ENHANCEMENT BY CYCLOOXYGENASE INHIBITORS OF PLATELET-ACTIVATING-FACTOR PRODUCTION IN THAPSIGARGIN-STIMULATED MACROPHAGES

1 Thapsigargin stimulated the accumulation of cell-associated platelet -activating factor (PAF) and extracellular prostaglandin E(2) (PGE(2)) in rat peritoneal macrophages. PAF in the conditioned medium was less than the detectable amount. To obtain further insight into the mechan ism of PAF accumulation, the role of PGE(2) in PAF accumulation was in vestigated. 2 When macrophages were incubated in medium containing tha psigargin (30 ng ml(-1), 46.1 nM) and cyclo-oxygenase inhibitors such as indomethacin, naproxen or ibuprofen, the PAF content of the cells a t 10 min was increased in a concentration-dependent manner in accordan ce with inhibition of PGE(2) production. The stimulation of PAF accumu lation by cyclo-oxygenase inhibitors was significant at 10 min. Withou t stimulation by thapsigargin, cyclo-oxygenase inhibitors did not incr ease PAF accumulation. 3 In thapsigargin-stimulated macrophages, when PGE(2) (10(-7) M) was added to the medium, the cyclo-oxygenase inhibit or-induced stimulation of PAF accumulation at 10 min was markedly inhi bited. 4 The accumulation of PAF induced by thapsigargin alone at 10 m in was also inhibited by exogenous PGE(2) (10(-8) and 10(-7) M), or ar achidonic acid (10(-6) and 10(-5) M) in accordance with the increase i n PGE(2) production. 5 The accumulation of PAF induced by thapsigargin alone or by thapsigargin and indomethacin (10(-6) M) was inhibited by dibutyryl cyclic AMP. 6 These results indicate that the concurrently produced PGE(2) in thapsigargin-stimulated macrophages down-regulates PAF accumulation by increasing intracellular cyclic AMP levels, and th at cyclo-oxygenase inhibitors increase PAF accumulation by inhibiting PGE(2) production.