In primary hyperaldosteronism, the characteristic laboratory finding is hypokalemia with metabolic alkalosis. Which ion channel or transporter is directly responsible for the potassium wasting in the collecting duct?

Explanation

## Mechanism of Potassium Wasting in Primary Hyperaldosteronism **Key Point:** Aldosterone acts on the collecting duct principal cell to increase sodium reabsorption via ENaC, which creates a more negative luminal potential. This electrochemical gradient directly drives potassium secretion into the urine through **ROMK (renal outer medullary potassium channel)** — the channel directly responsible for K⁺ wasting. ### Aldosterone's Cellular Mechanism 1. **Aldosterone binding** — crosses cell membrane and binds mineralocorticoid receptor (MR) in the cytoplasm 2. **Gene transcription** — MR-aldosterone complex translocates to nucleus and upregulates ENaC subunits, Na-K-ATPase, and SGK1 3. **Increased sodium reabsorption** — ENaC opens, allowing Na⁺ influx into the principal cell, generating a lumen-negative transepithelial potential 4. **Potassium secretion via ROMK** — the lumen-negative potential is the direct electrochemical driving force for K⁺ exit through **ROMK** into the tubular lumen 5. **Hydrogen ion secretion** — intercalated cells increase H⁺ secretion → metabolic alkalosis ### Why ROMK Is the Direct Effector of Potassium Wasting The question specifically asks which channel is **directly responsible for potassium wasting**. ROMK (Kir1.1) is the apical potassium channel in the principal cell through which K⁺ physically exits into the tubular lumen. ENaC drives the electrochemical gradient, but it is ROMK that is the actual conduit for K⁺ secretion. This is confirmed by the fact that ROMK knockout models abolish aldosterone-stimulated K⁺ secretion (Giebisch & Stanton, Brenner & Rector's The Kidney; also referenced in Harrison's Principles of Internal Medicine). **High-Yield:** Amiloride blocks ENaC (reducing the driving force), while ROMK is the channel through which K⁺ actually leaves the cell into the lumen. Both are involved, but ROMK is the **direct** route of potassium wasting. ### Role of Each Option | Structure | Function | Role in Aldosterone Effect | | --- | --- | --- | | **ENaC** | Sodium entry into principal cell | Indirect — creates lumen-negative potential that drives K⁺ secretion | | **Na-K-ATPase** | Basolateral Na⁺ extrusion, K⁺ uptake | Indirect — maintains intracellular K⁺ for secretion | | **ROMK** | Apical potassium secretion channel | **Direct — the channel through which K⁺ is secreted into the lumen** | | **Aquaporin-2** | Water reabsorption | Not involved in K⁺ wasting | **Clinical Pearl:** Potassium-sparing diuretics act at two points: amiloride/triamterene block ENaC (reducing the driving force for K⁺ secretion), while spironolactone/eplerenone block the mineralocorticoid receptor upstream. ROMK itself is the final common pathway for K⁺ loss. **Mnemonic:** **ROMK = Route Of Massive K⁺** — it is the apical channel directly mediating potassium secretion in the collecting duct.