First, some pharmacodynamic definitions related to the animation:

Full Agonists: Compounds that are able to elicit a maximal response following receptor occupation and activation.

Partial Agonists: Compounds that can activate receptors but are unable to elicit the maximal response of the receptor system.

Inverse agonist: an agent which binds to the same receptor binding-site as an agonist for that receptor and reverses constitutive activity of receptors. Inverse agonists exert the opposite pharmacological effect of a receptor agonist.

Competitive and Irreversible Pharmacologic Antagonists

Competitive antagonists are drugs that bind to the receptor in a reversible way without activating the effector system for that receptor. In the presence of a competitive antagonist, the log dose-response curve is shifted to higher doses (ie, horizontally to the right on the dose axis) but the same maximal effect is reached (Figure 2-5A).

In contrast, an irreversible antagonist causes a downward shift of the maximum, with no shift of the curve on the dose axis unless spare receptors are present (Figure 2-5B). The effects of competitive antagonists can be overcome by adding more agonist. Irreversible antagonists cannot be overcome by adding more agonist. Competitive antagonists increase the ED50; irreversible antagonists do not (unless spare receptors are present).

Agonist dose response curve in the presence of competitive and irreversible antagonists