Step I Why does serotonin increase vascular permeability?

[whatdidigetinto]

I can understand why serotonin causes vasoconstriction (since it is released from platelets to stop bleeding), however why does serotonin lead to vasodilation at the arteriole? Shouldn't it also constrict the arteriole to control and minimize the bleeding?

 

[Spin]

I can understand why serotonin causes vasoconstriction (since it is released from platelets to stop bleeding), however why does serotonin lead to vasodilation at the arteriole? Shouldn't it also constrict the arteriole to control and minimize the bleeding?

Serotonin (as well as histamine, PAF, and bradykinin) does different things based on whether the endothelium is intact. So yes, if the endothelium is damaged, it causes vasoconstriction to reduce bleeding. If the endothelium ISN'T damaged, you don't need to worry about reducing bleeding. Instead, you need to increase blood flow so that white blood cells can extravasate and get to the injury.

You get vasodilation when intact endothelium in the arterioles is exposed to histamine, serotonin, bradykinin and/or PAF (released due to injury). Then endothelium releases nitric oxide (NO), causing smooth muscle dilation.

Also, PGI2 is a secondary effector of vasodilation: if the endothelium is intact, it can produce PGI2, which causes histamine, serotonin, bradykinin and/or PAF (the primary effectors) to cause arteriolar vasodilation. That makes sense if you want to increase blood flow and get more WBCs to the injured tissue.

 

[thinker]

Same thing with Acetylcholine. It vasodilates via NO if the endothelium is intact but vasoconstricts if it acts directly on the damaged endothelium