macula densa cell

**Location/anatomy:** Part of the juxtaglomerular apparatus (JGA), which has three components: (1) macula densa (tubular chemosensors), (2) juxtaglomerular (JG) cells (modified smooth muscle of the afferent arteriole; secrete renin), and (3) extraglomerular mesangial (lacis/Goormaghtigh) cells. Macula densa cells are tall, closely packed columnar cells whose nuclei crowd together, giving a "dense spot" on histology.

**Sensing mechanism:** Cells detect luminal [NaCl] (mainly Cl⁻) taken up via NKCC2 (the target of loop diuretics). Changes in tubular flow rate/NaCl delivery are the signal.

**Tubuloglomerular feedback (negative feedback):** - ↑ GFR or ↑ NaCl delivery → ↑ NaCl uptake by macula densa → ↑ adenosine (from ATP breakdown) → binds A1 receptors on afferent arteriole → afferent vasoconstriction → ↓ GFR (back to normal). Also ↓ renin release. - ↓ GFR or ↓ NaCl delivery (e.g., hypovolemia, hypotension, renal artery stenosis) → ↓ NaCl uptake → ↑ PGE2 and nitric oxide (NOS1 and COX-2 dependent) → afferent arteriole vasodilation (↑ GFR) and ↑ renin release from JG cells → ↑ angiotensin II → efferent constriction (↑ GFR) → ↑ aldosterone.

**Renin release is controlled by three inputs (high-yield):** (1) intrarenal baroreceptors in the afferent arteriole (↓ stretch → ↑ renin), (2) macula densa (↓ NaCl → ↑ renin), and (3) β1-sympathetic stimulation of JG cells (↑ renin).

**Clinical correlates:** - Loop diuretics (furosemide) block NKCC2 → macula densa falsely "senses" low NaCl → ↑ renin release and impaired tubuloglomerular feedback. They also blunt the macula densa's ability to constrict the afferent arteriole. - NSAIDs inhibit COX-2/PGE2 → blunt macula densa-mediated vasodilation and renin release; in volume-depleted patients, this can precipitate AKI (afferent constriction), and can cause hyporeninemic hypoaldosteronism/hyperkalemia. - ACE inhibitors/ARBs: remove angiotensin II efferent tone → ↓ GFR in bilateral renal artery stenosis. - SGLT2 inhibitors: ↑ distal NaCl delivery → macula densa senses ↑ NaCl → afferent vasoconstriction → ↓ intraglomerular pressure (an acute eGFR dip, with renoprotective effect long term). - Bartter syndrome (NKCC2/ROMK/ClC-Kb defects) mimics chronic loop diuretic use: hypokalemic metabolic alkalosis, hypercalciuria, with ↑ renin/aldosterone and juxtaglomerular hyperplasia, but normal to low BP. - Gitelman syndrome affects the DCT (NCC), mimicking thiazides (hypocalciuria, hypomagnesemia).

**Pearls:** Macula densa = "sensor of NaCl in the tubule"; JG cells = "renin-secreting effectors"; the macula densa signals the afferent arteriole of the same nephron (this is the basis of tubuloglomerular feedback).