Arterial walls generate from prostaglandin endoperoxides a substance (prostaglandin X) which relaxes strips of mesenteric and coeliac arteries and inhibits platelet aggregation

Fresh arterial tissue generates an unstable substance (prostaglandin X) which relaxes vascular smooth muscle and potently inhibits platelet aggregation. The release of prostaglandin (PG) X can be stimulated by incubation with arachidonic acid or prostaglandin endoperoxides PGG₂ or PGH₂. The basal release of PGX or the release stimulated with arachidonic acid can be inhibited by previous treatment with indomethacin or by washing the tissue with a solution containing indomethacin. The formation of PGX from prostaglandin endoperoxides PGG₂ or PGH₂ is not inhibited by indomethacin. 15-hydro-peroxy arachidonic acid (15-HPAA) inhibits the basal release of PGX as well as the release stimulated by arachidonic acid or prostaglandin endoperoxides (PGG₂ or PGH₂). Fresh arterial tissue obtained from control or indomethacin treated rabbits, when incubated with platelet rich plasma (PRP) generates PGX. This generation is inhibited by treating the tissue with 15-HPAA. A biochemical interaction between platelets and vessel wall is postulated by which platelets feed the vessel wall with prostaglandin endoperoxides which are utilized to form PGX. Formation of PGX could be the underlying mechanism which actively prevents, under normal conditions, the accumulation of platelets on the vessel wall.