PLATELET-ACTIVATING-FACTOR PREFERENTIALLY STIMULATES THE PHOSPHOLIPASE A(2) CYCLOOXYGENASE CASCADE IN THE RABBIT CORNEA

Platelet-activating factor (PAF) is formed in the cornea after injury as well as by infiltrating inflammatory cells. We have studied the eff ects of PAF on the release and metabolism of arachidonic acid (AA) in the rabbit cornea. Corneal lipids were labeled in vivo by injecting [H -3]AA and subsequently incubated in vitro with 100 nM PAF in the prese nce or absence of 10 mu M BN50727, a PAF antagonist. The AA and eicosa noids released by incubated corneas were analyzed by high-performance liquid chromatography (HPLC). Tissue lipids were examined by mono- and bidimensional thin-layer chromatography (TLC). Within 5 min, PAF stim ulated AA release to 76% above control levels. BN50727 inhibited the A A release elicited by PAF at all time points studied. The decreased co ntent of [H-3]AA in phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI) following PAF exposure and the lac k of stimulation by PAF on the release of [H-3] linoleic acid suggest that the cytosolic phospholipase A(2) was activated. PAF also stimulat ed depletion of AA from the inositol lipids, phosphatidylinositol-4-ph osphate (PIP) and phosphatidylinositol-4,5-biphosphate (PIP2) and incr eased content of [H-3]AA into diacylglycerol (DAG) and phosphatidic ac id (PA). This reaction indicates that PAF could also mediate activatio n of other phospholipases in the cornea. In addition, PAF preferential ly stimulated the cyclooxygenase pathway. The PAF antagonist BN50727 m ainly suppressed the PAF-stimulated release of PGE(2). The antagonist did not inhibit lipoxygenase activity even after 30 min of PAF stimula tion. These results suggest that PAF activates a phospholipase A(2)/cy clooxygenase pathway in the cornea via a PAF-receptor mechanism. PAF a ntagonists could be of therapeutic importance in the management of cor neal inflammation.