Agonists, Antagonists, and Partial Agonists

What is an agonist

An agonist produces a physiological effect by activating a drug receptor. When an agonist bonds to a receptor, it will stimulate the same response that stimulation of the receptor by the body's own chemicals would initiate.

What is an antagonist

An antagonist produces a physiological effect by preventing drug receptor activation. When an antagonist bonds to a receptor, it will block the response that stimulation of the receptor by the body's own chemicals would initiate.

What is a partial agonist

Partial agonists bind to and activate receptors so they function as agonists. The activation they produce, though, is less than that of a full agonist. When both an agonist and partial agonist are given together, they compete for receptor sites. The result is that the partial agonist acts somewhat as an antagonist by blocking sites that would be fully activated by agonists. This is why a partial agonist are also sometimes called an agonist-antagonist.

If activation of a receptor causes vasoconstriction, what physiologic action would result if someone takes an antagonist that binds to that receptor?

Vasoconstriction would not occur. (This may or may not lead to vasodilation depending on whether receptors for vasodilation are also being stimulated

If activation of a receptor causes vaso-constriction, what physiologic action would result if a person takes an agonist drug that activates that receptor?

Vasoconstriction would occur