Which of the following is the primary mechanism by which histamine causes increased vascular permeability in acute inflammation?

Explanation

## Mechanism of Histamine-Induced Increased Permeability **Key Point:** Histamine causes acute inflammatory edema by inducing endothelial cell contraction, not cell death. This contraction creates intercellular gaps in post-capillary venules, allowing fluid and plasma proteins to escape into the interstitium. ### Cellular Mechanism Histamine binds to **H1 and H2 receptors** on endothelial cells, triggering: 1. Increased intracellular calcium (Ca²⁺) 2. Activation of myosin light-chain kinase (MLCK) 3. Contraction of the endothelial cell cytoskeleton 4. Widening of intercellular junctions ### Timeline and Characteristics **High-Yield:** Histamine-induced permeability is: - **Immediate onset** (within seconds to minutes) - **Transient** (resolves within 15–30 minutes) - **Reversible** (no cell death occurs) - **Occurs primarily in post-capillary venules** (not arterioles or capillaries) ### Comparison with Other Mediators | Mediator | Mechanism | Onset | Duration | Site | |----------|-----------|-------|----------|------| | Histamine | Endothelial contraction | Immediate | 15–30 min | Post-capillary venules | | Bradykinin | Endothelial contraction + direct effect | Immediate | 15–30 min | Post-capillary venules | | Leukotrienes (LTC₄, LTD₄, LTE₄) | Endothelial contraction | Immediate | Prolonged | Post-capillary venules | | TNF-α, IL-1 | Endothelial injury + leukocyte adhesion | Delayed | Hours | Multiple sites | | Complement (C3a, C5a) | Endothelial contraction + leukocyte activation | Immediate | Variable | Post-capillary venules | **Clinical Pearl:** Antihistamines (H1 blockers) are effective in histamine-mediated acute inflammation but ineffective against bradykinin, leukotrienes, or complement-mediated permeability — explaining why they work well in allergic reactions but not in severe inflammatory conditions. **Warning:** Do not confuse histamine-induced permeability (reversible contraction) with severe inflammatory mediators (TNF-α, IL-1) that cause endothelial injury and prolonged leakage. [cite:Robbins 10e Ch 2]