Aggregation and/or oxygenated products of arachidonic acid are not required for collagen-induced deacylation of phosphatidylcholine in human platelets.

In the present study the effects of collagen on platelet aggregation and arachidonic acid (AA) mobilization, specifically from phosphatidylcholine (PC), were investigated in the presence and absence of BW755C, a selective inhibitor of cyclo-oxygenase and lipoxygenases. The inhibition of cyclo-oxygenase and lipoxygenase(s) by BW755C (75 microM) resulted in severe impairment in collagen-induced platelet aggregation. In the presence of BW755C, the aggregation response amounted to 14, 26, 37 and 49% of the corresponding controls (without BW755C) at 10, 25, 50 and 100 micrograms of collagen respectively. On the contrary, the amount of AA released from PC, which ranged from 3.5 to 8.6 nmol/10(9) platelets, in response to the action of collagen (10-100 micrograms) remained unaffected by the presence of BW755C. Substantial amounts of non-esterified AA were detected in the free fatty acid fractions obtained from collagen-stimulated platelets in the presence as well as in the absence of BW755C. However, the presence of BW755C caused a greater accumulation of free AA (mass) and this ranged from 4 to 16 nmol, depending upon the amount of collagen. In addition, small increases in free stearic and oleic acids were observed in collagen-stimulated platelets as compared with unstimulated platelets. The amount of AA lost from PC represented 67, 80, 49 and 52% of the free AA obtained at 10, 25, 50 and 100 micrograms of collagen respectively. Our results adhesion of platelets to collagen fibres may be responsible for much of the AA release from PC Furthermore, these results demonstrate that aggregation and/or cyclo-oxygenase/lipoxygenase metabolites are not obligatory for the release of AA from PC in collagen-stimulated human platelets.