Histamine is released from mast cells in allergic reactions and causes increased vascular permeability. However, in the early phase of acute inflammation (first 30 minutes), histamine-mediated permeability is confined to venules but not capillaries or arterioles. What structural feature of venular endothelium explains this selectivity?
- A Venular endothelial cells uniquely express caveolin-1 vesicular transport systems activated by histamine that bypass tight junctions
- B Venular endothelium contains abundant Weibel-Palade bodies that, upon histamine stimulation, release von Willebrand factor and P-selectin simultaneously with morphological contraction
- C Capillary and arteriolar endothelium express constitutive eNOS that generates NO, antagonizing histamine-induced contraction and maintaining barrier integrity
- D Venular endothelial cells have the highest density of H1 receptors and lack tight junctions, allowing histamine-mediated contraction to open intercellular gaps ✓
Explanation
Post-capillary venular endothelial cells express the highest density of H1 histamine receptors among vascular beds and have a morphology permitting active contractile responses — histamine binding triggers IP3-mediated Ca2+ release, activating actomyosin-based cell contraction and opening intercellular junctions to allow plasma protein leakage (exudation). Arterioles have muscular walls where smooth muscle contraction dominates, and capillary endothelium lacks the contractile machinery and H1 receptor density to respond in this manner. This selectivity defines the histamine-mediated immediate transient response (0-30 min) of acute inflammation as a venular phenomenon. Weibel-Palade bodies release VWF and P-selectin but this is the mechanism for leukocyte recruitment, not primarily vascular permeability.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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