## Primary Stimulus for Renin Release **Key Point:** The juxtaglomerular (JG) cells respond to THREE main stimuli, but decreased sodium chloride (NaCl) delivery to the macula densa is the PRIMARY intrarenal mechanism. ### Mechanism of Detection The macula densa cells sense NaCl concentration via the Na-K-2Cl cotransporter (NKCC2). When GFR falls or renal perfusion decreases: 1. Glomerular filtration rate drops 2. Less NaCl is delivered to the distal convoluted tubule and macula densa 3. Macula densa cells detect this decrease 4. Renin is released from adjacent JG cells ### Three Stimuli for Renin Release | Stimulus | Mechanism | Relative Importance | |----------|-----------|---------------------| | **Decreased NaCl at macula densa** | Intrarenal baroreceptor; most sensitive | PRIMARY | | **Decreased renal perfusion pressure** | Baroreceptor in afferent arteriole | Secondary | | **Increased sympathetic activity** | β₁-adrenergic receptors on JG cells | Secondary | **High-Yield:** The macula densa mechanism is the most physiologically important under normal conditions. It acts as a "sensor" of tubular fluid composition, not just pressure. **Clinical Pearl:** This is why ACE inhibitors and ARBs cause a reflex increase in renin—they remove negative feedback from angiotensin II, allowing the macula densa signal to dominate. **Mnemonic:** **MAC** = **M**acula densa senses **A**bsolute **C**hloride (and sodium) [cite:Guyton & Hall Physiology 14e Ch 27]
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