The platelet cyclooxygenase metabolite 12-L-hydroxy-5, 8, 10-hepta-decatrienoic acid (HHT) may modulate primary hemostasis by stimulating prostacyclin production

Although HHT accounts for approximately one third of the arachidonic acid (AA) metabolites produced by stimulated platelets, no well defined function has been attributed to this product. We report that HHT stimulates prostacyclin production by endothelial cells, and have identified the mechanism for this effect. In human umbilical venous endothelial cells, HHT (0.5 and 1 microM) stimulated prostacyclin (RIA for 6KPGF1 alpha) by 32 +/- 22% (1SD) and 42 +/- 38% (P less than 0.05 and less than 0.01). Similar changes were observed when the effect of HHT on exogenous 1-14C] AA metabolism in fetal bovine aortic endothelial cells (FBAECs) was studied. Kinetic analyses revealed that HHT affected vascular cyclooxygenase. HHT (1 microM) increased Vmax in test microsomes (706 +/- 21 pmol/mg/min, mean +/- 1SE) when compared to controls (529 +/- 20; P less than 0.02). No concomitant effect on Km was observed. A further effect of HHT on AA release from endothelial cell membrane phospholipids was noted. Prelabeling experiments revealed that HHT (1 microM) increased the ionophore stimulated release of AA from FBAECs (20952 +/- 555 cpm/well control mean +/- 1SE vs 25848 +/- 557 for paired HHT treated cells; P less than 0.05). The effect of HHT on platelet AA metabolism was next studied. Preincubation of washed platelets with HHT (1 microM) did not enhance thrombin or arachidonic acid induced platelet TXB2 formation. In platelets prelabelled with 1-14C]AA, HHT (1 microM) had no effect on AA release post thrombin stimulation. Conversion to cyclooxygenase metabolites was also not enhanced. HHT stimulates vascular prostacyclin without a concomitant effect on platelet AA metabolism. HHT may thus be an important local modulator of platelet plug formation.