Which chemicals are believed to trigger vascular spasm after vessel injury?

1. In vascular spasm the smooth muscle in the vessel wall contracts dramatically. 2. The trigger is believed to be a group of chemicals called endothelins. 3. They are released by the vessel-lining cells and by pain receptors responding to the injury. 4. Prostaglandins do maintain vasoconstriction later, but they come from platelets during plug formation rather than initiating the spasm, which is what makes them the closest trap. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.5 Hemostasis_