Coronary vasoconstrictor extracted from blood plasma stimulates platelet lipoxidase activity

The effects of a coronary vasoconstrictor, obtained from human blood plasma, on aggregation and arachidonate metabolism by human platelets was determined. At low concentrations, the vasoactive factor stimulated formation of prostaglandins, thromboxane B2, and 12-L-hydroxyeicosatetraenoic acid in both intact platelets and in platelet microsomal enzyme preparations. As factor concentration was increased, thromboxane B2 formation decreased, but production of the other products continued to rise. Low concentrations of factor initiated platelet aggregation, whereas high concentrations prevented arachidonate-induced aggregation. Low levels of factor could induce aggregation via stimulation of thromboxane A2 production. Increases in formation of 12-L-hydroperoxyeicosatetraenoic acid at high factor concentrations could inhibit formation of thromboxane A2 and thus prevent aggregation.