What does the term 'antagonist' refer to in pharmacology?

In pharmacology, the term 'antagonist' specifically refers to a substance that binds to a receptor and inhibits its activity. This means that when an antagonist is present, it prevents the natural ligand (often a hormone or neurotransmitter) from activating the receptor, thereby blocking the biological response that would typically occur. This action is critical in many therapeutic contexts where it is necessary to diminish or negate the effects of certain endogenous substances.

For example, in the case of opioid receptors, antagonists like naloxone can reverse the effects of opioid overdose by displacing the opioids from their binding sites and blocking their action. This mechanism illustrates the importance of antagonists in managing various medical conditions.

The other choices describe different pharmacological actions. Enhancers are substances that facilitate or increase the activity of receptors, increasing the physiological response. Increasing the bioavailability of another drug is related to how much of a drug reaches its active site, separate from the action of antagonists. Lastly, altering the metabolism of a drug pertains to how substances are processed in the body, influencing their efficacy and duration of action, but it does not directly define the role of an antagonist.