Which ion channel is responsible for the most common cause of prolongation of the action potential duration (APD) in cardiac myocytes?

Explanation

## Ion Channels and Prolongation of Action Potential Duration (APD) **Key Point:** The rapid delayed rectifier potassium channel (IKr) is responsible for the **most common cause** of APD prolongation in cardiac myocytes. Blockade or dysfunction of IKr — whether drug-induced or genetic — is the predominant mechanism underlying long QT syndrome (LQTS) and pathological APD prolongation. ### Role of IKr in Cardiac Repolarization The IKr channel (encoded by the *hERG* gene, KCNH2): - Carries the rapid component of the delayed rectifier K⁺ current during **phase 3** (repolarization) - Is the dominant repolarizing current that terminates the action potential plateau - Is exquisitely sensitive to blockade by a wide range of drugs (antiarrhythmics, antibiotics, antihistamines, antipsychotics) - Loss-of-function mutations cause **LQT2**, the second most common form of congenital LQTS ### Why IKr Blockade Is the "Most Common Cause" of APD Prolongation | Mechanism | Example | Effect on APD | |-----------|---------|---------------| | IKr blockade (drug-induced) | Amiodarone, sotalol, quinidine, erythromycin, haloperidol | ↑↑ APD (most common clinical cause) | | IKr loss-of-function mutation (LQT2) | *hERG* mutation | ↑↑ APD (most common genetic cause) | | L-type Ca²⁺ channel gain-of-function (LQT8) | Timothy syndrome | ↑ APD (rare) | | INa late current increase (LQT3) | *SCN5A* mutation | ↑ APD (less common) | The stem asks for the **most common cause of APD prolongation** — this is unambiguously IKr dysfunction. Drug-induced IKr blockade accounts for the vast majority of acquired long QT syndrome cases, and *hERG* mutations account for ~30–40% of congenital LQTS (LQT2), making IKr the single most implicated channel across both acquired and inherited forms. ### Comparison with L-Type Calcium Channel The L-type Ca²⁺ channel maintains the **plateau phase** (phase 2) and is the physiologic determinant of baseline APD duration. However, it is **not** the most common cause of pathological APD prolongation. Gain-of-function mutations in L-type Ca²⁺ channels (Timothy syndrome / LQT8) are exceedingly rare. **High-Yield (KD Tripathi / Harrison):** IKr (hERG) is the most pharmacologically vulnerable repolarizing channel in the heart. Its blockade is the leading mechanism of drug-induced QT prolongation and torsades de pointes — a major reason why all new drugs are screened for hERG liability during development. **Clinical Pearl:** When a patient on multiple medications develops a prolonged QT interval, the first suspect is always IKr blockade. Common culprits include Class IA/III antiarrhythmics, macrolide antibiotics, fluoroquinolones, antifungals, and antipsychotics. **Mnemonic:** **hERG = Heart's Emergency Repolarization Gate** — when it fails (blocked or mutated), repolarization is delayed, APD lengthens, and arrhythmia risk rises.