During anaphylaxis, how does histamine affect vascular smooth muscle?

During anaphylaxis, how does histamine affect vascular smooth muscle?

• A. constricts
• B. no effect
• C. relaxes
• D. depends on receptor

Answer: (C)

Histamine in anaphylaxis causes relaxation of vascular smooth muscle, leading to vasodilation. This happens when histamine acts on endothelial cells (primarily via H1 receptors) to trigger nitric oxide release, which diffuses to the underlying smooth muscle and promotes relaxation. Some contribution from H2 receptors, increasing cAMP in smooth muscle, also supports relaxation. The net result is decreased systemic vascular resistance and hypotension, with increased vascular permeability contributing to edema. So the best answer reflects vasodilation, not constriction or no effect.

Histamine in anaphylaxis causes relaxation of vascular smooth muscle, leading to vasodilation. This happens when histamine acts on endothelial cells (primarily via H1 receptors) to trigger nitric oxide release, which diffuses to the underlying smooth muscle and promotes relaxation. Some contribution from H2 receptors, increasing cAMP in smooth muscle, also supports relaxation. The net result is decreased systemic vascular resistance and hypotension, with increased vascular permeability contributing to edema. So the best answer reflects vasodilation, not constriction or no effect.