A 28-year-old woman from Bangalore presents with hypokalemia (K^+^ 2.8 mEq/L), metabolic alkalosis (pH 7.52, HCO~3~^−^ 38 mEq/L), and hypertension (BP 160/100 mmHg). Urine potassium is 80 mEq/day (high). Plasma renin and aldosterone are both elevated. Renal ultrasound is normal. On examination, she has muscle weakness. A 24-hour urine cortisol is normal. Which segment of the nephron is the PRIMARY target of the elevated aldosterone causing potassium wasting in this patient?

Explanation

## Clinical Context: Secondary Hyperaldosteronism with Hypokalemia This patient has secondary hyperaldosteronism (elevated renin and aldosterone) with hypokalemia, metabolic alkalosis, and hypertension — consistent with renovascular hypertension or other renal artery stenosis. Aldosterone is driving potassium wasting. ## The Collecting Duct Principal Cell: The Aldosterone Target **Key Point:** Aldosterone acts on the collecting duct principal cells (not intercalated cells) to increase expression of: 1. **ENaC** (epithelial sodium channel) on the apical membrane → increases Na^+^ reabsorption 2. **Na^+^/K^+^-ATPase** on the basolateral membrane → increases K^+^ secretion into the lumen **High-Yield:** Aldosterone-mediated K^+^ secretion occurs in the collecting duct principal cells. Enhanced Na^+^ reabsorption creates a negative electrical gradient in the tubular lumen, driving K^+^ secretion via ROMK (renal outer medullary potassium) channels. ## Mechanism of Aldosterone-Induced Hypokalemia ```mermaid flowchart TD A[Elevated Aldosterone]:::action --> B[Binds mineralocorticoid receptor<br/>in collecting duct principal cell]:::action B --> C[Increases ENaC expression<br/>and Na-K-ATPase activity]:::action C --> D[Increased Na+ reabsorption<br/>from tubular lumen]:::outcome C --> E[Increased K+ secretion<br/>into tubular lumen]:::outcome D --> F[Hypokalemia]:::urgent E --> F D --> G[Metabolic alkalosis<br/>H+ secretion increases]:::outcome ``` **Clinical Pearl:** The combination of hypokalemia + metabolic alkalosis + hypertension + high renin/aldosterone suggests secondary hyperaldosteronism (e.g., renovascular hypertension), NOT primary hyperaldosteronism (where renin is suppressed). ## Comparison: Aldosterone Targets in the Distal Nephron | Segment | Cell Type | Aldosterone Effect | Ion Transported | |---------|-----------|-------------------|----------------| | **Collecting duct** | **Principal** | **Increases ENaC & Na-K-ATPase** | **Na^+^ ↑ reabsorption, K^+^ ↑ secretion** | | Collecting duct | Intercalated (α) | H^+^ secretion (via H-ATPase) | H^+^ secretion (independent of aldosterone) | | Distal convoluted tubule | — | Minimal direct aldosterone effect | — | **Mnemonic — "PRINCIPAL CELLS LOSE POTASSIUM"** — Principal cells are the aldosterone-responsive cells that secrete K^+^ and reabsorb Na^+^. [cite:Guyton & Hall Ch 29; Harrison 21e Ch 280]