Which of the following is the most common mechanism of receptor-mediated drug action in the body?

Explanation

## Receptor-Mediated Drug Actions **Key Point:** Agonist-mediated receptor activation is the most common and physiologically fundamental mechanism by which drugs produce therapeutic effects. ### Classification of Receptor Ligands by Intrinsic Activity | Ligand Type | Intrinsic Activity (α) | Mechanism | Clinical Prevalence | Examples | |---|---|---|---|---| | **Agonist** | α = 1.0 | Binds receptor → full activation → signal transduction | **Most common** | Epinephrine, acetylcholine, morphine, dopamine agonists | | **Partial agonist** | 0 < α < 1.0 | Binds receptor → submaximal activation | Moderate | Buprenorphine, aripiprazole | | **Antagonist** | α = 0 | Binds receptor → no activation (competitive or non-competitive) | Common | Propranolol, atropine, metoclopramide | | **Inverse agonist** | α < 0 | Binds receptor → reduces basal activity below resting | Rare | Timolol (β-blocker with inverse activity) | ### Why Agonists Are Most Common 1. **Physiological replacement** — restores or mimics endogenous neurotransmitter/hormone function 2. **Direct pathway activation** — simplest mechanism to produce desired effect 3. **Therapeutic breadth** — used across all organ systems (cardiovascular, CNS, endocrine, GI) 4. **Predictable dose–response** — follows Michaelis–Menten kinetics; easier to titrate **High-Yield:** The majority of therapeutic drugs are **agonists**. Antagonists are used to block unwanted effects, not to initiate new ones. ### Comparison of Mechanisms ```mermaid flowchart TD A[Drug binds receptor]:::outcome --> B{Intrinsic activity?}:::decision B -->|α = 1.0| C[Full agonist]:::action B -->|0 &lt; α &lt; 1.0| D[Partial agonist]:::action B -->|α = 0| E[Antagonist]:::action B -->|α &lt; 0| F[Inverse agonist]:::action C --> G[Maximum response]:::outcome D --> H[Submaximal response]:::outcome E --> I[No response/blockade]:::outcome F --> J[Below-baseline response]:::outcome ``` **Clinical Pearl:** - **Agonists** increase function (e.g., β~2~-agonists in asthma open airways) - **Antagonists** decrease function (e.g., β-blockers reduce heart rate) - **Partial agonists** are useful when full agonism would cause toxicity (e.g., buprenorphine in opioid dependence) - **Inverse agonists** are rare; used when basal receptor activity itself is pathological