED50 while decreases the Emax.
Katzung pharmacology, says that the irreversible antagonist may change the ED50 when spare receptors are present. How is that? What difference would it make? Could anyone explain the change graphically?
Spare receptors are said to be present when the Kd (dissociation constant, from the drug-receptor binding curve) is more than the ED50 (from the dose-response curve). This may happen due to
- prolonged effect of a drug-receptor interaction or
- when the receptors are actually in excess of the drug
ED50 = effective dose at 50% of maximum effect, Kd = Dissociation constant.