Platelet-activating factor stimulates metabolism of phosphoinositides in horse platelets: possible relationship to Ca2+ mobilization during stimulation.

Stimulation of horse platelets with platelet-activating factor (PAF) induces a rapid degradation of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. Addition of 0.1 microM PAF for 5 sec to platelets prelabeled with 32P induces a 50% loss of [32P]PtdIns(4,5)P2. 32P-Labeled phosphatidylinositol 4-monophosphate (PtdIns4P) and [32P]phosphatidylinositol (PtdIns) also are decreased, albeit at a slower rate. Loss of 32P radioactivity correlates with a net loss of fatty acids from both polyphosphoinositides. Stimulation of platelets with PAF also produces formation of [32P]phosphatidic acid and [32P]lysophosphatidylinositol. The initial disappearance of inositol lipids is subsequently followed by resynthesis, as evidenced by increased incorporation of 32P into PtdIns(4,5)P2, PtdIns4P, and PtdIns. The resynthesis of the inositides increases with time and is proportional to the concentration of PAF. Prostacyclin (1 microM) inhibits (i) the formation of phosphatidic acid and lysophosphatidylinositol and (ii) the resynthesis of polyphosphoinositides induced by 0.03 microM PAF without affecting the initial loss of PtdIns(4,5)P2. The loss of inositol lipids appears to be a primary event of platelet activation. The initial loss of polyphosphoinositides might be linked to the initiation of cellular activation by mobilizing membrane-bound Ca2+, whereas the subsequent formation of these lipids might be involved in mechanisms to prevent overstimulation of the cell.