## ACE Inhibitors and Renal Hemodynamics **Key Point:** ACE inhibitors reduce angiotensin II formation, causing preferential relaxation of the efferent arteriole and a decrease in glomerular filtration pressure. This is the mechanism by which they reduce proteinuria but may also reduce GFR acutely. ### Renal Hemodynamic Changes with ACE Inhibition ```mermaid flowchart TD A[ACE Inhibitor administered]:::action --> B[Decreased Ang II formation]:::outcome B --> C[Loss of efferent arteriolar vasoconstriction]:::outcome C --> D[Reduced glomerular capillary pressure]:::outcome D --> E[Decreased GFR]:::outcome B --> F[Afferent arteriole dilates more than efferent]:::outcome F --> G[Increased renal blood flow]:::outcome D --> H[Reduced intraglomerular hypertension]:::outcome H --> I[Reduced proteinuria]:::action ``` ### Mechanism of Glomerular Pressure Reduction | Component | Effect of ACE Inhibition | Physiological Consequence | |-----------|--------------------------|---------------------------| | Efferent arteriole | Vasodilation (loss of Ang II constriction) | ↓ Glomerular capillary pressure | | Afferent arteriole | Vasodilation (less Ang II, local prostaglandins) | ↑ Renal blood flow | | Net GFR | Decreases (especially in CKD) | ↓ Filtration pressure overcomes ↑ blood flow | | Intraglomerular pressure | Decreases | ↓ Proteinuria (therapeutic goal) | **High-Yield:** In patients with severe renal artery stenosis or advanced CKD, ACE inhibitors can cause acute kidney injury because they eliminate the compensatory efferent vasoconstriction that maintains GFR when renal perfusion is low. ### Why Option 3 Is Incorrect (The Exception) ACE inhibitors do **NOT** directly inhibit proximal tubular sodium reabsorption. They work by reducing angiotensin II, which is a stimulus for tubular Na+ reabsorption. The consequence is: - **Indirect effect:** Less Ang II → less stimulation of NHE3 → relatively increased natriuresis - **NOT a direct tubular inhibition** like thiazides or loop diuretics - The natriuresis seen with ACE inhibitors is primarily a consequence of reduced GFR and hemodynamic redistribution, not direct tubular blockade **Clinical Pearl:** The acute rise in serum creatinine (5–30% increase) seen in the first 1–2 weeks after starting an ACE inhibitor in CKD patients is expected and usually stabilizes. It reflects the drop in GFR from loss of efferent vasoconstriction, not drug toxicity. **Warning:** Do not confuse the mechanism of ACE inhibitors (hemodynamic) with loop or thiazide diuretics (direct tubular transport inhibition). [cite:Harrison Principles of Internal Medicine 21e Ch 297]
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