Given answers
Interactions of agonists and antagonists
Competitive antagonists can displace agonists from their binding site. This reduces the effect of the agonist (at a certain concentration on the dose-response curve). An increase in the concentration of the agonist can in turn displace the antagonist from the receptor and enhance the effect of the agonist. This shifts the dose-dependency of the agonist's effect in the presence of an antagonist to the right, and there is an apparent decrease in the agonist's affinity.
Non-competitive antagonists do not bind to the receptor; they inhibit the effect at another site in the signaling cascade (e.g. by closing ion channels). The maximum effect is reduced, and the dose-response curve is scaled without being shifted.
Partial agonists induce a reduced effect at the same receptor occupancy compared to a full agonist; they have lower "efficacy." When a full agonist is administered in the presence of different concentrations of a partial agonist, the maximum effect is always reached.
A slightly more complex situation occurs when a full agonist is administered in the presence of different concentrations of a partial agonist. In the presence of high concentrations of the full agonist (and thus pronounced effect), increasing concentrations of the partial agonist lead to displacement of the full agonist and thus to a decrease in the maximum effect. In the presence of low concentrations of the full agonist (and thus low effect), administration of increasing concentrations of a partial agonist leads to an increase in the effect, which, however, will not exceed the maximum effect of the partial agonist.