The stimulatory effect of prostacyclin (PGI2) on isolated rabbit and rat aorta is probably associated to the generation of a thromboxane A2 (TXA2) "like-material".

Contractile responses to prostacyclin (PGI2) as well as the generation of PGI2 "like-material" in isolated aorta from rabbits and rats, were studied. PGI2 produced a biphasic response on thoracic aorta isolated from rabbits and rats, i.e. vasodilatation at low concentrations and distinct contraction at higher ones. The rabbit aortic arch responded with relaxation in the presence of a wide range PGI2 concentration. Inhibitors of prostaglandins and thromboxane A2 (TXA2) biosynthesis such as corticosterone, indomethacin, acetylsalicyclic acid, imidazole and L-8027, abolished the stimulatory effect of PGI2 in the thoracic aorta from rats and rabbits, while alpha adrenoreceptor blockers failed to modify the vasoconstricting influence. The basal generation of PGI2 "like-material" by thoracic aorta was significantly higher in rats than in rabbits. Thoracic aortic strips from rabbits diminished the antiaggregatory capacity of 5 ng/ml of PGI2 during 40 sec and the effect disappears within 2 min. The foregoing results suggest that in thoracic aorta from rats and rabbits the vasoconstrictor effect of PGI2 could be evoked by the production of an unstable constricting prostanoid with aggregatory capacity, presumably TXA2.