Prostaglandin H2 (PGH2), initially isolated from the incubation of arachidonic acid with ovine seminal vesicle microsomes, acts as a potent vasoconstrictor. It serves as the precursor for all 2-series prostaglandins (PGs) and thromboxanes (TXs). Moreover, as a TP receptor agonist, PGH2 irreversibly aggregates human platelets at concentrations of 50-100 ng ml.