A potent inhibitor of platelet aggregation released by endothelial cells is?

• A. Epinephrine
• B. Prostacyclin
• C. Ristocetin
• D. Thromboxane A2

Answer: (B)

Endothelial cells release a potent inhibitor of platelet aggregation called prostacyclin (PGI2). It works by raising cyclic AMP levels in platelets, which dampens calcium signaling and prevents platelets from activating and sticking together. This helps keep blood flowing smoothly in uninjured vessels and counteracts unintended clot formation. While thromboxane A2 promotes platelet activation and vasoconstriction, prostacyclin does the opposite, maintaining the balance in favor of inhibition when appropriate. The other substances listed do not serve as endothelial inhibitors in the same way: epinephrine can enhance aggregation, ristocetin is a lab reagent that induces aggregation under specific conditions, and thromboxane A2 is a strong promoter of aggregation. Thus prostacyclin from the endothelium is the correct choice.